Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-09-03
2001-07-03
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S607000
Reexamination Certificate
active
06255527
ABSTRACT:
The present invention relates in general to a carbonylation process for the production of acetic acid and in particular to a process for the production of acetic acid by the carbonylation of methanol and/or a reactive derivative thereof in the presence of a Group VIII noble metal as catalyst, a hydrocarbyl halide as co-catalyst and optionally a promoter.
Homogeneous liquid phase processes for the production of acetic acid by the Group VIII noble metal catalysed, hydrocarbyl halide co-catalysed reaction of carbon monoxide are well-known. The process using rhodium as the noble metal catalyst is described in, for example GB-A-1,233,121; EP-A-0384652; and EP-A-0391680. The process using iridium as the noble metal catalyst is described in, for example, GB-A-1234121, U.S. Pat. No. 3,772,380, DE-A-1767150, EP-A-061997, EP-A-0618184, EP-A-0618183, EP-A-0657386 and WO-A-95/31426. Carbonylation processes for the production of acetic acid in the presence of either a rhodium or an iridium carbonylation catalyst are operated on a commercial scale at several locations worldwide.
Howard et al in Catalysis Today, 18 (1993) 325-354 describe rhodium and iridium-catalysed carbonylation of methanol to acetic acid. The continuous rhodium-catalysed, homogeneous methanol carbonylation process is said to consist of three basic sections; reaction, purification and off-gas treatment. The reaction section comprises a stirred tank reactor, operated at elevated temperature and pressure, and a flash vessel. Liquid reaction composition is withdrawn from the reactor and is passed through a flashing valve to a flash tank where the majority of the lighter components of the liquid reaction composition (methyl iodide, methyl acetate and water) together with product acetic acid are vaporised. The vapour fraction is then passed to the purification section whilst the liquid fraction (comprising the rhodium catalyst in acetic acid) is recycled to the reactor (cf
FIG. 2
of Howard et al). The purification section is said to comprise a first distillation column (the light ends column), a second distillation column (the drying column) and a third distillation column (the heavy ends column) (cf
FIG. 3
of Howard et al). In the light ends column methyl iodide and methyl acetate are removed overhead along with some water and acetic acid. The vapour is condensed and allowed to separate into two phases in a decanter, both phases being returned to the reactor. Wet acetic acid is removed from the light ends column as a sidedraw and is fed to the drying column where water is removed overhead and an essentially dry acetic acid stream is removed from the base of the distillation zone. From
FIG. 3
of Howard et al it can be seen that the overhead water stream from the drying column is recycled to the reaction section. Heavy liquid by-products are removed from the base of the heavy ends column with product acetic acid being taken as a sidestream.
It is with the reaction section and the operation thereof that the present invention is concerned. In terms of the process outlined above it is specifically with the reactor and its operation that the invention is primarily concerned. During continuous operation it has been customary to feed carbon monoxide on demand under pressure control, and methanol to a reactor containing a liquid composition comprising specific standing concentration of methyl acetate, water, methyl iodide co-catalyst, Group VIII noble metal catalyst, optionally one or more promoters, and comprising the remainder of the composition acetic acid. In the reactor carbonylation occurs to produce acetic acid which is removed in the liquid reaction composition, and thereafter acetic acid is recovered as hereinbefore described. Unconverted carbon monoxide is vented from the reactor and after recovery of volatile components therefrom is generally discarded. At methyl acetate concentrations in the liquid reaction composition of less than about 6% w/w, which levels are generally associated with the use of rhodium catalysts, practically all the methyl acetate is converted by carbonylation to acetic acid. Under such circumstances little, if any, difficulty is experienced in controlling the reactor temperature. However, at methyl acetate concentrations of at least 5% w/w, typically 8% w/w or greater, which levels are generally associated with the use of iridium catalysts, not all the methyl acetate in the liquid reaction composition is converted and the potential therefore exists for uncontrollable exotherms arising from an ever increasing demand for carbon monoxide and the presence of unconverted methyl acetate reactant. Under such circumstances the plant may trip, which is undesirable because it interrupts production. Unsteady reactor temperature also leads to instability in reactor carbon monoxide uptake. This leads to the requirement to vent carbon monoxide to flare for control purposes, resulting in loss of carbon monoxide conversion efficiency. Reaction temperature control at high methyl acetate concentrations is therefore a significant problem. A solution to the problem is the provision of a mechanism to limit the amount of carbon monoxide available to the reactor to avoid uncontrollable exotherms.
Accordingly the present invention provides a method of controlling the carbon monoxide flow to a reactor wherein acetic acid is produced continuously by feeding carbon monoxide through a control valve and methanol and/or a reactive derivative thereof, there being maintained in the reactor a liquid reaction composition comprising at least 5% w/w methyl acetate, a finite concentration of water, from 1 to 30% w/w methyl iodide, a Group VIII noble metal catalyst, optionally at least one promoter and acetic acid comprising the remainder of the composition which method comprises the steps of:
(i) measuring the carbon monoxide flow through the control valve;
(ii) performing a background calculation to arrive at a time-averaged carbon monoxide flow rate;
(iii) adding a constant value to the time-averaged carbon monoxide flow to arrive at a maximum allowable carbon monoxide flow rate; and
(iv) feeding information comprising the calculated maximum allowable carbon monoxide flow rate to a control system which operates in a manner such that the carbon monoxide flow rate to the reactor can not exceed the calculated maximum flow rate at any time.
In one embodiment the method comprises activating the control valve through a low signal selector responsive to inputs from either a reactor pressure controller or a carbon monoxide feed flow controller, the flow-controller being governed by the maximum allowable carbon monoxide flow rate, as calculated by a calculation block functioning to determine a time-averaged carbon monoxide flow rate and add a constant value thereto, the input to the signal selector and hence operation of the control valve being through the flow controller when the carbon monoxide feed rate to the reactor is higher than the maximum allowable carbon monoxide flow rate and through the pressure controller when the carbon monoxide feed rate to the reactor is lower than the maximum allowable carbon monoxide flow rate. In this embodiment the flow-controller is not normally in control of the flow valve operation because the flow rate is normally below the maximum allowable carbon monoxide flow rate as determined by the calculation block and the pressure controller is operative through the selector. In the event of a reactor disturbance involving greater carbon monoxide uptake the carbon monoxide flow output becomes lower than the pressure controller output and the flow controller takes control through the selector.
There are other ways in which the method of the present invention may be applied. Thus, for example, the flow controller may be in permanent charge of operation of the control valve in which case the calculation block sets a value for the maximum allowable carbon monoxide flow such that the flow through the control valve does not exceed this value.
An advantage of the process according to the present invention is that
BP Chemicals Limited
Deemie Robert W.
Killos Paul J.
Nixon & Vanderhye
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