[0001] A process is provided for controlling quench column pH. More specifically, the process includes adding acid to a quench column to provide a pH of about 3.5 to about 7 in a condensate from one or more quench column aftercoolers.

BACKGROUND

[0002] Acrylonitrile is an important commodity chemical used mainly as monomer for the manufacture of a wide variety of polymeric materials such as polymers for acrylic fibers used in textiles, and in resins such as ABS and SAN resins. Worldwide, acrylonitrile is produced in amounts exceeding four million metric tons per year. The most commonly used process for manufacturing acrylonitrile or other olefinically unsaturated nitrile, such as methacrylonitrile, is to react a suitable hydrocarbon, such as propylene or propane for the manufacture of acrylonitrile, or isobutylene for the manufacture of methacrylonitrile, in an ammoxidation reactor in the presence of ammonia using air or other source of molecular oxygen as an oxidant. Such oxidation reactions, also called ammoxidation reactions, typically use a solid, particulate, heterogeneous catalyst in a fluidized catalyst bed to catalyze the ammoxidation reaction and provide the desired acrylonitrile or methacrylonitrile in acceptable conversion and yield. In addition to producing an olefinically unsaturated nitrile, such ammoxidation reactions also generally produce other organic compounds such as acetonitrile, hydrogen cyanide (HCN) and other co-products. Processes for the catalytic ammoxidation of a hydrocarbon feed to acrylonitrile are disclosed, for example, in U.S. Pat. Nos. 4,503,001; 4,767,878; 4,863,891 and 5,093,299, all of which are incorporated herein by reference.

[0003] The processes widely used in commercial practice for recovering the products of such hydrocarbon ammoxidation, such as the ammoxidation of propylene to form acrylonitrile, generally comprise the steps of: a) contacting the effluent from an ammoxidation reactor in a quench tower or column with an aqueous quench liquid to neutralize ammonia and cool the gaseous effluent; b) contacting the quenched gaseous effluent with water in an absorber, forming an aqueous solution comprising the ammoxidation products; c) subjecting the aqueous solution to a water extractive distillation in a distillation column, and d) removing a first overhead vapor stream comprising the unsaturated nitrile and some water from the top of the column, and collecting a liquid waste stream containing water and contaminants from the bottom of the column. Further purification of the olefinically unsaturated nitrile, such as acrylonitrile, may be accomplished by passing the overhead vapor stream to a second distillation column to remove at least some impurities from the acrylonitrile, and further distilling the partially purified acrylonitrile.

[0004] The effluent from the ammoxidation reactor generally contains a certain amount of ammonia. Therefore, the quench liquid used in the quench column may also contain a strong mineral acid, such as sulfuric acid, to react with and thereby form a water soluble salt of ammonia, such as ammonium sulfate. The used or spent quench fluid containing the ammonium sulfate and other components is typically treated or disposed of in an environmentally safe manner.

[0005] Control of pH in the quench column is important. Any ammonia that passes through the reactor unconverted must be neutralized. If not neutralized, the ammonia may react with acrylonitrile to form various polymers and cause fouling. The ammonia may also aid in polymerization of HCN. A lack of effective pH control in the quench column results in loss of product.

SUMMARY

[0006] A process controlling quench column pH provides a quench column stream having a steady and nearly constant pH. The continuous pH measurement of a condensate stream in the present process eliminates problems with direct pH

measurement of quench liquid. For example, quench column liquid itself includes characteristics that result in frequent failure of pH probes and sample line plugging. Surprisingly and unexpectedly, quench column condensate provide a cleaner stream which is directly related to quench column pH and tracks quench column pH with small offsets. [0007] A process for controlling quench column pH includes adding acid to the quench column to control pH in a quench column effluent from one or more quench columns. In one aspect, the process includes measuring a pH of quench column condensate and adjusting an amount of acid added to the quench column to maintain a pH of about 3.5 to about 7, in another aspect, about 3.5 to about 6, and in another aspect, about 5 to about 5.5 in the condensate. Measurement of condensate pH provides a consistent and accurate procedure for controlling acid addition to the quench column.

[0008] A process for reducing ammonia in a quench effluent from a quench column includes conveying the quench effluent to a quench column aftercooler; measuring a pH of condensate from the quench column aftercooler; and adding acid to the quench column. In this aspect, the process includes adding acid to the quench column to provide a pH of about 3.5 to about 7, in another aspect, about 3.5 to about 6, and in another aspect, about 5 to about 5.5 in the condensate from the quench column aftercooler.

[0009] A process for controlling quench column pH includes measuring a pH of a condensate from a quench column effluent, wherein the condensate has about 5 weight percent or less acrylonitrile, and/or about 1 weight percent or less HCN, and/or about 0.05 weight percent or less ammonia, and/or about 0.01 weight percent or less dissolved sulfate; and adding acid to the quench column. In this aspect, the process includes adding acid to the quench column to provide a pH of about 3.5 to about 7, in another aspect, about 3.5 to about 6, and in another aspect, about 5 to about 5.5 in the condensate from the quench column.

[0010] A process for controlling ammonia neutralization in a quench column includes adding acid to the quench column and measuring a pH of a condensate from a quench column effluent, wherein the condensate has about 5 weight percent or less acrylonitrile, and/or about 1 weight percent or less HCN, and/or about 0.05 weight percent or less ammonia, and/or about 0.01 weight percent or less dissolved sulfate, wherein the acid added to the quench column provides about 90 % or more ammonia neutralization. In this aspect, the process includes adding acid to the quench column to provide a pH of about 3.5 to about 7, in another aspect, about 3.5 to about 6, and in another aspect, about 5 to about 5.5 in the condensate from the quench column effluent.

[0011] A system for controlling quench column pH includes a quench column configured to supply a quench column effluent to a quench column aftercooler, the quench column aftercooler configured to provide a condensate; a pH sensor for monitoring pH of the condensate from the quench column aftercooler; and a controller electronically connected to the pH sensor and to an acid control valve, the acid control valve configured to control acid flow to the quench column; wherein the controller is configured to increase or decrease acid flow through the acid control valve.

BRIEF DESCRIPTION OF FIGURES

[0012] The above and other aspects, features and advantages of several aspects of the process will be more apparent from the following figure.

[0013] Figures 1 generally illustrates a quench column and aftercooler.

[0014] Corresponding reference characters indicate corresponding components throughout the drawing. Skilled artisans will appreciate that elements in the figure are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various aspects. Also, common but well-understood elements that are useful or necessary in a commercially feasible aspect are often not depicted in order to facilitate a less obstructed view of these various aspects.

DETAILED DESCRIPTION

[0015] The following description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of exemplary embodiments. The scope of the invention should be determined with reference to the claims. [0016] Reference to the process and apparatus for carrying out the process is described further with respect to the Figure as follows.

[0017] As shown in FIG. 1, the quench column 10 includes a first portion 28 and a second portion 30, the first portion 28 being located below the second portion 30. The first portion 28 of the quench column 10 includes an inlet 32 configured to receive a gas stream or reactor effluent 12. The gas stream or reactor effluent 12 may include acrylonitrile and ammonia. The second portion 30 of the quench column 10 includes a multi-level spray system 34 that is configured to receive an aqueous stream or quench liquid 16. The aqueous stream or quench liquid 16 may include an acid 36.

[0018] In one aspect, the process includes adding acid to a process stream prior to the process stream entering the quench column. In this aspect, the process includes adding acid 36 via line 38 to a quench liquid 16 at a juncture 40. Acid 36 may include any suitable acid, e.g., sulfuric acid (such as 98% sulfuric acid).

[0019] Quench liquid 16 may include effluent or quench column bottoms stream exiting bottom 42 of the quench column 10 and through line 44. In an aspect, effluent or quench column bottoms stream may include a concentration of ammonium sulfate of about 45 % by weight or less, in another aspect, about 10 to about 25% by weight, and in another aspect, about 15 to about 21% by weight.

[0020] Water may be added via line 46 to the quench column 10 through an inlet 48, or otherwise may be added to the quench liquid 16 or elsewhere in the liquid recycle loop formed by streams 16 and 44. Water may also be added to the quench column 10 via a line 68. In this aspect, the quench column may be any type of quench column known in the art, and the quench column may include packing or trays.

[0021] Quench liquid 16 may be circulated through a line 44 and back to lines 18, 20, 22, and 24, using a pump 50. In this aspect, the quench column may include multiple return lines, such as for example, in one aspect, 2 or more, in one aspect, 4 or more, in one aspect, 6 or more, and in one aspect, 8 or more. An exit stream 67 may be withdrawn as part of the quench column bottoms stream exiting through a line 44, in order to maintain a relatively constant mass flow in the liquid recycle loop by offsetting the liquid added via lines 38 and 46. An exit stream 67 removes formed neutralization reaction products (e.g. , ammonium sulfate) and is also useful for preventing the accumulation of unwanted products in the liquid recycle loop, such as corrosion products. The exit stream 67 may be drawn from line 44 at a discharge point 52.

[0022] A multi-level spray system 34 includes at least a first spray bar 54, corresponding to line 18, and a second spray bar 56 corresponding to line 20. As shown in FIG. 1, the multi-level spray system 34 may include a spray bar 58, corresponding to line 22, and a spray bar 60, corresponding to line 24. Spray bars 54, 56, 58, and 60 extend substantially across a diameter 62 of the quench column 10. As shown, spray bar 54 is located below spray bar 56, and substantially parallel to spray bar 56. Spray bar 58 is located above spray bar 56, and below spray bar 60. Spray bar 58 is substantially parallel to spray bar 60. In this aspect, the multi-level spray system 34 may include multiple spray bars and spray nozzles.

[0023] Spray bars 54, 56, 58, and 60 may each include a series of spray arms (not shown in FIG. 1). Spray arms may extend substantially across diameter or chords of the quench column 10 that are perpendicular to diameter 62 of quench column 10. Each spray arm may include two or more extenders (not shown in FIG. 1). Each extender may extend substantially perpendicular to its respective spray arm. Each extender may include spray nozzles, wherein each spray nozzle faces downward. In an aspect, each nozzle 47 of the spray system 34 may be configured to downwardly spray a hollow cone spray of the quench liquid 16, wherein each hollow cone spray defines a center equidistant from the walls of the hollow cone spray. In an aspect, the nozzles of each spray bar may be spaced so that a portion of a first hollow cone spray of quench liquid from a first nozzle of the first spray bar overlaps with a portion of a second hollow cone spray of quench liquid from a second nozzle of the first spray bar to provide an overlap of the quench liquid.

[0024] In another aspect, the quench column may include packed sections of multiple trays in place of multi-level spray system 34. In this aspect, quench liquid 16 is circulated to the quench column above and/or below the packed or tray section of the column.

[0025] Cooled effluent gas containing acrylonitrile (including co-products such as acetonitrile, hydrogen cyanide and impurities) along with mist may then rise up from the multi-level spray system 34 to the mist eliminator 26. The mist eliminator 26 is configured to remove mist from the cooled effluent gas. The mist eliminator 26 is located downstream of the second portion 30 of the quench column 10. The mist eliminator 26 may include a water spray system (not shown). The water spray system is configured to spray water to a surface of the mist eliminator 26, wherein collection of droplets is reduced and formation of polymer and corresponding fouling on surfaces of the mist eliminator 26 is reduced.

[0026] The quenched or cooled effluent gas comprising acrylonitrile (including co- products such as acetonitrile, hydrogen cyanide and impurities), after passing through mist eliminator 26, may exit quench column 10 as a gas stream 70. In one aspect, the quench column effluent is the gas stream 70.

[0027] A gas stream 70 may be sent to one or more entrainment separators 82 and one or more quench column aftercoolers 80. The process may include the use of quench column aftercoolers, for example, such as shell and tube, finned tube, box type, plate type, spiral type, and double pipe type. Condensate 85 may be removed from the quench column after cooler 80 at outlet 90. The process further includes conveying condensate 85 via pump 95 back to the quench column aftercooler 80. A portion of the condensate 85 may be sent to downstream equipment such as an absorber or recovery column (not shown). The pH of the condensate 85 is measured prior to entering downstream equipment.

Process stream 110 is a vapor effluent from the aftercooler 80 which may be sent to an absorber.

[0028] In another aspect, the system and process may include a pH control loop. A pH meter 115 continuously monitors the pH of the condensate 85 and controls addition of acid with an acid addition valve 118. The acid addition valve 118 to provides acid to the quench column 10 in an amount to maintain the pH of the condensate 85. In this aspect, the process includes adding acid to the quench column to provide a condensate pH of about 3.5 to about 7, in another aspect, about 3.5 to about 6, in another aspect, about 3.5 to about 5, in another aspect, about 3.5 to about 4.5, in another aspect, about 3.5 to about 4, in another aspect, about 5 to about 5.3, and in another aspect, about 5 to about 5.5. Maintaining the condensate in this pH range has been found to be an effective way and consistent method for controlling quench column pH in an acceptable level.

[0029] A process that includes measuring condensate pH from one or more quench column aftercooler is applicable to any quench column configuration. Examples of quench column configurations include a one stage quench, a two stage quench, and a multiple stage quench. Selection of pH range will depend on the type of quench. For example, a one stage quench may include a pH control of about 5 to about 5.5, and a two stage quench may include a pH control of about 3.5 to about 7.

[0030] In one aspect, the process includes measuring condensate pH from one or more quench column aftercoolers. Measurement of condensate pH may be more stable and consistent as condensate may include any one or more of the following:

about 5 weight percent or less acrylonitrile, in another aspect, about 2.5 weight percent or less acrylonitrile;

about 1 weight percent or less HCN, in another aspect, about 0.5 weight percent or less HCN;

about 0.05 weight percent or less ammonia, in another aspect, about 0.025 weight percent or less ammonia; and/or

about 0.01 weight percent or less dissolved sulfate, in another aspect, about 0.005 weight percent or less dissolved sulfate.

[0031] In another aspect, the process provides about 90 percent or more ammonia neutralization, in another aspect, about 95 percent or more ammonia neutralization, and in another aspect, about 99 percent or more ammonia neutralization.

[0032] In another aspect, the process includes a one stage column. The quench column may have a temperature of about 60 to about 90 °C, and in another aspect, about 65 to about 85 °C. Operating pressure of the quench column is about 0.025 to about 0.045 MPa(G), and in another aspect, about 0.03 to about 0.04 MPa(G).

[0033] While the invention herein disclosed has been described by means of specific embodiments, examples and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.