Encyclopedia of Renewable Energy. James G. Speight
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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 is 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.
Butane Vapor-Phase Isomerization
Isomerization applications are to provide additional feedstock for alkylation units or high-octane fractions for gasoline blending. Straight-chain paraffins (n-butane) are converted to respective iso-compounds by continuous catalytic (aluminum chloride and noble metals) processes. Natural gasoline or light straight-run gasoline can provide feed by first fractionating as a preparatory step. High volumetric yields (>95%) and 40 to 60% conversion per pass are characteristic of the isomerization reaction.
The butane vapor phase isomerization process is a process for isomerizing n-butane to iso-butane using aluminum chloride catalyst on a granular alumina support and with hydrogen chloride as a promoter. A non-regenerable aluminum chloride catalyst is employed with various carriers in a fixed-bed or liquid contactor. Platinum or other metal catalyst processes utilize a fixed-bed operation and can be regenerable or non-regenerable. The reaction conditions vary widely depending on the particular process and feedstock, 40 to 480°C (100 to 900°F) and 150 to 1,000 psi; residence time in the reactor is 10 to 40 minutes.
The butane vapor phase isomerization process is a process for isomerizing n-butane to iso-butane using aluminum chloride catalyst on a granular alumina support and with hydrogen chloride as a promoter.
Butanol
Butanol (butyl alcohol) is a four-carbon alcohol that can be obtained by fermentation from the same feedstocks used in ethanol fermentation. Some properties of butanol are superior to ethanol for fuel – butanol has more energy/ gallon than ethanol, it does not absorb water easily which unlike ethanol may allow it to be transported via gasoline pipeline, and butanol-gasoline blends have lower vapor pressure than ethanol-gasoline blends which is important in reducing evaporative hydrocarbon emissions. The obvious advantages of butanol are the high octane rating (over 100) and high energy content, only approximately 10% lower than gasoline, and subsequently approximately 50% more energy-dense than ethanol, 100% more so than methanol. The major disadvantage of butanol is the high flash point (35°C, 95°F).
Traditionally, butanol is fermented from Clostridium acetobutylicum via the so- called ABE (acetone-butanol-ethanol) fermentation. ABE fermentation yields 3 parts acetone, 6 parts butanol, and 1 part ethanol (3:6:1). However, butanol from ABE fermentation is less economic than ethanol essentially because the fermentation is impeded by low concentrations of the products (end-product inhibition) requiring larger process stream volumes, reactors, and tanks.
See also: Alcohols, Ethanol, Methanol, Propanol.
Butene
Butene, also known as butylene, is an alkene with the formula C4H8 (CH3CH=CHCH3 or CH3CH2CH=CH2) (Table B-31). The butene derivatives are colorless gases that are typically obtained by catalytic cracking of long-chain hydrocarbon derivatives during refining of crude oil. Cracking produces a mixture of products, and the butene is extracted from this by fractional distillation.
Table B-31 The defend isomers of butene (C4H8).
All four of these isomers are gases at room temperature and pressure but can be liquefied by lowering the temperature or raising the pressure. These gases are colorless, but do have distinct odors and are highly flammable. Although not naturally present in crude oil, they can be produced from by the catalytic cracking of crude oil and higher- molecular-weight crude oil products. Although they are stable compounds, the carbon-carbon double bonds make them more reactive than similar alkane derivatives, which are more inert compounds in various ways. Because of the double bonds, these 4-carbon alkene derivatives can act as monomers in the formation of polymers, as well as having other uses as petrochemical intermediates.
See also: Alkenes.
Butene Dehydrogenation
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