A pilot unit for steam cracking equipped with a transfer line heat exchanger (TLE) that allows for the study of coke deposition in both the reactor and the TLE is presented. The reactor and TLE are made of Incoloy 800HT. The duration of a coking run typically amounts to 32.4 ks. The influence of different dimethyl disulfide (DMDS) addition procedures, i.e., continuous addition, presulfidation and presulfidation followed by continuous addition, on CO production and on coke deposition in the reactor and in the TLE during naphtha cracking is investigated. Presulfidation reduces CO production. However, to obtain a low and stable CO production, continuous addition of sulfur is required. The influence of sulfur addition on coke formation in the reactor can strongly differ from its influence on coke formation in the TLE. In the reactor, as well as in the TLE, the observed influence of sulfur addition is complex and strongly depends on the technique used. The optimal operating conditions for reducing CO production and minimizing coke formation consist of presulfidation followed by continuous dosing.
Coke formation under transfer line exchanger conditions, that is, at temperatures from 623 to 873 K and atmospheric pressure, is studied in an electrobalance setup. The coking rate is initially very high (catalytic coking) and drops after a few hours to a constant value. At the studied conditions the observed coking behavior on 15Mo3 alloy pigs can be explained via a catalytic mechanism only, and contributions of the free-radical mechanism and the condensation mechanism are insignificant. Experiments with ethane and naphtha steam cracking effluents and with well-defined reaction mixtures show that the coking rate is independent of the partial pressure of ethene (0-2.7 × 10 4 Pa), ortho-xylene (0-1.0 × 10 4 Pa), heavy aromatic hydrocarbons (0-3.0 × 10 2 Pa), and also 1,3-butadiene (0-2.7 × 10 4 Pa). The rate of coke deposition depends only on the temperature and the ratio of the partial pressures of water to dihydrogen. The activation energy for initial coke formation was estimated to be ∼90 kJ/mol, a value close to the experimentally determined diffusion energy of carbon in iron.
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