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The ‘process step scoring’ method for making quick capital estimates
Eng&ering
rrtd Process Economics 2(!911)
259
259-261
@Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
THE ‘PROCESS STEP SCORING’ METHOD FOR MAKING QUICK CAPITAL ESTIMATES J.H. Taylor ICI Ltd., Mond Division, The Heath, Runcom, Cheshire (U.K.)
Abstract
Preliminary screening of the many alternative proposals that arise in R & D requires economic assessment methods that are quick, can be carried out with a minimum of information and yet are reasonably reliable. The ‘Process Step Scoring’ method described here requires only outline information about the process and has an accuracy within 95% confidence limits of +36% to -26% which is good enough for many preliminary evaluations.
The method is based on a system in which a complexity score accounting for factors such as throughput, corrosion problems and reaction time is estimated for each process step, and these are combined to give an overall ‘costliness index’. The capital is then derived from a relationship between this index, capacity and capital cost which was based on experience ot 45 recent projects in the U.K., covering a broad spectrum of plant sizes and technology.
INTRODUCTION
Preliminary screening of the many alternative proposals that arise in R & D requires capital estimating methods that are quick, can be carried out with a minimum of information and yet are reasonably reliable. For most situations the traditional engineering methods of estimating capital are too cumbersome, since to achieve a reliable result, detailed flowsheeting and equipment sizing of all the main plant items is required, necessitating considerable chemical engineering effort. A more practical way of making a quick capital estimate for research decision purposes at an eady stage is to seek out a similar process of known capital cost and then make appropriate modifications for the differences in detail using various rules of thumb. This ‘direct analogy’ approach can be very effective but it requires experienced staff who have extensive and detailed knowledge of a wide variety of chemical processes and
their associated capital costs. The dissemination of detailed capital and process information, even within a company, is normally restricted for security reasons and the movement of staff poses problems in identifying a source of the necessary expertise. There is therefore a need for a quick, yet reasonably reliable, estimating method which incorporates the knowledge of a broad spectrum of achieved costs on past projects and yet can be used by relatively inexperienced people. The ‘Process Step Scoring’ method described here aims to fulfil this need. The basic principle of the ‘Process Step Scoring’ method is the postulation that the capital cost of a plant is related to a costhness index (I), expressing the complexity and nature of the chemistry involved in the process, and to its size by a relationship of the form: Capital cost = constant X costliness index X (capacity)P
where p is a fractional power.
The costliness Index, which is a unique function for a particular process, is determined by the expression: N I = c
(1)
cl.39
where IVis the number of ‘significant process steps’ (e.g. filter, react, distill and S is a ‘complexity score’ determined for each process step to take account of factors such as throughput, materials of construction, reaction time, temperature, pressure and multistreaming. Costliness indexes normally range from about 10 for a plant to make a simple chemical such as formaldehyde up to 200 or more for a plant for a complex drug or pesticide. In principle the ‘Process Step Scoring’ method relates capital cost directly to the chemistry involved in the process without considering engineering aspects on the type of equipment required. In this respect it differs from other published quick costing methods [ l-51.Another tignificant feature is that each process step is s:ored independently for throughput, corrosion etc. The relationship between capital, ‘costliness index’ and plant size was derived by carrying
out a regression analysis on 45 U.K. projects completed during the past 12 years. These cover a broad spectrum of the types of process encountered in the Chemical Industry. They include processes for organic solvents, intermediates and monomers, complex biologically active compounds, polymeric materials and inorganic materials. Most of the common chemical reactions are covered, for example, chlorination, fluorination, oxidation, hydrolysis, amination, hydrogenation, dimerisation, addition and cracking reactions. Plant sizes are evenly distributed over the range 300 to 250 000 ton/year. No extrapolations on the basis of assumed scale-up power factors were made; in fact an important finding from the work has been that the traditional 0.6 power factor for scale-up is too high for correlations of this type. (This is to be discussed in a subsequent communication.) DESCRIPTION
The method 1sexplained below step-by-step and is then exemplified by a worked sample. Step 1 /IDefine the process to be diagram like the one in Fig.
5 (WCYCLE3) t 4t
AQ SOLN OF BY PRODUCTS (EFFLL’ENT-TO WORL(S DISPOSAL “NIT)
t
C~IMPOUND”C~’
Fig. 1. Hypothetical flow diagram for the reaction: A+B-rCinAsohrentS,
by a
261 The diagram should show the mam process steps, relative throughputs (ton/ton product), temperatures, pressures, materials of construction, reaction times, multi-streaming and any special conditions such as explosion hazards. Step 2. List the significant process steps. The following unit operations/unit processes would usually be regarded as ‘significant process steps’: - Chemical reactions, such as chlorination, oxidation, nitration. - Neutralise or acidify (but not minor pH adjustment). - Storage/handling of a raw material, product, by-product, intermediate or recycle stream. Also effluents when these are not assumed to be discharged directly to outside the battery limits (e.g. to river or Works effluent disposal system). - Filter, screen or centrifuge. - Distil, evaporate, fractionate or strip. - Crystallise or precipitate. - Formulate. - Compress. -- Vaporise. - Dry or spray dry. - Mill or grind. - Scrub or absorb. - Pack into special small containers (not sacks or drums). - Dissolve, mix, slurry or blend when required as a specific pre- or post-treatment, e.g. dissolving before spray drying. (But not when an integral part of another process step such as, say, solvent extraction). - Quench (but not normal cooling of a reaction mixture). - Condense when used to separate a component from a gaseous stream containing inerts (but not for normal condensation in stills, quenches or reactors). - Phase separation of a reaction mixture (but not when part of a still or extraction system). - Extract or leach. The ‘si,gnificant process step’ refers to the operation that is performed on the material
flow and not to the equipment that is necessary. The fact, for example, that sevoral reactions or unit operations could be done in the same vessel is of no significance from the point of view of this method; similarly the need in some continuous processes for several reactors in series is ignored. This is because the method relates the number of process steps/complexity directly to the capital costs for the optimum process at the scale concerned. Therefore, it is most important when listing the significant process steps to disassociate one’s mind completely from the equipment that might be used. If any special process steps not covered above are encountered, such as electrolysis, fibre spinning and extrusion, these should be costed separately either by ‘direct analogy’ or an engineering estimate. Step 3. Score each ‘significant process step’ on throughput, materials of construction, temperature, pressure, multi-streaming, reaction (or storage) time and any special conditions, according to Table 1. Relative throughput: This is the totai (i) weight flow into the process step per unit weight of final product. Internal recycles such as those in stills or absorbers are ignored. The throughput score i I dsuatly dominant compared to those for the other factors. (ii) Materials of construction: For a single process step there can be several construction materials involved (e.g. ELMS vessel with Ti stirrer). The score is based on the dominant material (ELMS). Knowledge of the precise materials of construction required is not necessary; the score can be based on a subjective judgement as to the magnitude of the corrosion problem e.g. no problem - mild steel, score 0; major problem - titanium, score 3. (iii) Temperature: Score based on the temperature extreme. (iv) Pressure: Score based on the pressure extreme. With gas phase reactions, elevated pressures up to about 50 atms are assumed to have no effect on the cost of a process
Score -1 0.6
r 3
usb
20
0
US
Stoic 1 if a major problem.
-2 0.35
-3 0.2
--~-
2
1.7 5 2 -25 500 0.1 100 SSc, Keebusb RLusd,EbLUSe, PVC Nickel Uonel PbLUSg
-7.5 1100 0.01 50” ELMSf
3 9 14
5
s
200 Titanium Hastelloy
-125 1700
3
25 8
8
700 Precious metals Tantallum
2330
4
14 42
11
!SOO
5
23 69
6
40 120
I
8 67
110
9
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 IS 16 Cosfline=? i!+*: (I) a4 0.6 0.8 1 1.3 1.7 2.2 2.8 3.7 4.8 6.3 8.1 10.6 14 18 23 30 39 51 66 ___.. ----___--____________~__ ---~~ ------__ = Mild steel. cSS = Stainless steel. dRLUS = Rubber lined mild steel. eEbLMS = Ebonite lined mild steel. :v~LUS = oFor liquid phase reactions only. All others swe = 0. b Znamel lined mild steel. @bLUS = Lead Lined mild steel.
scorn (S)
Conversion of score to costliness index
,
Special conditions: (a! E*p!csic!!, da-tI odom o: tcxicity prblems. (b) Reactions in tluid beds. Score 1.
Multistreaming. No. of streams
Relative thm *’ put (t/t product) Reaction time in h (reaction, crystallisation, etc) Storage time in weeks Temperature extreme CC) Uin Temperature extreme CC) Max Pressure extreme (atm) Uin Pressure extreme (atm) Max Materials of construction
-___
Scoring for complexity of signiticant process steps ---.---.. . -.
TABLE 1
step: this is because the increased wall thickness is offset by reduced vessel size. Multistreaming: This might be considered necessary for reasons such as poor reliability, uncertainty of market growth or limitations on equipment size. Reaction time/storage time: These have different scales. The reaction time scale refers to liquid phase process steps. The cost of gas phase reactions are usually effected only slightly by the residence time; no scoring scale is therefore included for them. (vii) Special conditions: These could include (a) explosion, odour, dust or toxicity problems, (b) fractionation of materials of similar boiling points, (c) reactions in fluid bed+, (d) film evaporation and (e) tight specification. At present these are assessed on an empirical basis. Step 4. Convert the total score for each ‘significant process step’ into a costliness index using the conversion table (at foot of Table 1). For a single process step the costliness index (I) is given by: I = 1.3s
where S is the total score for the step. Thus a
DESIGN
Fig. 2. A graph of eq. 2.
CAPACITY
IN
TON / YEAR
includes storage) that would be needed for a completely new plant in the U.K. built up from the ground. The capacity is the design capacity, not the achieved capacity. An alternative ‘direct analogy’ type of approach not employing the above derived relationship is to compare the costliness index (I) with that (I,) of a similar process of known capital cost (C,) and capacity (A I); the required capital cost (C) bging given by: (3)
This has the advantage that it eliminates the personal factor in assessing the ‘significant process steps’ - one of the main difficulties in applying the method. Other possible defects in the method are also avoided, providing a closely similar process is used for the comparison. Step 7. Make allowances, where necessary, to cover inflation, off-site facilities and site development, use of existing structure and construction of plants overseas. (a) Inflation: Appropriate infiation from Jan 77 should be added using suitabie indices such as the one published regularly in this Jouma!. tb) Off-sites and site development: Off-site facilities includes for exampie workshops, supply of basic services and effluent treatment. Site development includes for example roads, railways and drainage. The extra capital expenditure required for these could u rider some circumstances add 50% or more to the capital. On the other hand it could be virtually zero if the plant is located on a suitable developed Works site with all the services available in ample supply. It is suggested that in preliminary estimates when there are usual’ o indications on the type of site that would be used, a notional allowance of 30% should be added.to cover off-sites and site developme..:. Cc) Use of existmg structure: With small plants
of a few hundred ton/year capacity making non-complex chemicals, use can frequently be made of an existing structure. This would reduce the capital by around 25%. With small plants, further savings are possible by using some existing equipment, manhandling of materials, less ngorous standards and general improvisation. Obviously the saving possible will vary considerably but a typical figure could be taken as 50% (including structure). Cd) Plants overseas: Capital costs for plants overseds can be substantially higher or lower than those in the U.K. and the current exchange rate is not therefore necessarily appropriate. Location indices have been discussed in this Journal. Warning on sca~eupr: The costliness index, I, can change on scale-up as a result of more multistreaming, or the addition of more process steps (e.g. to recover solvents). If I does change, extrapolation of the cost to other scales cannot be achieved by simple use of the scale exponent but I must be recalculated for the new stakes. EXAMPLE Suppose it is required to cost the process outlined in Fig. 1, for single stream plants of 2000 and 8000 ton/year and for an 8000 ton/
year plant with a two stream reaction stage: (a) Single stream plant of 2000 ton/year: Costliness index for 2000 ton/year plant = 2 1, asshown in Table 2. Using eq. 2 (or Fig. 2) the ‘battery limits’ capital for a 2000 ton/year plant at Jan 77 pricts is =21X42x1.31 (1000 &) =2.1.16x 10” Adding 30% as a notional allowance to cover off-sites and site development gives f. 1.50 X lo6 (Jan 77). (b) Single stream plant of 8000 ton/year: The costliness index is 2 1 as for the 2000 ton/ year plant. The ‘battery limits’ capital = 21 X 42 X 2.25 .’’ =E 1.98X 10b
TABLE 2 Calculation _-__
of costliness index for plocess in Fig. Throughput
MofC
1
Rractionl storage
Pressure/ temp.
Other
0
0 0 0 0
1 (toxic) 0 0 0 0
1 0 0 0
0 0 0 0
Total score
index
1 1 3 3%
1.3 1.3 2.2 2.5 0.8
7 4 0 4
6.3 2.8 1.0 2.8
-I
21.0
Adding 30% for off-sites, the capital = .E 2.57 X 10t (Jan 77) (c) 8000 ton/year plant with twin
about 5 process steps (N.B. the more steps the better). (ii) Modification or extension of axisting plants. (iii) Fully batch operated plants of abnormally high capacity (3000 ton/year or more). (iv) FuHy continuous plants of abnormally low capacity (500 ton/year or less). (vl Plants involving appreciable solids handling on the large scale (5000 + ton/year). (vi) Plants involving special operations such as electrolysis, fibre spinning, extrusion, when these are likely to represent a substantial proportion of the cost. ACCURACY The accuracy of the method, for a specified process definition, determined from the regression analysis, is as follows: Y5% Confidence limits Standard deviation
The method can be used for complete new plants of capacities ranging from 300 to 250 000 ton/year. However, at the current level of development, It cannot be used for the following: (i) Very simple plants involving less than
+36% and -26% 1.5%
This order of accuracy is generally adequate for the preliminary screening of R & D projects. If, for example, there are several proposed routes to a new compound, the method can be used to reduce these to a short list of two
which can then be evaluated in depth using ‘engineering type’ estimating methods. In the early stages of an R & D project when the process is poorly defined the ‘Process Step Scoring’ method can in many cases give a more reliable result than ‘engineering type’ estimates. This is because cursory assessment of the plant equipment required, often grossly underestimates the complexity of the process, thus leading to an estimated cost which is much too low. Errors of thi? type are less likely with the ‘Process Step Scoring’ method, because costs for a specified process step are determined by correlating directly with costs achieved in practice for steps of analogous complexity - thus analysis of the fine details of the step are already built into the method. It is also noteworthy that preliminary ‘engineering type’ estimates are essentially based on analogy, in that usually 20% of the cost (the main plant equipment items (it rlgorosly estimated and then the remaining 80% (foundations, structure, instrument, piping, design etc.) is estimated by multiplying by a factor based on experience. Though the ‘Process Step Scoring’ method gives reasonably satisfactory results for complete plants, it can lead to an answer which is grossly in error (e.g. by a factor of 2), if applied to a single process step. The factorial type of engineering methods can also give similarly inaccurate results when applied to a single step and deficiency in this respect should not be considered to invalidate the method. Satisfactory results are obtained with complete plants primarily because of the ‘swings and roundabouts’ effect. Accuracy will tend to be best for plants with a large number of process steps. In its present form the ‘Process Step Scoring’ method assumes that all process steps are equal in cost for given throughputs, temperatures etc. An attempt has been made to improve accuracy by employing different ‘. ghting factors with respect to reactions, separations and storage/handling. The improvement in accuracy obtained was only marginal (standard deviation from 15.2% to 14.1%), and probably
not significant. Also, surprisingly, the improved accuracy was obtained by giving more weight to storage/handling and less to reactions. The main source of error in estimating the capital for a specified plant is likely to arise from personal differences as to what constitutes a ‘significant process step’. Though the list of these given on page 261 is intended to be as fbll as possible, it is recognised that difficulties can still arise. However, unless the plant being considered only in rolves a few process steps, this ‘personal facto] ’ need not represent a sizeable error. SHORT-CUT METHODS
Sometimes there is not sufficient time or the data available to apply the ‘Process Step Scoring’ method as described. Accordingly, relationships have been derived for capital cost as a function Of:
the number of reactions, and the number of ‘significant process steps’. These short-cut but cruder relationships are outlined below: (i) Number of reactions:
(i)
(ii)
Capital in 1000 E = 1120 (Capacity in 1000 ton)“” Number of reactions
(6)
95% Confidence limits +155% Standard deviation = 52%
N.B. Subsidiary reactions such as pH adjustment are not taken as reactions. fii) Number of ‘significant process steps’ Capital in 1000 f = 115 (Capacity in 1000 tonjo*” Number of process steps
(7)
95% Confidence limits +96% and -49% Standard deviation = 44%
In some assessments of R & D projects an indication of the minimum likr’y capital cost is all that is necessary. In these circumstances the short-cut relationships can prove useful. For example, say it was known that a new product requiring 4 reactions stages could only be viable if the capital cost (battery limits) for
a i
000
ton/yearpiantWBS iP,:ISthan
E 1.5 mil-
lion. From eq. 6, the most likely capital cost is f. 4.5 million and from the 55% confidence limits there is only a 5% chance of the capital being less than 2 1.8 million. On this basis, it would be concluded that the new product was very unlikely to be an economic proposition.
3 Wilson, G.T., 1971. Capital lnrestment for Chemical Plant, British Chemical Engineering and Process Technology, 16(10): 931. 4 Allen, D.H. and Page, R.C., 1975. Revised ischniquo for Tredesigu Cost Estimating, Chemical Engineer&g, March 2. S Bridgwater, A.V.. 1974. Rapid Cost Estimation in the Chemical Process Industrits, Paper presented at 3rd International Cost Engineering Symposiuc, The A ,ocirtiotr of Cost Engineers.
ACKNOWLEDGEMENTS
BIOGRAPHICAL NOTE
The author is indebted for helpful discussions to many colleagues within ICI Ltd, particularly those in Mond Division’s Research Economics Section.
John Taylor is a chemist by training, with the degrees of R.Sc. and Ph.D. from Liverpool Un.versity. He is employed in the Research bi Development Department of ICI’s Mond Division. For uea-rly 10 years he has had the position of Research Economist, in which he is responsible for the economic evaluation of a range of the Division’s R&D projects. in this activity he has gained a Sroad spectrum of experience on the costs of chemical orocesses and the factors affecting them.
REFERENCES 1 Zevnik, F.C. and Iluchanan, ILL., 1963. GeneraIised Correlation of Process Investment, Chemical Engineering Progress, 59(2): 70. 2 Stallworthy, EA., 1970. The Viewpoint of a Large Chemical Manufacturing Company, The Chemicai Engineer, June. 182.
rrtd Process Economics 2(!911)
259
259-261
@Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
THE ‘PROCESS STEP SCORING’ METHOD FOR MAKING QUICK CAPITAL ESTIMATES J.H. Taylor ICI Ltd., Mond Division, The Heath, Runcom, Cheshire (U.K.)
Abstract
Preliminary screening of the many alternative proposals that arise in R & D requires economic assessment methods that are quick, can be carried out with a minimum of information and yet are reasonably reliable. The ‘Process Step Scoring’ method described here requires only outline information about the process and has an accuracy within 95% confidence limits of +36% to -26% which is good enough for many preliminary evaluations.
The method is based on a system in which a complexity score accounting for factors such as throughput, corrosion problems and reaction time is estimated for each process step, and these are combined to give an overall ‘costliness index’. The capital is then derived from a relationship between this index, capacity and capital cost which was based on experience ot 45 recent projects in the U.K., covering a broad spectrum of plant sizes and technology.
INTRODUCTION
Preliminary screening of the many alternative proposals that arise in R & D requires capital estimating methods that are quick, can be carried out with a minimum of information and yet are reasonably reliable. For most situations the traditional engineering methods of estimating capital are too cumbersome, since to achieve a reliable result, detailed flowsheeting and equipment sizing of all the main plant items is required, necessitating considerable chemical engineering effort. A more practical way of making a quick capital estimate for research decision purposes at an eady stage is to seek out a similar process of known capital cost and then make appropriate modifications for the differences in detail using various rules of thumb. This ‘direct analogy’ approach can be very effective but it requires experienced staff who have extensive and detailed knowledge of a wide variety of chemical processes and
their associated capital costs. The dissemination of detailed capital and process information, even within a company, is normally restricted for security reasons and the movement of staff poses problems in identifying a source of the necessary expertise. There is therefore a need for a quick, yet reasonably reliable, estimating method which incorporates the knowledge of a broad spectrum of achieved costs on past projects and yet can be used by relatively inexperienced people. The ‘Process Step Scoring’ method described here aims to fulfil this need. The basic principle of the ‘Process Step Scoring’ method is the postulation that the capital cost of a plant is related to a costhness index (I), expressing the complexity and nature of the chemistry involved in the process, and to its size by a relationship of the form: Capital cost = constant X costliness index X (capacity)P
where p is a fractional power.
The costliness Index, which is a unique function for a particular process, is determined by the expression: N I = c
(1)
cl.39
where IVis the number of ‘significant process steps’ (e.g. filter, react, distill and S is a ‘complexity score’ determined for each process step to take account of factors such as throughput, materials of construction, reaction time, temperature, pressure and multistreaming. Costliness indexes normally range from about 10 for a plant to make a simple chemical such as formaldehyde up to 200 or more for a plant for a complex drug or pesticide. In principle the ‘Process Step Scoring’ method relates capital cost directly to the chemistry involved in the process without considering engineering aspects on the type of equipment required. In this respect it differs from other published quick costing methods [ l-51.Another tignificant feature is that each process step is s:ored independently for throughput, corrosion etc. The relationship between capital, ‘costliness index’ and plant size was derived by carrying
out a regression analysis on 45 U.K. projects completed during the past 12 years. These cover a broad spectrum of the types of process encountered in the Chemical Industry. They include processes for organic solvents, intermediates and monomers, complex biologically active compounds, polymeric materials and inorganic materials. Most of the common chemical reactions are covered, for example, chlorination, fluorination, oxidation, hydrolysis, amination, hydrogenation, dimerisation, addition and cracking reactions. Plant sizes are evenly distributed over the range 300 to 250 000 ton/year. No extrapolations on the basis of assumed scale-up power factors were made; in fact an important finding from the work has been that the traditional 0.6 power factor for scale-up is too high for correlations of this type. (This is to be discussed in a subsequent communication.) DESCRIPTION
The method 1sexplained below step-by-step and is then exemplified by a worked sample. Step 1 /IDefine the process to be diagram like the one in Fig.
5 (WCYCLE3) t 4t
AQ SOLN OF BY PRODUCTS (EFFLL’ENT-TO WORL(S DISPOSAL “NIT)
t
C~IMPOUND”C~’
Fig. 1. Hypothetical flow diagram for the reaction: A+B-rCinAsohrentS,
by a
261 The diagram should show the mam process steps, relative throughputs (ton/ton product), temperatures, pressures, materials of construction, reaction times, multi-streaming and any special conditions such as explosion hazards. Step 2. List the significant process steps. The following unit operations/unit processes would usually be regarded as ‘significant process steps’: - Chemical reactions, such as chlorination, oxidation, nitration. - Neutralise or acidify (but not minor pH adjustment). - Storage/handling of a raw material, product, by-product, intermediate or recycle stream. Also effluents when these are not assumed to be discharged directly to outside the battery limits (e.g. to river or Works effluent disposal system). - Filter, screen or centrifuge. - Distil, evaporate, fractionate or strip. - Crystallise or precipitate. - Formulate. - Compress. -- Vaporise. - Dry or spray dry. - Mill or grind. - Scrub or absorb. - Pack into special small containers (not sacks or drums). - Dissolve, mix, slurry or blend when required as a specific pre- or post-treatment, e.g. dissolving before spray drying. (But not when an integral part of another process step such as, say, solvent extraction). - Quench (but not normal cooling of a reaction mixture). - Condense when used to separate a component from a gaseous stream containing inerts (but not for normal condensation in stills, quenches or reactors). - Phase separation of a reaction mixture (but not when part of a still or extraction system). - Extract or leach. The ‘si,gnificant process step’ refers to the operation that is performed on the material
flow and not to the equipment that is necessary. The fact, for example, that sevoral reactions or unit operations could be done in the same vessel is of no significance from the point of view of this method; similarly the need in some continuous processes for several reactors in series is ignored. This is because the method relates the number of process steps/complexity directly to the capital costs for the optimum process at the scale concerned. Therefore, it is most important when listing the significant process steps to disassociate one’s mind completely from the equipment that might be used. If any special process steps not covered above are encountered, such as electrolysis, fibre spinning and extrusion, these should be costed separately either by ‘direct analogy’ or an engineering estimate. Step 3. Score each ‘significant process step’ on throughput, materials of construction, temperature, pressure, multi-streaming, reaction (or storage) time and any special conditions, according to Table 1. Relative throughput: This is the totai (i) weight flow into the process step per unit weight of final product. Internal recycles such as those in stills or absorbers are ignored. The throughput score i I dsuatly dominant compared to those for the other factors. (ii) Materials of construction: For a single process step there can be several construction materials involved (e.g. ELMS vessel with Ti stirrer). The score is based on the dominant material (ELMS). Knowledge of the precise materials of construction required is not necessary; the score can be based on a subjective judgement as to the magnitude of the corrosion problem e.g. no problem - mild steel, score 0; major problem - titanium, score 3. (iii) Temperature: Score based on the temperature extreme. (iv) Pressure: Score based on the pressure extreme. With gas phase reactions, elevated pressures up to about 50 atms are assumed to have no effect on the cost of a process
Score -1 0.6
r 3
usb
20
0
US
Stoic 1 if a major problem.
-2 0.35
-3 0.2
--~-
2
1.7 5 2 -25 500 0.1 100 SSc, Keebusb RLusd,EbLUSe, PVC Nickel Uonel PbLUSg
-7.5 1100 0.01 50” ELMSf
3 9 14
5
s
200 Titanium Hastelloy
-125 1700
3
25 8
8
700 Precious metals Tantallum
2330
4
14 42
11
!SOO
5
23 69
6
40 120
I
8 67
110
9
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 IS 16 Cosfline=? i!+*: (I) a4 0.6 0.8 1 1.3 1.7 2.2 2.8 3.7 4.8 6.3 8.1 10.6 14 18 23 30 39 51 66 ___.. ----___--____________~__ ---~~ ------__ = Mild steel. cSS = Stainless steel. dRLUS = Rubber lined mild steel. eEbLMS = Ebonite lined mild steel. :v~LUS = oFor liquid phase reactions only. All others swe = 0. b Znamel lined mild steel. @bLUS = Lead Lined mild steel.
scorn (S)
Conversion of score to costliness index
,
Special conditions: (a! E*p!csic!!, da-tI odom o: tcxicity prblems. (b) Reactions in tluid beds. Score 1.
Multistreaming. No. of streams
Relative thm *’ put (t/t product) Reaction time in h (reaction, crystallisation, etc) Storage time in weeks Temperature extreme CC) Uin Temperature extreme CC) Max Pressure extreme (atm) Uin Pressure extreme (atm) Max Materials of construction
-___
Scoring for complexity of signiticant process steps ---.---.. . -.
TABLE 1
step: this is because the increased wall thickness is offset by reduced vessel size. Multistreaming: This might be considered necessary for reasons such as poor reliability, uncertainty of market growth or limitations on equipment size. Reaction time/storage time: These have different scales. The reaction time scale refers to liquid phase process steps. The cost of gas phase reactions are usually effected only slightly by the residence time; no scoring scale is therefore included for them. (vii) Special conditions: These could include (a) explosion, odour, dust or toxicity problems, (b) fractionation of materials of similar boiling points, (c) reactions in fluid bed+, (d) film evaporation and (e) tight specification. At present these are assessed on an empirical basis. Step 4. Convert the total score for each ‘significant process step’ into a costliness index using the conversion table (at foot of Table 1). For a single process step the costliness index (I) is given by: I = 1.3s
where S is the total score for the step. Thus a
DESIGN
Fig. 2. A graph of eq. 2.
CAPACITY
IN
TON / YEAR
includes storage) that would be needed for a completely new plant in the U.K. built up from the ground. The capacity is the design capacity, not the achieved capacity. An alternative ‘direct analogy’ type of approach not employing the above derived relationship is to compare the costliness index (I) with that (I,) of a similar process of known capital cost (C,) and capacity (A I); the required capital cost (C) bging given by: (3)
This has the advantage that it eliminates the personal factor in assessing the ‘significant process steps’ - one of the main difficulties in applying the method. Other possible defects in the method are also avoided, providing a closely similar process is used for the comparison. Step 7. Make allowances, where necessary, to cover inflation, off-site facilities and site development, use of existing structure and construction of plants overseas. (a) Inflation: Appropriate infiation from Jan 77 should be added using suitabie indices such as the one published regularly in this Jouma!. tb) Off-sites and site development: Off-site facilities includes for exampie workshops, supply of basic services and effluent treatment. Site development includes for example roads, railways and drainage. The extra capital expenditure required for these could u rider some circumstances add 50% or more to the capital. On the other hand it could be virtually zero if the plant is located on a suitable developed Works site with all the services available in ample supply. It is suggested that in preliminary estimates when there are usual’ o indications on the type of site that would be used, a notional allowance of 30% should be added.to cover off-sites and site developme..:. Cc) Use of existmg structure: With small plants
of a few hundred ton/year capacity making non-complex chemicals, use can frequently be made of an existing structure. This would reduce the capital by around 25%. With small plants, further savings are possible by using some existing equipment, manhandling of materials, less ngorous standards and general improvisation. Obviously the saving possible will vary considerably but a typical figure could be taken as 50% (including structure). Cd) Plants overseas: Capital costs for plants overseds can be substantially higher or lower than those in the U.K. and the current exchange rate is not therefore necessarily appropriate. Location indices have been discussed in this Journal. Warning on sca~eupr: The costliness index, I, can change on scale-up as a result of more multistreaming, or the addition of more process steps (e.g. to recover solvents). If I does change, extrapolation of the cost to other scales cannot be achieved by simple use of the scale exponent but I must be recalculated for the new stakes. EXAMPLE Suppose it is required to cost the process outlined in Fig. 1, for single stream plants of 2000 and 8000 ton/year and for an 8000 ton/
year plant with a two stream reaction stage: (a) Single stream plant of 2000 ton/year: Costliness index for 2000 ton/year plant = 2 1, asshown in Table 2. Using eq. 2 (or Fig. 2) the ‘battery limits’ capital for a 2000 ton/year plant at Jan 77 pricts is =21X42x1.31 (1000 &) =2.1.16x 10” Adding 30% as a notional allowance to cover off-sites and site development gives f. 1.50 X lo6 (Jan 77). (b) Single stream plant of 8000 ton/year: The costliness index is 2 1 as for the 2000 ton/ year plant. The ‘battery limits’ capital = 21 X 42 X 2.25 .’’ =E 1.98X 10b
TABLE 2 Calculation _-__
of costliness index for plocess in Fig. Throughput
MofC
1
Rractionl storage
Pressure/ temp.
Other
0
0 0 0 0
1 (toxic) 0 0 0 0
1 0 0 0
0 0 0 0
Total score
index
1 1 3 3%
1.3 1.3 2.2 2.5 0.8
7 4 0 4
6.3 2.8 1.0 2.8
-I
21.0
Adding 30% for off-sites, the capital = .E 2.57 X 10t (Jan 77) (c) 8000 ton/year plant with twin
about 5 process steps (N.B. the more steps the better). (ii) Modification or extension of axisting plants. (iii) Fully batch operated plants of abnormally high capacity (3000 ton/year or more). (iv) FuHy continuous plants of abnormally low capacity (500 ton/year or less). (vl Plants involving appreciable solids handling on the large scale (5000 + ton/year). (vi) Plants involving special operations such as electrolysis, fibre spinning, extrusion, when these are likely to represent a substantial proportion of the cost. ACCURACY The accuracy of the method, for a specified process definition, determined from the regression analysis, is as follows: Y5% Confidence limits Standard deviation
The method can be used for complete new plants of capacities ranging from 300 to 250 000 ton/year. However, at the current level of development, It cannot be used for the following: (i) Very simple plants involving less than
+36% and -26% 1.5%
This order of accuracy is generally adequate for the preliminary screening of R & D projects. If, for example, there are several proposed routes to a new compound, the method can be used to reduce these to a short list of two
which can then be evaluated in depth using ‘engineering type’ estimating methods. In the early stages of an R & D project when the process is poorly defined the ‘Process Step Scoring’ method can in many cases give a more reliable result than ‘engineering type’ estimates. This is because cursory assessment of the plant equipment required, often grossly underestimates the complexity of the process, thus leading to an estimated cost which is much too low. Errors of thi? type are less likely with the ‘Process Step Scoring’ method, because costs for a specified process step are determined by correlating directly with costs achieved in practice for steps of analogous complexity - thus analysis of the fine details of the step are already built into the method. It is also noteworthy that preliminary ‘engineering type’ estimates are essentially based on analogy, in that usually 20% of the cost (the main plant equipment items (it rlgorosly estimated and then the remaining 80% (foundations, structure, instrument, piping, design etc.) is estimated by multiplying by a factor based on experience. Though the ‘Process Step Scoring’ method gives reasonably satisfactory results for complete plants, it can lead to an answer which is grossly in error (e.g. by a factor of 2), if applied to a single process step. The factorial type of engineering methods can also give similarly inaccurate results when applied to a single step and deficiency in this respect should not be considered to invalidate the method. Satisfactory results are obtained with complete plants primarily because of the ‘swings and roundabouts’ effect. Accuracy will tend to be best for plants with a large number of process steps. In its present form the ‘Process Step Scoring’ method assumes that all process steps are equal in cost for given throughputs, temperatures etc. An attempt has been made to improve accuracy by employing different ‘. ghting factors with respect to reactions, separations and storage/handling. The improvement in accuracy obtained was only marginal (standard deviation from 15.2% to 14.1%), and probably
not significant. Also, surprisingly, the improved accuracy was obtained by giving more weight to storage/handling and less to reactions. The main source of error in estimating the capital for a specified plant is likely to arise from personal differences as to what constitutes a ‘significant process step’. Though the list of these given on page 261 is intended to be as fbll as possible, it is recognised that difficulties can still arise. However, unless the plant being considered only in rolves a few process steps, this ‘personal facto] ’ need not represent a sizeable error. SHORT-CUT METHODS
Sometimes there is not sufficient time or the data available to apply the ‘Process Step Scoring’ method as described. Accordingly, relationships have been derived for capital cost as a function Of:
the number of reactions, and the number of ‘significant process steps’. These short-cut but cruder relationships are outlined below: (i) Number of reactions:
(i)
(ii)
Capital in 1000 E = 1120 (Capacity in 1000 ton)“” Number of reactions
(6)
95% Confidence limits +155% Standard deviation = 52%
N.B. Subsidiary reactions such as pH adjustment are not taken as reactions. fii) Number of ‘significant process steps’ Capital in 1000 f = 115 (Capacity in 1000 tonjo*” Number of process steps
(7)
95% Confidence limits +96% and -49% Standard deviation = 44%
In some assessments of R & D projects an indication of the minimum likr’y capital cost is all that is necessary. In these circumstances the short-cut relationships can prove useful. For example, say it was known that a new product requiring 4 reactions stages could only be viable if the capital cost (battery limits) for
a i
000
ton/yearpiantWBS iP,:ISthan
E 1.5 mil-
lion. From eq. 6, the most likely capital cost is f. 4.5 million and from the 55% confidence limits there is only a 5% chance of the capital being less than 2 1.8 million. On this basis, it would be concluded that the new product was very unlikely to be an economic proposition.
3 Wilson, G.T., 1971. Capital lnrestment for Chemical Plant, British Chemical Engineering and Process Technology, 16(10): 931. 4 Allen, D.H. and Page, R.C., 1975. Revised ischniquo for Tredesigu Cost Estimating, Chemical Engineer&g, March 2. S Bridgwater, A.V.. 1974. Rapid Cost Estimation in the Chemical Process Industrits, Paper presented at 3rd International Cost Engineering Symposiuc, The A ,ocirtiotr of Cost Engineers.
ACKNOWLEDGEMENTS
BIOGRAPHICAL NOTE
The author is indebted for helpful discussions to many colleagues within ICI Ltd, particularly those in Mond Division’s Research Economics Section.
John Taylor is a chemist by training, with the degrees of R.Sc. and Ph.D. from Liverpool Un.versity. He is employed in the Research bi Development Department of ICI’s Mond Division. For uea-rly 10 years he has had the position of Research Economist, in which he is responsible for the economic evaluation of a range of the Division’s R&D projects. in this activity he has gained a Sroad spectrum of experience on the costs of chemical orocesses and the factors affecting them.
REFERENCES 1 Zevnik, F.C. and Iluchanan, ILL., 1963. GeneraIised Correlation of Process Investment, Chemical Engineering Progress, 59(2): 70. 2 Stallworthy, EA., 1970. The Viewpoint of a Large Chemical Manufacturing Company, The Chemicai Engineer, June. 182.