Coking time

Yield of coke is lower as a result of partial combustion of coal. The process cannot be used to produce good coke from blends of inferior coals. The process lacks in flexibility in operation. The process needs a large coking time of about 2-3 days. 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

Figure 5) NMR intracrystalline self-diffusion coefficient Di ( ) and NMR desorption diffusivity Dd of methane sorbed in ZSM5 with increasing coking time, (reproduced with the permission from reference 18)...

The self-diffusivity of propane in the coked polycrystalline grains unveils details of coke formation in polycrystalline particles. Figure 44 shows that coking reduces the translational mobility inside the grains. The effect of intracrystalline coke deposition on the translational mobility of propane is indicated in Fig. 43. After a coking time of 1 h, only a slight increase of Di ,ra with increasing observation times occurs. After 12 h, a decrease is ob-... 

Fig. 44. Self-diffusivity of propane (10 CjHg per u.c., 293 K) as a function of the observation time t of self-difTiision for the polycrystalline grains shown in Fig. 43 after different coking times by n-hexane cracking (732) O, starting ZSM-5 Cl h, 3.6 wt% C , 12 h, 4.3 wt% C.

During mesityiene coking, the carbonaceous compounds are found to be exclusively deposited on the outer surface. For n-hexane, two stages of the coke deposition become visible At shorter coking times n-hexane is mainiy deposited in the intracrystalline space, tnus simultaneously affecting a retardation of intracrystaliine diffusion ana tracer desorption. In a second stage, similar to the behaviour observed with mesityiene, coke is predominantly deposited on the crystallite surface. 

Figure 9 shows the self-diffusion coefficients oi propane in the poly-crystalline grains after different coking times corresponding to different amounts of coke deposited <33>. In complete agreement with the proposed model, the time dependence is determined, by the amount oi coke deposits In the ire sc specimen... 

Table 5 Evolution of the 4 the coking time amount of coke and of the H/C ratio with...

Figure 26 compares values for the intracrystalline mean lifetime and Tin,ra " for methane in ZSM-5 type crystallites after different coking times and of the values of [145,187]. Depending on the applied coking compound,... 

Figure 2 shows the influence of the pH2 on the coking rate rc) and coke concentration. The coking rate formation curves have been calculated from the numerical derivative of the experimental coke-time curves. It can be seen in Figure 2 how all the rc vs. 

Fig. 8 Values for the intracrystalline mean life time tintra ( M) and the quantity (A,A) for methane in H-ZSM5 which has been coked by n-hexane (filled symbols) and mesitylene (open symbols) as a function of the coking time (methane concentration 12 molecules per unit cell measuring temperature 296 K). From [116] with permission...

After a coking time of about 20 h, a pusher machine, which travels on rails alongside on one side of the battery, removes the door of the respective coking chamber while a so-called coke guide car simultaneously opens the door on the reverse side of the chamber. The hot coke is then pushed out of the oven by the coke guide car into a coke quenching car for conveyance to the quench tower where the hot coke is cooled by wet quenching with water. The cooled coke is then finally transported to a blast furnace. 

Figure 6.5.5 shows the influence of coking time on measured and calculated temperature profiles in a coking chamber 0.43 m wide. 

These three aspects are not considered here in order to derive simple solutions for the influence of different parameters such as the chamber width on the coking time. Subsequently, the coke formation process is simply reduced to a transient heat transport process between two plane walls (heated brick wall and coking chamber), each with constant material properties, and three different (border) cases are inspected. 

In this (border) case, the coking time increases proportionally with the width Wq, Eq. (6.5.6), and thus the productivity of each coking chamber would be independent of chamber width ... 

Figure 6.5.9 Influence of chamber width on coking time (simple power law, exponent n).

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