Aspen Model Documentation
•About This Process
•Physical Property Models and Data
•Selected Simulation Results: Blocks
PEP Process Module 1 19 Aug 1999
This Aspen Plus model simulates the production of acetic acid by low pressure methanol carbonylation in the presence of a heterogeneous rhodium (Rh) catalyst and the promoter methyl iodide. It is intended to resemble the Chiyoda/UOP Acetica™ process, a novel heterogeneous methanol process for the production of acetic acid. This technology is based on a heterogeneous Rh catalyst in which the active Rh complex is chemically immobilized on a polyvinylpyridine resin. In the Aspen Plus model, the plant (base case) is designed to produce 800 million lb/yr (363,000 t/yr) of acetic acid from methanol carbonylation. The process consists of both the carbonylation and purification sections. Results from the Aspen Plus simulation shows that the purity of acetic acid obtained is 98.4%. Vent gas (4,304 lb/hr) from two absorbers and heavy by-products (1,150 lb/hr) from the bottom of the heavy-ends stripper are sent to incinerator. The process also generates approximately 65,000 lb/hr of low-pressure (150 psig) steam that supplies a significant portion of the plant energy requirements.
PEP Process Module 2 19 Aug 1999
About This Process
Current global capacity for acetic acid exceeds 6.4 million t/yr. Downstream applications for the acid are primarily as a precursor for vinyl acetate monomer synthesis, for acetic anhydride production, and as a solvent in purified terephthalic acid production. Methanol carbonylation to produce acetic acid using a homogeneous Rh catalyst accounts for more than 55% of global capacity, most of which uses the low-pressure process developed by Monsanto and licensed by BP Amoco. Together, BP Amoco and Celanese are responsible for more than 40% of the global acetic acid production. Other major players in the field include Millennium Chemicals and Acetex; several other producers have smaller capacities.
BASF introduced homogeneous methanol carbonylation in 1960. The process used an iodide-promoted cobalt catalyst and operated at an elevated temperature of 230°C (446°F) and pressure of 882 psia (60 atm). In 1970, Monsanto commercialized an improved homogeneous methanol carbonylation process using a methyl iodide- promoted Rh catalyst. Compared with the BASF process, the Monsanto process operates at much milder conditions of 150– 200°C (302– 392°F) and 440– 880 psia (30– 60 atm), and exhibits superior results with ~100% conversion of methanol and 98– 99% selectivity to acetic acid. The Monsanto process was later improved by the addition of a lithium or sodium iodide promoter to enable operation in a reduced water environment. The reduced water condition mitigates the water gas shift reaction and results in fewer by-products and reduced hydrogen iodide (HI) formation. As a result, raw materials use is improved, and downstream purification costs are lower. We evaluated the Monsanto process in PEP Report 37A, Acetic Acid and Acetic Anhydride (March 1973), and PEP Review 78-3-4, Acetic Acid by Low Pressure Carbonylation of Methane (January 1980).
The use of other less expensive catalyst such as nickel, palladium, and ruthenium in homogeneous systems has also been investigated. In 1996, BP Amoco commercialized the iridium-based Cativa technology, which operates with reactor water levels that are comparable to the improved Monsanto process. The new catalyst is most effective when used in combination with lithium and ruthenium. The Cativa technology is installed in Sterling’s Texas City acetic acid plant, which has a capacity of 990 million...