Reactor decoking

In the process, ethylbenzene is dehydrogenated to styrene in a fixed-bed catalytic reactor. The feed stream is preheated and mixed with superheated steam before being injected into the reactor at a temperature above 490°C. The steam serves as a dilutant and decokes the catalyst, thereby extending its life. The steam also supplies the necessary heat for the endothermic dehydrogenation reaction. For our model we have chosen six reactions to represent the plant data. 

Coke formation is a problem in all TLXs, both in the cone of the TLXs and in the tubes. Coke increases the resistance to heat transfer so that less heat can be transferred (and less steam can be generated). Eventually, the thickness increases to a degree where the TLXs must be cleaned, usually manually. The reactor tubes and the TLXs must be designed and then operated such that the needs for decoking coincide for the two. As a rule, one TLX is provided for two reactor tubes. 

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

A prediction model of fouling rate of the furnace was developed to decide optimal decoking cycles of the furnace. It is a very effective tool, of course, for the design of the new plant with three hours cycle of a reactor swing. 

The use of membrane reactors allows process conditions which cannot be obtained with more conventional processes (see Chapter 10 and overviews [13]) and which allow improved yields and selectivities, the use of two simultaneously occurring reactions (e.g. the main reaction and a decoking reaction to eliminate carbon deposits), controlled supply of reactant, etc. 

Following the results of the adiabatic reactor concept it is expected that high selective membranes will further improve the economics. However, it should be recognised that the process conditions in an isothermal concept are more severe than in an adiabatic concept. In particular, decoking conditions can be a problem in using high selective membranes. Detailed calculations on the isothermal membrane reactor concept are being performed and will be reported in future. 

Visbreaking is a relatively mild thermal (noncatalytic) cracking process that is used to reduce the viscosity of residua. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing overhead. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Coke formation can occur and the coke accumulates on visbreaker walls periodic decoking (cleaning) is necessary. 

Coke and carbon oxides, both undesirable by-products, are always formed to some extent In commercial pyrolysis units. The carbon oxides, are produced when part of the coke reacts with steam that Is used as a diluent with the hydrocarbon feedstock. Most, If not all, of these undesired products are formed by surface reactions that reduce the yields of olefins and other desired products. Coke also acts to Increase heat transfer resistances through the tube walls, and most pyrolysis units must be periodically shut down for decoking of the tubes. During decoking, pure steam or steam to which a small amount of oxygen (or air) Is added Is fed to the reactor, and the coke Is oxidized to produce carbon oxides. 

Fixed bed decoking involves time-dependent profiles of the oxygen concentration and the carbon load both within the particles (pore diffusion) and within the fixed bed (moving reaction zone). The reaction zone migrates through the reactor, which may lead to overheating of the catalyst, if the velocity of the zone is too fast. To model the coke burn-off process in the adiabatic fixed bed the so-called one-dimensional pseudo-homogeneous reactor model can be used. 

In steam cracking processes, the amount of coke produced and deposited on the heated pipeline walls depends on the type of fuel employed, operation conditions and the metallurgic nature of pipelines. In addition, coke is also produced in heat exchangers (where temperatures ean be between 400 and 700°C)." Coke deposits with a thickness of some millimeters/centimeters make heat transfer difficult, so the temperature in the reactor must be increased which in turn leads to higher coke formation. Moreover, coke accumulation favors a pressure drop which results in reduced production of olefins. Over time, production must be frequently stopped to remove coke (decoking) from the reaction system. Decoking is carried out with a mixture of water and air to burn the coke. This process is undesirable as it results in a drop in the production of olefins, is expensive to maintain and reduces the longevity of the pipelines. 

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