I
M. G. FRYBACKI and J. A. HUFNAGEL Catalytic Construction Co., Philadelphia 2, Po.
Distillation Equipment Design The engineer who keeps his design i n proper perspective with respect to the rest of the process and who keeps i n mind the importance of auxiliary equipment to satisfactory operation is the one most likely to develop a trouble-free design
FROM
a design standpoint, the field of distillation can be divided into two broad categories. Under such division, one category would contain those units where the feed stream is to be divided into a multiplicity of cuts. Highly efficient separation between cuts is normally not required. Examples of this type of distillation are crude towers and catalytic cracking unit fractionators found in the petroleum industry. The second category would contain units which require separation into overhead and bottom streams, either or both of which may have relatively stringent specifications. Such designs require close attention to theoretical requirements to effect the desired separation. This discussion is devoted primarily to the practical design aspects of units falling into this latter category. The proper starting point in the design of any column, regardless of the type of fractionation, is the careful consideration of its relationship to the remainder of the process of which it is a part. Distillation columns very rarely function as complete and independent process units ; instead they are generally used for stream splitting, purification or recovery at various points in an over-all process design. Such an examination provides information on two very important items pertinent to the design of any column; namely, the maximum capacity the column will be expected to handle and the operating flexibility it must possess in order to fractionate satisfactorily throughout the range of throughput and feed composition changes which may be reasonably expectcd from the over-all plant operation. 1 Present address, Iranian Oil Refining Co., Ltd., Abadan Refinery, South Iran.
654
There is obviously no point in designing a column to handle throughput 40 to 50% above normal operating capacity if other major items of equipment such as furnaces and reactors, which are not easily altered, are designed for a lower value. Aside from being costly, the use of excessive capacity factors may be a major contribution to poor tower performance, particularly with certain types of trays. Conversely, the engineer who makes an arbitrary decision without regard to the general design philosophy for the entire plant and discovers too late that his column is the unit “bottleneck” finds himself in a very vulnerable and embarrassing position. Thus, in general, the column should have the same capacity factor as other major equipment in the process and will generally be about 20 to 25% above the anticipated normal operating capacity. Having established the maximum capacity for design, the next question to be considered is the one of required flexibility. When the plant under consideration is based on proved commercial practice, actual operating experience provides the guide to fluctuations in throughput and composition which can be expected as a part of normal operating variation. \$‘hen the column is a part of a new and untried process a certain amount of “crystal ball” gazing is required. However, it is believed that a little thought as to the likely variation in such items as feed stock quality, process control, and end product specifications will be very helpful in determining the flexibility limit which the distillation column should possess. One final aspect of the distillation column design in relationship to the rest of the process must be taken into con-
INDUSTRIAL AND ENGINEERIN6 CHEMISTRY
sideration-i.e., the thermal condition of the feed. For most cases, to ensure proper balance between the rectifying and stripping sections, it is desirable that the feed be at or near its bubble point or partially vaporized with the per cent vaporization no greater than the overhead product, expressed as per cent of feed. Vaporization beyond this point will generally require an unnecessarily high reflux and condenser duty in order to provide adequate stripping at the bottom of the column. Feed introduced well below its bubble point will require a high reboiler duty in order to provide adequate reflux. It is far cheaper to adjust feed conditions by external heat exchange than to have to adjust for unbalanced heat load in the tower design proper. Having established maximum design capacity, operating range, and thermal conditions of the feed, the engineer is then ready to consider the design of the distillation equipment. For the purposes of this presentation it is assumed that the calculations to determine minimum reflux ratio and minimum number of theoretical stages have been previously determined.
Column Design The first question to be answered is the type of column to be used, packed or tray. This can generally be resolved rather quickly on the basis of unit capacity. Packed columns are rarely used for distillation where the column di- ’ ameter is greater than about 3 feet owing to cost and problems of liquid distribution in larger diameter columns. Extensive work is being done on the des’ign of liquid distributors in an effort to eliminate poor liquid distribution as a
restriction to the wider use of packed columns and progress is being made in this direction. Larger diameter packed towers of u p to 7 or 8 feet have been used in the field of absorption for such systems as amine absorption where the severe frothing characteristics of the system present problems in a tray-type column. In addition, expanded metal-type packing has been used in larger diameter distillation columns, but it has not found wide acceptance in industry. One item worthy of mention in connection with packed columns is the entry of plastics into the field of industrial packings. Polyethylene tellerettes and polypropylene pall rings are now available on the market. The pall rings may be obtained in 5 / 8 - , 1-, 11/2- and 2inch sizes. Although a maximum allowable operating temperature of about 250’ F. somewhat limits their application in the distillation field, such features as lower weight, lower cost, and minimum loss in handling make them serious competitors to other types of packing in the absorption and liquid extraction fields. The polypropylene pall rings, for instance, weigh about 10% of equal sized stainless steel Raschig rings and cost about one third as much. Because towers in industrial usage are preponderantly of the tray type, the design considerations here are limited to columns of that type. Before considering the type of tray to be employed, the design reflux ratio and corresponding number of theoretical stages or trays should be determined. I t is at this point that the engineer frequently rubs his hands in glee. By the use of a liberal safety factor the design reflux ratio becomes a ‘‘bottomless pit” into which he can toss all his fears
and uncertainties over such items as the validity of his equilibrium data and the adequacy of his assumptions concerning capacity requirements. However, he, cannot completely relax without some knowledge of the extent to which he has departed from the optimum reflux ratio and what such departure is going to cost. The poini is worthy of some elaboration. Several authors have commented in a general way on optimum reflux ratio. Robinson and Gilliland ( 5 ) give an example in their text of the determination of optimum reflux ratio for the distillation of a methanol-water mixture wherein the most economical reflux ratio was determined to be 15y0 above the min-
imum. Albright (7) used a digital computer to determine that optimum reflux ratio for a pentane splitter column was in the range of 120 to 13Oy0 of minimum reflux. For this presentation a large debutanizer column has been selected for the purpose of illustrating optimum reflux ratio and the magnitude of investment and operating cost differentials as a function of reflux ratio, for a relatively extreme case (Figure 1 ) . Direct material and labor costs for the various component parts of the system are shown at the bottom of the chart. Heat exchange costs increase linearly with increased reflux over the range shown. Column costs will go through VOL. 52, NO. 8
AUGUST 1960
655
Four Main Types of Trays with Their Respective Properties Are Compared with the Bubble Tray
Type
No liquid downcomers Turbogrid Ripple tray Valve type Flexitray Ballast Uniflux Sieve Trays ' / 1 6 - to '/a-inch holes
Vapor Capacity Relative t o Bubble Tray
Tray A P Relative to Bubble Tray
Relative t o bubble tray
20-407, greater
Relatively narrow Poor at low vapor load
Approx. same at 60S070 flood or above, less at lower vapor rates
Good
Low For turbogrid about 1/3 @ 60-10070 flood
I/ 2
20-507, greater
5-107, higher at optimum 107, higher at optimum
Good
Slightly lower at high vapor rates Low
*./a
10-207, greater
Wide (20-S570 flood) Relatively wide Good above 50% flood
20-40% greater
Relatively wide
Approx. 10% higher
Poor
IO 80 90 40 5 0 6 0 70 80 PEd CENT AROVC I4INIMW REFLIJX
Figure 1. Investment and operating costs vs. reflux ratio for 120,000 B.P.S.D. debutanizer shows that the most economic design trend i s toward additional trays and lower reflux
a minimum and approach infinity at both ends of the curve; a t minimum reflux an infinite number of theoretical stages is required and a t the other extreme of minimum number of stages aninfinite reflux is required. I t may be noted that minimum investment for this design occurs a t around 10 to 15% above minimum reflux ratio.
656
Dirty Service Performance
High efficiency, range
80 I
0
Efficiency
cost Relative to Bubble Tray
Operating costs were determined by amortizing the tower cost over a 10year period and assuming maintenance and taxes at 5 and 3y0 of investment, respectively. From the curve a t the top of the chart it may be seen that minimum operating cost occurs at about 1OYo above minimum reflux. However, it is believed that in most instances this value is too low for safe design. Errors in k data and in plate efficiency determinations must be allowed for. For relatively difficult separations where alpha is low, a small error in K values can introduce a relatively large error in the required number of theoretical stages. Here again, consideration of the distillation equipment in relationship to the process as a whole will provide an indication of how liberal the designer can afford to be with reflux ratio safety factor. If the distillation section represents an appreciable part of the process and the cost of distillation is a significant percentage of total plant operating cost, it is obviously desirable to hold the reflux ratio as close to minimum as possible and still ensure satisfactory operation. On the other hand, if distillation costs represent a small part of total cost and equilibrium and tray efficiency data for the system are rather nebulous, a larger safety factor is justifiable. If time does not permit calculations to be made, it is recommended that a design reflux ratio of about 125% of minimum be employed. For the column just discussed, increasing design reflux ratio from 125% to 150% of minimum would increase the investment some $75,000 and the annual operating cost by $31,000. Also, utility costs alone are in the range of $300,000 per year. Thus in slightly over 3 years they will have exceeded the initial investment cost of the equipment. This again stresses the economic importance of designing for minimum heat input consistent with satisfactory performance. For towers which can be
INDUSTRIAL AND ENGINEERING CHEMISTRY
Questionable
Low
'/a
2 /:4
expected to have a reasonably long operating life, the most economic design route will be toward additional trays and lower reflux. Type of Tray, Today, more than ever before, supremacy of the bubble-cap tray in the field of industrial distillation has been seriously challenged and the engineer is confronted with what at times seems a bewildering array of tray types from which to make his selection of the best type for any given design. Few subjects have exceeded tray design and tray Performance in volume of published technical literature. Many companies have, through years of trial and error, developed their own pet designs and are extremely reluctant to deviate from them. In many cases, this attitude stems frcm painful memories of earlier days of frustration and lost revenue through column performance failures. However, if the engineer is to be of maximum value to his management, he must a t least be in a position to acquaint them with the design which will give them the optimum, both from an economic and performance standpoint, based on the best current information available to him. The greatest contribution which can be made here would be to consider the most important items to be checked in making tray selection and to provide a brief comparison of the tray types, pointing out where it is possible to generalize, the salient features and advantages of each. Here then are a few of the points to be considered in making tray selection : Operating Range. Does the tray have a sufficiently wide operating rangei.e., does it maintain high efficiency over the anticipated range of vapor a n d liquid load? I t is important here to consider the range of vapor to liquid ratios that will be encountered because some trays have greater sensitivity to changes of this variable than others. Type of Service. Is the tray suitable to the type of service-i.e., clean or
D I S T I L L A T I O N EQUIPMENT
Check these points before making tray selections Operating Range Type of Service Maintenance Ease liquid Holdup Pressure Drop cost
...
then choose from the various types now available Flexitray Ballast Kaskade Nutter Uniflux Sieve Tray Turbogrid Ripple Tray
dirty? If it is known that the tray will be in dirty service such as having to handle suspended solids carried over from other areas of the process, or products of corrosion, is the tray susceptible to plugging under these conditions? Maintenance Ease. If the tray is to be in dirty or corrosive service, can it be readily cleaned or replaced? Liquid Holdup. Is low liquid holdu p important from the standpoint of possible chemical reactions such as polymerization or thermal decomposition when subjected to high retention times at elevated temperatures? Pressure Drop. Is low pressure drop a requirement such as is the case when vacuum is to be employed? Does the tray meet this requirement? Cost. Last but not least, what is its cost in relationship to other types which meet all other requirements? I n considering the various types of trays available today, it is possible to divide them into two broad categories : those with separate liquid downcomers and those without. I n the first category can be placed: the bubble-cap tray, the valve-type trays such as the Koch “Flexitray,” the Glitsch “Ballast” tray, and the Nutter tray, the Uniflux, and the sieve, or as it is sometimes called, the perforated tray. The second category contains such proprietary trays as the Turbogrid and the ripple tray. The chart (p. 656) compares four types of trays with respect to capacity, efficiency, ability to perform in dirty
service, pressure drop, and cost. I t is extremely dificult to generalize in this manner because it is possible to design for appreciable ranges of capacity at the expense of efficiency, or vice versa, for a given type of tray. However, it is felt that the comments as provided describe the general tray characteristics reasonably well. Physical characteristics of all of the trays have been extensively described and will not be repeated here. Considering first the trays with no liquid downcomers, the flow of vapor and liquid occurs as jets of vapor or liquid through the holes or slots. The jets of either phase are random in time and location so that a uniform balanced flow of both phases over the column cross section takes place. Thus liquid or froth height is fairly constant over the entire tray area and no hydraulic gradient across the tray exists. The trays possess high vapor and liquid handling capacities as the result of utilization of the entire tower cross section for vapor flow and the nonexistence of crossflow resistance to liquid flow. Performance in dirty service is good due to the high degree of turbulence a t the deck. Pressure drop is appreciably lower than for bubble trays, being quoted for Turbogrid trays as about one third the bubble cap drop over the range of 60 to 100% of tray capacity. The trays in this category are as cheap as any type, costing approximately 40 to 50% less than bubble-cap trays. The major disadvantage rests with relatively narrow operating range. Efficiency is generally poor at vapor rates below 50% of flood. However, for those designs where little variation in rate is anticipated the trays in this category must be given serious consideration. The term flooding, as used in this discussion, refers to jetting or entrainment resulting from excessively high vapor rates. The next group of trays to be considered are the valve type, as represented by the Flexitray and the ballast tray. They are characterized by relatively high capacity, providing some 20 to 50% more vapor capacity than bubble cap trays of the same diameter. Liquid handling capacity is also high, relative to the bubble tray, due to lower liquid gradient across the tray. Some doubt has been expressed as to the suitability of these trays for dirty service, the theory being that the valves might tend to stick. However, the author knows of several installations where they have replaced bubble-cap trays in dirty service and have provided longer operating periods before cleaning was required than the preceding bubble caps. This, coupled with the feature of simpler cleaning at least qualifies the trays as
satisfactory in dirty service. Pressure drop is slightly lower than for bubble caps at high vapor rates and may be somewhat higher at very low rates. Operating range is generally accepted as being wider than the bubble tray with efficiency of some 5 to 10% higher at the optimum point. Costs run about 30y0 less than bubble trays having the same diameter. The next tray to be considered is the Uniflux. It contains the essential features of the bubble cap, but is modified to minimize some of the undesirable features of the bubble-cap tray. Channels with one slotted edge are installed in long straight rows and oriented so that the escaping vapors assist in moving liquid across the channels. It is claimed that this compensates for hydraulic gradient and all slot submergence is uniform across the tray. Slot areas run 1 2 to 14% and riser area is two to four times as great as for conventional bubble caps. Thus high loads may be handled at relatively low pressure drop. The tray appears to have excellent performance characteristics at loads of 50% of flood or higher. There is some question as to whether it has quite as wide an operating range as the bubble cap tray due to some rather low reported efficiencies at low vapor rates for some systems. Very little has been published about the tray characteristics in dirty service. By visual inspection of the tray design it would seem that it might be more difficult to clean in place than some of the other types of trays. Cost is reported to be about 60% of a conventional bubble-cap tray. The final type of tray is the sieve or perforated tray. Perforated trays have been used perhaps longer than any other type. Until recently, the belief existed that hole size should be small, in the range of 3 / 3 2 to l/4 inch, in order to achieve high efficiency and stability. However, contrary to this popular opinion and established commercial practice, recent confidential studies have shown the efficiency and flexibility of sieve trays with larger diameter holes, in the range of */zto 1 inch to be good. The possibility of satisfactory design of sieve trays with large diameter holes may well stir renewed interest in their application because the plugging tendencies with which the small hole diameter trays were plagued would disappear and other features of the tray such as pressure drop, cost, and dirty service performance compare favorably with competing tray types. Sizing Column. After selection of the type of tray to be used, the next step is the determination of column sizing. Entrainment is the basic variable on which practically all tower sizing correlations are based. Th: well-known VOL. 52, NO. 8
0
AUGUST 1960
657
Figure 2. Tray spacing must b e considered concurrently with the determination of column size in designing a tower
Brown and Souders correlation for bubble cap trays has been extensively used since its introduction in 1934 and many companies today still utilize their basic equation as a basis for tower sizing tables where the constants have been determined by experience over the years and vary for different types of operation such as atmospheric distillation, vacuum distillation, and absorption. I t has been pretty well proved by subsequent studies (2) that the Souders and Brown correlation is not one of entrainment. Nevertheless, equations of this type where the value of the constant is empirical, are of value as a “quickie” method for obtaining approximate tower diameter. Nelson presents such a chart ( 4 ) based on a general literature survey (Figure 2 ) . Curve 1 is for maximum rates with perforated, sieve-type, Turbogrid, and Kaskade trays. Curve 2 is defined as maximum for perfectly designed bubble trays operating at most favorable liquid loads: also, normal performance of Kaskade, Turbogrid, perforated, and similar plate constructions. Curve 3 is for normal performance of bubble plates through normal range of liquid loads a t atmospheric and higher pressures. The lowest values of K apply to high liquid loads and wide cap spacings. Sizing data from two private sources are included as the two heavy curves and show reasonably good agreement. Curve 4 presents the original Souders and Brown values for K dating from 1934, when tower design was not fully understood. More recently Zenz ( 6 ) and Bolles (2) have utilized Colburn’s method of determining minimum column cost and economic optimum entrainment. Bolles’ method for column sizing is based on the
658
concept of determining the vapor handling capacity of a standard cap without overloading the slots. Required slot area for a given loading is determined and area is obtained using the developed ratio of slot to allocated cap area. For a first estimate, allocated cap area is assumed to be 60y0 of total tower area. Simplified charts are provided for making tray layouts and equations, and standards are provided for determining satisfactory tray dynamics. The method is considered to be excellent and is recommended for bubble-cap tray design. For sizing columns where perforated trays are contemplated, the article by Huang and Hodson (3) gives an excellent presentation of the available
data for the design of small hole-size trays. When proprietary trays are contemplated, the company engaged in the sale of the particular tray should be consulted. They are generally most accommodating in providing methods for at least rough-sizing columns based on the use of their tray. One must consider tray spacing concurrently with the determination of column size because there is a reciprocal relationship between tower diameter and tray spacing. An examination of a number of correlations seems to indicate that flooding vapor load varies approximately as the 0.6 power of tray spacing. Bolles has pointed out that it is generally mechanical factors which decide tray spacing. For large columns adequate space must be provided between trays for inspection, cleaning, and maintenance. The authors concur with his suggestion that for columns of 5 feet and over a minimum of 24-inch spacing be used. Number of Actual Trays Required. The effect of tray design on efficiency must be determined in order to arrive at the number of actual trays needed for a given separation. A number of authors have pointed out that “well designed” bubble trays can tolerate appreciable variation of such things as cap size, cap spacing, and the like without appreciable effect on tray efficiency. However, it should be remembered that it is still possible to develop a poor tray design. A word of caution is voiced here about over optimism in rating efficiency, when using multiple pass trays. At present, the trend is toward larger capacity single plants or process trains, the principal purpose being to minimize plant investment and maintenance costs.
DOWNCOMER SEAL Multiple-pass trays can tolerate appreciable variation, but they are efficient only i f there i s good contact between phases
INDUSTRIAL AND ENGINEERINGCHEMISTRY
DOWNCOMER
\
DOWNCOMER SEAL
\
0000000 oooooo/ 0 0 0 0 0 0 dmNv,fiNva3 00 00 0
DISTILLATION EQUIPMENT
And don't auxiliaries:
forget
=
these
Pumps Exchangers Condensers Distillate Drums Instruments
. . . when designing distillation equipment
Consequently, large diameter columns are frequently encountered where vapor and/or liquid rates may be high. More and more frequently 3- and 4-pass trays are encountered where high liquid loads exist, as efforts are made to keep down investment costs. This is not meant as a condemnation of multiple pass trays, but merely a plea to remember that distillation involves mass transfer between vapor and liquid phases, that concentration gradient is the driving force, and that high efficiency is synonymous with good contact between the phases. Occasionally this is forgotten in zealous attempts to obtain adequate liquid handling capacity such as was the case with the tray design shown below. Forty-three per cent of this 4-pass tray which was located in the stripping section of the particular column is devoted to liquid handling capacity. The low cap area limited the boil up rate which could be attained without flooding. Entrainment was high and efficiency low. The wisdom of only two rows of caps in the side sections is questioned as it is also believed to have contributed to low efficiency through low liquid concentration gradient across the side sections. The author knows of no studies on efficiency of multi-pass trays which would provide a basis for applying correction factors, if required, to the present methods of determining efficiency. The Fractionation Research Institute has such a study on its future program which should provide a valuable contribution to the field. Auxiliary Equipment. The tower auxiliaries are those very necessary appurtenances such as pumps, exchangers, distillate drums, and instruments. These items very frequently get pretty quick treatment from the process engineer. This is partly because he has probably just completed the fairly rigorous task of designing the column proper and partly because of a feeling that design errors here are not so costly and more easily remedied. While this philosophy is true in a general way, it should be remembered that lost production time is ex-
Pressure drop considerations dictate design and location of condensers
pensive and a serious deficiency in any of the auxiliaries will result in either a capacity limitation or a complete shutdown for correction. Considering first the heat exchange equipment, the principal items are the overhead condenser and the reboiler. I n the early days, it was customary to mount condensers above the column and to employ gravity flow of reflux back to the column. There are two objections to this design for plant scale equipmentnamely, the cost of mounting the exchanger at the high elevation and of providing the necessary access structure for cleaning and maintenance, and the lack of positive control of reflux rate. Today the more common practice is to
-
locate the condenser at lower levels and to employ pumped back reflux. Design and location of the condensers will vary depending upon pressure drop considerations. If low pressure drop is advantageous, split or divided flow condensers may be employed. If appreciable noncondensables are present in a low pressure drop system, the condenser will be located immediately above the reflux drum (see diagram above). If pressure drop is not a problem a cheaper and frequently employed practice is to locate the condensers at grade, below the level of the reflux drum. When this arrangement is used on systems involving partial condensation, care must
~~
Direct-Fired Reboilers Horizontal Thermo-Syphon vs. the Kettle Type Advantages Best for absolute minimum pressure drop Flexible in operation
Low cost
Disadvantages Insufficient accuracy of design
Low surge volume
Vertical Thermo-Syphon Advantages More foolproof design Lends itself to high pressure columns
Disadvantages Elevated column Short tube lengths More difficult to clean
VOL. 52, NO. 8
o
AUGUST 1960
659
OLUMN
COLUMN
c
7 1
COLUMN
F-i
HE AT INO MEDIUM MEDjUM
H-hc
cc
-
=io‘LU
c
BOTTQMs PRODUCT
B O T T O ~ SPRODUCT
BOTTOMS PRODUCT
ONCE - THROUGH HORIZONTAL THERMOSYPHON
RECIRCULATING HORIZONTAL THE R MO SY PH 0 N
K E T T L E REBOILER
These two general types of reboilers are the most widely used in column design today
be exircised in the sizing of the twophase transfer line from the condenser outlet to the drum. Velocities must be high enough to ensure that slug flow \trill not occur because this would produce pressure surge, making tower control difficult and rzsult in poor tower performance. A velocity range of 30 to 35 feet per second has been found satisfactory for light hydrocarbon systems. Transfer line sizing is also important for total condensation systems with two immiscible fluids such as oil and rvater to prevent phase separation in the line, causing a liquid seal of the heavier liquid to be formed at the base of the loop. Such a seal will also cause pressure surging. This is a convenient point to make a few general comments regarding equipment arrangement. The accent today is very much on designing for minimum maintenance cost. This has led to a departure from the former practice of boxing distillation columns in with steel structure which in itself becomes a maintenance item. The philosophy today is toward the maximum utilization of mobile equipment such as cranes and lift and straddle trucks. T o use this type of equipment for removal of such items as relief valves, block valves, and piping sections, towers should be openmounted and accessible. Thus, the practice today is to mount the column on a concrete pedestal with vertical
660
ladders and radial platforms at manways. A connection is generally placed at the top of the column for the use of a portable hoist for raising and lowering tray sections and maintenance tools for work inside the column. Wherever possible, auxiliaries such as exchangers are mounted low for more rapid access and open area ways provided for easy access to pumps. While such arrangements require more area and somewhat higher investment than the vertical or “stacked” layout, experience has indicated that the payout is rapid in terms of lower maintenance costs. In the discussion of auxiliary equipment, the reboiler should be considered in terms of four general typ-s : forced circulation, direct fired, thermosyphon (both vertical and horizontal), and the kettle type. Forced-circulation reboilers are generally used for small columns where space limitations exist or where viscous liquids are being handled because such materials do not lend themselves to economical application of a thermo-syphon design. The use of direct-fired reboilers is generally restricted to those cases where high bottom temperatures are required that cannot be attained by normally available mediums of exchange. They are not more widely used because of the greater potential hazard of thermal decomposition and the fact that they require higher investment and more plot
INDUSTRIAL AND ENGINEERING CHEMISTRY
plan space than the shell and tube exchanger types. The two most prevalent types in use today are the horizontal thermo-syphon and the kettle type which are shown above schematically. A consideration of the pros and cons of the horizontal thermo-syphon shows its chief advantage to be lower cost, and its main drawback to be the accuracy of design required to ensure trouble-free operation. System pressure drop must be carefully determined because it sets the circulation rate which must be high in order to minimize fouling. On the other hand, economics dictate that the pressure drop be kept as low as possible because an increase in design pressure drop can be achieved only by raising column liquid level through elevation of the column proper. Normally, circulation rate is based on maximum vaporization of 20 to 25YG,and pressure dropparticularly for large columns-is held to a maximum of 0.25 p.s.i. Liberal safety factors should be used in pressuredrop calculations for the system because high pressure drop can produce very serious consequences such as inadequate reboil capacity and tower flooding if a once-through design with total tray trapout is employed. The kettle-type reboiler is the most suitable of all types where pressure drop must be kept to an absolute minimum. Tower height can be kept to a minimum.
DISTILLATION EQUIPMENT CENTRIFUGAL SLURRY P U ~ P
CASE AND IMPELLER WEAR RING FLUSHING
COMBINATION THROAT BUSHING AND LANTERN RING
This type of pump i s frequently used in column bottoms where service conditions are severe
This type is more flexible in operation because high percentage vaporization, a t times up to SO%, can be tolerated. Bottoms product is drawn from the reboiler where surge volume for the weir type unit is quite low. This is a liability when the bottoms stream constitutes the feed stream to additional processing equipment because under such conditions surge times in the order of 15 to 30 minutes are desirable. O n the other hand, the kettle reboiler presents minimum difficulty with respect to tower flooding so long as the vapors returning to the column are distributed properly. Perforated plates are frequently used for this purpose. The final reboiler type to be considered is the vertical thermo-syphon. The question is frequently raised as to when to use vertical thermo-syphons and when to use horizontal. I n general, it can be said that the vertical thermo-syphon is a more foolproof design which is particularly applicable where plot plan space is tight. Because vaporization takes place in the tubes, it lends itself to high pressure columns where the high pressure can be confined to the less expensive tube side. The major disadvantages of the vertical reboiler are: The column must be elevated more to provide the head required for circulation. Tube lengths must be relatively short which tends toward higher investment ‘ through larger diameter shell requirements. It is generally considered to be more difficult to clean. However, very satisfactory results have been obtained by in-
stalling channel end down, removing the shell, and cleaning the bundle in place. Thus this point is believed to be somewhat overemphasized.
Pumps Before turning to the last major category of auxiliary equipment-i.e., pumps, it becomes necessary to discuss briefly the very important subject of instrumentation. No matter how well designed a column may be, if it is poorly controlled, performance will suffer. T o cover the subject adequately would require a paper devoted solely to instrumentation. Instrumentation should be worked out before pumps are specified. Control valves, orifices, and other measurement devices should be located in order to ensure that their existence i s taken into consideration when determining pump head requirements. I t is very disconcerting, after being imbued with that satisfied feeling that comes from a job well done in the design of a column, to find that the operation is very erratic because the reflux control valve is wide open. The reason-insufficient pump head to provide the pressure drop required by the valve for optimum throttling range and control. With regard to the pumps themselves, centrifugal pumps are generally employed in reflux service. Frequently relatively large liquid volumes are handled and head requirements are not high. The centrifugal type fills the requirement nicely. With column bottoms pumps, selection of the best pump is not so clear cut.
Frequently the service is pretty severe, often combining high temperature, corrosive liquids, suspended solids, and the possibility of high back pressures due to plugging. When these severe conditions exist, the cost of providing the best pump for the job will be high. However, this is a poor place to economize, and it is good policy to buy on the basis of trouble-free service rather than price. In general, the choice will be between rotary pumps and specially designed centrifugals. The rotary type has the advantage for those systems where high back pressure due to settling out of suspended solids in line and exchangers can occur because the pump can be used as a ram u p to the limit of the driver horsepower. O n the other hand, it is a positive displacement pump and care must be taken to insure that it is adequately sized for those systems where there is any tendency toward foaming. Centrifugal pumps designed specifically for handling slurries (see diagram at left) have given very satisfactory performance. Special flushing connections are provided to prevent abrasive material from coming into contact with case and impeller rings and with the packing. Because boiling liquids are being handled, it is desirable to design for low suction line friction. Pumps should be located as close to the bottom of the column as safety will permit and should be oriented so as to require a minimum number of pipe bends. This is particularly important for vacuum tower service. One final comment regarding pumps is concerned with net positive suction head (N.P.S.H.). Because N.P.S.H. required by the pump varies with capacity, the values a t design maximum capacity should be ascertained and final elevations for the column and distillate drum should be checked to ensure that the N.P.S.H. requirements can be met. literature Cited
43) Huang, C., Hodson, J. R., Petrol. Refiner 37,104-118 (February 1958). (4) Nelson, W. L., “Petroleum Refinery Engineering,” p. 494, McGraw-Hill, 1958. (5) Robinson, Gilliland “Elements of Fractional Distillation,” McGraw-Hill, 1950. ( 6 ) Zenz, F. A., Petrol. Refner 36, 179-81 (March 1957).
RECEIVED for review March 15, 1960 ACCEPTED March 15, 1960 Division of Industrial and Engineering Chemistry, 137th Meetixig, ACS, Cleveland, Ohio, April 1960. VOL. 52, NO. 8
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