Steam cracking Severity

Operating problems with palladium eatalysts have been associated with increasingly high volumes of acetylene in the process gas, whieh is a result of increased steam cracking severity to improve ethylene yields. Both front-end and tail-end reactors now include several adiabatic beds, with interbed cooling, to control reaction and remove the excessive heat evolved as acetylene is hydrogenated. 

Significant products from a typical steam cracker are ethylene, propylene, butadiene, and pyrolysis gasoline. Typical wt % yields for butylenes from a steam cracker for different feedstocks are ethane, 0.3 propane, 1.2 50% ethane/50% propane mixture, 0.8 butane, 2.8 hill-range naphtha, 7.3 light gas oil, 4.3. A typical steam cracking plant cracks a mixture of feedstocks that results in butylenes yields of about 1% to 4%. These yields can be increased by almost 50% if cracking severity is lowered to maximize propylene production instead of ethylene. 

Products from steam cracking naphtha at high severities ... 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

PVC production, on the other hand, is carried out by first high severity steam cracking of gas oil to produce ethylene. Vinyl chloride monomer (VCM) is then... 

Steam-cracking reactors typically consist of several steel tubes, perhaps 100 m long and 4 in. in diameter in a tube furnace with reactants and steam fed through the several tubes in parallel. The ceramic fined furnace is heated by burning natural gas at the walls to heat the tubes to 900°C by radiation. The reactor is fed by ethane and steam in a ratio of 1 1 to 1 3 at just above atmospheric pressure. The residence time in a typical reactor is approximately 1 sec, and each tube produces approximately 100 tons/day of ethylene. We will return to olefins and steam cracking in Chapter 4. 

These processes are specifically designed for ethylene production but they also yield C4 hydrocarbons as coproducts. The amount of C4 compounds produced depends on the feedstock, the cracking method, and cracking severity. Steam cracking of naphtha provides better yields than does catalytic cracking of gas oil. With more severe steam cracking both butenes and overall C4 productions decrease, whereas the relative amount of 1,3-butadiene increases. Overall C4 yields of 4-6% may be achieved. 

Steam cracker tar (SCT) is a by-product from the steam cracking of naphtha or gas oils to produce ethylene. The characteristics and yield of SCT is dependent on the feed characteristics, the plant design and severity of cracking,... 

Should MTBE be banned, what would be the logical replacement(s) There are several options available. Several refiners opted to build MTBE capacity and avoid purchasing the ether on the open market. MTBE units were an option to use the facility s isobutylenes. Several licensed processes can be used to convert existing MTBE units. Kvaerner and Lyondell Chemical Co. offer technologies to convert an MTBE unit to produce iso-octane, as shown in Fig. 18.27.12 Snamprogetti SpA and CDTECH also have an iso-octene/iso-octane process. These processes can use various feedstocks such as pure iso-butane, steam-cracked C4 raffinate, 50/50 iso-butane/iso-butene feeds, and FCC butane-butane streams. The process selectively dimerizes C4 olefins to iso-octene and then hydrogenates the iso-octene (di-iso-butene) into iso-octane. The processes were developed to provide an alternative to MTBE. The dimerization reactor uses a catalyst similar to that for MTBE processes thus, the MTBE reactor can easily be converted to... 

The location of the minimum indicates the most economic steam dilution. Higher cracking severity and heavier feed stock shifts the optimum steam dilution to higher values. Existing naphtha furnaces operate mostly with steam dilution between 0.5 and 0.6. An example of the reduction of steam dilution for existing furnaces is discussed at the end of this chapter. 

Steam cracking of ethane is the most widely used process for making ethylene. U.S. 6,578,378 (to Technip-Coflexip) gives a typical ethane cracker product composition and describes an improved separation process for ethylene recovery. U.S. 5,990,370 (to BP) gives yields for ethane, propane, and mixtures. U.S. 5,271,827 (to Stone Webster) gives details of furnace design and yields for a naphtha feed. Several other separation schemes for ethylene and propylene recovery are described in the literature. Estimate the cost of production for a new steam cracking facility that produces 1 million metric tons per year of ethylene and 600,000 metric tons per year of propylene. What feedstock would you recommend ... 

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