Coke deposition control

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

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. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Coke builds up on the catalyst since the start up of operation. In the first weeks of operation, an amount between 5% and 8% of coke accumulates on the catalyst. The rate of deposition decreases with time on stream, a careful monitoring of temperature and of feed/H2 ratio is the basis for controlling deposition. Coke deposition primarily affects the hydrogenation reactions (and so denitrogenation), but the deposition rate determines the catalyst life. As mentioned above, deactivation is compensated by an increase in temperature (and some times in pressure, when denitrogenation has to be adjusted, as well). However, increasing severity, increases coke deposition and shorten catalyst life. 

Catalysis of 12-membered zeolites, H-mordenlte (HM), HY, and HL was studied In the alkylation of biphenyl. The para-selectlvltles were up to 70% for Isopropylblphenyl (IPBP), and 80% for dllsopropylblphenyl (DIBP) In HM catalyzed Isopropylatlon. Catalysis of HY and HL zeolites was nonselectlve. These differences depend on differences In pore structure of zeolites. Catalysis of HM to give the least bulky Isomer Is controlled shape-selectlvely by sterlc restriction of the transition state and by the entrance of IPBP Isomers. Alkylation with HY and HL Is controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealuminatlon of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. 

Evaluation of Fuel Properties with Coke Deposition. Considerable effort has been spent in the development of a test based on either fuel properties or bench-scale apparatus which will correlate with the coking performance of fuels in engines and provide a control method for fuel specifications. As few planned engine data are available, much of the effort has been to relate the results to single-combustor testing. 

In operation, preheated feedstock meets a controlled stream of hot. regenerated catalyst. Vaporized oil and catalyst ascend in the riser, such that the catalyst particles are suspended in a dilute phase. Essentially all of the cracking occurs in the riser. The catalyst particles are separated from the cracked vapors at the end of the riser and the catalyst containing a coke deposit is relumed in the regenerator. The cracked vapors puss through one or more cyclones located in the upper portion of the reactor and proceed to Ihe fractionator (main column) thai produces the side streams indicated. 

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... 

The coke deposited on the catalyst is burned off in the regenerator along with the coke formed during the cracking of the gas oil fraction. If the feedstock contains high proportions of metals, control of the metals on the catalyst requires excessive amounts of catalyst withdrawal and fresh catalyst addition. This problem can be addressed by feedstock pretreatment. 

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. 

Introduction of Re and S increased selectivity more effectively than coke formation. Coke deposited on metal sites first. Presulfiding deactivates metal sites toward coke formation. Metal sites control overall deactivation.76 ... 

Steam reforming of hydrocarbons has become the most widely used process for producing hydrogen. One of the chief problems In the process Is the deposition of coke on the catalyst. To control coke deposition, high steam to hydrocarbon ratios, n, are used. However, excess steam must be recycled and It Is desirable to minimize the magnitude of the recycle stream for economy. Most of the research on this reaction has focused mainly on kinetic and mechanistic considerations of the steam-methane reaction at high values of n to avoid carbon deposition ( L 4). Therefore, the primary objective of this studyis to determine experimentally the minimum value of n for the coke-free operation at various temperatures for a commercial catalyst. 

In this study we address hydrocracking of VGO at moderate hydrogen pressures (30 bar) and elevated temperatures (450°C) using catalysts with little or no acidity [1]. The moderate pressures are attractive from a capital investment point of view. A potential drawback could be that the severe conditions lead to considerable coke deposition on the catalyst. In order to control the level of catalyst coking a careful balance of catalyst and process parameters is a prerequisite. 

In order to control and monitor the coke deposition during heavy oil processing a quantitative model is mandatory. Often the pragmatic approach of a direct relationship between process parameters and the catalyst performance with time is used. It has been argued, however, that the "performance with time model beneficially starts from a description of the coke deposited followed by a relation between coke content and performance [2). The latter approach will be followed in this paper. 

Three different zeolites (USY-zeolite, H-ZSM-5 and H-mordenite) were investigated in a computer controlled experimental equipment under supercritical conditions using the disproportionation of ethylbenzene as test reaction and butane or pentane as an inert gas. Experiments were carried out at a pressure of 50 bar, a flow rate of 450 ml/min (at standard temperature and pressure), a range of temperatures (573 - 673 K) and 0.8 as molar fraction of ethylbenzene (EB) in the feed. The results showed that an extraction of coke deposited on the catalysts strongly depends on the physico-chemical properties of the catalysts. Coke deposited on Lewis centres can be more easily dissolved by supercritical fluid than that on Brnsted centres. 

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]. 

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. 

The catalyst system a ZSMS zeolite, which operates with a LHSV of 15 has an overall life 1.5 years. Infrequent intennedia r enerationsl controlled combustion are required to rmnove the coke deposits formed. These operations are carried out when the reaction tonperature reaches 315 C... 

Subsequent investigations, including IINS, were carried out to characterize the various resistances of such cokes to controlled after-treatments, such as oxidation or hydrogasification processes, as a basis for determining the feasibility of catalyst reactivation. The presence of metallic contaminants (iron, cobalt, and nickel) was of relevance, not only to the deposition of cokes and the catalytic transformation of the carbon structure, but also to the dynamic processes in the controlled decomposition of the material in catalyst regeneration procedures 50). 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

In Fig. 41, distinct differences in the amounts of chemisorbed pyridine can be seen for HZSM-5 samples coked by n-hexane and mesitylene cracking. Whereas coke from mesitylene only slightly inhibits the pyridine chemisorption, coke from n-hexane leads to a much stronger inhibiting effect. This result supports the model of controlled coke deposition derived... 

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