GB2100747A - Process for the fluid catalytic cracking of a hydrocarbon feedstock - Google Patents

Abstract

In a process for the fluid catalytic racking of a hydrocarbon feedstock in reactor system comprising an endothermic cracking zone (6, 7) and in exothermic regeneration zone (13), heat is exchanged between said zones not only by the circulation of catalyst (5, 6, 11), but also by passing the fresh feedstock (1) through heat exchange coils (16) in the regeneration zone (13) before contacting hot regenerated catalyst in a liftpot (4) and entering the reaction zone (6, 7). The temperature in the regeneration zone is thus kept below 725 DEG C, whereas the feedstock is preheated to at least 200 DEG C, preferably to 325-400 DEG C. Heavy and high metal-containing feedstocks may be processed in this way. <IMAGE>

Description

SPECIFICATION Process for the fluid catalytic cracking of a hydrocarbon feedstock The present invention relates to a process for the fluid catalytic cracking of a hydrocarbon feedstock in a reactor system comprising a cracking zone and a regeneration zone in communication with each other.

The fluid catalytic cracking process has for many years been considered one of the major gasoline producers for the refining industry. Typically the feedstocks employed have been distillates, e.g. gas oils and operating conditions have been selected for relatively high conversion of the gas oil feedstocks to permit maximum yields of gasoline. The cracking of gas oils to gasoline is fairly well understood and the limitations imposed on the feedstock gas oils are quite well defined. The major limitations currently placed on gas oil feedstocks are the amount of carbon or "coke" precursors and the amount of metallic compounds contained in the feedstocks.

These coke formers or precursors, which are typically high molecular weight condensed ring hydrocarbons, are primarily a function of the crude type and the boiling range of the material. Results obtained by Conradson or Rambsbottom carbon residue analyses represent approximate measures of these compounds and of the coking propensity of the raw oil. Normally it is desirable to limit the amounts of these compounds in the gas oil feedstocks to 0.5% by weight or less by the Conradson method. The reason is that if the coke precursor compounds in the gas oil are permitted to increase much, more coke may be deposited on the catalyst in the hydrocarbon reaction zone that is required for the process heat balance.Temperatures in the regeneration zone, where the coke is oxidized to CO and/or CO2, can become excessive and, equally important, the catalyst particle temperatures can rise to the point that the catalytic structure is damaged or destroyed with a resulting loss in activity.

The second major limitation imposed on gas oils is the content of organic nickel(Ni)-, vanadium(V)-, and iron(Fe)-containing compounds therein. These compounds, commonly called "porphyrins", are distilled into the high-boiling fractions of vacuum gas oils. Additionally, inorganic metal compounds may be present. Typically, the metal content should be limited to the extent that the Nickel Equivalent defined by the equation NE = Ni + 0.3 x V, and expressed in ppm by weight, is less than 0.4. The circulating catalyst adsorbs the metals mentioned almost completely and becomes poisoned. These metals, in their active state on the catalyst, depress the yield of primary gasoline product, promote various dehydrogenation reactions, and can produce large quantities of hydrogen and coke.This can lead to tremendous increases in volumes of unwanted gases and very quickly overload gas compressors and gas recovery facilities.

When one compares residual oils, such as whole crudes or atmospheric reduced crudes to typical gas oils, it is apparent that in addition to being more difficult if not impossible to vaporize completely, the residual oils contain higher amounts of "coke" precursors (as determined by Conradson or Ramsbottom carbon residue) and larger quantities of metals Ni, V and Fe. In spite of the potential processing difficulties imposed by the presence of such contaminants, the refiner has been prompted by the tightening of crude supplies to expand the characteristics of the feedstock charged to the fluid catalytic cracking process beyond those of the relatively clean distillates such as gas oils.In order to increase charge stock availability to meet gasoline and fuel oil demands higher-boiling feeds which were previously considered only marginal or unsuitable because of such contaminants are receiving increasing considerations.

The reaction conditions of the fluid catalytic cracking process typically comprise a temperature in the range of from 4250 to 5500C, a pressure in the range of from atmospheric to 4 bar abs., and a catalyst-to-oil ratio below 7. The catalyst-to-oil ratio is the quotient of the circulation rate of the catalyst and the feed rate of the hydrocarbon oil, expressed in the same mass units.

In fluid catalytic cracking units, it is necessary to remove heat at a controllable rate from the regeneration zone where spent catalyst is regenerated by burning off carbonaceous deposits, with an oxygen-containing gas, such as air, in order to maintain equilibrium cracking conditions since the exothermic heat of regeneration imparted to the catalyst is transmitted to the fresh oil feed to the cracking reactor. It is also necessary to remove heat continuously to prevent undue regeneration temperature levels tending to sinter and deactivate the catalyst by surface area reduction. When heavy hydrocarbon feeds, such as atmospheric residues, vacuum residues, and heavy crude oils are catalytically cracked, a greater amount of carbon deposits on the catalyst particles than in the cracking of feeds such as gas oil.When the spent catalyst from the catalytic cracking of such heavy hydrocarbon feeds is regenerated by combustion with an oxygen-containing gas in the regeneration zone, the heat removal problem is further aggravated since more heat is released in the regenerator than that which can be utilized in the process.

With the introduction of more active zeolite-based cracking catalysts the above signalled problems with respect to the heat balance, in particular at maximum yield conditions, have become more serious, since now the maximum product yield is achieved at a lower coke make, and therefore the heat balance is disturbed again. To avoid overcracking of the feedstock, when using the more selective and active zeolite-based cracking catalysts, a reduction in cracking severity is necessary. The cracking severity is usually expressed by the "severity parameter" (C/O/Sv), wherein C is the catalyst circulation rate, 0 is the oil feed rate and Sv is the space velocity. C/0 is commonly called the catalyst-to-oil ratio. All rates are expressed in mass units, usually tons/day. The severity reduction may be effected in several ways.

One method of lowering the cracking severity is to reduce the catalyst circulation rate C. However, this results in a rise of the temperature of the catalyst in the regeneration zone because a reduced quantity of catalyst has to pick up about the same quantity of heat released by the same amount of coke in the exothermic regeneration reaction in order to provide the necessary heat for the endothermic cracking reaction. As already observed excessive regenerator temperatures cause damage to the catalyst because of the resulting higher catalyst particle temperatures, and may also cause damage to the regenerator itself. Therefore, the further lowering of the cracking severity will be constrained by the maximum allowable regeneration temperature.On the other hand, increasing the oil feed rate 0 will cause problems with respect to the mixing and vaporization of the feedstock, which is effected by direct contact with regenerated, hot catalyst and which is usually carried out in a so-called liftpot and a socalled riser zone. Since the hot regenerated catalyst coming from the regeneration zone is used for supplying the required heat for feedstock vaporization, a reduction of, in general, the catalyst-to-oil ratio will adversely affect the temperature in the regeneration zone and the said feedstock vaporization.

Particularly with the heavy hydrocarbon feedstocks mentioned this causes problems, as an insufficient vaporization of the feedstock leads to an increased coke-make in the cracking zone. Although a certain coke-make is required in the fluid catalytic cracking process because of the desired heat for feedstock vaporization, an excessive coke-make causes a too high catalyst temperature during regeneration.

Another possible method to lower the cracking severity is to increase the space velocity Sv, i.e. the oil feed rate divided by the reactor catalyst inventory, for instance by decreasing the reactor catalyst inventory. However, this results in the same problems as mentioned above with respect to reducing the catalyst-to-oil ratio. Moreover, there is a limit to the reduction of catalyst inventory, imposed by the necessity to keep the upper level of the fluidized bed above the draw-off standpipes of the reactor.

In catalytic cracking reactor systems it is of course desirable to regenerate the catalyst particles as completely as possible and also to utilize all the heat that can be freed from the coke deposit by combusting it completely. In practice, however, the combustion is often performed incompletely, i.e.

carbon is burnt to carbon monoxide in the regeneration zone. The carbon monoxide is then oxidized to carbon dioxide in a separate flue gas combustor, coupled to a waste heat boiler, to recover the chemical and the sensible heat present in the flue gas. The reason for this complicated scheme is that a complete combustion directly in the regeneration zone would release so much heat that would damage the catalyst and/or the regenerator materials. If it were possible to keep the temperature in the regeneration zone below 7250C, a generally accepted upper limit, while practising the so-called "complete CO-combustion", then this would mean a tremendous simplification of the catalytic cracking process. Unfortunately, most presently used heavier feedstocks produce so much coke deposit on the catalyst that complete CO-combustion is not possible, when using those feedstocks.

From the above it appears that the operation of heat-balanced, fluid catalytic cracking units is constrained by several operational conditions which adversely affect each other and that there is little flexibility left to vary feedstocks and/or catalysts. As it is moreover desirable to operate the process such that dependent on the market demands it yields either an optimum amount of middle distillates or an optimum amount of gasoline, there is clearly a need for an improved fluid catalytic cracking process in which the flexibility of the operation is not constrained by the overall heat balance requirement of the process, as determined by such parameters as the maximum regenerator temperature, the minimum regenerator temperature, the capacity of the equipment used, the activity of the catalyst, and the coking tendency of the feedstock.

It is thus contemplated to provide a process for the fluid catalytic cracking of hydrocarbon oils, and in particular to process hereby more difficult-to-crack and/or more coking feedstocks. Such feedstocks include "residual oils", meaning all hydrocarbons, regardless of their initial boiling points, which contain heavy bottoms, such as tars, asphalts (bitumens), asphaltenes, resins, etc. Accordingly, a residual oil can be a whole crude, an atmospheric reduced crude, or even the bottoms fraction, boiling above about 570-6000C, remaining after vacuum column distillation. But more unconventional feedstocks, such as propane and butane deasphalted oils, extracts and thermally cracked flashed distillates are also included.In general, feedstocks and/or feedstock mixtures with an average carbon residue (as determined by the Conradson Carbon Residue of Petroleum Products test, ASTM designation D 18965) up to 2.0 mass %, and a Nickel Equivalent (as defined hereinabove) up to 1.4 ppm can be processed according to the process of the present application. Like it is contemplated to process a wide range of feedstocks, it is likewise intended to use a wide range of catalysts. Conventional cracking catalysts, e.g.

amorphous catalysts based on natural or synthetic clays or on silica-alumina, can be used, but preferably more active and/or more selective cracking catalysts comprising crystalline aluminosilicate or ironsilicate zeolites, usually composited with a siliceous matrix, such as silica-alumina, silica-gel, etc., are considered for use. The process of the invention therefore allows for a rather great variation in feedstocks, catalysts, and products, consequently. The activity/selectivity of a particular catalyst composition is conveniently expressed by its zeolite content. The zeolite content can be determined conveniently for instance in the way as described in French patent specification 2,330,756.The zeolitecontaining catalysts preferably contain 120% of zeolite, in particular a natural or synthetic alkali metal aluminosilicate zeolite, such as type X, type Y or type L, wherein at least a substantial portion of the alkali metal has been replaced by hydrogen or other metal ions, the remainder being silica-alumina.

Since the application of too much zeolite could result in overcracking, i.e. an excessive gas make, the zeolite content preferably lies between 1 and 10% by mass, based on the total catalyst composition.

It is therefore an object of the present invention to provide a process for the fluid catalytic cracking of a hydrocarbon feedstock, in particular a heavy residual one, in which the heat balance of a reactor system as defined hereinafter is not solely dependent on the heat transfer from the regeneration zone to the cracking zone by the circulating catalyst.

It is further an object to provide a process by which it will be possible to produce an optimum yield of either gasoline or of middle distillates dependent on market demands from heavy hydrocarbon feedstocks by applying in particular zeolite-based catalysts.

It is another object to provide a process for the fluid catalytic cracking which may be operated with complete CO-combustion when regenerating the spent coke-containing catalyst.

A general object of the present invention is to increase the operational flexibility of the process, independent of the type of feedstock processed or the catalyst used.

These objects are accomplished according to the invention, by keeping the temperature in the regeneration zone below 7250C through removal of heat from the bed of the catalyst being regenerated through indirect heat exchange with the fresh hydrocarbon feedstock to be processed.

The present invention thus relates to a process for the fluid catalytic cracking of a hydrocarbon feedstock in a reactor system comprising at least a cracking zone and a regeneration zone in communication with each other, by contacting the said hydrocarbon feedstock in the cracking zone with a hot cracking catalyst, thereby causing at least part of the feedstock to be vaporized and subsequently to be cracked to lower-boiling hydrocarbon products under cracking conditions comprising a temperature in the range of from 4250 to 5500C, a pressure in the range of from atmospheric to 4 bar abs. and a catalyst-to-oil ratio below 7, after which the lower boiling hydrocarbon products are separated from spent catalyst and recovered while the spent catalyst containing coke deposits is passed to the regeneration zone where an oxygen-containing gas is passed through a bed of the catalyst, thereby oxidizing the coke deposits with the production of heat, thus regenerating and heating the catalyst, after which the hot, regenerated catalyst is returned to the cracking zone, in which process the temperature in the regeneration zone is kept below 7250C by removing heat from the bed of the catalyst being regenerated through indirect heat exchange with the fresh hydrocarbon feedstock, thereby heating said feedstock to a temperature of at least 2000 C, whereafter-said feedstock is introduced into the cracking zone and evaporated at a temperature above 2000C by directly contacting it with the hot, regenerated catalyst.

The main advantage of the process in accordance with the invention is that the heat balance will no longer depend solely on the heat transfer from the regeneration zone to the cracking zone by means of the catalyst circulation. By pre-heating the feedstock in the regeneration zone a possibility is being offered for withdrawing heat from the circulating catalyst, but meanwhile still supplying this heat to the endothermic cracking reaction. Therefore, the present process will allow an increased flexibility in cracking severity without exceeding regeneration zone temperature limitations. The vaporization upon contact of preheated feedstock and hot catalyst takes places now substantially faster, thereby minimizing the amount of liquid hydrocarbons in the catalyst pores.The liquid hydrocarbons remaining in the catalyst pores are primarily the cause of the excessive coke deposits when using heavy feedstocks.

It is observed that it is known as such to withdraw heat liberated during catalyst regeneration, for instance, it is known to generate high pressure steam through indirect heat exchange between water or steam supplied to coils, i.e. a plurality of interconnected tubes, in the catalyst bed. In such an arrangement, however, the heat is irreversibly withdrawn from the reactor system. In the present invention such heat liberated is of use in the said reactor system by preheating the feedstock to be processed in the same system.

The products of the cracking process according to the invention comprise in general lower molecular weight and lower boiling compounds than found in the original feedstock. These products find particular utility as feedstreams for petrochemical, polymer, gasoline, and alkylate manufacture. The cracking process can be optimized for a maximal production of middle distillates, i.e. kerosine and gas oils, approximate boiling ranges 140--3000C, and 180--3700C, respectively, or for a maxima gasoline and naphtha production, boiling range approximately 30--2000C, but some gaseous compounds, such as the lower olefins, will always be formed too.

It is possible to effect the indirect heat exchange of the bed of the catalyst being regenerated with the fresh hydrocarbon feedstock in several ways. One may, for instance, circulate a heat-exchange fluid through heat-exchange pipes located in the regeneration zone, and subsequently through a heat exchanger through which the feedstock is led too. This solution, however, requires a substantial investment, and is in general not preferred. Preferably, the indirect heat exchange of the catalyst being regenerated with the fresh hydrocarbon feedstock is effected by passing said feedstock through one or more heat exchange pipes located in the bed of the catalyst being regenerated.

It has surprisingly been found that the presence of the pipes in the fluid bed (occupying about 20% of the bed area) has moreover a positive influence upon the bubble size and the bubble distribution of the air in the bed. The horizontal tubes disrupt the bubble growth by dividing the rising bubbles into a larger number of smaller bubbles, and this in turn results in an increase in the mass transfer coefficient from the air to the catalyst particles, meaning that more coke can be burnt at the same throughput of air.

The size and the thickness of such pipes, as well as the kind of material to be used will be obvious to experts in the art of designing heat exchange equipment for the petroleum processing industry.

It has been found that during normal operation, while the average temperature of the feedstock rises to a temperature above 2000C, the velocity adjustment of the feedstock in the heat exchange pipes and the heat transfer within the hydrocarbon feedstock are such that the temperature of the feedstock nowhere rises too high. Maximum film temperatures at the inner pipe surface of about 4000C occasionally occur, but such temperatures do not cause any noticeable thermal cracking of the hydrocarbons, since the heat is immediately transferred to the bulk of the oil, and the temperature consequently drops.

The initial temperature of the feedstock to be processed in the catalytic cracking unit, i.e. the temperature before the preheating according to the invention, depends on its source and the degree of cooling it underwent during its production. In refineries it is common practice to minimize heat losses, and therefore the cracking feedstock, e.g. the residues of an atmospheric or a vacuum distillation unit, are transferred as quickly as they become available to the fluid catalytic cracking unit, preferably via insulated lines. The (initial) feedstock temperature would then be about 200 or 2500C, respectively. The same applies, mutatis mutandis, to other feedstocks such as gas oils, deasphalted oils and the like.

Sometimes oil-fired charge-preheaters may have to be used, for instance when the initial temperature of the feedstock is so low that the feedstock -- even when preheated according to the invention -- is not heated to vaporization temperatures by the direct contact with the hot, freshly regenerated catalyst mass. On the other hand, charge pre-coolers may have to be used too sometimes, for instance when the feedstock is difficult to crack and highly coke-producing, which would result in a too high temperature of the catalyst particles. Preheating the charge is costly, and precooling the charge, apart from being laborious, decreases the vaporization of the feed upon contact with the hot catalyst. As mentioned before, poor vaporization leads to an increased coke-make and a decreased product yield.Some very high coke-making hydrocarbon feeds therefore are not suited for catalytic cracking according to the prior art methods, but they can be cracked by preheating the charge in the regeneration zone according to the invention.

According to a preferred embodiment of the process of the invention the fresh hydrocarbon feedstock prior to the indirect heat exchange with the catalyst being regenerated may have a temperature of at least 80 and not higher than 250"C. Preferably, the fresh feedstock is heated by said heat exchange to a temperature in the range of from 2 50--425 OC. Feedstocks of 80--1 500C are called relatively cold, and those of 1 50-2500C are called relatively hot. The fresh feedstock is heated preferably to a temperature in the range of from 325-4000C in accordance with the process of the invention. Temperatures in the range of from 350-4000C should not be sustained for too long a period, since then thermal cracking could start to occur.The danger of this is minimal however, as long as the pre-heated feedstock is fed directly to the reaction zone.

As explained hereinbefore, it is desirable but often impossible to operate a fluid catalytic cracking process with complete CO-combustion. This is particularly difficult when one wants to process heavier coke-making feedstocks in reactor systems that were designed for lighter feedstocks, such as gas oils.

Due to the present method of heat withdrawal from the regeneration zone, operation with complete COcombustion is possible, using a wider range of feedstocks. The application therefore also relates to a process, wherein the temperature in the regeneration zone is kept below 7250C while oxidizing any coke deposit on the spent catalyst completely to carbon dioxide.

Although a temperature of 7250C is considered to be the upper limit for the temperature in the regeneration zone, it is obvious that it is safer to operate at temperatures below 7250C, e.g., to maintain an upper limit of 7000C. On the other hand, the temperature in the regeneration zone should not fall too low, not below e.g. 6100C, for in that case the combustion of the coke deposits would take place too slowly, and, moreover, the catalyst mass would not be hot enough to be able to vaporize the feed in the cracking zone.

It is contemplated within the scope of the present invention to cool part of the preheated feedstock by means of indirect heat exchange with a suitable cooling medium such as water, to remove any excess heat which cannot be accommodated in the reactor system. This may be the case with very high coke-making feeds and particularly where the present process is operated with complete COcombustion during spent catalyst regeneration. In such cases so much heat may have to be withdrawn from the catalyst bed to prevent the temperature from rising about 7250C that at least part of the preheated hydrocarbon feedstock will have to be cooled again externally by indirect heat exchange with a cooling medium like water. The heat thus contained in the cooling medium may be used in other units of the refinery in which the catalytic cracking unit is located. In other words, in a special embodiment of the present process, at least part of the feedstock after having been heated through indirect heat exchange with the catalyst being regenerated is cooled outside the regeneration zone by indirect heat exchange with a cooling medium.

Thus pre-heated and subsequently partially re-cooled feedstock may be fed to the reaction zone directly, and in case only part of the feedstock was re-cooled, after blending with the other portion of the feedstock, that was not re-cooled.

The flexibility of the process and the temperature stability of the feedstock entering the reaction zone is increased however, when a certain amount of recycle is applied. Preferably at least part of the feedstock heated and cooled in the above-described way is recombined with fresh feedstock to be heated through indirect heat exchange with the catalyst being regenerated.

If (some of) the feedstock is heated and cooled again, it turns out to be uneconomical if the feedstock that is to be cooled, is cooled to a temperature higher than the temperature of the fresh hydrocarbon feedstock, for then one could have easier cooled with the feedstock itself, e.g. simply by blending fresh and heated feedstocks. Therefore, it is preferred that at least part of the heated feedstock is cooled to a temperature not higher than the temperature of the fresh feedstock to be heated.

In the latter case at least part of the heated feedstock is cooled to a temperature preferably.in the range of from 800 to 1 500C depending on the particular temperature of the feedstock to be heated.

As mentioned above, all or part of the heated feedstock may be cooled after the heating in the regeneration zone. For process economical reasons it is preferred however that a third to a tenth of the said heated feedstock is cooled and blended with fresh feedstock. This amounts to a recycle ratio of 0.11 to 0.50, when the recycle ratio is defined as the ratio of recycled feed to fresh feed.

In some instances it is advantageous to burn a so-called "torch-oil" in the regeneration zone. This oil is a portion of the reaction product which is very hard to crack and which is less suited for recycling with fresh feed, in contradistinctioi. to the so-called cycle oils. it is advantageously used then as a heating oil, e.g. in the regeneration zone, in particular if one aims at an optimum yield of middle distillates. The added extra heat is easily transferred to the feed according to the process of the invention. The application therefore also relates to a process wherein a hydrocarbon fuel is burnt in the regeneration zone.

The invention will now be illustrated with the aid of the figures. Figure 1 depicts a simplified flow scheme of a fluid catalytic cracking reactor system. Figure 2 is a graphic illustration of the effect of preheating the feedstock on the catalyst-to-oil ratio and the regenerator temperature, and will be discusssed in detail in Example 2.

Referring now to Figure 1 , fresh feedstock is supplied through a line 1, and via regenerator coils 1 6 and a line 17, including a heat exchanger 18, the feedstock is preheated and introduced into line 3. It is possible to recycle part of the feedstock via a line 1 9 to pass through regenerator coils 1 6 again, resulting in a more stable temperature of the feedstock in line 1 7 and a more stable temperature of fluid bed 13. The regenerator coils 1 6 are submerged in the fluidized bed 1 3 of catalyst particles being regenerated.

In some instances it may be advantageous to install another heat exchanger in line 1, either before or after branch 2, in order to cool or heat excessively hot or cold fresh feedstocks. It is also possible to install the heat exchanger 1 8 in the recycle line 19, in particular when processing very heavy, cokemaking feedstocks, for then a high temperature of the feed in lines 3 or 1 7 (to facilitate the vaporization of the heavy feedstock in the liftpot) has to be combined with a large cooling effort in the regenerator, because of the extra coke being combusted. The feedstock is passed through line 3 to a liftpot 4, wherein the feedstock is contacted with hot, regenerated catalyst supplied through a standpipe 5.The feedstock evaporates and travels with the catalyst particles through a riser reactor 6, in upward plugflow fashion, to a fluid bed 7 in a reactor vessel 8. The gaseous and vaporized liquid cracked products are removed through the top of the vessel 8 via a conduit 9. Conduit 9 is connected to cyclones (not shown) removing entrained solid catalyst particles from the gas stream and returning those particles to the fluid bed 7 via diplegs (also not shown). The spent catalyst particles are stripped from any adhering hydrocarbons in a stripper vessel 10, located under the reactor vessel 8. Then the stripped, spent catalyst particles are fed via a standpipe 11 to a regenerator vessel 1 2. In this vessel a fluidized bed 1 3 of catalyst particles is maintained by an oxygen-containing gas stream, usually air. supplied via a nozzled pipe 14.The carbon deposits on the catalyst particles are burnt off in the bed 1 3 and through the standpipe 5 the regenerated catalyst particles return to the liftpot 4. The combustion gases are removed at the top of the regenerator vessel 12 through a conduit 1 5.

In the process according to the invention, line 2 is not used, at least not necessarily. It may, of course, be and generally is present as a remainder of a conventional reactor system, in case of a revamp of such conventional system for implementing the present process, and it will be used with advantage for start-up purposes, or as a safety relief line. To these ends there is provided a valve (not shown) at the junction of lines 1 and 2, enabling one to direct the feedstock wholly or partially into line 2 as desired.

EXAMPLE 1 The fluid catalytic cracking unit of a refinery is first operated conventionally, but with complete CO-combustion. However, in this mode of operation it is only possible to process cold feed (in the range of 50--800C), as any increase of the feed temperature results in an unacceptable increase of the regenerator temperature. In the regenerator a high oxygen slip (excess oxygen) has to be maintained to cool the regenerator bed.

In order to apply the process of the present invention this unit is revamped by installing preheating coils in the regenerator. Because of the particular size and shape of this regenerator, two parallel sets of three coils in series are installed. The coils occupy the cross-section of the vessel between the top of the manhole and the lowest level of the draw-off bin. Each coil has its own feed inlet and outlet and each coil constitutes a self-supporting beam requiring only end supports. Furthermore, each coil is constructed such that it is able to pass through the manhole. The tube alloy selected permits operation at a maximum regenerator temperature of 7000C and a design pressure of 25 bar abs. In the design the necessary attention is given to thermal expansion aspects.The total length of piping inside the regenerator is sufficient to heat up the feed from 200 to 3700C at a feed rate of 3000 tons/day.

The reaction conditions and the results of the measures taken can be seen in the following Table:

Operation Conventional with Operation Preheating reactor temperature: OC 510 510 reactor pressure: bar 1.3 1.3 catalyst-to-oil ratio 9 6 max. regenerator temperature: OC 660 660 initial temperature of feed: OC 50-80 200 liftpot entrance temperature of the feed: OC 50-80 370 regenerator oxygen slip: % vol 5 2 regenerator air/coke ratio 1 9.5 1 5 Conradson number cf feed: S/o mass 0.4 2.0 0.4 0.4 Ni-equivalent of feed: ppm 0.4 0.4 1.4 0.4 catalyst zeolite content*: % mass 3 3 3 5 * Determined as described in French pat. spec. 2,330,756.

Obviously the operational flexibility increases as a result of the preheating according to the invention. The table illustrates three cases according to the invention: 1) more coke-producing feedstocks can be processed, 2) more metal-containing feedstocks can be processed, and 3) more active/selective catalyst can be used.

Moreover less oxygen is used in the regenerator for cooling purposes, and warmer feeds can be processed, thus obviating the need to pre-cool the feedstock.

EXAMPLE 2 In Figure 2, the effect of preheating the feed in regenerator coils 1 6 according to the invention is shown. The regeneration zone temperature Treg is plotted as a function of the catalyst-to-oil ratio, C/O.

Clearly, a reduction of the catalyst feed rate C results in a rise of Tree, which is to be kept below 7250C.

The reduction of C is necessary, however, to lower the cracking severity, e.g. when using more active/selective catalysts.

Two cases are illustrated for a fluid catalytic cracking reactor system according to Figure 1. Series A in conventional heat-balanced catalytic cracking operation, using line 2, and series B in accordance with the invention, by-passing line 2 and preheating the feed in the regeneration zone 12, without heat exchange in exchanger 1 8 or recirculation through line 1 9. In all measurements the feed was introduced in line 1 at a temperature of 2000 C, and the reactor/riser outlet temperature was kept at 5200 C. Curve A shows the results of the heat-balanced operation of series A, and curve B shows the results of the heat-balanced operation of series B, while preheating the feed in the regenerator coils to 4000 C. Curve B is shifted towards lower C/0 ratios and lower regenerator temperatures, proving that in the process according to the invention more selective/active catalysts and/or more coke-producing feedstocks can be used, without exceeding the maximum regenerator temperature of 7250C.

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Claims (14)

1. A process for the fluid catalytic cracking of a hydrocarbon feedstock in a reactor system comprising at least a cracking zone and a regeneration zone in communication with each other, by contacting the said hydrocarbon feedstock in the cracking zone with a hot cracking catalyst, thereby causing at least part of the feedstock to be vaporized and subsequently to be cracked to lower-boiling hydrocarbon products under cracking conditions comprising a temperature in the range of from 4250 to 5500C, a pressure in the range of from atmospheric to 4 bar abs. and a catalyst-to-oil ratio below 7, after which the lower boiling hydrocarbon products are separated from spent catalyst and recovered while the spent catalyst containing coke deposits is passed to the regeneration zone where an oxygen containing gas is passed through a bed of the catalyst, thereby oxidizing the coke deposits with the production of heat, thus regenerating and heating the catalyst after which the hot, regenerated catalyst is returned to the cracking zone, in which process the temperature in the regeneration zone is kept below 7250C by removing heat from the bed of the catalyst being regenerated through indirect heat exchange with the fresh hydrocarbon feedstock, thereby heating said feedstock to a temperature of at least 2000 C, whereafter said feedstock is introduced into the cracking zone and evaporated at a temperature above 2000C by directly contacting it with the hot, regenerated catalyst.

2. A process as claimed in claim 1 , wherein the indirect heat exchange of the catalyst being regenerated with the fresh hydrocarbon feedstock is effected by passing said feedstock through one or more heat exchange pipes located in the bed of the catalyst being regenerated.

3. A process as claimed in claim 1 or 2, wherein the fresh hydrocarbon feedstock prior to the indirect heat exchange with the catalyst being regenerated has a temperature of at least 800C and not higher than 2500C.

4. A process as claimed in claim 1-3, wherein the fresh feedstock is heated by said indirect heat exchange to a temperature in the range of from 250-4250C.

5. A process as claimed in claim 4, wherein the fresh feedstock is heated to a temperature in the range of from 325-4000C.

6. A process as claimed in any one of claims 1-5, wherein the temperature in the regeneration zone is kept below 7250C while oxidizing any coke deposit on the spent catalyst completely to carbon dioxide.

7. A process as claimed in any one of claims 1-6, wherein the temperature in the regeneration zone is kept in the range of from 610-7000C.

8. A process as claimed in any one of claims 1-7, wherein at least part of the feedstock after having been heated through indirect heat exchange with the catalyst being regenerated is cooled outside the regeneration zone by indirect heat exchange with a cooling medium.

9. A process as claimed in claim 8, wherein at least part of the heated feedstock is cooled and recombined with fresh feedstock to be heated through indirect heat exchange with the catalyst being regenerated.

10. A process as claimed in claim 8 or 9, wherein at least part of the heated feedstock is cooled to a temperature not higher than the temperature of the fresh feedstock to be heated.

11. A process as claimed in claim 10, wherein at least part of the heated feedstock is cooled to a temperature in the range of from 80-1500C.

12. A process as claimed in any one of claims 7-11, wherein a third to a tenth of the said heated feedstock is cooled and blended with fresh feedstock.

13. A process as claimed in any one of claims 1-12, wherein a hydrocarbon fuel is burnt in the regeneration zone.

14. A process as claimed in claim 1, substantially as hereinbefore described with reference to the Examples and the drawings.