Hybrid technologies: examples of applications

Membranes in (petro)chemical related industries

Nowadays, membrane technologies are becoming more frequently used for separation of wide varying mixtures in the (petro)chemical-related industries and can compete successfully with traditional schemes. Especially the development of novel materials for gas membrane manufacturing such as organic polymeric, hybrid organic-inorganic and inorganic will expand the use of membrane technology into new fi elds of applications.

In the petrochemical industry, olefins such as ethylene and propylene are the most important chemicals used for the production of polyolefi ns such as polyethylene, polypropylene, styrene, ethyl benzene, ethylene dichloride, acrylonitrile, and isopropanol. An important step in the manufacture of olefins is large-scale separation of the olefin from the corresponding paraffin. Furthermore, dehydrogenation, oxidative coupling of methane, steam reforming of methane and water gas shift reaction are some important reactions in petrochemical industry.

Membrane gas separation is attractive because of its simplicity and low energy cost, but it has one major drawback and that is a reverse relationship between selectivity and permeability. Nano composite
membranes, in which selectivity and permeability can simultaneously be improved, solve this problem. Petrochemical waste streams may contain phenolic compounds or aromatic amines. They are highly toxic and at high concentrations are inhibitory to biological treatment. Membrane aromatic recovery system (MARS) is a relatively new process for recovery of aromatic acids and bases.

Wastewater in petrochemical industry is currently treated by activated sludge process with pretreatment of oil/water separation. Tightening effluent regulations and increasing need for reuse of treated water have generated interest in the treatment of petrochemical wastewater with the advanced membrane bio-reactor (MBR) process.

Traditional chemical engineering separation methods rely mostly on differences in physical properties (e.g., boiling point, size, solubility) between the components of a mixture. If physical properties are similar or if a high specificity is required, separation methods that rely on chemical differences, rather than physical differences, may be useful. Already in the early nineties, possibilities of combining membrane separation and distillation gained interest. Although most membrane processes cannot produce high-purity products, it may be possible to take advantage of the energy efficiency associated with them to perform part of the separation.

Chemical synthesis could also be combined with a closely coupled membrane separation device. This
would be most useful for equilibrium processes and would require a membrane selective for the particular product. An example is the selective production of para-xylene by an equilibrium redistribution
of mixed isomeric xylenes coupled with selective transport of the product through a membrane.

The most intimate combination of a separation process with chemical synthesis occurs in a membrane reactor, in which the membrane and catalyst are one and the same. Membrane reactors can potentially increase the efficiency of chemical synthesis because the reaction and separation steps are combined into a single process.

Synergistic processes that combine chemical synthesis with distillation, sorption, and membranes can, in principle, lead to more energy-efficient and materialsefficient chemical processing, especially for equilibrium-controlled reactions.