IL28081A - Fluid-bed ammoxidation of propylene or isobutylene to acrylonitrile or methacrylonitrile,respectively - Google Patents

Description

nascnm Fluid-bed aramoxidntion of propylene or leobutylen© to acrylonitrile or nethacryloni trile, respectively THE STANDARD OIL COMPANY 0: 26628 - la - This invention relates to the prevention of catalyst decomposition in the top portion of a fluidized solid catalytic reactor and more particularly pertains to a process for the fluid-bed amraoxidstion of propylene or isobutylene to acrylonitrile or methacrylonitrile, respectively, wherein decomposition and build-up of catalyst in a relatively quiescent zone at the top of a fluidized solid catalyst reactor, is substantially prevented.

Fluidized solid catalyst reactors in which a gaseous reactant component contacts a fluidized solid catalyst and particularly in fluidized solid ammoxid ion and oxidation reactors commonly employe! one or more single or multistage cyclone separators to remove solid catalyst particles from the effluent gases ne r the top of the reactor. These cyclones, associated piping, duct work, and supporting members present a large amount of surface area upon which catalyst fines or dilute phase solid catalyst can accumulate. The flat, horizontal surfaces near the top of the reactor are particularly prone to accumulate substantial quantities of catalyst. The catalyst which accumulates on these surfaces and particularly the stagnant or immobilized catalyst tends to be chemically reduced by prolonged contact with the effluent gases and this has several undesirable consequences* The heat generated by the reduction of the catalyst and/or reoxidation of the reduced anal st causes the catalyst to be damaged by loss of surface area which is an important physic 1 characteristic of the catalyst and to suffer damage in other ways. In some cases, the fine catalyst particles which occumulate at the top of the reactor become fused and agglo When the catalyst becomes fused, it often becomes sticky causing more dilute phase catalyst in its vicinity to stick to the sticky, fused surface and ultimately to suffer the same fate. As this process continues, the entire cyclone region in the top or head of the reactor may become packed with reduced and fused catalyst. Mechanical vibration at times causes large and small pieces of the fused catalyst in the reactor head to break loose and fall into the dense phase catalyst bed of the reactor proper. The presence of these large pieces of catalyst in the reactor frequently disrupt the normal patterns of gas flow and catalyst circulation causing inefficiency in conversion and, in extreme cases, causing damage to the remainder of the catalyst.

The present invention eliminates the problems discussed above, by providing a process for the fluid-bed ammoxidation of propylene or isobutylene to acrylonitrile or methacrylonitrile, respectively, in the presence of excess oxygen wherein a low-boiling gaseous monoolefin is converted to a higher boiling unsaturated nitrile, without coke formation, in a reactor having a relatively quiescent zone above a turbulent reaction zone of a powdery catalyst, characterized by introducing inert gas into said quiescent zone in an amount sufficient to provide an escape velocity of from 0.1 to 10 feet per second into said turbulent zone, simultaneously maintaining the temperature of said quiescent zone at least 200°P below the mean temperature of said turbulent zone.

One embodiment of the process according to the present invention involves the positioning of a horizontal plate of planar configuration at an elevation within the reactor at or near the top of the primary or first stage cyclone horn or horns. This horizontal plate extends over essentially the entire open cross-section area of the reactor and is fitted snugly around dip legs, cyclone bodies and other internals.

Another aspect of this invention involves providing a small percentage of open area in the horizontal plate preferably removed froa any of the cyclones, associated piping, duct work and supporting members. The exact position of this open area in the horizontal plate is not critical just so long as the enclosed gas volume in the head of the reactor is in free communication with the main reactor gas stream.

In addition, an inert purge gas is bled continuously into the region above the horizontal plate at such a rate that the average lineal velocity of the purge gas passing out through the open area in the horizontal plate into the reactor proper is in the range of 0.1 to 10 ft./sec. The open area in the horizontal plate comprises about 0.1 and 10$ of the tctel cross-sectional area of the reactor. In any case, it must be at least large enough to permit the pressure between the main reactor volume nd the volume above the horizontal plate to equalize a change in oressure of the main reactor volume without developing a pressure differential during the equalization process of greater than about 5 p.s.i.g.

It has been discovered that practice of the present invention prevents the usual deposition, stagnation, reduction and fusion of catalyst in the region of the reactor above 1he primary cyclone horns. Costly damage to the catalyst and ultimate shut-down of the reactor due to disruptive fused Especially outstanding advantages ^re experienced when the invention is practiced with catalysts comprising the element antimony (such as antimony oxide inteorabination with one or more elements from the group uranium, iron, manganese, tin, bismuth, cerium, thorium, molybdenum, tungsten, vanadium, tellurium, selenium, etc.) and particularly the catalysts disclosed in U.S. Patents Nos. 3,200,084; 3,200,031; 3,193,750; 3,197,419 and 3,244,642, In one preferred embodiment, the invention is practiced as followsΪ (1) A hori ontal plate of 1/8** or 1/2 cerbon steel pl te is fitted over the open cross section of the reactor at the elevation of the top of the primary cyclone horns. t is fitted around dip lege, cyclone bodies and other internals such that sufficient gap is provided between the plate and the internal member in question to allow for thermal expansion and contraction. (2) Additional holes are provided in the horizontal plate to bring the total open area (sum of g ps around members and additional holes) within the range of 0.5 to 5$ of the reactor cross section. (3) Provision is made to feed inert gas from an inert gas generator into the hcadjof the reactor above the horizontal plate at such a rate that the average linear gas velocity of the inert gas passing through the open areas in the horizontal plate is within the range of 0.3 to 1.0 ft./sec. (calculated at the temperature and pressure of the reactor) · EXAMPLE (A) In a control reaction which is outside the scope of the present invention, acrylonitrile was prepared from a mixture of propylene, air and ammonia in the presence of an antimony oxide-uranium oxide catalyst.

The reactor was a cylindrical tube constructed of steel having a diameter of 11 feet and a height of ?ibout 50 feet.

The catalyst bed depth was 22 feet under static conditions and the catalyst was a fluidized solid combined antimony oxide-uranium oxide catalyst on a silica carrier more fully described in U.S. Pauent No. 3,198,750. The reactor contained perforated horizontal plates or sieve-trays spaced at two foot intervals and each of the sieve-trays had about 30?¾ open area with 5/8 diameter holes as more fully described in U.S. Patent Wo. 3»230,246. The feed consisted of propylene: air:ammonia in the mole ratio of 1:10.5-11·5:1·15, respectively. The reaction temperature was 920 - 940°F. and n initial pressure of about 16 - 18 p.s.i.g. was reached in the upper portion of the reactor. The top of the reactor was equipped with a multiple cyclone arrangement but without a horizontal plate. A feed rate of 1.5 feet per second was the superficial linear velocity. The superficial linear gas velocity is defined as Volume of feed in cubic feet per second Reactor cross-sectional area in cubic feet and is expressed as feet per second.

The reaction was started up and a total conversion of 90-95 of propylene was achieved. An initial per pass conversion of propylene to acrylonitrile of 66-68$ was obtained.

Initially a normal catalyst loss of about 100 pounds per day was observed in the effluent and this was continuall replaced. B the end of the ihree months operation period, however, apparent catalyst losses were from 200—500 pounds per day with no more than 100 pounds per day showing up in the effluent. The per pass conversion of propylene to acrylo-nitrile had also dropped to the range of 60-63 by this time and temperatures in excess of 1,000°F. had developed in the top of the reactor. The operation was shut down and it was found th- t the region in the vicinity of the cyclones and particularly on the horizontal surfaces was completely filled with fused, reduced c alyst and thnt several tons of fused, reduced catalyst, which was damaged beyond use, were deposited in the cyclone region.

(B) The process of the present Invention is illustrated with a repeat of (A) with the exception that a horizontal plate was fitted over the open cross section of the reactor at the elevation of the top of the primary cyclone horns, and nitrogen was fed into the head of the reactor above the horizontal plate at a linear gas velocity of about 0.5 ft./sec. In this exper_tent no more than the normal 100 pounds per day of catalyst were lost in the effluent, a normal 66-68 per pass conversion of propylene to acrylonitrile was achieved, no great temperature increase occurred in the top of the reactor, and no buildup of fused, reduced catalyst in the cyclone region of the reactor was observed after more than three months of continuous operation. 28081/2 CASE 3814-7

Claims (2)

1. A process for the fluid-bed ammoxidation of propylene or isobutylene to acrylonitrile or methacryloni-trile , respectively, in the presence of excess oxygen wherein a low-boiling gaseous monoolefin is converted to a higher boiling unsaturated nitrile, without coke formation, in a reactor having a relatively quiescent zone above a turbulent reaction zone of a powdery catalyst, characterized by introducing inert gas into said quiescent zone in an amount sufficient to provide an escape velocity of from 0.1 to 10 feet per second into said turbulent zone, simultaneously maintaining the temperature of said quiescent zone at least 200°P below the mean temperature of said turbulent zone whereby clumping of said catalyst due to fusion, and adhesion of fused catalyst to internals in the top portion of the reactor, is substantially prevented.

2. A process according to claim 1, characterized by the fact that the pressure differential between the inert gas and the gases in the remainder of the reactor is no greater than about 5 p.s.i.g.