GB2087920A

GB2087920A - Process and apparatus for the distillation and thermal cracking of a crude oil feedstock

Info

Publication number: GB2087920A
Application number: GB8036126A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-11-11
Filing date: 1980-11-11
Publication date: 1982-06-03
Also published as: GB2087920B

Abstract

Feedstock 31 is fed to a flash- settler 11 having three sections 21, 23 and 25. Liquid is withdrawn from section 25 and fed to a thermal cracking reactor 13 via a heater 12. The reaction products are quenched to inhibit cracking by means of a cooling stream 36 prior to their introduction to section 21 of the flash-settler 11. Coke is removed from section 21 as a slurry 37. Gases flashed off from the feedstock and from the reaction products are fed to a distillation column 14 where they are separated. Liquid from section 23 is either removed as product, allowed to overflow into section 25 to be recycled, or fed to a reduced pressure column 51 in which heavy gas oil may be removed as a product 53. The top product 56 is fed to the distillation column 14, the bottom product 57 is recycled to the reactor 13 via the heater 12. <IMAGE>

Description

SPECIFICATION Process and apparatus for the distillation and thermal cracking of a crude oil feedstock The present invention relates to a process and apparatus for the distillation and thermal cracking of a crude oil feedstock for example the simultaneous primary separation by means of distillation of crude oil into its main petroleum fractions and the thermal cracking of the heavier fractions.

In petroleum refining the initial stage of processing is generally to separate crude oil into its main petroleum fractions, namely, gas, naphtha, gas oils and reduced crude by means of distillation. Sometimes, intermediate fractions are separated and these fractions are often further processed. The further processing is frequently the technique referred to as thermal cracking which may range from the relatively light process of visbreaking to the relatively severe process of coking. The feedstock for these processes may be reduced crude or in some cases vacuum residue.

Vacuum residue is the term given to the bottom products from the processing of reduced crude in a distillation column under reduced pressure to separate petroleum fractions referred to as heavy gas oils from the vacuum residue.

In known applications, the distillation process and the thermal cracking processes are carried out separately.

In one known process of crude oil distillation, the crude oil is heat exchanged against previously separated petroleum fractions, passed through a fired heater and then passed to a distillation column. Sufficient heat is provided in the fired heater to effect the distillation. The distillation column normally operates at a pressure slightly above atmospheric pressure and the feed temperature to the column is normally restricted to about 3450C to avoid thermal cracking. The quantity of reduced crude produced is usually about half the feedstock.

In a known process for the thermal cracking of reduced crude, the feedstock is passed directly into the bottom of a distillation column as is the outlet stream from a cracking reactor. The bottoms from the distillation column comprises the heavier fractions from the feedstock and the recycle. This is passed to a fired heater where it is heated to about 50O0C and then passed to a reactor, generally referred to as a coking drum.

From the reactor the fluid product is returned to the distillation column where the lighter fractions flash off and are distilled into separate petroleum fractions.

These stages suffer from a number of disadvantages. There is usually excess heat in the distillation system and it is fairly common to recover this heat by means of steam generation an inter-condenser. Clearly, this is wasteful in terms of energy. Furthermore, the process is regarded as dirty and there is a considerable deposition of solids in the form of coke, both in the heater and in other items of equipment, in addition to that formed in the coking drum, which also causes fouling.

It is an object of the present invention to simplify these processing steps and to reduce the combined thermal energy requirements.

It is a further object of the invention to increase the flexibility to accommodate a wide range of feedstocks and to enable a wide range of products to be obtained.

It is a further object of the invention to reduce the general fouling tendency due to deposition of coke outside the reactor, since this tends to impair process performance and tends to make process control more difficult.

It is a further object of the present invention to improve the product yield structure.

According to the invention there is provided a process for the distillation and thermal cracking of a crude oil feedstock comprising: introducing the feedstock to a separation vessel; withdrawing liquid, including heavier fractions, from the separation vessel; heating at least a portion of the liquid withdrawn from the separation vessel; reacting the heated liquid in a reactor where the heavier fractions are at least partially cracked; feeding an outlet stream, including cracked products, from the reactor to the separation vessel; withdrawing gas from the separation vessel and transferring the gas to a distillation column where the gas is separated into various fractions, and cooling the cracked products prior to their being fed to the separation vessel.

Preferably, the cracked products are cooled using liquids from the separation vessel, and such cooling may be effected within the reactor. Both liquid product and coke may be withdrawn from the separation vessel.

The process may include the further step of feeding liquid, including cracked products, to a reduced pressure column where the lighter fractions may be separated. The bottom product from the reduced pressure column may be recycled to the reactor and/or may be used to cool the cracked products. The top product from the reduced pressure column may be fed to the distillation column or may be removed as product.

Preferably, the separation vessel is a flashsettler which may be divided into three sections by means of two weirs. Liquid may overflow from the first to the second and to the third section over the first and second weirs respectively. Gases from the feedstock and from the reactor cracked products may be flashed off in the flash settler.

The coke may be separated from the liquids by means of the first weir.

Preferably, the feedstock is heated prior to its introduction to the separation vessel by heat exchange with product streams from the distillation column and/or the reduced pressure column which may provide inter-condensing and condensing heat exchange within the distillation systems.

It will be appreciated that the reactions which take place in thermal cracking are complex and are only partially understood. However, it is well known that some take longer than others. Ideally, when some light fractions have formed in the reactor they should be removed to minimise further cracking of these fractions into unwanted gaseous components. To remove selectively from the reactor those fractions which have cracked to the optimum degree is not practical, consequently, the residence time in the reactor represents a compromise between minimising the quantity of recycle without causing excessive cracking exposure.By providing the heat input required to effect both distillation of the feedstock and cracking at essentially one point, namely, a heater for the reactor feed, larger recycle quantities and shorter residence times can be catered for and thus a more desirable product yield may be obtained.

It is possible, therefore, that in the process according to the invention, the thermal energy consumed is less than the equivalent combined thermal energy consumed in crude distillation and thermal cracking when carried out separately. In a conventional thermal cracking process, the outlet temperature from the reactor is about 475 CC and normally contains thermal energy in excess of that required to distil the cracked liquid into the main petroleum fractions. Generally, in crude oil distillation, thermal energy is given to the distillation system. It will be appreciated that the process in accordance with the invention may exploit this imblance so that the excess heat in the thermal cracking part of the process may be used in vaporising feedstock.

A further advantage of the process of the present invention is that the process may operate with only one atmospheric distillation column to separate the fresh crude feedstock and the cracked liquid into the light petroleum fractions.

Furthermore, the heat exchanger requirements may be a good deal simpler.

A further advantage of the process in accordance with the present invention is that only streams in the vapour phase may enter the atmospheric distillation column, other than reflux, with the result that fouling in this major item of equipment may be minimised. Coke particles and other solids will tend to deposit in the flash-settler.

A still further advantage is that the process is particularly flexible and can accommodate fresh feedstock ranging from light crude to reduced crude and also be used to carry out processes ranging from mild cracking (visbreaking) to severe cracking (coking).

According to a second aspect of the present invention, there is provided apparatus for the distillation and thermal cracking of a crude oil feedstock comprising a separation vessel, a reactor and a distillation column; the separation vessel having a feedstock inlet, a gas outlet leading to the distillation column, a cracked products inlet leading from the reactor and a liquid outlet leading to the reactor, the apparatus further including means for heating liquid withdrawn from the separation vessel liquid outlet and means for cooling cracked products prior to their entry into the separation vessel.

The invention may be carried into practice in various ways and two embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a first embodiment of a process in accordance with the invention; and Figure 2 is a schematic diagram similar to Figure 1 showing a second embodiment.

Referring to Figure 1, the apparatus comprises, essentially, a flash-settler 11, a cracker heater 12, a cracker reactor 1 3 and an atmospheric distillation column 14. The flash-settler 11 is typically 6 metres wide and is separated into three sections 21,23 and 25 by means of two weirs 22 and 24 which are arranged so that liquid can overflow from section 21 into section 23 and then into section 25. The sections 21,23 and 25 are typically 25 metres, 3 metres and 3 metres in length respectively.

The process operates as follows. The feedstock 31 is heat exchanged against a stream withdrawn from the distillation column 14 in a heat exchanger 33. Some of the stream is withdrawn as product in stream 32, while the remainder is returned to the column 14, providing cooling and/or inter-condensation at this section of the column 14. The heated feedstock is passed to section 25 of the flash-settler 11 where it is mixed with a recycle stream overflowing from section 23 of the flash-settler. The flash-settler typically operates at a pressure of 2 atm, and the temperature in section 25 is typically 2000 C. A portion of this liquid from section 25 is fed to the reactor 13 via outlet stream 34 and heater 12.

In the reactor 13, the heavy fractions in the liquid are subjected to a temperature of about 5000C and a pressure of about 7.0 atm and cracking takes place. The reactor outlet stream 35 is fed to section 21 of the flash-settler 11, and liquid from section 25 is used to quench the stream entering section 21. This may take place by means of a cooling stream 36 as the liquid leaves the reactor 1 3 or may take place within the reactor 1 3 by means of a cooling stream 36'. The liquid is quenched from about 4750C to between 3000C and 3600C so that cracking of the streams entering the flash-settler 11 will have essentially stopped, but not over quenched so as to reduce vapour flashing off in section 21. Quench liquid may also be drawn from sections 21 and/or 23.

The temperature in section 21 should be kept as close to 3450C as is practically possible.

The quantity of quench liquid in stream 36 or 36' can be up to 2 2 times greater than the quantity fed to the heater 12, although this is dependent on the temperatures in section 25.

The reactor outlet 35 includes cracked fluid which may contain coke particles. As this enters section 21 of the flash-settler, coke particles can be allowed to accumulate and at the end of an operational run removed after the flash-settler 11 has been drained off and purged or alternatively coke particles can be removed from section 21 in slurry form via slurry outlet 37. The slurry may be led to a drain tank (not shown) and the separated liquid returned, or alternatively, the slurry may be returned to the reactor if the reactor is specifically designed to receive coke particles in slurry form.

The temperatures in sections 25 and 23 are typically at about 275cm.

The size of section 21 and in the form of a weir 22 ensures that the coke is trapped in section 21 and only liquid is allowed to overflow into section 23. If this liquid is not recycled it can be withdrawn as a liquid product 42.

A substantial proportion of the lighter fractions in the feedstock 31 will flash off in section 25 of the flash-settler. These will combine with vapour from the reactor outlet stream 35 which will flash off in section 21, and these are then passed to the distillation column 14 from gas outlet 43. Here they are separated into the main petroleum fractions, namely, gas 38, naphtha 39, light gas oils in product stream 32 and other gas oils 41.

Intermediate products may be drawn off, if required.

Figure 2 shows a second embodiment which may be used, for example, where a heavy gas oil fraction is required as a product. In this case, the system further includes a reduced pressure column 51. The system operates in substantially the same way as that described with reference to Figure 1 except for the incorporation and interaction of the reduced pressure column 51.

Liquid from section 23 of the flash-settler 11 is fed to the reduced pressure column 51 via liquid product outlet 42'. A reduced pressure of about 0.2 atm is induced in the column 51 by means of a positive displacement compressor 52, though any suitable vacuum source may be used, for example a steam injection device. Heavy gas oil is flashed off and removed via gas oil product outlet 53. This stream is heat-exchanged against the feedstock stream 31 in a heat exchanger 54.

The overhead vapours 55 from the reduced pressure column 51 can be recovered as a product or alternatively are fed to the atmospheric distillation column 14 and introduced via a vapours inlet line 56 at a suitable point, typically a few trays up from the bottom of the distillation column 14.

The bottoms 57 from the reduced pressure column 51 can either be passed directly to the cracker heater 12, withdrawn as a product or returned to section 25 of the flash-settler 11.

Figure 2 illustrates the bottoms 57 being passed directly to the cracker heater 1 2.

In an alternative method of operation, some of the bottoms 57 from the reduced pressure column 51 are used as quench liquid, since the liquid will be at a temperature of about 225"C, instead of liquid in the cooling stream 36 or 36' from section 25 of the flash-settler 11. The liquid from section 25 would then all be passed to the cracker heater 12.

Claims

1. A process for the distillation and thermal cracking of a crude oil feedstock comprising: introducing the feedstock to a separation vessel; withdrawing liquid, including heavier fractions, from the separation vessel; heating at least a portion of the liquid withdrawn from the separation vessel; reacting the heated liquid in a reactor where the heavier fractions are at least partially cracked; feeding an outlet stream, including cracked products, from the reactor to the separation vessel; withdrawing gas from the separation vessel and transferring the gas to a distillation column where the gas is separated into various fractions, and cooling the cracked products prior to their being fed to the separation vessel.

2. A process as claimed in Claim 1 in which the cracked products are cooled using liquid from the separation vessel.

3. A process as claimed in Claim 2 in which the cooling is effected within the reactor.

4. A process as claimed in any of Claims 1 to 3 in which coke is withdrawn as a product, from the separation vessel.

5. A process as claimed in any preceding claim in which liquid product is withdrawn from the separation vessel.

6. A process as claimed in any preceding claim including the further step of feeding some of the liquid withdrawn from the separation vessel, including cracked products, to a reduced pressure column where lighter fractions are separated.

7. A process as claimed in Claim 6 in which bottom product from the reduced pressure column is recycled to the reactor.

8. A process as claimed in Claim 6 or Claim 7 in which the cracked products are cooled using bottom product from the reduced pressure column.

9. A process as claimed in any of Claims 6 to 8 in which top product from the reduced pressure column is fed to the distillation column.

10. A process as claimed in any preceding claim in which the separation vessel is a flashsettler in which gases from the feedstock and from the reactor cracked products are flashed off, and in which coke is separated from the liquids by means of a weir.

1 A process as claimed in Claim 10 in which coke is withdrawn from the separation vessel as a slurry and is recycled to the reactor.

12. A process as claimed in any preceding claim in which feedstock is heated prior to its introduction to the separation vessel by heat exchange with product streams from the distillation column and/or the reduced pressure column providing inter-condensing within the distillation system and/or the reduced pressure column systems.

13. A process for the distillation and thermal cracking of a crude oil feedstock substantially as herein specifically described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings, or as modified as described.

14. Apparatus for the distillation and thermal cracking of a crude oil feedstock comprising a separation vessel, a reactor and a distillation column; the separation vessel having a feedstock inlet, a gas outlet leading to the distillation column, a cracked product inlet leading from the reactor and a liquid outlet the reacter having a liquid inlet arranged to receive at least some of the liquid withdrawn from the separation vessel, the apparatus further including means for heating liquid withdrawn from the separation vessel liquid outlet and means for cooling cracked products prior to their entry into the separation vessel.

1 5. Apparatus as claimed in Claim 14 further including a reduced pressure column and a liquid outlet from the separation vessel leading to the reduced pressure column.

1 6. Apparatus as claimed in Claim 1 5 including a liquid recycle conduit from the bottom of the reduced pressure column adapted to lead liquid from the reduced pressure column to the reactor.

1 7. Apparatus as claimed in Claim 1 5 or Claim 1 6 including a reduced pressure column gas outlet for transferring gas to the distillation column.

18. Apparatus as claimed in any of Claims 14 to 1 7 further including heat exchangers between the feed-stock stream one or more outlet streams from the distillation column and/or from the reduced pressure column.

19. Apparatus as claimed in any of Claims 14 to 1 8 in which the separation vessel is a flashsettler having one or more weirs dividing the flashsettler into sections, the weirs being arranged to separate solids from liquids and to allow liquids to flow from one section into another.

20. Apparatus as claimed in any of Claims 14 to 1 9 including a liquid recycle conduit from the separation vessel, adapted to lead liquid from the separation vessel to cool the cracked products.

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21. Apparatus for the distillation and thermal cracking of a crude oil feedstock constructed and arranged substantially as herein specifically described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings, or as modified as described.