Coke deposit removal

The cracked products leave as overhead materials, and coke deposits form on the inner surface of the dmm. To provide continuous operation, two dmms are used while one dmm is on-stream, the one off-stream is being cleaned, steamed, water-cooled, and decoked in the same time interval. The temperature in the coke dmm is in the range of 415—450°C with pressures in the range of 103—621 kPa (15—90 psi). Overhead products go to the fractionator, where naphtha and heating oil fractions are recovered. The nonvolatile material is combined with preheated fresh feed and returned to the furnace. The coke dmm is usually on stream for about 24 hours before becoming filled with porous coke, after which the coke is removed hydraulically. 

Catalysts that do not contain potassium lose activity very quickly because of coke deposition on the surface of the catalyst. Chemical changes that occur when the catalyst is removed from the operating environment make it very difficult to determine the nature of most of the promoter elements during the reaction, but potassium is always found to be present as potassium carbonate in the used catalyst. The other promoters are claimed to increase selectivity and the operating stabiUty of the catalyst. 

Regeneration of noble metal catalysts to remove coke deposits can successfully restore the activity, selectivity, and stabiUty performance of the original fresh catalyst (6—17). The basic steps of regeneration are carbon bum, oxidation, and reduction. Controlling each step of the regeneration procedure is important if permanent catalyst damage is to be avoided. 

Hydrocarbons and carbonized or coke deposits can be removed by chromic acid. The chromic acid oxidizes the binders holding the deposits together. Use a 10 to 20% solution for 12 to 24 hours at 190 to 200 °F. Chromic acid cannot be effectively inhibited and is not suitable for cleaning copper, brass, aluminum, zinc, or cast iron because these are all rapidly attacked. 

Water and organic molecules occluded during the synthesis were removed from the intracrystalline volume as follows. The solids were slowly heated (5°C/min) in a N2 flow up to 550°C and held at this temperature for 2h. The coke deposit resulting from the non-oxidative degradation of the organics was then... 

As already discussed, if a catalyst is deactivated via coking, it could be regenerated so that its initial catalytic activity is restored. The treatment at high temperatures in an oxygen-rich atmosphere can burn off the coke deposited and the catalyst may regain its activity. Moreover, if the reduced activity is a temporary event caused by an inhibitor, die removal of the inhibiting substance of the feed can restore the catalyst to its initial potential. 

The technical development of petroleum coking has been inseparable from the development of thermal cracking. Untold millions have been spent on research and development trying to eliminate the formation of petroleum coke. Added millions have been spent learning how to prevent coke from forming in heating coils and making the coke deposit where it could be removed most conveniently (18). 

Some of the principles used in Thermofor catalytic cracking have been applied to a coking operation. Coke itself, instead of catalyst, is the solid circulated. The coke is heated in the regenerator of the unit and more coke is deposited on the hot moving solid in the reactor of the unit. Appropriate proportions of the coke are removed continuously as the process proceeds (84). 

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

It is desirable to operate the molecular sieve bed in the adsorption mode at the same temperature as in the desorption mode. Sista and Srivastava (16) show that temperatures in excess of 533 K are needed to desorb by vacuum C12 to C32 n-paraffins from type 5A molecular sieves at a pressure of 13 Pa (0.1 mm Hg). Only 5% of 2-C32 is removed at 636 K. Asher e al. (14) show that, whereas it is possible to remove 98% of Cg/C g n-paraffins from type 5A molecular sieves with ammonia at 589 K, only 79% removal is attained with C15/C33 n-paraffins even though the temperature is higher (658 K). Some of the retained material over a long period of exposure to high temperature gradually forms a carbonaceous deposit which reduces the adsorption capacity of the molecular sieve this coke deposit must be occasionally removed by a controlled oxidation step which eventually reduces molecular sieve life. Desorption rates increase with... 

In the regenerator, coke deposited on the catalyst is partially burned to form carbon monoxide in order to reduce iron tetroxide and to act as a heat supply. In the desulfurizer, sulfur in the solid catalyst is removed and recovered as molten sulfur in the final recovery stage. 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

Coke deposition begins on metal site and then continues on acidic catalysts sites. When combustion of coke is carried out in order to regenerate the catalyst, it is removed from the metal sites first.70 ... 

Finally, coke deposits were efficiently removed from the catalyst by steam and carbon dioxide gasification, which restored the initial catalytic activity. 

Reversible deactivation, either by active site removal or by pore blockage, is observed when the amount of coke deposited is limited and readily eliminated. 

Coke, deposited on a catalyst, may be removed by one of several reactions oxidation, reaction with water to form carbon monoxide and hydrogen methanation, and the Boudouart reactions. 

Figure 7 shows the relative areas of different acid centres on ZSM-5 in dependences on temperature and pressure. It can be found that the acid centres were less reduced at 623 K under supercritical conditions than those at normal pressure. Especially on Lewis centres the coking tendency is weak. This implies that the coke deposited on Lewis centres may be loosely built and can be easily removed by supercritical fluid. At 673 K the acid centres of ZSM-5 disappeared almost totally. This indicates that coking tendency increases more quickly with increasing temperature than the ability of coke extraction. 

The use of a fluidized-bed reactor has a number of advantages in the MTO process. The moving bed of catalyst allows the continuous movement of a portion of used catalyst to a separate regeneration vessel for removal of coke deposits by burning with air. Thus, a constant catalyst activity and product composition can be maintained in the MTO reactor. Figure 12.10 demonstrates the stability of a 90 day operation in the fluidized-bed MTO demonstration unit at the Norsk Hydro Research Center in Porsgrunn, Norway. A fluidized-bed reactor also allows for... 

Supercritical fluid extraction is a potentially viable technique in removing carbonaceous coke deposits from hydrotreating catalysts. 

To reactivate a coked catalyst, the coke can usually be removed by burning off the deposits at a controlled temperature with a mixture of air and an inert diluent, such as nitrogen or steam. The temperature level at which the coke deposits ignite has to be determined experimentally. The allowable 02 content in the air-diluent mixture can be calculated [2]. 

Coke deposition on the catalyst is increased. There needs to be efficient ways to regenerate the catalyst and to remove the excess heat produced in the regenerator. 

In the case of a solid heat carrier, the preheated feedstock is placed in contact with a refractory mass raised to a hi temperature. Cracking lowers the temperature and generates coke deposits that must be removed. The state of the solid and the inifiai operating condidons are restored by combustion. These operations can take place m the same reactor c> clically on a hxed r ractory (Wulff process) or in distinct units in which the solid exists in the form of moving or fluidized particle beds. In this case the hydrocarbon feedstock is injected in the combustion gases. 

In industrial practice, however, the most widespread technique consists in passmg a mixture of hydrocarbons and steam through tubes placed in a furnace. The hydrocarbons, which are raised to high temperature, are pyrolysed and the resulting products are separated after a rapid quench. Coke deposits are periodically removed by controlled combustion. This is the technology of steam cracking, which is the main focus of this chapter. 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

The catalyst is sensitive to sulfur and arsenic poisoning (the Utter being a permanent poison). Natural gas must, therefore be desulfurized. Carbon and coke deposits also damage the catalyst and must be removed by steam or by burning off with air. 

---