PROCESS FOR THE PRODUCTION OF NITRILES AND OXIDES

Abstract: Nitriles and oxides are provided from hydrocarbons by reaction with oxygen, air or preferably oxygen-enriched air and ammonia where a nitrile is desired, in the presence of a suitable catalyst. An alkane, eg propane, is converted to an alkene in a dehydrogenator. The product stream is introduced into an ammoxidation/oxidation reactor. The product formed therein is recovered in a conventional quench tower. The gaseous effluent from the quench tower is treated in a PSA unit to form a gas stream containing the unreacted alkane and alkene as well as a minor amount, of oxygen and nitrogen. The gas stream, which may or may not contain hydrogen depending on the adsorbent in the PSA unit, is introduced into a selective oxidation unit to remove the remaining oxygen and then recycled to the dehydrogenator. Hydrogen may be introduced into or removed from the PSA effluent, depending on the choice of the adsorbent, to obtain an optimum concentration for introduction into the dehydrogenator. A particularly preferred PSA system for use in the subject process comprises two parallel PSA units containing different adsorbents such that the gas stream formed in one contains all of the hydrogen in the gas phase from the quench tower. The feed to the PSA units is divided disproportionately so that the combined PSA effluent recycle streams contain the optimum concentration of hydrogen for the dehydrogenator.

Description: The present invention is directed to a process for producing nitriles and oxides from alkanes, an oxygen-containing gas and ammonia in the presence of a suitable catalyst under conditions which achieve high efficiency and selectivity toward the desired product. The production of nitriles and oxides by ammoxidation and oxidation, respectively, of an appropriate alkene in the presence of a suitable catalyst is well known. The production of acrylonitrile, for example, from a gaseous feed of propylene, ammonia and air is described by Bruce E Gates et al in Chemistry of Catalytic Processes , McGraw-Hill (1979), pp 380-384. The feed is sent to an ammoxidation reactor where, in the presence of a suitable catalyst, acrylonitrile is produced along with lesser amounts of other nitrogen-containing compounds. The effluent from the ammoxidation reaction is quenched with water and the desired products are obtained in the liquid phase. The gas phase by-products, typically oxygen, carbon dioxide, carbon monoxide, nitrogen and unreacted hydrocarbon, are combined with natural gas and sent to a boiler for combustion as disclosed, for example, in Yoshino et al, US Patent No 3,591,620 and Callahan et al, US Patent No 4,335,056. Such a process is illustrated in Figure 1 of the accompanying drawings. More recently, Khoobiar et al, in US-A-4,609,502 disclosed a cyclic process for producing acrylonitrile using propane as a starting material which is initially dehydrogenated catalytically in the presence of steam to form propylene. This is in contrast to most conventional dehydrogenation processes and which avoid steam primarily due to the costs involved. After ammoxidation, the effluent is quenched, the desired product removed, and the off-gases, including propylene and propane, are sent to an oxidation reactor ("oxidator") to remove oxygen by selective reaction with hydrogen to form water vapour. The gas mixture exiting the selective oxidation reactor includes substantial amounts of methane, ethane and ethylene, which are by-products of dehydrogenation, and unreacted propylene in addition to carbon oxides. As an option, this gas mixture is split and a portion is sent to a separator which removes only carbon dioxide. A portion of the effluent from the separator is purged to remove light hydrocarbons. The nonpurged stream is mixed with the remainder of the oxidator effluent, fresh propane and steam, if necessary, sent to the dehydrogenator where the propane is converted to propylene. (Such a process is illustrated in Figure 2 of the accompanying drawings.) Another option is to cool and liquefy the C₃ hydrocarbons therefrom and then vaporise them prior to recycle. The aforementioned process suffers from several disadvantages. For example, there is no practical way to selectively remove by-products of propane dehydrogenation, such as methane, ethane, ethylene and the like, thereby preventing their accumulation in the system other than by removing them in the purge stream. The removal of these gases in a purge stream will result in a loss of some of the circulating propane and propylene. As the process is being carried on in a continuous manner, this loss of starting material causes a significant decrease in the yield of propylene. As mentioned above, propane and propylene can be recovered from the purge stream prior to venting. This requires additional refrigeration apparatus to cool and liquefy propylene and propane. The separated C₃ hydrocarbons must be vaporised prior to recycle. These operations add to the capital costs and power requirements of the process. Another disadvantage of the Khoobiar et al process stems from the use of the selective oxidation reactor to treat the gaseous effluent from the quencher. The gases exiting the quencher are at ambient temperature and must be heated prior to introduction into the oxidation reactor in order to promote oxygen removal. Because there is a significant amount of oxygen in the quench effluent, the heat of reaction generated in the oxidation reactor can result in excessive temperatures in the system. There are three options to alleviate this problem. First, the amount of oxygen entering the oxidation reactor can be reduced by other means. Second, multiple reactors can be utilised with a cooling means between each pair of reactors. Third, a portion of the effluent from the reactor is passed through a cooling means and recycled to the feed to reduce the internal temperature of the reactor. None of these measures is attractive from the viewpoint of cost and efficiency. The oxidation reactor in the Khoobiar et al process is operated with oxidation catalysts such as noble metals (eg platinum). Olefins and carbon monoxide, which are generated in the dehydrogenation reactor, are known to deactivate these catalysts, as disclosed in Catalytic Processes and Proven Catalysts , Charles L Thomas, Academic Press (1970) pp 118-119. Accordingly, multiple oxidation reactors must be used to allow for frequent regeneration of the catalyst which represents yet another addition to production costs (US-A-4,609,502, column 4, lines 51-56). The invention is particularly concerned with providing a process for the production of nitriles and oxides in which a simpler oxidation reactor may be used. According to the present invention there is provided a process for the production of nitriles and oxides comprising: (a) forming an alkene from a gaseous alkane in a catalytic dehydrogenator; (b) introducing a first gaseous stream comprising said alkene, oxygen, and, optionally, ammonia into a reactor and reacting them in the presence of a suitable catalyst to produce a gaseous effluent containing said nitrile or oxide; (c) quenching said effluent in a liquid to form a liquid phase containing said nitrile or oxide and a gaseous phase; (d) recovering said nitrile or oxide from said liquid phase; (e) separating the gaseous phase under pressure by pressure swing adsorption thereby to form a second gaseous stream comprising said unreacted alkane and alkene, and a minor amount of oxygen; (f) introducing said second gaseous stream into a catalytic selective oxidation unit to remove the remaining oxygen in said stream; and (g) recycling the effluent from the selective oxidation unit to the dehydrogenator. The term "suitable catalyst" indicates a catalyst that will catalyse the production of the desired product under the conditions utilised in the reactor. In the event an oxide, eg ethylene oxide, is desired, an oxidation catalyst is utilised. On the other hand, to form a nitrile, the feed to the reactor additionally includes ammonia and the catalyst is an ammoxidation catalyst. These catalysts and their use are conventional and well known to one of ordinary skill in the art. Illustrative of products, and their respective starting gaseous alkanes, which can be advantageously produced by the method of this invention are acrylonitrile from propane, methacrylonitrile from isobutane, and ethylene oxide from ethane. The process according to the invention is now further described by way of example with reference to Figure 3 of the accompanying drawings which: illustrates in a block diagram a process for producing acrylonitrile. In the interest of brevity, the subject process will be described with reference to the production of acrylonitrile from propane, but is in no way intended to be limited thereto. The process of the present invention is illustrated in Figure 3. The present process provides the efficiency of recycle afforded by the process illustrated in Figure 2 and is similar in a generic sense in that it contains the same kinds of functional units, yet is substantially more efficient and, unexpectedly, capable of effectively utilising air or oxygen-enriched air as a feed to the ammoxidation/oxidation reactor. The configuration of the subject process eliminates the substantial loss of efficiency inherent in the process of Figure 2 by the use of the purge stream. Further, it is not contemplated in the present process to utilise steam in the dehydrogenator, thereby providing an additional saving of energy. Referring to Figure 3, propane is fed into the dehydrogenator where it is converted to propylene. For increased catalyst life, it is preferred to introduce a hydrogen-containing gas into the dehydrogenator with the propane feed. The required amount of hydrogen can conveniently be provided through the recycle stream from the PSA unit as will be discussed below. The hydrogen-containing gas can, if desired, be provided as a discrete stream. The catalyst utilised in the dehydrogenator can be any conventional dehydrogenation catalyst, preferably one or more Group VIII noble metals such as platinum on an alumina support. The effluent product stream from the dehydrogenator comprising unreacted propane, propylene and hydrogen, is fed into a conventional ammoxidation/oxidation reactor along with pure oxygen, air or, preferably, oxygen-enriched air and ammonia. The ammoxidation/oxidation reactor utilised in the present process is conventional and may employ either a fixed or fluidised catalyst bed. A typical example of an ammoxidation reactor is disclosed in Angstadt et al, US Patent No 4,070,393 and Gates et al, ibid, pp 381-383, each incorporated herein by reference. The reactor contains a conventional ammoxidation catalyst, such as bismuth-molybdenum oxide, iron-antimony oxide, uraniumantimony oxide precipitated on silica and the like. Other suitable catalysts are disclosed, for example, in Chemistry of Catalytic Processes , Gates et al, McGraw Hill (1979) pp 349-350, and Yoshino et al, US Patent No 3,591,620, incorporated herein by reference. Additional suitable catalysts are known to those skilled in the art. The ammoxidation reaction is conducted at a temperature of from about 375° to 550°C, preferably from about 400° to 500°C, at low pressures, typically in the range of from about 3 to 30 psig, preferably from about 136 to 239 kPa (about 5 to 20 psig). The reactants are passed through the reactor at a relatively low velocity, typically in the range of from about 53.34 to 67.06 (1.75 to 2.2 ft/sec). The oxygen-containing gas feed may be pure oxygen, air or oxygen-enriched air. In accordance with this invention, oxygen-enriched air preferably contains from about 30 to about 80, most preferably from about 55 to 65, percent by volume of oxygen. Such mixtures may be produced by adjusting the capacity of a conventional oxygen-producing unit, eg a conventional pressure swing adsorption unit, or by mixing pure oxygen with air in the proper proportions. The ratio of oxygen to propylene in the feed is suitably in the range of from about 1.6:1 to 2.4:1 by volume. In the production of a nitrile, the ratio of ammonia to propylene in the feed is suitably in the range of from about 0.7 to 1.2:1 by volume. The effluent from the ammoxidation reactor comprises a major amount of acrylonitrile and minor amounts of acrolein, hydrogen cyanide, acetonitrile, and carbon oxides and nitrogen, when present in the feed, as well as unreacted oxygen, propylene and propane. This gaseous mixture is quenched or scrubbed with a liquid, such as water, to dissolve the water-soluble compounds for subsequent separation and recovery of acrylonitrile, acetonitrile and hydrogen cyanide. The gas phase effluent from the quench step is introduced into a pressure swing adsorption (PSA) unit. PSA is a well known process for separating the components of a mixture of gases by virtue of the difference in the degree of adsorption among them on a particular adsorbent retained in a stationary bed. Typically, two or more such beds are operated in a cyclic process comprising adsorption under relatively high pressure and desorption or bed regeneration under low pressure or vacuum. The desired component or components may be obtained during either of these stages. The cycle may contain other steps in addition to the fundamental steps of adsorption and regeneration, and it is commonplace to have two or more adsorbent beds cycled out of phase to assure a pseudo continuous flow of desired product. It is preferred to pass the quench tower effluent through a conventional dryer (not shown) to remove moisture therefrom prior to introducing it into the PSA unit. It is may be necessary to raise the pressure of the quench tower effluent in a compressor or other suitable means prior to introducing it into the PSA unit. The compressor increases the pressure of the quench tower gaseous effluent to the operating pressure of a PSA unit, typically from about 122 kPa to 446 kPa (about 3 to 50 psig), preferably from about 239 kPa to 377 kPa (about 20 to 40 psig. These ranges may vary to an extent depending on the adsorbent in the PSA unit. The adsorbent in the PSA unit may be any art-recognised material which will adsorb propane and propylene to a substantially greater degree than carbon oxides, nitrogen and oxygen, or vice versa. Silica gel or a conventional molecular sieve material, such as 4A zeolite, are preferred adsorbent materials. Silica gel is a particularly preferred material wherein oxygen-enriched air is utilised as a reactor feed material. The PSA unit produces a recycle stream and a waste stream. The latter is comprised of oxygen, carbon oxides, ie carbon monoxide and carbon dioxide, nitrogen wherein the feed to the ammoxidation/oxidation reactor is air or oxygen-enriched air and C₁₋₂ hydrocarbons, ie methane, ethane and ethylene, as well as trace amounts of propane and propylene. The PSA recycle stream contains propane, propylene, a minor quantity of oxygen, typically less than about 1 percent by volume, and a minor quantity of nitrogen. The use of an adsorbent in the PSA unit which will adsorb propane and propylene, such as a silica gel, will cause substantially all of the hydrogen in the quench tower effluent to be removed in the waste stream. Therefore, it is necessary to add an appropriate quantity of hydrogen to the recycle stream before introducing it into either the dehydrogenator or the selective oxidator. Wherein an adsorbent, which adsorbs carbon oxides, nitrogen and oxygen, such as 4A zeolite molecular sieve, is used as the adsorbent in the PSA unit, the hydrogen will be withdrawn in the recycle stream. Therefore, it will not be necessary to add hydrogen thereto. In fact, it will typically be necessary to remove some hydrogen therefrom to prevent excessive build-up in the system. In a preferred embodiment of the subject process, a PSA system comprising parallel PSA units each containing one of the two types of adsorbers mentioned above is utilised wherein the feed is disproportionately divided between them so that the combined effluents contain the desired amount of hydrogen. The waste streams are combined and discarded. The amount of hydrogen required in the recycle feed to the dehydrogenator or the selective oxidator will vary with the catalyst and can be determined by operation of the system utilising a given catalyst. The selective oxidation reactor is of conventional configuration and contains a known catalyst capable of selectively catalysing the reaction of oxygen and hydrogen to form water, ie the oxidation of hydrogen, without causing oxidation of the desired hydrocarbons, ie propane and propylene in the PSA effluent. Such catalysts and their use are well known in the art. Suitable catalysts include noble metals or base metals, particularly platinum or palladinm on alumina. As previously stated, the oxidation reactor utilised in the present process requires only a modest capital expenditure in comparison with the multiple bed unit contemplated in the process illustrated in Figure 2 since the PSA effluent in the subject process contains less than one percent by volume of oxygen. Typically, the oxygen content of the PSA effluent in the present process is of the order of from about 0.01 to 1 percent by volume. Since the oxygen content is at such a low level, a small oxidation reactor consisting of a single bed without the need for catalyst regeneration over a period of several years is more than adequate in the method of this invention. Utilising a system as shown in Figure 3 for the production of acrylonitrile utilising propane as the starting material, the flow rates at various points in the system were determined and are presented in Table I. The flow rates are expressed in mole percent based on 100 moles of acrylonitrile produced. The propane feed was virtually 100 percent propane. The fresh feed added to the dehydrogenator effluent prior to introduction into the ammoxidation reactor was 32.88 percent of ammonia and 67.12 percent of pure oxygen. The data expressed in Table I represents operation of the system under conditions such that 60 percent, 80 percent and 97 percent, respectively, of the propylene in the feed to the ammoxidation reactor is converted to different products, including acrylonitrile, in the ammoxidation reactor. In Table I, Point A is the feed into the dehydrogenator after the stream from the oxidation reactor has been combined with fresh propane, Point B is the combined feed into the ammoxidation reactor, Point C is the ammoxidation reactor effluent, Point D is the quench tower gaseous effluent to the PSA unit, Point E is the recycle stream from the PSA unit and Point F is the PSA unit waste stream. As previously mentioned, the amount of hydrogen in the feed to the dehydrogenator will vary with the catalyst and reaction conditions used, and may be negligible. For purposes of the comparative results given in Tables I, II and III, hydrogen was not included in the dehydrogenator feed, Point A. Again utilising a system as shown in Figure 3 for the production of acrylonitrile with propane as a starting material, the oxygen feed to the ammoxidation reactor was changed to an equal mixture of pure oxygen and air which produced oxygen-enriched air containing approximately 60 percent by volume of oxygen. The flow rates at various points in the system were determined and are presented in Table II. The data expressed in Table II represents operation of the system under conditions such that 60 percent and 80 percent, respectively, of the propylene in the feed to the ammoxidation reactor is converted therein to different products, including acrylonitrile. Utilising a system as shown in Figure 2 for the production of acrylonitrile using propane as the starting material, the flow rates at various points in the system are presented in Table III. The propane, ammonia and pure oxygen feeds were as in Table I. The system was operated to convert 60, 80 and 97 percent, respectively, of the propylene feed to the ammoxidation reactor to products. In Table III, Point A is the feed into the dehydrogenator after the recycle stream has been combined therewith, Point B is the feed into the ammoxidation reactor, Point C is the ammoxidation reactor effluent, Point D is the quench tower gaseous effluent to the selective oxidation reactor, and point E is the selective oxidation reactor effluent. During this simulation, steam was not added to the dehydrogenator feed since it is conventionally considered to have an adverse effect on the efficiency of the process. The effluent from the selective oxidation unit is divided and an appropriate fraction determined in accordance with the operating conditions of the ammoxidation reactor is introduced into a separator for the removal of carbon dioxide. It will be appreciated that Point E represents the feed into the separator as well. Point F is the effluent from the separator. In Khoobiar et al, US Patent No 4,609,502, it is stated that a sufficient quantity of the effluent from the separator will be purged to remove the net production of carbon oxides and light hydrocarbons, Col 3, lines 65-69. Accordingly, depending on the operating conditions of the ammoxidation reactor, from about 30 to 80 percent of the separator effluent will have to be purged. The remaining effluent is combined with the remainder of the selective oxidation effluent for reintroduction into the dehydrogenator. The combined streams are point G in Table III. The process of this invention is advantageous in that it is very efficient and is cost attractive in comparison to prior art processes. It is readily apparent from the data presented in Tables I, II and III that, in contrast to the subject process, the process illustrated in Figure 2 continually removes propane and propylene from the system, thereby sharply reducing the efficiency thereof. It is stated in the Khoobiar et al patent that propane and propylene are removed from the purge stream before it is vented. This would require an additional sizeable capital expenditure for the refrigeration equipment required for the recovery procedure as well as an on-going cost in pover to operate the recovery unit. The process of the invention has a comparatively small incidence of build-up of any of the components of the various gaseous streams formed or separated at any stage thereof. Further, the subject process can be utilised with air or an oxygen-enriched air feed, heretofore not feasible with a closed loop system. Unexpectedly, the subject process operates at particularly enhanced efficiency with an oxygen-enriched air feed.

Claims: A process for the production of nitriles and oxides comprising: (a) forming an alkene from a gaseous alkane in a catalytic dehydrogenator; (b) introducing a first gaseous stream comprising said alkene, oxygen, and, when a nitrile is required, ammonia into a reactor and reacting them in the presence of a suitable catalyst to produce a gaseous effluent containing said nitrile or oxide; (c) quenching said effluent in a liquid to form a liquid phase containing said nitrile or oxide and a gaseous phase; (d) recovering said nitrile or oxide from said liquid phase; (e) separating the gaseous phase under pressure by pressure swing adsorption thereby to form a second gaseous stream comprising said unreacted alkane and alkene, and a minor amount of oxygen; (f) introducing said second gaseous stream into a catalytic selective oxidation unit to remove the remaining oxygen in said stream; and (g) recycling the effluent from the selective oxidation unit to the dehydrogenator. A process in accordance with claim 1, wherein the recycled effluent from the selective oxidation unit is combined with fresh alkane feed before introduction into the dehydrogenator. A process according to claim 1 or claim 2, wherein the alkane is propane, the alkene is propylene, ammonia is added in step (b), the catalyst is an ammoxidation catalyst and the nitrile produced is acrylonitrile. A process according to any one of the preceding claims, wherein oxygen is added in step (b) as pure oxygen. A process according to any one of claims 1 to 3, wherein oxygen is added in step (b) as air or oxygen-enriched air, said second gaseous stream thereby additionally including a minor amount of nitrogen. A process according to claim 5, wherein oxygen-enriched air contains from 30 to 80 percent of oxygen by volume. A process according to claim 6, wherein the oxygen-enriched air contains from 55 to 65 percent by volume of oxygen. A process according to any one of the preceding claims, wherein the said gaseous phase includes hydrogen, the separation by pressure swing adsorption removes substantially all of the hydrogen from the gaseous phase to form a second gaseous stream, substantially free of hydrogen and the second gaseous stream is combined with a hydrogen-containing stream prior to introduction into the dehydrogenator. A process in accordance with Claim 8, wherein the hydrogen-containing stream is combined with the second gaseous stream upstream of its introduction into the selective oxidation unit. A process according to claims 1 to 7, wherein the said gaseous phase includes hydrogen and substantially all the hydrogen is separated into the second gaseous stream, and said second gaseous stream is treated to remove excess hydrogen prior to recycle into the dehydrogenator. A process in accordance with claim 1, wherein in step (e), the gaseous phase includes hydrogen and is divided into two streams which are each separately separated by pressure swing adsorption such that the second gaseous stream produced by one separation contains substantially all the hydrogen in the respective divided stream, whereas the second gaseous stream produced by the other separation includes substantially no hydrogen, the relative sizes of the two gaseous streams being such that on being mixed the two second gaseous streams contain a predetermined proportion of hydrogen. A process according to claim 11, wherein the separation by said pressure swing adsorption which produces a hydrogen-containing second gaseous stream employs a 4A zeolite molecular sieve adsorbent and the other separation by pressure swing adsorption employs a silica gel adsorbent. A process according to claim 1 or claim 2, wherein ethylene oxide is formed by dehydrogenating ethane to form ethylene and oxidising the ethylene to form the ethylene oxide.

Inventors: MACLEAN DONALD L, RAMACHANDRAN RAMAKRISHNAN, SATCHELL JR DONALD P, MACLEAN, DONALD L., RAMACHANDRAN, RAMAKRISHNAN, SATCHELL JR., DONALD P.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1989
Main classification: C07C253
IPC codes: C07C253, C07D301, C07C255, C07B61