Steam-air decoking

Both Mitsubishi and Mitsui TLEs differ drastically from other designs. Mitsubishi offers a TLE with an integral steam dmm and cyclone for vapor—hquid separation. The pyrolysis gas flows in the shell side, and is claimed to accomplish the decoking of the furnace and the transferline exchanger in one operation. The Mitsui quench cooler uses three concentric tubes as the tube element, and requires steam—air decoking to clean the TLE (58,59). 

Coke/carbon Alkaline solutions of potassium permanganate or steam-air decoking Steam/air decoking is used on fiimace tubes where coke is removed by controlled burning... 

Double-fired, multi-point steam (or water) injection, online spalling, bi-direction steam/air decoking and other techniques enable a three-year run length for the heater and 5% savings. 

Coking heater tubes are coked. A sudden loss in heater feed will lead to coking of the tubes. If the firing rate is rapidly reduced, severe tube damage may be avoided. One can count on the need to steam-air decoke the tubes after the coker is forced down because of a foamover. 

Aft FlSdfnTih g oiit coke from inside the tu es (steam-air decoking), rememrer to Water vash th tubes to remove residual sodiurh Sf You can see themes white deppsits inside therotitlet elbow ... 

A typical coking heater run length is nine months to a year. At the end of the run the heater must be taken off-line for steam-air decoking. However, the very name "steam-air decoking is misleading. cdke 96+ shd be burned off wifh ir at all it.should bfcspall... 

Steam-air decoke the tubes. This very effective method of reducing firing requires a two- or three-day shutdown to complete. 

Coking cycles, 33-72 Coking heater, 73-80, 89 mass ve-locily/heat flux, 75 feed interruption, 75-77 velocity steam, 77 sodium, 77-78 light resid, 78 foamover, 78 parallel passes, 78 steam-air decoking, 78-79 online spalling, 79-80 run length, 89... 

Steam velocity, 294 Steam-air decoking (coking heater), 78-79... 

Steam-air decoking tubes Onstream washing convective lubes Infrared thermograph survey SO3 dew-point limit Minimize atomizing steam Install soot blowers Fix convective section air leaks... 

There are a great number of lube and seal oil systems in these units. The fluid cokers may have up to three cyclones or venturis to separate the coke fines from the overhead products. Because the products leaving these units will have to be cooled, waterside problems will occur at these temperatures (formation of iron oxides and hardness deposits will require chemical treatment). The furnace itself will be steam/air decoked, but the fuel lines will have to be periodically cleaned to reduce burner fouling. 

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... 

Figure 10. Coke yields at 850°C in successive steam cracking/air decoking cycles...

Anon., Decoking Heater Tubes with Steam, Air Saves Time. .. Pet Processingf Aug. 7, 1946, p. R-677. 

Decoking of the heater tubes in the coil visbreaking is easier and done with a steam-air. 

Regular decoking is required to deal with coke deposits in both pyrolysis coils and the TLE. The furnace is shut down and high-pressure steam and air are fed... 

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. 

Pyrolytic coke is dense with a very small surface area. Hence, it is not possible to remove after ageing by steaming [499] and it may still be difficult to bum off in air. Steam crackers are regenerated by Ifequent on-line decoking with steam [374] instead of off-line decoking with steam and air. 

Decoking The coke built up in many hydrocarbon furnace tubes during operation is removed by injecting steam and air into the tubes. 

Exhibit 7-43 also shows the product outlet line and its decoking hook-up. Line A is set up in its usual operation configuration line B is set in the decoking mode. As the steam and air are blown through the radiant tubes, any waste is carried to the sewer or, if preferred, to a drum for removal. The vent line is run to the stack. Once again, the plant layout designer... 

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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