US 2547021 A
Description (OCR text may contain errors)
April 3, 1951 4 R. c. LASSIAT ETAL CATALYTIC CRACKING 0F HYDROCARBONS AND APPARATUS THEREFOR 3 Sheets-Sheet 1 Filed March 2, 1946 INVENTORS mama/v0 a. LASS/AT GEORGE/1. KELSO JAMESE. EVA/V5 AGENT April 3, 1951 R. c. LASSIAT ET AL 2,547,021
CATALYTIC CRACKINGOF HYDROCARBONS AND APPARATUS THEREFOR Filed March 2, 1946 s Sheets-Sheet s v I INVENTORS RAYMOND 6. L485!!! 7 GEORGE/1. KEL S0 EGENT Patented Apr. 3, 1951 CATALYTIC CRACKING OF HYDROCARBONS AND APPARATUS THEREFOR Raymond C. Lassiat, Swarthmore, George H. Kelso, Upper Darby, and James E. Evans, Wallingford, Pa., assignors to Hcudry Process Corporation, Wilmington, Del.,
of Delaware a corporation Application March 2, 1946, Serial No. 651,662
8 Claims. 1
This invention relates to those treatments of fluids with static contact masses where the treatment occasions a change of heat in thesystem. The invention is particularly concerned with the type of treatment exemplified by a conversion of petroleum by contact with porous solid catalysts where the operation gives rise to a net exothermic heat effect.
Since the present invention relates especially to commercial catalytic cracking operations, it will be described in terms of such operations, although it is to be understood that the invention is not restricted to such operations but includes other hydrocarbon conversion processes such as reforming, dehydrogenation with or without added hydrogen, vapor phase treating of cracked gasolines and the like, other endothermic catalytic operations, and similar processes.
An especially efiective process for the catalytic cracking of hydrocarbons in which the hydrocarbons are contacted with a static bed of solid catalyst has been frequently described in considerable detail (see, for example, The Design and Operating Features of Houdry Fixed-Bed Catalytic Cracking Units, by R. H. Newton and H. G. Shimp, Transactions of the American Institute of Chemical Engineers, volume 41, page 197, April 25, 1945, and the references there cited). Briefly summarized, the process consists of passing hydrocarbon materials, such as a gas oil, a heavy naphtha, or a reduced crude, over a static or fixed bed of porous solid catalyst in a converter provided with means for indirect heat exchange. The hydrocarbon material is catalytically cracked to high octane gasoline, light hydrocarbons, and a" carbonaceous deposit which remains on the catalyst. The carbonaceous deposit accumulates as the cracking continues and reduces the ability of the catalyst to function efiiciently. Therefore, after a suitable period of time, the flow of hydro carbon material is stopped, the converter and catalyst purged of hydrocarbon vapors, and the catalyst is regenerated by passing air over it to burn ofi the carbonaceous deposit. The activity of the catalyst is thus restored and, after air is purged from the converter, the cycle of operation is repeated. Inasmuch as the exothermic heat evolved during regeneration is greater than the endothermic heat of cracking, the excess heat must be removed from the converter by some means. In commercial operations, this has been done by an indirect heat exchange fluid circulated through heat exchange elements in the form of finned imperforate conduits regularly disposed throughout the catalyst mass.
After extended periods of use such as is encountered in commercial operations, the catalyst progressively deteriorates and must eventually be removed from the converter. The cause of the deterioration of the catalyst is not fully known, although it has been found that the type of hydrocarbon material, the partial pressure of process steam, and the temperature of regeneration are factors. Since the converters must be cooled and partially dismantled in order to change the catalyst, an appreciable length of time is consumed in the change of catalyst, during which time the converter is unproductive. The time required to change the catalyst is therefore of distinct economic importance, since the productive capacity of the refinery will thereby be infiuenced.
According to our invention, we crack hydro carbon materials by passing them over a fixed or static contact mass which has both catalytic activity and high volumetric heat capacity. We use such a contact mass in conjunction with a novel converter in which heat exchange elements are arranged and constructed in relation to the contact mass, or rather the volumetric heat capacity of the contact mass, so as to effect efficient heat control. We have found that, by using a combination of indirect heat exchange elements and a contact mass of selected heat capacity, such controlled heat capacity being obtained by using controlled proportions of catalyst and high heat capacity material or by using a single material having both catalytic and high heat capacity properties, the heat exchange elements may be of simple design and arrangement. These features in combination with other features of the invention pointed out in the following detailed description, provide a converter from which the contact mass can be discharged speedily and simply.
In carrying out the process of our invention for cracking hydrocarbons, We prefer to operate so that the coke deposit as specified below is less than 5.8 grams per liter of contact mass per each ten minutes of cracking time. Such a coke deposit is produced while maintaining conditions of cracking that are included in the ranges; 700 to 950 F. and preferably in the range 750 to 900 F. about the atmospheric to pounds per square inch gauge pressure and preferably about atmospheric to 50 pounds per square inch gauge; space rates of about 0.4 to 8 volumes of liquid oil per hour per volume of solid catalyst present in reactor and preferably 0.5 to 4 space rate; and amounts of added steam ranging from 0 to about 25 weight per cent of the charge stock. Any or all of the more severe conditions of cracking (higher temperatures, higher pressures, low space rates and a small amount, if any, of added steam) are used for more refractory or lower boiling stocks. The higher conditions of temperature can be used where a high content of arcmatic or olefinic material in the product is desired. In general, the various condition of cracking are interrelated and it is within the scope of our invention to vary any or all of these factors in order to produce a desired severit of cracking as indicated by the amount of coke deposit.
The invention can be more easily understood by a consideration of th drawings which illustrate specific embodiments of the invention as applied to the particular example of catalytic cracking.
In the drawings:
Figure 1 is a vertical section of an assembled converter, taken generally along line I-I of Figure 2. For clearer understanding, the converter is illustrated somewhat diagrammatically and process fluid distributing conduits and heat exchange elements are shown both in section (right hand side) and in full (left hand side) While a fin has been omitted from each of the heat exchange elements in the full view.
Figure 2 is a horizontal section of the assembled converter, taken along line 2-2 of Figure 1. To simplify the drawings, process fluid distributing conduits and the heat exchange elements have been omitted in the upper portion of Figure 2, although the converter, in operation, would be completely filled with such conduits and elements. The arrangement of process fluid distributing conduits and heat exchange elements in Figure 2, as in Figure 1, is schematic; examples of actual arrangements being shown in Figures 5 and 6.
Figure 3 is an enlarged vertical section of the details of a process fluid distributing conduit or distributor of the type that communicates with the upper process fluid manifolding chamber.
Figure 4 is an enlarged vertical section of the details of process fluid distributing conduit or distributor of the type that communicates with the lower process fluid manifolding chamber.
Figure 5 is a horizontal section of a unitary pattern of arrangement of both types of distributors and of heat exchange elements.
Figure 6 is a horizontal section of an alternate unitary pattern of arrangement of distributors and modified heat exchange elements.
Figure '7 is a horizontal section of a portion of the boundary zone of a converter wherein the heat exchange elements have been modified to provide for the abnormal conditions in that zone.
Figure 8 is a representation of a type of contact mass which we use in combination with our converter.
Referring to Figures 1 and 2 the converter consists of a cylindrical casing shown generally at I which may be provided with an outer covering (not shown) of heat insulating material. The converter is divided by tube sheets H, I2, 13 and I4 into various chambers or process zones.
A top tube sheet II and a contact mass retaining tube sheet I2 define a chamber I in which a contact mass M is placed in accordance with our invention and whose nature will be described -more fully below. The top tube sheet forms with the removable top of the casing It an upper -;process fluid manifolding chamber I7; the contact mass retaining tube sheet I2 and the main tube sheet I3 define a lower process fluid manifolding chamber I8; the main tube sheet I3 and the bottom tube sheet I4 define an upper heat exchange fluid manifolding chamber I9; and the bottom tube sheet I4 and the bottom of the casing 20, which may be removable, define a lower heat exchange fluid manifolding chamber 22. The top tube sheet II and the bottom tube sheet I l are held between the cooperating flanges of the casing Ii] and the top I6 and the bottom '20 respectively. Within the chamber I5 for the contact mass, there are placed distributors 21, communicating with manifolding chamber I1, and distributors 32, communicating with manifolding chamber I8, for the introduction, distribution and removal of process fluids to and from the contact mass placed within chamber I5. Also within chamber I5 are heat exchange elements 33 for the circulation of an indirect heat exchange fluid to control the temperature of aforesaid contact mass. Spacers 23 and 2| rest on tube sheet I3 and thereby space and support tube sheet I2, spacer 23 forming the vertical wall of the lower process fluid manifolding chamber I8,
A shell 24 which, as seen in Figures 2 and 7-, is shaped to conform to the outer boundary of the pattern of distributors and heat exchange elements, is placed within thecontact mass chamber I5. Shell 24 is attached to the casing by means of bolts IE! which hold the shell 24 in place and space the shell from the casing I0, thereby providing a space for insulation 25. The shell 24, which is preferably of light metal SO that it can be easily formed into the appropriate shape, may be secured to tube sheet I2 so as to prevent the passage of fluids through the insulation 25. The insulation 25 may be any appropriate material but is preferably relatively impenetrable, such as insulating concrete. The main tube sheet I3, which is transversely disposed within the casing II) in spaced relation between the contact mass retaining tube sheet I2 and the bottom tube sheet I4, is sealed in a suitable manner, as by welding, to the inner periphery of the casing Ill. Process fluids can be passed in and. out of the casing It! by means of a port 26 in the removable top It of the casing. Distributors 2'5, one of which is described in greater detail in connection with Figure 3, extend through the upper tube sheet I! and are held in position by means of plates 28. These distributors are removably attached to the contact mass retaining tube sheet I2 by threaded closure plugs 29. To aid in an even distribution of process fluids into or from the contact mass M, the distributors are provided with metering orifices 38. Process fluids are also passed to and from the contact mass by distributors 32 which are removably attached to the contact mass retaining tube sheet I2 as shown in Figure 4 and are provided with metering orifices 3! in a similar manner to distributors 21. Conduits 38, for the passage of process fluids in or out of the converter, pass through the two indirect heat exchange fluid manifolding chambers I9 and 22 and communicate with the lower process fluid manifolding chamber I8. These conduits extend through the bottom of the casing 20 to suitable apparatus for additional treatment or use of the process fluids. The up-- per ends of conduits 38 are rolled into the main tube sheet I3 or otherwise suitably secured or attached thereto. Any liquid that accumulates in chamber I8 may be drained out by conduits 38. Conduits 38 may be eliminated, if desired as when drainage of process liquids is not necessary, in which event the process fluids are introduced to or removed from manifolding chamber l8 by ports 42.
From what has been said above, it will be seen that process fluids may be passed through the converter either from top to bottom or from bottom to top by means of the elements shown. When the converter is being employed for cracking hydrocarbon oils, the process fluids, which comprise the oil in vapor form during the reaction period and air or other oxidizing gas during the regeneration period, are preferably caused to flow through the converter in the following manner. duced through conduits 33 whence it flows successively through the chamber l8, into conduits 32, through perforations 3!, and into contact mass M. The fluid leaves the converter through perforate conduits 2?, chamber H and port 25. It is preferred, however, to pass the hydrocarbons through the converter in the direction described, and to pass the oxidizing gas through in the reverse direction.
To carry a heat exchange medium in indirect heat exchange relationship with the contact mass, heat exchange elements, in the form of imperforate conduits 33, are provided. The open lower ends of the heat exchange elements 33 are rolled into holes in the main tube sheet E3 and extend through the lower process fluid manifolding chamber I8 into the contact mass chamber I with their closed upper ends 36 in proximity to the top tube sheet H. In the particular design illustrated in Figures 1 and 2, flns 34, which provide a controlled amount of heat exchange surface, are welded longitudinally along the outer periphery of heat exchange elements 33 for substantially the verticalextent of the portion of the heat exchange elements within the contact mass. Placed within each heat exchange element 33 and spaced therefrom by spacers (not shown), is a conduit of smaller diameter 35 whose open lower end is rolled into the bottom tube sheet [4 and whose open upper end is in proximity to the closed upper ends 36 of the heat exchange element 33. Orifice plates 37 are placed within the open upper ends of conduits 35 in order to increase the resistance to the flow of heat exchange fluid and, by the consequent increase in pressure drop, to minimize the effect of varying resistance to rlow in the various conduits and thus insure the even distribution of heat exchange fluid to the various conduits within the casing.
The temperature of the converter and its contents is regulated by means of an indirect heat exchange fluid circulated in indirect heat exchange relationship with the contact mass and through tubes or conduits placed around the exterior of the casing. Heat exchange fluid in line 4| is pumped by pump :14 through throttling valve 45 and line 46 to a header 4?, which extends, as shown in Figure 2, in a circular fashion around the exterior of the case. The heat exchange fluid flows upwardly through conduits 48 to an upper header 49 from which it is removed by linelifi.
I Tubes or conduits 18 are placed in direct contact with the exterior of the casing and may be flattened to insure better contact. Tubes 48 may be welded or secured by other means to the exterior of the casing if so desired and serve to maintain the exterior wall of the casing at substantially the temperature of the heat exchange medium which is also circulated within the cham- Either fluid may be alternately introchamber 22, its flow being adjusted by means of throttling valve 53. The heat exchange fluid then passes up conduit 35, through orifice 31, and down the annular space between elements 33 and to the upper heat exchange fluid manifold l9. During its passage through the portion of the heat exchange element within the chamber l5 for the contact mass, and particularly during its downward passage in contact with the inner periphery of heat exchange element 33, the heat exchange fluid is in indirect heat exchange relationship with both the contact mass and heat conducting fins 35. The heat exchange fluid passes out of chamber it through ports 54 and line 55 and can be returned, together with similar fluid from the header 49, to the pump 44. The heat exchange fluid can be adjusted in temperature by heat exchangers placed at any point in its flow outside of the casing. Vents 56 and 51, communicating with the manifolding chamber l9 and with the header &9 respectively, furnish a means of venting the heat exchange system to remove gases which might interfere with the circulation of heat exchange liquid.
Any suitable type of fluid heat exchange medium may be used for controlling the temperature of the contact mass and the exterior wall of the casing. The heat exchange fluid can be a single phase composition which does not or is not permitted to vaporize, including fused salts such as the alkali nitrates or nitrites or eutectic mixtures of these materials such as disclosed in U. S. P. 2,375,761, granted May 15, 1945, to John R. Bates. Certain metals and metallic alloys may be used, or materials which undergo a change of state as by vaporization. Water, diphenyl, mercury, chlorinated hydrocarbons or other inert liquids can be used in the liquid, vapor or mixed phase state. The flow or circulation of the heat exchange fluid can be regulated or stopped in either the heat exchange elements in the con 1: tact mass or the heat exchange tubes fastened to the exterior of the case by means of valves 45 and 53. As previously mentioned, orifices 31 compensate for any inequality in the length or pressure drop in conduits 35 and act to prevent inadequate flow of the fluid in longer or obstructed conduits and thereby insure an even distribution of heat exchange fluid through all of the conduits with an ensuing evenness of temperature throughout the contact mass despite variations in pressure above orifice ill.
In Figure 3 is shown in detail the manner in which distributors 2! are attached to the top and contact mass retaining tube sheets I! and 2. Distributor 2?, which is a perforate process fluid distributing conduit, is closed at the lower end by a threaded closure plug 29 so that the lower process fluid manifolding chamber i5 is out of communication with the interior of distributor 21. This closure plug is Welded inthe end of tube 2! and has a threaded extension which extends into and engages a threaded hole or aperture 15 in the contact mass retaining tube sheet 52. The distributor is removably attached to the top of tube sheet H by means of a plate I? (indicated schematically by 28 in Figures 1 and 2) which bears on sealing ring is and is tightened by nuts '29 on bolts which are in threaded engage ment with the top tube sheet H. When nuts 19 are tightened the plate Tl bears against ring 18, which may be of any suitable packing material that willstand' high temperature and is preferably metallic such as stainless steel. Plate 1'! is madeof a corrosion resistant metal such as stainlesssteel and is designed to provide a clearance space 83 between distributor 2? and plate ll. The aperture in tube sheet 1 I through which distributor 21 passes is somewhat larger than distributor 2'! and communication is thus provided between the chamber 15 for the contact mass and the upper process fluid manifolding chamber I! by clearance 83 which provides a metered flow. Clearance 83 acts similarly to orifices, 38 by controlling the amount of flow of process fluids to and from the contact mass. Orifices 30 and clearance 83 arev designed and sized to provide equal streams of process fluids relative to; the amount of contact mass traversed by these streams, with due consideration being given to the various pressure drops along the path of the process fluids. Thus the sum of the flow through the topmost set of orifices 30 and clearance 83 provide the same flow relative to the amount of contact mass as do the lower sets of orifices. A conventional packing may also be used in place of clearance 83 in which event, all process fluids pass through orifices 3B which, accordingly are sized to give equal distribution. Welded or otherwise secured in the top of .distributor 21 is a member 82 to which is securely attached tube 84 as by rolling the open end of tube 84 into member 82. As can be seen by reference to Figure 1, tube 84 forms a means whereby fluids can be passed down tube 84, out of the open end of tube 84 at the bottom of conduit 2'! and then upwardly along the annular space 85 between tube 34 and conduit 21 and out the metering orifices 30. By using the reverse flow arrangement shown in Figure 3, the temperatures of process fluids and of top of distributor 21 instead to the bottom by the reverse flow arrangement of Figure 3.
In Figure 4 is shown a detailed view of distrib- V utor 32 which is a perforate process fluid distributing conduit, closed at the upper end by removable closure plug 93. Distributor 32 is joined to the contact mass retaining tube sheet [2 by means of a plug 9| which has a hexagonally shaped aperture for the insertion of a wrench. Distributor 32 is welded or otherwise suitably attached to plug 9! and the assemblage is then threaded into the contact mass retaining tube sheet i2. Also secured to the plug Si is an inner tube 92 which serves to convey the process fluids to the upper end of distributor 32 and operates in a similar manner to tube Ed in distributor 21. The metering orifices 3! in distributors 32 are placed at levels which alternate with the levels at which the metering orifices 39 in distributors 21 are placed.
Distributors 32 and 21 form two sets of perforate conduits which serve to introduce and remove the process or reactant fluids to and from the contact mass. Distributors 21, which communicate with the upper process fluid manifolding chamber Ii, will be designated, for convenience. as B tubes and distributors 32. which communicate with the lower process fluid manifolding chamber I8, as C tubes. Thus, by placing the metering orifices at diflerent levels in the B and C tubes, the contact mass is, in efiect, divided into separate horizontal sections, through each-oi? which the process fluids flow in parallel. Thus, levels in the contact mass may be considered as being spaced equidistantly in a sequence, I, 2, 3 4, 5, 6, where l and 6 are the top and bottom of the contact mass. The orifices in a B tube can be placed at levels I, 3. and 5 and the orifices in a C tube at 2, 4, 6. In a converter designed as shown in Figures 1 and 2 We prefer to space the metering orifices so that the distance between the orifices in a single tube is between 4 and 20, feet, thus dividing the contact mass into 2 to 10 foot sections. However, other lengths of path through the contact mass, such as lengths of path as short as 6 inches, or as great as 15 to 20 feet, may be used where the conditions render such lengths of path desirable.
The arrangement of the fluid distributing conduits and the heat exchange elements is an important feature of the structure of the converter and, in accordance with the invention, is related to the type of contact mass employed. We have found that use of the contact masses described below together with arrangements of distributors and simplified heat exchange elements, such as those illustrated in Figures 5 and 6, provide superior temperature control of the contact mass with ensuing process advantages while providing for a quick and complete discharge of the contact mass. As noted above, the preferred unitary patterns of arrangement are shown in Figures 5 and 6. Figure 5 illustrates a unitary pattern in which a distributor 21, in communication with the upper process fluid manifolding chamber I1, is placed at the center of a rectangle at the corners or vertices of which are placed distributors 32 which are in communication with the lower fluid process manifolding chamber. Distributor 2! is also the center of a hexagon at Whose vertices are arranged heat exchange elements 33. On the outer periphery of the heat exchange elements 33 are fins 34 which, in the embodiments illustrated in Figures 5 and 6, are rectangular in transverse section and attached longitudinally to the heat exchange elements by some suitable means such as welding. Fins 34 may be formed integrally in the manufacture of the heat exchange elements 33 or may be attached subsequently or, instead of being solid metallic members as shown in Figure 5, may be hollow and filled with the indirect heat exchange fluid which circulates through the heat exchange element 33. Fins 35 extend radially from the vertical axis of the heat exchange element and are spaced approximately apart in the embodiment shown in Figure 5. In the unitary pattern of Figure 5, the fins in each case are directed toward one of the distributors. Removal of one of the freely rotatable distributors, either of the type 21 or 32, leaves an aperture in the contact mass retaining tube sheet 12 to which these distributors are removably attached (see Figures 3 and 4). The contact mass disposed between the heat exchange elements and their fins can then flow downwardly and out of the contact mass chamber l5 through such an aperture. The unitary pattern shown in Figure 5 is arranged and disposed so that the fins permit downward and lateral flow of contact mass from around several tubes toward a single aperture in the tube sheet formed by removal of a distributing conduit. We have found that removal of every distributor is not necessary and the contact mass will discharge quickly and completely when approximately 20% of the distributors are removed. It should be noted that, as the unitary pattern is repeated, distributor 21, which is the center of the unitary pattern illustrated, becomes the corner of a rectangle of similar distributors 21 in the center of which is distributor 32, surrounded by a hexagon of heat exchange elements. If the terminology of Figures 3. and 4 is used, the chamber for the contact mass is filled with two sets of d.stributors, identified as B and C tubes together with heat exchange elements which may be designated as K tubes. The perforate conduits are arranged in an interlocking pattern such that each B tube is at the center of a quadrangle at the corners or vertices of which there are four C tubes, while each C tube is at the center of a quadrangle at the corners or vertices of which there are four B tubes, each C and B tube being located at the center of a hexagon at the vertice of which there are six K tubes. The relationship of the contact mass M to the unitary pattern is shown by the circular section in Figure 5, an enlargement of which is shown in Figure 8.
In Figure 6 is shown another method of the arrangement of the fins on the outer periphery of heat exchange elements 33. Figure 6 illustrates a unitary pattern with a distributor 32 as the center of a hexagon at the alternate vertices of which are heat exchange elements having six instead of three fins, the fins thereby being 60 apart. It should be noted, however, that this arrangement still does not interfere with the proper flow of the contact mass toward the apertures formed by the removal of the distributors.
The above features of our converter permit an easy and complete discharge of a granular contact mass in the following fashion. When the catalyst has deteriorated past the point where the hydrocarbon charge stocks available can be economically processed, the converter is purged of combustible vapors and allowed to Cool to a temperature which permits handling of the equipment. The removable top it is taken off and the top tube sheet H is removed after unscrewing nuts 19 and removing plates 71. The vertical walls of the lower process fluid manifolding chamber i8 formed by spacer 23 are provided with a plurality of apertures used in connection with the discharge of the contact mass. These apertures are provided with closure plugs 40 and communicate with ports 42 placed in the wall of the casing l0. Ports 42 are provided with pressure .tight covers 43 which, together with closure plugs 40, ar kept in place during the normal operation of the converter and are removed for the discharge of the contact mass. After removing covers 43 and plugs 40, some of the distributors of both types, 21 and 32, which are freely rotatable without interference with any of the other structures in the reaction chamber i even when the contact mass is in the reaction chamber, are dsengaged from the catalyst retain'ng tube sheet i 2 and the catalyst allowed to discharge through the resulting apertures in tube sheet 12 into chamber 18 and thence through conduits 38. Alt/ernately, part or all of the contact mass may be removed from chamber [9 through ports 42.
During the period when the chamber is being filled with the contact mass, the tops of heat exchange elements and distributing conduits can be held in place by spacers which may be removed before the top tube sheet H is replaced.
The above described procedure for discharging and recharging contact mass has decreased the time consumed by this operation by as much as four fifths of that previously necessary. This 10 decrease in oiT-stream time represents considerably improved commercial operation. In Figure 7 is shown a modified pattern or arrangement of the two sets of distributors and the heat exchange elements at the boundary of the reaction zone. At the boundary, unusual conditions of temperature may occur. ample, in catalytic cracking operations, we have found that local overheating is sometimes encountered near the reactor shell. Such local overheating causes excessive deposition of coke on the catalyst within the zone affected. Upon burning, the excess of coke results in increasingly higher temperatures, even to the point of damaging either or both the catalyst and adjacent metal structure. To eliminate these difficulties, we employ lower volumetric concentrations of catalyst adjacent the boundary of the reaction zone than in the remainder of the reaction zone, while simultaneously increasing the heat capacity of the boundary portions thus modified. Thus, the ratio of heat capacity material to catalyst or the volume of the heat conducting fins attached to the heat exchange elements or both may be increased. In some cases, it is desirable to omit the catalyst completely from a small portion of the reaction chamber adjacent the boundary and use only refractory material. Figure 7 illustrates one method of improving the temperature control at the boundary of the reaction zone. The fins attached to heat exchange elements 33 are increased in volume in the manner shown by fins 95. Where the heat exchange elements are close to sheet 24 which forms the boundary, a short stubby fin 96 can be used. As shown in Figure 7, various arrangements of the more massive fins and 96 can be used. These more massive fins have the eiiect of decreasing the temperature gradient along their radial extent with the result that the adjacent contact mass is more effectively cooled. However, these fins still are designed so that they do not obstruct the fiow of contact mass when it is'dis charged from the bottom of the reaction zone.
One of the features of the invention is the contact mass used in connection with converters similar to that described above. We have found that contact masses having at least 0.45 British thermal unit per liter per degree Fahrenheit permit greatly improved converter design and operation. (The volumetric heat capacity, as used throughout this specification, is calculated as the product of the speciic heat, measured at the temperature of operation, times the apparent or bull; density; i. e., the weight of a unit volume of the material in the form (such as granules, pieces or pellets) under consideration.) We select or prepare the contact mass M to be used in the converter with regard to its catalytic properties and its volumetric heat capacity. We prefer to use contact masses which, when used for the catalytic cracking of hydrocarbon material boiling above the gasoline range, produce, under the conditions of time, temperature and pressure used in the process, gasoline to the extent of at least 30 and preferably above 40 volume percent of the charge stock without excessive formation of gas and coke. We have found that the volumetric heat capacity of the contact mass is related to the heat exchange elements, particularly in regard to the extent of and average spacing or clearance between the heat exchange surfaces. By using contact masses which havea volumetric heat capacity of at least 0.45 British thermal unit per liter per degree Fahrenheit (B. t. u./li-
For exl 1 ter/FJ, we can space adjacent non interscting heat exchange surfaces to provide clearances averaging an inch to two'inches or greater, with a'consequent simplification of converter design. Such clearances provide not only easy discharge, but also dense packing of the contact mass and hence efficient utilization of the space available for the contact mass. Thus, a comparison of the packing of contact masses in converters cons'tructed with various clearances between the sur faces of the heat exchange elements showed that the contact mass occupied the space available one type of our novel converter (clearance over 1 /4 inches) almost completely (99%) whereas a similar contact mass packed in a converter having average clearances of approximately of an inch occupied only 90% of the available space. We have found that converters employed catalytic cracking having heat exchange ele inents constructed and arranged to have clearances averaging an inch or over, such as 1 /4 to 2" inches, when used in conjunction with contact masses having volumetric heat capacities of over 0.45 B. t. u./liter/F., show excellent temperature control when heat exchange surfaces of between 0.3 and 0.7 square foot per liter of contact "mass are provided. We have also found that by increasing the volumetric heat capacity of the contact mass to 0.5 B. t. u./liter/F. or higher, we can further increase the clearances between nonadjoining surfaces of heat exchange elements to values such as average clearances of about 2 inches and greater, and/or use intermediate values of the ratio of the heat exchange surface to contact mass, such as 0.4 to 0.6 sq. ft./liter of contact mass. Even with the increased clearances, thermal control of the contact mass is excellent and low thermal gradients are encountered within the mass.
One form 'of the contact mass, according to our invention, comprises a mixture of porous solid surface active material or catalyst and in ert refractory and preferably nonep'oro'us material for heat capacity, the nature and proportions of which will be hereinafter described. The mixture of the two materials may be a mixture of particles of the two materials, the sizes of the particles being selected to obtain the maximum average dispersion of one material in the other, or the mixture may be more intimate and may be obtained by forming aggregates of the proper size from a mixture of finer materials. As examples of the latter, an active clayv may be mixed with a fine powder of a material having adequate volumetric heatcapa-city and the resultant mixture extruded, formed into pellets and baked to harden the composite mass, or a fine powder of an inert refractory material may be added to a synthetic inorganic gel before or during the gelation stage, and the resulting gel formed or cast in the form of beads or pellets or the two materials may be mixed in powder form and pelleted with a pelleting machine. The contact mass is used in the form of particles of such size that the contact mass fills the converter evenly and as completely as possible. We use particles or pieces of between 1 and 10 millimeters in size but preferably between 3 and 7 millimeters, the larger sizes being more adapted. to converters having larger average clearances,
'such as 2 inches or greater, and to the type of contact mass in which both a catalyst and an inert refractory material are incorporated in a single piece.
When the invention is employed in connection attics-ihave an adverse catalytic effect.
32 with catalytic cracking or hyarecarb n mat the" porous solid surface ac'tive material p propriately a cracking catalyst of natural or en}; thetic origin. Among the various catalysts which can be used are natural or synthetic active al'uminosilicates such as clays containing clay minerals such as montmorillonite, and the like which may be activated by acid or alkaline treat-#- ment, or synthetic silica-alumina, silica-'zirconia or sili'ca alumina-zirconia gels. Other suitable catalysts are silica-urania gels, silica tho'ria gels, zirconium phosphate and the like. An inert refractory material should be selected which does not react chemically with the catalyst or with the fluids to be contacted or produced by the process, and which can withstand the temperatures used in the process without physi cal change or deterioration and which hasaa adequate volumetric heat capacity. When the invention is used in connection with catalytic cracking, th specifications of the inert refrac t'ory material should includeability to withstand temperatures of l000 to 2000 F. and preferably above 1200 F., and a volumetric heat capacity of greater than 0.6 British thermal unit per liter per degree Fahrenheit (measured at the operating temperature) although heat capacities in the range greater than 0.8 B. t. u./liter/ F. are pre* ferred. In general, suitable materials are dense preferably non-porous materials with specific heats of greater than 0.20 and preferably above 0.25 (measured at the mean temperature of operation of the process). Suitable materials include various oxides of the second, third and fourth groups of the periodic table, for example, alumina, silica, beryllia and zirconia. Preferred forms of these materials, which possess the desired characteristics of cheapness, availability and high heat capacity, include corundum, fused quartz, magnesia, mixtures of these materials, and commercial modifications of these materials such as sand, Alund-um, Corhart and the like. The inert refractory solid may consist of one or more components and it may be prepared by fusion, precipitation, gelation or other methods. I 'n fusible metals can be used in the free state, but the metal should be selected so that itdoes not Various suric'ates, carbides, dead burned ores, natural refractories, crushed igneous rocks, dense or fused in"- act'i've clays, and the like may also be used. Thus,
a suitable material can be selected for the various modifications of this invention in accordance with the conditionsof operations or the mate'- -rials used in the modification.
When a mixture of catalyst and inert refractory mate'rialis used for a contact mass forcatalytic cracking, presently commercially available catalysts are generally materials having volumetric heat capacityof about 0.35 to 0.42 B. t, u;/
liter/F. In such a case, an appropriate mix ture consists of about 40 to volume percent .of the catalyst and about 60 to 10 volume percent contact mass such as is described above. The dark particles 98 are crushed and sized particles of an inert refractory material such as is obtained by the fusion of a mixture of refractory oxides such as silica, alumina and zirconia. The pellets 99 are molded pieces of a synthetic or natural porous solid catalyst. The contact mass can be a single material which has the proper heat capacity and catalytic activity. In such a form, the contact mass may be a natural product, which may have been chemicallytreated to produce the desired characteristics or may be a synthetic product, such as an inorganic hydrogel, which, either by the method of formation of the gel or by subsequent processing, has a density and a specific heat such that its volumetric heat capacity is greater than 0.45 B. t. u./liter/ E. Additional advantages are realized when contact masses of whatever origin are used which have a higher heat capacity such as 0.5 B. t. u./liter/ F. or above.
We have found that the use of contact masses as described above enable us to employ a converter of simplified design which has considerable advantages in the ease of discharge of the contact mass and yet maintains and even surpasses the performance of more intricate and complex converters in regard to temperature control and processing capacity. The following example illustrates an application of our invention and advantages thereof.
EXAMPLE A converter A, embodying the principles of the unitary pattern of process fluid distributing conduits and heat exchange elements shown in Figure 5, was constructed and filled with a contact mass which comprised a mixture of 72 volume per cent of pellets, 4 mm. in size, of a synthetic cracking catalyst composed of silica and alumina and 28 volume per cent of crushed pieces of about the same size of a commercial refractory material known as Corhart (a fused alumina containing about 25% silica). The volumetric heat capacity of the contact mass was calculated to be .55 B. t. u./liter/F. at 1000 F. lhe converter, which was cooled by indirect heat exchange with a molten mixture of inorganic salts, had an average clearance of about 2% inches between adjacent nonintersecting surfaces of heat exchange elements and had a ratio of 0.45 square foot of heat exchange surface per liter of contact mass. In order to evaluate the utility of converter A, hydrocarbon charge stocks of varying boiling ranges and from various types of crudes were cracked in converter A. Similar stocks were cracked in a converter B, which also used a molten salt heat exchange system. Converter B had over twice the ratio of heat exchange surface to contact mass (1.2 square feet per liter of contact mass) as did converter A and had average clearances between the heat exchange surfaces or fins of approximately of an inch. Converter B Was filled with a contact mass comprising only synthetic cracking catalyst of the same type and activity as used in converter A. The timing cycle used in the cracking operations was 10 minutes on-stream (cracking phase), 10 minutes for burning off coke by air (regeneration phase) and two 5 minute periods between the alternate on-stream and regeneration periods for valve changes and purging.
The results of cracking the various charge stocks showed that the process using converter A at least equalled the process using converter B in regard to quality and quantity of products at equal or greater throughputs while having superior temperature control of the contact mass. The data below in Table I are illustrative of the results and were obtained from cracking operations on similar heavy gas oil fractions from an East Texas crude oil; the process listed under converter B being performed under typical commercial conditions of charging rate of feed stock and amount of coke deposit, other conditions being adjusted to obtain a high conversion of charge stock to motor gasoline. Runs in converter B, the results of one of which are shown in Table I, proved that a higher charging rate, relative to the catalyst, could be maintained in converter A than in converter B at the same level of conversion of charge stock to motor gasoline.
Table I Process using Yields 1 Con- Converter A i verter B Space velocity (Volumes of liquid charge stock (at 60 F.) per hour volume of catalyst present in reactor) 1.25 l. 1 Motor gasoline, vol. per cent 42. 0 42. 2 Octane Number (AS'IM), clear 78. 9 78.8 Liquid recovery, vol. per cent 90. 4 87.0 04, weight per cent 8.0 9. 6 C3 and lighter, weight per cent. 4. 6 6. 3 Coke, weight per cent of charge 4. 4 4. 3 Coke, grams coke/liter contact mass 5. 7 6. 9
1 Based on 10 pounds Reid vapor pressure.
The results in Table I show that the process conducted in our novel converter A showed a higher liquid recovery and a lower percentage of less desirable light gases than did the process conducted in converter B at the same level of conversion of charge stock to motor gasoline and coke. Moreover, converters of the type of converter A have a 35 to 40% higher volumetric efficiency than converters of the type of B (where volumetric efficiency is based on the amount of contact mass per unit volume of converter) due to the simplification of converter design and improved packing of the contact mass. Thus, as shown by the 14% increase in charging rate for converter A, in our improved converters, we have a higher throughput or processing capacity for the same size of converter when using contact masses having ratios of catalyst to inert material of 2 /2 or 3 to 1 and above while at even lower ratios we can maintain at least equal throughput at the same time that we have the above advantages.
Further investigation showed another feature of our invention. When converter A is used for a cracking operation, the temperature variation of the contact mass is less than that in converter B when the same amount of coke per unit of catalyst is involved. Since the amount of coke burned per cycle governs the heat liberated per cycle, slightly more heat was removed by the indirect heat exchange fluid from converter A than was removed from converter B with less variation in the average temperatures of cracking and regeneration, even though converter B had over twice as much heat exchange surface per unit volume of catalyst of contact mass. The following data were observed in the two converters during runs at the same space velocity with the same charge stock and using the same timing cycle and contact masses described above.
Table I1 Convert- Converter A er B Coke deposited, grams/liter catalyst 7. 7 7.0 Temperature rise of contact mass above minimum cracking temperature, F.:
0-2 minutes (regeneration) 51 128 0 69 196 0-6 105 155 0-8 115 116 Difierence between In regeneration and minimum temperature of cracking of contact mass, F 115 197 Maximum temperature of regeneration of contact mass for cracking temperature of 850 F. 956 l, 034 Temperature diilerence between contact mass and indirect heat exchange fluid, F. at end of cracking period 42 0 The operation in converter A resulted in much lower maximum regeneration temperature for the same temperature of cracking and less variation between the temperatures during cracking and regeneration. As shown by the last line of Table II, the molten salt temperature is lower in converter A than in converter B for the same temperature of conversion or cracking with resultant beneficial efiects on the molten salt system.
In a typical operation laying down 7.0 grams of coke per liter of active catalyst, about 210 B. t. u. (-due to burning of the coke) are liberated in every half-hour cycle of the type described above. Of this heat, 80 to 90% may be removed by the indirect heat exchange system. The remainder of the heat is used in the endothermic heat of cracking, and in raising the temperatures of the reactants and products. Excess heat is thereby removed from the contact mass at an hourly rate of about 370 B. t. u./liter of catalyst. In cracking operations in order to limit the regeneration temperatures, we prefer to limit the coke deposit so that stable operation results when the excess heat is removed at a rate of the order of 400 B. t. u. per hour per liter of contact mass; this rate being independent of the timing of the cycle and corresponding, in the case of the contact mass described above in connection with converter A, to about 8 grams of coke per liter of catalyst per half-hour cycle or about 5.8 grams of coke per liter of contact mass for the half-hour cycle. Under such conditions, the life of the catalyst is prolonged due to low maximum temperatures of regeneration and yet adequate processing capacity is maintained.
As will be seen from the results given above, the use of the contact masses and converters of our invention results in several advantages. For 1 the same molten salt temperature, the on-stream temperature is higher with a resultant increase in conversion without an increase in the maximum regeneration temperature. If high conversion temperatures are desired in converters of other designs. the salt temperature must be raised and the system containing the molten salt thereb having resultant tendency toward accelerated metal corrosion. The lower regeneration maximum reduces the temperatures to which metals within the converter including conduits are subjected with a resultant beneficial effect on the useful life of the converters. Considerable advantage can be gained by use of the invention in various other types of converters,'such as those utilizin a path of suitable length straight through the entire depth of contact mass instead of a plurality of paths such as provided in the converters of the drawing.
Inasmuch as many modifications and variations 16 of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, only such limitations should be imposed on the invention as are indicated in the appended claims.
We claim as our invention: 1. In a cyclic process for the catalytic cracking of hydrocarbons using a static bed of catalytically active contact material which process comprises an endothermic cracking period, during which said bed of catalytically active contact material is contacted with said hydrocarbons and concomitantly accumulates a carbonaceous deposit, and an exothermic regeneration period, during which said carbonaceous deposit on the contact mass is removed by oxidation; the improvement which comprises passing said hydrocarbons under cracking conditions through a bed of selected heat capacity of at least 0.45 British thermal unit per liter per degree Fahrenheit, said contact mass consisting of from 40 to percent of siliceous hydrocarbon cracking catalyst and from 10 to 60 percent of an inert refractory material of high volumetric heat capacity, and continuously removing heat from said contact mass during the entire cracking and entire regeneration periods by fiowing heat exchange fluid at a temperature substantially lower than that of the contact mass at any time through a plurality of indirect heat exchange zones regularly arranged and spaced apart within said bed, said heat exchange zones extending horizontally and vertically into said bed and having between 0.3 to 0.7 square foot of heat exchange surface per liter of contact mass, the boundary between said heat exchange zones and said bed being devoid of protuberances restricting downward and lateral flow of said contact mass to discharge points at the bottom of said bed.
2. The improvement of claim 1 in which said contact mass consists of discrete particles of between 1 and 10 millimeters in size and consisting of an intimate mixture of said catalyst and said inert refractory material in the form of a powder.
3. The improvement of claim 1 in which the peripheral portion of the horizontal cross section of said bed contains less catalyst per unit volume throughout the vertical extent of said bed than does the remainder of said bed.
4. Apparatus for contact treatment of process fluids at controlled temperatures comprising a vertical casing with a removable top, a top; tube sheet and a contact mass retaining tube sheet transversely disposed within said casing, the top tube sheet being removable and forming in conjunction with the removable top of the casin an upper process fluid manifolding chamber, the contact mass retaining tube sheet being in parallel spaced relation to said top tube sheet and forming in conjunction therewith a chamber. for a contact mass, said contact mass retaining tube sheet forming the top of a lower process fluid manifolding chamber spaced immediately therebelow, process fluid distributing conduits within the casing communicating with the upper process fluid manifolding chamber and removably attached to the top tube sheet and extending through the vertical extent of the chamber for the contact mass, said conduits havin closed lower ends and being threaded into openings in the contact mass retaining tube sheet but out of communication with the lower process fluid manifolding chamber, additional process fluid distributing conduits positioned within the chamber for the contact mass and with closed top ends in proximity to the top tube sheet and with open lower ends threaded in openings in the contact mass retaining tube sheet and communicating through open bottom ends with the lower process fluid manifolding chamber, both of the above types of conduits being freel rotatable about their vertical axis without interference with other portions of the structure or with the contact mass and being apertured with metering orifices at various points, indirect heat exchange fluid conduits with closed ends in proximity to the top tube sheet and with open lower ends fastened to the main tube sheet, said indirect heat exchange fluid conduits extending through and peripherally sealed to the contact mass retaining tube sheet, the portion of said indirect heat exchange fluid conduits within the reaction chamber having fins on the outer periphery thereof for substantially the vertical extent thereof, said fins being arranged and disposed to allow substantially complete and easy downward discharge of a granular contact mass toward said openings in the contact mass retaining tube sheet provided by removal of only a fraction of said process fluid distributing conduits and having no obstructions to the lateral flow of contact mass outwardly from said indirect heat exchange conduits, said indirect heat exchange fluid conduits and said fins constituting heat exchange elements having an outside surface of between 0.3 to 0.7 square foot per liter of contact mass and having average clearances of greater than one inch between adjacent surfaces, means for circulating a heat exchange fluid through said indirect heat exchange fluid conduits, conduits for the flow of process fluids to be contacted which extend outside the casing and communicate with the lower process fluid manifold, the removable top of the casing being provided with a port communicatin with the upper process fluid manifold, the casing being provided with ports communicating with the lower process fluid manifold and having removable pressure tight covers.
5. The apparatus according to claim 4 in which the indirect heat exchange fluid conduits are circular tube and the fins on the outer periphery thereof have a substantially rectilinear transverse section and extend radially from the axis of said circular tube and are arranged to provide clearances of greater than one inch between adjacent fins.
6. In apparatus for treatment of process fluids with granular contact mass at controlled temperatures which apparatus comprises a contacting chamber defined by upper and lower horizontal tube sheets and adapted to contain a granular contact mass, upper and lower process fluid manifoldin chambers disposed immediately above and below said contacting chamber and horizontally coextensive therewith, two sets of perforated process fluid distributing conduits within the contact mass chamber communicating with the upper and lower fluid process manifolds respectively through openings in said upper and lower tube sheets respectively, said sets of conduits forming a means for passing process fluids from one process fluid manifolding chamber to the other process fluid manifolding chamber through a predetermined section of the contact mass, vertical heat exchange elements within the 18 municating with the upper process fluid manifolding chamber have closed lower ends, which lower ends close openings in said lower tube sheet, and are removable from said openings in said lower tube sheet solely by manipulation at a 10- cation above said lower tube sheet; wherein each of said heat exchange elements has fins on the outer periphery thereof for substantially the vertical extent thereof, said fins being arranged and disposed to allow substantially complete and easy discharge of a granular contact mass through the openings in said lower tube sheet provided by removal of only a fraction of the process fluid distributing conduits removably engaged therewith, said fins having no obstructions to the lateral flow of contact mass outwardly from said indirect heat exchange conduits; and wherein all of said process fluid distributing conduits are removable and freely rotatable without interference with the remainder of the structure and in the presence of contact mass in the chamber.
'7. Apparatus for treatment of process fluids with a granular contact mass at controlled temperatures comprising a vertical casing with a removable top, a top tube sheet and a contact mass retaining tube sheet transversely disposed within said casing, the top tube sheet forming in conjunction with the removable top of the casing an upper process fluid manifolding chamber, the contact mass retaining tube sheet being in parallel spaced relation to said top tube sheet and forming in conjunction therewith a contacting chamber for containing a granular contact mass, said contact mass retaining tube sheet forming the top of a lower process fluid manifolding chamber spaced immediately therebelow, a first set of process fluid distributin conduits in the contacting chamber commun cating with the upper process fluid manifolding chamber and extending through the vertical extent of the contacting chamber, said conduits having closed lower ends, which lower ends engage openings in the contact mass retaining tube sheet so as to prevent communication with the lower process fluid manifolding chamber, said first set of process fluid distributing conduits being engaged with said contact mass retaining and said top tube sheets so as to be removable solely by manipulation from a location above said top tube sheet,
a second et of process fluid distributing conduits in the contacting chamber havin closed top ends in proximity to the top tube sheet and open lower ends removably engaged with the contact mass retaining tube sheet and communicating through open lower ends with the lower process fluid manifolding chamber, each of the process fluid distributing conduits of said first and second sets being freely rotatable about its vertical axis without interference with other portions of the structure or with the contact mass and being apertured with metering orifices at various points, a main tube sheet positioned in parallel relation below said contact mass retaining sheet and defining the bottom of the lower process fluid manifolding chamber, indirect heat exchange fluid conduits with closed ends in proximity to the top tube sheet and with open lower ends fastened to the main tube sheet, said indirect heat exchange fluid conduits extending through and peripherally sealed to the contact mass retaining tube sheet, the portion of said indirect heat exchange fluid conduits within the reaction chamber having fins on the outer periphery thereof for substantially the vertical extent thereof, said fins being arranged and disposed to allow subfstanftially hammers, and easy of, ajgranular contact nass toward said ol ifi s ,in the, contactmassretaining tube sheetprovided by removal crp'mya fraction of said :first set of ,process fluid distributing conduits and having no obstructions to theilateralflow of contactmass outwardly from said indirect heat exchange conduits said indirect heat exchange fluidconduits ments having an outside isurface of LbetWeenOB to 9.? square foot perliter of contact mass and having average clearances of greater than one inch between adjacent surfaces, means for circulating a heat exchange fluid through said indirect heat exchange fiuid conduits, conduits for the flow ofr process fluid to be contacted which extendoutside the casing and communicate with the lower process fluid manifold, the removable top of the casing being provided with a port pommunicating with the upper process fluid manifold, the casing being provided with ports communicating with the lower processfiuid manifold and having removable pressuretight covers. 8. Theapparatus according to claim '7 in which the indirect heat exchange conduits are circular and said fins constituting heat exchange ele- I Y ,t'ub es and th'e" on ihaveasub a l rl t lin a'nd t nd r di ll iromj Maxis, i tul0e and are arrangedto proyideicl rancesof "greater than one inch betweenfadjacent fins.
.RAYMO DC. LAI SI ORGE H: KEL
JAMES E. EVANS.
, FER N E .E TEP v The following references "are er'reeard inti'ie file of this patent; a, UNITED STATES PATENTS