Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 272

Butenes, 1-butene, cis-2-butene, trans-2-butene, and iso-butene, also known as butylenes, have a variety of commercial uses. Iso-butene is a primary reactant in the production of methyl tertiary butyl ether (MTBE), a major additive in reformulated gasoline and used to reduce emissions from automobile exhaust. Butenes are oligomerized and hydrogenated to produce higher alkanes for gasoline blend stock uses and can be reacted further to produce other commercially important products. It is estimated that 90% of butene consumption is in motor fuel applications such as alkylate, polymer gasoline, and oligomerized gasoline blend stocks. Butenes are also blended directly into gasoline and mixed with propane and butanes in LPG. Approximately 10% of the available butenes are used in chemical production where the most important products are butadiene, sec-butyl alcohol, butyl rubber, and polybutylene elastomer.

The butene derivatives are produced as by-products of many refinery processes. Due to the huge volumes of crude oil subjected to catalytic cracking, catalytic crackers are the single largest source of mixed butenes that are typically used for MTBE production. Cracking catalysts and conditions are sometimes formulated and selected to especially maximize the production of iso-butene. Steam cracking of olefins is another major source of by-product butenes.

Butenes are dehydrogenated further to produce butadiene. Butadiene is one of three copolymers in acrylonitrile-butadiene-styrene (ABS) plastic and styrene-butadiene (SB) rubber. Dehydrogenation reactions are endothermic, and those of butane and butene are no exception.

One goal of the various processes for producing either butenes or butadiene is to maximize feedstock conversion and simultaneously selectivity to the desired product isomer. For example, while mixed butenes are typically used for MTBE and polygas synthesis, polybutylene production requires higher purity 1-butene. The yield of each isomer is controlled by the reaction conditions employed. The recovered yield is controlled by the downstream separation steps applied to the mixture of product and un-reacted starting materials. The practical result of these sometimes conflicting demands are a wide range of conversion technologies and separation approaches, each more or less optimized for a specific end use application.

Maximization of the conversion of feed to product can be accomplished by reducing the vapor pressure of the products in the reactor. A common practice is to add steam to the reactor. This not only reduces the partial pressure of the products driving the conversion higher; it is typically also utilized to import the needed heat of reaction into the reaction vessel. Steam is used because it can be easily separated from the reactor effluent through condensation. Another approach is to selectively remove one of the products of the reaction from the reactor, in this case the hydrogen.

Hydrogen is removed by oxidation to produce water and also to supply some of the required heat of reaction. The direct addition of small amounts of oxygen into the reactor, typically in a specially designed reaction zone, usually with a catalyst present has been described. The risk with this solution is the indiscriminate reaction of the oxygen with either the reactants or the products.

Another approach is to control the side reactions of products with oxygen is to use a membrane reactor to effectively segregate the reactant and product hydrocarbon molecules from the oxygen hydrogen scavenger. A third approach is to utilize redox agents along with the dehydrogenation catalysts that provide a supply of reactive but not free oxygen for reaction with the product hydrogen. One difficulty with this solution is that the redox agents are reduced and consumed in the process and must be regenerated in a separate processing step.