WASTE MINIMIZATION THROUGH IMPROVED PROCESS THERMODYNAMICS: CRUDE OIL FRACTIONATION by David B. Manley The University of Missouri, Rolla, Missouri

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1 WASTE MINIMIZATION THROUGH IMPROVED PROCESS THERMODYNAMICS: CRUDE OIL FRACTIONATION by David B. Manley The University of Missouri, Rolla, Missouri INTRODUCTION Crude oil distillation is the first unit encountered in virtually every refinery. Figure 1 shows a typical refinery flow diagram including a crude unit containing a atmospheric section, a vacuum section, and a stabilizer section 3. Figure 2 shows the crude fractionation unit in more detail including in this particular case two stages of vacuum distillation 6. Fourteen million barrels per day of crude oil are currently processed in the United States 1. About seven million barrels per day are imported 1. About seventy million barrels per day are processed worldwide 31. Conventional atmospheric and vacuum crude distillation units require about 100,000 BTUs (British Thermal Units) per barrel processed for furnace energy to accomplish these separations 2. Since a barrel of oil contains about 6,200,000 BTUs of energy 3, this required furnace energy amounts to the energy content of about 1.6 percent of the processed oil. At current oil

2 prices of about $15 per barrel 1 the energy cost of processing is then $0.24 per barrel. As shown in table 1 the annual energy cost for the United States for this purpose is about $1.24 billion, and worldwide the annual cost is about $6.18 billion. Furnace effluents are a major contribution to refinery waste and consequent environmental pollution 6 depending primarily on the type of fuel used (high sulfur fuel oil, low sulfur fuel oil, natural gas, etc.). There are a number of ways to minimize this

3 waste by incremental process improvements and post process waste treatment and disposal such as stack gas scrubbing 7. Even when using relatively clean fuels and post process waste treatment furnace effluents are still significant 7. Regulatory efforts to quantify the environmental impact of these effluents have led to the concept of "cost of waste 5,8,9." Waste costs from Nevada regulations 9 for furnace effluents (CO 2, SO x, NO x, TSP, VOCs, and CO) from conventional and "clean" crude distillation units 7 are given in table 2 resulting in national costs ranging from a maximum of $2.698 billion per year (conventional process with high sulfur fuel oil) to a potential minimum of $0.808 billion per year ("clean" process with natural gas fuel). The sulfur content of the fuel is a major contribution which can be reduced by preprocessing or changing the fuel. The major contribution from carbon dioxide is a consequence from using fossil energy. Since about one half of the crude oil processed in the United States is imported, the $1.24 billion per year energy cost for crude oil distillation significantly effects the national balance of payments, with important consequences to the national economic competitiveness in the world economy. Obviously, a significant increase in oil price such as occurred in 1973 and 1979 would have a major impact on this cost. In addition, the strategic military implications of reliance on importation of a critical commodity such as crude oil are virtually incalculable as evidenced by the recent Gulf war. The energy efficiency of crude oil processing is consequently a significant factor in the United States' internal economy, environmental cleanliness, international industrial competitiveness, and national security. The first and second laws of thermodynamics allow the minimum

4 possible furnace energy required for crude oil distillation to be calculated using the concepts of "reversible thermodynamics 10 " and a computer simulation of a typical crude unit including "pseudocomponents" to estimate thermodynamic properties 11. As shown if figure 3 the relevant equations yield a theoretical value of about 3,000 BTUs per barrel for a minimum furnace temperature of 700 o F. About 3% of the conventional process value. This maximum energy efficiency can never be completely achieved in practice, but it serves as a target for potential process improvements. It has been shown 12,13 theoretically that any multicomponent distillation process, such as crude oil distillation, can be designed to approach thermodynamic reversibility by using the modern technological concepts of distributed distillation, pinch technology, and high efficiency column internals to address mass, heat, and momentum transfer inefficiencies respectively 14,15. If a large reduction in furnace energy requirements could be commercially implemented throughout the United States, then significant improvements in the worthwhile objectives of economic competitiveness, environmental protection, and national security could be achieved. REVIEW OF PROCESSES Many crude distillation processes were proposed in the early 1900s 19 before the industry settled on the currently used conventional pipestill unit 20. Conventional sharp split distillation of crude oil has been commercialized 4 in both a "direct" sequence of distillation columns as shown in figure 4 and in an "indirect" sequence of distillation columns as shown in figure 5. The direct sequence or "shell still" process has the

5 advantage that each product is vaporized from the crude oil at the minimum temperature thus minimizing heating; but, because the products must be fractionated to meet specifications, considerable refluxing and remixing with the crude oil occurs thus requiring revaporization at higher temperatures. The indirect sequence or "pipe still" process virtually eliminates the refluxing and remixing but requires heating the entire charge

6 to the maximum temperature in order to vaporize all the products before fractionation. The entire charge must then be cooled from the maximum process temperature. Although combinations of these flowsheets are used, they all retain some of the disadvantages of each because conventional "sharp split" distillation is used. Use of a preflash tower 22 to bypass a light fraction around the furnace helps to improve efficiency. Another inefficiency is removal of all the vacuum gas oils at the lowest pressure (highest vacuum). Use of two vacuum towers 23 to remove the light vacuum gas oils at an intermediate vacuum as shown in figure 2 helps to reduce this inefficiency. Each of these innovations increases the complexity and cost of the process, but incrementally improves the energy efficiency. Figure 6 shows a flowsheet of a modern atmospheric pipe still, indirect sequence, which has been mechanically integrated into two column vessels 4. In figure 6 the cold crude oil is preheated by heat exchange with the product streams and with the products 4 from the vacuum unit as shown in figure 7. The heat integration of the crude charge preheat train can be optimized using "PINCH" technology 14,17,21 and typically can reduce furnace heat by about 35% to around 65 MBTU/BBL at the expense of increased heat exchanger area.

7 Theoretically 12,13 a completely distributed distillation design as shown in figure 8 for ethylene recovery 24 will achieve the highest energy efficiency and the smallest size unit. In a completely distributed distillation sequence the most volatile and least volatile products entering each column are separated. As this strategy propagates through the process the "christmas tree" of distillation columns in figure 8 is generated. In contrast to the conventional direct sequence of figure 4, in the distributed design refluxing and remixing of vaporized products into the heavy crude is minimized; and, in contrast to the conventional indirect sequence of figure 5, in the distributed sequence each product is heated to the minimum possible temperature. Appropriate procedures for the design of a distributed distillation unit have been developed by the author 15 and others 24,25, but are not published in general. One key element in distributed distillation design is the efficient implementation of interstage heat exchange through the use of dephlegmators 26 such as used in ethylene recovery 28. Conventional interexchangers can also be used. A second key element in distributed distillation design is the opportunity for the mechanical consolidation of columns through the use of internal partitions 29 which have been commercialized in Europe 30. Using these technologies the typical process composite heating and cooling curves (from PINCH technology 14 ) can be respectively lowered and raised in temperature by significant amounts. By then redesigning the heat exchanger network large energy and capital reductions can be achieved because more energy is reused as it cascades through the process. Capital savings are enhanced by the mechanical consolidations of equipment.

8 The design procedures for thermodynamically reversible multicomponent distillation processes have been developed for cryogenic processes because of the strong economic incentive generated by the high cost of refrigeration utilities. These same

9 procedures can be productively applied to above ambient temperature processes such as encountered in the refining industry and specifically to crude oil distillation to save furnace energy and capital costs. In ethylene recovery, for example, the main processing problem is the removal of valuable condensable products such as ethylene and propylene from noncondensable gases such as hydrogen and methane. Because the noncondensable gases dramatically lower the condensation temperature of ethylene and propylene, high pressures (550 psia) and low temperatures (-215 o F) are required for recovery with associated high capital and utility costs. In crude oil distillation, by analogy, the main processing problem is the removal of valuable volatile products such as gasoline and naphtha from nonvolatile liquids such as gas oils and asphalt. Because the nonvolatile liquids dramatically raise the boiling temperature of gasoline and naphtha, low pressures (25 mmhg) and high temperatures (850 o F) are required for recovery with associated high capital and utility costs. The two processes are thermodynamically mirror images of each other, and the same processing concepts can be used to improve both. A "first order" distributed distillation process as shown in figure 9 is appropriate for crude oil processing. To reduce complexity with minimal impact on thermodynamic reversibility the large number of prefractionators present in the completely distributed process of figure 8 is truncated. The prefractionation is "first order" because only one product is distributed in each column of the prefractionation train. For example, the first column which separates LPG from naphtha distributes gasoline between the overhead and bottoms (a gasoline distributor column). The overhead from the gasoline distributor column is further distilled in a stabilizer (LPG column) to remove LPG from gasoline. The bottoms from the gasoline distributor column is further distilled in a prefractionator which separates gasoline from kerosine and distributes naphtha (a naphtha distributor column). The procedure is then repeated as the heavier fractions are removed at progressively lower pressures. Because the prefractionation columns are designed to separate relatively wide boiling fractions they are very lightly refluxed and are consequently relatively thermodynamically efficient. The first order distributed distillation process for crude oil fractionation is still relatively complex and requires a large number of columns and heat exchangers.

10 However, if the hot products are used to supply heat to the prefractionator reboilers, then this process is relatively efficient and requires about 50,000 BTU/BBL processed. It is quite similar to that proposed by Elf 18 and shown in figure 10. Elf Aquitaine of Paris, France has proposed a "Progressive Separation Scheme for Crude Oil Distillation 16,17,18 " which uses some of the concepts of distributed distillation and which achieves a furnace energy requirement of about 50,000 BTUs per barrel of oil. About one half of the value for a conventional

11 process. In addition, the Elf authors claim some reduction in the capital cost of the plant. However, the Elf process as published does not include many of the above mentioned modern technological concepts for reversible multicomponent distillation. Using these concepts and practical design practices 3,4 the author has simulated further improved processes achieving furnace energy requirements as low as 20,000 BTUs per barrel which approaches the theoretical limit of 3,000 BTUs per barrel. As discussed above furnace energy for a conventional process is about 100 MBTU/BBL of capacity. The 1993 capital cost for a unit 1,2 of this kind is about $750 per barrel per day of capacity. Improvements in heat integration of the crude preheat train can reduce the furnace energy to about 65 MBTU/BBL of capacity 7,14,17,21. The major inefficiency of this conventional process is the heating of all the crude charge including the LPG and gasoline to a maximum temperature of about 700 o F. The Elf process 18 achieves its energy efficiency at the expense of greater complexity by using more prefractionation and more pressure levels. However, the estimated 1993 capital cost of a new Elf progressive distillation crude unit 1,16 is about $600 per barrel per day of capacity. This reduction in capital cost over a conventional unit

12 is debatable, but ostensibly the reductions in equipment size outweigh the increased complexity costs. The first order distributed distillation process can be further improved through thermal integration as shown in figure 11. To reduce pressure drop and the number of heat exchangers, the prefractionation columns can be refluxed from the final fractionators; and two pairs of final fractionators can be thermally coupled in a multieffect arrangement. This directly reuses the thermal energy in the bottom column to reboil the upper column and also contributes to the thermal efficiency of the process. Interreboilers can also be used to improve the heat integration.

13 Results from thermally integrating a distributed distillation process for crude oil fractionation are shown by the before and after process composite heating and cooling curves in figure 12 which shows how the heating and cooling curves are moved apart through improved process design. Table 3 shows resulting furnace duty, total heat exchange, and total column volume for a conventional process, a conventional process with maximum heat integration, a distributed distillation process, and a distributed distillation process with maximum thermal integration. For the conventional process significant reductions in furnace duty and total heat exchange can be accomplished through increased heat integration, but the economic benefits are mitigated by the reduced temperature driving force for heat exchange which increases heat exchange area. The total volume of the columns is unchanged. For the advanced distillation design using distributed distillation alone the furnace duty, total heat exchange, and column volume are all significantly reduced in comparison with the conventional case without decreasing the temperature driving force. If in addition the temperature driving forces are reduced through increased heat integration a large further reduction in furnace duty can be achieved at the expense of increased heat exchanger area.

14 The thermally coupled first order distributed distillation process of figure 11 requires less capital and energy than the thermally unintegrated process of figure 9, but still has a relatively large number of pieces of equipment requiring considerable pumping, piping and operating maintenance. The thermally integrated first order distributed process can also be mechanically integrated as shown in figure 13. This retains the high thermal efficiency of the thermally integrated process, but reduces capital and operating expense by consolidating columns of equal pressure in single shells through the use of partitions 30. Mechanically integrated columns such as these are in use in Europe 31, and have been observed by the author; but they are not yet used in the United States. The thermally and mechanically integrated first order distributed distillation process is not unique as additional prefraction ("second order" distributed distillation), additional pressure levels, and additional heat integration could have been used. As additional complexity is introduced the thermal efficiency improves to approach the theoretical limit of 3,000 BTU/BBL, but at some point the capital and operating cost of the complexity outweighs the savings due to thermodynamic efficiency. Having simulated and analyzed a number of these configurations, it is the author's opinion that the optimum design for a grassroots unit will achieve about 35,000 BTU/BBL furnace energy at about $500/BBL 1993 capital investment. This is a 65% reduction in energy and a 33% reduction in capital as compared to a conventional design.

15 DISCUSSION There are a number of technical reservations regarding the distributed distillation processes for crude oil. First, the simulations and designs are based on a "pseudocomponent" characterization of the crude oil which is only approximately correct and does not account for decomposition reactions, unidentified components, exchanger fouling, and corrosion effects. In addition the simulations are based on the equilibrium stage simplification for distillation which is only approximately correct especially for an extremely complex mixture such as crude oil. Second, the control strategies for distributed distillation are not well developed 32,33. In a first order distributed distillation design each product specification must be met separately in more than one separate distillation column. In figure 9 for example, gasoline loss must be controlled not only in the top of the LPG column and the bottom of the gasoline column, but also in the bottom of the second prefractionation (naphtha distributor) column. While making control more difficult this flexibility improves the ability of the process to handle different feedstocks and also increases the degree of fractionation between the products. The improved fractionation

16 can be used to increase product yields 23 which significantly improves plant profitability. Finally, the mechanical partitioning while conceptually reasonable requires considerable hardware development and testing. There are also economic reservations regarding the distributed distillation processes for crude oil. Because crude oil processing is not increasing in the United States there is very little demand for new units, and replacement of an existing unit no matter how inefficient is very difficult to justify based on projected energy savings alone. Net present values for replacement of a conventional process with either the Elf or a new advanced process are given in table 4 as calculated from the formula: NPV = I + 1 S ( 1+ r r S r(1 + r) t ) where I = capital cost S = operating savings r = interest rate / cost of capital t = project life In both cases negative net present values are calculated, although the new process results are marginal. However, if environmental effects (cost of waste) are included, then the economics are considerably improved and yield respectably large

17 positive net present values. Incorporation of these incentives will require government legislation as they are not native in a free market economy. Also, significant economic incentive must exist to mitigate the risk of commercializing a new process. Of course, any increase in energy prices would have a dramatic and direct impact on the economic evaluations. Also, if the existing unit were so physically deteriorated that it must be replaced anyway, then the proposed new process would be the economic choice. Another alternative is to retrofit existing units for improved thermal efficiency at minimal capital investment. This may prove viable especially if one unit can be incrementally expanded to replace two existing smaller units. The distributed distillation process is especially adaptable to this service because the additional columns can be added incrementally to the existing process. Partitions can also be installed in existing columns with minimal modifications to the superstructure. And existing exchangers can be reused in the modified process. However, each revamp situation is unique and requires an individual analysis to evaluate. Outside the United States there is a significant market for crude oil distillation technology, and this may be of value to the engineering and construction industry in addition to the international oil companies. Crude oil consumption is projected to increase by about one and one half percent per year for several decades 31, and this will generate demand for about 1000 MBPD of new refining capacity worldwide each year. At $600/BPD capital cost this amounts to $600MM capital investment each year for crude and vacuum units alone. If United States technology is used in these refineries then the royalties on this technology can potentially contribute to the national economy. The author has informally attempted to interest a number of oil companies and engineering companies in developing the proposed process with little success. Typical reactions are "How do I know it will work?" or "Nobody will buy it." or "The (perceived) capital cost is too high." or numerous technical reservations regarding engineering decisions. Fundamentally, without considering environmental or strategic costs, the economic incentive in the United States is just not sufficient to justify the risk and cost of development and demonstration; and at present there is no accepted mechanism for incorporating the relevant environmental or strategic costs into the economic equations. In addition, unless a technology is proven, it is usually not even considered as an available alternative for use in possible regulation 7. CONCLUSIONS Crude oil distillation is a large energy consumer and a large waste generator, but through the use of improved process thermodynamics significant reductions in energy consumption and

18 waste generation can be achieved. However, the improved process designs are unproven, projected capital cost reductions are uncertain, and energy savings at current prices will not justify the investment needed for process development and commercialization in the United States where refining capacity is not increasing. However, depending on the accepted economic cost of waste generation, combined energy and waste savings may be shown to provide sufficient incentive for implementation. REFERENCES 1 Statistics, Oil & Gas Journal, Pages 75-77, December 6, Foster Wheeler USA Corp., Crude Distillation and Vacuum Flasher, Hydrocarbon Processing, Page 111, September, Gary, J. H. and G. E. Handwerk, Petroleum Refining Technology and Economics, 2nd Edition, Marcel Dekker Inc., Appendix B.3, Pages , Nelson, W. L., Petroleum Refinery Engineering, 4th Edition, McGraw-Hill Book Company, Page 423, Iyengar, S., Air Emissions: Vermont Utility to Consider Cost of Pollution, Burlington Free Press, November 30, Sittig, M., Petroleum Refining Industry Energy Saving and Environmental Control, Noyes Data Corporation, Cindric, D. T., B. Klein, A. R. Gentry, and H. M. Gomaa, Reduce Crude Unit Pollution With These Technologies, Hydrocarbon Processing, Pages 45-54, August, Opting Into the Acid Rain Program: Proposed Rule, Federal Register, Environmental Protection Agency, Part II, 40 CFR Part 72, et al., Friday, September 24, Final Rule for Docket No , Public Service Commission of Nevada, Table 2, January 22, Smith, J. M., and H. C. Van Ness, Introduction to Chemical Engineering Thermodynamics, 4th Edition, McGraw-Hill Publishing Company, Chapter 16, Pages , Atmospheric Crude and Vacuum Towers, HYSIM Special Features and Applications Guide, Version C2.10, Hyprotech Ltd., Calgary, Canada, Pages R1-R19, January, Petyluk, F. B., V. M. Platonov and D. M. Slavinskii, Thermodynamically Optimal Method for Separating Multicomponent Mixtures, International Chemical Engineering, Volume 5, Number 3, Pages , July, 1965.

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