GB2085911A

GB2085911A - Thermal cracking of heavy hydrocarbons

Info

Publication number: GB2085911A
Application number: GB8033027A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-13
Filing date: 1980-10-13
Publication date: 1982-05-06
Also published as: GB2085911B

Abstract

A heavy hydrocarbon feed, such as petroleum oil or coal tar, is passed from an inlet 13, e.g. at 480 DEG C and 40 atm into an inner vessel 12 within an outer pressure vessel 12. The cracked hydrocarbon and any coke produced pass through an open bottom end 17 of the inner vessel into the outer vessel, which is maintained at a temperature insufficient to sustain the cracking reaction, e.g. by cooling and recycling liquid withdrawn from it. The product is withdrawn, either continuously or intermittently via an outlet 18 or 23. The coke produced may be fluidised in the outer vessel or may be allowed to settle. In the former case the coke is withdrawn as a slurry and allowed to settle outside the reactor system, while in the latter case the coke is removed through the bottom outlet 21. <IMAGE>

Description

SPECIFICATION Method and apparatus for the thermal cracking of heavy hydrocarbons The present invention relates to a method and apparatus for the cracking of heavy hydrocarbons, for example heavy petroleum oil or coal tar. The invention may be applied to any thermal cracking process varying in severity from so-called visbreaking to a full coking reaction. The invention is particularly applicable to the treatment of the heavy oils or bottom fraction obtained in petroleum distillation.

In petroleum distillation it is desirable to make full use of all the fractions produced. In the case of the heavier fractions, it is desirable to crack these into lighter products since these have more uses. This is generally carried out by heating the heavy oil in the absence of oxygen to such a temperature that the hydrocarbon molecules split to form lighter compounds. In a visbreaking process the cracking is not particularly severe with the result that comparatively little solid coke is formed. However, in a full coking reaction the heavy oil is cracked to such a degree that large quantities of coke are produced.

In one known process for cracking heavy oil, the feedstock is introduced to the base of a reactor column at a temperature of about 480"C where cracking takes place. The lighter products, in the form of gases and liquids, are withdrawn from the top of the reactor and coke builds up within the reactor. In the case of visbreaking, this build-up is slow, taking perhaps up to a month or more, though in the case of coking, the build-up is rapid tailing perhaps only 24 hours.

This process suffers a number of disadvantages. The product recovered almost inevitably contains certain quantities of coke, which tend to block the apparatus downstream of the reactor. Also, cracking carries on in the product outlet, since the temperature is usually still sufficiently high, resulting in further coke production and deposition outside the reactor. Furthermore, the continual coke buildup results in a constantly changing reactor volume, with the result that the reaction parameters must be continuously adjusted in order to maintain a constant product composition. Finally, the coke builds up as a solid deposit which must be removed from the inside of the reactor by the use of high pressure water lances. This is costly, time consuming and necessitates the reactor being taken out of commission, at regular intervals to remove the coke deposits.

In a second known process the feedstock is introduced to a fluidised bed reactor in which the coke produced is fluidised. The product is removed from the top of the reactor. The coke is continuously withdrawn from the base of the reactor and transferred to the top of a second vessel to the base which hot air and steam are introduced. This burns off part of the coke, and heats the rest of the coke which is continuously withdrawn from the vessel and fed back to the top of the reactor, thus providing heat for the endothermic cracking reaction.

This suffers from the disadvantages that the products withdrawn again tend to include some coke and again, further coke production can take place in the outlet. Also, this process is generally a low pressure process and can only effectively be carried out for a full coking reaction. Thus, the product range tends to include a greater proportion of lighter products which are less desirable, and the process is far less flexible.

It is an object of the present invention to provide a method and apparatus which does not suffer from the above disadvantages.

It is a further object to produce an apparatus which is capable of batch operation, semi-continuous operation and fully continuous operation.

According to one aspect of the present invention there is provided apparatus for the thermal cracking of heavy hydrocarbons comprising an outer vessel, an inner vessel located within the outer vessel, a feedstock inlet to the inner vessel, a gas outlet from the inner vessel, one or more product outlets from the outer vessel, a discharge port near the bottom of the outer vessel for the discharge of coke and means for cooling the contents of the outer vessel, there being an opening at the base of the inner vessel to allow cracked products, including coke, to leave the first vessel and to enter the second vessel.

Preferably, the outer vessel is spherical and may be a pressure vessel, while the inner vessel is preferably comparatively thin walled.

There may be filtered outlets at the base of the outer vessel for draining the liquid product and fluid inlets at the base of the outer vessel to introduce fluid to the outer vessel thereby fluidising the coke. Preferably, the outer vessel is a good deal larger than the inner vessel to enable coke to settle and to accumulate if required.

According to a second aspect of the invention there is provided a method of thermally cracking heavy hydrocarbons which comprises passing a liquid feedstock of heavy hydrocarbons, at a temperature and pressure such as will allow cracking of the hydrocarbons to occur, into an inner vessel, the inner vessel being located within an outer vessel; allowing the cracked hydrocarbons, together with any coke produced, to pass out of the inner vessel into the outer vessel; maintaining the contents of the outer vessel at a temperature at which cracking of the hydrocarbons is inhibited (e.g.

325"C.); removing cracked product from the outer vessel, and removing coke from the bottom of the outer vessel.

Preferably, the contents of the outer vessel are cooled by withdrawing liquid from the outer vessel, cooling the withdrawn liquid and reintroducing it into the outer vessel, In one preferred method the coke is withdrawn continuously or intermittently while the cracking reaction is taking place whereby the process is in continuous operation.

In a second preferred method, the reaction is terminated, the liquid product is subsequently drained from the base of the outer vessel through one or more outlets having a filter, and the coke is then removed from the bottom of the outer vessel. In this case, the coke may be purged with steam prior to its removal but after the product has been drained.

In a third preferred method the fluid is withdrawn from the outer vessel and is reintroduced at the base of the outer vessel thereby fluidising the coke. The coke may be continuously or intermittently withdrawn as a slurry. Fluidisation may be effected by passing a portion of the cooling liquid through the fluid inlets at the base of the outer vessel. The outlets having filters may serve as the fluidisation fluid inlets. The cooling fluid need not necessarily be withdrawn from the outer vessel but may be any suitable fluid.

in a fourth preferred method the liquid cracked products, the coke and the cooling fluid may be withdrawn from the base of the outer vessel as a slurry continuously or semicontinuously to a settling tank. Vapourised cracked products may be withdrawn from the top of the outer vessel under pressure control.

Thus it may be seen that the invention may result in the minimal deposition of coke in the region of the reactor where cracking mainly occurs, and consequently where most coke is formed, namely in the inner vessel. Thus the volume of the reactor and space velocity of the reactants remain relatively constant over the operating cycle. Furthermore, since the inner vessel can be fabricated in sections, which can be assembled and disassembled into pieces that can be conveniently passed through a port in the outer vessel, the inner vessel can be readily removed and replaced by another vessel of different dimensions and therefore maintenance and modifications can be carried out fairly easily. This feature offers considerable flexibility to accommodate desired changes to the reactor performance including the possibility of using different feedstocks.

The retention time of the fluid passing through the inner vessel can be varied, and consequently the severity of cracking, by controlling the liquid level in the outer vessel, since the liquid level in the inner vessel is directly dependent on the liquid level in the outer vessel. It may also be seen that a large volume can be provided to accommodate coke accumulation within the main body of the reactor, namely in the outer vessel without subjecting the fluid to be cracked to excessive cracking exposure. Since the cracking is inhibited in the outer vessel, this will have the beneficial effect of discouraging coke particles formed in the inner vessel from coagulating into a mass in the outer vessel and forming constrictions in the outlet ports of the outer vessel also reducing the potential of coke to form in subsequent downstream items of equipment.

The invention may be carried into practice in various ways and one possible apparatus and a number of possible methods will be described by way of example with reference to the accompanying drawing in which the single figure is a schematic sectional view of apparatus in accordance with the invention.

The reactor shown in Fig. 1 comprises a spherical outer vessel 11 capable of withstanding up to 40 atmospheres pressure and an inner vessel 12 having thin walls in comparison with the outer vessel 11. The inner vessel 1 2 is suspended within the upper part outer vessel 11 or attached to the upper part of the outer vessel 11 by any suitable means.

The outer vessel 11 is 10.0 metres in diameter while the inner vessel 1 2 is approximately cylindrical, being 6.0 metres high and 3.5 metres in diameter.

A feedstock inlet 1 3 leads into the inner vessel 1 2 leaving a clearance 14 for the escape of gases. The inner vessel 1 2 has a generally cylindrical upper portion 1 5 and a generally frusto-conical lower portion 1 6 terminating in a wide-mouthed bottom opening 17, to minimise solids deposition in this region.

The outer vessel 11 has a liquid product outlet 18 in its upper region and a series of cooling fluid inlets 1 9 (only one being shown) arranged symmetrically about the outer vessel 11. The outer vessel 11 also has a bottom outlet 21, a series of fluidisation fluid inlets 22 (two being shown) at its base, and a series of fluid outlets 23 having filter elements 24 also at the base of the outer vessel 11. The inlets 22 may be used to withdraw the liquid cracked products, coke and cooling fluid as a slurry on a continuous or semi-continuous basis cracked vapour product may be withdrawn under pressure control from a gas outlet 25. All the inlets and outlets are provided with suitable valves (not shown) where necessary.

The reactor may be used in a number of ways to crack thermally, heavy hydrocarbons.

In one such method, a feedstock of heavy hydrocarbons is introduced to the inner vessel 12 at a temperature of about 480"C and at a pressure of about 40 atm. The liquid level in the outer vessel is adjusted so that the residence time is about 3 to 4 minutes. The feedstock is cracked lightly and the lighter products, together with the small amount of coke produced leave the inner vessel 1 2 through the aperture 1 7 and enter the outer vessel 11.

The contents of the outer vessel 11 are maintained at a temperature of about 320 at which temperature, thermal cracking is inhibited, so that as soon as the cracked products enter the outer vessel 11, further cracking does not take place. The temperature in the outer vessel is maintained by withdrawing liquid product from the outlet 18, cooling a proportion of this liquid by any suitable means, and returning this to the outer vessel via the fluid inlets 19, which should preferably be directed towards the bottom opening of the inner vessel. The remainder of the liquid product withdrawn is removed as a product of the process. Since the liquid product outlet 1 8 is located near the top of the outer vessel 11, the amount of coke present in the liquid product stream may be minimised.Thus downstream contamination and blockages may also be minimised.

The coke produced settles to the bottom of the outer vessel 11 in the form of particles and is withdrawn intermittently through the bottom outlet 21. This can be carried out during the reaction process or during shutdown. In the latter case, the process is shut down and the outer vessel 11 is drained of liquid product through the filtered outlets 23.

When this has been completed, the coke is purged with steam, cooled and withdrawn via the bottom outlet 21.

In another method, the feedstock is introduced in the same way as in the first method described above, however, the liquid level in the outer vessel 11 is adjusted so that the residence time is about 20 minutes, so that a full coking reaction takes place. In this case a large quantity of coke is produced.

Liquid product is withdrawn through the liquid product outlet 1 8 and as described above, some is removed as a product stream, while the remainder is cooled and reintroduced to the outer vessel 11. However, in this case, the recycled product is introduced through the fluidisation fluid inlets 22, thereby fluidising the coke particles. Thus, the liquid product will be in the form of a slurry of coke particles and so this is fed to a separation tank (not shown) where the liquid product is removed, partly for recycling to fluidise the coke in the outer vessel 11 and partly as a product stream. The coke is removed from the separation tank.

As an alternative, in this second method, the fluidisation may be arranged so that the liquid product withdrawn from the liquid product outlet 1 8 is substantially free from coke.

In this case, the coke slurry is withdrawn from the base of the outer vessel 11 through one of the inlets 22 and transferred to a separation tank (not shown). Again, the separated liquid is reintroduced to fluidise the coke in the outer vessel, and the coke is removed after it has settled.

In these above described methods cracked vapour products may be withdrawn from the outer vessel 11 through the gas outlet 25 under pressure control and also cracked vapour products flashed-off in the separation tank removed through a vapour outlet (not shown).

Where the processes above are batch processes, two reactor systems may be used in sequence so that the process may be semicontinuous.

Claims

1. Apparatus for the thermal cracking of heavy hydrocarbons comprising an outer vessel, an inner vessel located within the outer vessel, and feedstock inlet to the inner vessel, a gas outlet from the inner vessel, one or more product outlets from the outer vessel, a discharge port near the bottom of the outer vessel for the discharge of coke and means for cooling the contents of the outer vessel, there being an opening at the base of the inner vessel to allow cracked products, including coke, to leave the first vessel and to enter the second vessel.

2. Apparatus as claimed in Claim 1 in which the outer vessel is spherical.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the inner vessel is thinwalled compared with the outer vessel.

4. Apparatus as claimed in any preceding claim in which the cooling means comprises a liquid outlet from the outer vessel leading to a liquid cooler to cool the liquid withdrawn, and a liquid inlet arranged to lead the cooled liquid back into the outer vessel.

5. Apparatus as claimed in any preceding claim further including at least one liquid outlet located at the base of the outer vessel and including a filter.

6. Apparatus as claimed in any preceding claim further including fluid inlet ports at the base of the outer vessel arranged to fluidise coke particles in the outer vessel and a fluid outlet to withdraw slurries.

7. Apparatus for the thermal cracking of heavy hydrocarbons constructed and arranged substantially as herein specifically described with reference to and as shown in the accompanying drawing.

8. A method of thermally cracking heavy hydrocarbons which comprises passing liquid feedstock of heavy hydrocarbons, at a temperature and pressure such as will allow cracking of the hydrocarbons to occur, into an inner vessel, the inner vessel being located within an outer vessel; allowing the heavy hydrocarbons to react in the inner vessel; allowing the cracked hydrocarbons, together with any coke produced, to pass out of the inner vessel into the outer vessel; maintaining the contents of the outer vessel at a temperature at which cracking of the hydrocarbons is inhibited; removing cracked product from the outer vessel, and removing coke from the bottom of the outer vessel.

9. A method as claimed in Claim 8 in which the contents of the outer vessel are cooled by withdrawing liquid from the outer vessel, cooling the withdrawn liquid, and reintroducing it into the outer vessel.

1 0. A method as claimed in Claim 8 or Claim 9 in which the coke is withdrawn continuously or intermittently while the cracking reaction is taking place whereby the process is a continuous operation.

11. A method as claimed in Claim 8 or Claim 9 in which the reaction is terminated, the liquid product is subsequently drained from the base of the outer vessel through one or more outlets having a filter, and the coke is then removed from the bottom of the outer vessel.

1 2. A method as claimed in Claim 11 in which the coke is purged with steam prior to its removal but after the product has been drained.

13. A method as claimed in Claim 9 in which fluid is withdrawn from the outer vessel and is reintroduced at the base of the outer vessel thereby fluidising the coke.

14. A method as claimed in Claim 1 3 in which the coke is continuously or intermittently withdrawn as a slurry.

1 5. A method of thermally cracking heavy hydrocarbons substantially as herein specifically described with reference to the accompanying drawing.