Published September 2002
Vinyl Acetate Monomer (VAM) is conventionally produced in gas-phase by reacting acetic acid, oxygen and ethylene in the presence of an alumina- (or silica) supported palladium catalyst. The palladium is used in conjunction with gold along with an alkali metal. The oxyacetylation (also termed as acetoxylation) of ethylene for VAM production is carried out in fixed-bed reactor.
A new approach to produce VAM is to use a fluidized-bed process in which the reactants in gas phase are contacted continuously over (small-sized) supported catalytic particles under fluidized conditions. This approach has been practically demonstrated in the form of a commercial plant by BP Chemical Limited (BP) at Hull (United Kingdom). For a fluidized-bed VAM process, which runs under more or less the same reaction conditions i.e. 302 – 320°F (150 –160°C) and 130 – 150 psia (9.1 – 10.5 kg/cm2) as a fixed-bed process, claimed benefits include increased catalyst life due to decreased hot spots which is typical about fixed-bed reactor and technology's capability of on-process addition and withdrawal of (make-up) catalyst in and out of reactor that maintains the catalyst performance and thus eliminates complete catalyst change-outs requiring reactor shutdowns. Another benefit as process commercializer BP claims emanates from higher production rate, which may be achieved as higher oxygen levels can be safely fed into the fluidized- bed reactor without producing flammable concentrations of reactants in the reaction mixture.
As for catalyst, whose exact formulation is proprietary and currently not in public domain, a higher activity, increased selectivity to VAM and excellent attrition resistance are the major attributes added to the system. The catalyst essentially consists of palladium (as key component), cerium and (or) gold as catalyst promoter, and sodium or potassium as co-promoter. A VAM selectivity of 91.8 – 98.9% (based on molar conversion of ethylene) is claimed with a catalytic activity ranging from 1858 to 2774 gm/hr of VAM per kg of catalyst. VAM selectivity and catalyst activity generally vary in opposite directions, other things being the same. The VAM recovery section of the newprocess is largely the same as in conventional fixed-bed plant. A reduction in capital investment of up to 30% is claimed for grass roots plant based on this new technology.
Our estimates indicate that a grassroots 600-million lb/yr (272,000 metric ton/yr) fluidized bed plant may yield up to 25% capital savings over a conventional fixed-bed VAM plant of above capacity as described in PEP Report 15B (published in January 1996).