GB2094336A

GB2094336A - Method and apparatus for the thermal cracking of heavy hydrocarbons

Info

Publication number: GB2094336A
Application number: GB8106098A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-02-26
Filing date: 1981-02-26
Publication date: 1982-09-15
Also published as: GB2094336B

Abstract

A method and apparatus for the thermal cracking of heavy hydrocarbons, including means to decoke the reactor. The reactor comprises an outer vessel 11 and within the outer vessel 11 an inner vessel 12 having an opening 13 at its base. The inner vessel 12 is rotated slowly about its vertical axis and high pressure fluid jets are directed at its surface to dislodge coke 18 deposited on the inside and outside wall. The inner vessel 12 may be rotated by means of a hydraulic motor or by directing the jets tangentially against blades on the inner vessel 12. <IMAGE>

Description

SPECIFICATION Method and apparatus for the thermal cracking of heavy hydrocarbons The present invention relates to a method and apparatus for the thermal cracking of heavy hydrocarbons such as heavy petroleum oil or coal tar and in particular to the de-coking of such a reactor.

A preferred reactor for cracking hydrocarbons comprises an outer pressure vessel an inner vessel located within the outer vessel. The feedstock is fed to the inner vessel where cracking takes place and the products are discharged from the bottom of the inner vessel into the outer vessel where cooling takes place and any coke produced settles out. Products are withdrawn from the outer vessel.

Such a reactor is described in the present applicant's co-pending British Patent Application No. 80.33027 (Serial No. ) whose disclosure is incorporated herein by reference.

Although coke deposition may be minimised in such a reactor, certain amounts of coke do tend to deposit under certain circumstances, for example in full coking reactions involving heavy oils, when large amounts of coke are produced. The deposits may build up on the walls of the inner vessel, particularly on the inner wall.

Conventional methods of removing deposited coke from a coking drum include the use of a device which is lowered into the coking drum and which permits a high velocity water jet to be rotated and directed in such a way that it sweeps the inner surface of the coking drum and causes the coke deposits to be dislodged. This technique suffers the disadvantage that the coking drum must be at least partially cooled and purged and subsequently opened to provide access to the decoking device. This is a slow and uneconomical operation.

It is an object of the present invention to enable a reactor, in particular the inner vessel of the preferred form of reactor described above, to be de-coked without the necessity to open, cooi and purge the reactor.

According to the present invention, appartus for the thermal cracking of heavy hydrocarbons comprises an outer vessel and, within the outer vessel, an inner vessel having an opening at its base, the inner vessel being rotatable about a vertical axis, and nozzles located within the outer vessel and being directed at the surface of the inner vessel.

Preferably, the inner vessel is supported on structural members within the outer vessel by means of a ball race.

One of a number of methods may be employed to rotate the inner vessel, however it is preferable that the inner vessel should be caused to rotate slowly but positively against a varying resistant force. The inner vessel could be quite large, for example 5.0 metres diameter and 1 5 metres long.

Together with the deposited coke its weight could be several tons.

One method may be to use a high velocity fluid jet to impinge tangentially to the axis of rotation of the inner vessel against a series of blades which are fixed to the inner vessel outside wall. By a combination of varying the number of jets and the pressure of fluid entering the nozzles the imparted force to cause the inner vessel to rotate can be conveniently varied and the speed of rotation controlled.

Another method to impart the necessary motive force to cause the vessel to rotate is to provide a fixed hydraulic motor which may interact with a circumferential attachment to the inner vessel causing it to rotate. Such a method may use established rack and gear arrangements.

To prevent the various nozzles from being occluded during the coking cycle, a small but constant fluid flow may be maintained through the nozzles.

This method of removing deposited coke may use as the fluid jets the hydrocarbon fluid normally used in the process of thermal cracking as feedstock or coolant recycle steam since the outer vessel need not be opened during the de-coking operation, provided that the fluid jets do not contain solids that may cause occlusion in the nozzles and provided the fluid is at a lower temperature than that at which cracking occurs.

It will be appreciated that using the method of the present invention, the use of water jets may be avoided while the fluid jets used dislodge coke deposited on the walls of the inner vessel, by sweeping both the inside surface and those areas of the outside surface of the same vessel where coke may tend to deposit.

The invention may be carried into practice in various ways and one embodiment will now be described by way of examples with reference to the accompanying drawing in which the single figure is a diagrammatic cross-section through a reactor in accordance with the present invention.

The reactor comprises an outer vessel 11 and an inner vessel 12. The inner vessel 12 has an opening 13 at its base and a flange 14, about twothirds of the way up from its base, attached to its outer surface. The outer vessel 11 has a support ring 1 5 located about two-thirds of the way up from its base by means of interior structural supports 16. The flange 14 is supported on the support ring 1 5 by means of a ball race indicated schematically by reference numeral 17.

A number of fixed nozzles 21,22,23,24 are located within the outer vessel and are arranged to direct fluid at the surfaces of the inner vessel 12. A hydraulic motor (not shown) is provided to rotate the inner vessel 12.

In use, as the cracking reaction continues within the inner vessel 12, coke may deposit on the surfaces of the inner vessel 1 2 as shown at 1 8. To remove this deposit, the liquid level in the outer vessel 11 is first lowered to approximately that of the base of the inner vessel 12. The inner vessel 12 is rotated slowly but positively about its vertical axis by means of the motor and fluid is discharged at a high velocity through the nozzles 21-24. The fluid impinges upon the inner vessel 12 and effectively sweeps its surface, thus removing the coke from both the inside and outside surfaces of the inner vessel 12. This operation may be carried out intermittently when required without cooling and purging the contents of the reactor vessel.

The fluid used may be the hydrocarbon feedstock or may be recycle coolant to the reactor.

The reactor system is not opened during the coke dislodgement operation and the introduced fluid mixes readily with the reactor contents without undue adverse effects provided it does not contain solids which will cause occlusion in the nozzles or conduits leading to the nozzles.

By positioning the rotational support system and the motive force system which rotates the inner vessel about two thirds up from the bottom of the inner vessel, so that both these systems are normally below the liquid level during the cracking reaction, coke deposition on these driving systems will be substantially reduced because the temperature in this region will normally be about 3450C. Also, by maintaining a small but constant flow of fluid through the nozzles such that the temperature leaving the nozzles is less than about 3450C the temperature at which thermal cracking starts to occur, the nozzles will be kept free from occlusions.

In a modified form of reactor, the inner vessel 12 is caused to rotate by means of blades attached to its surface. Nozzles are arranged to direct their fluid jets against these blades in a tangential direction with respect to the axis of the inner vessel 12.

Claims

1. Apparatus for the thermal cracking of heavy hydrocarbons, which comprises an outer vessel and, within the outer vessel, an inner vessel having an opening at its base, the inner vessel being rotatable about a vertical axis, and nozzles located within the outer vessel and being directed at the surface of the inner vessel.

2. Apparatus as claimed in Claim 1 in which the inner vessel is supported on structural members within the outer vessel by means of a ball race.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the inner vessel has a series of blades attached to its outer surface and nozzzles which are fixed relative to the outer vessel are arranged to direct fluid jets tangentially with respect to the inner vessel, to impinge upon the blades, thereby effecting rotation of the inner vessel.

4. Apparatus as claimed in Claim 1 or Claim 2 in which the inner vessel is rotatable by means of a hydraulic motor acting upon the inner vessel.

5. 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 drawings.

6. A method for the removal of coke deposited on the surface of the inner vessel of a reactor for the thermal cracking of heavy hydrocarbons, the reactor comprising an outer vessel and, within the outer vessel an inner vessel having an opening at its base, the method comprising rotating the inner vessel about a vertical axis and directing fluid jets at the surface of the inner vessel while it is rotating.

7. A method as claimed in Claim 6 in which the fluid is fluid feedstock or coolant recycle.

8. A method as claimed in Claim 6 or Claim 7 in which the inner vessel is provided with a series of blades and the fluid jets are directed tangentially with respect to the inner vessel to impinge upon the blades, thereby effecting rotation of the inner vessel.

9. A method as claimed in Claim 6 or Claim 7 in which the inner vessel is rotated by means of a hydraulic motor.

10. A method as claimed in any of Claims 6 to 9 in which the level of liquid within the reactor is lowered to approximately that of the bottom of the inner vessel.

11. A method for the removal of coke substantially as herein specifically described with reference to and as shown in the accompanying drawings.