Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-09-13
2003-11-04
Padmanabhan, Sreeni (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S519000, C562S607000, C560S232000, C422S106000, C422S110000, C422S234000
Reexamination Certificate
active
06642413
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for monitoring a continuous acetic acid and/or methyl acetate production.
More precisely, the invention relates to a method of improving the monitoring and control of a process for the preparation of acetic acid and/or methyl acetate.
Of the acetic acid manufacturing processes in common use, one of the most widely used in industry is the carbonylation of methanol or, more generally, a carbonylatable derivative of methanol with carbon monoxide. This reaction is carried out in the liquid phase under carbon monoxide pressure, carbon monoxide being one of the reactants, in the presence of a homogeneous catalyst system.
The rhodium-based carbonylation process is a known process which is exploited in industry and has formed the subject of numerous articles and patents, e.g. U.S. Pat. Nos. 3,769,329 and 3,813,428.
European patents EP 618 183 and EP 618 184, and European patents EP 785 919 and EP 759 022, describe a process for the carbonylation of methanol in the presence of an iridium-based catalyst system which may also contain rhodium.
A carbonylation process based on iridium and ruthenium, currently exploited in industry, is described in European patent EP 643 034.
The aim of improving these methanol carbonylation processes was to increase the productivity of the catalysts and reduce the acetic acid manufacturing costs.
The so-called “low water content” processes actually permit a considerable increase in acetic acid production, thereby limiting the level of investment required and lowering the operating costs by reducing the energy required to separate the acetic acid from the various constituents of the reaction mixture, particularly water.
Conventionally these different processes for the carbonylation of methanol in the liquid phase and in the presence of a homogeneous catalyst system are carried out in installations comprising three separate zones, as described in the article by M. J. HOWARD et al., Catalysis Today, 18 (1993) 325-354.
The reaction zone consists of a stirred continuous reactor operating under pressure (5-200 bar) and at elevated temperature (150-250° C.).
The methanol feed and a number of recycle streams are introduced at the bottom of the reactor. The carbon monoxide is dispersed in the reactor.
The liquid reaction medium produced is then sent to the second zone, called the vaporization zone or flash zone, in which the liquid is partially vaporized at a pressure below the reaction pressure.
This creates a flash, or adiabatic expansion, in which the majority of the light constituents (methyl iodide, methyl acetate and water) are vaporized together with the acid produced.
The flash zone makes it possible to separate the gas from the liquid; the vaporized stream then passes into the third zone, called the separation zone, while the liquid stream (essentially acetic acid containing the catalyst) is recycled into the first zone.
The purification zone can comprise one or more distillation columns; it makes it possible to separate the acetic acid and/or methyl acetate from the other constituents and to recycle the streams into the reaction zone.
In addition, a gas bleed at the top of the reactor makes it possible to monitor the level of carbon monoxide partial pressure and to remove the gaseous reaction by-products together with the inert gases present in the carbon monoxide feed.
The result of optimizing the current processes for the preparation of acetic acid is to maximize the acetic acid production in already existing equipment. To do this, the reaction is carried out with the concentrations of the various constituents of the reaction medium being kept at predetermined values so as to take advantage of the most appropriate kinetic conditions.
It then becomes essential to monitor the reactor and the recycle streams.
A variety of recent articles and patents have started to tackle this area.
The paper delivered by D. Z. TOBIAS at the IEEE Conference on Advanced Process Control, VANCOUVER, Apr. 29-30 1999, entitled “Adaptive Process Control of an Acetic Acid Reactor”, shows the use of an external exchanger for making the material balance independent of the calorific balance, since without this exchanger it is impossible to vary the reactor output rate in order to maintain the reactor temperature, i.e. the reactor output rate is in effect dictated by the material balance.
The desired objective is to stabilize the reactor temperature in order to increase the production of the unit; the installation of a new monitoring system thus made it possible to reduce the standard deviation of the temperature from 3.6 to 0.8° C.
In continuous operations, it is customary to supply the carbon monoxide on demand under the control of the total pressure in the reactor.
The object of European patent application EP 0 983 752 is to install a monitoring system which aims to keep the carbon monoxide flow rate below a calculated maximum value representing an acceptable maximum flow rate.
The aim of European patent application EP 0 999 198 is to maintain the composition of the reaction medium, particularly the water and methyl acetate concentrations, in the carbonylation reactor by recycling a rich acetic acid stream coming from the acid purification zone.
Patents EP 0 846 674 and FR 2 750 984 describe the optimization of the CO consumption by introducing a second carbonylation reactor (finishing effect) between the 1st reactor (reaction zone) and the flash zone (vaporization zone).
U.S. Pat. Nos. 5,352,415 and 5,374,774 describe acetic acid manufacturing processes in which the levels of the reactor and the flash zone and the water concentration in the reaction medium are monitored.
U.S. Pat. No. 5,831,120 provides a technical solution for avoiding the accumulation of water in the reaction medium to give a target water concentration in the carbonylation reactor.
These various documents teach that, in the conventional process developed by MONSANTO, the heat of reaction was extracted via expansion of the reactor output, or flash, between the reactor and the flash zone. Thus, to monitor the reactor temperature, there was a fixed relationship between the flow rate of methanol entering the reactor and the flash rate. This led to a small variability in the level of liquid in the reactor and the intermediate process streams.
With the introduction of low water content processes, this system becomes inappropriate because the flash vaporization rate increases considerably due to the fact that the quantities of water evaporated are smaller and that they have to be replaced with much larger quantities of organic products whose latent heat of vaporization is lower than that of water.
This led to the installation of cooling exchangers for absorbing part of the heat of reaction, but this in turn increased the variability of the liquid levels in the reactor and the flash zone, as well as the liquid flow rates.
This great variation in liquid flow rates, in particular, leads to deficiencies in the process because it may be necessary to decrease production in order to reduce the stream entering the purification zone.
Furthermore, these fluctuations cause variations in the water concentration in the reactor and, as the reaction kinetics depend on the water concentration in the case of low water content processes, this again increases the risks of instability of the system.
The aim of patent application EP 1 002 785 is to maintain the methyl acetate concentration in the reactor at a predetermined value by adjusting the ratio of methanol to carbon monoxide, which regulates the feed rate of methanol in the reactor.
International patent application WO 00/37405 provides a method of monitoring the process by measuring the concentrations of the various components of the reaction solution by means of an infrared analyzer and, in response thereto, adjusting the concentrations of at least the catalyst species, the methyl iodide, the methyl acetate and the water in order to optimize the acetic acid production.
Whatever the case may be, the problem of monitori
Acetex Chimie
Dennison, Schultz & Dougherty
Forohar Farhad
Padmanabhan Sreeni
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