A process for reduction of waste material during manufacture of acrylonitrile

Abstract: A process for the reduction in the amount of waste material generated during the manufacture of acrylonitrile comprising introducing an additional amount of oxygen containing gas, preferably air, in the substantial absence of any oxygenate compounds, into the upper portion of the fluid bed reactor to react with at least some of the unreacted ammonia to reduce the amount of unreacted ammonia present in the reactor effluent.

Description: The present invention is directed to substantial reduction of unreacted ammonia and the corresponding reduction in ammonium sulphate and resulting waste products produced from the unreacted ammonia during the manufacture of acrylonitrile by the direct ammoxidation of an unsaturated or saturated hydrocarbon, preferably propylene or propane, ammonia and oxygen in a fluid bed reactor containing an ammoxidation catalyst. In particular, the present invention is directed to the addition of an oxygen-containing compound, preferably air, at a specific location in the fluidised bed reactor during the manufacture of acrylonitrile to reduce substantially the amount of ammonia remaining in the gaseous effluents exiting the fluidised bed reactor during the manufacture of acrylonitrile. This process is carried out in the absence of any additional oxygenated hydrocarbon compound such as methanol. This substantial reduction in the generation of ammonium sulphate during the practice of the manufacture of acrylonitrile leads to significant environmental and economic advantages. There are several patents which are directed to the injection of methanol into a fluid bed reactor to produce hydrogen cyanide. In addition, these references further disclose the injection of methanol into an acrylonitrile fluid bed reactor to produce hydrogen cyanide while manufacturing acrylonitrile. For example, US Patents 3,911,089 and 4,485,079 each teach the ammoxidation of methanol to produce hydrogen cyanide by injection of methanol into a fluid bed reactor containing an ammoxidation catalyst suitable for the manufacture of acrylonitrile. Moreover, Japanese Patent Applications 74-87474, 79-08655 and 78-35232 all are related to similar methods of increasing or making hydrogen cyanide during the manufacture of acrylonitrile. Japanese Patent Application 74-87874 also suggests that a secondary effect of their procedure is the decrease of the amount of sulphuric acid used for neutralisation. All of these patents are primarily concerned with the production of additional hydrogen cyanide. Finally, US Patent 5,288,473 and copending application US Serial No. 08/104,752 filed August 11, 1993, all assigned to the assignee of the present invention, are directed to the substantial reduction of the amount of unreacted ammonia exiting the fluid bed reactor which utilised oxygenate compounds such as methanol while Chinese Patent CN 1032747 to Sun et al discloses multi-staged air feed to a reactor to increase acrylonitrile conversion. The present invention is directed to a specific procedure for injection of an oxygen-containing gas, preferably air, into the fluid bed reactor to obtain a substantial reduction in the amount of unreacted ammonia exiting the fluid bed reactor during the manufacture of acrylonitrile without any decrease in the acrylonitrile yield. The process is performed in the absence of any oxygenated hydrocarbon compounds such as methanol. It is a primary object of the present invention to substantially reduce the amount of ammonium sulphate generated during the manufacture of acrylonitrile. It is a further object of the present invention to substantially reduce the amount of unreacted ammonia exiting the reactor effluents during the manufacture of acrylonitrile. Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and advantages of the invention may be realised and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing objects in accordance with the purpose of the present invention comprises a method of reducing the amount of unreacted ammonia exiting the reactor during the manufacture of acrylonitrile comprising: a) introducing into the lower portion of a fluid bed reactor ammonia, air and a hydrocarbon selected from the group consisting of propylene and propane to react in the presence of a fluid bed catalyst to produce acrylonitrile, said air to propane or air to propylene ratio being at least 9.3:1; b) introducing at a point in the reactor above at least 50 percent of the calculated expanded fluid catalytic bed height an additional amount of oxygen-containing gas above the amount of air added to the lower portion of the reactor, c) reacting said additional amount of oxygen-containing gas with at least some of the unreacted ammonia present in the reactor to reduce the amount of free ammonia present in the reactor effluent, the additional amount of oxygen-containing gas being substantially free of any oxygenate compounds; d) passing the reactor effluent containing acrylonitrile into a quench column to cool the reactor effluent with water to remove unwanted impurities; and e) recovering acrylonitrile from the quench column. Two addition benefits obtained from the practice of the present invention are that (1) no additional acrolein, an undesirable by-product of acrylonitrile production, is manufactured and (2) additional hydrogen cyanide, a desirable by-product in most cases, is produced. In preferred embodiment of the process of the present invention the oxygen-containing gas is air. The point of injection of the additional oxygen-containing gas into the reactor is at a point above at least 50 percent of the calculated expanded fluid catalytic bed height, preferably above at least 70 percent of the calculated expanded bed height, most preferably above 85 percent, especially preferred being above 90 percent. In another preferred embodiment of the present invention the additional oxygen-containing gas is introduced above 100 percent of the calculated expanded bed height. In a preferred embodiment of the present invention the amount of additional oxygen-containing gas injected into the fluid bed reactor is sufficient to react with at least 15 percent of the unreacted ammonia present in the upper portion of the fluid bed reactor, preferably at least 25 percent, especially preferred being at least 40 percent. The term oxygenate compounds as defined for purposes of this invention include carboxylic acids, ketones, alcohols, esters or mixtures thereof. The present invention is characterised by the fact that no significant amounts of these materials are present during the practice of the process of the present invention. The significance of the process in the present invention is that it provides a simple and economic procedure for the substantial reduction of ammonia breakthrough (i.e. unreacted NH 3 ) in a fluid bed reactor along with the attendant advantages of (1) reducing ammonium sulphate as a by-product during the manufacture of acrylonitrile, (2) achieving a reduction of NH 3 breakthrough without the use of costly oxygenate compounds and (3) achieving a reduction of NH 3 breakthrough without producing additional amounts of undesirable byproducts. This can lead to significant economic advantages in the practice of the acrylonitrile process if one cannot practice deepwell injection. Currently, the waste stream emanating from the quench column contains (NH 4 ) 2 SO 4 in a fairly high concentration which makes the disposal of these streams in an economic and environmentally acceptable manner difficult. The minimisation of this ammonium salt from this stream can make these streams acceptable to treatment by waste treatment procedures which do not require severe conditions or expensive materials of construction (e.g. incineration), or if deepwell injection is not available leading to significant economic and environmental advantages. Reference will now be made in detail to the present preferred embodiment of the invention. The present invention reduces the production of ammonium sulphate generated during the manufacture of acrylonitrile by adding an additional amount of oxygen-containing gas, preferably air, substantially free of any oxygenate compounds, preferably in the absence of any oxygenate compounds, to the fluid bed reactor at a location which allows for the reaction of at least some of the excess ammonia in the reactor with the additional oxygen without substantially affecting the efficiency of the acrylonitrile production. It is important to appreciate that the injection of the oxgyen-containing gas to the upper portion of the reactor is an actual injection of an additional amount of oxygen into the reactor above the amount added to the lower portion of the reactor during normal operating conditions used to produce acrylonitrile. That is, if the normal operating conditions required an air to propylene ratio of 9.5:1 in the bottom of the reactor, this ratio would be maintained while additional oxygen would be injected into the reactor at the upper location. The reduction of ammonium sulphate from the waste stream emanating from the quench column of an acrylonitrile plant can dramatically improve the environmental impact and economics associated with the practice of the acrylonitrile process. In the practice of the present invention, air is injected into the fluid bed reactor in the upper portion of the reactor in the catalyst zone (at least 50 percent of the expanded bed height) or above (i.e. a height above 100 percent of the expanded catalyst bed height) from a sparger at a position where it will have an opportunity to react with a substantial amount of the excess ammonia but not be competitive with the main propylene ammoxidation reaction occurring in the lower portion of the catalyst bed. For the purposes of the present invention, the term fluid bed reactor is intended to not only encompass conventional fluid bed reactors but also any reactor capable of maintaining the catalyst in a fluid state such as circulating fluid bed reactors, transport line reactors, riser reactors or recycle reactors. The additional oxygen can be injected into the reactor in any direction although a downward angle is preferred. The air sparger may be constructed from conventional materials (e.g. steel/steel alloys) with the number of nozzles being sufficient to allow for good mixing without affecting flow distribution in the reactor. In a preferred embodiment of the present invention, the location of the additional oxygen-containing gas should be at a level calculated at 70 percent of the level of the expanded catalyst bed height, most preferably being over 90 percent of expanded catalyst bed height. The term expanded catalyst bed height as used in this application means the catalyst bed height while the catalyst is in the fluidised bed state. That is, the bed height when the gaseous components are present in the fluidised bed reactor and mixing with catalyst. Each propylene/propane ammoxidation catalyst operates at somewhat different feed ratios and operating conditions for maximum acrylonitrile yield and/or economic considerations. The amount of excess ammonia exiting from the propylene ammoxidation reactor will vary somewhat depending on the catalyst used. The level of additional oxygen-containing gas to be added will vary according to the catalyst types and the nature of the reactor. Accordingly, in the practice of the present invention the amount of additional oxygen-containing gas injected into the reactor will be dictated by the conditions and the catalyst used. Typically, any ammoxidation catalyst may be utilised in the practice of the present invention although catalysts which operate under a conventional air/propylene ratio (e.g. above 9.3:1) are preferred. For example, catalyst such as those disclosed in US Patent Nos. 3,642,930; 4,485,079; 3,911,089; 4,873,215; 4,877,764; Japanese Patent Application Nos. 74-87474 and 78-35232 are suitable for the practice of the present invention and are incorporated herein by reference. As stated previously, each propylene/propane ammoxidation catalyst will operate at somewhat different feed ratios and operating conditions. During the practice of the process of the present invention the standard operating condition at which the existing propylene/propane catalyst has been operated should not have to be changed but can be changed depending upon feed and catalyst conditions. Conventional operating conditions and feed ratio for the manufacture of acrylonitrile as set forth in US Patents 3,911,089 and 4,873,215 are suitable and herein incorporated by reference. For purposes of illustration only, the following examples are set forth to describe the process of the present invention. For the avoidance of doubt, Examples 1, 4, 5, 10, 11, and 15 to 18 do not fall within the scope of the present invention and are included herein for comparative purposes only. In Example 1, 4, 5, 10, 11 an additional amount of air oxygen-containing gas (air) is not added, contrary to the requirements of the present invention. In Example 15 to 18, the air to propylene ratio initially employed in the bottom of the reactor is less than 9.3:1. Example 1 (Comparative) Approximately 11,340 Kg (12.5 tons) of a propylene ammoxidation catalyst was charged to a fluidized bed acrylonitrile reactor. After several days onstream, feeds of air/ammonia/propylene in the mole ratio of 9.3/1.21/1.0 were passed through the catalyst bed at 434°C (813°F) reactor bed temperature, 1215.9 kPa (12psig) top pressure at a weight hourly space velocity (WWH) of 0.085 hr-1. Twenty-four hours after adjustment of feed ratios and reactor temperature, baseline recovery run (See Table I below) indicated propylene conversions of 98.1 per cent, per pass conversions to acrylo 80.49 per cent, to acrolein 0.7 per cent, to acrylic acid 1.3 per cent Notably, 8 per cent of the feed ammonia was burning, with ammonia breakthrough of 7.76 g/Nm 3 (0.22 g/sef) and sulfuric acid usage in downstream quench operation of 89.9 kg/hr (0.22 gpm) to neutralize excess ammonia. Example 2 At the same conditions of Example 1, and with no methanol (oxygenate) vapor injected, air was injected through an air sparger at a position in the bed equal to 90 per cent of the catalyst's expanded bed height, at dilute air to propylene molar ratio of 0.52. Recovery run (See Table 1 below) gave 99.1 per cent C 3 = total conversion, 80.5 per cent PPC to acrylo, 5.43 per cent PPC to HCN, 0.6 per cent PPC to acrolein, and 1.3 per cent PPC to acrylic acid from propylene. Notably, 7 per cent of feed ammonia was burned, and sulfuric acid usage dropped from 89.9 to 65.4 kg/hr (0.22 to 0.16 gpm), or 27 per cent ammonia breakthrough reduction. Example 3 At the same conditions of Example 2, and with no methanol (oxygenate) vapor injected, air was injected through a separate air sparger at a position in the bed equal to 90 per cent of the catalyst's expanded bed height, at dilute air to propylene molar ratio (DPAR) of 0.5 l. The bottom air/C 3 = molar ratio was then decreased from 9.3 to 9.1 to see if the total air/C 3 = could be lowered without penalizing AN or HCN yield while still reducing sulfuric acid consumption. Recovery run (See Table 2 below) gave 99.2 per cent C 3 = total conversion, 79.9 per cent PPC to acrylo, 5.6 per cent PPC to HCN, 0.6 per cent PPC to acrolein, and 1.3 per cent PPC to acrylic acid from propylene. Notably, 1.3 per cent of feed ammonia was burned, and sulfuric acid usage dropped from 89.9 to 69.4 kg/hr (0.22 to 0.17 gpm), or 23 per cent ammonia breakthrough reduction compared to without separate air (See Table 2 below). This showed the total air/C 3 = ratio could be lowered from 10 to 9.48 without significant deterioration of AN or HCN yield, lessening the penalty for a higher dilute phase air ratio. Example 4 (Comparative) and Example 5 (Comparative); Example 6 to 9 The next set of experiments was directed at quantifying the benefits of separate air addition as a function of amount of air added, all other conditions being constant. At the same conditions of Example 1, except feeds of air/ammonia/propylene in the mole ratio of 9.4/1.22/1.0, initial baseline and final baseline (Examples 4 and 5, Table 3 below indicated propylene conversions of 98.7 per cent, per pass conversions to acrylo 80.7 per cent, to HCN 5.1 per cent, to acrolein 0.8 per cent, to acrylic acid 1.7 per cent for propylene. Notably, 5 per cent of the feed ammonia was burning, with ammonia breakthrough of 6.35 g/Nm 3 (0.18 g/sef) and the sulfuric acid usage is downstream quench operation of 69.4 kg/hr (0.17 gpm) to neutralize excess ammonia. With no methanol (oxygenate) vapor injected, air was injected through a separate air sparger at a position in the bed equal to 90 per cent of the catalyst's expanded bed height, at dilute air to propylene molar ratios of 0.1, 0.2, 0.4 and 0.6. An average of four recover runs (Examples 6 to 9, Table 3 below) at a increasing DPA ratios gave an average 99.2 per cent C 3 = total conversion 80.7 per cent PPC to acrylo, 5.4 per cent PPC to HCN, 0.8 per cent PPC to HCN, 0.8 per cent PPC to acrolein, and 1.5 per cent PPC to acrylic acid from propylene. Notably, 15 per cent of feed ammonia was burned, and sulfuric acid usage dropped from 69 4 kg/hr (0.17 gpm) (0 per cent ammonia breakthrough (ABR)) to 49.0 kg/hr (0.12 gpm) (30 per cent ABR) to 0.10 (40 per cent ABR) as dilute phase air addition was increased. The relationship between dilute phase air to propylene ratio and sulfuric acid usage, all other reactor conditions being constant (e.g. bottom air/C 3 = ratio, NH 3 /C 3 = ratio, catalyst bed temperature, reactor top pressure and catalyst inventory) is set forth below in Table 3. Example 10 (Comparative) and 11 (Comparative); and Examples 12 to 14 The next set of experiments was directed at demonstrating applicability of dilute phase air (additional air at elevated point in reactor) over a wider range of ammonia ratio. The experiments illustrated in Examples 4 to 9 were repeated except at a lowered ammonia to propylene ratio of 1.17. At the same conditions of Examples 4 to 9, except feeds of air/ammonia/propylene of molar ratio 9.3/1.17/1.0, average oftwo baseline recovery runs (Examples 10 and 11, Table 4 below) indicated propylene conversions of 98 7 per cent, per pass conversions to acrylo 80.2 per cent, to HCN 5.0 per cent, to acrolein 1.2 per cent, to acrylic acid 1.8 per cent from propylene. Notably, 10 to 13 per cent of the feed ammonia was burning, with ammonia breakthrough of 2.1 g/Nm 3 (0.06 g/scf) and sulfuric acid usage in downstream quench operation of 28.6 kg/hr (0.07 gpm) to neutralize excess ammonia. With no methanol (oxygenate) vapor injected, air was separately injected through air sparger at a position in the bed equal to 90 per cent of the catalyst's expanded bed height, at dilute air to propylene molar ratio (DPAR) of 0.1, 0.2 and 0.3. An average of three recovery runs (Examples 12 to 14, Table 4, below) at increasing DPA ratios gave an average 99.2 per cent C 3 = total conversion, 80.1 per cent PPC to acrylo, 5.1 per cent PPC to HCN, 1.1 per cent PPC to acrolein, and 1.9 per cent PPC to acrylic acid from propylene. Notably, 15 percent of feed ammonia was burned, and sulfuric acid usage dropped to 24.5 kg/hr (0.06 gpm) (65 per cent ABR) to 20.4 kg/hr (0.05) (71 per cent ABR) to 16.3 kg/hr (0.04) (76 per cent ABR) as dilute phase air addition was increased, relative to baseline acid usage at 1.22 ammonia to propylene ratio. The relationship between dilute phase air to propylene ratio and sulfuric acid usage, all other reactor conditions being constant (e.g., bottom air/C 3 = ratio, NH 3 /C 3 = ratio, catalyst bed temperature, reactor top pressure and catalyst inventory) is set forth in Table 4 below: Examples 15 to 18 (Comparative) The next set of experiments was directed at lowering the bottom air/C 3 = molar ratio with increasing dilute phase air ratio, to see if the total air/C 3 = ratio could be lowered without penalizing AN or HCN yield while still reducing sulfuric acid consumption At the same conventions of examples 4 to 9, dilute phase air was injected at 90 per cent of the catalyst's expanded bed height, at dilute air to propylene molar ratios of 0.2, 0.4 and 0.6, while decreasing the bottom air C 3 = ratio from 9,1 to 8.9 to 8.7 to 8.6, respectively. In this way, the total air C 3 = was kept approximately constant at 9.3 to 9.4. An average of three recovery runs (Examples 15 to 17, Table 5 below) at increasing DPA ratios gave an average 98 per cent C 3 = total conversion, 80.4 per cent PPC to acrylo, 4.8 per cent PPC to HCN, 0.9 per cent PPC to acrolein, and 1.4 per cent PPC to acrylic acid from propylene. Notably, 15% of feed ammonia was burned, and sulfuric acid usage increased from 77.6 kg/hr to 85.8 kg/hr (0.19 to 0.21 gpm), relative to baseline acid usage at 1.22 ammonia to propylene ration. The relationship between dilute phase air to propylene ratio and sulfuric acid usage, all other reactor conditions being constant (e.g. bottom air/C 3 = ratio, NH 3 /C 3 = ratio, catalyst bed temperature, reactor top pressure and catalyst inventory) was as follows: These experiments showed that total air/C 3 = ratio could be kept constant, however, as the bottom air/C 3 = was reduced, total C 3 = conversion decreased which has a negative effect on HCN and AN yields. Further, sulfuric acid addition could not be reduced using dilute phase air addition in this manner. This showed that only a small reduction in the bottom air/C 3 = ratio is possible without adversely affecting reactor product distributions and yields.

Claims: A method of reducing the amount of unreacted ammonia exiting the reactor during the manufacture of acrylonitrile comprising: a) introducing into the lower portion of a fluid bed reactor ammonia, air and a hydrocarbon selected from the group consisting of propylene and propane to react in the presence of a fluid bed catalyst to produce acrylonitrile, said air to propane and/or air to propylene ratio being at least 9.3:1; b) introducing at a point in the reactor above at least 50 percent of the calculated expanded fluid catalytic bed height an additional amount of oxygen-containing gas above the amount of air added to the lower portion of the reactor; c) reacting said additional amount of oxygen-containing gas with at least some of the unreacted ammonia present in the reactor to reduce the amount of free ammonia present in the reactor effluent, the additional amount of oxygen-containing gas being substantially free of any oxygenate compounds; d) passing the reactor effluent containing acrylonitrile into a quench column to cool the reactor effluent with water to remove unwanted impurities; and e) recovering acrylonitrile from the quench column. A method according to claim 1 wherein the oxygen-containing gas is air. A method according to either claim 1 or claim 2 wherein the point of injection of the additional oxygen-containing gas is above at least 70 percent of the calculated expanded catalytic bed height. A method according to claim 3 wherein the point of injection of the additional oxygen-containing gas is above at least 80 percent of the expanded catalytic bed height. A method according to claim 4 wherein the point of injection of the additional oxygen-containing gas is above at least 90 percent of the expanded catalytic bed height. A method according to any one of the preceding claims wherein the amount of additional oxygen-containing gas injected into the reactor is sufficient to react with at least 15 percent of the unreacted ammonia. A method according to claim 6 wherein the amount of additional oxygen-containing gas injected into the reactor is sufficient to react with at least 25 percent of the unreacted ammonia. A method according to claim 7 wherein the amount of additional oxygen-containing gas injected into the reactor is sufficient to react with at least 40 percent of the unreacted ammonia. A method according to any one of the preceding claims wherein the hydrocarbon is propylene. A method according to any one of the preceding claims wherein the additional oxygen-containing gas is injected into the reactor at a downward angle.

Inventors: MCDONEL TIMOTHY R, REILING VINCENT G, RINKER JEFFREY E, SARNA JOSEPH C, MCDONEL, TIMOTHY R., REILING, VINCENT G., RINKER, JEFFREY E., SARNA, JOSEPH C.
Applicants: STANDARD OIL CO OHIO, THE STANDARD OIL COMPANY
Application year: 1995
Main classification: C07C253
IPC codes: B01J12, B01J8, C07C253, C01C3, C07C255