US2768934A

US2768934A - Heat exchanger system

Info

Publication number: US2768934A
Application number: US273431A
Authority: US
Inventors: Sylvan B Schapiro; Mark C Hopkins
Original assignee: American Oil Co
Current assignee: American Oil Co
Priority date: 1952-02-26
Filing date: 1952-02-26
Publication date: 1956-10-30
Anticipated expiration: 1973-10-30

Description

Oct. 30, 1'956 s. B. scHAPlRo ET AL 2,768,934

HEAT EXCHNGER SYSTEM 2 shetS-sheet 1 Filed Feb. 26, '1952 Sylvan B. Schop/ro BY Mark 6. Hopkins A TTOR/VEY Oct. 30, 1956 s. B. scHAPlRo ET AL 2,768,934

HEAT EXCHANGER SYSTEM Filed Feb. 26, 1952' l 2 Sheets-Sheet 2 ATTORNEY United States ima'r EXCHANGER SYSTEM Application February 26, 1952, Serial No. 273,431

3 Claims. (Cl. 196-52) This invention relates to an improved heat exchanger system and it pertains more particularly to an improved method and means for avoiding the build-up of deposits on surfaces of heat exchangers, such for example as are employed for preheating charging stock to fluid catalytic cracking processes with hot slurry oil produced in such processes.

In uid catalytic cracking systems employing solid catalyst of small particle size, some catalyst particles are carried overhead from the reactor to a point near the base of a main fractionator or scrubber tower wherein such solid particles are scrubbed out of the partially condensed product stream by heavy gas oil components thereof. The slurry of solid catalyst particles leaves the base of the scrubbing zone of the fractionator at a temperature of about 600 to 650 F. This hot slurry is then cooled by heat exchange with incoming charging stock, a part of the cooled slurry being returned to a higher level in the fractionator (the upper part of the scrubbing zone) for effecting partial condensation of reactor efiiuent and removing solids therefrom, while another portion of the cooled slurry is sent to a settling zone in order to remove as much as possible of the heavy gas oil from the solids before the solids are returned to the catalytic cracking system. The heavy gas oil portion of the product, which is also called slurry oil, is undesirable as a component of Vthe cracking charging stock because it consists predominantly of polycyclic aromatics which tend to form excessively large amounts of coke on the catalyst when contacted therewith under cracking conditions.

VEver since the advent of the fluid catalytic cracking process in about 1941, the fouling of the fresh feed side of the heat exchangers has constituted a costly and vexatious problem. It has been the practice to pass the fresh feed through the shell side of the exchanger and the slurry through the tube side thereof since the slurry is more viscous than the fresh feed and because of the belief of skilled operators that solids could riot be satisfactorily handled on the shell side because of their tendency to settle out in certain portions of the shell and erode other portions of the exchanger. Heretofore it has been the universal practice to clean periodically the shell side of the exchangers by means of a caustic or steam treatment but such cleaning operations have not really solved the problem; not only are these treatments troublesome and expensive, but they fail to remove all of the deposits and each exchanger gets less and less eflicient as the run proceeds despite the frequent cleaning operations. An object of our invention is to avoid the necessity of such cleaning operations, to avoid the continuous build-up of deposits, and to enable heat exchangers to operate at maximum efficiency throughout an entire run length of at least a year or more. On an economic basis, our object is to effect savings in operating costs of as much as $200,000 to $300,000 a year on each catalytic cracking unit of approximately 30,000 barrels per day capacity.

While the precise composition of the deposits cannot rice be determined, it appears that such deposits are at least partially due to oxygen absorbed in the gas oil feed stock; considerable reduction yin deposits may be elected by simply stripping the incoming charging stock with fuel gas to eliminate, as far as possible, any oxygen which might be dissolved in said stock. Such stripping, however, does not completely avoid fouling difficulties and although it may be employed as a part of our technique, our invention may make it possible to avoid the expense of stripping oxygen from the original charging stock.

ln practicing our invention, we provide an arrangement of valves and piping in the heat exchanger system so that the materials iiowing through the tube side and shell side of the exchanger, respectively, may be periodically reversed. The combined solvent action of the slurry oil and scouring action of the solids containedtherein will substantially entirely remove deposits which accumulate on the fresh feed side of the exchanger after a short period of operation so that by periodically reversing materials llowing through the separate sides of the exchanger at intervals of about l to 20 days, the heat exchangers may operate at substantially design heat transfer coecient throughout the entire run life. ln new installations, it may be desirable to modify `somewhat the design of the heat exchangers so that the resistance to ow will be more nearly uniform on the shell side and the tube side, respectively, when charging stock and slurry oil are alternately passed therethrough; however, our invention is applicable to shell and tube type heat exchangers currently employed iii catalytic cracking units and although a small amount of solids may settle out in corners where baies join the shell, there is no plugging problem and passage of the slurry oil through the shell side is remarkably eifective in cleaning deposits therefrom. Usually, we prefer to 0perate with the fresh feed passing through the shell side of the exchangers most of the time; a relatively short operation of about 2 hours to 6 days, with the slurry passing through the shell side, is sumcient to remove accumulated deposits.

some cleaning action on the shell side is obtainable by returning the solids from the slurry (after the bulk of the slurry oil has been separated therefrom by settling and decantation) to the fresh feed before the fresh feed enters the heat exchanger system. In this method of operation, however, there is constant ow of solids through the shell side of the exchanger and hence a greater tendency toward undue erosion therein. Furthermore, this method of operation does not utilize the solvent action of the slurry oil in helping to remove any solids which might accumulate.

The invention will be more clearly understood from the following detailed description of a specific example thereof as applied to a 30,000 barrel per day lluid catalytic cracking unit. Since units of this type are well known to those skilled in the art, this description will be limited to the fresh` feed preheat section thereof as illustrated in the accompanying drawings wherein Figure l is a schematic ow diagram illustrating the initial fresh feed stripping heat exchanger system and preheat furnace, and

Figure 2 is a more detailed diagram illustrating the structure of each heat exchanger.

Referring to Figure 1, about 30,000 barrels per day of gas oil charging stock is introduced through line 10 to stripping tower ll which is operated at about 30 p. s. i. g. and at a temperature of about F. About 22,000 cubic feet per hour of fuel gas is introduced into the stripper through line 12, the stripper overhead being Withdrawn through line 13 to a low pressure separator. The purpose of the stripping operation is to remove dissolved oxygen from the incoming charging stock and

.valves

17a and 17a `are open.

vthe base of tower 11 passes by line 14- to pump yl5 and thence by line 16 to the exchanger system. As heretofore operated, the charging stock stream was evenly divided with half of it passing through valve 17, the shell -side of exchanger iii, line 19, the shell side of exchanger y20,\l ine 21, furnace preheat tube 22 and transfer line l23, catalyst being picked up from a standpipe into the transfer line and carried by the preheated vapors into the cracking reactor. Overhead from the reactor was lntroduced in the base of the scrubbing section of a fracltionator vwherein the heaviest components of the product were condensed and'catalyst particles were scrubbed out of the ascending vapor stream, about 33,00() barrels per day of slurry oil with suspended solids being withdrawn from the base of the scrubbing zone at a temperature of -about 600 to 650 F. Half of this slurry was introduced by line 24- to the tube side of exchanger Ztl, thence by line 25 to the tube side of exchanger i8, and was then discharged by line 26, a part of the cooled slurry being sent to a settler for removing, by decantation, as much as possible of the slurry oil before returning the solids to the cracking system and another part of the cooled slurry being introduced into the fractionator at the top of the scrubbing section to condense the highest boiling :components of the products introduced thereto and effect further removal of solids. The other half of the charging stock and slurry were handled in a similar manner as designated by like reference characters with 'anf added prime When operated in the manner hereinabove described, the shell side of exchangers 18 and 20 (also 18 and 26') gradually became fouled by deposits to such an extent that the heat transfer coeicient was markedly impaired; it was necessary periodically to bypass each exchanger land to clean out the shell side thereof. The clean-out methods heretofore employed were not only time-consuming and expensive but they were not entirely satisfactory because cach cleaning operation left a little more deposit than was left by the previous cleaning so that during the course of the run, more and more deposits Iaccumulated on the shell side of the exchanger despite frequent cleaning operations.

In Iaccordance with our invention, additional piping and valves are added to the heat exchange system and for the purpose of simplicity, reference characters will be applied only to the valves since reference to the valves will designate any added lines in which these valves occur. Valves 17a and l7a are provided to enable the charging stock to enter the tube side of the exchangers through the inlets where slurry oil was previously introduced thereto. Valves 23 and 28 are added to stop slurry flow through lines 25 and 25', respectively, when Valves 23a and 28a are provided for bypassing slurry to the shell side inlets when

valves

28 and 28 are closed. Valves 29 and 29 are provided to stop flow through

lines

26 and 26 when charge is introduced thereto through valves 17a and ll7a; when

valves

29 and 29 are closed,

valves

29a and 29a' are open so that the charge stream may be returned to

lines

19 and 19. When the flow of charge is through valves 29a `and 29a', valves Si) and 34B are closed and material leaving the shell side of the exchanger lare passed through valves 30a and 34M to points in

lines

26 and 26 beyond closed valves 29 and 29'.

From the above description, and referring only to exchanger 18, it will be seen that by simultaneously closing valves 17 and 23 and opening valves l7a and 28a the charging stock can be made to enter the tube side of the exchanger where slurry oil previously entered, and slurry oil made to enter the shell side where charging stock previously entered. As soon as gas oil charging stock introduced through valve 17a reaches valve 29, valve 29 is closed and valve 29a opened while simultaneously valve 30 is closed and valve 30a is opened. rThe system will now be operating vwith all of the original valves closed and the alternate valves opened (a, indicating `alternate valves) so that there is a reversal of materials tiowing through the shell Vand tube side of the exchanger, respectively, without any interruption in the overall ow of these streams through the system.

Corresponding valves and lines are provided for exchanger Zt. Valve 3l may be closed so that the gas oil charge may pass through valve 31a to the exchanger inlet through which slurry previously entered. When flow is thus reversed, valve 32 is closed and valve 32a -is opened so that slurry will enter the shell side of the exchanger to the inlet where gas oil charge previously entered. When the rerouted gas oil reaches valve 33 this valve is closed and valve 33a is opened; simultaneously valve 34 is closed and valve Ma is opened. The valve connections for exchanger 20 are the same as those hereinabove described for exchanger 20, the cor responding valves being indicated by 4an added prime Referring to Figure 2, each heat exchanger in this particular case is that previously installed in a catalytic cracking system 4and comprises a shell 35 about 30 inches in diameter provided with upper and lower tube sheets 36, `vhich require about 175 tubes 37, each 11A inches outside diameter and 16 feet long. Between the tube sheets are staggered baies 3S for causing sinuous ow of liquid through the shell side of the exchanger from shell inlet 39 to shell outlet 4t?. A transverse bale 41 separates the space below the bottom tube sheet into inlet and outlet zones so that liquids which enter the tube side of the exchanger at 42 pass upwardly through the right hand bank of tubes, then downwardly through the left hand bank of tubes and out through outlet 43. The valved lines employed in accordance with our invention are also illustrated in Figure 2 and further description thereof is unnecessary in view of the description hereinabove given.

From the foregoing `description it will be apparent that at any time, by mere switching of valves, the gas oil `can be made to ow through the tube side and slurry o-il through the shell side, or vice versa. Furthermore, the flow through the system can be so controlled that the gas oil charge may flow through the shell side of one exchanger land the tube side of the other while the slurry oil is passing through the tube side of one and the shell side of the other; when operated in this manner, both the pressure drop between pump 15 and the preheater furnace and the pressure drop between

line

24 and 26 will remain constant regardless of the switching of valves. However, the pressure drop diiferences in switching the liquids from the shell to the tube side of the exchanger are suiiiciently small so that valves may be changed on any particular heat exchanger without materially disturbing flow through the remaining heat exchangers. In the example herein described the invention is applied to an existing heat exchanger and it will be understood, of course, that for maximum simplicity of operation, the heat exchanger should be designed to give equal pressure drop through the shell sides and tube sides, respectively, for a given stream.

By reversing the valves in the manner hereinabove described, as soon as a significant impairment of heat exchange coefficient is observed the combined action of the suspended solids and the solvent nature of the slurry oil removes such deposits in a relatively short period of time, which may range from about 2 hours to as much as 6 days. The valves may then be again reversed to their initial position and the heat exchanger system operated until impairment of heat transfer coefficient is again indicated at which time this process may be repeated. Thus, without ever bypassing any exchanger and, in fact, without taking any exchanger off stream, the heat exchangers may' be operated continuously with substantially maximum heat transfer coecient.

The exchanger system hereinabove described was designed to have the fresh

feed enter exchangers

18 and 18 at a temperature of about 100 or 110 F. and leave

exchangers

20 and 20 at atemperature of about 350 F., the hot slurry entering exchangers 20 Iand Z0 at about 605 F. and leaving

exchangers

18 and 18 at about 442 F. As initially operated, these exchangers became so fouled'on the fresh feed side that at the end of the run the fresh

feed leaving exchangers

20 and 20 was preheated to only about 170 F. With fresh feed stripping -in tower 11 it was possible to maintain an outlet temperature of at least about 250 F. provided that the shell sides of the exchanger were periodically cleaned. By operating in accordance with our invention, it should be possible to maintain the outlet temperature within about 10 or 15 degrees of the designed figure of 350 F. at all times by reversing the inlet of the streams from shell to tube side and vice versa as soon as the temperature in

lines

21 and 21 drops by as much, for example, as to 15, e. g. 10 F.

In a test run on a 30,000 bbl./ day uid catalytic cracking unit, one of the exchangers (corresponding to 18) was initially operated with fresh feed (containing solids from the Dorr settler) in the shell side and slurry oil in the tube side. In 30 days the corrected overall heat transfer coefficient (Us) gradually dropped from about 38 to 29. When flow was transferred so that slurry passed through the shell side and fresh feed through the tubes, said coefficient rose to 33 in a matter of hours, in 2 days it had reached 37 and in 4 days was up to 40 (i. e. was more efficient than at the beginning of the run). During the next 4 days the overall heat transfer coeicient gradually dropped back to 36 because of deposit formation on the tube side. At that time (8 days from the previous ow transfer) the ow was again transferred to that originally employed, and in a matter of hours the overall heat transfer coetiicient was back up to 40.

For comparison, other portions of the same charging stock (containing the same amount of solids per unit volume) and other portions of the same slurry oil were passed through another heat exchanger with the charging stock on the shell side throughout the whole test period. In this exchanger the initial corrected overall coeiiicient (Us) was about 50 and in 20 days it had dropped to 28, in the next 8 days it dropped to 27, and at the end of 40 days it was down to 25. This comparative run shows that the use of solids in the charge has some beneficial effect in retarding'accumulation of deposits, but that it fails to keep the exchanger surfaces clean in the manner exhibited by the flow transfer technique.

An important feature of our invention is that of removing the slight deposits formed on the fresh feed side of the exchanger by subsequent ow of hot slurry therethrough so that the removed deposits are discarded from the system with decanted slurry oil and are not carried downstream to lthe tubes of the preheat furnace. Experience has shown that when heavy deposits occur on the fresh feed side of the exchangers, there is a tendency for deposits to form downstream of said exchangers, and particularly in the tubes of the preheat furnace. By removing deposited material with the hot slurry, the preheat furnace tubes are thus protected so that they operate at increased efficiency for longer periods of time.

The actual quantity of solids in the slurry oil may vary throughout a relatively Wide range, depending upon the efficiency of the cyclone separators in the upper part of the reactor of the fluid catalytic cracking system. Usually, the amount of solids carried by the slurry oil is in the range of 0.1 to 1.0 pound per gallon or from about 4 to 42 pounds per barrel, such particles being activated 6 clay, synthetic silica-alumina or silica-magnesia catalysts having a particle size in the range of about 1 to 100 microns. The net amount of slurry continuously produced is passed to a settling zone (e. g. a Dorr settler) so that as much as possible of the slurry oil may be decanted therefrom and the catalyst particles may be r'eturned to the system with a minimum amount of such slurry oil. It is known that by introducing the settled Vsolids to the fresh feed before the feed passes through the preheat coils, the scouring action of the solids helps to prevent `deposition of coke in said preheat tubes. By introducing the solids into the fresh feed before the fresh feed enters the heat exchange system a scouring effect may, likewise, be obtained on the fresh feed side of the heat exchangers. However, it is not always desirable to introduce solids into the fresh feed at this stage of the process and furthermore, as hereinabove pointed out, the fresh feed does not have the solvent power for deposits that is exhibited by the hot slurry oil and by ernploying the hot slurry to remove the slight initial deposits laid down by the fresh feed, such deposits are eliminated from the system with discarded slurry oil so that they cannot cause coking in the preheat tubes or form undue amounts of coke on the catalyst in the reactor. By delaying the switching of outlet valves (after inlet valves have been reversed) until the gas oil has replaced the slurry content of that side of the exchanger before reversing the outlet valves of the exchanger, it is possible to minimize any loss of gas oil with decanted slurry oil and also to minimize the entry of any slurry oil into the charging stock stream; while there may be a slight intermingling of gas oil and slurry oil, this is of no serious consequence since the reversals of inlets occur only periodically at relatively long intervals so that in overall operations the amount of gas oil lost with slurry oil and the amount of slurry oil contaminating the gas oil charge is relatively insignificant.

While we have described and shown a manually operated valve system for alternating the flow of uids from shell to tube and tube to shell sides of the exchanger, it should be understood, of course, that such valves may be automatically operated in any known manner in response to a predetermined temperature drop in the charging stock outlet. Pneumatic, electrical and other types of automatic controls are well known to those skilled in the art and require no detailed description herein. Also, instead of employing separate alternative valves in each instance, a three-way valve may be employed so that a single valve structure will accomplish the function of two separate valves. Other modifications and alternative arrangements will be apparent from the above description to those skilled in the art.

We claim:

1. In a fluid catalytic cracking system including a furnace tube, a reactor, a scrubbing tower and at least one heat exchanger comprising a vessel, a plurality of tubes therein, and tube headers to which the tubes are secured, the space around the tubes between the headers being called the shell side and the space beyond and within the tubes being called the tube side of said exchanger, wherein charging stock is passed through one side of said heat exchanger and thence through the furnace tube to the reactor and eiluent from the reactor is introduced into the scrubbing tower wherein solid catalyst particles are scrubbed out of partially condensed product by heavy gas oil components thereof to form a hot slurry which in turn is passed through the other side of said heat exchanger to preheat said charging stock, the charging stock normally passing through the shell side of the exchanger and the hot slurry through the tube side thereof and said exchanger having only one inlet line and only one outlet line, respectively, for the shell side and only one inlet line and only one outlet line, respectively, for said tube side, the improvement which comprises a by-pass line from the shell inlet F line to the tube inlet line, a by-pass line from the tube inlet line to thershell inlet-line, a by-passline from the shell outlet line to the tube outlet line, a by-pass line from the tube outlet line to the shell outlet line, and valves in each of said lines so that the ow of charging stock and slurry through the tube side and the shell side may be periodically alternated without changing direction of flow in the exchanger.

2. The system of claim 1 which includes a second exchanger connected in series with the rst named exchanger, said second exchanger having inlet, outlet and alternate lines and valves corresponding to those of the rst named exchanger whereby the liquid charge may always flow through the shell side of one exchanger when it is owing through the tube side of the other so that the pressure drop through the exchanger system always remains substantially constant.

3. The system of claim 1 which includes a stripping tower, a charging stockv inlet leading tothe upper part of said tower, a fuel gas inlet at an intermediate part of said tower whereby liquid may be accumulated in the lower part thereof, a gas outlet at the top of said tower, av pump, and connections for pumping accumulated stripped charging stock from the lower part of said tower to saidrheat exchanger.

References Cited in the le of this patent UNITED STATES PATENTS 1,006,197 Frasch Oct. 17, 1911 2,396,109 Martin Mar. 5, 1946 2,490,750 Grewin et al. Dec. 6, 1949 2,490,759 Tyden Dec. 6, 1949 2,493,494 Martin Jan. 3, 1950 2,57 6,843 Lockman Nov. 27, 1951 2,647,570 Lockman Aug. 4, 1953 2,723,948 McCurdy Nov. 15, 1955

Claims

1. IN A FLUID CATALYTIC CRACKING SYSTEM INCLUDING A FURNACE TUBE, A REACTOR, A SCRUBBING TOWER AND AT LEAST ONE HEAT EXCHANGER COMPRISING A VESSEL, A PLURALITY OF TUBEES THEREIN, AND TUBE HEADERS TO WHICH THE TUBES ARE SECURED, THE SPACE AROUND THE TUBES BETWEEN THE HEADERS BEING CALLED THE "SHELL SIDE" AND THE SPACE BEYOND AND WITHIN THE TUBES BEING CALLED THE "TUBE SIDE" OF SAID EXCHANGER, WHEREIN CHARGING STOCK IS PASSED THROUGH ONE SIDE OF SAID HEAT EXCHANGER AND THENCE THROUGH THE FURNACE TUBE TO THE REACTOR AND EFFLUENT FROM THE REACTOR IS INTRODUCED INTO THE SCRUBBING ROWER WHEREIN SOLID CATALYST PARTICLES ARE SCRUBBED OUT OF PARTIALLY CONDENSED PRODUCT BY HEAVY GAS OIL COMPONENTS THEREOF TO FORM A HOT SLURRY WHICH IN TURN IS PASSED THROUGH THE OTHER SIDE OF SAID HEAT EXCHANGER TO PREHEAT SAID CHARGING STOCK, THE CHARGING STOCK NORMALLY PASSING THROUGH THE SHELL SIDE OF THE EXCHANGER AND THE HOT SLURRY THROUGH THE TUBES SIDE THEREOF AND SAID EXCHANGER HAVING ONLY ONE INLET LINE AND ONLY ONE OUTLET LINE, RESPECTIVELY, FOR THE SHELL SIDE AND ONLY ONE INLET LINE AND ONLY ONE OUTLET LINE, RESPECTIVELY, FOR SAID TUBE SIDE, THE IMPROVEMENT WHICH COMPRISES A BY-PASS LINE FROM THE SHELL INLET LINE TO THE TUBE INLET LINE, A BY-PASS LINE FROM THE TUBE INLET LINE TO THE SHELL INLET LINE, A BY-PASS LINE FROM THE SHELL OUTLET LINE TO THE TUBE OUTLET LINE, A BY-PASS LINE FROM THE TUBE OUTLET LINE TO THE SHELL OUTLET LINE, AND VALVES IN EACH OF SAID LINES SO THAT THE FLOW OF CHARGING STOCK AND SLURRY THROUGH THE TUBE SIDE AND THE SHELL SIDE MAY BE PERIODICALLY ALTERNATED WITHOUT CHANGING DIRECTION OF FLOW IN THE EXCHANGER.