Heavy coking

Figure 11.9 Picture of reaction zone in an original reactor with significant pressure increase in <4h due to heavy coking.

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Table II compares two atmospheric resids, West Coast and Kuwait, in a traditional manner. The obvious differences include sulfur, nitrogen, asphaltenes, total metals and mid-boiling point. Apart from sulfur content, one might surmise a greater catalyst demand by the West Coast feedstock in that the boxed values suggest heavy coke laydown and metals deposition. Neither of the sulfur values is boxed because there is no indication as to (1) what fraction of the sulfur is refractory or "hard" sulfur, nor (2) the degree of desulfurization to be achieved.

Figures 1A and 1B show the adsorption isotherms of xenon on the Na, H-ZSM-5 and H-ZSM-5 zeolites, respectively. From the comparison, one sees that xenon uptake decreases slightly (about 10%) with coke content in the Na, H-ZSM-5 with a low (1%) coke content, on zeolite H-ZSM-5, and decreases only slightly more with heavy coking (12%).

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

Catalytic distillation essentially eliminates catalyst fouling because the fractionation removes heavy coke precursors from the catalyst zone before coke can form and foul the catalyst bed. The estimated ISBL (inside battery limits) capital cost for 35,000bpd CDHydro/CDHDS unit with 92% desulfurization is US 25 million, and the direct operating cost including utilities, catalyst, hydrogen, and octane replace-... 

To complete this study we performed a dynamic experiment on the diffusion of DMP in the 10% coked pellet. It should be recalled that the kinetic diameter of the probe is 0.6 nm after heavy coking it is reasonable to think that the diameter of the hexagonal openings (0.78 nm) of the Y zeolite is seriously reduced, leading to a decrease in the rate of diffusion of the molecule in the pellet, it being possible then to study this diffusion by 1-D NMR imaging. 

The nature of crude oils depends on their source. Initial separation into components is carried out by atmospheric and vacuum distillation. Heavy ends are particular boiling point cuts, which can include atmospheric gas oil (250-350°C), atmospheric residues (350°C+) vacuum gas oil (350-5S0°C) and vacuum residues (5S0°C+). The descriptions are based on boiling points and, within a particular distillation cut, various chemical species can be identified. These include saturated and unsaturated hydrocarbons, aromatic and polyaromatic hydrocarbons and inorganic atoms such as V, Ni, and S, which are associated with large organic molecules [5]. As a result of this complexity, the composition of the boiling cuts is often described in terms of their content of oils, resins and asphaltenes [6,7,8], the relative amounts of which vary depending on the cut and the source of the crude [6] Of these species, asphaltenes are particularly important in the present context since they are known to be associated with heavy coke formation [7,8]. 

Thermal dehydrochlorination of 1,1-dichloroethane at about 820 K is generally used for the production of vinylchloride. However, the process suffers from heavy coke deposition on the reactor walls, and catalytic reactions operating at lower temperatures were investigated in industry. Carbons were found to catalyze the dehydrochlorination (DHC) of alkyl chlorides to the corresponding alkenes. This reaction had been studied in 1933 for its suitability in the production of vinyl chloride. A list of early patents is given in ref. 170. Formation of 1-butene from... 

The optimum conditions of radiation processing allow production up to 80 mass% motor fuels including up to 20% gasoline and 60% diesel fractions. The by-products of radiation-induced cracking are heavy coking residue (up to 10%) and the gas mixture (up to 10%), containing ethylene and other unsaturated compounds valuable for chemical industry. 

Bleken et al. (2011) have compared the methanol conversion of four kinds of catalysts with 10-membered ring three-dimensional pore structure (i.e., IMF, TUN, MEL, and MFI). They found that although aU catalysts have 10-ring cross channel, but there are differences between them in terms of life time and coke compositions. Since the cross channels in IMF and TUN structure have wide space near the cross, which allows the formation of heavy coke species, the zeolites (IM-5 and TUN-9) having IMF and TUN structure appear rapid deactivation in methanol conversion reaction. On the contrary, zeolites with MEL and MFI structure (such as ZSM-11 and ZSM-5), in which the space near the cross is relatively narrow and Hmits the formation of coke compositions, have a long Hfe time in the methanol conversion reaction. In this case, there are no heavy coke compositions in the channels, and the deactivation is mainly caused by the coke formation on external surface of zeolites. 

In addition to coke formation catalyzed by acids, heavy coke formation is observed in the reaction of methanation, Fisher-Tropsch synthesis, and steam reforming over metallic catalysts such as Fe and Ni. ° ... 

All modern refineries have conversion units, designed to transform black effluent streams into lighter products gas, gasoline, diesel fuel. Among these conversion units, coking processes take place by pyrolysis and push the cracking reaction so far that the residue from the operation is very heavy it is called coke . 

The main feedstock for catalytic reforming is heavy gasoline (80 to 180°C) available from primary distillation. If necessary, reforming also converts byproduct gasoline from processes such as visbreaking, coking, hydroconversion and heart cuts from catalytic cracking. 

The coking process produces electrode quality coke from vacuum residues of good quality (low metal and sulfur contents) or coke for fuel in the case of heavy crude or vacuum residue conversion having high impurity levels. 

Here again, this is not a refining process, properly speaking. Partial oxidation is one of the processes for the ultimate conversion of heavy residues, asphalts, coke and even coal. 

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

Cavanaugh, T.A., D.E. Blaser and R.A. Busch (1978), Fluid coking/Flexi-coking, a flexible process for upgrading heavy crudes . Japanese Petroleum Institute (JPI) Conference, Tokyo. 

As a whole, a given crude is generally used to make products most of which have positive added values. This is particularly the case for motor fuels and specialty products. However, some of the products could have negative added values, as in the case of unavoidable products like heavy fuels and certain petroleum cokes. 

Heavy oil, ie, grade nos. 4, 5, and 6, and residual fuel oils light oils, ie, no. 2 heating oil, kerosene, and jet fuel and petroleum coke are deflvered at... 

Catalytic Processes. A second group of refining operations which contribute to gas production are the catalytic cracking processes, such as fluid-bed catalytic cracking, and other variants, in which heavy gas oils are converted into gas, naphthas, fuel oil, and coke (5). 

In the semidirect process, (Fig. 23) the taw coke oven gas is cooled to condense tar and ammonia Hquor. The heavy layer, tar phase, is pumped to storage and the aqueous layer containing free and fixed ammonia is subsequendy processed in a stiH operation. Free ammonia is that which is in a form which readily dissociates by heat. Fixed ammonia is in a form which requites the presence of an alkaH, such as milk of lime, to effect the ammonia release. 

Several utility-scale demonstration facilities having power outputs in the 300-MW class have been constmcted in the United States and Europe. These started accumulating operating experience in 1995 and 1996. Other IGCC plants have been constmcted, including units fueled by petroleum coke and refinery bottoms. Advanced 500-MW class IGCC plants based around the latest heavy-duty combustion turbines are expected to be priced competitively with new pulverized-coal-fined plants utilising scmbbers. 

A typical primary distillation product pattern at a coke-oven tar-processing plant is given in Table 1. At some coke-oven distilleries, only one fraction, designated naphthalene oil, is taken between 180 and 240°C. Two fractions, light creosote or middle oil (230—300°C) and heavy creosote or heavy oil (above 300°C), are taken between the naphthalene oil and pitch. 

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