Decoking after

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. 

As will be discussed in more detail later, current methods of decoking contribute to decreased longevity of the metal coils. Further coking rates immediately after decoking are high for perhaps 1 day. 

NMR spectrometry of Xenon-129 adsorbed in coked samples of the totally protonated H-ZSM-5 zeolite and the modified Na, H-ZSM-5 showed variations attributable to differences in coke distribution. 129Xe NMR spectrometry is extremely useful for probing microporous materials. Ito et al.(2) demonstrated, for example, that NMR spectrometry of adsorbed xenon in coke-fouled H-Y zeolite could probe the deposits after coking and the nature of the internal surfaces after decoking. The NMR results in this study are consistent with a distribution of coke restricted by size selectivity of the acidifying medium. 

Fig. 3. XRD patterns of commercially decoked catalyst A - extrudates before attrition B — extrudates after attrition C = fines after attrition ...

Fig. 2. Effect of reaction temperature on cyclohexene conversion when using catalysts fresh (A) spent (B) decoked before (D) and after (E) leached by 2%.

The coke drums are typically installed in pairs for each fired heater. When the on-line coke drum is full of a high-density hydrocarbon residue known as petroleum coke, the heated feed is switched to an empty drum. After being filled, the full coke drum is isolated from the process flow and goes through a sequence of steps to remove the coke and prepare the drum for the next cycle, which can be from as little as 10 hours to more than 24 hours. Therefore, DCU is a semi-batch operation, in which the batch stage is represented by the filling and decoking steps sequence of the drums. Table 1 presents the sequence of typical steps and the associated duration of a 20 hours cycle DCU. 

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. 

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. 

But it is somewhat unsettling to pay a person for eight hours of work and only have him on the job for half that time. In one refinery, management attempted to rectify this matter by assigning the decokers to general maintenance work after they completed cutting a full coke drum. Suddenly, the time to cut a drum of coke increased from four to seven hours. 

When the process operators begin draining down the coke drum, the decoking crew s job begins. Its first task is to remove the top head after ascertaining that the coke drum is essentially depressured. This is done by opening a bleeder on the top head. 

---