Steam cracking formation

Steam cracking reactions are highly endothermic. Increasing temperature favors the formation of olefins, high molecular weight olefins, and aromatics. Optimum temperatures are usually selected to maximize olefin production and minimize formation of carhon deposits. 

Modem steam cracking reactors use 4-in. steel tubes 100 ft long in a tube furnace heated to -850°C. Pressures are approximately 2 atm (sufficiently above 1 atm to force reactants through the reactor), and residence times are typically 1 sec. Water (steam) interacts very httle with the hydrocarbons by homogeneoirs reactions, but more water than alkane is typically added to reduce coke formation. 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

This table, points out the low production of propylene in comparison with the steam cracking of naphtha, the high coproduction of acetylene ranging up to nearly 150 kg t ofethyiene instead of the 15 kg/t observed in naphtha steam cracking, and the substantial formation of fuel oil and tars. 

The ThermoWood process has been developed in Finland (Finnish ThermoWood Association, 2003) and both hardwoods and softwoods are available in two classes - ThermoWood D (durability) and ThermoWood S (stability), where the ThermoWood D grade is treated at a higher temperature to allow use in exterior applications out of ground contact (hazard class 2). The process uses steam to limit the extent of pyrolysis, and control crack formation. A similar system called... 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

Only about 3wt% of ethane is observed in the steam cracking products, indicating that the formation of ethylene is the preferential fate of ethyl radicals. Note that most of ethane is formed via ethyl radical H-abstraction reactions, while less than 10% is due to the recombination reaction of methyl radicals. Similarly, propane formation is mostly due to the H-abstraction reactions of propyl radicals and only marginally to the recombination of methyl and ethyl radicals. 

Finally, it is relevant to observe that this dissolution presents strong analogies with a condensation process discussed and stressed by several authors (Cai et al., 2002) as being responsible for coke formation/deposition in the TLE tube outlet section at operating temperatures of 350 450°C. Indeed this mechanism can be explained on the basis of the solubility of heavy species of the process fluid phase in the soft polymer. There has also been research into the computer generation of a network of elementary steps for coke formation during steam cracking process (Wauters and Marin, 2002). 

Inhibition of Coke Formation in Ethylene Steam Cracking... 

The effect of the decoking operation on coke formation in subsequent cracking runs was also studied and the results shown pictorially in Figure 10. On an uncoated Incoloy 800 tube, the high coke yield remained approximately constant throughout four successive steam cracking/air decoking cycles. This tube was subsequently coated with silica to produce an immediate xlO... 

A process for forming thin, adherent silica coatings in high alloy steel tubing has been developed. The silica coatings substantially reduce the rate of coke formation in laboratory pyrolysis tubes operating under ethylene steam cracking conditions. 

The effect of surfaces on the gaseous and solid products of the steam cracking of propane has been studied. The chemical nature of the surface near the reactor inlet has a significant effect on the reaction products while the surface near the exit does not. The material of the reactor tube appears to catalyze gas phase reactions as well as coke formation and gasification. Pretreatment of the reactor tube alters the chemical nature of the surface and, as a result, alters the effect of the material on the reaction products. 

The present studies were initiated in order to investigate the effect of the reactor surface on the product distribution and on the tendency for coke formation during the steam cracking of propane in a tubular reactor. Attention has been focused on correlating various effects which can arise in the system. Previous studies of the pyrolysis of propane has been reviewed recently (17, 18), and the findings of the present work are related to these studies later in this paper. 

As a result of these considerations, it would be expected that surface effects during steam cracking would be apparent in the formation of gaseous, liquid and solid products. The present studies show that this is indeed the case. 

Figure 3. Coke formation during steam cracking of propane in the quartz reactor at 850°C on foils made from different materials. Key O, steel V, Co A. Mo , Cu and 0, quartz. Conversion of C3HB — 98%. Feed gas as in Figure 2.

The present studies have been concerned with the overall effect of surfaces on reactions occuring during steam cracking. The formation of gaseous and solid products has been related to the nature of the reactor surface. Steam cracking is, however, a high temperature pyrolysis reaction in which free radical intermediates play an important role. No attempt has been made to relate the experimental results to the nature and amount of free radicals present in the system. 

Figure 7. Coke formation during steam cracking of propane at 840°C on a steel foil (Sandvik ISRelO) in a preoxidized steel reactor. The reactor surface and the foil were preoxidized for 95 min using 46% Ot in Nt at 840°C. Conversion of C3Hs 89%. Feed gas as in Figure 4.

Figure 9. Coke formation during steam cracking of propane at 850°C on steel foils (Sandvik 15RelO) in a preoxidized quartz reactor.

Using the quartz liner, coke formation on the foils was substantially less and so were the difference between the coke formation of preoxidized and prereduced foils. Quartz has been found to be catalytically inert and any effect on steam-cracking of propane should not and does not reflect the chemical nature of the surface. Gaseous and solid products obtained in the quartz reactor are shown in Figures 2, 3 and 9. The gas phase product spectrum is typical of a quartz reactor (Figure 2) and is unaffected by the foil material. The initial rate of coke formation on the foil depends on the foil material, but the rate of coke formation on all foils appears to approach a value similar to that for coke formation on coke. 

Carbonaceous deposition during steam cracking is the net result of steady state formation and removal processes. If the measured oxidation rates in water vapour did represent the removal of the deposit in situ, then this would be an extremely rapid process over the temperature range at which deposition on radiantly heated process tubes is most significant. Thus, 1 mm thickness of deposit would be oxidised by 362 mm Hg water partial pressure in 300 h at 800°C, 33 h at 900°C and 5 h at 1000°C. If a hydrocarbon, or its decomposition products, enhanced the oxidation rate these times could be decreased. Coke removal by thermal oxidation cannot be ignored, therefore, although its extent would depend on specific plant operating conditions. 

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