Economic Incentive for Intermittent Operation of Air Separation Plants

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Ind. Eng. Chem. Res. 2008, 47, 1132-1139

Economic Incentive for Intermittent Operation of Air Separation Plants with Variable Power Costs Jason Miller and William L. Luyben* Department of Chemical Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015

Stephane Blouin AdVanced Control and Operations Research, Process and Systems R&D, Praxair Inc., Tonawanda, New York 14151

This article presents a simplified economic analysis of the effect of hourly variations in power costs on energy and capital costs of cryogenic air separation plants. The objective is to see what ratio of peak-tominimum energy costs is required to make intermittently operated air separation plants economically attractive. The study focuses on super-staged argon-column air separation plants producing both gaseous and liquid oxygen and nitrogen, along with high-purity liquid argon. Preliminary results indicate that power price ratios between about 2 and 7 are required, depending on the process and economic assumptions. The analysis uses thermodynamic ideal-work calculations to predict energy requirements for plants that produce varying amounts of liquid and gaseous products. These products must be provided to customers even when the plant is not running, so excess liquid oxygen and nitrogen must be produced and stored during the period when the plant is running. The actual power consumption of the base 24-hour plant is used to calculate a thermodynamic efficiency (about 33%), which is then used for the modified plants. Capital costs for the conventional continuously operated 24-hour plant are assumed to be some multiple of the energy costs for the 24-hour plant evaluated at the minimum power price (Pmin). Capital costs for the 12-hour plant and the 8-hour plant are scaled up from the capital cost of the 24-hour plant. The analysis assumes a three 8-hour power-pricewindow cost structure. The economics are evaluated over a range of power price ratios. In the most optimistic case, when the annual capital cost for the 24-hour plant is assumed to be 30% of the total annual cost, the equipment cost is scaled using the conventional 0.6 power factor and instantaneous start-up is assumed, the 8-hour plant becomes economically viable at a power-price ratio of 2.13. When the instantaneous start-up assumption is removed and one assumes that only the minimum power price is observed on weekends, the 8-hour plant becomes economically viable at a power price ratio of 4.15. 1. Introduction The dominant operating cost in air separation is energy. The cost of electric energy can vary drastically from hour to hour during the day as industrial and domestic demands change with ambient temperature and activity. The ratio of the cost at the peak period of demand to the cost at the lowest period of demand can be quite large (greater than 3) depending on the location of the plant. One approach to reducing energy cost is intermittent plant operation: run at higher than normal throughputs when power is cheap (at night) and shut down when power is expensive. Of course, a prerequisite for intermittent operation is an agile process that can start up very quickly. In this preliminary analysis, we assume that oxygen, nitrogen, and argon product purities can be achieved in either 0 or 2 h. The analysis uses thermodynamic ideal-work calculations to predict energy requirements for plants that produce varying amounts of liquid and gaseous products. These products must be provided to customers even when the plant is not running, so excess liquid oxygen and nitrogen must be produced and stored during the period when the plant is running. The base plant operates continuously (24 h). Two other plants are considered in which the plants operate for 12 h (one-half of the time) or 8 h (one-third the time). The capacities of the 12-hour * To whom correspondence should be addressed. Tel.: 610-7584256. Fax: 610-758-5297. E-mail: [email protected].

and 8-hour plants must be twice and three-times, respectively, that of the 24-hour plant. The production of more liquid for storage when the plant is running to provide products when the plant is not running increases the power requirements. However, the cost of a unit of power is smaller because the plant is only operating during periods of less-expensive energy. The capital investments are increased using a scale-up factor. 2. Process Description A typical super-staged argon cryogenic air separation plant (Figure 1) includes a double distillation column with a side column to recover high-purity liquid argon. Feed air (from the atmosphere) is compressed and passed through an adsorbent bed of molecular sieves to remove water, carbon dioxide, acetylene, ethylene, butane, and other heavier hydrocarbons. This helps alleviate the potential dangers of hydrocarbonoxygen mixtures and prevents the freezing of material in the system. The feed-air stream is split, with a good portion being expanded in the lower column turbine, after being cooled in the primary heat exchanger against returning cold oxygen and nitrogen product streams, along with the waste nitrogen stream. The air expansion provides refrigeration for the plant. This stream provides vapor air feed to the high-pressure column. The air that is not expanded is also cooled in the primary heat exchanger and provides liquid air for both the high-pressure and low-pressure columns.

10.1021/ie070593n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008

Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1133

Figure 1. Simplified Schematic of Cryogenic Air Separation Plant. Table 1. Work and Efficiency Results for Base 24-Hour Plant stream type temperature (K) pressure (MPa) flowrate (F/F24) enthalpy (kJ/mol) entropy (kJ/mol‚K)

air feed 295.37 0.1 1.0000 -0.09 0.0044

GN2 product 310.35 0.9 0.0155 0.29 -0.0174

Wideal (kw) 4184

Wreal (kw) 12 474

η (%) 33

LN2 product 80.85 0.9 0.1219 -11.84 -0.1093

The lower column (high-pressure column) operates at approximately 85 psia (0.586 MPa) and separates the air into a high-purity nitrogen stream (top) and oxygen-enriched liquid stream (bottom). The nitrogen stream is referred to as the shelf transfer and the enriched oxygen stream is called the kettle transfer. The upper (low-pressure) column operates at approximately 20 psia (0.138 MPa) and produces high-purity nitrogen (top) and oxygen (bottom) product streams. The oxygen product from the bottom of the upper column typically contains 99.9% oxygen with the remainder being argon. The nitrogen product from the upper column typically contains ppm-level impurities of oxygen. The oxygen liquid-product stream is pumped to a higher pressure. A portion is vaporized in the primary heat exchanger and provides high-pressure gaseous oxygen product, and the remainder goes to the product oxygen liquid-storage tank. Reflux for both columns is generated at the top of the lower column (i.e., shelf transfer acts as reflux for the upper column). Additional reflux for the upper column is

waste N2 product 309.75 0.1 0.6436 0.33 -0.0004

LO2 product 128.05 1.5 0.1468 -10.67 -0.0903

GO2 product 308.95 1.4 0.0628 0.20 -0.0208

LAr product 96.05 0.1 0.0093 -10.48 -0.0953

provided by a liquid nitrogen-add stream, which is combined with the shelf transfer. The liquid nitrogen-add stream is provided by drawing from the liquid nitrogen storage tank or by recycling liquid from a nitrogen liquefier. A nitrogen liquefier includes a series of compression, expansion, and heat exchange equipment.1 Nitrogen vapor at the top of the lower column is condensed against boiling liquid oxygen in the bottom of the upper column by heat exchange in a reboiler-condenser. Argon boils between oxygen and nitrogen, which results in a peak argon composition in the lower portion of the upper column. A vapor side stream is drawn from the upper column near the argon peak and is fed to the argon column. The argon column produces a liquid argon product that is drawn a few stages from the top of the column. This argon product stream contains ppm-level impurities of oxygen and nitrogen. The product stream is drawn several stages from the top of the column to prevent too much nitrogen from entering the product argon stream. Reflux for the argon column

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Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008

with

Wideal ) ∆H - T0∆S )

(∑

T0

Figure 2. Simplified Flowsheet 24-Hour Plant.



is provided by heat exchange in the argon condenser between the vapor at the top of the argon column and the oxygenenriched liquid (kettle transfer) from the lower column. This stream, after expansion to the lower pressure, has a lower boiling point than the argon. The liquid from the bottom of the argon column is fed to the upper column. The oxygen-enriched liquid and vapor from the cold (boiling) side of the argon condenser are also fed to the upper column. 3. Preliminary Calculations The basic approach is to calculate the thermodynamic ideal work for each plant. The ideal work increases as the fraction of liquid products increases. The ideal work of separation of air into the three pure products at atmospheric conditions is 3

Widealmix ) -FairRT0

∑ xj ln xj j)1

(1)

where Fair ) air feed rate (mol/s) R ) 8.314 J/mol‚K xj ) mole fraction of component j in the air feed j ) 1 (nitrogen), j ) 2 (oxygen), and j ) 3 (argon) T0 ) 295.37 K In a more general sense, the ideal work is computed on the basis of changes in conditions between the feed and the products

)

PnHn - FairHair -

(∑

)

PnSn - FairSair (2)

where H ) enthalpy (J/mol) S ) entropy (J/mol‚K) T0 ) 295.37 K Pn ) molar flow rate of product n (mol/s) In the limiting case where vapor oxygen, nitrogen, and argon are produced at atmospheric conditions, the enthalpy change for the ideal plant is negligible and eqs 2 and 1 are equivalent. Thus, the ideal work of mixing described by eq 1 is inherently captured by eq 2. Using this approach, the ideal work for the base 24-hour plant was determined. All of the stream conditions, including pressure, temperature, and flow rate, were obtained from steady-state data for an actual operating plant. The enthalpies and entropies of each stream were obtained from Aspen Plus process modeling software using the Peng-Robinson equation of state.2 The inlet and outlet flow rates, enthalpies, and entropies were then used to determine the ideal work for the plant using eq 2. The power consumption for the plant compressors obtained from the plant data was used to determine the actual work for the plant. The ratio of the ideal work to the actual work is the thermodynamic efficiency (∼33%) for the plant. A summary of the stream conditions that were used, along with the ideal work, actual work, and efficiency, is given in Table 1. The flow rates given in Table 1 have been normalized with respect to the feed-air rate for the base 24-hour plant (F24). Using the thermodynamic efficiency that was determined for the base 24-hour plant, work calculations for the 8-hour and 12-hour plants were performed. As mentioned above, the 8-hour and 12-hour plants will be producing higher fractions of liquid oxygen and nitrogen while operating to meet vapor and liquid product demands when idle. Simplified schematics, which show the flow rates and work requirements for each plant, are given in Figures 2 -4. The flow rates in Figures 2-4 are described in relation to the feed-air rate for the base 24-hour plant (F24). It was assumed that the same stream pressures, temperatures, enthalpies, and entropies (Table 1) would exist in each plant. One can note that the daily energy consumption for the 8-hour plant is the highest, whereas that for the 24-hour plant is the lowest. This is caused by the 8-hour plant having the highest fraction of liquid products while running and the 24-hour plant having the lowest. Producing higher fractions of liquid inherently requires more work. 4. Economic Analysis 4.1. Assumptions. A. Process Assumptions. 1. The thermodynamic efficiency is the same for all of the plants. 2. The capacity and size of the 12-hour plant is twice that of the base-case 24-hour plant. The capacity and size of the 8-hour plant is three times that of the base-case 24-hour plant. 3. Enough liquid oxygen and nitrogen are produced and stored during the period when the plant is running to provide both oxygen and nitrogen products (gas and liquid) when the plant is not running. 4. The nitrogen liquefier is running whenever the plant is running.


5. Enthalpy and entropy data for feed and product streams are obtained from Aspen Plus process modeling software using the Peng-Robinson physical property package.2 6. Component mole balances perfectly close, with the composition of air assumed to be 78.11 mol % nitrogen, 0.93 mol % argon and 20.96 mol % oxygen. 7. Product streams are assumed to be pure. 8. Plants are assumed to start up instantaneously. This assumption will be relaxed in a later section of this article. B. Sizing and Economic Assumptions. 1. There are three 8-hour power-price windows during each 24 h day. The base-case 24-hour plant encounters the minimum power price for 8 h, the average of the maximum and minimum power price for 8 h, and the maximum power price for 8 h. The 12-hour plant encounters the minimum power price for 8 h and the average of the maximum and minimum power price for 4 h. The 8-hour plant encounters only the minimum power price when it is operating for 8 h. 2. The peak power price is some multiple of the minimum power price. The third power price is the average of the maximum and minimum power prices. The economics are evaluated over a range of Pmax/Pmin ratios. 3. The annual capital cost (capital investment divided by payback period) for the 24-hour plant is assumed to be some multiple (R) of the annual energy cost of the 24-hour plant evaluated at Pmin. The annual capital cost for the 12-hour plant is assumed to be some multiple (β) of the annual capital cost for the 24-hour plant. The annual capital cost for the 8-hour plant is assumed to be some multiple (κ) of the annual capital cost for the 24-hour plant. 4. Total annual costs (annual capital cost plus annual energy cost) for the three plants are calculated for different peak-tominimum power prices to assess at what ratio the intermittent plants become economically attractive. 4.2. Calculations. On the basis of the assumptions listed above, the following relationships can be developed to determine the annual costs for the 24-hour, 12-hour, and 8-hour plants:

ACC24 ) RW24Pmin

(3)

AEC24 ) (1/3)W24[Pmax + Pmin + 0.5(Pmax + Pmin)] (4) TAC24 ) AEC24 + ACC24 ) (1/3)W24[Pmax + Pmin + 0.5(Pmax + Pmin)] + RW24Pmin (5) ACC12 ) βACC24 ) βRW24Pmin

(6)

AEC12 ) W12[(2/3)Pmin + (1/6)(Pmax + Pmin)]

(7)

TAC12 ) AEC12 + ACC12 ) W12[(2/3)Pmin + (1/6)(Pmax + Pmin)] + βRW24Pmin (8) ACC8 ) κACC24 ) κRW24Pmin

(9)

AEC8 ) W8Pmin

(10)

TAC8 ) AEC8 + ACC8 ) W8Pmin + κRW24Pmin (11) where Pmin ) minimum power price ($/kwh) Pmax ) maximum power price ($/kwh) Wn ) yearly energy requirements for plant n (kwh/year), n ) 24, 12, or 8 ACCn ) annual capital cost for plant n ($/year), n ) 24, 12, or 8 AECn ) annual energy cost for plant n ($/year), n ) 24, 12, or 8

TACn ) total annual cost for plant n ($/year), n ) 24, 12, or 8

R ) ratio of ACC24/AEC24 evaluated at Pmin β ) ratio of ACC12/ACC24 κ ) ratio of ACC8/ACC24 By manipulating the above equations, one can obtain a general relationship for the critical power price ratio (Pmax/Pmin) at which TAC12 becomes lower than TAC24, TAC8 becomes lower than TAC24, and TAC8 becomes lower than TAC12. The critical power price ratios are functions of R, β, κ, W12, W24, and W8 and are given in eqs 12-14. Note that if the plants have the same efficiency, which has been assumed, the critical powerprice ratio is not a function of the efficiency, and therefore the ideal work can be used instead of the real work in eqs 12-14. However, the efficiency does impact the magnitude of the TAC of each plant.

( ) ( ) ( ) ( ) [ ( ) ] ( ) [( ) ( ) ] Pmax Pmin

Pmax Pmin

Pmax Pmin

5 W12 1 6 W24 2 1 1 W12 2 6 W24

(12)

W8 1 W24 2

(13)

R(β - 1) +

)

12-24

) 2 R(κ - 1) +

8-24

)6 R

8-12

W24 W8 5 (κ - β) + W12 W12 6

(14)

4.3. Results. A comparison of the TAC for the three plants as a function of the economic parameters R, β, and κ is given in Figure 5. The parameter values used in part a of Figure 5 are R ) 1 (i.e., ACC24 and AEC24 (at Pmin) are equal), β ) 2 (i.e., ACC12 ) 2ACC24), and κ ) 3 (i.e., ACC8 ) 3ACC24). A value of R ) 1, which corresponds to the annual capital cost equaling the annual energy cost, has been derived from an article by Castle.3 However, it is unclear if the author uses the exact definition for R described in this article and under what power price structure the value was determined. Nevertheless, a 50/ 50 split between annual capital and energy costs seems like a logical starting point. Other values of R are explored below. Likewise, direct scaling of the annual capital cost to the plant capacity (i.e., β ) 2 and κ ) 3), which would be the case if parallel units were used, seems to be a reasonable starting point. Other scale-up relationships are explored below. With these economic parameters, the 8-hour plant begins to yield the lowest TAC at a power price ratio of about 6.78. All of the TAC plots assume a minimum power price of $0.03/kwh. A second value for R (0.43) has been derived from a article by Scharle and Wilson.4 With this value, 70% of the TAC would be attributed to energy. Again, it is unclear if the authors use the exact definition for R described in this article and under what power price structure the value was determined. Also, the plants described by Scharle and Wilson did not include argon production. However, it seems quite possible that an energyintensive process such a cryogenic air separation could have an annual energy cost that is higher than the annual capital cost. From part b of Figure 5, one can observe that changing R from 1 to 0.43 reduces the critical power price ratio at which the TAC curves for the 8-hour and 12-hour plants intersect to a value of 3.55. The economics are also improved (part c of Figure 5) if one assumes that capital costs scale up by the ratio of the capacities to the power of 0.6 (i.e., β ) 20.6 and κ ) 30.6), as is conventionally used (see Peters, Timmerhaus, and West).5 In

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Figure 5. TAC Analysis - Instantaneous Start Up. Table 2. Cost Breakdown - Most Optimistic Case

Pmax/Pmin

ACC 8-hour plant (MM$/yr)

AEC 8-hour plant (MM$/yr)

TAC 8-hour plant (MM$/yr)







1.0 1.5 2.0 2.5 3.0

2.73 2.73 2.73 2.73 2.73

3.53 3.53 3.53 3.53 3.53

6.26 6.26 6.26 6.26 6.26

2.14 2.14 2.14 2.14 2.14

3.47 3.76 4.05 4.34 4.63

5.61 5.90 6.19 6.47 6.76

1.41 1.41 1.41 1.41 1.41

3.28 4.10 4.92 5.74 6.56

4.69 5.51 6.33 7.15 7.97

this case, the critical power price ratio is about 3.48. Thus, a similar change in the critical power price ratio is observed by adjusting β and κ, as was observed by adjusting R. Under the most favorable conditions (part d of Figure 5), when the annual capital cost for the 24-hour plant is assumed to be 30% of the total annual cost and the equipment cost is scaled using the 0.6 power factor, the critical power price ratio is about 2.13. A breakdown of the annual capital and energy costs for the most optimistic case is shown in Table 2. 4.3.1. Effect of Startup Time. Thus far, we have assumed that the plants start up instantaneously. However, it typically takes about 2 h for the oxygen and nitrogen product streams to reach the desired product purities. Thus, there is a period of time for the 8- and 12-hour plants when no product is being drawn. To meet the product demands, the 8- and 12-hour plants would have to have higher throughputs than those described in Figures 3 and 4 or run for 10 and 14 h per day, respectively. If one assumes that the plants run for 10 and 14 h, the extra 2 h of operation for both the 8-hour and 12-hour plants fall in the Pavg power price period. Thus, in contrast to Figure 5 where

instantaneous start-up was assumed, the TAC for the 8-hour plant becomes a function of the power price ratio (part b of Figure 6). Using the most optimistic economic parameters (R ) 0.43, β ) 20.6, and κ ) 30.6) and assuming a 2 h start-up time, the critical power ratio is about 3.25 compared to 2.13 for instantaneous start-up. Alternatively, the capacities of the 8-hour and 12-hour plants could be increased so that the required products could be produced in 6 and 10 h of operation, respectively, during which products are produced from the columns. With this mode of operation, the critical power price ratio is about 3.02 (part c of Figure 6). Because the 24-hour plant runs continuously with no daily start up, the TAC does not change when the start-up time is included in the analysis (compare parts a, b, and c of Figure 6). One should also keep in mind that it takes about 10 h for the product argon to reach the desired purity given the current plant design. In this analysis, it has been assumed that it takes only 2 h. In a recent paper,7 we will discuss some design and control changes that can be implemented to reduce the time required to achieve argon product purity to about the time required for that of oxygen and nitrogen.


Figure 6. TAC Analysis - Effect of Start-Up Time and Weekend Prices.

4.3.2. Effect of Weekends. Thus far, it has also been assumed that three 8 h power price windows are observed each day of the week. However, this is typically not the case on weekends, where only Pmin is observed. When this is taken into account, the critical power price ratio assuming a 2 h start-up period rises to about 4.15 (part d of Figure 6). In this case, we have assumed that the 8-hour and 12-hour plants would operate for an extra 2 h per day instead of increase in capacity. 4.3.3. Example Using Actual Historic Power Price Data. Throughout this article, we have assumed a power price structure having three 8 h windows, with the price encountered in the second 8 h window being the average of the first (Pmin) and third (Pmax). However, actual energy prices will vary from hour to hour and from location to location.6 In this section, we present a concrete example of how this actual variability affects the economic analysis. Actual power price data was obtained from PJM Interconnection8 for the Baltimore and central Maryland region (Baltimore Gas and Electric Company). The data set contains hourly power price data for the entire year of 2006. The average hourly power price over the entire year can be observed in Figure 7. The average power price for the entire year, which determines the annual energy cost for the 24-hour plant, was $0.0574/kWh. The least expensive 12 h block of time, which is when one would operate the 12-hour plant, occurred between 10:00 p.m. and 10:00 a.m. During this time the average power price was $0.0448/kWh. The least expensive 8-hour block of time, which is when one would operate the 8-hour plant, occurred between 10:00 pm and 6:00 a.m., with an average power price of $0.0381/kWh. Thus, the annual capital cost for the 24-hour plant, which is based on the minimum power price (section 4.2), will use $0.0381/kWh for Pmin.

The second 8-hour block of time had an average power price of $0.0626/kWh, while the last 8-hour block had an average power price of $0.0715/kWh (i.e., Pmax). Thus, for this set of data, the power price observed in the second 8-hour block is not the average of Pmax and Pmin. Given the above power price data and assuming instantaneous start-up along with the most favorable economic parameters (R ) 0.43, β ) 20.6, κ ) 30.6), the lowest TAC is obtained with the 12-hour plant (7.90 × 106 $/yr), followed by the 8-hour plant (7.95 × 106 $/yr), and finally the 24-hour plant (8.06 × 106 $/yr). In relation to the base 24-hour plant, the incremental ROI eq 15, assuming a 4-year payback period, of the 12-hour plant is approximately 4.4%, whereas that of the 8-hour plant is 1.7%. Thus, the modified plant designs with intermittent operation are not economically attractive given the 2006 powerprice data for the Baltimore area. However, as mentioned above, power-price variability changes from location to location and there are most likely regions where the economics are more favorable. The economics have to be evaluated separately for each particular location of interest.

incROI ) 100

(

TAC24 - TACN

t (ACCN - ACC24) *

)

(15)

where: incROI ) incremental return on investment (%) TAC24 ) total annual cost of base 24-hour plant ($/yr) TACN ) total annual cost of modified plant with N ) 8 or 12 ($/yr) ACC24 ) annual capital cost of base 24-hour plant ($/yr) ACCN ) annual capital cost of modified plant with N ) 8 or 12 ($/yr) t* ) payback period (years)

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Figure 7. Average Hourly Power Price - Baltimore, MD 2006.

Figure 8. Effect of Fraction of Liquid Products on Critical Power Price Ratio.

4.3.4. Effect of Higher Fraction of Liquid Products. The product distribution given in Table 1 shows that 70% of the product oxygen and 89% of the product nitrogen are sold in the liquid phase. However, depending on demand, the plant may be designed to produce higher or lower fractions of liquid products. If one assumes that all of the product oxygen and nitrogen is sold in the liquid phase, the critical power price ratio is about 2.08 (Figure 8). If one as sumes that all of the oxygen and nitrogen is sold in the vapor

phase, the critical power price ratio rises to 3.2 (Figure 8). As above, we have assumed that all of the plants have the same thermodynamic efficiency. As one would expect, plants that supply larger fractions of liquid products are more amenable to intermittent operation. The results shown in Figure 8 assume instantaneous start-up and the most favorable economic parameters. Here, we define the critical power price ratio as the smallest power price ratio at which the lowest TAC is that of the 8-hour plant.


5. Conclusions In conclusion, this article discusses the use of thermodynamic ideal-work calculations to predict energy requirements for cryogenic air separation plants that produce varying amounts of liquid and gaseous products. The work calculations were used to perform a simplified economic analysis to determine the effect of hourly variations in power costs on energy and capital costs. The objective has been to see what ratio of peak-to-minimum energy costs is required to make intermittently operated air separation plants economically attractive. Preliminary results indicate that power price ratios between about 2 and 7 are required, depending on the process and economic assumptions. Clearly, the TAC for the given plants is highly dependent on the economic assumptions that are made. However, the general approach described in this article is generic and can be applied to specific design cases. For example, one needs to consider the plant’s desired liquid-to-vapor product splits. As shown above, a plant producing a higher fraction of products as liquids would be more amenable to intermittent operation. One would also need to consider the location of the plant as the power prices may change more frequently and to a greater extent in different regions. Also, intermittent operation does not necessarily require modified plants that can start up faster. If existing plants are running below capacity, production rates can be increased during periods when power prices are at a minimum and the plants shut down when power prices are at a maximum. 6. Extensions and Future Work The cases in this article have all assumed that the plants are capable of rapid start-up. In the least favorable scenario, it was assumed that oxygen, nitrogen, and argon all take about 2 h to reach the desired product purities. Given the existing plant design, oxygen and nitrogen do reach product purity in about 2 h, but argon takes approximately 10 h. Thus, intermittent operation would probably not be economically viable because the 8-hour and 12-hour plants would produce significantly less

argon than the continuously operated 24-hour plant. In a recent paper,7 we discuss methods for reducing the time required to achieve the desired argon product purity. The methods involve both design and control changes. The design changes involve the addition of storage tanks to collect argon-column liquid during shutdown and the reintroduction of the collected liquid during the subsequent start up.9,10 Thus, the extra capital required to build and operate the agile plant will have to be included in the economic analysis. Literature Cited (1) Olszewski, W. J.; Union Carbide Corporation. Gas Liquefaction Process and Apparatus. U.S. Patent 3,677,019, July 18, 1972. (2) Peng, D. Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15, 59-64. (3) Castle, W. F. Air Separation and Liquefaction: Recent Developments and Prospects for the Beginning of the New Millennium. Int. J. Refrig. 2002, 25, 158-172. (4) Scharle, W. J.; Wilson, K. Oxygen Facilities for Synthetic Fuel Projects. J. Eng. Ind. 1981, 103. (5) Peters, M. S.; Timmerhaus, K. D; West, R. E. Plant Design and Economics for Chemical Engineers, 5th ed.; McGraw-Hill, Inc.: New York, 2002; p 242. (6) Miller, J.; Luyben, W. L.; Belanger, P.; Blouin, S.; Megan, L. Improving Agility of Cryogenic Air Separation Plants. Ind. Eng. Chem. Res. 2008, 47, 394-404. (7) Li, Y.; Flynn, P. C. Power Price in Deregulated Markets. Power Engineering Society General Meeting, 2003; Vol. 2, pp 874-879 (8) PJM Interconnection website, www.pjm.com. (9) Billingham, J. F.; Bonaquist, D. P.; Dray, J. R.; Lockett, M. J.; Beddome, R. A.; Praxair Technology, Inc. Rapid Restart System for Cryogenic Air Separation Plant. U.S. Patent 6,272,884 B1, August 14, 2001. (10) Smith IV, O. J.; Espie, D. M. Air Products and Chemicals, Inc. Recirculation of Argon Sidearm Column for Fast Response. U.S. Patent 6,070,433, June 6, 2000.

ReceiVed for reView April 26, 2007 ReVised manuscript receiVed November 15, 2007 Accepted November 19, 2007 IE070593N

Economic Incentive for Intermittent Operation of Air Separation Plants

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