The present invention is directed to an improved process for the manufacture of acrylonitrile or methacrylonitrile. In particular, the present invention is directed to improved operation of the heads, or HCN separation, column in the acrylonitrile and methacrylonitrile recovery process. Applicant has discovered a previously unknown relationship between the formation of undesirable polymeric HCN in the heads column and the formation of an aqueous second liquid phase in the heads column above the feed tray. The present invention is directed towards preventing the formation of the aqueous phase in the heads column above the feed tray, since the presence of this aqueous phase causes the formation of unwanted and detrimental polymeric HCN. Previous art was directed at reducing the pressure of the heads tower, resulting in lower operating temperatures and perceived reduction in the polymerization rates of HCN. The instant invention is directed at disrupting the mechanism of the HCN polymerization, which occurs as ionic polymerization in the aqueous phase. By practicing the present invention, unwanted polymerization of HCN may be reduced, fouling of the heads column may be greatly diminished or eliminated, and increased production of desirable products may be achieved.
The present invention is directed to an improved process for the manufacture of acrylonitrile or methacrylonitrile. In particular, the present invention is directed to the improvement in the recovery and operation of hydrogen cyanide separation column utilized during the manufacture of acrylonitrile or methacrylonitrile.
Recovery of acrylonitrile/methacrylonitrile produced by the ammoxidation of propane, propylene or isobutylene on a commercial scale has been accomplished by quenching the reactor effluent with water followed by passing the gaseous stream containing acrylonitrile or methacrylonitrile, as well as byproduct HCN, resulting from the quench to an absorber where water and the gases are contacted in counter-current flow to remove substantially all the acrylonitrile or methacrylonitrile. The aqueous stream containing HCN and the acrylonitrile or methacrylonitrile is then passed through a series of distillation columns and associated decanters for separation and purification of product acrylonitrile or methacrylonitrile from a vapor stream containing substantially all the HCN.
Typical recovery and purification systems that are used during the manufacture of acrylonitrile or methacrylonitrile are disclosed in U.S. Pat. Nos. 4,234,510 and 3,885,928, assigned to the assignee of the present invention and herein incorporated by reference.
It is the primary object of the present invention to provide an improved process for the recovery and operation of byproduct HCN in the manufacture of acrylonitrile or methacrylonitrile.
Another object of the present invention is to provide an improved process for the recovery of acrylonitrile, methacrylonitrile, or HCN obtained from the reactor effluent of an ammoxidation reaction of propane, propylene or isobutylene comprising passing the reactor effluent through an absorber column, a recovery column and a heads column wherein the improvement comprises operating the heads column in a manner which inhibits the formation of an aqueous phase above the feed tray of the heads column.
An additional object of the present invention is to provide an improved process for the recovery of acrylonitrile, methacrylonitrile, or HCN obtained from the reactor effluent of an ammoxidation reaction of propane, propylene or isobutylene by operating the heads column in a manner which inhibits the formation of an aqueous phase above the feed tray of the heads column, such as increasing reflux ratios; using a side decanter to split and remove the aqueous phase from the column; using a cooler feed stream to increase the stripping in the column; increasing the number of stripping trays; using an intermediate condenser above the feed to supplement the overhead condenser; subcooling the reflux stream; increasing reboiler and overhead condenser duties to increase reflux flow rates; control operating pressure to shift the equilibrium between the two liquid phases; and other methods known to those skilled in the art that would increase reboiler duty, and the associated stripping effectiveness of the heads column. Increasing the hydrogen cyanide reflux or concentration of hydrogen cyanide above the feed tray can also be achieved through higher HCN production levels for eliminating the second liquid phase. Any increased tray efficiency also allows more stripping effectiveness and is helpful in eliminating the undesired second liquid phase.
Yet another object of the present invention is to provide an improved process for the recovery of acrylonitrile, methacrylonitrile, or HCN obtained from the reactor effluent of an ammoxidation reaction of propane, propylene or isobutylene comprising passing the reactor effluent through an absorber column, a recovery column and a heads column wherein the improvement comprises feeding extra HCN to the heads column, either by operating the ammoxidation reactor in a manner to produce a higher concentration of HCN to other products, or by recycling HCN to the heads column, to permit operation of the heads column in a manner that reduces or eliminates the formation of the undesirable aqueous phase.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which 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 realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, the process of the present invention comprises transporting the reactor effluent obtained during the ammoxidation of propane, propylene or isobutylene to a quench column wherein the hot effluent gases are cooled by contact with an aqueous spray, passing the cooled reactor effluent overhead to an absorber column wherein the HCN and crude acrylonitrile or methacrylonitrile is absorbed in water, passing the aqueous solution containing the HCN and acrylonitrile or methacrylonitrile, plus other impurities to a first distillation column (recovery column), where a significant portion of the water and impurities are removed as a liquid bottoms product, while HCN, water, a minor portion of impurities and acrylonitrile or methacrylonitrile are remove as an overhead vapor stream. This overhead vapor stream is further cooled using a heat exchanger, and directed to a decanter, to separate and condense liquids which are returned to the recovery process, while the remaining vapor stream is directed to a flare, incinerator, or other disposal process. The organic stream is fed to the heads column for separation of HCN from acrylonitrile.
In a preferred embodiment of the present invention, the process is performed with the reactor effluent obtained from the ammoxidation of propane or propylene, ammonia and oxygen to produce acrylonitrile.
In a still preferred embodiment of the present invention, the reactor effluent is obtained by the reaction of propane, propylene, ammonia and air in a fluid bed reactor while in contact with a fluid bed catalyst. Conventional fluid bed ammoxidation catalyst may be utilized in the practice of the invention. For example, fluid bed catalyst as described in U.S. Pat. Nos. 3,642,930 and 5,093,299, herein incorporated by reference, may be utilized in the practice of the present invention.