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HARRY W. HAINES, Jr. W a l t e r Kidde EngineersSouthwest, Inc., Houston, Tex.
Distillation The Practical Aspects
Here i s a logical, orderly plan for selection and installation of a distillation system which bridges the gap between the process design engineer and the proiect engineer
ASSUMING
that the engineer understands distillation theory and process design techniques, it is important that he know how to select the correct system for the desired separation. After this selection has been made, the engineer is faced with the problems of specifying, purchasing, controlling, automating, arranging, and installing the columns, reboilers, condensers, and other auxiliary equipment. Frequently, he is required somewhere along the line to present management with capital and operating cost estimates to establish budget appropriations. Large organizations may have routine procedures for carrying out these functions, whereas in the small company the engineer may be on his own. A junior engineer can encounter considerable difficulty, especially when his formal education did not teach him these functions and if he does not have a definite pattern developed for moving from the process design through the construction stages of the project. T h e practical aspects of distillation necessarily involve considerable experience in project engineering, which ties together the mechanical design, specification and selection, instrumentation, layout, and in some cases, erection of equipment. ThisB article,
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then, stresses the importance of project engineering techniques needed for effective application of prior theory and process designs.
Choose the Correct System
Prior to initiating the process design himself, or assigning this responsibility to others, the engineer must select an appropriate system for carrying out the desired separation. Although engineers seldom pick the wrong setup: it may help here to review the fundamental arrangements available, for a clear understanding of their functions ( 8 ) . Conventional Fractionation. Sormal mixtures of binary or multicomponent character can be separated rather sharply in a conventional system with a
INDUSTRIAL AND ENGINEERING CHEMISTRY
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liquid, vapor, or partially vaporized feed (below. center). The engineer should recognize that the degree of separation in this case varies with the number of trays in the column and the reflux ratio employed under operating conditions. Charge Reflux. A good bottoms purity may be obtained with this system, at the expense of yield, by charging liquid a t the top of the column (see below). Most engineers will recognize this setup as being commonly employed as a side stripper on a drawoff stream from a fractionator. There is no rectifying section or reflux in this system. The operation is primarily one of topping o f f the light component and is particularly attractive if the overhead is small or can be recycled. Bottom temperature or quan-
DlSTlLLATlON EOUIPMENT tity of heat input is the only operating variable when the number of trays have been specified. Charge Reboiler. A good overhead purity may be obtained with this system, at the expense of yield, by charging vapors or a highly vaporized feed at the bottom of the column (shown below).
or water can be admitted into the bottom of the column (shown below).
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Most engineers will recognize this setup as being commonly employed for crude distillation, or to remove a very small amount of heavy ends in rerunning operations. There is no stripping section or boilup in this system. The operation is primarily one of cutting out the heavy component and is particularly attractive if the bottoms are small, or can be recycled. Amount of reflux is the only operating variable available for controlling overhead purity when the number of trays have been specified. Steam Distillation. Vacuum distillation is often used to separate heat sensitive materials, but steam distillation offers another approach (shown below).
small amounts of volatile material from a liquid feed (shown below). Strippers of
Operating pressure within the column must be capable of condensing steam in the bottom section. This system is effective only if the steam is immiscible with the bottoms products, or when the bottoms can be wasted economically. Steam Stilling. This operation can be used successfully to bring the endpoint down on an extremely high boiling point material such as thermal-cracked naphthas (shown below). I t is best
this type are normally not equipped with a condenser or reboiler, and the overhead is taken off as a gas. Absorber Oil Stripping. When it is necessary to remove absorbed material from an absorber oil, the stripper is equipped with a reboiler and condenser (see below). Open steam is employed
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-. w suited economically in situations where large amounts of exhaust steam are available. Open steam supplies all of the heat required and is partially condensed in a charge reflux tower. Low Temperature Fractionation. Refrigerated fractionation is commonly employed for separations that are otherwise impossible for distillation because the critical points would be exceeded (see below). Liquid air fractionation
and the reboiler supplies the balance of the heat required for stripping. The condensed liquid overhead is decanted to remove water from the product, and a portion of the product is employed as column reflux. Absorption. If the process is intended to be nonselective, readily condensable materials in a gas stream may b e . removed by oil absorption in a column that is not equipped with a condenser or reboiler (see below). Intercoolers and refrigeration may be used to increase the degree of absorption.
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Any inert gas could be employed in this system, steam is most commonly used because of its availability, and the immiscibility of water with hydrocarbons permits effective separation in the liquid phase with small product losses. Steam has the effect of reducing the partial pressure of the material to be separated. Pressure Steam Distillation. When it is necessary to decrease the bottom temperature of a fractionator operating under pressure, or increase reboiler load handling capacity by raising its mean temperature difference, open steam
02
is a classic example, which operates as a combination pressure and atmospheric system. Pressure in the lower column is regulated so that liquid oxygen at the bottom of the atmospheric column will condense nitrogen at the top of the pressure column. Stripping. Inert gas or open steam can be employed effectively to remove
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Rectifying Absorption. A reboiler must be added to an absorption column, if the process is to be made selective (bottom p. 663). Heavier portions of the wet gas can be selectively absorbed away from the lighter portions in this system by adjusting the heat supplied to the column. Rectifying absorbers can operate with either a wet gas or rich oil feed. Crude Distillation. Specialized arrangements are required for separation of a complex mixture into multiple products (shown below). Practically all
Azeotropic Distillation with Entrainer. Azeotropic distillation requires two columns-an azeotrope tower and an entrainer recovery tower (see above).
dense into two liquid phases with compositions on either side of the azeotrope composition, the separation can be made without an entrainer (below).
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petroleum crude is processed initially in a system of this type, and catalytic cracking effluent is processed similarly. The section of the main column above the feed is similar to a charge reboiler column, whereas the section below the feed is a stripper for the bottoms product. Normally, the amount of charge vaporized in the heater is approximately equal to the combined overhead and side stream products. Little o r no reflux enters the section below the feed, which operates with stripping steam. The steam removes light components from the bottom product and reduces the heater outlet temperature to prevent thermal decomposition of the charge. Side streams are taken from the top section to individual strippers for removal of components which are lighter than those desired, inasmuch as the side streams constitute rough cuts. Extractive Distillation. Difficult or impossible separations can often be conducted by the addition of a solvent which alters the relative volatility of the two components (top, right). Extractive distillation requires two columns-an extraction tower in which the desired separation is made and a solvent stripper (conventional fractionator) to separate absorbed material from the solvent. I n the extraction tower, the lower section operates as a rectifying absorber with a partially vaporized feed. The upper section acts as an enriching column to prevent solvent from going overhead.
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INDUSTRIAL AND ENGINEERING
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When the entrainer is used as reflux in the azeotrope tower, it breaks the azeotrope in the feed and allows the entrainer to carry one of the components overhead. The overhead condensate separates into two phases, and the entrainer rich phase is used for reflux to maintain the required entrainer concentration on the top trays. The other phase goes to the recovery tower, a charge reflux column, where the entrainer is recovered in the overhead. Exceptions to this system occur when it is necessary to use entrainer in the
Azeotrope vapors leave overhead from the first tower, exhausting one of the components from the bottoms. When these vapors are condensed and separated, the phase rich in bottoms product is employed as reflux. ‘The other phase goes to a charge reflux, or recovery column, in which the valuable component is removed overhead and returned to the decanter. Multiple Tower Distillation. An example here is the use of three columns to separate the feed into four sharply fractionated products, where respectively heavier components are taken overhead
charge for the entrainer to be effective. In some cases, overhead from the azeotrope tower may condense into a single phase and entrainer may appear in the bottoms, requiring additional recovery methods. Azeotropic Distillation without Entrainer. If the azeotrope vapors con-
(see below). I n some cases, economics will dictate a different arrangement where volatility is not the criterion for determining the sequence of separation. It is generally more economical to perform the easiest separation in the first tower where the feed rate is greatest
CHEMISTRY
DISTILLATION EQUIPMENT and the more difficult separations in subsequent towers. The choice can be made on the basis that the reflux to feed ratio is constant for a given degree of separation between any two components.
Specify Equipment Clearly
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During the mechanical design, equipment selection, and purchasing phases of a distillation plant project, it is important for the engineer to specify clearly the equipment needed in an orderly format. This is best accomplished through the use of process information sheets, vessel sketches, and equipment schedules. This information should become a permanent project record for subsequent evaluation of vendors’ quotations. Process Information Sheets. Project secrecy, or the need for protecting proprietary information, will often cause the engineer to withhold data required by the vendor to make intelligent suggestions or quotations. This difficulty can be avoided with process information sheets designed to release only the physical data needed to arrive at adequate tower diameters and tray spacings. Process information sheets are not a substitute for working out the process design of a column. Proper specification of internal design conditions, however, will enable vendors to size and guarantee performance with respect to column throughput and diameter. This is particularly helpful if the column contains float valve or other types of proprietary trays, where correlation data may not exist in the literature. In addition, the process engineer can check his results against those prepared by the vendor. A good process information sheet should reveal the controlling tray location (top or bottom), number and type of trays desired, internal design conditions, and foaming tendency of the liquid. Standardized Column Sketches. Where it is possible to use them, standardized forms and procedures save time and money. Column sketches are a typical example. These sketches can be prepared in advance of a project to represent columns of uniform diameter and columns having a change in diameter. Some vendors will supply printed forms to their customers which can be used as a part of the quotation request. When mechanical design of the column has been completed, the mechanical data is transferred to these standardized sketches which become a permanent record of the project. Vessel Sketch Sheets. If a column has unusual features that cannot be
represented on standardized column sketches, vessel sketch sheets may be preferred. The latter also have a standard format to ensure complete listing of mechanical data. The more useful type of vessel sketch sheets are printed on cross-hatched paper for convenience in free-hand work, normally employed for preliminary estimates. A good sketch sheet is complete in its listing of information, and will have the flexibility required to illustrate accumulators, decanters, and other vessels auxiliary to distillation columns. Vessel Schedules. Sketches are excellent for representing individual items of equipment, but the project engineer should have equipment schedules summarizing pertinent data for all vessels indicated on the flow sheet. Equipment quotations can be expedited and readily checked from schedules showing all mechanical information related to columns, accumulators, decanters, and auxiliary vessels. A comprehensive schedule will immediately indicate the absence of important specifications for a particular vessel. Exchanger Schedules. In many cases, heat exchanger schedules may be the only project tabulation for these items of equipment. Nevertheless, they serve a twofold purpose. Exchanger schedules can be used for obtaining vendor quotations and for tabulating the utilities requirements needed to prepare operating cost estimates. Most manufacturers of heat exchange equipment furnish specification sheets which generally follow a standardized form developed by TEMA. O n large projects, however, there is a definite advantage in transferring this information to schedules. Pump Schedules. Although many engineers use individual specification sheets for pumps, a master tabulation (schedule) has many advantages. Vendors can quote directly from the schedule, which also tabulates utility requirements needed to make operating cost estimates.
actual operation, many or all of these variables may change continuously or abruptly with time. The system, then, must be controlled so that it will return as quickly as possible to the point conditions for which it was designed. The problem is further complicated in situations where concentration cannot be measured directly for control purposes, and some secondary variable related to composition, such as temperature, is selected for measurement. Furthermore, the static design of a distillation column must make allowances for the fact that the composition pattern (gradient) will actually float during plant operation. Distillation columns can be controlled effectively if the position of the composition pattern inside the column is regulated; when the desired reflux or reboil ratio is maintained at all levels of throughput; and the rate of heat input at the reboiler is related to the rate of heat removal at the condenser.
(75). Position of the Composition Pattern. Temperature control a t some point in the column will control the position of the composition pattern, as the two are related at a given operating pressure (below). If the temperature profile
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Select Appropriate Control Methods At some stage in the project, before or after mechanical design of the column, appropriate control methods must be selected. This function can become one of general confusion or misconception unless the engineer fully appreciates the dynamic nature of the system. A vast majority of distillation calculations are prepared with the tacit assumption of a static system-that is to say, flow rates, temperatures, pressures, and concentrations are fixed at point rather than transient conditions. In
has a break point this is normally a sensitive and suitable point for the control element. However, the temperature profile may show only a gradual change, but this does not eliminate the possibility of temperature control. Vapor pressure bulbs, filIed with liquid of the desired composition at the control point, may be used. VOL. 62, NO. 8
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Vacuum distillations are an exception. When the load on the column changes, a single temperature measurement may be seriously affected by the pressure change at the control point. Hence, a temperature difference method is recommended. However, the temperature difference between two control points due to a change in composition must be greater than the temperature difference due to pressure drop, otherwise a false signal will occur. In lieu of temperature control, the position of the composition pattern can be fixed by removing a controlled fraction of the feed as either top (see below)
tion pattern will satisfy the material balance set by the relative flow rates.) These methods, however, cannot be used to produce high purity material, owing to the limitations in sensing very small changes in flow rates. Fixed Reflux or Reboil Ratio. A fixed reflux ratio can be obtained by proportioning the flow of reflux to the flow of top product, bottom product, or feed (see below). I n the case of a r - 4
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total condenser, a reflux splitter is commonly employed to maintain a fixed ratio between reflux and top product (see below). For partial
L or bottom (shown below) product.
can be fixed by two temperature controllers, one regulating the heat supplied to the reboiler, the other regulating the flow of reflux (see below).
Relating Heat Input a n d Removal. Top and bottom products of a specified purity cannot be produced unless a definite relationship exists between the rate of heat input at the reboiler and the rate of heat removal at the condenser. Both have an effect on the profile and position of the temperature pattern. In cases where temperature control of the composition pattern is not used, the heat input and removal relationship must be controlled by level controls or other methods (shown below).
Sim-
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ilarly, the top and bottom product may be withdrawn in a fixed ratio (see below). (Only one position of the composi-
t condensers, a controlled relation must be established between the heat load and the flow of top product, bottom product. or feed. A fixed reboil ratio is obtained by proportional control of the heat load on the reboiler and the flow of top product, bottom product, or feed (shown be 1OM') .
The reflux-to-reboil ratio, which has an effect on the composition profile,
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Chromatographic Control. In an effort to work directly with compositions, vapor-phase chromatographs (below)
INDUSTRIAL AND ENGINEERING CHEMISTRY
have been used to control fractionating columns, notably depropanizers, debutanizers, and deisobutanizers ( 6 ) . A periodic type of control was developed which is not truly continuous because of the time lag required to complete each analysis (about 5 minutes). How-
DISTILLATION EOUIPMENT ever,“it provided an effective method of relaying changes in set point to a temperature recorder-controller, which in turn regulated the flow of heat to the reboiler.
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PNEU~ZATIC
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t With the advent of high-speed chromatograpic analyzers (above) that operate on cycles of a minute or less and produce a continuous output signal, automatic control is now possible (7). Refractometer Control. Differential refractometers (see below) have also
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concentration and over a wide range of concentrations. The multicomponent mixture is simplified as nonkey components tend to exhibit constant composition zones in the column. In theory, the feed stream, reflux stream, or reboiler heat input can be regulated to develop a specific composition at a fixed point in the column. In practice, however, many columns have no freedom of manipulation of feed rate or feed composition, and reflux manipulation usually gives twice as much dead time as heat input manipulation. (Vapor passes through the column more rapidly than liquid, hence the rate of energy propagation is greater when the heat input is regulated.) Manipulation of reboiler heat input, therefore, is by far the most effective control method. Commercial plant results have shown that analytical control methods of this type give superior performance over temperature control methods. Al-
though temperature control is reliable, low in cost, and has a short dead time, economics in commercial superfractionation favor analytical control.
look for Automation Opportunities The modern engineer should train himself to take advantage of computer control in cases where the equipment will pay for itself. Certain guides can be established for computer economics (74). One method suggests that the economics be based on a 1-year payout-considering tangible fuel savings of the increased value of upgraded product. As a rule of thumb, one can assume that gross savings must equal twice the cost of the computer to produce a 1-year payout. Investment credits due to a lower cost of process equipment should also be examined by the prospective purchaser of a new process unit, as computer con-
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+ been used to control depropanizers, debutanizers, deisobutanizers, and other separations (79). In these cases, control has been obtained by analysis of the light key component below the feed, or analysis of the heavy key component above the feed point. Sampling near the feed point has its advantages. Terminal composition behavior is improved due to early recognition of compositibn transients, as they “float” from the feed point toward the column terminals. Analytical requirements are less stringent because the control component is analyzed at a high
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trol may allow significant equipment savings. Similarly, an increase in the capacity of existing equipment may be realized. For the most part, computer applications of immediate interest will cost no more than $5000 to $10,000 in the case of analog equipment. Control of Internal Reflux. Several equipment manufacturers ( 7 7) now have analog computers on the market for control of internal reflux in fractionation columns (top p. 667). These devices, using standard electric and pneumatic instrument components, are the most effective means yet devised for determining and controlIing internal reflux flow rate. They provide rapid corrections for variations in reflux temperature originating in the condensing system, without additional equipment other than normal column instrumentation. The reflux computer controller measures external reflux flow, and the temperature differential between towertop temperature and external reflux temperature to calculate the internal reflux flow. The computed value is then used as a signal for control purposes. Substantial savings can be obtained with less reboiler heat requirements, a reduction in off-specification product, faster on-stream time, continuous column operation nearer the flood point, and lower equipment and maintenance costs due to elimination of condenser temperature controls. Capacity Control. Analog computers are also being used to manipulate operating capacity in fractionation towers (bottom p. 667). One such instrument (78) computes the actual vapor rate from measured product and reflux rates, the vapor capacity as a function of measured pressure, and the per cent of vapor capacity actually utilized. The computed percentage is recorded and can be used to control the tower to any desired percentage of capacity. Pumparound Heat Removal. Frequently, it is necessary to remove excess heat from a fractionating column to
assure proper separation, and, if pumparound heat removal is employed, this operation in theory can be controlled with an analog computer (left below). In this type of operation, a trim cooler is ordinarily employed for adjusting the amount of heat removed, inasmuch as temperatures and flow rates of the main coolant stream normally depend upon operations elsewhere in the plant. Even though the trim cooler is automatically regulated by appropriate temperature controls, the amount of heat removed from the tower may not remain constant. Temperature of pumparound leaving the tower, temperature of pumparound returning to the tower, and its flow rate, can be measured
Bottoms Cutpoint. When cutting deeply into a crude oil for maximum output, the still operator attempts to maintain a specified flash zone temperature to obtain a constant bottoms flow rate. The number of different crudes to be run and feed rate changes to the atmospheric still complicates the operator’s problems. The job is even more complicated if flow measurements are not temperature or density compensated. One analog system has been proposed, where the compensated bottoms flow is divided by the compensated total crude flow for calculating per cent bottoms (see below). The computer automatically uses the correct assay curve previously set on a crude selector
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6OfTOMS
but they are not the primary variables. However, if fed into an analog computer that calculates the rate of heat removal, these variables will control the operation. The computer output goes to a conventional controller which adjusts the quantity of heat removed by the trim cooler.
CUTPOINT
switch and reads out bottoms cutpoint Feed Forward Control. Analog computers are suitable for feed forward control, where the computer automatically changes the feed plate location. reboiler temperature and reflux ratio set point (top p. 669). A feed stock analysis and a feed stock temperature measurement are taken and fed into the computer, which has previously been instructed on the steps it must take for different feed compositions and temperatures. In this application, the computer receives advance notice that a different feed stock will shortly enter the tower and makes changes accordingly, keeping tower upsets to a minimum. Arrange Equipment Efficiently
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Every engineer should understand the principles required to maintain maximum flexibility and compactness of equipment arrangement at a reasonable first cost. Aside from the process design, no single factor is as important as the physical layout of equipment. A modern plant erected today is likely to
DISTILLATION E Q U I P M E N T
remain in service for 20 years or longer, and any errors in the beginning may be costly if not impossible to rectify later. Units generally can be arranged for grouped layout where similar equipment are grouped together in separate areas to facilitate operation or maintenance, or in a flow line pattern where equipment are arranged in the layout as they appear on the flow sheets. The grouped layout is normally preferred for larger units with a large number of towers, drums, exchangers, and pumps. Small units, or large units with relatively few exchangers or pumps, will effectively fit into the flow line pattern (73). Today, engineers are placing increasing emphasis on facilities for minimizing maintenance costs as one of the most important features of unit design. There is definitely a trend toward the use of mobile maintenance equipment to maximum extent, and a trend away from built-in handling facilities. For maximum use of mobile maintenance equipment, the unit should be designed with its equipment “horizontal,” or within reach of the mobile maintenance equipment. Working space and accessways must be provided for passage of maintenance equipment within battery limits. Control valves, instruments, and other equipment requiring operator attention should be placed at grade, or as close thereto as possible, for operating convenience. In both the grouped and flow line layout, equipment should be placed in no more than two adjacent rows. Limiting the layout in this manner allows mobile equipment to approach very close to at least one side of any tower, exchanger, or pump row. A large crane can operate with complete freedom if accessways have a minimum width of 25 feet, and the area is clear of overhead piping or other obstructions.
A 25-foot dimension also allows unhindered two-way passage of trucks. Where two or more pipe racks cross an accessway, a clear space of 60 feet between each rack is required for raising and lowering a crane boom. Tower and Vessel Layout. I t is generally good practice to keep one unit at least 100 feet rrom an adjacent unit to allow welding on one while the other is in operation (72). For fractionating columns with elevated condensers mounted on common steelwork between each pair of columns, the centerto-center spacing between columns should be equivalent to 7 or 8 times the average of the two column diameters. In many cases skirt heights of 8 to 16 feet will be required to assure adequate N.P.S.H. for pumps. Davits should be provided on top of towers to handle safety valves and trays, with dropout space for removing trays. Davits should also be provided on all manhole covers. Horizontal reflux accumulators should be spaced, center-to-center, at least two diameters apart. Large horizontal storage vessels can be center-to-center spaced about 11/2 diameters apart, provided this rule does not restrict the clear space between vessels to less than 6 feet. When hazardous materials such as light hydrocarbons, or highly toxic materials are employed, it is a good idea not to group together more than four large horizontal storage vessels. Small vertical accumulators should be center-to-center spaced about 3 to 4 diameters apart, whereas large vertical vessels may have a spacing of 2l/2 to 3 diameters. Exchanger Layout. Heat exchangers should be located at grade and stacked two high, only when using two shells in series, and not higher than 12 feet for small exchangers. Where exchangers
are stacked two high, provide lifting lugs a t 45 degrees on each side of the lower exchanger channel end. Flooded-type condensers and coolers at grade are preferred in lieu of overhead condensers and coolers, where operating pressures are high enough to permit this type of installation. Provide bypass or valving on fouling exchangers at grade to allow tube bundle removal during operation of the unit. Avoid locating exchangers under drums or the unit structure, and, when so located by necessity, the channel end must be clear of overhead obstructions. Horizontal heat exchangers should be spaced about 3 to 4 diameters apart at grade, or have a center-to-center spacing of 2 to 2l/z diameters if elevated. When overhead condensers are located a t elevations above 50 feet, a permanent trolley and hoist should be provided. If fouling is likely to occur in overhead condenser service, valves are recommended on each half of a condenser group to permit in-place chemical cleaning or tube bundle removal during operation of the unit. I n general, vertical exchangers should be avoided unless required for process reasons. When vertical exchangers are used, a center-to-center spacing of 2%/2to 3 diameters is recommended. Pump Layout. Except where bad suction conditions exist or for individual ungrouped pumps, groups of pumps should have a clear distance of 15 feet from towers, drums, and exchangers. Fairly large horizontal pumps usually are placed with a %foot center-to-center axis spacing. For medium size pumps a 7-foot spacing is adequate, whereas small pumps need only 4 feet. Alternately, one should allow 5 feet of clear space between medium size pumps and 2l/2 feet of clear space between small pumps, exclusive of piping.
Make an Accurate Cost Estimate From an engineer-constructor’s viewpoint, there is no really satisfactory substitute for the detailed cost estimate prepared by his estimating department. This type of estimate can be worked up only from equipment schedules, complete engineering drawings, and a bill of materials by the “take-off” method. I t may cost many thousands of dollars and involve a small contingency of several per cent. In contrast, the operating company is obliged to make “horseback” estimates to determine project feasibility before thework goes to the engineer-constructor. The quickestimate must contain sufficient accuracy for budgetary approval and require a minimum of time in preparation, VOL. 52, NO. 8
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Table I. Estimating Time Should Be Distributed in Proportion to the Value o f the Equipment Involved % Columns Exchangers Pumps
Tanks
49 33
10 8 100
so that numerous projects can be screened throughout the year. The quick estimate is an area of extreme difficulty for the inexperienced engineer who does not have a grasp of significant us. insignificant cost elements. However, a few basic rules can be established as guideposts. Take care of the big items first with a high degree of accuracy and spend as little time as possible on the small items, which tend to average out in their inaccuracies. Then use a different set of estimating factors to check the possibility of discrepancies. Equipment Prices. If the average engineer will take time to examine typical equipment ratios in distillation units (Table I), it is obvious where the pricing effort should be placed (2). Equipment prices may be determined from vendors’ quotations, which is the most accurate method, but this is a time consuming process. Some engineers prefer to make their own estimates in the early stages of screening a project to avoid gaining a reputation in the trade of always looking and never buying. If prices are available on similar equipment of a different size, the “six-tenths’’ factor becomes a handy tool. The “six-tenths” rule says that the ratio of the costs is directly proportional to the 0.6 power of the relative capacities ( 3 ) . This rule is not without some theoretical basis, where vessels are concerned, inasmuch as the surface-to-volume ratio is a 0.66 power function and related to the weight of metal used for fabrication of the vessel. Engineers should exercise caution, however, in applying this method. It may not be accurate for capacities varying as much as tenfold. A “break point” may exist in fabrication costs somewhere between the two capacities in question. No attempt should be made to use this method when different materials of construction are involved or in cases where widely different temperature and pressure conditions exist. In any event it is always a good idea to check tower or vessel costs on a metal weight basis, adding to this the cost of trays or internals, manholes, and nozzles. Some excellent descriptions of this method exist (5, 70).
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Many of the leading chemical publications and magazines devoted to cost estimating contain a wealth of information that enables one to estimate prices for heat exchangers and pumps. Curves have been published a t regular intervals relating heat exchanger costs on a square foot basis and pump costs on a gallons-per-minute basis. Organizations receiving quotations a t frequent intervals will probably have sufficient information in their files from which similar curves can be constructed and kept u p to date with reasonable accuracy. In proper perspective, the engineer should recognize that a 5Ye error in the cost of distillation columns is roughly equivalent to a 25Y0 error in the cost of pumps, dollanvise. This another way of saying that half of his time should be allocated to estimating column costs and 10% of his time estimating pump costs. Unit Estimating Factors. For units of a similar nature, estimating factors can be developed from previous job records which allow the engineer to base his total estimate on delivered equipment prices (Table 11). This is probably the easiest and most widespread system in use today (7, 76). Special materials of construction or extreme pressure and temperature conditions should not be treated so casually, however, as the labor factor tends to “overshoot” in this method. Refinements can be added to this method by separating the factors into labor and materials components and applying multipliers as a compensating feature for special materials of construction. Equipment Estimating Factors. In recent years “installed cost” charts have appeared in the literature for specific items of equipment (77). Fortunately, these charts automatically compensate for special materials of construction by the use of suitable parameters. An estimate prepared by the “unit factor” method should always be checked against these charts.
Table 11. For Units of a Similar Nature, Estimating Factors Can Be Developed from Previous Job Records % Equipment (delivered) 100 43 Installationn 86 Piping 15 Instrumentation 8 Insulation 13 Electrical 30 Building 5 Yard improvements __
A third check can be obtained with Hand’s method (Q), using equipment multipliers (Table 111). His method applied to all items in a distillation unit agree very well with typical unit estimating factors, in that total battery limits costs are approximately four times the equipment costs. Although no general rule can be established, any substantial deviation from this “fourfold amount” should be subject to question or explanation ( 4 ) .
Table 111. Hand’s Method Provides a Reasonably Accurate Short Cut to Estimation of Installed Equipment Costs Multiplying Factors
Columns Pressure vessels Heat exchangers Pumps Instruments
4
4 3‘/2 4 4
literature Cited (1) Aries, R . S., Newton, R. D., “Chemical
Engineering Cost Estimation,” McCraw-Hill, New York, 1955. (2) Bach, N. G., Chem. Eng. 65, 155-9 (September 1958). ( 3 ) Bauman, H. C., IND.ENG.C H E w 50, NO. 8, 69A-71A (1958). (4) Bromberg, I., Petrol. Refiner 37, 141-3 (December 1958). (5) Brownell, L. E., Young, E. H., “Process Equipment Design,” Wiley, New York, 1959. (6) Chem. Eng. 6 6 , 34 (November 1959). (7) Fourroux; M. M., Karasek, F. W‘.: Wightman, R. E., Oil t3 Gas J.5 8 , 96-9 (March 21. 10hO’i. ~...
(8) Gordon, K. F.,‘ Davies, J. A., Refining Engr. 31, C-22-4 (August 1959). (9) Hand, W. E., Petrol. Refiner 37, 331-4
(September 1958). (IO) How, H., Chem. Ene. - 55.. 122 (January 1948). (11) Lupfer, D. E., Berger, I). E., ZSA Journal 6, 34-9 (June 1959). (12) McGarry, J. F., Petrol. Refiner 37, 109-10 (October 1958). (13) Marancik, J. V., Petrol. R e f n u 37, 339-42 (September 1958). (14) Moore, E., Weber, N. E., Pelrochem. Znd. 3, 10, 12-1 4 (January 1960). (15) Parkins, R., Cfiem. Eng. Progr. 5 5 , 60-8 (July 1959). (16) Peters, M. S., “Plant Design and Economics for Chemical Engineers,“ McGraw-Hill, New York, 1958. (17) Vilbrandt, F. C., Dryden, C. E., “Chemical Engineering Plant Design,” McGraw-Hill, New York, 1959. (18) Webber, W. O., Martin, R . L., Pink, J. F . , Hargett, J. T., “Analog Computer-Controller for Capacity Control of a Distillation Column,” Division of Refining, American Petroleum Institute, 24th Midyear Meeting, May 1959). (19) Wherry, T. C., Berger, D. E., Petrol. ReJiner 37, 219-24 (May 1958).
300
Engineering t construction
100 __ 400
Foundations, platforms and supports, and erection of equipment.
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review April 6, 1960 ACCEPTED April 12, 1960 Division of Industrial and Engineering Chemistry, 137th Meeting, ACS, Cleveland, Ohio?A4pril1960.
