Coking rates

Computer controls are likewise used for stove operation, to control deUvery of the hot blast. High hot blast temperatures are generally desirable, as these reduce the coke rate. Control of the flame temperature in the raceway is effected by controlled additions to the hot blast, primarily of moisture. Injectants into the tuyeres such as coal, oil, and natural gas are often used to replace some of the coke. The effect of these injectants on flame temperature must be accounted for, and compensation is performed by lowering moisture or adding oxygen. 

Worldwide demand for blast furnace coke has decreased over the past decade. Although, as shown in Figure 1, blast furnace hot metal production (pig iron) increased by about 4% from 1980 to 1990, coke production decreased by about 2% over the same time period (3). This discrepancy of increased hot metal and decreased coke production is accounted for by steady improvement in the amounts of coke required to produce pig iron. Increased technical capabihties, although not universally implemented, have allowed for about a 10% decrease in coke rate, ie, coke consumed per pig iron produced, because of better specification of coke quaUty and improvements in blast furnace instmmentation, understanding, and operation methods (4). As more blast furnaces implement injection of coal into blast furnaces, additional reduction in coke rate is expected. In some countries that have aggressively adopted coal injection techniques, coke rates have been lowered by 25% (4). 

Fig. 3. Weight of coke formed (AQ and coking rate (r) in ethane cracking as a function of time (51).

Aside from the above reforming reactions, a small amount of feed components are converted to polymeric hydrogen deficient products which deposit on the catalyst as "coke." A coke buildup results in activity and selectivity loss which ultimately requires catalyst regeneration. In semi-regenerative operation, the coking rate is maintained at a low level to provide cycles of at least three to six months. In cyclic units, coking conditions are inherently much more severe so that frequent regenerations are required. 

These factors have prompted two principal thrusts in ironmaking development. First, progress continues to be made in increasing blast furnace productivity and in decreasing coke rates. Coal (qv) injection to replace coke units has assumed a prominent role. Coal replaces coke on a nearly 1 1 mass basis, and coal injection rates of up to 250 kg/t of hot metal (thm) have been achieved. Injection of oxygen and other reductants besides coal are expected to be used more extensively. Increased additions of scrap, DRI, and HBI are expected to play a significant role in efforts to boost productivity and decrease coke rates. 

Higher pressure operation in the LC-Finer tends toward a decrease in the catalyst deactivation rate for conversion. This is believed to be due to a lower coking rate at a higher hydrogen partial pressure. 

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. 

Numerous laboratory and plant tests have been made in coils constructed of materials with nickel- or iron-free surfaces e.g., quartz glass, silicon-coated and aluminum-coated steels, or ceramics. Filamentous coke was often completely absent, and the overall levels of coking were much reduced. Furthermore, the coking rates were essentially identical during the entire pyrolysis run. 

The hydrogen content in the gas influences the coke formation rate [7] as well. In a recent article Hou et al. [19] showed the influence of the removal of hydrogen on coking rates in a membrane steam reformer using palladium membranes. The need of a minimum concentration of hydrogen is of special importance when operating a membrane steam reformer, because it limits the process conditions at which such a reactor can be operated. 

Coking rate is much slower than the main reactions rate. 

The temperature determines for a large part the coking rate and the composition of coke. This can be related to the influence of this parameter on ... 

A new reactor concept for the study of catalyst deactivation is presented, it consists of the combination of an electrobalance and a recycle reactor. With the electrobalance, the coke content on the catalyst is measured continuously. The recycle reactor operates gradientlessly at high conversion, with on-line gas chromatographic analysis of the effluent. Thus, the catalyst activity and product selectivities may be coupled directly with the coke content and the coking rate on the catalyst. 

Figure 7 Coking rate as a function of n-hexane conversion to propylene for five experiments at 450 °C.

Since DBT does not affect the coking rate, it is possible to measure HDS activity while coking the catalyst with pyrene. Results are shown in Fig. 7 for three repeat tests of HDS activity as a function of run length. The three tests were operated for different periods of time 40 hours, 65 hours and 110 hours. The resultant levels of carbon for the samples aged 40 and 110 run hours fit (9.5% wt and 13.5% wt, respectively) the data in Fig. 6 perfectly. However, the carbon level found for the 65 run hour aged sample was somewhat larger than expected, 15.7% wt vs. the expected 12.5% wt. The reason for deposition of the additional coke is unresolved. The data in Fig. 7 (solid boxes) show an unexpected activity drop between run hour 40 and 50. This activity drop is most likely caused by coke deposition on the catalyst (coke is the only source of deactivation during these runs ). We... 

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