Among currently employed processes for synthesizing acetic acid, one of the most used commercially is the catalyzed carbonylation of methanol with carbon monoxide. Preferred methods of practicing this technology include so-called “low water” processes catalyzed with rhodium or iridium of the class seen in U.S. Pat. No. 5,001,259, issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25, 1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; as well as European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. The features involved in practicing a low water carbonylation process may include maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, an elevated concentration of inorganic iodide anion over and above the iodide ion that is present due to hydrogen iodide in the system. This iodide ion may be a simple salt, with lithium iodide being preferred in most cases. U.S. Pat. Nos. 5,001,259, 5,026,908, 5,144,068 and European Patent No. EP 0 161 874 B2 are herein incorporated by reference.
Generally speaking, a methanol carbonylation production line includes a reaction section, a purification section, light ends recovery and a catalyst reservoir system. In the reaction section, methanol and carbon monoxide are contacted with a rhodium or iridium catalyst in a homogenous stirred liquid phase reaction medium in a reactor to produce acetic acid. Methanol is pumped into the reactor from a methanol surge tank. The process is highly efficient, having a conversion of methanol to acetic acid of typically greater than 99 percent. The reaction section also generally includes a flash vessel coupled to the reactor which flashes a draw stream in order to remove crude product from the reaction section. The crude product is fed to a purification section which includes generally a light ends or stripper column, a drying column, auxiliary purification and optionally a finishing column. In the process, various uncondensible vent streams containing light ends, notably methyl iodide, carbon monoxide and methyl acetate are generated and fed to the light ends recovery section. These vent streams are scrubbed with a solvent to remove the light ends which are returned to the system or discarded.
Despite advances in the art, catalyst deactivation and vent losses, especially carbon monoxide losses, remain persistent inefficiencies in methanol carbonylation systems. So also, there is always a need to reduce capital and operating expense associated with vent scrubbing and product purification.
In a traditional methanol carbonylation plant, a high pressure and low pressure absorber are included wherein acetic acid is used as the scrubber solvent. The acetic acid solvent must subsequently be stripped of light ends, usually in another purification column so that the acid is not wasted. Such columns are expensive because they must be made of a highly corrosion resistant material such as zirconium alloys and so forth. Moreover, stripping light ends from the acid requires steam and contributes to operating expense. Methanol has been suggested for use as a scrubber solvent in connection with a methanol carbonylation processes as well. In this regard, see U.S. Pat. No. 5,416,237 to Aubigne et al., entitled “Process for the Production of Acetic Acid”. It is noted in the '237 patent that noncondensibles from a flash tank vapor overhead may be scrubbed countercurrently with chilled methanol. The methanol scrubber solvent residual stream is added to pure methanol and then used as feed to the reactor. See Col. 9, lines 30-42. Chinese Patent Application Publication No. 200410016120.7 discloses a method for recovering light components in vent gas from production of acetic acid/acetic anhydride by way of scrubbing with methanol and acetic acid. Another system is seen in an industrial publication entitled “Process of 200 ktpa Methanol Low Press Oxo Synthesis AA” (SWRDICI 2006) (China) (referred to as SWRDICI below). In this research publication, there is shown a vent gas treatment system including a high pressure absorber as well as a low pressure absorber. Both absorbers of this system are described as being operated utilizing methanol as a scrub fluid.
European Patent No. EP 0 759 419 proposes to reduce vent losses by injecting methanol into the reactor vent stream and catalytically producing more product in a secondary reactor, which optionally contains heterogeneous catalyst.
Catalyst deactivation and loss is generally believed due to carbon monoxide-depleted or low pressure environments in the carbonylation system as are seen in the flasher. As carbon monoxide levels fall in the catalyst solution, rhodium increasingly takes the form of rhodium triiodide which precipitates. Various modifications have been proposed in the art to address this aspect of conventional processes, perhaps the most successful being the use of lithium iodide to enhance catalyst stability and reaction rates under low water conditions. Other proposed modifications are discussed below.
U.S. Pat. No. 5,770,768 to Denis et al. discloses carbonylation systems where recycle catalyst solution from the flasher is treated with additional carbon monoxide prior to return to the reactor to increase catalyst stability.
A high pressure “converter” reactor is proposed in Chinese Patent No. ZL92108244.4 as well as SWRDICI (noted above). The converter reactor illustrated in SWRDICI is coupled to the high pressure vent scrubber and is reported to allow the reaction to proceed to a greater extent prior to flashing.
In accordance with the present invention, there is provided an improved carbonylation system with staged reaction and pre-flash removal of light ends to increase productivity and operating efficiencies.