Coking and fractionating process

May 16, 1961 a. _{M EDELMAN ETAL} 2,984,618

COKING AND FRACTIONATING PROCESS

Filed July 21, 1958

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Allan M. Edelman

Harold N. Weinberg Inventors

B y /cut. a, (% Attorn ey

United States Patent Office

2,984,618

Patented May 16, 1961

2,984,618

COKING AND FRACTIONATING PROCESS

Allan M. Edelman, Brooklyn, N.Y., and Harold N. Weinberg, Milltown, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware

Filed July 21,1958, Ser. No. 749,917

Claims. (Cl. 208—100)

The present invention is concerned with the removal of deposited materials from a fractionation zone associated with a hydrocarbon conversion process. More particularly, it deals with a system for freeing the top portion of a fractionation zone treating the overhead products of a thermal cracking zone of deposited chloride salts.

Within the past several years, the well known fluid bed coking process for thermally cracking relatively heavy hydrocarbons into high valued, lighter fractions has been developed. The process, commonly known as fluid coking, consists chiefly in contacting a heavy feed stock with a highly turbulent mass of inert contact particles maintained at a coking temperature, e.g. 850 to 1200° F. The oil, upon contacting the hot solids, is pyrolyzed into vaporous reaction products and carbonaceous residue, the latter being deposited as a coating on the contact particles. Conventionally, a portion of the carboncoated solids are passed to a combustion zone wherein oxidation of the deposited carbon serves to heat the solids to sufficiently high temperatures so as to enable their recirculation to the coking vessel to supply the requisite thermal energy for the reaction process.

Typically, the hydrocarbon oil feed has an initial boiling point of 600° F. or higher, an A.P.I. gravity ranging from 0° to 20° and a Conradson carbon residue content of about 5 to 40 weight percent. The feed oil may be a crude, residuum, shale oil, vacuum bottoms, cycle oil, or the like, the process being adapted to handle a wide range of hydrocarbon materials. Suitable contact solids for fluid coking include, among others, coke particles, sand, ceramics and the like, and range from 0 to 1000 microns in size, being preferably 40 to 500 microns in diameter.

The vaporous conversion products, normally after having passed through a solids separation step, e.g. a cyclone separator, are withdrawn overhead from the coking zone, quenched, and thereafter subjected to rectification in a fractionation unit. Though it is generally desirable to have both the quenching and fractionation zones within a single unit, two or more distinct vessels may be utilized for the desired processing of the vapors. In the fractionation zone, various hydrocarbon fractions are separated from the coker vapors, e.g. a heavy end material, a gas oil product, a naphtha fraction, etc. Generally, the naphtha fraction remains as vaporous material in the fractionator, and is withdrawn from the uppermost part of the unit for subsequent condensation and partial recycle.

There is a wide range of temperatures in the scrubbing or fractionation zone, the temperatures ranging from about 625° to 750° F, in the area wherein the hot coker vaporous effluent is freshly introduced into the unit to approximately 150° to 300° F. in the upper portion of the fractionator which treats only the lighest of the reaction products. It is generally desired to utilize as low a temperature as possible at the top of the fractionator, i.e. substantially below 250° F., preferably less than about 230° F., in order to promote the efficient rectifica2 tion of the light naphtha constituents. When operating the top portion of the fractionation tower at higher temperatures, light products satisfying specification requirements are not readily obtained.

During the course of normal operations, it has been found that over an extended length of time there is a gradual buildup of fouling deposits in the upper confines of the fractionator, i.e. the coolest part of the unit. The deposits cause blockage of the vapor-liquid contacting 10 members, e.g. contact trays, thus resulting in a buildup in pressure drop across the members. In addition to causing poor rectification across the trays, the pressure buildup tends to result in dislocations throughout all portions of the conversion system which are pressure balanced on the 15 fractionator. Upon examination of these deposits found in the coolest section of the fractionator, it has been discovered that they are primarily chloride salts, principally ammonium chloride. The feeds to a coker are characteristically heavy, low-volatile hydrocarbons and they ²⁰ therefore contain the heavy, low-volatile hydrocarbon complexes of nitrogen which are present in crude petroleums. The heavy coker feeds also contain chloride salts. Although it is normal practice to desalt crudes, the coker feed, which is generally the heaviest fraction ²5 in the refinery, still contains a substantial amount of unremoved chlorides. It appears that under the high-temperature thermal cracking condition of the coking zone, the nitrogen complexes are converted to ammonia, and the hydrolyzable chlorides, e.g. MgCl₂, hydrolyzed to ³⁸ hydrogen chloride. The ammonia and hydrogen chloride gases react in the low temperature zone at the top of the fractionation zone to form ammonium chloride which deposits on the fractionation trays, thus leading to the problem the present invention serves to solve.

³⁵ In the past, the chloride deposits have been periodically removed from the top of the fractionator by a rather cumbersome method. Normal fractionation was substantially discontinued while the tower top was subjected to water washing for a relatively long period of time, i.e.

⁴⁰ 8 to 24 hrs. In addition to causing dislocations in normal operations, this water washing procedure necessitated the erection of various structures and piping connections above and beyond that associated with normal fractionation facilities. Further, the procedure offered ⁵ no direct indication of sufficient removal of the deposits since a pressure drop measurement indicating the degree of fouling could not be made across the contacting trays during the course of the water wash. Water washing is also an inherently dangerous operation in that any ^u liquid water which inadvertently passes to the higher temperature portions of the unit may flash off resulting in severe physical damage to the fractionator. Additionally, the dislocation of the process and the length of time required for the wash cause extensive degradation ⁰ of large quantities of valuable product so that the operation is extremely costly.

The present invention teaches a highly efficient manner of removing these chloride salt deposits from the gn upper portions of the fractionator. In accordance with ^υ the present invention, the temperature of the top portion of the unit is periodically raised above its normal operational level to a value above the sublimation point of the deposits. The elevated temperature is maintained for a time sufficient to cause sublimation of the fouling ⁰ chloride salts, e.g. ammonium chloride, the effective removal of the deposits being readily ascertained by a drop in pressure differential across the sections of the upper part of the fractionator. Upon removal of the deposits, normal temperature conditions, e.g. less than 230° F., ⁷⁸ are reinstated in the top of the unit. It has been found that coking systems which have been operated for up to two continuous months or more may be quickly rid of

2,084,618 deposited ammonium chloride salts by raising the temperature of the top portion of the fractionator to a temperature preferably in the range of 250-350° F, for a period of 3 to 8 hours. Normal lower temperatures necessary for desired light ends fractionation are then reinstituted. The deposits are thus removed in a very short time as overhead vapors without causing appreciable dislocations in the conversion process, and in a manner requiring little or no investment. Of course, the length of time employed for the removal of the deposits by elevation of fractionator top temperature will vary directly with the length of the period of conventional processing operation between cleanups. Though it is generally desired to perform deposit removal only once every month or two, if done frequently, the time required for cleanup can be reduced to an hour or even less.

It should be clearly noted that the present invention contemplates periodic elevation of tower top temperature for removal of chloride salts therefrom. Merely the continuous running of the fractionator at temperatures above the incipient sublimation temperature of the chloride salts is undesirable in that it results in poor fractionation of the naphtha overhead, thereby resulting in product loss as well as limiting the flexibility of the fractionator unit.

By way of clarifying nomenclature, the terms “fractionation unit,” “fractionator,” etc., are used to denote any zone employed for the rectification of a mixture of vapors by fractional condensation of its constituents.

The various aspects of the present invention will be made more readily apparent by reference to the following description, example and accompanying drawing.

Turning to the drawing, there is illustrated an integrated fluid coking system primarily comprising reactor 1, fractionator 3 and heater 4. Within the reaction vessel, there is maintained a dense turbulent, fluidized bed 5 of coke particles maintained at a temperature of 950° F. Fluidizing gas, such as steam, is introduced into the reactor by line 7 in sufficient amounts to keep the solids bed in a highly mobile state while preserving a solids density of about 20-60 pounds per cubic foot, e.g. 30 pounds per cubic foot. A suitably preheated heavy oil feed, such as a South Louisiana crude, is injected into the solids bed through multiple inlets, the feed being pyrolyzed to relatively light vapors and carbon upon contact with the hot solids. The contact solids are thus coated with the carbonaceous residue of the conversion process. Normally, a portion of the solids is withdrawn from vessel 1 through conduit 8 and circulated with the aid of multiple aeration taps to heater 4. Heater 4 is normally a combustion zone wherein the carbon deposited on the solids is oxidized. As depicted, heater 4 contains a fluid bed 10 of solids undergoing combustion, oxygen-containing gas, e.g. air being supplied through line 11. Of course, other types of heating zones, e.g. a transfer line burner, shot heater, etc., can be employed. If desired, extraneous fuels such as a heavy tar may be injected into burner 4 for oxidation in conjunction with, or in place of, the carbon-coated reaction solids. Flue gas is withdrawn overhead by line 14, cyclone separator 13 serving to remove entrained solids. The solids thus heated to a temperature of 50 to 200° F. above that in reaction bed 5 are recirculated to reactor 1 by line 15 thereby supplying requisite thermal energy thereto.

Normally, a portion of the coated solids is removed from the conversion system by outlets 9 and/or 12 and recovered as product. Generally, the relatively coarse, removed solids are replaced by an approximately equal number of extremely fine particles in order to preserve the fluidity of the reaction bed.

Returning to reaction vessel 1, the gasiform effluent of the reaction bed, i.e. volatilized and cracked hydrocarbons, fluidizing steam, etc., passes upwardly into dilute phase 16, thereafter being subjected to entrained solids removal by one or more cyclones 17. Separated fines are returned to the reaction bed by dipleg 18. The vapors are then subjected to cooling and fractionation.

As shown in the drawing, superimposed directly on reactor 1 is fractionator 3, the coker vapors being directly introduced into the fractionator by one or more cyclone outlets 19. Of course, fractionator 3 may be a distinct vessel or may even be a series of vessels, the first of which merely serving as a quenching stage. The hot reaction vapors at a temperature of about 850° F. are discharged into quench-scrubber section 2 fitted with plates 26 or the like. They are quenched to a temperature of about 625° to 750° F. in the area denoted A. Normally, a portion of the heavy ends materials which is condensed in the lower portion of the fractionator to form liquid holudup 21 serves as the quench media. The heavy ends are removed from the unit by line 22, a portion thereof cooled by cooler 24, and recycled to the fractionator by line 25. Heavy ends withdrawn through line 23 may be recycled to the coker, used as· fuel in heater 4, discarded or processed in a distinct, higher, severity conversion zone.

The cooled vapors, free of their heaviest fractions, pass upwardly through fractionator 3 and are rectified by contact with various liquid streams, e.g. recycled gas . oil, naphtha or extraneous oils. Various vapor-liquid contacting members, e.g. trays 26 and 35, positioned in the unit serve to promote intimate contacting between the downcoming liquid and upflowing vapors. A gas oil product, having a boiling range of about 430° to 1015° F., is normally withdrawn from collecting plate 27 by line 28. After cooling in cooler 29, a portion of the gas oil is conventionally recycled to the fractionator by line 31, outlets 32 serving to disperse the reflux gas oil across the unit. Product oil is recovered through line 30. Similarly, a heavy naphtha fraction (boiling range 300° to 430° F.) may be removed from the unit by collecting tray 33 and outlet 34. The temperature of the tower from the intermediate section of the unit, denoted B, to the beginning of the top section indicated by the letter C, ranges from approximately 550° to 230° F. Uncondensed light naphthas and other hydrocarbons are normally withdrawn from the top portion of the unit by line 36 at a temperature of about 190° to 230° F. and condensed in condenser 37. A portion of the relatively cool naphtha fraction is generally recycled to the top portion of the fractionator by lines 38 and 40, the naphtha product being removed through conduit 39. The light naphtha product has a boiling range of 100° to 300° F. Of course, the upper portions of the fractionator may be maintained at sufficiently low temperatures so as to condense the naphtha within the unit itself rather than withdrawing it as an overhead vapor stream.

It is in the upper portion of the fractionator, i.e. from the area denoted C upwards, that deposition of chloride salts, primarily ammonium chloride, has been found to occur. The low temperature of the top section of unit 3, i.e. less than 230° F., together with the appreciable vapor and liquid holdup time therein, has been found to favor the laydown of chloride salts from the upflowing vapors. Ammonium chloride thus deposits on bubble cap trays 35 as well as on the walls of the unit. The deposited salts gradually build up during the continuous operation of the fractionation zone. Thus, while the pressure drop across the top several trays in the fractionator was initially about 0.4 p.s.i.g. during startup, over the course of several months of continuous coker operation at standard conditions, the pressure drop may rise to 1.3 p.s.i.g. or more due to salt deposition.

In accordance with the present invention, periodically the temperature of the upper portion of the fractionator is raised from its normal level of less than 230° F. to above 250° F., e.g. 300° F. Theoretical considerations predict that within an extremely broad range of inert and inertlike diluent concentration (with respect to ammonia and hydrogen chloride gases), and within a broader range of

2,884,618 pressures than would be encountered in the practice of the coking process, the sublimation temperature of the various chloride salts lies between 230° and 250° F. These theoretical predictions are borne out in actual practice and it has been found that ammonium chloride deposits resulting from several months of continuous operations can be readily removed in about 4 hours by thus elevating the temperature, the chloride deposits being sublimated into the vaporous state, thereafter being withdrawn overhead through line 36. The rate of deposit removal is easily monitored by means of the pressure drop measurement across the top of the unit. A pressure drop of .0.4 p.s.i.g. is thus restored. Normal temperature conditions are thereafter reinstituted. Because of the short residence in condenser 37, little or no deposition of the 15 sublimed salts will occur therein.

The most convenient means of raising the temperature of the top portion of the fractionator is simply to reduce the naphtha recycle rate. Thus, while the naphtha recycle rate varies from about 0.3 bbl. to 2 bbls, per 20 barrel of naphtha product under normal conditions, during the cleanup period it is reduced to about 0.1 bbl. to 1 bbl. per barrel of naphtha product as determined from heat balance consideration, and limited by the minimum required liquid holdup on the top tray. Alternatively, a 25 relatively warm stream of extraneous hydrocarbon oil may be added to the upper portion of the unit. Numerous means of elevating the temperature of the top of the fractionator, such as a steam coil, will suggest themselves to those skilled in the art, and such means are to be con- 30 strued as falling within the scope of the present invention.

Tabulated below is a compilation of data applicable to the system described.

Table I ⁶

230° F. thus reducing free passage of upflowing gasiform effluent from said coking zone, the improved method of efficiently removing said solid fouling chloride deposit without substantially interfering with normal operation of the process which comprises periodically increasing the temperature of said deposit-containing section of said fractionation zone to a temperature between about 250° F. and 350° F., maintaining said fractionation section at a temperature between about 250° F. and 350° F. for at least about 1 hour to cause sublimation of said solid fouling chloride deposit and its removal overhead, and thereafter periodically restoring the lower normal temperature condition of said fractionation zone section.

2. In a process for converting heavy hydrocarbon oil into lighter gasiform materials by subjecting said oil to contact with a fluidized bed of inert particles maintained at a temperature of 850°-1200° F. in a coking zone, said oil being converted into gasiform materials and carbonaceous residue, wherein the gasiform effluent of said coking zone is subjected to fractionation in a fractionation zone, and wherein a fouling deposit of solid chloride material is formed in an upper section of said fractionation zone normally having a temperature of less than about 230° F. thus interfering with free passage of upflowing gasiform effluent from said coking zone, the improved method of efficiently removing said solid fouling chloride deposit without substantially interfering with normal operation of the coking process which comprises periodically increasing the temperature of said depositcontaining section in the upper portion of said fractionation zone to a temperature above about 250° F., maintaining said fractionation section at a temperature between about 250° F. and 350° F. for at least about 1 hour to cause sublimation of said fouling solid chlo35 ride deposit and its removal overhead, and thereafter periodically restoring the normal lower temperature condition in the upper portion of said fractionation zone.

3. A process according to claim 1 wherein reflux hydrocarbons are introduced into the upper portion of 40 said fractionating zone and wherein the temperature of said low. temperature section of said fractionation zone is periodically raised by reducing the reflux rate to the top of said fractionation zone.

Eeactlon Zone Temperature________

Normal Temperature of Upper Portlon'of'

Fractionation, ⁰ F___________

Temperature During Deposit ''Removal'

Period, ° F_____________________

Deposit Removal Period, Hours'-~ZZ2ZIZI2IZZI

Broad Range

Preferred Bange

800-1,600

Below 250

Above 250

850-1,200

180-230

250-350 3-8

4. In a process for converting a heavy hydrocarbon 45 oil into lighter materials by subjecting said oil to contact with a fluidized bed of inert particles maintained at a temperature of about 850°-1200° F. in a coking zone said oil being converted into gasiform material and carbonaceous residue, wherein the gasiform effluent of said 50 coking zone is subjected to fractionation in a fractionation zone, and wherein a deposit of salt material is formed in an upper section of said fractionation zone normally having a temperature of less than about 230° F. thus interfering with free passage of upflowing vapors 55 through said fractionation zone, the improvement which comprises removing said salt deposit without substantially interfering with normal operation of the fractionation process by periodically increasing the temperature of said salt deposit-containing upper section of said frac60 tionation zone to a temperature between about 250° F.

and 350 F., maintaining said fractionation section at a temperature between about 250° F. and 350° F. at least one hour to effect removal of said salt deposit overhead, and thereafter periodically lowering the tem65 perature and restoring the normal temperature condition in the.upper section of said fractionation zone.

5. The improved process of claim 4 wherein the temperature of said deposit-containing section of said fractionation zone is periodically raised to said value in 70 the range of about 250°-350° F. for a period of about 3-8 hours.

6. A process according to claim 2 wherein reflux hydrocarbons are introduced into the upper portion of said fractionating zone and wherein the temperature 75 of said low temperature section of said fractionation

While the above description has specifically related to the fractionation of the vaporous products of a fluid coking system, it may readily be utilized in other elevated temperature conversion processes wherein sublimed salts deposit in the cool portions of the product vapor fractionator. However, it is particularly useful in conjunction with a thermal cracking process since the very nature of the heavy oil feed to the process tends to result in the formation of various salts and complexes which may deposit in cool overhead sections.

By operating in accordance with the present invention, fouling deposits are quickly removed from the upper confines of the fractionator. Desired low temperature conditions may be maintained over an extended period with only a periodic cleanup operation of a few hours necessary for effective removal of deposited salts. No extraneous equipment or investment is required. Deposits are removed with a minimum of interference with standard operating conditions by a method which is readily controllable to give the desired results.