Patent Application
20160008784
The present invention relates generally to chlorination chemistry, more particularly to exothermic chlorination reactions and management of the heat generated thereby.
Chlorination is an integral part of many chemical operations. As an example, in current practice, crude trichlorosilane (TCS) for conversion to refined TCS and/or silane gas is produced by one of two alternate methods, commonly known as: 1) direct chlorination, and 2) hydrochlorination.
In the direct chlorination process, gaseous hydrogen chloride (HCl) in pure form and solid metallurgic silicon (MGSi) are fed to a reactor in roughly stoichiometric amounts, where the HCl reacts with the MGSi according to the following formula to produce crude TCS and, to a lesser degree, silicon tetrachloride (STC):
3HCl+1MGSi→1SiHCl3+H2
This reaction typically proceeds under operating conditions of about 3 barg and 300° C. to 330° C. HCl conversion, in the presence of a stoichiometric excess of MGSi, is quantitative at essentially 100% conversion per pass. The reaction is very fast, enabling a reactor hold-up time of about 50 seconds. The reaction is also highly exothermic. The heat of reaction is typically removed from a direct chlorination reactor by means of internal cooling coils and/or reactor cooling jackets disposed on the external shell of the reactor.
Heat removal poses significant equipment design and control issues. Improper design and control can result in hot spots and localized erosion. Hot spots are disadvantageous for a number of reactions. For example, they promote over-chlorination producing unwanted STC. They also cause the formation of undesired sintered metallurgic silicon, and the formation of agglomerations of silicon metal called “clinkers”. Sintered silicon is less reactive than unsintered silicon and reduces reactor productivity. Clinkers accumulate in the reactor and also reduce reactor productivity. Hot spots, when in the proximity of interior reactor metal surfaces, promote erosion of those surfaces. Erosion of the reactor shell can lead to reactor breach and dangerous emissions of reactor content to the ambient environment. Erosion of an internal cooling coil can lead to coil leaks and the introduction of the cooling medium into the reactor, requiring reactor shutdown and maintenance.
The HCl in a direct chlorination feedstock is typically comprised of HCl recovered from the TCS deposition process and off-gas recovery systems commonly found in polysilicon manufacturing plants. When TCS is decomposed to polysilicon in a continuous vapor deposition reactor (CVD reactor) STC is produced as an unwanted byproduct according to the following formula:
4HSiCl3→1MGSi+3SiCl4+2H2.
In the direct chlorination process, STC produced in the CVD reactor is subsequently converted back to TCS and HCl in an STC converter, operating at temperatures ranging from as low as 300° C. to 400° C. to as high as 900° C. to 1300° C., and pressures ranging from 1 barg to 10 barg, according to the following formula:
1SiCl4+1H2→1SiHCl3+HCl.
STC conversion is around 20% per pass. STC converter products are separated from each other in an off-gas recovery system. The recovered HCl is recycled back to the direct chlorination reactor for reconversion to TCS. The recovered STC is recycled back to the STC converter. The recovered H2 is substantially recycled back to the STC converter. The recovered TCS is recycled back to the CVD reactor.
The direct chlorination process enjoys several advantages compared to the hydrochlorination process discussed below. The direct chlorination process is inherently safer than the hydrochlorination process because it is operated at significantly lower pressure and temperature. Further, because the operating temperature and pressure are relatively low, the reactor shell and internal units can be made from inexpensive and readily available carbon steel—compared with expensive Incoloy 800H required for hydrochlorination. Disadvantages comprise the need to remove the exothermic heat of reaction, high potential for hot spots, erosion, and need for an STC converter. It is also difficult to scale up the reactor to achieve higher production rates and economies of scale.
In the hydrochlorination process, gaseous hydrogen, STC, and solid MGSi are fed into a reactor where the STC and hydrogen react with the MGSi according to the following formula to produce crude TCS:
3SiCl4+2H2+1MGSi→4SiHCl3.
This reaction typically runs around 33 barg and 500° C. to 550° C. The reaction is relatively slow and slightly endothermic. Reactor hold-up time runs around 90 seconds. No reactor heating is typically employed, since the endotherm is offset by pre-heating feedstock to a temperature slightly in excess of the desired reactor operating temperature. For example, if the desired operating temperature is 500° C., the reactor feedstock may be heated to 550° C. The endothermic heat of reaction is thus satisfied by the cooling of the gaseous feedstock material. Gaseous feedstock is comprised of hydrogen and STC, typically in a 1:1 to 2:1 H2:STC molar ratio. A large excess of STC is fed into the reactor because STC conversion per pass is relatively low, for example 20% conversion per pass. The large excess of STC in association with a large excess of hydrogen gas plus the TCS reaction product serves to agitate and fluidize the hydrochlorination reaction bed comprised of solid metallurgic silicon. The high degree of fluidization results in a relatively homogenous reactor content.
The effluent from the hydrochlorination reactor is separated into constituent parts in an off-gas system. Unreacted STC and hydrogen are recycled back into the hydrochlorination reactor. STC produced in the CVD reactor and separated in the CVD reactor off-gas treatment system is also fed into the hydrochlorination reactor for conversion to TCS. Small amounts of HCl may be produced in the CVD reactor. To the extent formed, this HCl or a portion thereof may be recycled to the hydrochlorination reactor.
Advantages of the hydrochlorination reactor compared to the direct chlorination reactor include the absence of cooling systems, thus enabling easy scale-up to large-sized highly productive reactors, greater economies of scale due to the larger sized reactors, absence of hot spots, reduced maintenance, and elimination of the requirement for an STC converter and its associated off-gas treatment system. Disadvantages include the requirement for much higher operating temperature and pressure with inherently greater safety hazard, longer reactor hold-up time—almost twice that for direct chlorination, large excess of reactants fed to the reactor requiring recovery and recycle efforts, relatively large-sized reactor, significantly more expensive fines recovery system, problematic fines recycle system (the down-comer leading from the cyclone back to the reactor tends to plug), very thick reactor walls required by the high operating temperature and pressure, and expensive materials of construction. The hydrochlorination reactor must be made from expensive and sometimes hard to obtain Incoloy 800H, because this is one of the few materials of construction capable of withstanding the reactors high temperature and pressure operating conditions.
Reactor design and operating conditions that enable efficient temperature management in a chlorination reaction, for example a reactor for the direct chlorination of metallurgic silicon by hydrogen chloride, are provided. The chlorination reaction is an exothermic reaction, and according to the present process the heat generated by the reaction is absorbed by a cooling fluid that is in admixture with the reactants and products, thus eliminating the necessity of cooling the reactor via internal or external cooling coils. Reactor temperature may therefore be managed by controlling, for example, the temperature and composition of the reactor feedstock. Exemplary feedstock compositions, temperatures and other input conditions are described below. Various means of producing the feedstock are described, including a method whereby the feedstock is the product of an upstream STC converter. The present disclosure provides a new reactor design and new operating conditions which capture the advantages of both direct chlorination and hydrochlorination, and eliminate the drawbacks of each. In the present disclosure, the terms direct chlorination or direct chlorination reactor may optionally be substituted for a reference to chlorination or chlorination reactor (chlorinator) to make clear that the present disclosure provides an improvement on a direct chlorination reaction wherein gas phase coolant is introduced into the direct chlorination reactor in order to perform a direct chlorination reaction under adiabatic conditions.
In one aspect, the present disclosure provides a process comprising:
In optional embodiments, the process may be further characterized by one or more of the following criteria: the reactor shell comprises carbon steel; the chloride donor is HCl; the coolant comprises STC; the coolant comprises TCS; the coolant comprises DCS; the coolant comprises hydrogen; the coolant comprises STC and hydrogen; chloride donor is in admixture with hydrogen and TCS; the chloride donor is in admixture with STC and TCS; the chloride donor is HCl in admixture with hydrogen; the chloride donor is HCl in admixture with hydrogen and STC; the chloride acceptor is MGSi particles; the chloride donor comprises HCl and the chloride acceptor comprises MGSi; the process is run adiabatically; the process is run near adiabatically; the input conditions for the HCl and the input conditions for the gas phase coolant comprise a temperature selected to maintain the reactor operating temperature within desired range; the the reactor lacks cooling surfaces in contact with the reactor shell or the reactor contents; the reactor lacks an internal cooling coil that provides cooling surfaces in contact with the reactor contents; the reactor lacks an external cooling jacket that provides cooling surfaces in contact with the reactor shell.
For example, the present disclosure provides a process comprising:
Where optionally: the gas phase coolant comprises at least one of hydrogen, STC, TCS and DCS; and/or the gas phase coolant comprises STC and hydrogen; and/or HCl is in combination with the gas phase coolant upon introduction to the reactor; and/or the operating temperature of the reactor is within the range of 250° C. to 450° C.; and/or the operating pressure of the reactor is within the range of 1 barg to 15 barg; and/or the HCl and the coolant enter the reactor at a molar ratio of coolant:HCl of 2:1 to 20:1; and/or the heat is not transmitted to a cooling coil or a cooling jacket; and/or the process is operated under adiabatic conditions. The temperature of the gas phase HCl and the temperature of the gas phase coolant, which may be the same temperature in the event that the HCl and coolant are in admixture prior to entering the reactor, may be selected so as to achieve a desired operating temperature for the reactor, where the operating temperature may be a constant temperature, e.g., varying no more than about ±5%, of about, for example, 320° C. For example, the process may be performed such that the chloride donor and the coolant are in admixture upon being introduced into the reactor, the admixture having a temperature within the range of 120-280° C. and being at least 30° C. less than the operating temperature of the reactor, the operating temperature of the reaction being a temperature within the range of 250-400° C. and the operating pressure being 1-15 barg, the admixture having a molar ratio of coolant:HCl of 2:1 to 20:1. Any of these values may be replaced with other values as set forth herein for the specified parameter. The composition and temperature of the admixture may be selected so as to maintain the operating conditions within the reactor at a steady state.
The process as disclosed herein may further comprise:
In addition, the process as disclosed herein may further comprise:
In another aspect, the present disclosure provides a system comprising:
Optionally, the system may be characterized by one or more of the following criteria: the 1st stage reactor is an isothermal reactor; the 1st stage reactor is an adiabatic reactor; the system further comprises a cooling unit to cool the first product gas to a desired input temperature to the chlorination reactor; the system further comprises an HCl source, where the HCl source is in fluid communication with a conduit in fluid communication with both the 1st stage reactor and the 2nd stage reactor; the system further comprises an off gas recovery system in fluid communication with the 2nd stage reactor, where the off gas recovery system may separate TCS from hydrogen; the off gas recovery system is in fluid communication with a hydrogen compressor, where the hydrogen compressor may compress the hydrogen separated by the off gas recovery system; the system further comprises a reactor where polysilicon may be prepared from TCS. The system may optionally include a temperature control means, e.g., a cooling jacket or a heating coil surrounding the conduit, to control the temperature of the first product gas before it enters the 2nd stage reactor. The system may optionally include a temperature monitoring means, e.g., a thermocouple, to monitor the temperature within the 2nd stage reactor.
In another aspect, the present disclosure provides a system comprising a chlorination reactor in fluid communication with a source of MGSi and also in fluid communication with a source of hydrogen chloride, the chlorination reactor also in fluid communication with at least one of a source of hydrogen and a source of STC. The hydrogen and/or STC provide the coolant of the present disclosure which allows the chlorination reaction taking place within the chlorinator to be performed in an adiabatic manner. In various optional embodiments, where any two or more of the following embodiments may be combined to describe an embodiment of the invention (so long as the embodiments are not mutually exclusive): the chlorination reactor is also in fluid communication with an STC converter, and the STC converter provides the source of STC (an STC converter is typically far from 100% efficient in converting STC to TCS, and accordingly the effluent from an STC converter will typically contain STC); the chlorination reactor is not in direct fluid communication with an STC converter, i.e., the effluent from an STC converter is not directly fed into the chlorination reactor, however, an STC converter may be present elsewhere in the system, e.g., an STC converter may be used to treat the STC that is present in the off gas from a CVD reactor that produces polysilicon by the Siemens process, in which case the product from this STC converter is not directly fed into the chlorinator but instead is fed into an off gas recovery system which generates a fraction enriched in STC which may be used as the source of STC in the chlorination reaction; the chlorination reactor is in fluid communication with a fractionation column, and an effluent from the fractionation column provides the source of STC, where this may occur, as mentioned previously, when the CVD reactor is in fluid communication with an STC converter, and the STC converter is in fluid communication with an off gas recovery system that includes a fractionation column; the system further comprises an off gas recovery system in fluid communication with the chlorination reactor, the off gas recovery system providing separation of components of off gas from the chlorination reactor, the off gas recovery system providing an off gas fraction enriched in hydrogen where at least some of the off gas fraction enriched in hydrogen provides the source of hydrogen to the chlorination reactor, in other words, the hydrogen present in the off gas from the chlorination reactor is recycled back into the chlorination reactor after having pass through an off gas recovery system that provides a fraction enriched in hydrogen; the chlorination reactor is a direct chlorination reactor which reacts hydrogen chloride with MGSi in a fluidized bed to produce TCS; the chlorination reactor in not in direct fluid communication with a STC converter such that the effluent from an STC converter is not directly introduced into the chlorination reactor; the system further comprises a source of STC in fluid communication with the chlorination reactor, whereby STC is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the system further comprises a source of hydrogen in fluid communication with the chlorination reactor, whereby hydrogen is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the system further comprises a chemical vapor deposition (CVD) reactor for producing polysilicon and a CVD off gas.
In another aspect, the present disclosure provides a process comprising a) performing a chlorination reaction in a chlorination reactor at a first temperature, where MGSi and hydrogen chloride are reacted together to provide a product gas comprising trichlorosilane; b) introducing a coolant selected from hydrogen and STC to the chlorination reactor, the coolant being introduced at a second temperature, the second temperature being less than the first temperature, the second temperature selected so that the chlorination reactor is performed under adiabatic conditions. In various optional embodiments, where any two or more of the following embodiments may be combined to describe an embodiment of the invention (so long as the embodiments are not mutually exclusive): the chlorination reactor is in fluid communication with an STC converter, and the STC converter provides the source of STC; the chlorination reactor is not in direct fluid communication with an STC converter; the chlorination reactor is in fluid communication with an off gas recovery system, where the off gas recovery system generates a fraction enriched in STC, and the fraction enriched in STC is introduced into the chlorination reactor to provide the source of STC to the chlorination reactor; an effluent from an STC converter is not directly introduced into the chlorination reactor; STC is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the chlorination reactor is in fluid communication with an off gas recovery system, where the off gas recovery system generates a fraction enriched in hydrogen, and the fraction enriched in hydrogen is introduced into the chlorination reactor to provide the source of hydrogen to the chlorination reactor; the process further comprises performing the Siemens process for polysilcion production.
In standard direct chlorination reactors, heat of reaction (released when HCl chlorinates metallurgic silicon) must be removed by heat transfer to a contained cooling fluid, where the cooling fluid is contained in internal cooling coils and/or conduits applied to the shell of the reactor forming a co-called cooling jacket. This system of heat transfer will be referred to herein as external cooling. External cooling makes scale-up difficult: the practical limit is a reactor size of one meter in diameter. The present disclosure provides chemical processes having improved temperature management, so that little to no external cooling is required. Accordingly, a chlorination reaction may be conducted in larger reactors, for example, reactors of, in various embodiments, 2, 2.5, 3 or more meters in diameter. Larger size reactors enable higher production rates from a single reactor with concomitant economy of scale.
The present disclosure provides a reactor design and operation in which little to no external cooling is required because the reactor is operated in a way that the feed to the reactor will absorb the heat of reaction yielding an adiabatic or near-adiabatic operating conditions at a temperature of about, for example, 300° C. to 350° C. This is possible because the molar feed rate exiting the reactor will be an order of magnitude greater than the molar feed rate exiting the standard direct chlorination reactor. Further advantageously, the reactor gaseous effluent may be comprised of STC, TCS, and hydrogen, which all have inherently high heat capacities. Thus, by appropriately choosing the proper feed temperature of the combined admixture, the energy released in the reaction may be exactly balanced by the energy required to heat the combined admixture to the desired reactor operating temperature.
The details of one or more embodiments are set forth in the description below. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Other features, objects and advantages will be apparent from the description, the drawings, and the claims. In addition, the disclosures of all patents and patent applications referenced herein are incorporated by reference in their entirety.
Features of the invention, its nature and various advantages, will be apparent from the accompanying drawings and the following detailed description of various embodiments.
In one aspect, the present disclosure provides a chlorination process. In the chlorination process, a chloride donor and a chloride acceptor are combined in a reactor and chloride is transferred from the donor to the acceptor with the release of heat. If the donor is HCl, the chlorination process is an exothermic reaction. Optionally, at least one of the chloride donor and chloride acceptor is a silicon containing compound. As examples, the chloride acceptor may be a silicon-containing compound selected from any one or more of metallurgic silicon (MGSi), silane, disilane, monochlorosilane, dichlorosilane and trichlorosilane. MGSi may be the chloride acceptor. MGSi is a commercially available material, available from many suppliers. It can be produced by a submerged-electrode arc furnace, using quartzite gravel as the starting material. The chloride donor may be HCl. HCl is a commercially available material, available from many suppliers. It can be produced by the Siemen's process for polysilicon manufacture.
An adiabatic process refers to any process occurring without gain or loss of heat within a system (i.e. during the process the system is thermodynamically isolated—there is no heat transfer with the surroundings). This is the opposite of a diabatic process, where there is heat transfer. An adiabatic process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time, so that there is no opportunity for significant heat exchange. In other words, a transformation of a thermodynamic system can be considered adiabatic when it is quick enough or so well insulated such that no significant heat is transferred between the system and the outside (e.g., to cooling coils containing a heat transfer medium). An exothermic adiabatic reaction is one where the temperature of the reaction products are greater than that of the reaction feed admixture. As used herein, the term internal coolant refers to coolant that is in admixture with the reactants and products of the chlorination reaction. Internal coolant may also be referred to as inherent coolant. Internal coolant allows for internal cooling of the reactor contents. The term external coolant refers to coolant that is not in admixture with the reactants and products of the chlorination reaction. The use of external coolant will be referred to herein as external cooling. An external coolant may, for example, be contained within the walls of a conduit that traverses the interior of the reactor. A cooling coil is an example of such a conduit, and the coolant within a cooling coil is an example of an external coolant. A cooling coil allows for external cooling of the reactor contents. Another example of external cooling is a cooling jacket which is fitted to the outside of the reactor shell. Each of a cooling coil and a cooling jacket is exemplary of active external cooling, whereby heat is transmitted to coolant conduit and then to coolant, where the coolant is actively circulated or otherwise caused to travel through the conduit. External cooling may also occur passively, which occurs when the reactor shell is incompletely insulated and some heat leaks away from the reactor directly into the environment.
In one aspect, the process of the present disclosure includes providing a reactor sited in an environment, where the reactor comprises a reactor shell that encloses the reactor contents. The reactor shell is the barrier between the inside and the outside of the reactor. The reactor shell envelops an internal controlled environment in which a chlorination reaction as discussed below, takes place. Accordingly, the reactor may be referred to herein as a chlorinator. The reactor may, but may not, have internal heating or cooling coils. In addition, or alternatively, the reactor may, but may not, have an external heating or cooling jacket. Internal coils, an external jacket, and indeed any means by which conduit for cool fluid is made to contact the hot reactor shell or hot reactor contents so that the cooling fluid absorbs and thereafter transports away and dissipates heat from the reactor, is optionally absent from the chlorinator of the present disclosure. In one embodiment, the reactor does not have any specialized heat removal equipment so that all of the excess heat, i.e., all of the heat that is not simply dissipated to the environment, is captured in heating a cooling fluid that is in admixture with the reactants and products of the chlorination reactor. The environment containing the reactor, i.e., within which the reactor is sited, may be ambient conditions of temperature and pressure, e.g., a temperature of about 20-30° C. and a pressure of atmospheric pressure.
The reactor may be thermally insulated so that heat from the reactor does not passively escape into the environment. An insulated reactor is advantageous for several reasons. For example, from the viewpoint of safety, an insulated reactor avoids the presence of a hot external reactor shell surface which may burn people in the vicinity of the reactor. In addition, when the reactor is sited outdoors in the absence of a covering roof and/or enclosing walls, rain and/or wind which contacts the reactor will not be able to impact the temperature of the reactor when that reactor is thermally insulated. A perfectly thermally insulated reactor allows for the process of the present disclosure to operate under adiabatic conditions. Perfect insulation is difficult and expensive to achieve, so the process of the present disclosure may proceed under near adiabatic conditions, whereby a small amount of heat passively escapes from the reactor into the environment. Under near adiabatic conditions, the outside of the reactor may be slightly above ambient temperature, for example, within +2° C., or +4° C., or +6° C. or +8° C. or +10° C. compared to ambient temperature.
In order to operate a reactor at the high pressure and high temperature typically associated with hydrochlorination reactors, and yet avoid or minimize corrosion due to HCl, Incoloy 800H has proven to be the construction material of choice. Development of increasingly larger hydrochlorination reactors has been limited by the mechanical and strength properties of Incoloy 800H. Current metallurgy and metal plate production technology has limited plate thickness to 80 mm. Addition of other alloying materials to increase high temperature tensile strength appears to also reduce ductility (i.e., increase embrittlement). The reactor and system of the present disclosure can utilize Incoloy 800H, but as will be described herein, the reactor and system of the present disclosure operates at lower temperatures than typically utilized for hydrochlorination reactions, and accordingly carbon steel and other materials may be used to form the reactor shell.
In addition to a reactor shell, the reactor may include a distributor tray. Levenspiel's “Fluidization Engineering” discloses several suitable distributor trays. The distributor tray may be located near the bottom of the chlorination reactor. In effect, the distributor tray provides for a “false bottom” beneath which is a plenum into which the reactor feed is directed. In general, the purpose of a distributor tray in a fluid bed reactor (FBR), also sometimes referred to as a gas distributor plate, is to spread out the inlet gases so as to have an even or uniform bubbling action across the bed diameter, but with a minimum of solids abrasive wear and a minimum of distributor plate pressure drop. An uneven spread of inlet gases causes excessive central bubble formation which leads to bypassing, and also allows “dead spots” to form where there is an insufficient amount of reaction gas available. As mentioned herein, the chloride acceptor may be MGSi. However, it is recognized that MGSi is abrasive in nature and the undue localized jetting of reaction gas may cause wear spots on the reactor's wall and some portions of the distributor plate. Also, a higher than necessary pressure drop increases compressor horsepower, and therefore wastes utilities. The use of a gas distributor plate addresses the problems associated with use of MGSi as the chloride acceptor.
Due to the high exotherm of chlorination reactions (on the order of 50-60 kcal/g-mole) a high degree of fluidization is necessary to maintain the MGSi particle surface at a low enough temperature such that particle-to-particle agglomeration does not occur. It is also necessary to have the inlet gas temperature rapidly move past the minimum 250-280° C. range, where the surface reaction can be extinguished, but yet keep the Stage 2 bulk gas off-gas temperature sufficiently low (e.g., under about 400° C.) so that TCS formation is favored over STC formation. The distributor tray helps achieve these advantageous effects, although the reactor may be operated without a distributor tray.
The reactor may optionally have an expanded head or a cyclone to handle the fines generated during the chlorination reaction. During the course of the reaction, some fine material comprising metallurgic silicon, and catalyst if present, may be blown out of the fluidized bed. These fines may be captured in a calming zone at the top of the reactor, which is integral to and formed by an expanded reactor head. The fines that have been captured in this calming zone fall back into the fluidized bed portion of the reactor where they participate in the chlorination reaction. Alternatively, the fines may be captured in a cyclone located inside the reactor at the top of the reactor, or located external to the reactor on the product transfer line leaving the reactor. Fine captured by the cyclone are directed back into the reactor through a down-comer for further reaction. In one embodiment, the reactor used in the process of the present disclosure contains a calming zone. In an alternative embodiment the reactor used in the process of the present disclosure includes or is in combination with a cyclone.
Compared to a reactor employed in a standard direct chlorination reaction, the reactor utilized in the presently disclosed process has multiple advantages. For example, since little to no heat removal from the reactor is required, the reactor may employ a simplified design having little to no heat handling equipment, thus reducing installed equipment cost and maintenance costs, and affording a longer operating life for the reactor. In addition, the reactor can be scaled up very easily to large size because heat removal is no longer the limiting design factor. In embodiments, the reactor is generally cylindrical having a diameter of greater than 1.0, or 1.5, or 2, or 2.5, or 3 meters. As a result, for example, it is possible to build a single reactor capable of producing enough crude TCS to produce 10,000 MTA polysilicon—compared to standard direct chlorination reactors which are constrained by heat removal problems to only ⅓rd or less of this capacity.
Compared to a reactor employed in a standard hydrochlorination reaction, the reactor utilized in the presently disclosed process also has multiple advantages. For example, the lower operating pressure and temperature of about 3 barg and about 300° C., compared to about 33 barg and 550° C. in a standard hydrochlorination reactor, translate into improved inherent safety and lower capital and operating costs. The relatively mild operating conditions available in the chlorination process of the present disclosure enable the use of low cost carbon steel for reactor shell construction in lieu of high cost INCOLOY™ alloys such as INCOLOY 800H™, thereby reducing capital expense for building the reactor by a factor about 10 times. Indeed, the process and systems described herein can utilize a chlorination reactor formed from carbon steel, e.g., 321 SS and 347 SS are acceptable materials from which to form the reactor. The problem of fines can be managed by using calming zones for metallurgic silicon recovery instead of more expensive and operationally problematic cyclones. Finally, compared to standard hydrochlorination reactors, the reactors of the present process may be smaller size and more productive.
In one aspect, the process of the present disclosure includes operating the chlorinator within a specified temperature range and/or a within a specified pressure range. In embodiments, the temperature within the reactor is maintained within the range of 250-450° C., 250-400° C., 250-350° C., 300-450° C., 350-450° C., 300-400° C., 300-350° C., or 350-400° C. The upper temperature is lower than the 500° C. typically used by reactors operating a standard hydrochlorination process. In embodiments, the pressure within the reactor is maintained within the range of 1-15 barg, 1-10 barg, 1-8 barg, 1-6 barg, 2-10 barg, 2-8 barg, or 2-6 barg. Each of the aforesaid temperature ranges may be combined with any of the aforesaid pressure ranges. For example, the reactor may be operated at 250-350° C. and 1-10 barg or 300-350° C. and 1-6 barg.
The operating temperature and pressure within the reactor will depend, to some extent, on how quickly the gaseous feedstock(s) are allowed to enter the reactor, and how quickly the gaseous products exit the reactor. In other words, the flow rate and hold up time may influence the operating pressure and temperature. The hold up time will depend, in part, on the size and configuration of the reactor. A longer thicker reactor will have a longer hold up time than a shorter thinner reactor. The chlorination reaction that takes place between the chloride donor and the chloride acceptor is a fast reaction at temperatures above about 250° C., allowing for a short hold up time, that is, a hold up time in the range of 10 to 100 seconds, or 10 to 90 seconds, or 10 to 80 seconds, or 20 to 100 seconds, or 20 to 90 seconds, or 20 to 80 seconds, or 20 to 70 seconds, or 30 to 100 seconds, or 30 to 90 seconds, or 30 to 80 seconds, or 30 to 70 seconds, or 30 to 60 seconds. In other embodiments, the hold up time is less than 100 seconds, or less than 90 seconds, or less than 80 seconds, or less than 70 seconds, or less than 60 seconds.
The temperature of the gas phase HCl and the temperature of the gas phase coolant, which may be the same temperature in the event that the HCl and coolant are in admixture prior to entering the reactor, may be selected so as to achieve a desired operating temperature for the reactor. Optionally, the operating temperature may be a constant temperature, i.e., varying no more than about ±5%. The operating temperature of the reactor may be, for example, about 300-350° C., or about 320° C.
In one aspect, the process of the present disclosure includes introducing a gas phase component to the reactor under input condition, where the component is primarily intended to function as a coolant. For convenience, the component will be referred to herein as the coolant where the coolant may be one chemical or a mixture of chemicals. Unlike traditional cooling fluids, the coolant as used herein is in admixture with the chloride donor and chloride acceptor. This is in contrast to the use of traditional cooling fluids, which never come into direct contact with the reactor contents, but instead travel through and remain within conduits. The conduits are formed of heat transmitting materials that make direct contact with either the reactor content itself, or with the reactor shell which is, in turn, in direct contact with the reactor contents, so that heat from the reactor is indirectly transmitted to the cooling fluid. The present process provides for direct transfer of heat from the reactor contents to the cooling fluid, i.e., heat of reaction does not travel through material which forms a conduit for the cooling fluid.
According to the present process, because the cooling fluid is in direct contact with the reactants and products of the reaction, the cooling fluid should advantageously not interfere with or preclude the chemistry of the desired chlorination reaction.
The amount of cooling fluid introduced into the reactor is preferably selected so as to be able to absorb all, or nearly all, of the heat generated by the chlorination reaction. In other words, in order to maintain a selected reactor operating temperature while introducing chloride donor and/or chloride acceptor into a reactor, the heat generated by the exothermic chlorination reaction should be approximately equal to the heat needed to raise the temperature of the cooling fluid to the operating temperature of the reactor. If too little cooling fluid is introduced, the reactor operating temperature will rise above a desired maximum, while if too much cooling fluid is introduced, the reactor operating temperature will fall below a desired minimum temperature. If the reactor operating temperature rises above a desired maximum, there can be serious undesirable consequence. For example, the material that forms the reactor shell is exposed to higher than anticipated temperatures, which may weaken the shell to the point of vessel failure. As another example, the chlorination reaction may produce undesired by-products, such as fused particles of MGSi, the presence of which may slow down the desired chlorination reaction and/or necessitate periodic removal of the fused particles which reduces the operating efficiency of the reactor, or excessive formation of STC thereby reducing yield to desired TCS product. If the reactor operating temperature falls below a desired minimum, there can likewise be serious undesirable consequences. The chlorination reaction, while being an exothermic reaction, nevertheless needs a certain amount of heat in order to proceed. If the temperature within the reactor is too low, the chlorination reaction will either not begin, or will proceed only partially. Accordingly, it is desirable to introduce cooling fluid into the reactor in the proper amount and temperature.
The input conditions of the cooling fluid will impact the effectiveness of the fluid to achieve the desired cooling of the reactor contents. For example, as the amount of cooling fluid is increased, all other factors being kept constant, then the reactor contents are liable to cool below the desired minimum. Conversely, if the amount of cooling fluid is decreased, all other factors being kept constant, then the reactor contents are liable to heat above the desired maximum. As the temperature of the cooling fluid is decreased, all other factors being kept constant, then the reactor contents are liable to cool below the desired minimum. However, if the temperature of the cooling fluid is increased, all other factors begin kept constant, the reactor contents are liable to heat above the desired maximum. In selecting the input conditions for the cooling fluid, it is preferred that those conditions provide for a gaseous cooling fluid. In other words, the temperature of the cooling fluid is preferably above the dew point of the fluid.
However, the temperature of the cooling fluid, while it is preferably above the dew point of the fluid, is also preferably below the operating temperature of the chlorinator. In various embodiments, the cooling fluid has a temperature which is less than but within 20° C., 40° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 100° C., 150° C., or 200° C. of the operating temperature of the chlorinator. In other embodiments, the temperature of the cooling fluid is at least 20° C., 40° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 100° C., 150° C., or 200° C. below the operating temperature of the chlorinator.
In addition to the amount and temperature of the cooling fluid, it is important to keep in mind the identity of the fluid and relative components thereof. Some cooling fluid components require more heat to increase their temperature from T1 to T2 than do other cooling fluid components. This ability of a material to absorb heat may be measured by molar heat capacity. The molar heat capacity of gas phase STC is about 90 J/(mol K) while the molar heat capacity of gas phase hydrogen is about 29 J/(mol K). Accordingly, a mole of STC absorbs more heat as it warms from T1 to T2, than does a mole of hydrogen. As the cooling fluid is enriched in STC at the expense of hydrogen, all other factors being equal, the cooling fluid has a greater capacity to absorb heat and so the reactor contents are liable to cool below the desired minimum. Conversely, as the cooling fluid is enriched in hydrogen at the expense of STC, the cooling fluid cannot absorb as much heat, and is less effective. This effect can be counter-acted by using more of a cooling fluid having a lower molar heat capacity. However, using more cooling fluid requires a larger reactor to contain the larger amount of cooling fluid, which adds to the capital cost of the system. Another disadvantage of using more cooling fluid is that the reactants and products become diluted, which reduces the operating efficiency of the system.
In one aspect, the feed to the chlorinator may be the off gas recovered from an STC converter. In an STC converter, STC and hydrogen are introduced and a mixture of TCS, STC, hydrogen and HCl are produced as an off gas. This off gas includes a chloride donor (HCl) in admixture with cooling gas (TCS, STC and hydrogen). An STC converter may be operated so as to achieve various relative amounts of HCl and cooling gas in the off gas, e.g., by varying the ratio of hydrogen and STC in the feed. Table 1 illustrates four (cases 1-4) exemplary STC converter off gas compositions that may be used as the feedstock for a chlorination reaction.
An illustration of the impacts of changing the molar ratio of cooling fluid to HCl entering the chlorination reactor is shown in the four cases provided in Table 1. More specifically, in each of the 4 cases of Table 1, the amount of HCl entering the reactor is constant at 30 kg-mol/hr, TCS is constant at 30 kg-mol/hr, and STC is constant at 70 kg-mol/hr, however a decreasing amount of hydrogen coolant is present in the feed in going from case 1 to case 4. In case 1, the feed has 370 kg-mol/hr of hydrogen, 30 kg-mol/hr of TCS and 70 kg-mol/hr STC in addition to 30 kg-mol/hr HCl. In cases 2-4, the amount of hydrogen present in the feed is reduced, so that proportionally, the feed has increasing amount of TCS and STC. As a consequence, in case 1, the cooling fluid:HCl molar ratio is 15.7, which decreases to 9.0, then to 5.7, and finally to 4.0 in case 4. As a consequence of decreasing the hydrogen content of the cooling fluid, while keeping the amounts of TCS and STC constant, the reactor temperature must be reduced from 220.5° C. in case 1 to 182.9° C. in case 2, to 150.7° C. in case 3, to 127.8° C. in case 4. The reactor feed temperature is that temperature at which the reactor can be run adiabatically at a given temperature; e.g., 320° C., at a given pressure; e.g., 3 bar(g).
Table 1 shows feedstocks to a chlorination reactor exemplary of STC converter off-gas products where the temperature of the admixture forming the coolant is adjusted to achieve constant operating reaction temperature of 320 degrees centigrade for a constant amount of HCl reaction. The molar ratio of coolant to HCl is 15.7 in case 1. In general, the molar ratio of coolant to HCl may range from about 3:1 to 20:1, or from 4:1 to 19:1, or from 5:1 to 18:1; or from 6:1 to 17:1, or from 7:1 to 16:1, of from 8:1 to 15:1. At the lower end, i.e., 3:1 molar ratio, the required feed temperature approaches the dew point. At the higher end, the size of the direct chlorination reactor for a given hold up time becomes excessive.
Table 2 illustrates six cases (cases 5 to 10) which show the effect of using pure hydrogen coolant. Exemplary molar ratios range from 8:1 to 20:1. Below about 8:1, the required feed temperature becomes too low and will quench the chlorination reaction.
Table 3 illustrates five cases (cases 11 to 15) which show the effect of using pure STC coolant. Exemplary molar ratios range from 2:1 to 20:1. Below about 2:1, the required feed temperature approaches the dew point (e.g., the dew point is 92.9° C. at a molar ratio of 2.3:1). The dew point represents the approximate lowest possible input temperature for the feed, since the feed should enter the reactor substantially in a gas phase so as to prevent reactor cold spots and related operational issues.
In cases 1-15 in Tables 1 to 3, the HCl feed rate is held constant and 100% of it is reacted to silicon products in the chlorinator. Also, the operating temperature of the chlorinator is maintained at 320 degrees centigrade by adjusting coolant temperature, and the operating pressure in all cases is 3 barg.
The process of the present disclosure may be run in such a way that the HCl and the coolant are in admixture before they enter the reactor. However, whether or not they are in admixture, in various embodiments, the HCl and the coolant may enter the reactor at a molar ratio of coolant:HCl of 2:1 to 20:1, or 3:1 to 18:1, or 4:1 to 16:1, or other ratios as disclosed herein.
It is noted that the dew point is a function not only of composition (e.g., higher hydrogen content raises the dew point, whereas higher STC content lowers the dew point), but of system operating pressure. Higher operating pressure lowers the dew point for any given mixture—and vice versa. Tables 1-3 are based on 3 barg because this is the pressure at which direct chlorination reactors are typically operated. However, the process of the present disclosure is flexible and can utilize other operating pressures as described elsewhere herein, including any given concentrations of the coolant mixture, any given pressure of the direct chlorination reactor, and any given molar ratio of coolant to HCl in the direct chlorination reactor feed. The chloride acceptor is metallurgic silicon in these examples, and it is fed to the chlorinator at 200° C. because that temperature is typically used in chlorination reactions. However different temperatures can be used—both higher and lower, in the process of the present disclosure. For example, the temperature of the chloride acceptor may be as low as 50° C., or 80° C., or 100° C., or 130° C., or 160° C., or 180° C. or 220° C. The upper temperature may be 300° C., or 280° C., or 260° C., or 250° C., 240° C., or 230° C., or 220° C., or 200° C., or 180° C., or 160° C., or 140C. Suitable ranges may be therefore be, for example, 180-220° C., or 160-240° C., or 140-280° C.
Relevant factors to consider in selecting input conditions to the reactor include the following: (a) The feed should be maintained above its dew point. For a given temperature, this may be accomplished, for example, by adding more coolant to the feed thereby increasing the ratio of coolant to HCl; (b) A suitable amount of coolant should be used so that the chlorination reactor does not become too large for a given required hold up time (typically on the order of 50 seconds); (c) The feed temperature should be selected so that the chlorination reactor runs within a desired operating temperature range.
In one aspect, the process of the present disclosure additionally includes operating the reactor in a semi-batch or continuous mode and/or as a fluidized bed reactor. The reactor may operate in a semi-batch or a continuous mode. In a semi-batch mode, the chloride acceptor, e.g., metallurgic silicon, is introduced into the reactor and maintained therein for a desired period of time, while chloride donor, e.g., HCl, is fed to the reactor, and gaseous reaction products are continuously withdrawn from the reactor. At the end of the desired period of time, the residual metallurgic silicon feedstock is withdrawn from the reactor, and a fresh metallurgic silicon feedstock is introduced into the reactor. Typically, the temperature and pressure within the reactor, when operated in a batch mode, will undergo significant fluctuation as the metallurgic silicon material is introduced and then withdrawn from the reactor. In a continuous mode, chloride acceptor and chloride donor are substantially, continuously fed to the reactor, and gaseous reaction products are continuously recovered from the reactor. In a continuous mode operation, the reactor is constantly maintained within a desired temperature and pressure range.
The reactor may be a fluid (a.k.a. fluidized) bed reactor. In one embodiment, the MGSi provides the particles which form the fluidized bed, however the bed may instead be formed from small metallic particles. The chloride donor, e.g., HCl, may enter the reactor via an inlet tube directed into the base of the reactor below the distributor tray. The chloride acceptor, e.g., MGSi, may be added through a dip tube descending into the upper part of the reactor, the middle of the reactor, or the lower part of the reactor but above the distributor tray. In addition to one or more inlets, the reactor has at least one outlet so that materials can be constantly fed into the reactor and allowed to exit the reactor.
In one aspect, the process of the present disclosure additionally includes introducing a chloride donor to the chlorinator under input condition. The input conditions describe the conditions of the chloride donor as it enters the chlorinator. For example, the donor may be in the gas phase. The donor may be at a temperature within the range of 30-450° C., 250-450° C., 250-400° C., 250-350° C., 300-450° C., 350-450° C., 300-400° C., 300-350° C., or 350-400° C. The donor may be under a pressure of 1-10 barg, 1-8 barg, 1-6 barg, 2-10 barg, 2-8 barg, or 2-6 barg. The donor may be pure, or it may be in combination with one or more other components such that the donor constitutes less than 50 mole %, or at less than 45%, or less than 30%, or less than 20%, or less than 15%, or less than 10%, or less than 5% of the material input into the reactor, on a molar basis. As mentioned elsewhere herein, the chloride donor may be HCl. Optionally, the donor may be in admixture with the coolant as they both simultaneously enter the chlorinator. In this case, the temperature and pressure of the admixture may be any of the values provided herein for the admixture components, i.e., the chloride donor and the coolant.
In one aspect, the process of the present disclosure additionally includes introducing a chloride acceptor to the chlorinator under input condition. The input conditions describe the conditions of the chloride acceptor as it enters the chlorinator. A preferred chloride acceptor is MGSi, and the input conditions to the chlorinator for the chloride acceptor will be illustrated by reference to MGSi. In general, the chloride acceptor will contain silicon as it is introduced to the reactor. The chloride acceptor will contain silicon and chloride as it exits the reactor. In one embodiment, the chloride acceptor is metallurgic silicon (MGSi) of at least 95%, or at least 97%, or at least 98%, or at least 99% purity. The MGSi is a particulate material that may be added to the reactor at ambient or elevated temperature and pressure. Within the chlorinator, the MGSi may be present in molar excess compared to the presence of the chloride donor within the reactor. The MGSi may not only function as the chloride acceptor, it may also function as the bed material in forming a fluidized bed within the reactor.
In direct chlorination, the transfer of chloride from the chloride donor to the chloride acceptor is an exothermic reaction. The exothermic heat of reaction is transmitted, at least in part and optionally in full, to the internal coolant that is in admixture with the reactant(s) and product(s) of the chlorination reaction. Some of the heat may optionally be transmitted directly to the environment or to external coolant. This may occur when the reactor shell is incompletely insulated, so that heat can escape from the reactor to the environment. This may also occur when cooling coils are allowed to contact the reactor and/or the contents of the reactor, where the cooling coils contain circulating or otherwise moving coolant that absorbs heat from the walls of the cooling coil and then carries away that heat to the environment. The process of the present disclosure provides that the heat of reaction is transferred to the environment and/or external coolant in minor part, in other words, no more than 50%, or no more than 40%, or no more than 30%, or no more than 25%, or no more than 20%, or no more than 15%, or no more than 10%, or no more than 5% of the heat of reaction is transferred to the environment and/or external coolant. This is in contrast to conventional chlorination reactors where at least 50% of the heat of reaction is transferred to external coolant. For example, internal cooling coils, or external cooling jackets are utilized in conventional chlorination reactors to achieve cooling of the reactor contents. The present process avoids or minimizes the use of cooling equipment in favor of utilizing an internal cooling fluid which is in admixture with the chloride acceptor and chloride donor to control the rate of heat production, and then matching that rate of heat production to the rate of heat transfer to the internal cooling fluid.
Optionally, the reactor may include external cooling means such as coils located either inside or on the surface of the reactor, where those coils contain circulating cooling fluid. However, in this situation, these cooling means absorb a minor amount of the heat generated within the reactor. In embodiments, the external cooling means absorb less than 50%, or less than 40%, or less than 30%, or less than 25%, or less than 20%, or less than 15%, or less than 10%, or less than 5% of the heat generated within the reactor. The presence of cooling coils may be useful in those unexpected situations where some additional fine tuning of the temperature within the reactor is desired. However, in the ordinary course, fine tuning of the temperature within the reactor is accomplished by adjusting the input conditions of the feedstock to maintain reactor temperature within a specified range.
The reactor may contain coils that are used to transmit heated fluid, in order to warm up the reactor and its contents to a temperature sufficient to initiate the chlorination reaction.
When the chlorinator is operating under adiabatic or near adiabatic conditions, the input conditions for the chloride donor, the chloride acceptor, and cooling fluid added therewith, are selected in view of the innate ability of the reactor to passively dissipate heat to the environment. In other words, once the desired reactor operating conditions are selected, e.g., 2-4 barg pressure and 275-325° C., input conditions for the chloride acceptor, donor and cooling fluid are selected in order to achieve these operating conditions, keeping in mind that the reactor may have an innate ability to dissipate heat, particularly if the reactor is not completely isolated from the environment.
An exemplary chlorinator and its operation are illustrated by reference to
The upper region 108 and lower region 110 are adjacent to one another, as shown in
The reactor 100 is in fluid communication with a conduit 102 and a conduit 104, both used to introduce reactants to the reactor 100, and a conduit 106 used to permit egress of product from the reactor 100. Both of the conduits 102 and 104 may be used to introduce feedstock material to the reactor. For example, the conduit 102 and the conduit 104 may introduce feedstock material into a fluidized bed 116 that is present within the lower region 110 of the reactor 100. In a preferred embodiment the conduits 102 and 104 introduce feedstock materials into a region of the fluidized bed 116 that is located approximately half way down the lower region 110, or more than half way down the lower region 110. The conduit 102 may be used to introduce a gas phase feedstock including a chloride donor, while the conduit 104 may be a feeder dip-tube that is useful for introducing a particulate feedstock including a chloride acceptor, to the fluidized bed 116 within reactor 100. The conduit 106 is useful in allowing egress of the gaseous product mixture from the reactor 100, where the conduit 106 may be in fluid communication with the reactor 100 at the upper region 108 of the reactor, optionally at the top 112 of the reactor 100 as shown in
With the configuration of reactor 100 and conduits 102, 104 and 106 as shown in
The reactor 100 in
In one aspect, the chloride donor is present as a part of the effluent that is generated by one or more STC converters. In other words, the STC converter(s) may be a single converter or a multiplicity of converters acting in parallel and/or series. For example, the entirety of the effluent from an STC converter may be directed into the chlorinator, although in other embodiments, at least 95 wt. %, or at least 90 wt. %, or at least 85 wt. %, or at least 80 wt. % or at least 75 wt. %, or at least 70 wt. %, or at least 65 wt. %, or at least 60 wt. %, or at least 55 wt. % or at least 50 wt. % of that effluent is directed into the chlorinator. Although, as just mentioned, less than 100% of the effluent from an STC converter may be used as a feedstock for the chlorinator, it should be noted that in any event it is not necessary to select a particular composition of effluent to enter the chlorinator as a feedstock. In other words, it is not necessary and preferably does not occur that the composition of the effluent from the STC converter is altered by purification or refinement into a different composition. A notable feature of an embodiment of the present process is that the crude effluent from an STC converter may be used directly as the feedstock for the chlorinator. This is advantageous because the use of crude effluent avoids the undesirable capital costs and operating expenses of a refinement system that might be used to purify/fractionate the crude effluent into a different composition. Along these lines, it may be mentioned that in one embodiment, the crude effluent is combined with one or more additional gas streams, termed the auxiliary gas streams. An auxiliary gas stream may comprise chloride donor, for example, HCl.
The STC converter receives STC and converts it, in whole or part, to an effluent comprising TCS and HCl, where HCl is a chloride donor. In optional embodiments, the effluent may also comprise, for example, one or both of hydrogen (H2) and STC. The STC converter may be operated in an equilibrium or a non-equilibrium mode, and it may likewise be operated in catalytic or thermal (i.e., non-catalytic) mode. Thus, the STC converter may be operated in tandem with the chlorinator, in which case for convenience the STC converter may be referred to as the 1st stage reactor, and the chlorinator may be referred to as the 2nd stage reactor.
STC converters are well known in the art, and are presently operating in silicon producing facilities around the world. The STC converter in the foregoing description may be a standard hot converter (i.e., one that runs at high temperatures, as high as 900° C. to 1300° C.), or more preferably a low temperature, catalytic converter (that runs in the range of 300° C. to 700° C.). Accordingly, the effluent from the STC converter may likewise be in the range of 300° C. to 1300° C., depending on how the STC converter is operated. When it is desired to feed this 1st stage effluent into a 2nd stage reactor that is operating at a temperature below the temperature of the 1st stage effluent, it is desirable to cool the effluent from the 1st stage reactor using a cooling unit. In embodiments, the effluent from the 1st stage reactor is cooled to a temperature within one of the following ranges: 75-400° C., 125-400° C., 200-400° C., 150-350° C., 200-350° C., 150-300° C., 200-300° C., 150-250° C., 200-250° C., about 200° C., i.e., 175-225° C. After cooling, the effluent from the 1st stage reactor, optionally after combination with one or more auxiliary gas streams, may be directed into a 2nd stage reactor. The 2nd stage reactor may be optionally operated at a temperature in excess of the temperature of the effluent from the 1st stage reactor, or optionally of the cooled effluent from the 1st stage reactor. Optionally, the cooling unit may comprise a heat interchanger, using the heat in the 1st stage reactor off-gas to pre-heat the feed to the 1st stage reactor.
By using the entire (or at least most of the) unrefined effluent from an STC converter as all or part of the feedstock for a chlorinator as disclosed herein, and by adjusting, as necessary, the temperature of that effluent to a temperature as described above, e.g., about 200° C., the heat capacity of the 1st stage effluent and the mass of the 1st stage effluent combine to absorb the heat produced during the chlorination reaction that takes place in the 2nd stage chlorination reaction, thereby limiting the temperature rise in the 2nd stage reactor. As a result, the effluent from the 2nd stage reactor has a temperature of, for example, about 320° C., without recourse to any external cooling means, e.g., cooling coils. The combination of controlled chlorination reactor feed temperature and higher molar flow rate, due to the addition of coolant as described elsewhere herein, which is higher than that used in conventional direct chlorination reactors per mole of chloride donor in the feed, is advantageous in that it reduces the potential for the formation of hot spots within the reactor.
In one aspect, the present disclosure provides a process comprising:
In analogy, the present disclosure also provides a system comprising:
The systems and methods of the present disclosure may include a temperature control unit, also known as a temperature controller. In general, temperature control can be achieved by many methods, and the design, manufacture and operation of temperature controllers is well known in the art. Heat exchangers, cooling towers, chillers, boilers, electric and other types of heaters, and heat pumps are a few of the well-known temperature controllers used in industrial processes, any of which may be used in the systems and methods of the present disclosure. Temperature controllers may be used to cool a fluid or to heat a fluid, including heating or cooling to convert between gaseous and liquid states. For example, a vaporizer may be used to heat a liquid fluid to a gaseous state. Temperature control may be achieved with a heat exchanger. Heat exchangers are commercially available the plate, spiral and shell-and-tube varieties, any of which may be used to provide heat exchange.
The systems and methods of the present disclosure may include an off gas recovery system. The off gas recovery system receives a mixture of gases and refines that mixture to provide product gas(es) that are enriched in one or more of the components of the original mixture. The off gas recovery system may have multiple components. For example, a separator may be part of an off gas recovery system. A separator acts on a mixture of component parts of a feedstock to separate the components from one another. The design and operation of a separator will depend on the physical properties of the component that is being utilized to achieve the separation, and the component that is being separated. For example, the separator may be able to separate the component parts based on the boiling points of the components. In this case, the separator is commonly called a distillation unit. The design, manufacture and operation of distillation units, also known as distillation columns, is well known in the art. A distillation column typically achieves separation of component gases based on the difference in the boiling points of the components, and is conveniently in the form of a column. A decanter is another type of separator which may be used to separate liquids condensed from a non-condensable gas stream (e.g., hydrogen). Decanters are normally designed for continuous operation. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are the vertical decanter and the horizontal decanter. The feedstock may be cooled in order to achieve liquefaction of all or much of the condensable components of the feedstock. A refrigerator/decanter unit refers to a decanter which is in fluid communication with a refrigeration unit such that the condensable components in the feedstock going to the decanter are cooled to a liquid state. The design, manufacture and operation of decanters are well known in the art. The off gas recovery system may include an HCl recovery column which functions to separate hydrogen chloride from chlorosilanes using distillation. Distillation is a method of separating mixtures based on differences in those components boiling temperature at the same pressure. This is a physical separation process where no chemical reaction occurs. Furthermore, this may be a continuous distillation process in which the liquid mixture is continuously fed into the process and the separated fractions are removed continuously as output streams as time passes during the operation. Continuous distillation produces at least two output fractions, including at least one overhead distillate fraction and one bottom fraction. The distillate fraction being the lighter component that boils at a lower temperature is removed as a vapor and the bottoms fraction which boils at a higher temperature is removed from the bottom as a liquid. A distillation column may be employed to separate one or more components from a mixture on the basis of boiling point. A distillation column heats the mixture such that the more volatile components are separated from the less volatile components.
A chlorinator in tandem combination with an STC converter, i.e., a 2nd stage reactor in combination with a 1st stage reactor, respectively, is illustrated in
In
In one embodiment, which is shown in
The 1st and 2nd stage reactors of
The off-gas recovery system 240 separates components of the effluent that exits reactor 200. That effluent is typically composed of hydrogen, dichlorosilane (DCS), trichlorosilane (TCS), and silicon tetrachloride (STC) with either no or a de minimis amount of hydrogen chloride. The effluent components may be separated by condensation and/or distillation, i.e., by taking advantage of the difference in the boiling points of the effluent components. The fraction having the lowest boiling point, which consists primarily of hydrogen, exits the off gas recovery system 240 through conduit 244 and may be recycled to the 1st stage reactor as discussed below. The fraction having the highest boiling point, which contains short-chain polysilicon compounds and other high boiling waste material, exits the off-gas recovery system 240 through conduit 246 and is ultimately sent to waste treatment. The fraction having an intermediate boiling point, which consists primarily of dichlorosilane (DCS), trichlorosilane (TCS) and silicon tetrachloride (STC), exits the off-gas recovery system 240 through conduit 242 and enters the TCS refining unit 250.
The TCS refining unit 250 as shown in
As mentioned previously, the off-gas recovery system 240 separates components of the effluent that exits reactor 200, and that effluent is typically composed of hydrogen, DCS, TCS and STC, and either little or no hydrogen chloride. The gas recovery system 240 separates these components by condensation and/or distilling them, and the fraction having the lowest boiling point, which consists primarily of hydrogen, exits the off gas recovery system 240 through conduit 244. Conduit 244 delivers the hydrogen into a compressor 260 which compresses the gas into a higher pressure, lower volume form. The compressed gas exists the compressor 260 and is delivered into the 1st stage reactor 220 through a conduit 262. Overall, as seen in
The CVD reactor 230 will generate polysilicon and an off gas that exits the reactor 230 and enters an off gas recovery system 270 through conduit 232. The off gas from the CVD reactor 230 typically consists of hydrogen, hydrogen chloride, DCS, TCS and STC. The off gas recovery system 270 separates the components of this off gas on the basis of boiling point. For example, the component with the lowest boiling point temperature will consist primarily of hydrogen, and will exit the system 270 via conduit 276. This hydrogen will enter a hydrogen compressor 265 and the resulting compressed hydrogen travels to the CVD reactor 230 via conduit 267 which feeds into conduit 252.
Optionally, although not shown in
The next highest boiling fraction from off gas recovery system 270 will consist primarily of hydrogen chloride, and it will exit the system 270 via conduit 278. Conduit 278 may feed into conduit 222 (as shown) or conduit 202 (not shown), which also carries hydrogen chloride, and the combined fractions may be delivered into the 2nd stage reactor 200. The next highest boiling fraction will consist primarily of DCS and TCS, and this fraction will exit system 270 via conduit 272. Conduit 272 may feed into conduit 252, which also carries DCS and TCS (from the TCS refining unit 250), and the combined fractions may be delivered into the CVD reactor 230. The lowest boiling fraction will consist primarily of STC, and this fraction will exit system 270 via conduit 274. Conduit 274 may feed into conduit 224 and thereby provide a source of STC for the 1st stage reactor (STC converter) 220.
In addition to, or in the alternative to, obtaining hydrogen chloride from the off gas recovery system 270, the 2nd stage reactor 200 may obtain hydrogen chloride from a tank of hydrogen chloride. For example, the plant operator may purchase a pressurized tank of hydrogen chloride, shown as the HCl reservoir 280 in
Optionally, and not shown in
As mentioned previously, the 1st stage reactor 220 (STC converter) is connected directly or indirectly (via conduit 224) to conduit 262, conduit 254 and conduit 274, where any one or more of the conduits may deliver feedstock(s) to the reactor 220. Such feedstock(s) include STC and H2. In various embodiments, the feedstock(s) may be combined to create an admixture within the 1st stage reactor 220; and the feedstock(s) may be combined to create an admixture having an H2:STC molar ratio, where in various embodiments this H2:STC molar ratio may be in the range of 0.1:1 to 10:1, or 0.5:1 to 5:1, or 1:1 to 4:1 or 1.5:1 to 3.5:1 or 2:1 to 3:1. In addition, the 1st stage reactor 220 may be in contact with temperature control means, not shown in
The operational units shown in expanded form in
In addition to illustrating the tandem combination of a 1st and 2nd stage reactor as shown in
The tandem operation of the 1st and 2nd stage reactors, as illustrated in
While the present disclosure provides a tandem combination of chlorinator reactor and STC converter, the chlorination process of the present disclosure is not limited to the combination of an STC converter and a chlorination reactor. Feedstock to the chlorinator need not come entirely or even partially from an STC reactor. For example, the feedstock may be an admixture including portions of hydrogen, STC, TCS, and HCl formed from pure materials in sufficient amounts and provided at suitable temperature such that the direct chlorination reactor may be operated at desired operating temperatures without the need for external cooling mechanisms. An exemplary system and operation thereof that does not include the tandem combination of chlorinator reactor and STC converter is illustrated in
In
In
The off-gas from the chlorinator 300 may be refined and/or recycled in various ways, including the process illustrated in
The present disclosure provides, in one embodiment, a system comprising a chlorination reactor in fluid communication with a source of MGSi and also in fluid communication with a source of hydrogen chloride, the chlorination reactor also in fluid communication with at least one of a source of hydrogen and a source of STC. The hydrogen and/or STC provide the coolant of the present disclosure which allows the chlorination reaction taking place within the chlorinator to be performed in an adiabatic manner. In various optional embodiments, where any two or more of the following embodiments may be combined to describe an embodiment of the invention (so long as the embodiments are not mutually exclusive): the chlorination reactor is also in fluid communication with an STC converter, and the STC converter provides the source of STC (an STC converter is typically far from 100% efficient in converting STC to TCS, and accordingly the effluent from an STC converter will typically contain STC); the chlorination reactor is not in direct fluid communication with an STC converter, i.e., the effluent from an STC converter is not directly fed into the chlorination reactor, however, an STC converter may be present elsewhere in the system, e.g., an STC converter may be used to treat the STC that is present in the off gas from a CVD reactor that produces polysilicon by the Siemens process, in which case the product from this STC converter is not directly fed into the chlorinator but instead is fed into an off gas recovery system which generates a fraction enriched in STC which may be used as the source of STC in the chlorination reaction; the chlorination reactor is in fluid communication with a fractionation column, and an effluent from the fractionation column provides the source of STC, where this may occur, as mentioned previously, when the CVD reactor is in fluid communication with an STC converter, and the STC converter is in fluid communication with an off gas recovery system that includes a fractionation column; the system further comprises an off gas recovery system in fluid communication with the chlorination reactor, the off gas recovery system providing separation of components of off gas from the chlorination reactor, the off gas recovery system providing an off gas fraction enriched in hydrogen where at least some of the off gas fraction enriched in hydrogen provides the source of hydrogen to the chlorination reactor, in other words, the hydrogen present in the off gas from the chlorination reactor is recycled back into the chlorination reactor after having pass through an off gas recovery system that provides a fraction enriched in hydrogen; the chlorination reactor is a direct chlorination reactor which reacts hydrogen chloride with MGSi in a fluidized bed to produce ICS; the chlorination reactor in not in direct fluid communication with a STC converter such that the effluent from an STC converter is not directly introduced into the chlorination reactor; the system further comprises a source of STC in fluid communication with the chlorination reactor, whereby STC is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the system further comprises a source of hydrogen in fluid communication with the chlorination reactor, whereby hydrogen is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the system further comprises a chemical vapor deposition (CVD) reactor for producing polysilicon and a CVD off gas.
In a related embodiment, the present disclosure provides a process comprising a) performing a chlorination reaction in a chlorination reactor at a first temperature, where MGSi and hydrogen chloride are reacted together to provide a product gas comprising trichlorosilane; b) introducing a coolant selected from hydrogen and STC to the chlorination reactor, the coolant being introduced at a second temperature, the second temperature being less than the first temperature, the second temperature selected so that the chlorination reactor is performed under adiabatic conditions. In various optional embodiments, where any two or more of the following embodiments may be combined to describe an embodiment of the invention (so long as the embodiments are not mutually exclusive): the chlorination reactor is in fluid communication with an STC converter, and the STC converter provides the source of STC; the chlorination reactor is not in direct fluid communication with an STC converter; the chlorination reactor is in fluid communication with an off gas recovery system, where the off gas recovery system generates a fraction enriched in STC, and the fraction enriched in STC is introduced into the chlorination reactor to provide the source of STC to the chlorination reactor; an effluent from an STC converter is not directly introduced into the chlorination reactor; STC is introduced into the chlorination reactor along with hydrogen chloride and MGSi; the chlorination reactor is in fluid communication with an off gas recovery system, where the off gas recovery system generates a fraction enriched in hydrogen, and the fraction enriched in hydrogen is introduced into the chlorination reactor to provide the source of hydrogen to the chlorination reactor; the process further comprises performing the Siemens process for polysilcion production.
In one aspect, the present disclosure provides a chlorination process where the temperature of the gaseous reactor product, i.e., the gas product formed inside the reactor where the chlorination process takes place, is substantially controlled by controlling the temperature and composition of the gaseous feedstock(s) that comprise a chloride donor, particularly in the case where the reactor contains a molar excess of chloride acceptor, for example MGSi. Optionally: the gaseous feedstock comprises hydrogen and STC; the feedstock comprises hydrogen and TCS; the feedstock comprises hydrogen and HCl; the feedstock comprises STC and ICS; and/or the feedstock comprises hydrogen, STC, TCS, and HCl. In addition, and also optionally, the chlorination reactor is operated in an adiabatic mode and further optionally the reactor has no provision for heat removal by means of cooling surfaces internal or external to the reactor surface itself, e.g., the reactor has no internal or external cooling coils, or external cooling jacket or jackets on the shell. In addition, and also optionally, the feedstock to the chlorination reactor comes in whole or in part from an STC converter, wherein various embodiments, that portion of the feedstock coming from an STC converter comprises hydrogen, STC, TCS, and HCl; where the STC converter and chlorination reactor are in fluid communication with off-gas recovery equipment and hydrogen recycle equipment.
As described herein, one aspect of the present disclosure is a two-step process: (a) in a 1st stage reactor, referred to as an STC Converter, STC is converted to HCl and TCS in the presence of hydrogen gas; (b) in a 2nd stage reactor, the gaseous product of the 1st reactor is fed to a direct chlorination reactor where the HCl formed in the 1st stage reacts with metallurgic silicon to produce TCS. The STC converter may operate at low temperature, catalytic, non-equilibrium conditions. Viewed as a combined process, the feed to and the product exiting the system is identical to the feed to and products exiting a hydrochlorination reactor. However, the high pressure, high temperature, and high hold-up time required by a traditional hydrochlorination reactor are eliminated because the 2nd stage direct chlorination step runs at low pressure (i.e., 3 to 10 barg) and low temperature (e.g., 320 C), and because the direct chlorination reaction is relatively fast.
By lowering the temperature of the product leaving the 1st stage reactor to about 100-220° C., or about 200-210° C., e.g., 205° C., the 2nd stage reactor can be run as an adiabatic reactor. This is advantageous for the following reasons:
All external (e.g., cooling shell and/or cooling coil) heat transfer can be eliminated from the system which will greatly simplify design and operation, and reduce maintenance. (The cooling requirement associated with current generation direct chlorination reactors limits ease of scale-up and is a maintenance head-ache.)
The chlorination reactor of the present disclosure can be easily scaled up to large size, for example, the reactor may have a 3 meter to 4 meter diameter, capable of producing enough crude TCS to support 10,000 MTA polysilicon production.
By operating the 1st stage reactor to provide a product with a temperature of about 185° C., the heat capacity of the products exiting the 1st stage reactor and the mass of the material present in the 1st stage product combine to absorb the heat of the 2nd stage chlorination reaction, thereby limiting the temperature rise in the 2nd stage reactor. As a result, and in the absence of external cooling means, the 2nd stage reactor provides product that exits at about 320° C. As a side benefit, the larger flow rate into the 2nd stage chlorination reactor (approximately one order of magnitude greater molar flow rate compared to a convention HCl only chlorination reactor) eliminates hot spots within the reactor and by-products due to over-chlorination.
In the absence of either external cooling or internal cooling, the adiabatic temperature rise is so great that the chlorination process would be unworkable at best and dangerously unsafe at worst. For example, without heat removal either through the walls of the direct chlorination reactor or into internal coils filled with cooling medium, the adiabatic temperature rise due to HCl chlorination would increase the reactor temperature to 1790° C. This is unworkable because above 400° C. in such a system the chlorination reaction principally produces STC—not TCS. Since STC is an undesired by-product, this system is uneconomic. However, such a system would be dangerously unsafe to operate because the materials of construction—even if Inconel 800 H is used—it cannot withstand this high a temperature and the reaction vessel would catastrophically fail. The present process provide a process that utilizes internal cooling in order to safely operate the process and avoid the capital and maintenance expense associated with cooling coils and cooling jackets.
By including coolant in the feed to a chlorination reactor, in controlled amounts relative to HCl content, the coolant comprising one or more of hydrogen, STC, and TCS, in a molar ratio ranging from 2:1 coolant to HCl to 15:1 coolant to HCl, for example; or 3:1 to 5:1, or 5:1 to 7:1, or 8:1 to 10:1, and by cooling the feed to temperatures ranging from 100° C. to 300° C., or from 130° C. to 270° C., or from 150° C. to 250° C., or from 180° C. to 230° C., the chlorination reactor may be operated an adiabatic mode at desired operating temperatures ranging from 250° C. to 400° C., or from 300° C. to 350° C.
The present disclosure provides the following specific embodiments, which are not limiting on the invention, but are exemplary of the embodiments disclosed herein.
All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, if any, are incorporated herein by reference, in their entirety. The processes and systems for non-equilibrium trichlorosilane production as disclosed in PCT/US2012/064,568 (published as WO 2013/074425), and as disclosed in U.S. Application No. 61/559,657, are each optionally incorporated herein by reference.
Any of the various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/779,613 filed Mar. 13, 2013, which application is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/024620 | 3/12/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61779613 | Mar 2013 | US |
