Patent Application
20130156675
The present disclosure concerns embodiments of a system and reactive distillation method for producing silane and hydrohalosilanes of the general formula HySiX4-y (y=1, 2, or 3).
Mono-silane (SiH4), chlorosilane (H3SiCl) and dichlorosilane (H2SiCl2) are useful chemicals for the production of electronic devices based on high purity crystalline silicon. These silicon bearing gases are thermally decomposed to form the high purity silicon material. The production of high purity silane is presently practiced on a commercial scale by a process shown generally in
2H2+3SiCl4+Si→4HSiCl3 (1)
Then, in a second step, trichlorosilane is converted to the high purity silane product in a series of distillation separations and catalytic redistribution reactions which also produce silicon tetrachloride as a co-product. The silicon tetrachloride is recycled to the first step.
4HSiCl3→3SiCl4+SiH4 (2)
The silane is then pyrolyzed in any of several ways to form ultra-pure silicon and, if the process is close coupled, the by-product hydrogen is recycled to the first step.
Overall, the process is characterized as being highly efficient in the use of raw materials. However, the process is also characterized as being rather complicated and uses many distillation columns, some of which must operate at high pressure to achieve the desired results. It had been described in U.S. Pat. No. 3,968,399 that silane could be produced directly from trichlorosilane in a single step process wherein a solid redistribution catalyst also served as the contact surface in a fractional distillation column. While it was not so named in that patent, the process was the essential embodiment of a “reactive distillation” process in that both chemical reaction and distillation separation were conducted in the same apparatus.
However, there are several practical restrictions that must be addressed when combining distillation separation and the catalytic redistribution reaction. First, the kinetics of distillation, that is, the rate at which a vapor and liquid will interact to form an equilibrium mixture, is quite rapid, on the order of fractions of a second, whereas the chemical kinetics of the redistribution reaction, even with an outstandingly active catalyst, is measured in several minutes to achieve equilibrium. Thus the question is raised as to how to determine the amount of volume to devote to the reaction zone, to provide adequate space time for the reaction, amount of catalyst, etc. relative to the amount of vapor-liquid contact area or distillation separation stages. In the case of solid catalysts, the question becomes even more complex since the activity of the catalyst gradually changes with time, slowing the kinetics and thus altering the carefully thought out design based on the initial kinetic rates. Second, fixed beds of particles can develop flow restrictions over time due to tramp solids or from migration of smaller catalyst particles. This increased flow restriction must be addressed for a practical unit operation. Third, the temperature range at which the chemical reaction is presented with favorable kinetics and which do not result in undesirable side reactions, is rather narrow. In the context of a co-existent distillation operation, operating pressure and compositions restrain the location of the catalyst. In U.S. Pat. No. 3,968,399, for example, the rate of production of silane was very low because the reactive distillation operation was conducted at sub-ambient temperatures. Whereas U.S. Pat. No. 4,676,967 operates the redistribution reaction at temperatures chosen to maximize the chemical reaction rates and thus minimize the volume of catalyst required. Since the temperature within and associated with a distillation operation is a function of the vapor/liquid composition as well as the overall system pressure, the temperature limitation on the chemical reaction translates into limits on the system operating pressure as well as the location at which the chemical reagents are passed in contact with the catalyst. The addition of heaters or coolers to condition the reagent streams before they pass through the catalyst bed and to then reverse that heat effect before the reactor product returns to the distillation environment imposes an added energy and complexity burden on the process. Fourth, since rejection of thermal energy is necessary for any distillation separation, rejecting the energy to economically available ambient air or available cooling water is greatly preferred over rejecting the energy at sub-ambient or even cryogenic temperatures. This temperature restriction further limits the operating pressure and compositions in the reactive distillation system.
U.S. Pat. No. 6,905,576 puts forth a scheme whereby silane is produced in a reactive distillation system that utilizes an “intermediate condenser.” However, the inventors of U.S. Pat. No. 6,905,576 failed to realize that by purposefully restricting the production of the lower boiling components (SiH4 and H3SiCl) in the first reaction zone, the complexity of the process could be substantially reduced along with reduced refrigeration and process pumping requirements. Finally, in an economical process to produce silane, at least some portion of the process must be operated at elevated pressure in order to use economically available heat rejection means and to avoid sub-ambient temperatures as much as possible. While U.S. Pat. No. 3,968,399 was demonstrated at atmospheric pressure, the production rate was very low and the cooling requirements to effect the distillation meant a coolant temperature well below −70° C. U.S. Pat. No. 6,905,576 claims operation at elevated pressure, but achieves the higher pressure by requiring a gas pump (compressor) or by use of lower temperature refrigeration. The process described in U.S. Pat. No. 6,905,576 purposefully forces the production of silane in a “first redistribution reactor” which necessitates the use of either a low temperature condenser, to deliver only a condensed liquid, or a compressor to pump the vapor to the higher pressure. Higher pressures are best achieved by using a pump to transport liquid chlorosilane reagents through the system, rather than relying upon a compressor to pump the highly reactive silane gases. Compressing silane or chlorosilane vapors requires special and very expensive considerations for the compressor hardware.
Notwithstanding the overall goal to produce silane from silicon and hydrogen, it the process sequence should present a chemical labyrinth such that no compound except silane, or the desired hydrohalosilanes, can pass through to the final product. The process should present at least one method for removing any given contaminant from the silane. Since the number of contaminant possibilities is very large, a set of purification techniques should be used that, taken together, will result in no impurity being present at a level higher than about 100 parts per billion parts silane, and for some selected impurities such as boron and phosphorus, the level of impurities should be below about 20 parts per trillion parts silane in order to provide an ultimate silicon product acceptable for electronic applications. It is fortunate that only a few compounds have boiling points close to that of silane, such that distillation offers a very powerful tool for purifying silane. However, there are key impurities, mainly the hydrides of boron and phosphorus that boil too close to silane to permit the extreme purification required for ultra-pure silane useful in electronic applications. For these impurities as well as possibly others, especially those which may themselves be chemically transformed during the process, additional purification means should be included within the overall process sequence to assure that the final silane product is of the exceptional purity required for the most demanding applications. As each additional process step adds to the capital and operational costs of the process, a method which can combine or eliminate process steps or hardware would offer an attractive economic alternative.
Described herein are embodiments of a system and process that combine fractional distillation separation of hydrohalosilanes and catalytic redistribution of hydrohalosilanes in a novel configuration that minimizes the physical size and number of the process equipments, allows the use of an ambient heat sink for nearly all of the heat rejection, allows the redistribution catalyst to be monitored for effectiveness and changed out readily if it declines in activity, and incorporates a purification strategy of redundant means to remove any and all critical impurities from the silane to deliver an ultra-pure product. The detailed description will show how novel configurations of the process elements provide an ultra-pure silane product within the constraints of the constituent's physical properties and chemical stability while providing a process that is robust in design and is economic in terms of energy, raw materials and capital equipment utilization. The process also provides a product composition that has a lower halogen to silicon molar ratio than the reactant stream. In other words, if the reactant stream includes one or more hydrohalosilanes of formula HySiX4-y where X is a halogen and y is 1, 2, or 3, then the product composition will include a significant concentration of H2SiX4-z where z=y+1. For instance, when the reactant stream includes trichlorosilane, the product composition will comprise a reduced amount of trichlorosilane and an increased amount of dichlorosilane compared to the reactant stream.
Embodiments of the system include a first multi-zone fractional distillation column, a first catalytic redistribution reactor, and a first pump operable to pump a first distillate stream from the distillation column into the redistribution reactor. The first multi-zone fractional distillation column includes a reactant stream inlet, a first distillate stream outlet, a first product flow inlet, a bottom outlet, and a vapor outlet. At least one condenser is in communication with the vapor outlet. The first catalytic redistribution reactor includes a vessel defining a chamber, an inlet and a product flow outlet spaced apart from the inlet. The catalytic redistribution reactor does not include includes a pressure equilibrium outlet or a vapor return outlet.
In one embodiment, the system further includes a second catalytic redistribution reactor, and a second pump operable to pump a condensate from the first multi-zone fractional distillation column into the second redistribution reactor. The second catalytic redistribution reactor includes a vessel defining a chamber, an inlet and a product flow outlet spaced apart from the inlet, but does not include includes a pressure equilibrium outlet or a vapor return outlet. In another embodiment, the system further includes a second multi-zone fractional distillation column with an inlet operably coupled to a product flow outlet of the second redistribution reactor, an outlet positioned above the inlet, a purge stream outlet, and a bottom outlet.
In some embodiments, a reactant stream including one or more hydrohalosilanes of formula HySiX4-y where X is a halogen and y is 1, 2, or 3 is passed via a reactant stream inlet into a first multi-zone distillation column having at least a first distillation zone and a second distillation zone, wherein the first distillation zone is maintained at a temperature T1 corresponding to a boiling point of the reactant stream at a pressure within the column. A first distillate stream is pumped from the second distillation zone via a distillate stream outlet into a first catalytic redistribution reactor; the second distillation zone is maintained at a temperature T2 at which liquid and/or vapor in the second distillation zone has a halogen to silicon molar ratio between 2.8 and 3.2. A first product flow is produced by the first catalytic redistribution reactor, and the first product flow is returned to the first multi-zone distillation column at a point between the reactant stream inlet and the distillate stream outlet. Vapor is passed from an upper portion of the distillation column to a condenser to produce a condensate containing HzSiX4, where z=y+1.
In some embodiments, the condensate is pumped through a second fixed-bed catalytic redistribution reactor to produce a second product flow, which then passes into a second multi-zone fractional distillation column through an inlet positioned at a height corresponding to a distillation zone located within the second multi-zone fractional distillation column wherein the distillation zone has a temperature corresponding to a boiling point of the second product flow at a pressure within the region. Silane is withdrawn from the second distillation column through an outlet positioned above the inlet. In some embodiments, a purge stream containing gaseous impurities is withdrawn through a top outlet of the second distillation column.
In the drawings:
This disclosure pertains to that portion of the overall process for production of silane from metallurgical grade silicon and hydrogen wherein a mixture of hydrohalosilanes of formula HySiX4-y where X is a halogen and y is 1, 2, or 3 are converted into silane and silicon tetrahalide. For example, trichlorosilane and silicon tetrachloride resulting from a gasification process, reaction (1), may be converted into silane and silicon tetrachloride, reaction (2). Intermediate products including dihalosilane (H2SiX2) and halosilane (H3SiX) also can be isolated at various points in the process.
In particular, disclosed is a unique arrangement of two multi-zone fractional distillation columns combined with two fixed-bed catalytic redistribution reactors wherein the feed to the first reactor is controlled to have a halogen to silicon molar ratio greater than 2.8, such as between 2.8 and 3.2, and produce a condensate enriched in H2SiX2 and having a halogen to silicon molar ratio less than 2.0, which can be fed to a second catalytic redistribution reactor for further processing.
By this arrangement, which is achieved by design of the multi-zone distillation columns, there is a sufficiently low concentration of silane (SiH4) produced in the first reactor that a total condenser operating with ordinary coolant temperatures can be used on the first multi-zone distillation column. By selecting the system operating pressure, and hence the fractionation column temperature profile, the combined distillation and reaction operation can be conducted in a stable and predictable fashion using ambient air or commonly available cooling water for the condenser duty.
The intermediate product of this first distillation/reactor combination is pumped through a second fixed bed catalytic redistribution reactor where silane is produced in a mixture of hydrohalosilanes. All of the mixed hydrohalosilane stream passing to the second multi-zone distillation column passes through this second reactor. The redistribution catalyst, most favorably a weak base, macroreticular ion exchange resin, readily removes boron impurities from the hydrohalosilanes. The reactor beds also act as large sand filters to trap traces of silica solids that form from traces of oxygen or moisture present in industrial processes. The silica also acts to attract boron and other metallic species by chemisorption. The catalytic redistribution reaction combined with the chemisorption and physical filtration action of the catalyst bed prevent electronically active impurities from passing into the silane purification system. Providing this secondary purification immediately prior to the final silane distillation offers a redundant means for removing impurities and further guarantees the production of the highest purity silane. The hyper-pure silane is recovered in a high efficiency multi-zone distillation column as a side-draw liquid, while a small amount of silane is rejected as a vapor along with non-condensable impurity gases through a partial condenser. The result of these combined features is a process which has reduced energy consumption, reduced capital equipment investment and a process operation which can be easily monitored for its performance. The latter is particularly important for maximizing the unit's production quantity and quality.
This disclosure also pertains to a process wherein the trihalosilane is produced by the hydrohalogenation of silicon or where the final products can also include minor amounts of ultra-pure dihalosilane (H2SiX2) or halosilane (H3SiX). In the case of dihalosilane or halosilane, these components are present in enriched concentrations in the bottom streams of the multi-zone second distillation column. A side stream may be advantageously taken here and passed to a secondary set of distillation columns to deliver the desired amount and quality of these two hydrohalosilanes (
Impurities from the crude silicon feed stock are rejected in Zones 1 and 2. The impurity streams contain a halide value in addition to the impurity that is being rejected. To provide sufficient halide to replace that lost in both the impurity rejection as well as that contained in the by-product halosilane and/or dihalosilane streams, a make-up source of halide is required. The halide may be replenished by the addition of silicon tetrahalide, trihalosilane, hydrogen halide or halogen into Zone 1 of the process.
Optionally, the trihalosilane may be produced by hydrohalogenation of metallurgical grade silicon by the reaction of hydrogen halide and silicon as:
3HX+Si→HSiX3+H2 (3)
where X is a halogen. A significant co-product of reaction (3) is SiX4 which is generally present at about 15% of the total halosilane stream. Using this means to produce HSiX3 also requires an alternate outlet for the co-product SiX4 resulting from the reactive distillation process for preparing silane, SiH4. Among the alternative outlet means are conversion of the SiX4 to pyrogenic silica, preparation of organosilane alkoxylates, silica-based resins and other useful materials. In any of the processes, the mixed HSiX3/SiX4 stream need not be further refined to alter the ratio of HSiX3/SiX4 prior to the reactive distillation process. Only a minor alteration of the configuration of the reactive distillation column is necessary, and much energy is saved by not further refining the crude mixture of halosilanes.
A grade of silane suitable for solar-grade silicon production can be produced by a process and system illustrated by
A reactant stream (A) comprising one or more hydrohalosilanes of formula HySiX4-y where X is a halogen and y is 1, 2, or 3, from Zone 1, whether produced by the hydrogenation of SiX4 or produced by the hydrohalogenation reaction, enters the first multi-zone distillation column 2 at a reactant stream inlet 1. In some embodiments, reactant stream (A) comprises a mixture of HSiX3 and SiX4. Reactant stream (A) may be a liquid, a vapor, or a combination thereof. Reactant stream inlet 1 is a positioned at a height corresponding to the first distillation zone (Z1). The first distillation zone (Z1) is maintained at a temperature T1, which is close to a boiling point of the reactant stream at a pressure within the vessel. The second distillation zone (Z2) is maintained at a temperature T2 at which liquid and/or vapor in the second distillation zone (Z2) has a halogen to silicon (X:Si) molar ratio between 2.8 and 3.2. T2 is adjusted depending upon the pressure in the vessel. In some embodiments, the pressure within the vessel is from 450 kPa to 1750 kPa, and T2 is from 60° C. to 150° C.
A first distillate stream outlet 5 is provided and a pump 6 is used to transfer a first distillate stream through a first catalytic redistribution reactor 7. The first catalytic redistribution reactor 7 includes a vessel defining a chamber, an inlet 7a, a product flow outlet 7b spaced apart from the inlet 7a, and a fixed-bed catalyst disposed within the chamber between the inlet 7a and the product flow outlet 7b. The product flow outlet 7b is in communication with the first product flow inlet 8 of column 2. In the arrangement shown in
The reactor product (C) containing a mixture of hydrohalosilanes with the same X:Si ratio as stream (B), but with less trihalosilane than stream (B) and substantially free of silane, SiH4, is returned to multi-zone fractionation column 2 at a first product flow inlet 8 positioned between the reactant stream inlet 1 and the first distillate stream outlet 5. In some arrangements, the position of first product flow inlet 8 is selected to minimize the quantity of first distillate stream (B) flowing through first distillate stream outlet 5. In some embodiments, reactor product (C) has at least 5% less trihalosilane than stream (B), at least 10% less trihalosilane than stream (B), or at least 20% less trihalosilane than stream (B).
A condensate (F) containing a mixture of hydrohalosilanes substantially free of silane and silicon tetrahalide is withdrawn as a condensed liquid from the total condenser 28 and is fed by a pump 11 to a second packed-bed catalytic redistribution reactor 12. Condensate (F) comprises HzSiX4-z where z=y+1.
Second packed-bed catalytic redistribution reactor 12 includes a vessel defining a chamber, an inlet 12a, a product flow outlet 12b spaced apart from the inlet 12a, and a fixed-bed catalyst disposed within the chamber between the inlet 12a and the product flow outlet 12b. In the arrangement shown in
The second multi-zone fractional distillation column 14 includes a vessel defining a plurality of distillation zones, an inlet 13 operably coupled to the product flow outlet 12b of the second catalytic redistribution reactor 12, an outlet 19 positioned above inlet 13, a partial condenser 17 positioned above outlet 19, a purge stream outlet 18 positioned above partial condenser 17, and a bottom outlet 20. Inlet 13 is positioned at a height corresponding to a first distillation zone (Z3) located within column 14 wherein the distillation zone (Z3) has a temperature corresponding to a boiling point of the second product flow (G) at a pressure within the region. Ultra-pure silane (H) is produced as a vapor or a condensed liquid product at outlet 19 positioned between inlet 13 and a partial condenser 17. A small purge stream (I) containing non-condensable gases (hydrogen, nitrogen, methane) boiling lower than silane along with a minor amount of silane, may be taken from a purge stream outlet 18 above partial condenser 17. Stream (I) amounts to less than 10% of stream (H) and is used to purge low boiling point gases from the system. Even though stream (I) may be unsuitable for the most demanding electronic quality applications, it is sufficiently pure to be useful for production of silicon for solar cells or for other applications not requiring the highest purity silane.
The bottoms stream (D), containing a mixture of hydrohalosilanes and substantially free of silane, flows through pressure control device 21 to the first multi-zone fractional distillation column 2, and enters at inlet 21 a, which is positioned above first distillate stream outlet 5. Silicon tetrahalide (K) is delivered as a bottoms product from column 2 to be recycled to the hydrogenation zone, or is available for sale. Outlet 31 of column 2 provides an outlet for draining the column and/or removing non-volatile components.
The feed point, or inlet, 1 of reactant stream (A) to distillation column 2 is determined by the expected composition of the feed mixture and the separation profile of column 2. The higher the concentration of HSiX3, the higher in the column would be the feed point. As previously described, the optimal feed point would be at the location where the column temperature is close to the boiling point of the reactant stream (A) at the column's operating pressure. In some embodiments, the feed point is at a location where the column temperature is within 50° C. of the feed reactant stream's boiling point, such as within 40° C., within 30° C., or within 20° C. In practical applications, several feed points are usually provided so that adjustments may be readily made depending upon the efficiency of the upstream process. Likewise, the location of the first distillate stream outlet 5 may be altered from one of several points along column 2.
The recycle stream (D) from the second distillation column 14 contains substantial amounts of halosilane (H3SiX) and dihalosilane (H2SiX2), but is substantially free of silane, SiH4. Stream (D) enters column 2 above the outlet 5 for first distillate stream (B), and thus prevents the X:Si ratio in first distillate stream (B) from falling below the target range of 2.8-3.2.
By selecting the operating pressure of first multi-zone fractional distillation column 2 to be from 450 to 1750 kPa, such as from 450 to 650 kPa, the temperature at the first distillate stream outlet 5 can be controlled to be between 60° and 150° C., such as between 60 and 90° C. This range is high enough for fast reaction kinetics and low enough to provide long operating life of the weak base macroreticular ion exchange resin, typically used as the catalyst. With a more thermally durable catalyst, a higher operating pressure and thus a higher side-draw temperature could be used. However, the X:Si ratio should remain in the range of 2.8-3.2 to prevent significant amounts of silane from being produced in this first reactor.
If halosilane and/or dihalosilane are to be co-produced, a minor amount of stream (D) may be diverted as stream (J) to a two-column separation system (
Fourth distillation column 24 includes a vessel defining a plurality of distillation zones, an inlet 23 in communication with bottom outlet 22a to receive a bottoms stream (L) from third distillation column 27, a bottom outlet 25a located below inlet 23, and a top outlet 25b located above inlet 23. Inlet 23 is positioned at a height corresponding to a region located within the fourth distillation column wherein the region has a temperature corresponding to a boiling point of the second bottoms stream (L) at a pressure within the region.
As illustrated in
Each of the catalytic redistribution reactors 7, 12 may also be provided with a means to reverse the flow direction. Flow reversal or back-flushing is performed periodically to remove tramp solid impurities such as silica which can form from traces of moisture entering the process.
The following non-limiting example demonstrates an implementation of this process.
A process system arranged as in
In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the described embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims.
