Patent Application
20180162790
This disclosure relates to a modified propane dehydrogenation (PDH) system and process, which is configured to produce propene and any one of hydrogen, ammonia from the hydrogen, acrylonitrile from the ammonia, and/or additional alternate products, all from propane as a sole feed stock. The present disclosure further relates to a system and method for utilization of propene produced by a modified PDH system and process, in conjunction with a least one additional product also produced in the same modified PDH system and process, in order to produce either ammonia or acrylonitrile. The present disclosure further relates to the synergistic use of at least two sub systems and sub processes within the modified PDH process.
Propene (C3H6), also commonly known as propylene, is an important chemical intermediate chemical product. It can be used to produce numerous polymers. Around two thirds of the global demand for propene is consumed to produce polypropylene, which is in turn used, for example, to make plastic films and packing materials, or is used in the electrical and/or automobile industries, for example. The second-largest use of propene is in the production of acrylonitrile, with the third largest use of propene being in the production of propylene oxide. The industrial demand for propene is increasing, so producing propene is of great commercial interest for chemical companies.
Propene (or propylene) as an end product can either be produced from crude oil feed stock in refineries (which accounts for about 80% of all global propene/propylene production), or may be produced from propane (C3H8) feed stock in a conventional propane dehydrogenation (PDH) plant by an associated PDH process (which accounts for about 10% of all global propene/propylene production). For production of propene in a PDH system or plant, despite the relatively simple chemistry involved in the PDH process, industrial implementation of conventional PDH systems or plants is very complicated due to side reactions that occur, such as deep dehydrogenation, hydogenolysis, cracking, polymerization and coke formation. Side reactions impact the complexity of the process, operating costs and the purity of the products. The conventional PDH system and process converts propane into propene and reaction byproducts, including hydrogen, which byproducts are usually used as fuel that is burned to drive the PDH reaction itself. As a result, propene tends to be the only end product produced from the operation of a conventional PDH system and process. However, due to the rising exploitation of shale gas, the purchase price of propane (C3H8), which is the sole feed stock for the PDH process, has decreased over the last few years making it an attractive feed stock to use to produce propene (C3H6) and/or any other products capable of being made from propane (C3H8). Chemical companies are already planning to establish PDH plants in at least the United States to take advantage of this low price raw material obtained from shale gas.
In addition, another chemical product, Acrylonitrile, is one of the leading chemicals produced worldwide, with a worldwide production of about 6 million tonnes (or metric tons) in 2003. The most important applications and uses of acrylonitrile are in the production of acrylic fibers, thermoplastics such as styrene acrylonitrile (SAN) and acrylonitrile butadiene styrene (ABS), technical rubbers, adiponitrile, and various specialty polymers. Currently there are two main processes that may be used to make acrylonitrile, typically in stand-alone plants; (1) propene (or propylene) ammoxidation (see
Referring to
Referring to
Furthermore, production of ammonia is one of the most technologically complex and energy intensive industrial processes. In the conventional production of chemicals like urea, acrylonitrile, and other chemicals that require ammonia as a feed stock, the ammonia production itself accounts for approximately 80% of the total energy required for production of urea, acrylonitrile and any such other chemicals. As mentioned previously, currently both conventional PDH systems or plants and ammonia plants are independently implemented as separate stand-alone facilities for production of propene and ammonia respectively.
Thus, with the above background regarding propene and acrylonitrile production systems and methods, together with an understanding of the advantages and the disadvantages associated with each conventional system for their respective production, there exists a need for a new system and process that is capable of producing propene as well as another valuable intermediary or end product form a single process, using only propane as the sole feed stock. There also exists a need for a system that is capable of producing acrylonitrile that has the benefits of both of the previous acrylonitrile production processes (propene ammoxidation and propane ammoxidation), but without the associated disadvantages thereof. There is also a need for a system and process that can take advantage of synergies between a propene production system and additional downstream systems to produce propene and one or more valuable additional intermediary and/or end products, such as for example ammonia, methanol, urea, or acrylonitrile. There is also further a need for a system and process that can take advantage of efficiencies between a PDH system and process and additional downstream systems to produce propene and one or more valuable additional intermediary and/or end products in a greener/cleaner, cheaper, and more energy efficient manner than has been done in the past with stand-alone systems.
A modified propane dehydrogenation (PDH) system or plant of the present disclosure is configured to increase propane utilization efficiency in the PDH system or plant and process as well as ensure the efficient use of any product or byproduct from the modified PDH system or plant and its process.
As disclosed herein, a modified PDH plant or system is configured to produce propene by a modified PDH process, and includes a hydrogen (or dihydrogen (H2)) recovery unit configured to recover hydrogen from the PDH system, and convey a hydrogen stream from the hydrogen recovery unit to at least one additional downstream plant or system, for example an ammonia production system, an acrylonitrile production system, a urea production system, a methanol production system, or any other system or plant that may require hydrogen as a reactant. In the present disclosure, the terms hydrogen and dihydrogen may be used interchangeably and should be understood to be synonyms of each other.
The hydrogen recovery unit may provide for high efficiency and/or synergetic use of process fluids or process gasses. In particular, it has been found that many different favorable configurations or interconnections of different plants/systems may be realized if hydrogen in the waste gas stream is not used as fuel to be burned to operate the PDH system, but is instead removed from the waste gas stream and used as a reactant for additional processes downstream in a modified PDH system or process. In one aspect of the present disclosure, hydrogen produced by a modified PDH process of the present disclosure may be used as make-up gas fed to the front-end of an ammonia production system to produce an ammonia product. In this manner, eliminating the hydrogen production catalyst devices from ammonia manufacturing plants or systems results in substantial capital cost savings and reduction in total energy consumption and emissions.
In addition to the benefit of reducing operating and capital costs, a modified PDH plant or system of the present disclosure, in particular a plant or system configured to produce acrylonitrile, is also a greener process that generates lower carbon dioxide emissions than existing stand-alone plants or systems. The teachings of the present disclosure may be applied to the production of propene, ammonia, of acrylonitrile, of high purity hydrogen, urea, and of methanol, especially if combined with process carbon dioxide effluent from the PDH plant.
A hydrogen or dihydrogen recovery unit as disclosed herein is configured to separate out a stream of hydrogen from a stream of waste gas. This hydrogen stream is to be supplied from the hydrogen recovery unit in the propene production system of the modified PDH plant or system to another plant or system disposed downstream of the hydrogen recovery unit.
According to one embodiment, the modified PDH system or plant includes a cold box that is operatively disposed upstream of the hydrogen recovery unit or the hydrogen purification device. Alternatively, or in addition, the PDH plant may also include a CO2 removal system that is operatively disposed upstream of the hydrogen recovery unit or the hydrogen purification device. A system configured in this manner is advantageous over prior art systems in that, for example, when linking the propene production system of the modified PDH system or plant with an acrylonitrile (AN) plant that is disposed downstream of the hydrogen recovery unit, acrylonitrile may be produced using only propane as feed stock.
In one aspect of an embodiment of the present disclosure, the modified PDH system or plant and its respective process includes a fuel system configured to burn waste gas to operate the modified PDH system. In this embodiment the hydrogen recovery unit or the hydrogen purification device is operatively disposed upstream of the fuel system. The hydrogen recovery unit is configured to recover hydrogen from the waste gas stream in the modified PDH plant or system, thereby diverting the hydrogen gas from the waste stream upstream of the fuel system so that it is not burned as fuel in the fuel system. With this configuration, various waste gasses present in the waste streams, primarily methane (CH4), ethylene (C2H4), and ethane (C2H6), can be channeled directly to the fuel system.
In an aspect of one embodiment, the hydrogen purification device is disposed downstream of a CO2 removal system or a de-ethanizer stripper system of the PDH plant, the hydrogen purification device being operatively disposed between a cold box and a fuel system of the PDH plant, the hydrogen recovery unit being operatively connected to an ammonia synthesis plant or system. This configuration of the PDH plant or system enables the recovery of hydrogen at an optimal location or stage of the PDH process in the modified PDH system. In this manner, PDH technology can be integrated or interconnected with ammonia plant technology in a very synergistic and efficient manner.
As mentioned previously, conventional ammonia production, which occurs in a stand-alone plant, accounts for approximately 80% of the total energy required for production of urea, acrylonitrile and other chemicals that require ammonia as a feed stock reactant. By utilizing the hydrogen in the modified PDH system as a secondary product, ammonia can be produced in a more efficient way, especially by adding a hydrogen purification step 7.1, in order to produce high purity hydrogen (99.999%). In particular for acrylonitrile production systems, that system interconnection may provide for more energy efficient and economically beneficial production methods.
In particular, it has been found that such an interconnection between a PDH plant and another plant, such as an ammonia plant, may also be realized in an economically feasible way when a standalone ammonia plant is not feasible, for example when the ammonia plant is intended for small scale production. Such a use of hydrogen is also favorable in that, in another stage, the newly produced ammonia may be provided from the ammonia synthesis loop to an acrylonitrile plant as a feedstock to produce acrylonitrile. In particular, this efficient and beneficial use of hydrogen significantly minimizes carbon dioxide emissions of ammonia plants generated in process as byproduct, or in the vent stack of steam methane reformers.
The present disclosure may provide for the following configuration. A PDH process is provided for acrylonitrile production and mainly for another plant like a polypropylene plant, wherein at least 17.5% of total propene (i.e. excess propene) may be routed to the polypropylene plant. Two outcomes are possible: The hydrogen recovery/purifier unit may be designed in a quite cost effective or technically straight forward manner, and/or excess ammonia can be produced in addition to ammonia required for acrylonitrile production by current ammoxidation (SOHIO) process.
According to one embodiment, the hydrogen recovery unit is connected to an ammonia synthesis loop that produces ammonia and directs a stream of ammonia to still another plant or system further downstream, especially to an acrylonitrile production plant. Alternatively or in addition, the hydrogen recovery unit may be operatively connected to an acrylonitrile production plant, especially for example by being used as feed stock to an ammonia synthesis loop to produce ammonia, which ammonia is then sent to the acrylonitrile production plant to produce acrylonitrile. An Air Separation Unit (ASU) provides high purity nitrogen to the synthesis gas compression unit, together with the hydrogen from the PDH process, which is needed for ammonia production in the ammonia synthesis loop. In particular, nitrogen that is needed to produce ammonia is provided to a synthesis gas compression unit upstream of the ammonia synthesis loop.
According to one embodiment, an at least one additional plant is an acrylonitrile plant that includes the ammonia synthesis loop as a part thereof, wherein both the hydrogen stream from the hydrogen recovery unit and propene (or propylene) are provided to the acrylonitrile plant as feed stock. Thereby, the modified PDH process of the present disclosure enables high efficiency synthesis of additional chemical end products downstream from the PDH process.
According to one embodiment, the PDH plant includes a C3 splitter system that is configured to distribute or route propene (C3H6) directly to the at least one additional plant (e.g. the acrylonitrile plant or system) downstream of the C3 splitter system. This embodiment allows for the supply of both propene and hydrogen or ammonia (if the hydrogen stream is supplied as the feed stock to an ammonia synthesis loop to produce ammonia) in an efficient manner.
A C3 splitter system may be understood as an assembly for separating a feed mixture stream of propane and propene/propylene into separate streams of high purity propane (C3H8) and propene/propylene (C3H6).
One advantage of a system as disclosed herein includes the efficient use of propene and hydrogen products from a PDH process, as feed stocks to further efficiently produce additional chemical products downstream from the PDH process in a single overall system. A modified PDH system as disclosed herein includes a propene production system configured to produce both propene and hydrogen from propane, by PDH. To do so, the modified PDH system includes a hydrogen recovery unit and a hydrogen purification unit that are configured to recover high purity hydrogen during the PDH process. The system as disclosed herein further includes a second plant or system connected to the PDH plant, for example an acrylonitrile plant, that uses the propylene/propene and hydrogen from the PDH plant as feed stock to efficiently produce additional chemicals, for example acrylonitrile. An excess stream of propylene/propene is also generated by the PDH plant, which can be further processed onsite or sold as an intermediate propylene/propene product.
According to one embodiment, the system of the present disclosure is configured to produce propene and hydrogen or ammonia in a PDH plant/system, wherein a second plant may be configured to produce ammonia, or acrylonitrile, or acrylic acid. In this manner, considerable synergetic effects may be achieved for the production of one or more of the chemicals disclosed herein, as described above. Such a system may also be configured to produce propene and ammonia simultaneously based on a minimum molar ratio of propene:ammonia of 1:0.9897, by utilizing the hydrogen byproduct from said PDH process for ammonia synthesis as described above. Such a method allows for improving the propane utilization efficiency.
According to one embodiment, propene is provided both as partial propene stream to an acrylonitrile plant and as excess propene stream to any further plant or as end product, the excess propene stream being at least 1% of total propene output from the PDH process, and may be at least 16%, or at least 16.5%, or at least 17.5% depending on the excess ammonia required for the propene ammoxidation reaction. This allows for high efficiency use of propane and/or acrylonitrile production. Excess propene may be considered the molar ratio of the amount of propene not participating in acrylonitrile production to the total propene produced by the PDH process.
According to one embodiment, hydrogen recovered from the PDH process is mixed with nitrogen, especially nitrogen from an air separation unit ASU, especially in a molar ratio of 2.9:1 or 3:1. Thereby, ammonia may be produced with high yield.
According to one embodiment, a waste stream downstream of the hydrogen recovery unit is conveyed to a fuel system of the PDH plant/system, together with make-up fuel, where the waste stream and make-up fuel are consumed/burned in the fuel system to generate energy to run the PDH plant. Using the waste stream as feed to the fuel system minimizes the amount of make-up fuel that must be supplied to the fuel system for it to run at full capacity. The waste stream is comprised of any unneeded byproduct hydrocarbon chemicals resulting from the PDH process, including for example methane (CH4), ethylene (C2H4), and/or ethane (C2H6) separated out by the hydrogen recovery unit. Thereby, efficiency of PDH process is maintained at a high level.
According to one embodiment, up to 99.95% of hydrogen is recovered, especially downstream of the cold box system in the modified PDH system. This recovering rate provides for considerable synergies. According to one embodiment, at least 70% or 80% or 90% or 95% of hydrogen is recovered from the hydrogen rich gas product from the modified PDH process.
According to one embodiment, the recovered hydrogen is used to generate an ammonia stream as feed stock to an acrylonitrile production plant operatively disposed downstream of the ammonia production system. Alternatively or in addition, a method may comprise recovering hydrogen in order to provide ammonia in conjunction with propene to the second plant, especially to an acrylonitrile plant, wherein exclusively propane is used as feedstock.
According to one embodiment, recovering hydrogen is carried out for production of acrylonitrile, and/or for production of acrylic acid, and/or for methanol synthesis, and/or for polymerization reactions.
Such a use of hydrogen may provide for elimination of the energy-intensive front end of conventional stand-alone ammonia plants, resulting in significant operating cost savings relative to standalone ammonia plants.
The present invention is described in detail below with reference to the attached drawing figures, wherein:
Various embodiments now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments. However, the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. The following detailed description is not to be taken in a limiting sense.
Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. Furthermore, the phrase “in another embodiment” does not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments may be readily combined without departing from the scope or spirit of the present disclosure.
In addition, as used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. The term “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.”
Referring to
C3H8→C3H6+H2
This reaction is highly endothermic (124 kJ/mol) and high reaction temperatures are necessary to achieve high propane conversion. In one embodiment, a propane dehydrogenation may be performed, for example, over a Pt—Sn/alumina catalyst in one or more fixed bed PDH reformer(s) under nearly atmospheric pressure and temperatures of 580-650° C. Side reactions also taking place in the reformer may include the following:
C3H8→CH4+C2H4
In this reaction, propane (C3H8) is reacted to produce methane (CH4) and ethylene (C2H4).
C2H4+H2→C2H6
In this reaction, ethylene (C2H4) reacts with hydrogen (H2) to produce ethane (CH6).
C3H8+H2→CH4+C2H6
In this reaction, propane (C3H8) reacts with hydrogen (H2) to produce methane (CH4) and ethane (C2H6).
C3H8+6H2O→3CO2+10H2
In this reaction, propane (C3H8) is reacted with steam (H2O) to produce carbon dioxide (CO2) and hydrogen (H2).
As shown in
The cooled process gas 116 leaves the waste heat recovery system 2 and enters a raw gas compression unit 3 where it is compressed to around 32 bar. This compression is necessary for the subsequent distillation and CO2 removal systems downstream. From the raw gas compression unit 3, the compressed and cooled process gas 117 is then sent to a de-ethanizer stripper system 4. In the de-ethanizer stripper system 4, a stream of process gas containing any chemical component that is lighter than propene (C3H6), including carbon dioxide, 118 will be separated out from the compressed and cooled process gas 117 and sent to a CO2 removal unit 5A. And a stream of process gas containing propene (or propylene) (C3H6) and any chemical component heavier than propene 119, which includes primarily propane (C3H8), will be sent to the C3 splitter system 5B. In the CO2 removal unit 5A, carbon dioxide (CO2) 120 is separated out from the process gas 118. The remaining components in the stream of process gas 125 are hydrocarbons and hydrogen together with small amounts of propane and propene (or propylene). This stream of process gas 125 is then sent from the CO2 removal unit 5A to the cold box 6A to recover any fugitive propane and propene remaining therein.
A cold box 6A may be understood as an assembly for separating chemical components of fluids or mixtures by partial condensation. A cold box 6A may comprise one or more of a heat exchanger, a boiler, a distillation column, an expander or expansion brake turbine, or other separation technologies. In some embodiments, an expander may be a component of the cold box 6A. In other embodiments, the cold box 6A itself (excluding any expander) can be part of a rectification column, acting as a condenser where operated as low as −186° C. Avoiding decompression of a hydrogen rich stream and thereafter having to recompress the stream (especially for ammonia synthesis) may serve to prevent the wasting of energy.
The cold box 6A partially condenses the process gas 125 into a propene (or propylene) and propane rich process gas stream 130, and also separates out a light hydrocarbon (including hydrogen) stream 135. The propene (or propylene) and propane rich process gas stream 130 is sent from cold box 6A the back to the de-ethanizer stripper system 4 for additional separation as previously discussed. Referring now back to the C3 splitter system 5B, the feed stream of process gas 119 is separated or distilled into a high purity propane stream 140 (C3H8) and a high purity propene (or propylene) stream 145 (C3H6). The high purity propane stream 140 is recycled back to the depropanizer system 6B and joins the make-up propane stream 100, where unreacted high purity propane from the high purity propane stream 140 participates in the reaction as discussed above. The propene (or propylene) stream 145 that is separated out in the C3 splitter system 5B is an intermediate product that may be sold or further used in other chemical processes.
Referring again the cold box 6A, the light hydrocarbon stream 135 (including hydrogen) is sent to a fuel system 8, together with the waste stream of heavier hydrocarbons 150 that was separated out from the make-up propane 100 in the depropanizer system 6B and a stream of make-up fuel 155, where they are all consumed or burned to generate the energy necessary to operate the conventional PDH plant or system 90. Using the light hydrocarbon stream 135 and the waste stream of heavier hydrocarbons 150 from the depropanizer 6B as feed for the fuel system 8 minimizes the amount of make-up fuel 155 (e.g. natural gas) consumption required by the fuel system 8 for it to run at full capacity to accommodate the heat energy requirements of the PDH system or plant 90. The light hydrocarbon stream 135 is comprised of any unneeded byproduct hydrocarbon chemicals resulting from the PDH process, including for example methane (CH4), ethylene (C2H4), ethane (C2H6), and hydrogen (H2) that were separated out by the cold box 6A.
As discussed in the background section, one drawback of conventional PDH plants or systems is that it is only able to produce one end product (i.e. propene) using one feed stock (i.e. propane). In order to produce additional products (e.g. acrylonitrile) from the propene end product, additional stand-alone plants (i.e. ammonia plant), built at a considerable upfront capital cost and operational expense, are needed to produce other intermediary products and or a final end product. In the case of acrylonitrile production for example, propene produced in a stand-alone conventional PDH plant 90 must be transported (typically by semi-truck which incurs additional expense), together with ammonia produced in a separate stand-alone ammonia production plant (again by semi-truck at an additional expense) to the acrylonitrile plant, where they are both used together as feed stock to produce acrylonitrile.
Referring now to
As discussed previously, conventional PDH plants or systems produce only propene as the sole product with all remaining byproducts (including hydrogen) being used as fuel to run the PDH plant or system. The modified PDH plant or system 200 of the present disclosure not only produces propene product from the PDH process, but also either produces hydrogen as an end product, or uses the hydrogen byproduct produced in the PDH process as one of the two required feed stocks for one or more additional downstream subsystem or process within the modified PDH process, such as for example, an acrylonitrile production system 11, an ammonia production system or plant 170, a urea production system (not shown), or a methanol production system (not shown). In this manner, the modified PDH system 200 is able to simultaneously produce propene from the PDH process 200, as well as one or more of ammonia, acrylonitrile, urea, and/or methanol. Thus, this modified PDH plant or system 200 can simultaneously produce at least two valuable products from the single modified PDH system or plant and process, all while using propane as the sole feed stock to produce all of the aforementioned products.
Referring still to
Referring further to
In one embodiment, the separated/recovered stream of hydrogen recovered from the hydrogen recovery unit 7 has an ultra-high purity, for example greater than 99.9%, or greater than 99.99%, or greater than 99.999%.
The hydrogen recovery unit 7 may have various configurations (which may also include one or more hydrogen purification devices contained therein) and operate based on various exemplary technologies, several of which are discussed below and/or referenced in the table below. In the embodiment depicted in
As discussed above, several known exemplary hydrogen recovery and/or purification technologies and their associated operating conditions and ranges are listed below for reference:
Using the technologies discussed above, the hydrogen recovery unit may be configured to output ultra-high purity hydrogen having a pressure ranging from atmospheric pressure up to 200 bar, and a temperature ranging from −190° C. up to 400° C., and having a purity of up to 99.999%.
As between the previously disclosed technologies shown in the table above, it has been found that utilizing cryogenic separation technologies may be quite favorable in conjunction with the recovery and purification of hydrogen in a modified PDH system, since cryogenic separation may provide for a high hydrogen recovery rate (99+%) with hardly any noticeable impurities included (e.g. in the range of only 0.3%). The hydrogen stream from the hydrogen recovery unit may then be supplied to a PSA process or a membrane process, which can recover at least 96% of hydrogen from the waste gas feed stream at the aforementioned ultra-purity levels.
According to one embodiment, the hydrogen recovery unit includes a hydrogen purification device configured to purify hydrogen product produced by a PDH plant. The purity of such purified hydrogen is, for example, higher than 99.999% (ultra-high purity hydrogen). A hydrogen product having such a purity level allows for several procedural advantages, especially when used as a feed gas for ammonia production or acrylonitrile production. In particular, high purity hydrogen can be utilized as feedstock for additional processes downstream of the hydrogen recovery unit, for example ammonia production systems, acrylonitrile production systems, hydrotreating systems, urea production systems, and/or methanol systems where additional chemical products may be produced. Hydrogen at such purity levels is beneficial in that it can minimize hydrogen losses, especially in methanol or ammonia synthesis loops. A modified PDH plant that includes a hydrogen recovery unit as discussed above can provide a very competitive and economic system for the simultaneous production of high purity hydrogen and propene.
The hydrogen recovery unit (including the hydrogen purification device) may be configured based on the specific recovery method or technology that will be utilized, for example for a membrane system providing for a maximum recovery of up to 99% and a maximum purity of up to 99.99999%, or for pressure swing adsorption (PSA) providing for a recovery of 70% to at least 90% and a maximum purity of at least 99%, or for a cryogenic process providing for a maximum recovery of up to 99.95% and a purity of at least 99.9%.
In the embodiment shown in
It has been found that this location between the cold box 6A and the fuel system 8 is the optimal location at which to insert the hydrogen recovery unit 7 in order to achieve the most efficient and complete separation of hydrogen from the gas streams flowing through the various stages and systems in the PDH process. Having the hydrogen recovery unit 7 disposed downstream of the cold box 6A enables the recovery of as much as 99.95% of the hydrogen contained in the stream of light hydrocarbons and hydrogen 135 coming from the cold box 6A, with this recovered hydrogen having an ultra-high purity of 99.999%. In order to recover such a high percentage of hydrogen at such an ultra-high purity, the hydrogen recovery unit 7 may utilize one or more technologies, such as membrane separation technologies, especially those utilizing Pd/Ag membranes, cryogenic separation technologies, physical adsorption, and/or various combinations thereof.
However, while the above described embodiment shown in
Furthermore, while the embodiment shown in
Continuing on with reference to
As stated above, the simplified ammonia production plant or system 170 is simplified as compared to conventional ammonia production plants. Conventional ammonia production plants or systems typically utilize natural gas as both a fuel to be burned to run the conventional ammonia plant, and as a feed stock to create the hydrogen that is required to produce ammonia in the ammonia synthesis loop of the ammonia production plant. And the process of turning natural gas into hydrogen in the front-end of a conventional ammonia production plant, using for example coal gasification, steam methane reforming, or other conventional technologies located upstream of an ammonia synthesis loop, is not only very energy-intensive and expensive to run, but this front-end process also requires a significant capital investment in the expensive equipment needed just to turn the natural gas into hydrogen. For example, such a front end of a conventional ammonia production plant typically takes the natural gas feed stock and processes it in each of a purification and saturation device, a steam reformer, a CO conversion system, a desaturator system, a CO2 removal system, and a methanation system, all of which requires significant capital investment to purchase and install, and large amounts of energy to operate, which means routinely high operating costs.
In contrast, production of ammonia with the modified PDH plant or system 200 and process of the present disclosure eliminates this front end energy-intensive process, with its high operating cost, expensive front end equipment, and large capital expense, because the propene production system 157 of the modified PDH system 200 is able to generate all of the purified hydrogen feed that is needed by the simplified ammonia synthesis loop 10 in order to produce ammonia. Thus, to produce ammonia in addition to propene 145, the modified PDH plant 200 need only include this cheaper, greener/cleaner, and more energy efficient simplified ammonia production plant or system 170, without the front-end systems present in conventional ammonia plants. This at least ensures capital and operating cost savings, primarily due to elimination of a substantial part of the conventional ammonia plant.
In addition to the reduced capital and operation costs, another additional benefit of the modified PDH system or plant 200 of the present disclosure is that the simplified ammonia production plant or system 170 included in some embodiments, like that shown in
Referring further to
Any excess propene 12 produced in the propene production system 157 that is not needed for the production of acrylonitrile may be sold off, or further used as an intermediary or feed stock to produce other products requiring propene, such as for example acrylic acid, polypropylene, propene oxide, etc. For example, under certain operating parameters, at least 16.5% of the total mass of propene 12 produced by the modified PDH system 200 may be excess propene 12. In the acrylonitrile chemical reaction, a molar ratio of propene:ammonia of 1:1 may be considered. The actual ratio of propene:ammonia is between about 1:1 to 1:1.2 in most technologies of propene ammoxidation to minimize complete oxidation of propene. When an acrylonitrile plant requires a molar ratio of propene:ammonia (SOHIO ammoxidation process) of 1:1.2, then the excess propene could be at least 17.58%, for a PDH process built for acrylonitrile production. In case the acrylonitrile plant may require only a molar ratio of propene:ammonia of 1:1, then the excess propene could be at least 1%, for a PDH process built for acrylonitrile production.
However, it should be understood that the amount of excess propene that may be produced with acrylonitrile production is not limited to these exemplary percentages, and that alternate amounts may be produced depending on the operating parameters of the modified PDH system 200. As an another alternative, all of the propene produced by the modified PDH system 200 (including any that would otherwise be considered excess propene 12) may be used to produce acrylonitrile, and any shortage in the amount of ammonia required to react with the excess propene 12 could be made up by purchasing additional ammonia, so that the system produces only acrylonitrile as an end product. Including the acrylonitrile system 11 as part of the modified PDH process 200 enables the use of propane 100 as the sole feed stock for the production of any one or more of purified propene, hydrogen, ammonia from the hydrogen, and/or acrylonitrile from the propene and ammonia, and otherwise provides the most cost competitive, green, and energy efficient system and method for the production of acrylonitrile that exists to date.
As discussed above, an embodiment of the modified PDH process 200 shown in
However, referring to
Referring to
Referring to
Referring to
Furthermore, still additional benefits to the modified PDH system 200 of the present disclosure exist as compared to conventional PDH plants or systems and processes. For example, in embodiments of the modified PDH system 200 that include at least the propene production system 157 and the simplified ammonia production system 170, as well as embodiments that also include an acrylonitrile system 11, there exists a favorable and unanticipated process and system integration synergy between the different systems and processes (i.e. between the propene production system 157 and the ammonia production system 170, and/or the acrylonitrile plant 11). For example, in embodiments of the modified PDH system 200 that include the acrylonitrile plant 11, both of the ammonia production system 170 and the acrylonitrile production system 11 and their associated processes are net exporters of steam, while the propene production system 157 and its process is a net importer of steam. The propene production system 157 uses steam that is fed to the PDH reformer 1 to decrease the partial pressure of the propane needed for the reaction to occur in the PDH reformer 1. Also, steam is used to drive various steam turbines included in various of the subsystems in the modified PDH system 200, such as for example in the raw gas compression unit 3, in the syn. gas compressor 9, in an ammonia refrigeration compressor (not shown) that forms a part of the ammonia synthesis loop 10, and in still various others. By using the excess steam generated by the ammonia production system 170 and/or the acrylonitrile production system 11 in the manner disclosed above, the size of a boiler required by the modified PDH system 200 may be reduced or minimized, as compared to conventional PDH systems, further resulting in higher efficiency and lower CO2 emissions.
Still other advantages and synergies exist within the modified PDH system 200 of the present disclosure. When the ammonia production system 170 produces ammonia product, the ammonia product that is produced is fairly cold, for example around −32° C. However, in embodiments of the modified PDH system 200 that include the acrylonitrile system 11, the ammonia fed from the ammonia production system 170 to the acrylonitrile system 11 must be preheated before being sent to the acrylonitrile system 11, for example from about −32° C. to ambient temperature. Thus, in such embodiments, the cold ammonia produced in the ammonia production system 170 can be used as a heat sink in the ammonia refrigeration cycle of the ammonia synthesis loop 10. The resulting synergistic benefits to doing so are the elimination of a need for separate cooling water systems that would otherwise be used as a heat sink for the ammonia refrigeration cycle, achieving the required increase in the temperature of the ammonia before it enters the acrylonitrile system 11 without the need for an additional heater or heat exchanger to do so, and a reduction in overall energy use and dependency on utilities (e.g. electrical utilities).
For acrylonitrile plants, using the modified PDH system 200 of the present disclosure to produce acrylonitrile provides many technological advantages over using conventional ammoxidation of propane technologies that are currently used to produce acrylonitrile. First, production of acrylonitrile using the modified PDH system 200 has a propane utilization efficiency (defined as the mass of the valuable products produced from propane per unit mass of propane that is consumed as feed stock) of up to 88%, versus 49% for production of acrylonitrile by the ammoxidation of propane. Also, the modified PDH system 200 is a self-dependent process, in that it does not require the purchase of ammonia to produce acrylonitrile as does a conventional stand-alone acrylonitrile plant.
Included below are tables showing consumption and resulting product outputs for two exemplary embodiments of the modified PDH process of the present disclosure. However, it should be understood that such tables are included for exemplary illustration purposes only to show potential system capabilities of the modified PDH system, and are not to be read as limiting the scope of the present disclosure to any of the shown values in the tables. Alternate consumption and output values may be achieved by adjusting various operational parameters of the modified PDH system and/or including additional downstream systems. Further, in the below exemplary tables, “t/h” means tonnes per hour (or metric tons per hour), and “MMBtu/h” means million Btu per hour.
The below table shows the consumption and intermediary and/or end product output capabilities for an exemplary embodiment of a modified PDH system of the present disclosure, wherein excess propene is used to additionally make acrylic acid, and wherein 172,656 tons per anum (TPA) of acrylonitrile (AN) and 49,579 TPA of acrylic acid (ACA) are produced.
The below table shows the consumption and intermediary and/or end product output capabilities for an alternate embodiment of a modified PDH system 200 of the present disclosure. In this embodiment, all of the ammonia produced by the ammonia production system 170 from hydrogen recovered by the hydrogen recovery unit 7, as well as the propene 145 produced in the propene production system 157, to the acrylonitrile production system 11. The excess propene is sold/exported to an external customer. The design basis of this embodiment of a modified PDH system 200 is the production of 172,656 tons per anum (TPA) of acrylonitrile (AN) and 41,976 TPA of liquid propene.
