On purpose cracking units

Dehydrogenation. Dehydrogenation of / -butane was once used to make 1,3-butadiene, a precursor for synthetic mbber. There are currently no on-purpose butadiene plants operating in the United States butadiene is usually obtained as a by-product from catalytic cracking units. 

Large quantities of butadiene have become available over the past 30 yr, mostly as a by-product from the thermal cracking of naphtha and other heavy hydrocarbons. This marked shift has resulted in the shutdown of all on-purpose catalytic dehydrogenation units for butadiene production in North America, Western Europe, and the Far East. 

Figure 3.35 shows a process flow diagram of Phillips MTBE/ETBE/TAME process. This process is often called the Phillips Etherification Process. The reaction section (1,2) which receives methanol and isobutene concentrate, contains an ion exchange resin. The isobutene concentrate may be mixed olefins from a Fluid Catalytic Cracking Unit (FCCU) or steam cracker or from the on-purpose dehydration of isobutene (Phillips STAR process). High purity MTBE (99 wt%) is removed as a bottoms product from the MTBE fractionator (3). AH of the unreacted methanol is taken overhead, sent to a methanol... 

Ethylene Coproduction. Historically, butadiene was first prepared in pilot plant quantities via an uneconomical electric arc process. However, the primary source of butadiene in the world today is as a by-product of thermal pyrolysis of hydrocarbon feedstocks in ethylene production. In the United States, production of coproduct butadiene exceeded that of on-purpose butadiene for the first time in 1977 and by 1990 high cost on-purpose butadiene production was essentially eliminated in the United States (Fig. 1) (46,47). In 1996, the total US production of butadiene was 1.75 million, 93% of which was co-produced (47). Steam cracking of hydrocarbons yields varying amounts of butadiene, depending on the nature of the feedstock, the volume of ethylene produced, and the severity of the cracking operations (48-50). For example, when feedstocks are switched from atmospheric gas oils and napthas to propane and butane, yields of butadiene drop by as much as 60% (51). 

How is all this ethylene produced, and what is it used for Most hydrocarbons can be cracked to give ethylene. (See A Word About. . . Petroleum, Gasoline, and Octane Number on pages 1 02-1 03.) In the United States, the major raw material for this purpose is ethane. 

In the case of the studied FCC unit, it is found that several rules are already used for control purposes. It can be also seen from the results that the temperature of the catalyst is an important attribute because if it increases in the range of possible operation, the product quality, MON also increases. The temperature of the catalyst is fixed in the regenerator where the coke being on the catalyst s surface is burned away. The coke is formed in the reactor part of the FCC during the cracking of heavy hydrocarbons. 

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