Continuous Fractionating Columns for Pilot Plants JAMES K. CARPENTER AND RALPH W. HELWIG Gulf Research & Development Company, Pittsburgh, Pa.
T
Pilot plants in the petroleum and chemical industries material balances must be H E use of pilot plants are primarily used either to obtain preliminary design invery close to loo%, and in the petroleum and therefore leaks in all parts of formation or for solving current operating problems. In chemical industries for oborder that accurate data be obtained it is essential that the the equipment must be taining process design and continuous distillation equipment used in the.pilot plant avoided. An effective means evaluation data has been be quickly responsive to changes made in the rest of the of reducing leakage is to use increasing rapidly during unit, trouble-free in operation, and adaptable enough to welded construction wherever recent years. When conduplicate the required range of commercial operations. practical, both on the column tinous distillation equipment The design and construction features used on pilot plant itself and in its associated is incorporated in the pilot continuous fractionating columns charging a wide variety piping. To allow easy access plant it is necessary that to the column for cleaning opof hydrocarbon fractions are described in detail. These good design and construcerations, flanges of the ringcolumns, of the packed-tower type, range from 2.5 to 12 tion techniques be employed inches in diameter, up to 25 feet in height, and cover operjoint type are placed at both in order that accurate data ating pressures from 10 mm. of mercury absolute pressure, ends. Ring-joint flanges have may be obtained in a minito 300 pounds per square inch gage. Three distillation the advantage that they can mum of operating time. easily be made pressuretight, systems for high pressure, atmospheric, and vacuum opThe design methods preand tho metal ring gaskets erations are described, and diagrams show specific details sented are the results of exmay be re-used a number of of construction for such components as preheaters, reperience with the separation boilers, condensers, packing supports, thermowells, windtimes. The columns may be of both binary mixtures, such ings, and instrumentation. supported either at the top or as n-butane and isobutane, bottom, depending on the and complex mixtures, rangbuilding structure. Expaning from the depropanization of gasoline to the stripping of heavy sion on heating usually causes less difficultyif the column is supgas oils from tar. Since the control methods were adapted ported from the top and is allowed to hang free. Mechanically it specifically for the petroleum industry, it is realized that they is well to provide a high safety factor in the design of the column, will not apply to all situations encountered in other industries, since the operational limits are usually not well established until but it is felt that in general they will have wide utility. The after the column has been in service for some time. pilot plant distillation equipment described has capacities from Packing. Packed columns are very widely used in pilot plant 1 to 80 gallons per hour with fractionation columns ranging from work because they are more easily fabricated and have several 2.5 to 12 inches in diameter. significant advantages over bubble- or perforated-plate columns. Commercial columns are usually designed for a specific separaThese include greater ease of assembly and cleaning, lower tion with the tacit assumption that the column will be used for liquid holdup, lower pressure drop, and much greater adaptability. that separation for the major part of its useful life. However, Since the height equivalent of theoretical plates ( H E T P ) for a the design of pilot plant columns requires an entirely different given packing varies appreciably with the tower dimensions and approach because of the variety of work that an individual unit from one system to another, the exact number of theoretical is likely to encounter. The distillation equipment must be adaptplates being used in a packed column may not always be known. able enough to handle wide ranges of throughput and charge This uncertainty will usually be of less importance, however, than compositions. It also must be quickly responsive to changes the flexibility derived from the ease of changing the amount or made in the rest of the unit, and trouble-free in operation. type of packing in the column. A major disadvantage of packed columns is the reduction of efficiency caused by nonuniform COLUMN DESIGN AND CONSTRUCTION liquid distribution which may become serious in columns larger Pilot plant distillation columns are subjected to very severe than 12 inches in diameter. Use of liquid redistributors is ususervice from frequent disassembly for cleaning, inspection, and ally effective in overcoming this disadvantage. In the column change of packing. For this reason they are most serviceable if sizes normally encountered in pilot plant work, however, this fabricated from either carbon or stainless steel seamless tubing disadvantage is not of great importance and is greatly overwhich can easily be welded. For good pilot plant operation balanced by the advantages mentioned. The adaptability and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ease of cleaning and assembly are especially important in pilot plant work where clean charge stocks are not always used. Commonly used packings are Berl saddles and Raschig rings, although jack chain or carding teeth are also used frequently. h s in other distillation work the choice of the packing is influenced by the size of the column, the fractionation efficiency desired, the cost of the packing, and the cleanliness of the charge stock. Depending on the type of deposit laid down, it is sometimes advantageous to choose a packing which is cheap enough to be discarded in order to avoid the work involved in cleaning it. To ensure good vapor and liquid distribution the diameter of a piece of packing should be no greater than one eighth of the tower diameter ( 3 ) .
CREEN PACKING
Figure 1. Packing Supports
Packing Supports. The importance of a satisfactory design for a packing support is often underestimated. Although in some instances a flat screen has apparently been used without difficulty, experience has indicated that trouble-free operation is much more likely to result from the use of a steep, cone-shaped screen support. A flat screen of any strength frequently has even less free area than the packing and thus provides a restriction which easily can become a source of local flooding at boilups appreciably IoTer than those allowable for the packing. The cone formation pi ovides a much better opportunity for adequate disengaging of vapor and liquid and less chance of serious plugging from scale and broken packing. If the packing size is relatively small the disengaging process may be aided further by placing a layer of larger packing between the support and the desired packing. The construction of a satisfactory packing support is shown in Figure 1. In order that the bottom flange may be removed n-ithout allowing the packing to fall out of the column, the support is attached to the flange with screws. The top packing support rests on three removable pins, as also indicated in Figure 1. Theoretical Plates, The number of theoretical plates and the reflux ratio required are determined by conventional means whenever possible, but frequently when dealing with complex hydrocarbon mixtures a rigorous solution is impractical because of the large expenditure of time and effort required. In such instances previous experience is generally used as a basis for assigning values to these terms. The flexibility of a packed column is helpful in such a situation, since the proper number of plates may be obtained easily by changing the depth or type of packing as required. Column Throughput. To calculate the required diameter of the column i t is necessary to know the flooding velocity of the packing for the conditions expected. For rings, saddles, and wire hclices the correlation of 1,oho et al. ( 4 ) is generally adequate,
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whereas for these and other types of packing the correlation of Bain and Hougen ( I ) is frequently applicable. The two methods use similar variables, and although they do not give results that check perfectly, either one is probably satisfactory in view of the relatively large safety factor that is normally used in pilot plant design for providing flexibility. The flooding velocity should be evaluated for that portion of the column having the highest vapor and liquid load. For most ordinary distillation problems involving hydrocarbons i t will be found that the flooding velocity will be lower and the vapor load higher for the bottom of the column than for the top. Thus, the combination of these two factors frequently results in two to two and one half times more area being required for the bottom of the column than for the top. This situation will not always occur, however, and some systems will require more area at the top of the column. To avoid a localized disturbance that would result from introducing the feed directly into a packed section and that might induce localized flooding a t throughputs below the capacity of the column, it is common practice to flash the feed into an empty section of column by separating the packing into two sections by means of a packing support just above the feed inlet, as illustrated in Figure 6. Liquid redistributors are not used because the upper layer of packing serves more conveniently the same purpose in the small column sizes used here. Preheaters. The type of preheater to be used for pilot plant fractionating columns depends on the heat requirement. If the heat requirement is high, a gas-fired furnace may be used, but if the heat requirement is below 50,000 B.t.u. per hour, a coil immersed in an electrically-heated lead bath provides satisfactory performance. The latter type of preheater is shown in Figure 2. It has the advantage over a radiant-type preheater in that the outlet temperature is not greatly affected by changes in feed rate, provided the coil is of sufficient length. The temperature of the bath is easily controlled, either manually or automatically, rn
8” SCHEDULE 40 STEEL TUBING
PREHEATER COIL 3/8 SCHEDULE 40 STAINLESS STEEL TUB1
I 6 GAGE STAINLESS
Figure 2. Bath-Type Preheater
the heat capacity of the bath is high. Electrical heaters may either be immersed in the bath or fastened to the outside of the container. If high heat densities are used with external heaters, an air space should be provided between the heaters and the insulation by installing a 16-gage stainless steel sheet 0.5 to 1 inch from the outside of the heaters. This space allows radiation and ab- circulation to distribute the heat more uniformly and reduces the heater failures caused by high skin temperatures where contact with the. insulation would otherwise be made. Extra heaters above those actually required for preheat are provided for bringing the bath to operating temperature quickly and also for standby use if onc of the other heaters should fail.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Reboilers. The required reboilcr heat load is determined by conventional heat balances or is roughly approximated by multiplying the condenser heat load by a correction factor obtained from several heat balance calculations made on typical separations. This factor varies between 1 and 3, being increased by large temperature gradients in the column, large amounts of bottom product, and high reflux ratios. It is important, especially when using electrical heat, that adequate heat transfer surface be provided to avoid an excessive heat flux or reboiler wall temperature. Heat may be applied to the reboiler by means of General E l e c t r i c C a l r od h e a t e r s wound around the outside of the reb o i l e r below the Liquid level or by strip heaters attached to the outside surface of the 220 VOLT reboiler. If the reFigure 3. Interwound Reboiler boiler heaters exHeaters tend above t h e l i q u i d level, any surges in the level will cause a varying boilup rate and bottom temperature, and poor tower operation will result. The same precautions apply to installation of reboiler heaters as to the external heaters described above for preheaters. The heaters on the reboiler are interwound in the form of a spiral as indicated diagrammatically in Figure 3, in order that each heater will span the entire heating space and provide the maximum amount of reboiler surface effective for heat transfer. Extra heaters are also installed for start-up and standby use. It is advantageous to have the bottom flanges installed above the Liquid level to reduce the heat capacity and provide more heating surface for the reboiler liquid. The bottom temperature will thus be more responsive to changes in hest input. In most cases in pilot plant operation the holdup of the system should be a.~ low as possible to reduce the line-out time and to make the system responsive to changes in the operation. Since the liquid holdup in the reboiler may be excessive, some form of space filler or “dead man” is usually provided. The reboiler
PERFORATED SHIELD
16 GAGE S W N L E S S
STEEL SHIELD
Figure 4.
Level Controller and Reboiler
573
space may be partially filled by installing in the bottom a piece of pipe, closed a t both ends, which leaves an annular space with the desired holdup. If a displacement-type liquid level controller is used, the float may also be used to reduce the reboiler holdup. An installation of this type in a typical reboiler is shown in Figure 4. Gage glasses of the forged steel case-type are used on all reboilers for allowing visual observation of the liquid level. If the tower is operated under pressure, the gages should be. equipped with ball-type check valves for protection in case of a glass breakage. In installation it is important that the top line from the gage be connected to the column well above the maximum height of the boiling liquid which may be appreciably above the level indicated by the sight glass. It is also advantageous to use strip heaters on the sides of the gage to prevent excessive condensation within it. Good practice involves connecting the bottom of the gage glass to the reboiler by a line independent of the product takeoff line. Use of a separate line prevents the contamination of the bottom product by lighter material which may have condensed in the gage glass. Reboiler Liquid Level Controls. There are various methods of reboiler liquid level control that are satisfactory: 1. The liquid product may be pumped from the reboiler with a variable stroke piston pump. This method has particular advantage when the bottom product must be discharged to a high pressure. However, the liquid level is controlled manually by resetting the pump rate for changes in the bottom product rate. 2. A gear pump placed directly in the distillation column reboiler, as shown in Figure 5, provides satisfactory operation either in a vacuum or atmospheric pressure operation. To prevent vapor locking during vacuum operation the entire pump is installed inside the reboiler, and an additional external packing gland is provided on the drive shaft. Since the pump packing gland is at the suction pressure, there is no tendency for the pump to be air locked. Air that may leak through the external gland will pass up through the distillation column without entering the pump. No difficulty in pumping has been experienced while operating a t pressures as low as 6 mm. of mercury absolute and temperatures as high as 700” F. The standard-make gear pump, without alteration, is attached to the bottom manhole flange and can be removed easily for maintenance. A ball-check valve is placed in the pump discharge line to prevent air from entering the system during start-up periods or surges. The pump speed is not critical; any pump capacity as large as or larger than the maximum bottom product rate is satisfactory. 3. For the reboiler a displacement-type liquid level controller of standard make, as shown in Figure 4, has many advantages. The float may be placed directly in the reboiler, and as there is no packing gland between the float and the instrument, there is no danger of leakage. The float is large enough to rovide very sensitive control and is surrounded by a perforated sxield to eliminate bouncing of the float caused by the boiling liquid. Reboiler temperatures as high as 650 F. do not appear to have an adverse effect on the operation of the level controller. The control system is shown in Figure 6. The air output from the level controller operates a pneumatic valve which controls the flow from the reboiler. A gear pump operated at constant speed, with a relief valve on the discharge side to return excess liquid to the pump suction, provides a constant positive pressure on the upstream side of the pneumatic control valve. This displacement-type liquid level controller is a very versatile instrument, for it can be adjusted to a narrow proportioning band to give very accurate level control or to a wide band to give an averaging type of control which results in relatively uniform flow of product from the tower. This latter type of operation is especially desirable when the tower bottom product is fed to another distillation column or other piece of equipment requiring a uniform feed rate. Condensers. I n designing condensers for pilot plant fractionating columns it is generally satisfactory to use only the two film coefficientsinvolved. The correlations in Perry’s Chemical Engineers’ Handbook ( 5 )are complete and adequate for this purpose. A recent correlation by Donohue (S),however, is more convenient to use for the condensing hydrocarbon film coefficients over the range in which it is applicable. Good practice normally involves ample overdesign of condensers to reduce trouble with fouling of the heat transfer surfaces and to provide adaptability for other operations.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
For pilot plant work condensers are usually of three general classrs-the tube-bundle type, the pencil or finger type, and the coil type. The tuhe-bundle type is most commonly used for commercial work and is well adapted to high heat loads. If possible, a commercially available condenser is purchased. The tube bundle has the advantage that the insides of the tubes can be cleaned easily, but also has the disadvantage of requiring high coolant flows for obtaining high cooling-film coefficients. These large flows may be undesirable in many pilot plant applications.
JOINT FLANGE
Figure 5.
Bottom Product Pump for Vacuum Tower
The pencil- or finger-type condenser, used for condensing the overhead product in the system shown in Figure 6, has the advantage of being easy to fabricate, but also has the disadvantage of requiring high flows of coolant to gain high film coefficients. For a condenser having only a moderate heat load, a high coolant flow results in small change in temperature for the coolant, and this small temperature change limits the accuracy of the heat load determination when that IS desired. The coil-type condenser, used as a partial condenser in the system shown in Figure 6, passes the entire flow of coolant through the coil. High mass velocities and coolant film coefficients result. For applications in which a moderate heat load is encountered, this type of condenser is easy to fabricate and has the advantage that a more acrurate heat balance can be obtained than with the other types. A particular application on a distillation column may require either a partial condenser in which only the reflux is condensed or a total condenser in which both reflux and liquid product are condensed. The requirements for a total condenser are less exacting, as it is only necessary that sufficient capacity be provided. Thus, any of the types of the condensers can be used for this application. When a partial condenser is used, however, some means of automatic or manual control of the heat exchanged is normally desired. Where moderate heat loads are involved, realization of the desired control is much easier with a coil-type condenser than with the pencil or tube bundle types because of the much lower volume of coolant held in the coil type. Better tower operation is obtained, since a low coolant holdup allows control changes to take place more rapidly. The other types of condensers can be used with satisfaction if the heat load requires a relatively high coolant flow such that the condenser coolant is replaced ten or more times per hour. The coolant used in the condensers also depends on the application. Water is generally most convenient but is not normally used for temperatures much above its boiling point; for high temperatures a circulating oil system is very satisfactory. However, for low heat loads requiring low coolant rates, neither oil nor water may be satisfactory because of the difficulty of controlling small flows. In such an instance the use of compressed air is
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satisfactory, being easy to control and applicable over a wide temperature range. Heater Windings and Insulation. Column windings are normally of two types: (1) start-up windings for bringing the column temperatures up to the operating range before charging the feed and thereby reducing the time required for reaching onstream and lined-out conditions; ( 2 ) adiabatic windings for balancing the heat loss from the column. Both types can be used in the same column, or one winding can be applied to perform both functions if an autotransformer or equivalent device is used to vary the heat applied. Where separate windings are employed the start-up windings are usually placed as close as possible to the column metal, whereas adiabatic windings are normally placed betrreen layers of insulation. If an appreciable temperature gradient in the column is expected, both types of windings should be more concentrated-that is, wound closer-at the bottom of the column n-here the highest temperature and greatest heat loss is expected. For precision distillations, adiabatic operation is quite important. For the quality of separation frequently desired in pilot plant work, however, the adiabatic windings can be simplified. Rather than expending excessive operating time to maintain the tower exactly adiabatic by frequent adjustment of the amperage, it is often satisfactory to apply a winding having a fixed heat input which nil1 approximately balance the heat loss for the normal operations expected for the column. This method has the advantage of decreasing the number of variables for the operator to handle and is generally acceptable because a reasonable departure from adiabatic operation does not appreciably affect the operation as long as it is a steady departure. Any excess heat loss will merely have the effect of progressively increasing the internal reflux somewhat over the normal amount toward the bottom of the column, and this will not seriously affect the operation unless the heat loss is a large proportion of the heat that must be applied for boilup, as in the case of a large column operated a t low throughput and high temperature. Installation of this type winding is illustrated in Figures 1 and 7. An asbestos or magnesia-base insulation is normally applied in thicknesses of 2 to 4 inches for reducing the heat losses from the tower. An insulation resistant to high temperatures must frequently be used on the lower sections of the column, especially where frequent removal and reapplication of the insulation is necessary. INSTRUMENTATION
The automatic control of pilot plant columns is based on the same principles as commercial column control. The methods applying to commercial columns will usually T% orlr satisfactorily to control pilot plant distillation columns using standard industrial instruments. The relatively 1017 rates of flow and the wide variation in feed rates and composition encountered from run to run in the operation of pilot plant columns require special precautions in design in order to obtain satisfactory control. As indicated previousiy, it is important t o have the feed rate to the column held as constant as possible. Either a flow controller or an averaging-type level controller, or a combination of both, may be used satisfactorily. An averaging-type level controller is particularly advantageous if the feed rate is likely to vary. If there is sufficient spread in boiling point between the products to be separated, a temperature controller may be used to control the composition of the desired product. A top temperature controller varying the rate of flow of the coolant through a well designed partial condenser will control the top temperature accurately and thus maintain the top product specifications. It is important to have the condenser surface large enough that the change in heat removal will be proportional to the change in flow of coolant. In addition, the condenser coolant holdup
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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TEMPERATURE CONTROLLER
575
butanes through progressively higher boiling compounds in the gasoline, kerosene, and fuel oil ranges to a heavy tarlike residue. The tower throughput varies from run to run in the range of 2 to 8 gallons per hour. The feed to the tower is taken from an accumulator equipped with a liquid level controller adjusted for a relatively constant flow, since constant feed rate is one of the essential requirements for good tower operation. A schematic diagram of the fractionating system is shown in Figure 6. The feed to the column is heated in the bath-type preheater previously described. This preheater contains a OVERHEAD PRODUCT bCCUMULATOR 3/s-inch, Schedule 40 stainless steel coil 25 feet long immersed in a AND GAGE QLASS (I" SCHEDULE 40 lead bath. The temperature of the bath is controlled automatiSTAINLESS STEEL cally by an owoff temperature controller. Electrical heaters REFLUX TRANSFER LINE WINDINO-\ COOLANT with a total capacity of 6000 watts are available for heating the bath. One circuit of 2000 watts is connected through an autotransformer and also through the temperature controller relay. The two other 2000-watt circuits are connected directly to the power source. All circuits are protected by individual 112"BERL SADDLES circuit breakers. Ammeters are connected to all of the heater circuits to aid the operator and also to indicate a heater failure. The feed passes from the preheater through a heated transfer line and is flashed into the distillation column. The temperature of the feed is measured by a thermocouple just before entering the tower. The column is fabricated of 5-inch, Schedule 40 stainless steel tubing flanged at both ends with special oval STRIP HEATERS ring-joint flanges. The flanges are made as light as allowable, consistent with good safety practices, in order to reduce their TIC T1C BOTTOM_/v LCONTROL heat capacity. PRODUCT PUMP VALVE The column is packed with 7 feet of 0.5-inch Berl saddles with Figure 6 . Atmospheric Pressure Column 4 feet in the stripping section and 3 feet in the rectifying section. This relatively large packing is used to decrease the possibilities of trouble from any fouling bonditions that might be encountered. The packing is supported with 60" screen cones made from 4-mesh should be small in order that temperature lags will be held to a stainless steel screen. minimum. Although top temperature control is very simple The reboiler is of the type previously described and illustrated mechanically, it does have the disadvantage that a change of in Figure 4. Because the bottom product from this column is feed rate to the tower will change the reflux ratio, requiring a charged into another column for further fractionation, it is desirchange in heat applied to the column. In general, unless very able that the rate of discharge be as uniform as practical. The little fractionation is being used, small changes in reflux ratio level controller is thus set for an averaging type of control in change the efficiency of a distillation column very little. order that surges in the bottom level of this tower will tend to be If the bottom product specifications are more critical, a bottom smoothed out and not transmitted to the next distillation step. temperature controller varying the heat to the reboiler will give High reboiler capacity (high holdup) aids the level controller in satisfactory control. A reflux ratio controller, as shown in Figure this respect, but is undesirable from the viewpoint of short line8, or some equivalent device, would then be used to maintain a out time, as previously mentioned. Thus, a balance must be constant reflux ratio. Bottom temperature control has the adstruck in setting the capacity of the reboiler in order that optivantage that the reflux ratio can be made independent of the mum results can be obtained. charge rate. The condenser used on the column is a partial condenser conIn some instances temperature does not provide a satisfactory sisting of a a/,-inch, Schedule 40 stainless steel coil 30 feet long. means of controlling a distillation column. The difference in A partial condenser lends itself well to top temperature control boiling point between the products may be too :mall, or a definite which is used on this column to control the composition of the percentage of overhead or bottom product may be desired. overhead product. The temperature controller varies the rate of The column may then be controlled by material balance to procoolant to the eoil to maintain a constant top temperature. duce the correct amoun the desired produot. No matter The heat applied to the reboiler is held essentially constant and what method of control is used, thermocouples placed in the varied manually only t o adjust the efficiency of the separation of columns at various heights will indicate the temperature gradient the gasoline from the bottom product. For example, the 90% within the column and aid in its control. point on the A.S.T.M. distillation of the gasoline is held by setting To illustrate the more important features of construction and the top temperature on the column with the temperature coninstrumentation of pilot plant continuous distillation columns, troller, and the spread between the 90% point and the end point three examples of integrated fractionating systems performing is controlled by adjusting the heat to the reboiler. If trouble widely varying separations, and operating a t widely varying with flooding is encountered, operation with a constant amount conditions, are described. All these systems have yielded satisof heat on the bottom of the column may be particularly adfactory performance for a considerable length of time. vantageous, for if a heat input is used just below that required for flooding, safe operation at maximum practical throughput ATMOSPHERIC COLUMN for the column will be obtained. An atmospheric column has been designed to separate the The column is insulated with 3 inches of 85% magnesia insulagasoline from a wide-boiling range synthetic crude produced in a tion and is maintained at roughly adiabatic conditions by means 2-barrel-per-day catalytic cracking pilot plant. This synthetic of two electrical windings, one extending from the bottom of the crude consists of the liquid cracked products from the pilot, unit column to the flash point and the other from the flash point to the and contains hydrocarbons ranging from a minor portion of top. The heaters are operated with a fixed heat approximately TOWER VENT
M
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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equivalent to the heat loss from the column. The windings are wrapped closer at the bottom where the heat loss is higher. The location of the thermocouples on the column is shoir-n in Figure 6. Especially important are the flash, bottom, and top temperatures. To ensure good temperature measurement, the therniowells ale of small diameter ( l / 8 inch outside diameter by l / 1 6 inch inside diameter), stainless steel tubing, xvith at least 3 to 4 inches of immersion. Some typical data for this column are shonx in Table I. Comparison of the 90% and end points of the overhead product with the initial and 10% points of the bottom product shows that a sharp separation was made. The inspections given are for the composite products removed during the 15-hour lined-out period of an actual run chosen at random from the operations made in this unit and may be considered typical.
TABLE I. TYPICAL O P E R A T I K G D A T A FOR ATMOSPHERIC COLGMIN Column pressure, lb./?q. in. gage Temperatures, F. Flash Bottom Preheater Inlet Reboiler heat, matts Flow rates, cc./hour Charge Overhead product Bottom product Product inspectionsa
13800 7100 6700
A.S.T.hI. distillation (D86), O I?, Initial boiling point 10%
HIGH PRESSLRE COLUMN
The high pressure continuous distillation column described here is a part of a thermal cracking pilot plant and is used to stabilize the gasoline produced from the pilot plant to the required Reid vapor pressure. In addition to gasoline the feed to the column contains light hydrocarbons and a high percentage of propane when the pilot plant is operated as a Polyform unit. The charge rate to the column ranges from 1 to 5 gallons per hour. The efficiency of separation required is higher than in the atmospheric column previously described, as no propane is desired in the gasoline and the overhead product must be free from pentanes. Frequently, good depropanization or debutaniaation operations are also required. The column operates a t a sufficiently high pressure (usually 300 pounds per square inch gage) to condense propane for reflux at normal cooling water temperatures. A diagram of the distillation system is shown in Figure 7. Overhead vapors from a previous toiver are charged to this
10
373 483 654 467 479 2000
Top
E n d point Gravity, O.4.P.I. Reid vapor pressure, lb./sq. inch
Overhead Product
Bottom Product
94 123 146 172 198 228 259 294 331 374 414
468 492
502 507 514 523 535 549 565 585 623
(.
58.8
10.1
I
I
Inspections are not available for the actual streams of oil charged’and renioved from the column: the overhead product shown is for the rasoline removed from the top of the column after distillation on another tower to aeparate excessive light ends and give a Reid vapor pressure of 10 lb,/sq. inch; the bottom product shown is t h e light gas oil removed from the-top of the next tower and amounts t o 30% of the total bottom product froin the column described. a
V Z ’ SCHEDULE 8 0 REMOVABLE BELLO MOUNTING RING
RING JOINT FLANGE
AIR CONNECTION FROM CYCLE TIME SAMPLE LINE
k
TUBULAR TOTAL STAINLESS STEEL BELLOWS
3 SCHEDULE 8 0 STEEL TUBING
REFLUX TO TOWER
RECEIVER
Figure 8.
BLOWDOWN LINE
n
,.&
