CA1257844A - Visbreaking process - Google Patents

Abstract

"VISBREAKING PROCESS"

An improved visbreaking process having a lower utilities (heating fuel) cost is disclosed. The feed stream is heated by in-direct heat exchange against the bottoms stream of a first fractiona-tion column, which receives the effluent of the visbreaking heater.
A portion of this bottoms stream is then used as quench. This quench stream is hotter and has a higher flow rate than previous designs.
The feed is heated to a higher temperature by the indirect heat ex-change and therefore less fuel is required in the visbreaker fired heater.

Description

~LZ~7844 "VISBREAKING PROCESS"

FIELD OF THE INVE~TION

The invention relates to a hydrocarbon conversion process which may be employed in the refining of crude oils. The invention is an improved visbreaking process and therefore relates to the ther-mal processing o~ residual hydrocarbon streams normally produced by the fractional distillation of crude oil. The invention specifical-ly relates to the heat exchange which may be employed in such a pro-cess in order to minimize the fuel consumption within the process and to maximize heat recovery. The invention also specifically con-cerns the manner in which the effluent of the visbreaking heater or visbreaking reaction chamber is quenched with a lower temperature hydrocarbon stream to terminate the thermal cracking reaction.

INFORMATION DISCLOSURE

Visbreaking is a very well established commercial refining process. An extensive discussion of visbreaking and related thermal cracking processes is provided in the article appearing at page 101 of the May 1980 issue of Hydrocarbon Processinq. U.S. Patent 4,169,782 issued to H. L. Thompson presents a rather complete pro-cess flow diagram of a commercial visbreaking process. This refer-ence is also pertinent for its description of the materials which may be employed as the quench liquid which is admixed with the efflu-ent of the visbreaking heater.
An article appearing at page 109 of the April 13, 1981 edi-tion of the Oil and Gas Journal provides additional description of the visbreaking process. This article is especially pertinent for its ~257~34~

showing of the use of a portion of the bottoms stream of a fractiona-tor which receives the visbreaker effluent as the quench stream which is admixed into the visbreaker effluent. An article appearing at page 131 of the January 1979 issue of Hydrocarbon Processinq is also directed to visbreaking. This article is pertinent for the teaching which begins on page 135 in regard to the usefulness of quenching the visbreaking heater effluent stream and the various materials which can be employed as the quench stream.
The process flow diagram of a visbreaking unit shown in Figure 9 on page 22 of Volume 15 of the Second Edition of the Kirk-thmer Encyclopedia of Chemical Technoloqy shows various indirect heat exchangers employed in the process. The diagram is pertinent for indicating an awareness in the art of the desirability of heat-ing the charge stream by indirect heat exchange.

BRIEF SUMMARY OF THE INVENTION

The invention provides an improved visbreaking process by reducing both the capital and utilities cost of the process. These improvements are achieved by heating the feed stream to the visbreak-ing heater by indirect heat exchange in a manner which heats the stream to a higher temperature than in prior art processes. Since the feed stream is at a higher temperature when it enters the vis-breaking heater, less fuel must be consumed within the heater and the heater may be of smaller size.
A significant part of the subject process is the utiliza-tion of a quench stream having a higher temperature and higher flow rate than conventional visbreaking processes. The use of a larger quench stream provides an adequate temperature reduction of the vis-~ 257~344 breaker heater effluent even though the quench material is at a rela-tively high temperature.
The invention may be broadly characterized as a method of thermally processing a hydrocarbon stream which comprises the steps of heating a feed stream, which comprises a mixture of hydrocarbons having boiling points above 600F (315C), by indirect heat exchange against a hereinafter characterized first bottoms stream; passing the feed stream through a visbreaking zone, and admixing a resultant visbreak-ing zone effluent stream with a relatively high temperature quench stream to thereby form a first process stream; separating the first process stream in a first separation zone into the desired hydrocar-bon fractions including the previously referred to first bottoms stream; employing the first bottoms stream in said indirect heat ex-change and then dividing the first bottoms stream into at least said quench stream and a second process stream; and passing the second process stream into a second separation zone, and recovering a prod-uct stream from the second separation zone.

~RIEF DESCRIPTION OF THE DRAWING

The drawing is a simplified process flow diagram of a pre-ferred embodiment of the invention. The drawing has been simplified by the elimination of various process equipment customarily employed on such a process including flow control systems, temperature and pressure control systems, pumps, vessel internals, etc. This presen-tation of one preferred embodiment of the process is not intended to preclude from the scope of the subject invention those other embodi-ments set out herein.
A charge stream comprising a reduced crude oil as a vacuum ~,257844 bottoms fraction enters the process through line 1 and is first heated by indirect heat exchange in the exchanger 2. The charge stream is then further heated in the indireot heat exchange means 3 and is passed into the visbreaking heater 4. After being subjec~ed to the visbreaking conditions maintained in the heater and in an op-tional additional reaction chamber not shown, the visbreaking zone effluent stream carried by line 5 is admixed with a quench stream carried by line 6. The quench stream reduces the temperature of the visbreaking zone effluent stream below visbreaking temperatures.
The admixture of these two streams is passed through line 7 into a rectified flash tower or fractionator 8.
The hydrocarbons which enter the rectified flash tower are separated into a number of hydrocarbon fractions each having a dif-ferent boiling point range. Therefore, an overhead vapor stream is removed through line 9. This vapor stream is passed to the appropri-ate facilities for the recovery and separation of the naphtha boil-ing range hydrocarbons contained within this vapor stream. A side-cut stream is normally removed from an intermediate point in the tower as through line 10. This is normally a gas oil boiling range mixture of hydrocarbons. This stream is normally cooled by indirect heat exchange not shown and divided into a number of smaller streams.
Streams of the cooled gas oil are therefore passed into the tower through lines 11 and 12 to aid in the separation while a third stream may be removed from the process through line 13 as a product stream.
The remainder of the hydrocarbons which enter the flash tower are concentrated into a bottoms stream removed through line 14.
The undivided bottoms stream is cooled by indirect heat exchange against the charge stream. The flash tower bottoms stream is then ~,2~7844 divided into the quench stream passed through line 6 and a second stream passed into a secondary flash zone through line 15. The sec-ondary flash zone 16 is operated at a lower pressure than the flash tower 8. The hydrocarbons entering the secondary flash zone are therein separated into one or more lighter fractions such as a light and heavy gas oil. This is represented by the removal of a gas oil stream through line 17. The remainder of the entering hydrocarbons are concentrated into a second bottoms stream removed through line 18. Heat is recovered from this stream by indirect heat exchange against the charge stream and the second bottoms stream is then re-moved as a fuel oil after being blended with a suitable amount of a cutter or cutback oil from a means not shown.

DETAILED DESCRIPTION

Visbreaking is a mild thermal cracking type of hydrocarbon conversion process which is normally employed to reduce the viscos-ity and/or pour point of various heavy petroleum-derived hydrocarbona-ceous liquids. The visbreaking operation may be employed to decrease the amount of low value residual material produced in a petroleum refinery by upgrading a portion of the charge stock to a salable fuel oil product. It is also normal to recover some lighter hydro-carbons such as naphtha which are produced by the thermal cracking operation. The visbreaking process may employ a single fractiona-tion column as the initial separation zone or may be integrated with a vacuum fractionation column to recover additional amounts of light and heavy gas oils.
The visbreaking operation comprises the basic steps of heating the charge material to the relatively high temperature ~.2~i7~44 required for the mild thermal cracking operation and maintaining the charge stock at this temperature for a predetermined time, which i5 inversely proportional to the temperature employed. The material treated in this manner is then quenched to a temperature low enough to terminate the thermal cracking reactions and passed into the sepa-ration facilities.
As with all such processes in which a charge stream must be heated to an elevated temperature, the inherent inefficiency of heat recovery requires a net input of heat. In a thermal cracking process such as visbreaking, a large portion of this heat is con-sumed within the fired charge stock heater. The consumption of this fuel therefore represents a sizable part of the utilities cost of operating the process. It is an objective of the subject invention to provide an improved visbreaking process. It is a specific objec-tive of the subject invention to provide a visbreaking process hav-ing a lower utilities cost of operation due to a decrease in the fuel consumed in the visbreaking heater.
The feed stream to a visbreaking process is normally a heavy hydrocarbon stream such as a topped crude oil or a vacuum re-duced crude oil. These materials are normally referred to as resi-dual oils. Visbreaking may also be applied to heavy crude oils and other hydrocarbonaceous materials. However, this variety of mate-rials shares the common characteristic of containing heavy hydrocar-bons normally having boiling points, as determined by the appropri-ate ASTM distillation, above about 600F (315C). It is preferred that the charge stock to the visbreaking operation has a 10% boiling point above 50~F t260C).
The charge stock to the visbreaking operation is first ~L2~7~44 heated by indirect heat exchange in various heat recovery steps. It is then passed into a visbreaking zone which comprises the visbreak-ing heater and if employed in the process a reaction chamber or soak zone which basically increases the residence time of the heatea charge material at the desired temperature. Steam may be admixed with the feed stream to minimize coking within the heater tubes of the visbreaking furnace. The visbreaking heater and any reaction chamber are maintained at visbreaking conditions.- Visbreaking condi-tions in general include a temperature within the general range of about 800 to about 975~F (426~-523C), with temperatures above 900~F t482C) being preferred. Normal visbreaking conditions also comprise a pressure between about 25 and 400 psig (172-27~8 kPag) although higher pressures to about 1000 psig (6895 kPag) have been described in the literature The charge stock is preferably subjected to these visbreaking condi-tions for a period of about 20 to 65 equivalent seconds at a temperature above 900F (482C) while within the visbreaking zone. The effluent of the visbreaker heater is then preferably quenched, as with a ga, oil, to reduce its temperature by about 70 to 140~F (39-78C). A common variation in visbreaking is the use of a soaker drum in which the still-hot effluent of the visbreaker heater is retained for a preselected time prior to quenching. In these soaker-type visbreakers, the thermal conversion reactions continue within the drum thereby allowing a re-duction in the temperature required for the same degree of conver-sion. The exact conditions of temperature and pressure which are preferred will vary with such factors as the characteristics of the feed material and the degree of thermal cracking desired. Further in-formation on visbreaking may be obtained from many sources including the previously cited references.

~L257,8D~4 In the subject process, the feed stream is heated to the desired visbreaking temperature by a combination of indirect heat exchange against high temperature process streams and the use of a fired heater. The initial heating includes the exchange of the feed stream against the total bottoms stream of the first separation zone.
The initial heating preferably also includes an indirect heat ex-change against the b~ttcms stream ~f the s~cond separation zone.
However, the subject invention centers on the heat exchange with the bottoms stream of the first separation zone. This heat exchange heats the feed stream to a higher temperature than prior art methods therefore reducing the amount of heating required in the fired heater.
The ability of this heat exchange to produce a higher feed stream preheat temperature is due to the use of a "relatively high temperature" quench stream of residual (bottoms) material and to the use of a higher temperature unflashed bottoms stream. As used here-in, the tenm "relatively high temperature quench" is intended to re-fer to a quench stream having a temperature less than about 300 Fahrenheit degrees (167C) cooler than the visbreaker effluent stream. Th~
use of a hot quench stream requires the use of a larger amount of quench. The flow rate of the quench stream preferably exceeds that of the unquenched visbreaker heater effluent. Since the hot quench material is bottoms liquid from the separation zone, it will again be concentrated into the bottoms stream when it returns to the sepa-ration zone. Hence, the flow rate of the bottoms stream increases.
More heat can therefore be removed from the bottoms stream and used to heat the feed stream without cooling the bottoms more than is de-sired. The feed stream may thereby be heated to a higher tempera-ture even if the temperature of the bottoms stream is the same as ~Z57~344 in the prior art before and after the exchange. This method of in-creasing the flow rate of the heat exchange media is of course limited by the associated increases in pumping and piping costs and therefore must be optimized.
A preferred embodiment of the invention may be described as a visbreaking process which comprises heating a residual oil feed stream by indirect heat exchange against a hereinafter characterized first bottoms stream; passing the feed stream through a visbreaking zone, and then admixing the resultant visbreaker zone effluent stream with a relatively high temperature quench stream having a temperature above about 600F (315DC) and thereby forming a first process stream; passing the first process stream into a first separation zone in which enter-ing hydrocarbons are separated into different boiling point range fractions including the previously referred to first bottoms stream;
cooling the first bottoms stream in the previously described heat ex-change and then dividing the first bottoms stream into the previous-ly referred to quench stream and a second process stream; and pass-ing the second process stream into a lower pressure second separa-tion zone and recovering a product stream from the second separation zone.
In the subject process, the quenched effluent of the vis-breaking zone is passed into the first of two separation zones.
These zones may each have a number of configurations, with the de-sign of the separation zones varying with charge stock properties, desired products and process conditions, etc. Preferably, the first separation zone comprises a rectified flash tower. The quenched ef-fluent is directed into the bottoms void section of the rectified flash tower at a point some distance above the bottom of the column.

~2~q~44 This column is operated at a pressure of about 45 to 150 psig (310 to 1034 kPag)and at a bottom temperature of within the range of about 6~9 to 86Q~F
(365 to 460C). Preferably, the pressure is above 60 psig (414 kpag~. A~
used herein, specified pressures refer to the pressure found at the top of the separation vessel and temperatures refer to the bottom temperature of the vessel under consideration. A liquid phase is collected in the bot-tom of the column below the feed point and removed as a bottoms stream. The rectified flash tower is to have means to supply ade-quate cooling to the top section of the column to condense sizable amounts of liquid and effect countercurrent vapor-liquid flow. The upper rectification section is preferably separated from the lower section by a trap-out tray. The upper section preferably contains at least five fractionation trays and is fed reflux liquid at the top tray. A liquid stream removed at the trap-out tray may be cooled lS and returned to the upper section at a higher point which is interme-diate two fractionation trays to aid the separatory operation.
The ~ottoms stream removed from the first separation zone is subjected to an indirect heat exchange step in which it is cooled.
Preferably, this cooling is effected solely by heat exchange against the visbreaker feed stream. It is also preferred that the heat ex-change is performed before the bottoms stream is flashed to a lower pressure, such as the pressure of the second separation zone. After being cooled, the bottoms stream is divided into aliquot portions passed into the second separation zone and used as quench. The tem-perature of the bottoms stream after the heat exchange, and there-fore the temperature of the relatively high temperature quench stream, should be above about 600F (315C) and is preferably above 650F
(343C). More preferably, the quench stream has a temperature above ~257~344 approximately 680F (360C). The flow rate of the quench stream is set by the flow rate and temperature of the visbreaker effluent stream, the temperature of the quench stream, and the desired tem-perature decrease to be provided by the quench and may therefore be calculated.
The remainder of the bottoms stream of the first separa-tion zone is passed into a second separation zone often referred to as a secondary flash zone. This is preferably a void vessel having an upper vapor outlet and adapted to retain a liquid level below the feed point. A liquid is preferably sprayed into the vessel at a cen-tral location above the feed point. The secondary flash zone is op-erated at a lower pressure than the first separation zone. A broad range of temperatures for use in this zone is from about 644 to 752F (340 - 400C).
The pressure in tnis zone should be at least 30 psig (207 kPag) below that at which the bottom section of the rectified flash zone is operated. A range of pressures for this zone includes pressures from about 0 to 100 psig (0 to 689 kPag). The design and operation of the first and second separation zones and the other apparatus employed in the sub-ject invention are not of themselves unique.
It is believed well within the expertise of those skilled in the refining arts to design suitable process equipment. Neverthe-less, to ensure a proper understanding of the process, the following example based on engineering design (calculated) operation of a com-mercial scale unit are provided. The feed stream is a 20,000 barrels(3180 m3)

2~ per day stream of a reduced crude oil. In this example, the tempera-tures enclosed in brackets are the equivalent temperatures which would be expected in a prior art (low temperature) quench system.
The feed stream enters the process at approximately 480F (249C) and is ~2~7844 heated to about 550F (288C) by indirect heat exchange against the bo1;ton,s stream of the second separation zone, which is cooled from 650F to about 550~F
( ~43-28S C) . The thus-heated feed is then further heated to 710F (377C) t670F (354C)] by indirect heat exchange against the bottoms stream of the first separation zone. The feed is then passed into the visbreaker heater and heated to approxir,ately 925F (496C). The ef~luent of the vis-breaker heater is quenched to approximately 820F (438C) with the high tem-perature quench of the subject invention. This quench has a tempera-ture of about 700F (371C) r550F (28~C)]. To compensate for the higher tempera-ture of the quench liquid, the amount of quench is increased from a representative prior art weight ratio of quench to effluent of 0.65:1 to a ratio of 1.45:1. The flow rate of the bottoms stream is there-fore significantly increased. In this example, the increased tem-perature of the feed to the visbreaker heater (40F or 22C) reduces the cost of the visbreaker heater and produces a fuel savings of at least 10%. Although in this example some of the increased hPatin3 of the feed stream results from the use of an unflashed and therefore hotter bottoms liquid from the first separation zone, a significant portion of the improved heating results from the in-creased temperature of the quench stream and the corresponding high-er mass flow rate of the total bottoms stream.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of thermally processing a hydrocarbon stream which comprises the steps of:
(a) heating a feed stream, which comprises a mix-ture of hydrocarbons having boiling points above 600°F (315°C) to a temperature above 670°F (354°C), by indirect heat exchange against a hereinafter characterized first bottoms stream;
(b) passing the feed stream through a visbreaking zone, and admixing a resultant visbreaking zone effluent stream with a relatively high temperature quench stream having a temperature of above 600°F and less than 300°F (167°C) coder than visbreaking zone effluent stream to form a first process stream wherein the flow rate of the quench stream is greater than the flow rate of the visbreaking effluent stream;
(c) separating the first process stream in a first separation zone into the desired hydrocarbon fractions including the previously referred to first bottoms stream;
(d) employing the first bottoms stream in said indirect heat exchange and then dividing the first bottoms stream into at least said quench stream and a second process stream; and (e) passing the second process stream into a second separation zone, and recovering a product stream from the second separation zone.

2. The method of claim 1 further characterized in that the feed stream is heated by indirect heat exchange against a second bottoms stream, which is removed from the second separation zone, prior to being heated by heat exchange against the first bottoms stream.