WO1998036036A1

WO1998036036A1 - Delayed coking with external recycle

Info

Publication number: WO1998036036A1
Application number: PCT/US1997/023158
Authority: WO
Inventors: Gary C. Hughes; Jim R. Roth; Bruce A. Newman
Original assignee: Conoco Inc.
Priority date: 1997-02-13
Filing date: 1997-12-11
Publication date: 1998-08-20
Also published as: ZA9711637B; AU5898698A

Abstract

A delayed coking process for a coker feedstock having a high propensity toward fouling is disclosed. The process includes feeding a coker feedstock (10) to the lower section of a coker fractionator (12), adding a recycle stream (18) derived from an external source to the coker fractionator bottoms prior to passing the bottoms to a furnace (14), passing the heated coker fractionator bottoms to a coking drum (16) and returning the overhead vapors from the coking drum to the coker fractionator (12).

Description

DELAYED COKING WITH EXTERNAL RECYCLE

Background of the Invention

1. Field of the Invention This invention relates to delayed coking, and more particularly to a delayed coking process in which the feed is a material having a high propensity for coker furnace fouling.

2. Background Art Delayed coking is a process for treating various resid streams from refinery processes in order to enhance the value of the resid streams. The most common feeds in delayed coking are atmospheric and vacuum resid streams obtained during distillation of crude oil. In the basic delayed coking process as practiced today, feedstock is introduced to a fractionator, and the fractionator bottoms including recycle material are heated to coking temperature in a coker furnace. The hot feed then goes to a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes to form coke and volatile components. The volatile components are recovered and returned to the fractionator. When the coke drum is full of solid coke, the feed is switched to another drum, and the full drum is cooled and emptied by conventional methods.

In the design and operation of a delayed coker, the furnace is the most critical piece of equipment. The furnace must be able to heat the feedstock to coking temperatures without causing coke formation on the furnace tubes. When the furnace tubes become coked, the operation must be shut down and the furnace cleaned out.

Furnace tube fouling is a major concern in coking normal coker feedstocks. One approach to this problem is described in U.S. Patent No. 4,455,219 to Janssen et al, wherein an internal recycle stream having a lower boiling range than conventional coker heavy recycle is substituted for part of the conventional recycle. This approach has proven successful when used with conventional coker feedstocks.

In addition to the conventional coker feedstocks mentioned above, there is an interest in coking more intractable feedstocks such as solvent deasphalted pitch, visbreaker bottoms and deep vacuum resids. These feedstocks are very difficult to coke by conventional processing, primarily because they have a high propensity to foul the coker furnace, even when relatively high recycle rates are used to dilute the feed to the coker furnace.

Solvent deasphalting is a process in which a resid stream is mixed with a light hydrocarbon to extract deasphalted oil from the resid stream. The resulting pitch product from the deasphalting process is difficult to process or assimilate into other products. One way of processing solvent deasphalted pitch (SDA pitch) would be to feed it to a delayed coker. However, the pitch presents problems when heated to coking temperature in a coker furnace because of its high tendency to foul the furnace tubes.

Visbreaking is a process for upgrading a resid stream by thermal processing. The residue from a visbreaker is also difficult to coke because of furnace fouling problems.

Some vacuum distillation units now in service produce a "deep" resid having a very high boiling range, such as from 1000°F and up. These deep resids also present furnace fouling problems when they are coked. Accordingly, there has been a need for a process that would provide an acceptable run period for a coking process using a feedstock that is especially prone to produce furnace fouling problems. Summary of the Invention According to the present invention, a delayed coking process utilizing as the feedstock a stream that is prone to produce furnace fouling is made feasible by utilizing an external recycle or diluent stream. This external recycle or diluent stream is combined with the feed prior to heating the feed in the coker furnace. The external recycle or diluent has a lower furnace fouling propensity than an internal recycle stream would have, and when added to the feedstock provides sufficient protection against furnace fouling to enable run periods greater than could be obtained using internal recycle material. Description of the Drawings Figure 1 is a schematic flowsheet illustrating the process of the invention.

Figure 2 is a schematic flowsheet illustrating a specific variant of the process of the invention. Description of the Preferred Embodiments In conventional coking operations, streams internal to the delayed coker unit such as flash zone gas oil (also called natural recycle), heavy coker gas oil, light coker gas oil and coker naphtha are used to reduce fouling of the coker furnace. Typically, from 3 to 30 volume percent of internal recycle is added to the fresh feed to the unit prior to processing of the combined stream in the coker furnace. These internal recycle streams have proven both experimentally and commercially to reduce furnace fouling. When using low quality coking feedstocks having a particularly high propensity toward furnace fouling, such as the aforementioned solvent deasphalted pitch, visbreaker bottoms or deep resids, the internal recycle streams are not adequate to control furnace fouling, and largely for that reason those feeds have not been used as the primary feedstock in delayed coking operations. Small amounts of these feedstocks can be blended in with normal coker feedstocks, but the use of them as the primary source of feed to a coker has not heretofore been feasible because of furnace fouling problems.

In accordance with this invention, an external recycle or diluent is substituted for the normal internal recycle used in conventional coking. The external recycle or diluent may be an FCC slurry oil, a hydrotreated FCC slurry oil, an atmospheric or vacuum gas oil, an FCC light cycle oil, hydrocracker distillate, hydrotreated virgin and/or cracked gas oils and naphthas, and mixtures thereof. Additionally, a product stream from the coker fractionator may be removed from the coker unit, hydrotreated alone or in combination with other streams, and returned as all or part of the external recycle or diluent.

These external recycle streams, including coker fractionator streams that have been removed from the coker and hydrotreated, are better suited than the internal recycle streams to reduce the fouling tendency of low quality feedstocks processed in the coker furnace of a delayed coker.

The process of the invention in its broadest context is illustrated in Figure 1, wherein an intractable feedstock is fed via line 10 to the lower portion of coker fractionator 12. The bottoms stream from fractionator 12 is fed to coker furnace 14 where it is heated to coking temperature and then fed to one of a pair of coke drums 16. Overhead vapors from coke drum 16 are returned to the fractionator, and product streams are recovered from the fractionator. An external recycle or diluent stream is added to the furnace feed via line 18. The external recycle may be any of a variety of hydrocarbon streams as mentioned above, so long as it is not primarily a stream taken directly from the coker fractionator. The coker fractionator streams that would normally be used as recycle still contain a fairly high level of coke forming components, and as a result they do not provide the protection against furnace fouling that an external recycle stream provides. By using an external recycle stream that is low in coke-forming components, longer run periods between furnace cleanouts can be obtained even though the feedstock to the coker unit is primarily an intractable material such as SDA pitch, visbreaker bottoms or deep resid.

A variation of the process of the invention is illustrated in Figure 2, where an intractable coker feedstock is fed via line 10 to coker fractionator 12. The bottom stream from fractionator 12 is heated to coking temperature in furnace 14 and then coked in coke drum 16. Overhead vapors from coke drum 16 are returned to fractionator 12. A heavy coker gas oil stream from fractionator 12 is passed via line 20 to hydrotreater 22. The heavy coker gas oil may be combined with another hydrotreater feedstock via line 24, and part of the hydrotreated product may be recovered via line 26. Another part of the hydrotreated heavy coker gas oil, optionally combined with another recycle stream from line 28, is combined with fresh coker feed via line 30 as external recycle.

The actual source (s) of the recycle material may be a variety of hydrocarbon streams, so long as most of the recycle material is not taken directly from the coker unit without being treated to reduce the coke forming components.

EXAMPLE 1 In this example, an SDA pitch feedstock is fed to a coker fractionator. The bottom stream from the fractionator is combined with about 15 percent by volume of hydrotreated gas oil and then fed to a coker furnace, where it is heated to coking temperature and then fed to a coke drum. Coke drum overhead vapors are returned to the coker fractionator, and product streams are recovered from the fractionator. The coker unit run length is considerably increased over the run length that would be obtained using an internal recycle stream.

EXAMPLE 2

In this example, a coker feed comprised of visbreaker bottoms is fed to a fractionator. Fractionator bottoms are fed to a coker furnace and then to a coke drum. Overhead vapors from the coke drum are returned to the fractionator. A heavy coker gas oil stream from the fractionator is removed from the coker unit, combined with another refinery stream, and passed to a hydrotreater. A portion of the hydrotreater output is combined with the feed to the coker unit as external recycle. The coker run length is considerably greater than it would be if conventional heavy gas oil had been used as internal recycle in the process.

In the process of the invention, the external recycle may be combined with the coker feed ahead of the fractionator, as shown in Figure 2, or just prior to the furnace, as shown in Figure 1. The boiling range of the external recycle material determines the point of combination. Obviously, a lighter boiling material is best combined with the furnace feed near the furnace, rather than upstream of the fractionator, to avoid having most of the recycle be distilled off in the fractionator. While specific versions of the invention have been described above, the essential feature of the invention in its broadest aspect is that a recycle material from outside the coker unit is combined with an intractable coker feedstock to reduce fouling in the coker furnace. The external recycle material may one or more of a variety of hydrocarbon streams, so long as the external recycle material is more effective at reducing furnace fouling than an internal recycle stream would be.

The foregoing description of the preferred embodiments is intended to be illustrative rather than limiting of the invention, which is defined by the appended claims. We claim:

Claims

Clai 1. In a delayed coking process in which a coker feedstock is fed to the lower section of a coker fractionator, the bottoms from said fractionator are fed to a coker furnace, the feed from the furnace is passed to a coke drum, overhead vapors from said coke drum are returned to said fractionator, and product streams are recovered from said fractionator, the improvement wherein: (a) said coker feedstock has a high propensity toward furnace fouling; and (b) a recycle stream derived from a source external of said coking process is added to said feedstock prior to heating said feedstock in said furnace.

Claim 2. The process of Claim 1 wherein said recycle stream is added in an amount of from 3 to 30 percent by volume of said feedstock.

Claim 3. The process of Claim 1 wherein said recycle stream is selected from the group consisting of hydrotreated FCC slurry oil, atmospheric distillation unit gas oil, vacuum distillation unit gas oil, hydrotreated virgin or cracked gas oils, hydrotreated FCC light cycle oil, hydrotreated coker naphtha, hydrocracker distillate and mixtures thereof.

Claim 4. The process of Claim 1 wherein said recycle stream is at least in part comprised of a product stream from said coker fractionator that has been removed from the coker unit and subjected to hydrotreating prior to being added to said feedstock.

Claim 5. The process of Claim 1 wherein said feedstock is selected from the group consisting of solvent deasphalted pitch, visbreaker bottoms and deep vacuum resids.

Claim 6. The process of Claim 1 wherein said feedstock is solvent deasphalted pitch and said hydrocarbon stream is at least in part comprised of a heavy coker gas oil from said fractionator that has been removed from the coker unit and subjected to hydrotreating prior to being added to said feedstock.