Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/10—Preparation of carboxylic acids or their salts, halides or anhydrides by reaction with carbon monoxide
  • C07C51/12—Preparation of carboxylic acids or their salts, halides or anhydrides by reaction with carbon monoxide on an oxygen-containing group in organic compounds, e.g. alcohols
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/42—Separation; Purification; Stabilisation; Use of additives

Definitions

• This invention relates to a method of improving process control in the manufacture of acetic acid, and a method of manufacturing acetic acid utilizing improved process control.
• the carbonylation reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
• U.S. Patent 3,769,329 discloses that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations between about 14-15 wt.% are typically used. This is the so-called "high water” carbonylation process.
• U.S. Patent No. 5,144,068 discloses that, at low water concentrations, there is a synergistic effect between the methyl acetate concentration in the carbonylation reactor and the concentration of iodide salt used to stabilize the rhodium catalyst. It also teaches that an unexpected advantage of operating the reactor at high methyl acetate is a reduction in the formation of undesirable reaction products. In particular, propionic acid is reduced by an order of magnitude. Carbon dioxide and hydrogen, which are formed by the water gas shift reaction, are also reduced.
• Various means have been proposed for controlling the processes for the production of acetic acid. For example, U.S. Patent No.
• 5,474,774 discloses a system for controlling the liquid levels in a reactor-flasher combination used for the carbonylation of methanol to acetic acid.
• Liquid level control is achieved by proportional controllers or other controllers, which proportionally change the liquid flow rates from the respective reactor and flasher.
• An adjustment can be made to the level controllers to change the liquid flow rates by a function generator, which adjusts the flow rate, according to an empirically derived function, which correlates changes in methanol feed rate to liquid flow rates from the reactor and flasher.
• European Patent Application EP 1 002 785 Al describes a process for the production of acetic acid in which the methyl acetate concentration in the liquid reaction composition is maintained at a pre-determined value by monitoring the ratio of methanol and/or reactive derivatives thereof to carbon monoxide being converted to acetic acid and adjusting the methanol, and/or reactive derivatives thereof, feed rate in response.
• U.S. Patent No. 6103934 discloses an acetic acid production process with a control process which measures various reactor component concentrations, specifically the active catalyst species, methyl iodide, water, and methyl acetate with an infrared analyzer, and adjusting in response thereto, the concentrations of the catalyst species, methyl iodide, and water to control the acetic acid reaction.
• U.S. patent 6,255,527 Bl discloses an acetic acid production system with a method for controlling the carbon monoxide flow to a reactor by measuring carbon monoxide flowing through a control valve; performing a background calculation to arrive at a time-averaged carbon monoxide flow rate; determining a maximum carbon dioxide flow rate; and controlling the carbon monoxide flow rate so that it does not exceed the calculated maximum flow rate.
• the present invention relates to a process for monitoring and controlling reactor conditions during the production of acetic acid by the catalyzed carbonylation of methanol.
• the process of the present invention comprises measuring the density of the heavy phase of the light ends distillation column in the purification system of the carbonylation process.
• the density measurement is used to adjust the feed of methanol and/or to regulate the temperature in the reaction zone to optimize reactor conditions.
• the density measurement may also be used to adjust other parameters in the reactor system.
• the invention is also directed to the system for manufacturing acetic acid based on the process control procedure described.
• Monitoring the heavy phase density in accordance with the present invention may be performed near in time to removal of a sample or alternatively, the momtoring may be conducted online.
• Online monitoring refers to the analysis of heavy phase in real time or substantially real time either by direct insertion of a densitometer probe into the heavy phase process vessel or by rapidly circulating heavy phase process solution through a densitometer and subsequently returning this solution to the process.
• Off-line measurement refers to the irreversible removal of a heavy phase sample from the process and subsequent analysis being performed on laboratory instrumentation. Further, adjustment of component concentrations and reaction parameters as required should occur substantially immediately following characterization of the sample. This adjustment may be performed automatically in response to the heavy phase density measurement. Finally, it is preferred that the sampling be performed often to minimize undesirable drift from optimum reaction efficiency.
• Figure 1 is a schematic diagram of a typical continuous process for the production of acetic acid.
• Figure 2 is a diagram of the equipment arrangement used in accordance with an embodiment of the present invention.
• the present invention relates to a continuous 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 catalyst system which may comprise, for example, a Group VIII metallic element, particularly Rh, Ir, Co, Ni, Ru, Pd or Pt, and most often Rh or Ir, a halogen promoter, most often a hydrogen halide or organic halide, particularly an alkyl iodide such as methyl iodide, a stabilizer/copromoter, which is a salt of a metal of Group IA or IIA of the Periodic Table, or a quatenary ammonium or phosphosium salt, particularly an iodide or acetate salt and most often lithium iodide, or lithium acetate.
• a catalyst system which may comprise, for example, a Group VIII metallic element, particularly Rh, Ir, Co, Ni, Ru, Pd
• the continuous carbonylation process may be thought of as comprising three basic sections: the reaction, purification, and off-gas treatment sections.
• This process is represented schematically in Figure 1.
• the reactor 4 which may be a stirred tank reactor, is operated at adjustable elevated temperatures and pressures.
• the liquid reaction mixture is withdrawn from the reactor and passed to a flash tank 7 where the majority of the lighter components of the liquid reaction mixture (methyl iodide, methyl acetate, unreacted methanol, and water) together with product acetic acid are vaporized.
• the vapor fraction is then directed to the purification section and the liquid fraction (comprising the noble catalyst dissolved in acetic acid) and any remaining light components is recycled to the reactor 4.
• the purification section typically comprises a first distillation column 13 (the light ends column), a second distillation column (the drying column, not shown) and a third distillation column (the heavy ends column, not shown).
• a first distillation column 13 the light ends column
• a second distillation column the drying column, not shown
• a third distillation column the heavy ends column, not shown.
• methyl iodide, methyl acetate, and unreacted methanol are removed overhead along with some water and acetic acid in stream 16.
• the vapor is condensed and allowed to separate into two phases in a decanter 17, both phases are usually returned to the reactor.
• One phase of this condensate, the upper layer is the lighter phase 18 comprised primarily of water and acetic acid in which lesser amounts of methyl acetate and methyl iodide may be present.
• the light phase may be returned to the reactor in the light phase recycle line 22.
• the other phase is the heavy phase 19 comprised primarily of methyl iodide and methyl acetate in which lesser amounts of water, methanol, and acetic acid may be present.
• the heavy phase may be returned to the reactor in the heavy phase recycle line 21. It is the density of this heavy phase which provides feedback data for controlling the reactor composition in accordance with the present invention.
• the present invention involves monitoring the density of this heavy phase during continuous reactor operation as an indicator of the level of methyl acetate in the reaction zone. Based upon this density measurement, the continuous carbonylation process can be altered to prevent undesirable process excursions during the production of acetic acid.
• acetic acid is removed as a side stream 14 from the light ends column and is fed to a drying column (not shown) where water is removed overhead and an essentially dry acetic acid stream is removed from the base of the distillation zone.
• the overhead water stream from the drying column is recycled to the reaction section.
• Heavy liquid by-products are removed from the base of a heavy ends column with product acetic acid being removed as a side stream.
• the production of acetic acid is one such chemical process.
• the underlying reaction chemistry is intricate, involving a number of interrelated reactions, by-products and equilibriums, all of which must be properly balanced, one against the other, to make the process practicable and maximize efficiency of raw material utilization.
• the catalyst systems, such as coordination compounds of rhodium, iridium and the like, required for carbonylation are generally complex and expensive.
• carbonylation catalyst systems are extraordinarily sensitive to changes in any number of reaction parameters which, in turn, adversely affect catalyst stability and activity.
• monitoring systems based upon gas chromatography teclmology have been used.
• both of these types of systems have significant disadvantages in terms of the high level of maintenance and expense necessary to maintain the equipment in an accurate functioning state.
• the gas chromatography systems may require a considerable period of time to analyze a sample, resulting in a lag between the results and the actual status of the process system as mentioned above. It is therefore desirable to provide a monitoring system that facilitates more frequent monitoring of the chemical reaction in the production of acetic acid so that changes in the reaction zone can be detected early and to generate appropriate feedback to change the reaction conditions.
• the present invention provides this early detection capability by relying upon analytical samples taken from outside of the reaction zone. In accordance with the present invention, the heavy phase density is measured.
• the heavy phase density is a function of the concentration of methyl iodide and methyl acetate in the heavy phase.
• the concentration of methyl iodide and methyl acetate in the heavy phase is indicative of the concentration of methyl acetate in the reaction zone.
• the density of the heavy phase decreases because more methyl acetate is sent forward to the light ends distillation column and then concentrates, predominately in the heavy phase.
• the density of the heavy phase decreases as the methyl acetate concentration increases. Making use of this relationship, the present invention provides a convenient, safe, and relatively inexpensive method to continuously monitor the level of methyl acetate in the reaction zone.
• the present invention relates to control of a continuous carbonylation process based upon feedback derived from measuring the density of the heavy phase of the light ends distillation column in the purification section of the carbonylation process.
• continuous operation it is customary to feed carbon monoxide on demand under pressure control, and methanol to a reactor with a liquid composition comprising standard concentrations of methyl acetate, water, a catalyst system as described previously, with the remainder of the composition being acetic acid.
• carbonylation occurs to produce acetic acid which is removed with the reaction liquid. Thereafter acetic acid is recovered as described above. Unconverted carbon monoxide is vented from the reactor and after recovery of volatile components there from is generally discarded.
• methyl acetate concentrations in the reactor liquid of about 3 wt.% or less, which levels are generally associated with the use of rhodium catalysts, the reaction rate depends strongly on methyl acetate concentration. Under these circumstances little, if any, difficulty is typically experienced in controlling reaction section methyl acetate concentrations using reaction temperature. However, as methyl acetate concentrations approach 5 wt.% or greater, the reaction rate is significantly less dependent on methyl acetate concentration. In this situation, there is an increased potential for the reactor methyl acetate concentration to increase rapidly. This increase will cause significant disturbances in downstream equipment and the plant may trip which is undesirable because it interrupts production and creates a potentially dangerous situation.
• the present invention provides a means for preventing such process excursions during continuous processes for the production of acetic acid by providing a means to continuously monitor reactor methyl acetate concentration and to control the reaction process in response thereto in order to stabilize the methyl acetate concentration.
• a methyl acetate concentration of about 2 wt.% in the reaction zone generally corresponds to a heavy phase density in the range of about 1.90 to about 2.0.
• the density of the heavy phase will typically be in the range from about 1.70 to about 1.80. As mentioned above, as methyl acetate concentrations in the reaction zone during the production of acetic acid approach 5 wt.%, the likelihood of a reactor excursion is increased. At methyl acetate concentrations of about 6 wt.%, the density of the heavy phase will typically be in the range from about 1.5 to about 1.6.
• Table 1 sets forth a series of data depicting various methyl acetate concentrations over a range from about 2.0 wt.%> to about 7.0 wt.%> with corresponding heavy phase density values measured in a typical acetic acid production system.
• this data demonstrates that the density of the heavy phase will increase or decrease in the range from about 0.05 to about 0.1 with each 1 wt.%) decrease or increase in methyl acetate wt.% concentration in the reaction zone.
• the specific corresponding density range will vary from reaction system to reaction system. Accordingly, it will generally be necessary to calibrate the relationship between methyl acetate concentration and the heavy phase density for each reaction system in which the present invention is practiced.
• the present invention provides a means for accurately and continuously monitoring the reaction zone methyl acetate concentration and providing feedback to change or maintain reaction parameters in response to this feedback.
• the reaction parameters may be changed in a variety of ways. One such way is to change the methanol feed rate to the reactor. Another way is to change the reaction temperature. This is often accomplished by adjusting the temperature of a recycle stream flowing back to the reactor through one or more heat exchangers. Additionally, the temperature of the reaction system may be regulated, in whole or in part, by adjusting the temperature of other streams entering the reaction system, such as a pump-around stream. Other parameters which may be adjusted in accordance with the present invention include water concentration in the reactor, catalyst concentration, methyl iodide concentration, and CO partial pressure in the reaction system. By changing any of these variables, or a combination of more than one of the variables, it is possible to exert at least some control on the reactor methyl acetate concentration.
• the process of the present invention is programmed to respond (or an operator will manually respond) by decreasing the methanol feed rate to the reactor, which will lower the rate of formation of methyl acetate in the reactor.
• the system of the present invention may be programmed to respond (or an operator will manually respond) by raising the temperature in the reactor to increase the rate at which methyl acetate is converted to acetic acid.
• the system of the present invention is programmed to respond (or an operator will manually respond) by a combination of decreasing the methanol feed rate and raising the temperature in the reactor.
• the feedback mechanism in accordance with the present invention indicates that the methyl acetate concentration in the reactor is falling below desired levels, the information may be used to increase methyl acetate levels rather than lowering them.
• the process of the present invention is programmed to respond (or an operator will manually respond) by increasing the methanol feed rate to the reactor, which will increase the rate of formation of methyl acetate in the reactor.
• system of the present invention may be programmed to respond (or an operator will manually respond) by lowering the temperature in the reaction zone to decrease the rate at which methyl acetate is converted to acetic acid.
• system of the present invention is programmed to respond (or an operator will manually respond) by a combination of increasing the methanol feed rate and lowering the temperature in the reaction zone.
• FIG. 2 depicts an embodiment of the present invention demonstrating the placement of the components of the monitoring and feedback system.
• Any suitable density measuring device such as a nuclear or optical densitometer 20 is provided in the heavy phase heavy phase recycle line 21.
• the densitometer 20 may be any of various commercially available densitometers such as the commercial nuclear densitometers manufactured by Berthold.
• a suitable model is the Berthold LB 386-lC.
• the densitometer allows for the continuous monitoring of the heavy phase liquid density.
• samples of the heavy phase may be taken and their densities determined off-line by any suitable density measuring device, such as an optical densitometer. Using this density, as described above, the methyl acetate concentration in the reaction zone can be determined quickly and accurately.
• the system may be designed to allow the density measurement to be monitored by a system operator. Based upon desired methyl acetate concentrations in the reaction zone, the operator may use the measured density to determine when various changes are necessary in the process to maintain the methyl acetate concentration at the desired level. For example, in response to a heavy phase density measurement corresponding to a methyl acetate concentration higher than desired, the operator may choose to raise the temperature in the reactor by decreasing the flow rate to heat exchanger 10, which will have the corresponding effect of lowering the methyl acetate concentration. Alternatively, the operator may choose to decrease the methanol feed rate by increasing the flow through the methanol feed control valve 2.
• the operator may choose to adjust the methyl acetate concentration to a desired operating level by employing a combination of corrections involving temperature and feed rate control.
• the operator may seek corrective action in the opposite direction by increasing the flow rate through the heat exchanger 10 or increasing the flow rate through the methanol feed control valve or a combination of the two to raise the methyl acetate concentration.
• the system is equipped with an automatic feedback control process.
• the densitometer 20 may be wired, or otherwise in communication with, a control loop with a feedback control means 23 providing feedback to the control valve of the heat exchanger 10 and/or the control valve 2 for the methanol feed.
• the system maybe programmed to automatically control operation of the heat exchanger or the methanol feed or a combination of the two in response to deviations of the methyl acetate concentration from desired levels as indicated by the heavy phase density.

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• 2002
  • 2002-01-28 US US10/058,547 patent/US6677480B2/en not_active Expired - Lifetime
• 2003
  • 2003-01-09 BR BR0306690-8A patent/BR0306690A/pt not_active IP Right Cessation
  • 2003-01-09 ES ES03701257T patent/ES2327401T3/es not_active Expired - Lifetime
  • 2003-01-09 JP JP2003563990A patent/JP4263103B2/ja not_active Expired - Fee Related
  • 2003-01-09 MX MXPA04007238A patent/MXPA04007238A/es active IP Right Grant
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