Ethylene off-gas

Hydrogen and carbon dioxide recovery from steam-methane reformer ofF-gas, coke oven gas, ethylene ofF-gas, coal gasiheation, etc. PSA [16. 17]... 

Ethylene off-gas 80-90%voi H2 CO, CO2, CH4 balance Pressure 15-30 bar H2 purity 95-97% H2 recovery 80-90% Final destination make-up hydrogen compressor Hj purity 99.5% H2 recovery 80-90% Final destination primary somce of make up hydrogen for hydrocracker H2 purity 95% H2 pressure 15-30 bar Final destination make-up hydrogen compressor... 

The ethylene feedstock used in most plants is of high purity and contains 200—2000 ppm of ethane as the only significant impurity. Ethane is inert in the reactor and is rejected from the plant in the vent gas for use as fuel. Dilute gas streams, such as treated fluid-catalytic cracking (FCC) off-gas from refineries with ethylene concentrations as low as 10%, have also been used as the ethylene feedstock. The refinery FCC off-gas, which is otherwise used as fuel, can be an attractive source of ethylene even with the added costs of the treatments needed to remove undesirable impurities such as acetylene and higher olefins. Its use for ethylbenzene production, however, is limited by the quantity available. Only large refineries are capable of deUvering sufficient FCC off-gas to support an ethylbenzene—styrene plant of an economical scale. 

A dephlegmator process can be used to recover ethylene—ethane and heavier hydrocarbons from fluid catalytic cracking (FCC) unit off-gas (Fig. 7). Pretreated feed gas is cooled to about 230 K and then further cooled and rectified in a dephlegmator to recover 90 to 98% of the ethylene, 99 % of the... 

Fig. 7. FCC off-gas ethane/ethylene recovery process. C3 = propane refrigerant.

The vapor-phase Badger process (Eigure 10-2), which has been commercialized since 1980, can accept dilute ethylene streams such as those produced from ECC off gas. A zeolite type heterogeneous catalyst is used in a fixed bed process. The reaction conditions are 420°C and 200-300 psi. Over 98% yield is obtained at 90% conversion." Polyethylbenzene (polyalkylated) and unreacted benzene are recycled and join the fresh feed to the reactor. The reactor effluent is fed to the benzene fractionation system to recover unreacted benzene. The bottoms... 

A few ammonia plants have been located where a hydrogen off-gas stream is available from a nearby methanol or ethylene operation (e.g., Canadian plants at Kitimat, BC and Joffre, Alberta). Gas consumption at such operations range from 25 million to 27 million BTU per tonne of ammonia, depending on specific circumstances. Perhaps more important, the capital cost of such a plant is only about 50% of the cost of a conventional plant of similar capacity because only the synthesis portion of the ammonia plant is required. However, by-product carbon dioxide is not produced and downstream urea production is therefore not possible56. 

The Mobil-Badger process has also been modified to use dilute ethylene from FCC off-gas streams as a feedstock. The dilute ethylene process was first commercialized at Shell s Stanlow U.K. refinery in 1991 and has been in continuous use ever since. 

In the past, natural gas liquids - ethane, propane and butane -were the favoured feedstock for ethylene production. Propylene was extracted from the off-gas of some of the world s largest oil refineries in the same region. In recent times, naphtha crackers and flexible fiiel crackers have been built (the favoured approach in the Far East and Europe). However, as the following Figure 1.2 illustrates, natural gas liquids (ethane, propane and butane) aeeount for the major portion of the ethylene feedstoek. 

Off-Gas Analysis. Gas samples are initially cleaned of particulates and dried to 2% moisture before analysis. Carbon monoxide and carbon dioxide are measured continuously using a Horiba Mexa-300 CO analyzer and a Horiba Mexa-200 CO2 analyzer. Syringe samples are taken downstream of the CO2 analyzer for gas chromatographic analysis, A room temperature molecular sieve 13X column is used to analyze for carbon monoxide, oxygen, and nitrogen, A Poropak Q column at 130°C is used to analyze for carbon dioxide, methane, ethane, ethylene, sulfur dioxide, and hydrogen sulfide. 

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