WO2015200022A1

WO2015200022A1 - Control of ammonia and/or air feed into an ammoxidation reactor

Info

Publication number: WO2015200022A1
Application number: PCT/US2015/035791
Authority: WO
Inventors: Timothy Robert Mcdonel; Jay Robert COUCH; David Rudolph Wagner; Paul Trigg Wachtendorf; Thomas George TRAVERS
Original assignee: Ineos Europe Ag
Priority date: 2014-06-27
Filing date: 2015-06-15
Publication date: 2015-12-30
Also published as: TW201605772A; CN104028172B; EP3160634A1; KR20170023847A; CN107252663A; EA201692340A1; JP2017520579A; CN104028172A

Abstract

A process and system are provided for controlling an amount of ammonia and/or air provided to an ammoxidation reactor. The process includes maintaining a pH of a quench water bottoms and adjusting an amount of ammonia in a reactor feed to provide an ammonia to hydrocarbon ratio of about 1 to about 2 in the reactor feed. Further, the process may include adjusting an amount of air the reactor feed to provide an air to hydrocarbon ratio of about 9 to about 10 in the reactor feed.

Description

CONTROL OF AMMONIA AND/OR AIR FEED INTO AN AMMOXIDATION

REACTOR

[0001] A process is provided for controlling an amount of ammonia and/or air provided to an ammoxidation reactor. More specifically, the process includes maintaining a pH of a quench water bottoms and adjusting an amount of ammonia in a reactor feed to provide an ammonia to hydrocarbon ratio of about 1 to about 2 in the reactor feed. Further, the process may include adjusting an amount of air the reactor feed to provide an air to hydrocarbon ratio of about 9 to about 10 in the reactor feed.

BACKGROUND

[0002] In the commercial manufacture of acrylonitrile, propylene, ammonia and oxygen are reacted together according to the following reaction scheme:

CH₂=CH-CH₃ + NH₃ + 3/2 0₂→ CH₂=CH-CN + 3 H₂0

[0003] This process, which is commonly referred to as ammoxidation, is carried out in the gas phase at elevated temperature (e.g. , 350° to 480° C) in the presence of a suitable fluid bed ammoxidation catalyst.

[0004] Fig. 1 illustrates a typical acrylonitrile reactor used to carry out this process. As shown there, reactor 10 comprises reactor shell 12, air grid 14, feed sparger 16, cooling coils 18 and cyclones 20. During normal operation, process air is charged into reactor 10 through air inlet 22, while a mixture of propylene and ammonia from propylene inlet 34 and ammonia inlet 36 is charged into reactor 10 through feed sparger 16. The flow rates of these incoming gases are high enough to fluidize a bed 24 of ammoxidation catalyst in the reactor interior, where the catalytic ammoxidation of the propylene and ammonia to acrylonitrile occurs.

[0005] Reaction gases exit reactor 10 through reactor effluent outlet 26. Before doing so, they pass through cyclones 20, which remove any ammoxidation catalyst these gases may have entrained for return to catalyst bed 24 through diplegs 25. Ammoxidation is highly exothermic, and cooling coils 18 are used to withdraw excess heat and thereby keep the reaction temperature at an appropriate level. [0006] As further shown in Fig. 1, the first step in recovering acrylonitrile and other byproducts from the hot reaction gases passing out of a typical acrylonitrile reactor 10 is to cool them down by spraying them with quench water in quench column 30. These reaction gases contain unreacted ammonia, which are removed before these gases are further processed. For this purpose, sulfuric acid is added to the quench water, which reacts with this unreacted ammonia to produce ammonium sulfate in accordance with the following reaction:

H₂S0₄ + 2 NH₃→ (NH₄)₂S0₄

[0007] This ammonium sulfate dissolves in the quench water columns bottoms, which is discharged to waste through quench bottoms outlet line 31. The hot reaction gases, now being substantially cooler and essentially free of unreacted ammonia, exit from an upper portion of the quench column through quench gas outlet line 33 for further processing. Since the amount of unreacted ammonia in these gross reaction gases in reactor effluent outlet 26 can vary over time, the pH of the quench column bottoms water is monitored by pH monitor 37 and the amount of sulfuric acid added to the quench column adjusted by means of sulfuric acid control valve 40 and controller 42 to keep this pH at a desired level. Makeup water may be added to the quench as necessary through line 45.

[0008] In order for acrylonitrile reactor 10 to operate at maximum efficiency, the amount of ammonia being fed to the reactor at any particular time should be a slight molar excess of the amount needed to completely convert all of the propylene being fed to the reactor at that same time into acrylonitrile. Since the flowrate of incoming propylene can vary over time for a number of reasons, it is normal practice to continuously monitor this flowrate F and to continuously adjust the flowrate of incoming ammonia by means of ammonia control valve 32 and controller 38 in response to this measured propylene flowrate.

[0009] To further insure that a slight molar excess of ammonia is being fed to the acrylonitrile reactor, it is also desirable that a small but suitable amount of unreacted ammonia be present in the gross reaction product gases in reactor effluent outlet 26. For this purpose, the concentration of ammonia in these gases is periodically measured and the target or setpoint ammonia/propylene ratio, i.e., the NH₃/C3^~ratio, in controller 38 adjusted in response to this measured unreacted ammonia concentration. So, for example, if the measured concentration of unreacted ammonia is too low, then the NH₃/C₃ ^~ ratio setpoint programmed into controller 38 is increased slightly so that a slightly greater amount of ammonia is fed to the reactor relative to the propylene being fed on a continuous basis.

[0010] Periodic determination of the concentration of unreacted ammonia in reactor effluent outlet 26 is normally done on a routine basis, for example, several times per week. Accordingly, precise adjustment of the target NH₃/C₃ ^~ ratio in controller 38 in response to the concentration of unreacted ammonia in reactor effluent outlet 26 is inherently limited due to the inability to obtain data on this concentration on a more frequent basis.

[0011] This lack of information is not too troublesome when equilibrated catalysts are used, i.e., ammoxidation catalysts which have been used for some time and hence have achieved an equilibrium concentration of both oxygen and molybdenum. Even so, when changes are made to reactor operating conditions, such as a change in C₃ ^~ flow rate, information about unreacted ammonia in reactor effluent is not known until reactor effluent analysis is analyzed. Further, when fresh catalysts are used, this lack of precision can lead to significant problems, since fresh catalysts are known to exhibit significant ammonia burning, i.e., oxidation of ammonia directly into nitrogen oxides and water, which is both excessive and rapidly variable over time.

SUMMARY

[0012] A process for controlling an amount of ammonia provided to an ammoxidation reaction includes, providing a reactor feed to a reactor, the reactor feed including ammonia, oxygen, and a hydrocarbon selected from the group consisting of propane, propylene, isobutane and isobutylene, and combinations thereof; reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; providing the reactor effluent stream to a quench vessel; providing a quench liquid to the quench vessel;

contacting the gaseous stream with the quench liquid; monitoring a pH of quench water bottoms; and adjusting an amount of ammonia in the reactor feed to provide an ammonia to hydrocarbon ratio of about 1 to about 2 in the reactor feed.

[0013] A process for controlling an amount of air provided to an ammoxidation reaction includes providing a reactor feed to a reactor, the reactor feed including ammonia, oxygen, and a hydrocarbon selected from the group consisting of propane, propylene, isobutane and isobutylene, and combinations thereof; reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; monitoring an amount of oxygen in the reactor effluent; and adjusting an amount of air in the reactor feed to provide an air to hydrocarbon ratio of about 9 to about 10 in the reactor feed.

[0014] An ammoxidation process includes providing a reactor feed to a reactor, the reactor feed including ammonia, oxygen, and a hydrocarbon selected from the group consisting of propane, propylene, isobutane and isobutylene, and combinations thereof; reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; providing a quench liquid to the quench vessel; contacting the gaseous stream with the quench liquid; monitoring a pH of quench water bottoms, monitoring an amount of oxygen in the reactor effluent stream; adjusting an amount of ammonia in the reactor feed to provide an ammonia to hydrocarbon ratio of about 1 to about 2 in the reactor feed; and adjusting an amount of air in the reactor feed to provide an air to hydrocarbon ratio of about 9 to about 10 in the reactor feed.

[0015] A system for ammonia control in an ammoxidation reactor includes an ammoxidation reactor configured to supply a reactor effluent to a quench column; a pH sensor for monitoring pH of a quench water bottoms from the quench column; and a controller electronically connected to the pH sensor and to an ammonia control valve. The ammonia control valve configured to control ammonia flow to the ammoxidation reactor and the controller is configured to increase or decrease ammonia flow through the ammonia control valve. BRIEF DESCRIPTION OF FIGURES

[0016] The above and other aspects, features and advantages of several aspects of the process will be more apparent from the following figures.

[0017] Figure 1 is a schematic view illustrating fine control of the amount of ammonia being fed to a commercial acrylonitrile reactor; and

[0018] Figure 2 is a schematic view illustrating another aspect for fine control of the amount of ammonia being fed to a commercial acrylonitrile reactor

[0019] Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures 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 figures 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

[0020] 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.

Ammonia Control

[0021] Fine control of the amount of ammonia being fed to a commercial acrylonitrile reactor is accomplished in accordance with this invention by adjusting the NH₃/C3^~ ratio setpoint in controller 38 for controlling the operation of ammonia control valve 32 in response to the measured pH of the quench water bottoms in quench column 30. As shown in Figure 2, pH sensor 37 continuously monitors the pH of the quench water column bottoms in quench column 30. Sensor 37 is electronically connected to controller 38. In addition, controller 38 is programmed so that its predetermined NH₃/C₃ ^~ ratio setpoint, which is used for controlling ammonia control valve 32 in response to the measured flowrate of incoming propylene, F_1; is modified so that this predetermined set point is adjusted in response to the measured pH of the quench water bottoms in quench column 30.

[0022] As indicated above, the measured pH of these quench water column bottoms provides an accurate indication of the concentration of unreacted ammonia in the hot reaction gases in reactor effluent line 26. Accordingly, the present invention takes advantage of this phenomenon by changing the NH₃/C₃ ^~ ratio setpoint of controller 38 in response to this measured pH. So, for example, if this measured pH becomes too low, which indicates that more sulfuric acid is being fed to quench column 30 than is necessary which, in turn, indicates that the amount of unreacted ammonia in reactor effluent line 26 has decreased, the NH₃/C₃ ^~ ratio setpoint of controller 38 is automatically increased by a corresponding amount. This set point decrease causes a decrease in the relative amount of propylene fed to the reactor, and hence a corresponding increase in the relative amount of ammonia fed to the reactor, which in turn causes the amount of unreacted ammonia in the hot reaction gases in reactor effluent line 26 to increase back to its desired value.

[0023] In one aspect, a quench liquid is provided to the quench vessel through line 45. The quench liquid may include an acid to maintain a pH of the quench liquid of about 3 to about 6, and in another aspect, about 4.5 to about 6. The acid utilized may be sulfuric acid.

[0024] It can therefore be seen that, by adjusting the NH₃/C₃ ^~ ratio setpoint of controller 38 in this way, the amount of unreacted ammonia in reactor effluent line 26 is

automatically controlled in a continuous manner to insure that there is always a slight excess of ammonia in the reactor even though the relative ratio at which propylene and ammonia are consumed varies over time with respect to one another. In this aspect, the process includes adjusting an amount of ammonia in the reactor feed to provide an ammonia to hydrocarbon molar ratio of about 1 to about 2, in another aspect, about 1.25 to about 1.75, in another aspect, about 1.4 to about 1.6, and in another aspect, about 1.25 to about 1.3. A significant advantage, therefore, is that reliance on the NH₃/C3^~ ratio setpoint of controller 38 to insure that a proper amount of ammonia is always maintained in the acrylonitrile reactor occurs both automatically and continuously and hence is no longer dependent on a manual analytical test that occurs discontinuously. In one aspect, the system is configured such that a pH change resulting from increasing or decreasing ammonia flow through the ammonia control valve is detected by the pH sensor within a lag time of one hour or less. In another aspect, the lag time may be about 10 seconds to about 60 minutes, in another aspect, about 30 seconds to about 45 minutes, in another aspect, about 1 minute to about 30 minutes, in another aspect, about 1 minute to about 10 minutes, in another aspect, about 1 minute to about 5 minutes, and in another aspect, about 2 minutes to about 4 minutes.

Air Control

[0025] In another aspect, a process for controlling an amount of air provided to an ammoxidation reaction includes monitoring an amount of oxygen in the reactor effluent and adjusting an amount of air in the reactor feed to provide an air to hydrocarbon ratio of about 9 to about 12 in the reactor feed, in another aspect, a ratio of about 9 to about 11, in another aspect, a ratio of about 9 to about 10, in another aspect, a ratio of about 10.5 to about 11, in another aspect, a ratio of about 9.25 to about 9.75, and in another aspect, a ratio of about 9.4 to about 9.6. In a related aspect, the reactor effluent stream includes about 0.5 to about 1 weight % oxygen. The process may further include continuously measuring the amount of oxygen in the reactor effluent and continuously adjusting the molar ratio of air to hydrocarbon in response. Oxygen may be measured at any location downstream of the reactor, such as for example, between the reactor and quench column or downstream of the quench column. In one aspect, the oxygen monitor is electronically connected to controller 38. Controller 38 may be configured to increase or decrease air flow to the reactor. The system is configured such that an oxygen change resulting from increased or decrease oxygen flow is detected by the oxygen monitor within a lag time of one hour or less. In another aspect, the lag time may be about 10 seconds to about 60 minutes, in another aspect, about 30 seconds to about 45 minutes, in another aspect, about 1 minute to about 30 minutes, in another aspect, about 1 minute to about 10 minutes, in another aspect, about 1 minute to about 5 minutes, and in another aspect, about 2 minutes to about 4 minutes.

[0026] Ammonia control and air control may be utilized individually or may both be included in an ammoxidation process. In addition, it will be further appreciated that that the technology of this invention requires that no new equipment or structure be added to an existing acrylonitrile plant, since it can be implemented using only the equipment already in the plant, in particular controller 38, ammonia control valve 32 and pH sensor 37 for sensing the pH of the quench column water bottoms. All that is necessary to implement this invention is to electronically connect pH sensor 37 with controller 38 and reprogram this controller to adjust its NH₃/C3^~ ratio setpoint in response the signal generated by this sensor in accordance with the teachings of this invention, which are easy and inexpensive to do.

[0027] In another aspect, the process and systems described herein may be utilized with multiple size reactors and quench columns, including reactors having large diameters, such as for example, about 9 to about 12 meters, in another aspect, about 10 to about 12 meters, in another aspect, about 10 to about 11 meters, in another aspect about 9.4 meters and above, in another aspect, about 9.5 meters, and in another aspect, about 10.7 meters. In this aspect, a ratio of cross-sectional area of the ammoxidation reactor to a cross- sectional area of the quench column is about 1 to about 3, in another aspect, about 1.5 to about 2.5, and in another aspect, about 1.6 to about 1.9.

[0028] Although only a few embodiments of this invention have been described above, it should be apparent that many modifications can be made without departing from the spirit and scope of this invention. All such modifications are intended to be included within the scope of this invention, which is to be limited only by the following claims.

Claims

What is claimed is:

1. A process for controlling an amount of ammonia provided to an ammoxidation reaction, the process comprising:

providing a reactor feed to a reactor, the reactor feed including ammonia, oxygen, and a hydrocarbon selected from the group consisting of propane, propylene, isobutane and isobutylene, and combinations thereof;

reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; providing the reactor effluent stream to a quench vessel;

providing a quench liquid to the quench vessel;

contacting the gaseous stream with the quench liquid;

monitoring a pH of quench water bottoms; and

adjusting an amount of ammonia in the reactor feed to provide an ammonia to hydrocarbon molar ratio of about 1 to about 2 in the reactor feed.

2. The process of claim 1 wherein the reactor effluent stream includes acrylonitrile and ammonia.

3. The process of claim 1 wherein the quench liquid includes an acid.

4. The process of claim 3 wherein acid is added to the quench liquid to maintain a pH of the quench liquid of about 3 to about 6.

5. The process of claim 4 wherein acid is added to the quench liquid to maintain a pH of the quench liquid of about 4.5 to about 6.

6. The process of claim 3 wherein the acid is sulfuric acid.

7. The process of claim 1 wherein the pH quench water bottoms is measured continuously.

8. The process of claim 1 wherein the molar ratio of ammonia to hydrocarbon is adjusted continuously.

9. The process of claim 1 wherein a pH change resulting from increasing or decreasing ammonia flow through the ammonia control valve is detected by the pH sensor within a lag time of one hour or less.

10. The process of claim 1 wherein a ratio of a cross- sectional area of the

ammoxidation reactor to a cross-sectional area of the quench column is about 1 to about 3.

11. A process for controlling an amount of air provided to an ammoxidation reaction, the process comprising:

reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; monitoring an amount of oxygen in the reactor effluent; and

adjusting an amount of air in the reactor feed to provide an air to hydrocarbon molar ratio of about 9 to about 12 in the reactor feed.

12. The process of claim 11 wherein the reactor effluent stream includes acrylonitrile and oxygen.

13. The process of claim 11 wherein the reactor effluent stream includes about 0.5 to about 1 weight % oxygen.

14. The process of claim 11 wherein the amount of oxygen in the reactor effluent is measured continuously.

15. The process of claim 11 wherein the molar ratio of air to hydrocarbon is adjusted continuously.

16. The process of claim 11 wherein an oxygen change resulting from increased or decrease oxygen flow is detected by the oxygen monitor within a lag time of one hour or less.

17. An ammoxidation process comprising:

providing a reactor feed to a reactor, the reactor feed including ammonia, oxygen, and a hydrocarbon selected from the group consisting of propane, propylene, isobutane and

isobutylene, and combinations thereof;

reacting the reactor feed in the presence of a catalyst to provide a reactor effluent stream; providing a quench liquid to the quench vessel;

contacting the gaseous stream with the quench liquid;

monitoring a pH of quench water bottoms,

monitoring an amount of oxygen in the reactor effluent stream;

adjusting an amount of ammonia in the reactor feed to provide an ammonia to

hydrocarbon ratio of about 1 to about 2 in the reactor feed; and

18. The process of claim 17 wherein the reactor effluent stream includes acrylonitrile, ammonia, and oxygen.

19. The process of claim 17 wherein the quench liquid includes an acid.

20. The process of claim 19 wherein acid is added to the quench liquid to maintain a pH of the quench liquid of about 3 to about 6.

21. The process of claim 20 wherein acid is added to the quench liquid to maintain a pH of the quench liquid of about 4.5 to about 6

22. The process of claim 19 wherein the acid is sulfuric acid.

23. The process of claim 17 wherein the reactor effluent stream includes about 0.5 to about 1 weight % oxygen.

24. The process of claim 17 wherein the pH quench water bottoms is measured continuously.

25. The process of claim 17 wherein the molar ratio of ammonia to hydrocarbon is adjusted continuously.

26. The process of claim 17 wherein the amount of oxygen in the reactor effluent is measured continuously.

27. The process of claim 17 wherein the molar ratio of air to hydrocarbon is adjusted continuously.

28. The process of claim 17 wherein a pH change resulting from increasing or decreasing ammonia flow through the ammonia control valve is detected by the pH sensor within a lag time of one hour or less.

29. The process of claim 17 wherein an oxygen change resulting from increased or decrease oxygen flow is detected by the oxygen monitor within a lag time of one hour or less.

30. The process of claim 17 wherein a ratio of a cross-sectional area of the ammoxidation reactor to a cross-sectional area of the quench column is about 1 to about 3.

31. A system for ammonia control in an ammoxidation reactor, the system

comprising:

an ammoxidation reactor configured to supply a reactor effluent to a quench column; a pH sensor for monitoring pH of a quench water bottoms from the quench column; and a controller electronically connected to the pH sensor and to an ammonia control valve, the ammonia control valve configured to control ammonia flow to the ammoxidation reactor; wherein the controller is configured to increase or decrease ammonia flow through the ammonia control valve.

32. The system of claim 31 wherein the system is configured such that a pH change resulting from increasing or decreasing ammonia flow through the ammonia control valve is detected by the pH sensor within a lag time of one hour or less.

33. The system of claim 31 further comprising an oxygen monitor for determining oxygen concentration in reactor effluent, the oxygen monitor electronically connected to the controller.

34. The system of claim 32 wherein the controller is configured to increase or decrease air flow to the reactor.

35. The system of claim 31 wherein the system is configured such that an oxygen change resulting from increased or decrease oxygen flow is detected by the oxygen monitor within a lag time of one hour or less.

36. The system of claim 31 wherein the pH sensor provide a continuous measurement.

37. The system of claim 33 wherein the oxygen monitor provides a continuous measurement.

38. The process of claim 31 wherein a ratio of a cross-sectional area of the ammoxidation reactor to a cross-sectional area of the quench column is about 1 to about 3.