GB2169615A

GB2169615A - Conversion of alcohols and ethers to gasoline hydrocarbons

Info

Publication number: GB2169615A
Application number: GB08600295A
Authority: GB
Inventors: Paul Kuo-Bing Chao; Sergei Jurchak
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1985-01-09
Filing date: 1986-01-07
Publication date: 1986-07-16
Also published as: GB8600295D0; ZA86172B; NZ214587A

Abstract

In a thermally damped reactor for catalytically converting alcohols and ethers to hydrocarbons boiling in the gasoline range two sections, one upstream 8 and the other downstream 4 of a catalyst bed 2, contain a high heat capacity thermal absorptive material. The sections stabilize the temperature of the feed and the reaction products. <IMAGE>

Description

SPECIFICATION Process for control of a reactor This invention is concerned with the production of gasoline boiling-range hydrocarbons from an alcohol or ether starting material over a catalyst bed.

The catalytic conversion of methanol and dimethyl ether to gasoline is a well known process for manufacturing hydrocarbon fuels. For example, U.S. Patent 3,894,107 discloses the conversion of alcohols to aromatic hydrocarbons by contacting the alcohol with a zeolite catalyst having a silica to alumina ratio of at least 12 and a Constraint Index of 1 to 12. Useful zeolites are exemplified by ZSM5, ZSM-11, ZSM-12 and ZSM-21.

U.S. Patent 3,894,105 discloses the conversion of methanol and dimethyl ether to aromatic mixtures rich in tetramethyl benzene isomers, particularly durene using zeolite catalysts.

U.S. Patent 4,012,335 describes a catalyst bed which has positioned at either end of it a mass of ceramic spheres or balls which serve as a heat sink for the process conducted within the bed.

The catalytic conversion of methanol to gasoline is an exothermic reaction which has a high activation energy. Under typical operating conditions this can result in an inverse response to a temperature disturbance. Inverse response means that when the temperature of the feed to the reaction is suddenly decreased, a transient temperature rise will occur, and vice versa. A method of levelling out such fluctuations in the temperature of the feedstock is accordingly desirable.

A way has now been discovered to minimize temperature fluctuations associated with this process.

Accordingly, the present invention provides a process wherein an oxygenated hydrocarbon feedstock is contacted with a catalyst in a catalyst bed at a stabilized operating temperature, and is converted into gasoline boiling range hydrocarbons gasoline characterized by providing a first zone of heat absorptive inert material upstream of catalyst bed; equal to 0.05 to 1.0 times the volume of catalyst, and providing a second zone of heat absorptive inert material downstream of the catalyst bed, equal to 0.05 to 1.0 times the volume of catalyst; and passing feedstock through the first zone then through the catalyst bed and passing effluent from the catalyst bed through the second zone.

In Figure 1 is a flow sheet depicting the process of this invention.

Figure 2 is a graphical plot of calculated and experimental temperature profiles of an alcohol conversion reactor under steady state conditions. The numerals 1 are experimental data points. The solid curve is the calculated profile.

Figure 3 shows the calculated inverse of the maximum peak temperature of a zeolite catalyst for a steady state inlet temperature.

Figure 4 shows the frequency response for a zeolite catalyst bed.

Figure 1 is a flowsheet for a preferred embodiment of the invention. In Figure 1, reactor bed 1 contains a catalyst capable of converting methanol and/or methyl ether to aromatic hydrocarbons, some boiling within the range of gasoline. The catalyst section 2 can comprise a catalyst such as ZSM-5. Other catalysts are disclosed in U.S. Patents, 3,894,105 and 3,894,107. Perforated screen supports the catalyst bed above a lower section 4 filled with spheres or balls made up of heat absorbent material, such as a ceramic. Upper section 8 is filled with spheres or balls of heat absorbent material, such as a ceramic. Line 5 carries heated methanol vapor into reactor 1. Methanol flow is controlled by valve 6.The conversion of methanol to aromatic hydrocarbons is exothermic so recycle gas is circulated into line 5 through 7 to maintain the temperature in the reactor at 316 to 454"C (600 to 8500F). The recycle gas ordinarily is overhead gas recovered from reactor effluent. Line 9 carries effluent from reactor 1. Heat exchanger 10 and furnace 11 heat the recycled gas since the reaction requires a minimum temperature to sustain it. Under normal operating conditions methanol flows through valve 6, line 5 and catalyst bed 1 where methanol is converted with the release of heat into aromatic compounds.

The beds of inerts promote thermal stability in the reactor. Methanol conversion in the reactor can be represented by a simple first-order reaction in oxygenates with its activation energy determined by experiment. The experimental and calculated temperature profiles for a fixed bed adiabatic reactor are shown in Figure 2.

When the feed temperature is suddenly decreased, a transient temperature increase occurs in the catalyst bed. This is commonly referred to as wrong-way behavior or inverse response. This behavior complicates the control of adiabatic reactors and may cause irreversible damage to the zeolite catalyst. Figure 3 shows the effect of inverse response in the adiabatic zeolite reactor. A 0.55 C (1"F) drop in the feed temperature causes a temperature increase of 1.9"C (3.5"F). The temperature increase tends to be more severe for a larger drop in the feed temperature. These transient temperature increases reduce the catalyst life.

Figure 4 shows the response of a 2.4m (8 foot) catalyst bed (Curve 1) to temperature disturbances entering the bed at various frequencies. Because of the inverse response, the change in the catalyst bed outlet to inlet temperature (amplitude ratio) exceeds unity over a certain frequency range. The amplification depends upon the frequency of the disturbance. Relatively high frequencies cause large amplifications while low frequencies cause only small amplifications. This behavior may lead to a control in the wrong direction and complicates the development of rational control policies for the reactor. A control scheme which maintains reactor inlet temperature by adjusting reactor effluent flow is inherently unstable should temperature disturbances enter the reactor at frequencies where the catalyst bed acts as an amplifier.

Inherent stability to temperature perturbations can be achieved by placing a large thermal mass below the catalyst bed. Curve 2 in Figure 4 shows that at frequencies where the catalyst acts as an amplifier, the inert balls dampen the amplification due to their thermal mass and heat transfer properties. The frequency response for this is shown by Curve 3 in Figure 4. The monotonically declining amplitude ratio with frequency assures reactor stability. The inert balls dampen the amplification of the temperature disturbance caused by the catalyst bed and give stability to the reactor. Thus control will now be inherently stable. The frequency of a disturbance in a commercial unit is hard to estimate. It is also useful to include in the reactor a cushion to "absorb" or attenuate these disturbances.This is done by placing high thermal mass and relatively large size inert balls on top of the catalyst bed. These inerts reduce the probability that a large disturbance will be transmitted directly to the catalyst bed and decrease the degree of amplification which results from the catalyst bed. Disturbances will have only a minor effect on catalyst life.

The use of sections containing high thermal mass material both before and after a catalyst bed makes the reactor inherently stable and protects the catalyst from potential damage caused by temperature disturbances, particularly feedstock variations. This technique has wide applicability to catalysts which are sensitive to temperature when they are used to carry out exothermic reactions.

Each section containing the heat sorptive material may be 0.05 to 1 times the bulk volume of catalyst. This depends in part upon the heat sorption characteristics of the material. The ceramic balls, spheres or other heat sorptive material may be any material with a high heat capacity and/or density so as to have a material with high thermal mass.

Alumina, silica, quartz and other refractory material and mixtures thereof, are preferred.

Claims

1. A process wherein an oxygenated hydrocarbon feedstock is contacted with a catalyst in a catalyst bed at a stabilized operating temperature, and is converted into gasoline boiling range hydrocarbons gasoline characterized by: (a) providing a first zone of heat absoprtive inert material upstream of catalyst bed, equal to 0.05 to 1.0 times the volume of catalyst; and (b) providing a second zone of heat absorptive inert material downstream of the catalyst bed, and equal to 0.05 to 1.0 times the volume of catalyst; and (c) passing feedstock through the first zone into the catalyst bed and passing effluent from the catalyst bed through the second zone.

2. The process of Claim 1 wherein the heat absorptive material is selected from the group of alumina, silica, and quartz.

3. The process of Claim 1 or 2 wherein the heat absorptive material comprises spheres having an average diameter of 6 to 25mm.

4. The process of any preceding Claim wherein the oxygenated hydrocarbon feedstock is selected from the group of methanol and dimethyl ether.

5. The process of any preceding Claim wherein the catalyst comprises a zeolite.

6. The process of Claim 5 wherein the catalyst comprises ZSM-5.