GB2124249A - Process for fluid catalytic cracking of hydrocarbons - Google Patents

Abstract

In a fluid catalytic cracking process, the virgin make-up cracking catalyst is added to the fresh charge to be cracked as a suspension in a hydrocarbon. A cracking passivator such as an antimony additive can be mixed into the suspension. This process is efficient for the processing of hydrocarbon charges of high metal content, and enables catalyst losses to be reduced by about 50 to 80%. <IMAGE>

Description

SPECIFICATION Process for fluid catalytic cracking of hydrocarbons This invention relates to the fluid catalytic cracking of hydrocarbons and is especially useful for processing hydrocarbon charges contaminated with high contents of metal.

In the cracking of hydrocarbons a suspension of a hydrocarbon, a cracking additive, and a cracking catalyst, is constantly mixed with a fresh stream of hydrocarbons, and the mixture is caused to undergo cracking.

In the usual fluid catalyst cracking processes (referred to herein as FCC), the catalyst is brought into contact with the charge and the mixture flows through a riser (where the cracking reactions take place) towards the reactor. In the reactor the catalyst is separated, usually by means of cyclones, from the output of the reaction, and the latter goes to the distilling stage, while the catalyst is stripped and then sent to the regenerator.

The steam stripping treatment removes the less tightly held compounds adsorbed on the surface of the catalyst, while in the regenerator the coke lying on the surface of the catalyst is burnt. Active middle portions of the catalyst become obstructed, and a drop in catalytic action of the catalyst is thus bought about.

The particles of the catalyst removed by the combustion gases are also recovered by cyclones lying within the regenerator. After having been regenerated, the catalyst rejoins the process, and is mixed with a fresh stream of charge.

The output for distillation is led to a fractionator where it is distilled into gases, naphtha, light gas oil, heavy gas oil, clarified oil and slurry oil. Such slurry oil contains particles of the catalyst, dragged away by the reaction products upon separation from the catalyst at the reactor outlet.

Though the latest FCC units are equipped with sets of one or more stage cyclones, catalyst losses in such units are still considerable.

The fluid catalytic cracking process of this invention makes use of a catalyst charging method (described below) which enables a drastic cut in such losses to be made.

Because of the oil crisis, efforts have been worldwide to develop processes for making ever greater use of the available oil. Thus cracking processes are now used in which the heavier fractions of crude oil act as raw material, while at the same time catalysts that are more active and less susceptible to being poisoned than those used in the seventies are being developed.

The presence of certain metals, such as Ni and V, in the charge to be cracked, represents a serious obstacle when the usual catalysts are employed, since such metals soon poison them. Furthermore, in the heavier cuts of crude the proportion of such metals is much greater.

To overcome this difficulty, research had been undertaken towards finding a way of lessening the effect of such poisons upon the known catalysts. This led to the use nowadays of substances known as cracking passivators or additives. The presence of such substances in the reaction medium enables high Ni and V content hydrocarbon charges to be processed. Compounds containing B, Sn, Te, Sb, Mn, Bi, W, Te, In, Ge and others have such powers, and are sold for this purpose. Meanwhile further passivation process techniques have been described in publications on the subject. Fresh processes involving more effective use of crude oil, plus such techniques, have led to other discoveries in recent years.

The present invention provides a process for the fluid catalytic cracking of hydrocarbons in which catalyst losses can be drastically reduced, by about 50 to 80%.

In the new process catalyst fines (having a particle size of 0--40 microns) can be used in cracking reactions.

The new process is not adversely affected by the use of metal-contaminated hydrocarbon charges.

These results are achieved by a change in the way the catalyst is supplied to the process. Instead of employing the usual method of replacing catalysts losses, namely, by means of a pressurized silo leading to the regenerator, in this invention the catlyst is supplied wet, directly to the reaction medium.

By supplying the process with wet virgin catalyst, that is, with a suspension in a hydrocarbon, catalyst fines can play a part in the cracking reaction.

The new process enables stricter control to be kept over the quantities of catalyst and cracking additive (if any) which is added to the charge stream.

The catalyst is suspended in a stream of a hydrocarbon, for instance, heavy vacuum gas oil, heavy recycle gas oil, LCO or FCC diesel, naphtha, FCC fuel oil, etc., said suspension being mixed with the charge or else directly injected into the riser, after which the cracking takes place. When the charge is contaminated with metals, a cracking additive is used to inhibit any poisoning of the catalyst. The basis of such additives is usually antimony and there are several kinds on the market.

The process of the invention for the fluid catalytic cracking of hydrocarbons, comprises mixing fresh reactor charge with a suspension, in a hydrocarbon, of virgin cracking catalyst and optionally a cracking additive, and cracking the said charge.

The catalyst suspension may be injected directly into the riser and the charge then made to undergo cracking.

To determine the quantities of the suspension to be added to the process from time to time, certain operational aspects have to be considered.

The method of checking on the degree of metal contamination of the catalyst by measuring hydrogen and methane produced during the process is a well-known one.

When the catalyst is poisoned by Ni and V, dehydrogenation and the formation of coke take place at the same time, since both are catalysed by such metals. Therefore, a sign that the catalyst is being contaminated is that there is a higher proportion of hydrogen in the combustion gas.

Usually, the catalytic action is controlled by making piecemeal replacements on the spent catalyst with regenerated or even virgin catalyst and also by striving to keep the relationship between the hydrogen and the methane present in the combustion gas within bounds regarded as acceptable.

Preferably the hydrogen content is kept as low as possible.

For the purpose of comparing the performance of the new process with that of processes already described in the literature, a suspension was prepared, on an experimental scale, containing the catalyst, and the new process was carried out using the methods described below. Afterwards, for the sake of comparison, a known cracking process was effected.

It was found that in the new method, whether or not a cracking additive is added to the catalyst suspension, catalyst losses are substantially less. It was also found that with the new method, less cracking additive has to be used to arrive at the same hydrogen/methane ratio in the combustion gas, other variables in the process being the same. This means that about half as much cracking additive has to be employed.

Furthermore, it was discovered that lower hydrogen/methane ratios in the combustion gas could be achieved than those obtainable with the known kind of process, regardless of the quantity of cracking additive in the known process. This means that with the known process the hydrogen/methane ratio in the combustion gas cannot be brought down below a given figure no matter what quantity of cracking additive is employed.

The fluid catalytic cracking process of this invention is therefore more efficient than those previously described in the literature.

The new process will next be described with reference to the accompanying Figure 1 which is a schematic representation of the process of the invention. The apparatus shown comprises a cracking reactor 1 consisting of a riser unit and a product separator, a catalyst regenerator 2, and a tank 3 for preparing the suspension.

To the mixing tank 3, provided with a mechanical stirrer 4, a hydrocarbon 5 to act as the dispersing medium for the catalyst, a cracking additive 6 (if needed), and virgin catalyst 7 are supplied in suitable proportions. The stream of suspended catalyst 8 thus made is continuously mixed with the stream of fresh charge 9 and the joint charge stream 10 thus produced is fed into the cracking reactor 1, and more specifically, into the riser thereof.

The stream of reaction products 11 is separated and led to a fractionator (not shown in the figure) while the stream of spent catalyst 1 2 is led to the regenerator 2, in which regeneration of the catalyst takes place.

A stream of comburant gas 13 is fed to the regenerator 2 to burn up coke deposited on the spent catalyst, which gives rise to a stream of combustion gases 14, which is withdrawn from the regenerator 2. From the regenerator a stream of regenerated catalyst 1 5 runs back to and is combined with the charge stream 10 which enters the cracking reactor 1. In carrying out this invention heavy gas oil was employed as the charge and as the dispersing agent, and air as the comburant gas.

An alternative process consists in preparing a suspension of the catalyst in a hydrocarbon 8, either with or without a cracking additive, and in injecting this suspension continuously into the riser, that is, without going through the stage of mixing it with the stream of fresh charge 9.

The usual method which was employed for the sake of comparison was as follows. The catalyst stored in a silo was transferred to the regenerator by pressurizing the silo and the catalyst was added continuously to the process. The cracking additive was diluted with gas oil in a drum and continuously injected into the stream of the charge, in quantities large enough to arrive at the desired results. The mixture so secured was made to undergo cracking.

In order to substantiate results arrived at on the workbench, a prototype unit was tried out. The trials were intended to give more reliable figures than those ascertained at work bench scale, and in particular figures for catalyst losses, as well as for ideal operating conditions and a comparison with known processes.

The Examples given below show the results of such trials.

The catalysts that can be used for the new process are the usual ones, e.g.: low alumina, high alumina, zeolites, etc. The operating conditions for the trials were also the ones that usually apply.

Throughout the trials the quality of the charge was kept the same.

EXAMPLE I The purpose of this example is to check the efficiency of the feed system under the new process as compared with that for the known process. In tests 1 and 2 the catalyst was replaced continuously as provided for in the usual method, that is from the silo to the regenerator. In tests 3 and 4 the new method was employed, namely: A suspension of the catalyst in gas oil was prepared in a mixing tank provided with a mechanical stirrer. This suspension was fed continuously to the process, along with the charge.

The charge used for the tests was a heavy gas oil derived from Leona crude, while the catalyst used was high alumina. In all the tests, replacement was equal to quantity of catalyst lost.

TABLE (I) Test Charge (m3/day) Replacement (tons/day) 1 153 0.45 2 285 0.66 3 239 0.13 4 255 0.20 Ordinary comparison of test 3 against test 2 shows a drop of 80.3% in catalyst replaced, in the former.

EXAMPLE II In these tests the intention was to discover optimum operating conditions for the process. Runs were done in which the known process (run 5) was compared with that of the invention (run 6).

Reaction temperature, catalyst/oil ratio, and combined charge ratio were all kept as steady as possible.

A combined charge of 1.0 means that no recycling was done. Table II shows average results of the runs.

TABLE (II) Run 5 6 charge (m3/day) 186 186 charge temperature (OC) 165 169 circulation of catalyst (ton/min.) 3.0 2.4 catalyst/oil ratio 22.8 18.3 reaction temperature (OC) 492 490 dense stage 607 623 regenerator temperature 612 628 cyclone 620 639 combined charge ratio 1.0 1.0 hydrogen/methane ratio in combustion gas 3.17 2.72 cracking additive (kg/day) 24 8 replacement catalyst (kg/day) 200 100 * antimony type commercial additive g replacement exactly equal to catalyst lost These results show that, under the new process, catalyst losses were half as much. It should also be noted that even with only a third of the additive the hydrogen/methane ratio was lower, which means that the cracking additive was more efficiently used.

EXAMPLE III The object of this example was to find out if by raising the quantity of cracking additive employed, as in the known catalyst replacement method, the hydrogen/carbon (HC1) ratio would become lower than that arrived at under the new process.

In the new process a HaC, ratio of 2.7 was arrived at and 2.7 kg of Sb per day was used. Doses of Sb were worked out in terms of the commercial cracking additive employed. Table III shows results secured.

TABLE (III) kg Sb/day H2/Ct 1.80 3.8 2.76 3.17 3.50 3.3 4.00 3.5 In the known process no matter what quantity of additive was used, the HC, ratio could not be brought down below 3.17.

Claims (8)

1. Process for the fluid catalytic cracking of hydrocarbons which comprises mixing fresh reactor charge with a suspension, in a hydrocarbon, of virgin cracking catalyst and optionally a cracking additive, and cracking the said charge.

2. Process according to claim 1, in which the process is operated continuously and the said suspension is continuously injected into the fresh reactor charge.

3. Process according to claim 1 or 2, in which the said catalyst has a particle size of less than 40 microns.

4. Process according to any of claims 1 to 3, in which an alumina cracking catalyst is used.

5. Process according to any of claims 1 to 4, in which an antimony cracking additive is used with the cracking catalyst.

6. Process according to any of claims 1 to 5, substantially as hereinbefore described with reference to the accompanying drawings.

7. Process according to claim 1, substantially as described in any one of the foregoing Examples.

8. Catalytically cracked hydrocarbons produced by the process of any of claims 1 to 7.