FIELD OF THE INVENTION

[1] The invention relates to an improved process for the manufacture of acrylonitrile and methacrylonitrile. In particular, the invention is directed to improved processes that include product column level control.

BACKGROUND

[2] Various processes and systems for the manufacture of acrylonitrile and methacrylonitrile are known; see for example, U.S. Patent No. 6,107,509. As noted in U.S. Patent No. 6,107,509, conventional processes typically involve recovery and purification of acrylonitrile/methacrylonitrile produced by the direct reaction of a hydrocarbon selected from the group consisting of propane, propylene or isobutylene, ammonia and oxygen in the presence of a catalyst has been accomplished by transporting the reactor effluent containing acrylonitrile/methacrylonitrile to a first column (quench) where the reactor effluent is cooled with a first aqueous stream, transporting the cooled effluent containing acrylonitrile/methacrylonitrile into a second column (absorber) where the cooled effluent is contacted with a second aqueous stream to absorb the acrylonitrile/methacrylonitrile into the second aqueous stream, transporting the second aqueous stream containing the acrylonitrile/methacrylonitrile from the second column to a first distillation column (recovery column) for separation of the crude acrylonitrile/methacrylonitrile from the second aqueous stream, and transporting the separated crude acrylonitrile/methacrylonitrile to a second distillation column (heads column) to remove at least some impurities from the crude acrylonitrile/ methacrylonitrile, and transporting the partially purified acrylonitrile/methacrylonitrile to a third distillation column (product column) to obtain product acrylonitrile/methacrylonitrile. U.S. Pat. Nos. 4,234,510; 3,885,928; 3,352,764; 3,198,750 and 3,044,966 are illustrative of typical recovery and purification processes for acrylonitrile and methacrylonitrile.

[3] An acrylonitrile product column typically operates with a bottoms product flow rate that is relatively much lower than flow rates of other streams. The bottoms product flow rate is typically recycled in the acrylonitrile plant to a point where high- boiling impurities can be purged from the plant. Conventional methods present the technical challenge of keeping the bottom product flow rate relatively constant. Failing to keep the bottom product flow rate relatively constant may result in the acrylonitrile product going outside of a desired characteristic or predetermined specification with upset to other aspects of the acrylonitrile recovery and purification system. In addition to the desirability of maintaining a relatively constant flow rate, the flow rate of this bottoms product flow stream is too small to be used for effective control of liquid level in the product column sump.

[4] The technology described herein allows the product column bottoms flow to remain relatively constant, while still providing effective control of the product column sump level.

SUMMARY

[5] Accordingly, an aspect of the disclosure is to provide a safe, effective and cost effective method and apparatus that allows the product column bottoms flow to remain relatively constant, while still providing effective control of the product column sump level.

[6] In an aspect, a method is provided comprising the steps of feeding a feed stream to a product column, the product column comprising upper, middle, and bottom portions. The middle portion comprises a series of distillation trays. The method comprises distilling the feed stream in the products column to obtain a liquid/vapor product that is more pure than the feed stream. The method comprises removing the liquid/vapor product from the product column through a side stream line, the side stream line comprising a control valve. In an aspect, the method comprises controlling a bottom level of liquid in the product column by detecting the bottom level of liquid or vapor in the product column and using a bottom level controller to either control the control valve of the side stream line or reset a side stream flow controller, the side stream flow controller configured to control the control valve of the side stream line. BRIEF DESCRIPTION

[7] A more complete understanding of the exemplary embodiments of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features and wherein:

[8] FIG. 1 is a schematic flow diagram of an embodiment in accordance with at least one aspect of the disclosure;

[9] FIG. 2 is a schematic flow diagram of an embodiment in accordance with at least one aspect of the disclosure

[10] FIG. 3 is a schematic flow diagram of an embodiment in accordance with at least one aspect of the disclosure.

[11] FIG. 4 illustrates a flow diagram of a method in accordance with aspects of the disclosure.

[12] FIG. 5 illustrates a flow diagram of a method in accordance with aspects of the disclosure.

[13] FIG. 6 illustrates a flow diagram of a method in accordance with aspects of the disclosure.

DETAILED DESCRIPTION

[14] In an aspect, a method is provided comprising the steps of feeding a feed stream to a product column, the product column comprising upper, middle, and bottom portions. The middle portion comprises a series of distillation trays. The method comprises distilling the feed stream in the products column to obtain a liquid/vapor product that is more pure than the feed stream. The method comprises removing the liquid or vapor product from the product column through a side stream line, the side stream line comprising a control valve. The method comprises controlling a bottom level of liquid in the product column by detecting the bottom level of liquid in the product column and using a bottom level controller to either control the control valve of the side stream line or reset a side stream flow controller, the side stream flow controller configured to control the control valve of the side stream line.

[15] In an aspect, an apparatus is provided comprising a product column, the product column comprising an upper portion, a middle portion, and a bottom portion. The apparatus may comprise a series of distillation trays located in the middle portion of the product column, each distillation tray located at a height in the product column then is different than the other of the distillation trays. The apparatus comprises a bottoms level controller configured to detect a bottoms liquid level in the bottom portion of the product column. The apparatus comprises a product column side stream line off the middle portion of the products column and in fluid communication with a product outlet of the product column, the product column side stream line comprising a control valve. The bottoms level controller is configured to either control the control valve of the product column side stream line or reset a side stream flow controller, the side stream flow controller configured to control the control valve of the product column side stream line, wherein the bottom level of the product column is controlled by adjusting the control valve on the product column side stream line.

[16] Aspects of the disclosure are further discussed below with reference to the figures.

[17] Apparatus 100 is shown in FIG. 1. As shown in Figure 1 , feed stream 101 is fed through feed stream line 1 to product column 10. Feed stream 101 may comprise partially purified acrylonitrile/methacrylonitrile liquid. The partially purified acrylonitrile/methacrylonitrile liquid in feed stream 101 may come from any suitable source, such as a distillation or heads column (not shown), e.g., a distillation or heads column known in the art of production of acrylonitrile/methacrylonitrile. Feed stream line 1 is in fluid communication with inlet 29 of product column 10. In one aspect, the feed stream 101 is about 95 weight percent or more acrylonitrile and 5 weight percent or less water, in another aspect, 99 weight percent or more acrylonitrile and about 1 weight percent or less water, in another aspect, about 99.5 weight percent or more acrylonitrile and about 0.5 weight percent or less water, and in another aspect, about 99.9 weight percent or more acrylonitrile and about 0.1 weight percent or less water.

[18] Product column 10 comprises lower or bottom portion 28, a middle portion 31, and upper portion 30. Middle portion 31 comprises a plurality of trays 11, and is located between bottom portion 28 and upper portion 30. The plurality of trays may be any suitable number of trays to obtain a desired product side stream 103 comprising a predetermined amount of acrylonitrile by weight in side stream 103. For example, but not by way of limitation, the number of trays 11 in middle portion 31 may be between about forty-five (45) to fifty-five (55) trays, and in a preferred embodiment, about forty-seven (47) to fifty-three (53) trays, and in a more preferred embodiment, about forty-nine (49) to fifty-one (51) trays. Side stream 103 exits product column 10 through product outlet 33, which is in fluid communication with side stream line 3.

[19] Side stream 103 may be sent to a storage container (not shown). If the side stream 103 is vapor, it is first condensed in a side stream condenser to form liquid before sending to storage. The flow rate of side stream 103 may be any suitable flow rate. The flow rate of side stream 103 may equal the flow rate of feed stream 101 minus the flow rate of product column bottom stream 102 (i.e., the stream flowing through bottom outlet 35 and through bottom stream line 102 in fluid communication with outlet 35), and stream 105 moving through stream line 5. In an embodiment, the flow rate of side stream 103 may be about 80-97% of the flow rate of feed stream 101, in another aspect, about 85 to about 97%, in another aspect, about 90 to about 97%, and in another aspect, about 93 to about 97%. Product column 10 comprises a top outlet 37 located at the top of upper portion 30. Stream 106 exits through top outlet 37 and flows through stream line 6, which is in fluid communication with top outlet 37. Stream 106 may be condensed and cooled by cold water in heat exchanger 7. After being cooled in heat exchanger 7, stream 106 may be sent to vessel 8. From vessel 8, stream 106 may be transferred by pump 9. Stream 105 may be the portion of stream 106 that exits from juncture 39 of stream line 4 and that is not recirculated through stream line 4 as stream 104 to product column 10. Flow controller 30 may be configured to actuate and control valve 32. Valve 32 may be configured to adjust the flow rate of stream 105 flowing in stream line 5 by changing the size of the opening fluid flow through valve 32. Vessel 8 may comprise level controller 34. Level controller 34 may be configured to actuate and control valve 36. Valve 36 may be configured to adjust the flow of stream 104 flowing in stream line 4 by changing the size of the opening fluid flow through valve 36. In one aspect, the product composition is about 98 weight percent or more acrylonitrile and 2 weight percent or less water, in another aspect, 99 weight percent or more acrylonitrile and about 1 weight percent or less water, in another aspect, about 99.5 weight percent or more acrylonitrile and about 0.5 weight percent or less water, and in another aspect, about 99.9 weight percent or more acrylonitrile and about 0.1 weight percent or less water.

[20] The flow rate product column bottom stream 102 flowing through stream line 2 may be controlled, e.g., kept constant or relatively constant, by a flow controller 20. Flow controller 20 may be configured to actuate and control valve 21. Valve 21 may be configured to control the flow of stream 102 flowing in stream 2 by controlling the size of the opening of fluid flow through valve 21. Pump 27 may be used to pump bottom stream 102 from product column 10. In an embodiment, flow controller 20 may be configured to actuate and control pump 27 through a communication pathway (not shown in FIG. 1), and pump 27 may be configured to adjust the flow rate of liquid being transferred by pump 27.

[21] Product column 10 may comprise bottoms level controller 22. Bottom level controller 22 may be located below tray 25 of the plurality of trays 11. Tray 25 may be the first (1 ^st ) tray above bottom portion 28 of product column 10 (also called the product column sump level). Bottom level controller 22 may be configured to actuate and control a control valve 24. Control valve 24 may be configured to control the flow of stream 103 flowing through side stream line 3 by controlling the size of the opening of fluid flow through valve 24, and thus, effect level control of liquid 26 at bottom portion 28 of product column 10. [22] Thus, when bottom level controller 22 detects that the level of liquid 26 at bottom portion 28 is higher than a predetermined level or range of level, bottom level controller 22 actuates control valve 24 to open more from its initial opening by an appropriate amount and allow more liquid to flow out of product column 10 as side stream 103 through outlet 33 and side stream line 3, rather than flow down the trays to the sump at bottom portion 28. When bottom level controller 22 detects that the level of liquid 26 at bottom portion 28 is lower than a predetermined level or range of level, bottom level controller 22 actuates control valve to close more from its initial opening by an appropriate amount and allow less liquid to flow out of product column 10 as side stream 103 through outlet 33 and side stream line 3, thereby allowing more liquid to flow down the trays to the sump at bottom portion 28. In an embodiment, the predetermined level or range of level of liquid 26 at bottom portion 28 is about six (6) to eight (8) feet high.

[23] Feed stream 101 may enter product column 10 at predetermined location. In an aspect, feed stream 101 may enter product column 10 through inlet 29 and between tray 13 and tray 15 of the plurality of trays 11. Tray 13 may be the eighth (8 ^th ) tray to the fifteenth (15 ^th ) tray above the bottom portion 28 of product column 10.

[24] Side stream 103 may exit from product column through outlet 33 and between tray 17 and tray 19 of the plurality of trays 11. Tray 17 may be the thirty-third (33 ^rd ) tray to the thirty-seventh (37 ^th ) tray above bottom portion 28 of product column 10.

[25] Stream 104 may enter product column 10 through inlet 38 and above tray 23 of the plurality of trays 11. Tray 23 may be the highest tray of the plurality of trays 11 above bottom portion 28 of product column 10.

[26] The flow rate of feed stream 101 may any suitable flow rate dependent upon plant capacity. As previously noted, the flow rate of side stream 103 may equal the flow rate of feed stream 101 minus the flow rate of product column bottom stream 102, and stream 105. Stream 105 may be the portion of products column top stream 106 that is not recirculated as stream 104 to products column 10. In an embodiment, the flow rate of side stream 103 may be about 80-97% of the flow rate of feed stream 101. The flow rate of product column bottom stream 102 may be about 1% to about 10% of the flow rate of feed stream 101, in another aspect, about 1 to about 5%, in another aspect, about 1 to about 3%, and in another aspect, about 1.66 to about 2%. The flow rate of stream 105 may be about 2% to about 5% of the flow rate of feed stream 101. The flow rate of stream 104 may be about 133% to about 166% of the flow rate of feed stream 101. In another aspect, a ratio of reflux (stream 104) to side stream (stream 103) is about 1.5 to about 3, in another aspect, about 1.75 to about 2.75, in another aspect, about 1.5 to about 2.25, and in another aspect, about 2.

[27] FIG. 2 illustrates another embodiment in accordance with the disclosure.

Apparatus 200 is shown in FIG. 2. Apparatus 200 is the same as apparatus 100 shown in FIG. 1 , except that apparatus 200 further comprises flow controller 202 on the product column side stream 103. In this embodiment, a master or primary loop comprises level controller 22 measuring or detecting the level of liquid 26 at bottom portion 28, comparing that detected level to a predetermined set level, and changing the set flow rate of side stream flow controller 202. In this embodiment, a slave or secondary loop comprises the side stream flow controller 202 measuring or detecting the flow rate of side stream 103 flowing through stream line 3, comparing it to the set flow rate of side stream flow controller 202 (which is controlled by the master or primary loop discussed above) and actuate flow control valve 24, thereby adjusting the opening of flow control valve 24 to accordingly match the set flow rate of side stream flow controller 202. In other words, product column bottoms level controller 22 is configured to reset side stream flow controller 202. Side stream flow controller 202 is configured to actuate flow control valve 24 on side stream line 3.

[28] An advantage of this cascade control is to help maintain the level of liquid 26 at bottom portion 28 of product column 10 at a predetermined value or range. The position of opening of flow control valve 24 does not directly affect the level of liquid 26 at bottom portion 28. Rather, the flow rate of side stream 103 in stream line 3 is a direct variable that is required to maintain the level of liquid 26 at bottom portion 28. The slave or secondary loop is used because the flow rate of side stream 103 may be prone to higher frequency variation. The flow rate of side stream 103 may fluctuate, and if this happens, the flow measured by flow controller 202 may change faster than the level measured by level controller 22.

[29] In accordance with this embodiment, flow controller 202 is configured to control fluctuations in flow of side stream 103 to the set rate of flow controller 202, and level controller 22 is configured to control the fluctuations in level of liquid 26 at bottom portion 28 by increasing or decreasing the set rate of flow controller 202. The cascade type control in this embodiment uses two inputs to control valve 24, and allows apparatus 200 to adjust to both variable fluid flow out of product column 10 through side stream line 3 and/or set fluid flow rate of side stream 103 through stream line 3 to achieve the predetermined level of liquid 26 in bottom portion 28 of product column 10. Those skilled the art will recognize that in accordance with this disclosure, this cascade type control helps to reduce fluctuations in product column side stream flow 103, and resulting fluctuations in the level of liquid 26 in bottom portion 28, thereby maintaining the level of liquid 26 in bottom portion 28 at a desired, predetermined level or within a desired, predetermined range of levels.

[30] FIG. 3 illustrates another embodiment in accordance with the disclosure.

Apparatus 300 is shown in FIG. 3. Apparatus 300 is the same as apparatus 200 shown in FIG. 2, except that apparatus 300 further comprises a flow indicator 304 in communication with flow controller 302. Flow controller 302 is similar to flow controller 202 previously discussed with respect to FIG. 2. As shown in FIG. 3, flow indicator 304 is located on feed stream line 1. Flow indicator 304 is configured to detect the flow rate of feed stream 101 through feed stream line 1, and into product column 10. In this embodiment, flow indicator 304 is configured to supply to flow controller 302 with information relating to the flow rate of feed stream 101 through feed stream line 1 and into product column 10. Flow controller 302 is configured to process the information from flow indicator 304 and to adjust the set flow rate of product column side stream 103 in anticipation of changes in the product column bottoms level of liquid 26, i.e., the level of liquid 26 in bottom portion 28 of product column 10, that would likely occur without adjusting the set flow rate of product column side stream 103 through stream line 3. This feed-forward control system helps to reduce fluctuations in both the level of liquid 26 in bottom portion 28 of product column 10 and the flow rate of product column side stream 103 that would otherwise arise due to fluctuations in the flow rate of feed stream 101.

[31] Those skilled in the art will recognize that in accordance with the disclosure, a disturbance or fluctuation in feed stream 101 is measured or detected by flow indicator 304 and information relating to the fluctuation is transmitted to flow controller 302. Flow controller 302 is configured to receive the information from flow indicator 304 and account for the measured or detected fluctuation in the flow rate of feed stream 101.

[32] In an aspect, controller 68 may be configured to process one or more signals corresponding to a measured parameter, e.g., the level measured by level controller 22. Controller 68 may be configured to determine whether the measured parameter is above or below a predetermined parameter range, e.g., the level measured by level controller 22 is below or above a predetermined level range. Controller 68 may be configured to adjust operation of one or more devices via communication lines or wireless communications (not shown in FIG. 1, FIG. 2, and FIG. 3) if the measured parameter is below or above a predetermined parameter range. For example, controller 68 may be configured to adjust the amount of side stream 103 flowing through valve 24 when the level measured by level controller 22 is below or above a predetermined level range. Controller 68 may be configured to control operation of valve 24, such as controlling the size of the opening of valve 24. Those skilled in the art will recognize that controller 68 or a similar controller may be located remote from level controller 22 (as shown in FIG. 1, FIG. 2, and FIG. 3), or may be located at and comprise level controller 22. Controller 68 may be configured to control operation of product column bottoms level controller 22 and resetting of side stream flow controller 202 and 302 as described above. Controller 68 may be configured to control side stream flow controller 202 and 302 in actuating flow control valve 24 on side stream line 3. Controller 68 may be configured to control operation of flow controller 302.

[33] FIG. 4 illustrates a flow diagram of a method 400 in accordance with aspects of the disclosure. Method 400 may be carried out using apparatus previously described, e.g., the apparatus shown in FIG. 1. Step 401 comprises feeding a feed stream to a product column, the product column comprising an upper portion, a middle portion, and a bottom portion, wherein the middle portion comprises a series of distillation trays. Step 402 comprises distilling the feed stream in the products column to obtain a liquid/vapor product that is more pure than the feed stream. Step 403 comprises removing the liquid/vapor product from the product column through a side stream line, the side stream line comprising a control valve. Step 404 comprises detecting the bottoms liquid level in the bottom portion of the product column. Step 405 comprises controlling the bottoms liquid level in the bottom portion of the product column by adjusting the control valve.

[34] FIG. 5 illustrates a flow diagram of a method 500 in accordance with aspects of the disclosure. Method 500 may be carried out using apparatus previously described, e.g., the apparatus shown in FIG. 2. Step 501 comprises feeding a feed stream to a product column, the product column comprising an upper portion, a middle portion, and a bottom portion, wherein the middle portion comprises a series of distillation trays. Step 502 comprises distilling the feed stream in the products column to obtain a liquid/vapor product that is more pure than the feed stream. Step 503 comprises removing the liquid/vapor product from the product column through a side stream line, the side stream line comprising a control valve, wherein the control valve is controlled by a side stream controller. Step 504 comprises detecting the bottoms liquid level in the bottom portion of the product column. Step 505 comprises comparing the detected bottoms liquid level to a predetermined set level. Step 506 comprises resetting a set flow rate of the side stream controller to a new set flow rate when the detected bottoms liquid level is outside a predetermined range. Step 507 comprises adjusting the control valve in accordance with the new set flow rate. [35] FIG. 6 illustrates a flow diagram of a method 600 in accordance with aspects of the disclosure. Method 600 may be carried out using apparatus previously described, e.g., the apparatus shown in FIG. 3. Step 601 comprises feeding a feed stream to a product column, the product column comprising an upper portion, a middle portion, and a bottom portion, wherein the middle portion comprises a series of distillation trays. Step 602 comprises distilling the feed stream in the products column to obtain a liquid/vapor product that is more pure than the feed stream. Step 603 comprises removing the liquid/vapor product from the product column through a side stream line, the side stream line comprising a control valve, wherein the control valve is controlled by a side stream controller. Step 604 comprises detecting the flow rate of the feed stream to the product column. Step 605 comprises comparing the detected flow rate of the feed stream to a predetermined feed rate. Step 606 comprises resetting a set flow rate of the side stream controller to a new set flow rate when the detected flow rate of the feed stream to the product column is outside a predetermined feed rate range. Step 607 comprises adjusting the control valve in accordance with the new set flow rate of the side stream controller.

[36] While in the foregoing specification this disclosure has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the disclosure is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the disclosure. It should be understood that the features of the disclosure are susceptible to modification, alteration, changes or substitution without departing from the spirit and scope of the disclosure or from the scope of the claims. For example, the dimensions, number, size and shape of the various components may be altered to fit specific applications. Accordingly, the specific embodiments illustrated and described herein are for illustrative purposes only.