The invention relates generally to membrane separation processes and more specifically to a process for separating aluminum alkyls, such as trialkylaluminums, from olefins using pore size modified inorganic membranes.
Aluminum alkyls and especially triethylaluminum are used in the so-called Ziegler chain growth process to prepare linear alpha-olefins. The process involves the reaction of triethylaluminum (TEA) and ethylene at temperatures in the range of 200.degree.-500.degree. F. and pressures in the range of 2000 to 5000 psig to yield a mixture of tri-C.sub.2 -C.sub.20+ alkylaluminum compounds and C.sub.2 -C.sub.20 olefins. Linear alpha-olefins are then recovered from the alkylaluminum compounds by olefin displacement using ethylene, 1-butene or mixtures thereof as the displacing olefin. To the extent possible, the aluminum alkyls are separated from the olefins and recycled to the chain growth reactor. One limiting factor in the amount of aluminum alkyls that can be recovered is that when the boiling points of the aluminum alkyls and the olefins are similar, and/or when heat causes decomposition, separation by distillation is difficult. The aluminum alkyls can be removed by oxidation and hydrolysis to generate alcohols, but this causes a loss of the expensive aluminum alkyl materials and the alcohols are generally of little or no commercial value.
U.S. Pat. No. 3,149,179 discloses the use of membrane permeation to separate TEA from linear olefins. Preferred membranes are those selected from synthetically derived plastics which are predominantly hydrocarbon in nature and especially polyethylene. Polyesters are also mentioned. No working example is provided and the process is described in mostly general terms. In fact, we have found that a successful membrane separation of aluminum alkyls and alpha-olefins is difficult to achieve even considering the advances in membrane technology which have occurred in the approximately 30 years since the issuance of the U.S. Pat. No. 3,149,179. In our copending application Ser. No. 08/008,951, filed Jan. 26, 1993, a process is described which successfully separates aluminum alkyls from alpha-olefins by using a polyphenylene oxide-derived membrane at elevated pressures.