Process of Modification of a Feedstock in a Delayed Coking Unit

Abstract

A process is described of delayed coking optimized for greater yield of diesel oil from coke by means of modifications to the feedstock in a Delayed Coking Unit. According to the present invention in a first embodiment of the invention the feedstock includes the bottom product of the vacuum distillation tower, known in the prior art as vacuum residuum and the heavy gas oil from coke obtained in the fractionation tower and recycled to comprise the aforesaid combined feedstock. In a second embodiment of the present invention the feedstock consists of: the bottom residuum proceeding from the atmospheric distillation tower, known in the prior art as atmospheric residuum, and heavy gas oil from coke obtained in the fractionation tower and recycled to comprise the aforesaid feedstock of the unit.

Description

BACKGROUND OF THE INVENTION

The field of application of the present invention is delayed coking processes. Particularly in delayed coking processes wherein the yield of diesel oil is maximised whereas the yield of heavy gas oil from coke is minimised through modifications to the feedstock of a Delayed Coking Unit.

DESCRIPTION OF PRIOR ART

The process of delayed coking of residual petroleum fractions has been employed in the petroleum refining industry for some time. This process permits conversion of heavy petroleum fractions into lighter products having greater value added such as, for example, liquefied petroleum gas (LPG), naphtha, gas oils and coke.

In a conventional delayed coking process the new feedstock, generally a vacuum residuum, is fed to the bottom region of the fractionation tower wherein incorporation of the natural recycle occurs forming the combined feedstock of the Unit. Normally the natural recycle is employed to adjust the quality of heavy gas oil from coke to be sent to any Fluid Catalytic Cracking (FCC) Unit.

The combined feedstock is sent to a furnace wherein it must dwell for a very short time, of the order of a few minutes, such that the thermal cracking reactions can be initiated and the formation of coke in the furnace tubes be minimised.

On leaving the furnace at a temperature of the order of 500° C. the cracked feedstock is fed to the coke drum wherein the thermal cracking and coking or carbonisation reactions are completed. These reactions generate hydrocarbons lighter than those in the combined feedstock and coke. The reactions which take place in a coke drum are endothermic and the temperature of the effluents from the drum lie within a band of values from 430° C. to 455° C.

The coke formed accumulates in the drum until it requires to be removed following stages of steam purging and cooling with water. With the objective of removing the accumulated coke in a coke drum the effluent from the coke drum is diverted to another empty coke drum wherein the accumulation phase is initiated. Removal of the coke is carried out by means of high-pressure-water cutting devices.

The effluents from the coke drum are then sent to a fractionation tower of a Delayed Coking Unit wherein they are separated into:

• • A mixture of fuel gas, LPG and light naphtha exiting from the top of the fractionation tower, known for this reason in the prior art as top gases; and
  • Side drawings of heavy naphtha, light gas oil (LGO) from coke and heavy gas oil (HGO) from coke.

In order to achieve better operational yield special care is taken at some stages of the delayed coking process, i.e.:

• • It is desirable that coke formation occurs solely within a coke drum and not within the tubes of the furnace. Thus the combined feedstock dwells in the furnace for solely a few minutes in order to minimise the formation of coke within the tubes thereof; and
  • In order to prevent the reactions proceeding and possible undesirable deposition of coke in the outlet tubing of the coke drum a rapid cooling (quench) is carried out employing a stream of gas oil and/or residuum.

With the discovery of increasingly-heavy petroleums the delayed coking process in refineries has experienced an increase in its degree of importance, principally due to an increase in the yield of residuum from such petroleums.

The delayed coking process is well-known in the prior art. One of the oldest processes is disclosed by U.S. Pat. No. 3,563,884. The aforesaid patent describes a process wherein tar is utilised as raw material and a heavy gas oil recycle is provided for.

Some variations have been introduced based on the aforesaid invention. U.S. Pat. No. 4,213,846 discloses a delayed coking process for the formation of premium coke wherein the recycle is hydrotreated.

U.S. Pat. No. 4,177,133 describes a delayed coking process for the formation of premium coke wherein the new feedstock having passed through a preheating stage is subjected to flash distillation to remove non-crystalline substances.

U.S. Pat. No. 4,455,219 and U.S. Pat. No. 4,518,487 disclose a delayed coking process wherein part or all of the heavy hydrocarbon product commonly used as recycle is replaced by a lighter hydrocarbon, which same is combined with the new feedstock of the unit.

U.S. Pat. No. 4,661,241 describes a delayed coking process wherein the yield of coke is minimised and the yield of liquid products is maximised by means of the elimination of recycle.

U.S. Pat. No. 5,711,870 discloses a process of delayed coking wherein the fresh feedstock is mixed with water and, optionally, with a hydrogen donor such as methane or gas oil derived from the recycle in order to optimise the yield of liquid products and reduce the yields of coke and gas.

As may be observed there is a tendency to develop delayed coking processes with the objective of maximising the yield of liquid products, principally petrol, and reducing the yield of coke and gas. In order to achieve this objective there is a tendency to reduce the rate of recycle of the delayed coking process and increase the conditions of severity in the vacuum distillation tower in order to maximise separation of heavy vacuum gas oil.

In this manner the quality of production of a heavy vacuum gas oil suitable for use as feedstock for a Catalytic Cracking Unit is prioritised. This leads to the generation of increasingly heavy vacuum residuums at the bottom of the vacuum distillation tower.

Thus for refining programmes wherein there are excesses of gas oil and vacuum residuum and greater demand for light gas oil from coke, the state of the art is moving towards solutions making simultaneous conversion viable in order to maximise the yield of diesel oil from the refinery.

The diesel oil from a refinery comprises diverse streams, among them light gas oil from coke produced in a Delayed Coking Unit. As the present invention described below refers to diesel oil produced from light gas oil from coke, hereinafter such diesel oil will be referred to as diesel oil from coke.

SUMMARY OF THE INVENTION

The process of modification of a feedstock in a Delayed Coking Unit, subject of the present invention, considers a solution maximising the yield of diesel oil from coke and minimising the yield of heavy gas oil from coke by means of modifications to the feedstock of a Delayed Coking Unit.

According to the present invention the feedstock consists of: the bottom product from the vacuum distillation tower, known in the prior art as vacuum residuum, and the heavy gas oil from coke obtained in the fractionation tower and recycled in order to comprise the aforesaid combined feedstock. The percentage by volume of heavy gas oil from coke in the new feedstock lies within a band of values from 16% to 50%. Preferentially within a band of values comprised between 20% and 40%.

In a second embodiment of the present invention the feedstock consists of: the bottom residuum proceeding from the atmospheric distillation tower, known in the prior art as atmospheric residuum, and the heavy gas oil from coke obtained from the fractionation tower and recycled to comprise the aforesaid feedstock of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of modification of feedstock in a Delayed Coking Unit, subject of the present invention, will be better understood by means of the detailed description, given below solely as an example, in association with the drawings referred to below, which same are integral parts of this description.

FIG. 1 shows schematically a delayed coking process, according to the prior art.

FIG. 2 shows schematically a process of modification of a feedstock in a Delayed Coking Unit according to a first embodiment of the present invention.

FIG. 3 shows schematically a process of modification of a feedstock in a Delayed Coking Unit according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The description of the process of modification of a feedstock in a Delayed Coking Unit, subject of the present invention, will be provided in concordance with the identification of the respective components, based on the figures described above.

FIG. 1 shows schematically a delayed coking process according to the prior art. A fresh feedstock (1) is fed to a fractionation tower (2) whence diverse derivatives are removed such as, for example, fuel gas and LPG (3), light naphtha (4), heavy naphtha (5), light gas oil (6), medium gas oil (7) and heavy gas oil from coke (8).

The bottom product (9) of the fractionation tower (2) is fed to a furnace (10) such that the thermal cracking reactions may be initiated. The effluent from the furnace (11) is subsequently sent to a coke drum (12) wherein the thermal cracking and coking or carbonisation reactions are completed, generating coke and an effluent from the coke drum (13) comprising light hydrocarbons. The effluent from the coke drum (13) is subsequently sent to the fractionation tower (2).

At the commencement heavy gas oil from coke (8) is sent to a Fluid Catalytic Cracking Unit (not shown in the figure) wherein it is used as raw material for the production of petrol.

FIG. 2 shows schematically a process of modification of a feedstock in a Delayed Coking Unit according to the present invention. A fresh feedstock (1) is fed to a fractionation tower (2) whence several derivatives are removed such as, for example, fuel gas and LPG (3), light naphtha (4), heavy naphtha (5), light gas oil (6), medium gas oil (7) and heavy gas oil from coke (8).

A fraction (8′) of heavy gas oil from coke (8) is added to the bottom product (9) from the fractionation tower (2). The percentage by volume of the fraction (8′) of heavy gas oil from coke (8) in relation to the fresh feedstock (1) lies within a band of values from 16% to 50%. Preferentially the percentage by volume of the fraction (8′) of heavy gas oil from coke (8) in relation to the fresh feedstock (1) lies within a band of values from 20% to 40%.

The aforesaid fraction (8′) of heavy gas oil from coke (8) may be added to the bottom product (9) by means of a line external to the fractionation tower (2), according to the embodiment shown in FIG. 2.

Alternatively the aforesaid fraction (8′) of the heavy gas oil from coke (8) may be added to the bottom product (9) within aforesaid fractionation tower (2).

The feedstock thus combined (9′) is subsequently sent to the furnace (10) in order that the thermal cracking reactions may be initiated. The effluent from the furnace (11) is subsequently sent to a coke drum (12) wherein the thermal cracking and coking or carbonisation reactions are completed, generating coke and an effluent from the coke drum (13) comprising light hydrocarbons. The effluent from the coke drum (13) is subsequently sent to the fractionation tower (2).

FIG. 3 shows schematically a process of modification of a feedstock in a Delayed Coking Unit according to a second embodiment of the present invention. Petroleum (14) is fed to an atmospheric distillation tower (15) whence diverse derivatives are removed such as, for example, fuel gas (16), naphtha (17) and others not shown in this figure. In this manner the feedstock of the Delayed Coking Unit is the bottom residuum (18) from the atmospheric distillation tower (15), known in the prior art as atmospheric residuum, and a fraction (8′) of heavy gas oil from coke (8) proceeding from the fractionation tower (2) is added to the bottom product (9) of the fractionation tower (2).

EXAMPLES

The present invention may be better understood by means of the examples below. However the examples do not limit the present invention.

In the examples there have been employed an atmospheric residuum (AR) and a vacuum residuum (VR) having the properties according to Table 1:

	TABLE 1
	AR	VR
	RCR (% w/w)	7.3	15.0
	° API	14.3	9.5
	S (%)	0.67	0.74

Example 1

A vacuum residuum was processed in a pilot delayed coking unit without heavy gas oil from coke recycle. The temperature of the furnace was 0.500° C. and the pressure at the top of the coke drum was 2 kgf/cm²g. Volume yields of 51.3% for diesel oil from coke and of 20.2% for heavy gas oil from coke were obtained. The mass yield of coke was 24.5%.

Example 2

A vacuum residuum was processed in an industrial delayed coking unit having a furnace temperature of 500° C. and pressure at the top of the coke drum of 2 kgf/cm²g and a heavy gas oil from coke recycle rate of 8%. Volume yields of 54.9% for diesel oil from coke and of 14.6% for heavy gas oil from coke were obtained. The mass yield of coke was 25%.

Example 3

A vacuum residuum was processed in a pilot delayed coking unit having a furnace temperature of 500° C., pressure at the top of the coke drum of 2 kgf/cm²g and total recycle of heavy gas oil from coke. The volume yield was 68.2% for diesel oil from coke. The mass yield of coke was 26%.

Example 4

An atmospheric residuum was processed in a pilot delayed coking unit, without heavy gas oil from coke recycle, having a furnace temperature of 500° C. and pressure at the top of the coke drum of 2 kgf/cm²g. Volume yields of 53.5% for diesel oil from coke and of 27.7% for heavy gas oil from coke were obtained. The mass yield of coke was 13.5%.

Example 5

An atmospheric residuum was processed in an industrial delayed coking unit having a furnace temperature of 500° C., pressure at the top of the coke drum of 2 kgf/cm²g and a heavy gas oil from coke recycle rate of 25%. Volume yields of 62.9% for diesel oil from coke and of 14.0% for heavy gas oil from coke were obtained. The mass yield of coke was 15.2%.

Example 6

An atmospheric residuum was processed in a pilot delayed coking unit having a furnace temperature of 500° C., pressure at the top of the coke drum of 2 kgf/cm²g and total recycle of heavy gas oil from coke. The volume yield was 72.6% for diesel oil from coke. The mass yield of coke was 17%.

In the above examples there is noted an increase in yield in terms of diesel oil from coke and a reduction in yield of heavy gas oil from coke with an increase in the recycle rate of the process. In this manner by means of the herein described present invention there occurs a growing increase in the yield of diesel oil and a significant reduction in the yield of heavy gas oil from coke.

The description hereinbefore provided of the process of modification of a feedstock in a Delayed Coking Unit, subject of the present invention, must be considered solely as a possible embodiment or embodiments and any particular characteristics introduced therein shall solely be understood to be something described to facilitate comprehension. In this manner they cannot be considered to limit in any way the present invention which is restricted to the scope of the claims below.

Claims

1. A process for modification of a feedstock in a delayed coking unit wherein: characterised in that to the bottom product (9) of the fractionation tower (2) there is added a fraction (8′) of heavy gas oil from coke (8).

a fresh feedstock (1) is fed to a fractionation tower (2) wherefrom one or more derivatives are removed;

a bottom product (9) of the fractionation tower (2) is fed to a furnace (10) such that one or more thermal cracking reactions may be initiated;

effluent from the furnace (11) is subsequently sent to a coke drum (12) wherein the thermal cracking and coking and/or carbonisation reactions are completed, generating from the coke drum (13) coke and an effluent comprising light hydrocarbons;

the effluent from the coke drum (13) is subsequently sent to the fractionation tower (2);

2. A process according to claim 1 in which the one or more derivatives comprise one or more of fuel gas, LPG (3), light naphtha (4), heavy naphtha (5), light gas oil (6), medium gas oil (7) and heavy gas oil from coke (8).

3. A process according to claim 1 or 2, characterised in that the percentage by volume of the fraction (8′) of heavy gas oil from coke (8) in relation to the fresh feedstock (1) is from 16% to 50%, preferably from 20% to 40%.

4. A process according to claim 1, 2 or 3, characterised in that the fraction (8′) of heavy gas oil from coke (8) is added to the bottom product (9) by means of a line external to the fractionation tower (2).

5. A process according to any preceding claim, characterised in that the aforesaid fraction (8′) of heavy gas oil from coke (8) is added to the bottom product (9) within the fractionation tower (2).

6. A process according to any preceding claim, characterised in that the feedstock of a delayed coking unit is the bottom residuum (18) of the atmospheric distillation tower (15).