Distillation: processes – separatory – With measuring – testing or inspecting
Reexamination Certificate
1997-02-14
2003-08-12
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
With measuring, testing or inspecting
C203S002000, C203S003000, C203S100000, C203SDIG004, C202S160000, C202S206000
Reexamination Certificate
active
06605190
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to minimizing energy consumption in staged externally-controlled systems.
Staged externally-controlled processes where temperature is the control include distillation processes and chemical reactor processes. Other staged processes can be pressure controlled, such as membrane separation processes like isotope diffusion processes and reverse osmosis processes. Further, staged mechanical controls can apply to staged centrifugation processes.
At the present time, distillation processes account for more than 10% of industrial energy consumption in the United States. Any significant improvement in the efficiency of such processes would result in substantial savings of energy. The traditional fractional distillation column is structured with: (a) one point of heat input (i.e., a reboiler contained in a bottom tray); and (b) a point of heat removal (i.e., a condenser contained in a top tray). Articles suggesting the use of additional heat sources or heat sinks to improve the efficiency of distillation systems include:
1a. “Control of Sidestream and Energy Conservation Distillation Towers,” H. A. Mosler, Industrial Process Control (AIChE, NY, 1979);
1b. Conserving Energy in Distillation, T. W. Mix and J. S. Dweck, (MIT Press, Cambridge, 1982);
1c. Distillation Control, F. G. Shinskey (McGraw-Hill, NY, 1984);
2a. “Distillation with Intermediate Heat Pumps and Optimal Sidestream Return,” AIChE Jrnl. 32:1347-1359 (August 1986);
2b. “Minimum Energy Requirements of Thermally Coupled Distillation Systems”, AIChE Jrnl. Vol. 33, No. 4, (pp. 643-653, April 1987);
2c. “Heat Pumps for Distillation Columns,” A. Meili, Chemical Engineering Progress, 86:60 (1990);
2d. “Energy Requirements for Nonconventional Distillation Systems,” Z. Fidkowski and L. Krolikowski, AIChE Jrnl., 36, 1275 (1990);
2e. “Consider Thermally Coupled Distillation,” A. J. Finn, Chemical Engineering Progress, 89, 41 (1993);
2f. “On the Use of Intermediate Reboilers in the Rectifying Section and Condensers in the Stripping Section of a Distillation Column,” R. Agrawal and Z Fidkowski, Ind. Ch. Res., 35:2801-2807 (1996);
3a. “Thermodynamic Analysis of Rectification I and II, Reversible Model of Rectification & Finite Cascade Models,” Z. Fonyo, Int. Chemical Engng. 14:18 (1974) and 14:203 (1974);
3b. “The Ideal Column Concept: Applying Exergy to Distillation,” V. Kaiser and J. P. Gourlia, Chem. Eng., P. 45 (Aug. 19, 1985);
3c. Industrial Energy Management, V. Kaiser (Institut Francais du Petrole Paris, 1993);
3d. “Equipartition of Entropy Production: An Optimality Criterion for Transfer and Separation Processes”, D. Tondeur and E. Kvaalen, Ind. Eng. Chem. Res., V. 26, 50-56 (1987);
3e. “Analysis of Entropy Production Rates for Design of Distillation Columns,” S. Ratkje, E. Sauar, E. M. Hansen, K. M. Lien, and B. Hafskjold, I & EC Research, 34:3001-3007 (1995);
4. “Finite Time Thermodynamics: Limiting Performance of Rectification and Minimal Entropy Production in Mass Transfer,” A. M. Tsirlin, V. A. Kazakov, and R. S. Berry, J. P. Chm., 98:3300-3336 (1994);
5a. “Thermodynamic Length and Dissipated Availability,” P. Salamon & R. S. Berry. Physical Review Letters, 51:1127-1130 (1983); and
5b. “Quasistatic Processes as Step Equilibrations,” J. Nulton, P. Salamon, B. Andresen, and Qi Anmin, J. of Chem. Physics, 83, 334 (1985).
The articles listed above are categorized according to technical content in relation to the present invention. Articles 1a-1c are directed to general methodologies of adding heat pumps to effect energy savings from column operation. Articles 2a-2f describe specific examples of these methodologies, but are limited to one additional reboiler and condenser. In some of the examples, a heat pump uses the distillate as its working fluid (vapor recompression). In others, sidestream removal and readdition is used with possible thermal contact outside the column. Articles 3a-3e are directed to examples of multi-tray systems in which the possibility of heat addition and removal at each tray is raised. Their analyses are purely for comparison as idealized aids to analysis of real processes. The specific nature and amount of control at each stage is discussed only in article 3d, which does not present a means for effecting the equal entropy production criterion. This proposition is examined further in article 3e with disappointing results. Article 4 suggests controlling continuous (as opposed to staged) distillation process by adjusting the concentration profile. Articles 5a and 5b describe application of the equal thermodynamic distance principle in a multi-step process for one working fluid traversing a sequence of states.
The approach to heat integration can include the use of a heat pump with at most two points of contact with the column augmented by possible removal and readdition of the distillate material. Prior U.S. patents that address efficiency-improved distillation columns include, for example: F. G. Shinskey, U.S. Pat. No. 4,030,986, “Control for Maximizing Capacity and Optimizing Product Cost of Distillation Column”; G. Emmrich, et al., U.S. Pat. No. 5,080,761, “Method of Optimizing the Operation of a Distillation Column Provided with a Side Heating Device”; R. Agrawal, et al., U.S. Pat. No. 5,230,217, “Inter-Column Heat Integration for Multi-Column Distillation System”; and R. Agrawal, U.S. Pat. No. 5,289,688, “Inter-Column Heat Integration for Multi-Column Distillation System.”
SUMMARY OF THE INVENTION
The invention is directed to maintaining equal thermodynamic distances between stages in a thermodynamic process. The invention relates, in one embodiment, to a distillation system which minimizes exergy consumption in a distillation column by using heat exchange optimally distributed along the column. The system employs a plurality of thermostatted trays which are maintained at a sequence of temperatures specified by maintaining equal thermodynamic distances between the trays. The system can include a unique employment of heat pumps to attain the desired control. The total heat requirement at the boiler decreases by a factor between two and ten when the temperature of each tray in a column is controlled. The decreased heat requirement is accompanied by a similar reduction in the exergy loss.
In one aspect, the invention features a thermal distillation system. The system includes a plurality of thermostatically controlled trays distributed along a length of a distillation column and a means connected to the trays effective to maintain equal thermodynamic distances between the trays. The means can be a heat flow controller. The means can include a heat pump. The heat pump can be a tandem heat pump, which can be integrated in the design of the distillation column.
The heat pump can include an evaporator, a condenser, a compressor, and a throttling valve connected in series through a conduit. The throttling valve is adapted to circulate a refrigerant to perform a heat exchange cycle. The throttling valve is thermally connected to each of the trays and effective to control the amount of heat transferred to or extracted from the trays. The throttling valve can be an elongated small diameter tube.
In preferred embodiments, the distillation system includes: a distillation column having a bottom tray and top tray; a plurality of intermediate trays disposed between the bottom tray and the top tray, the bottom tray serving as a distillation bottom; a heat source in thermal contact with the bottom tray and effective to supply significant heat energy to the distillation bottom; and a first heat exchanger connected to the intermediate trays. The first heat exchanger can be distributed among the intermediate trays. The first heat exchanger can operate in a reversed Brayton cycle. Alternatively, the first heat pump can include a first conventional heat exchanger. The heat source can be an absorption heat pump and can supply significant heat energy to the column and the absorption heat pump.
The distillation system can also include a heat sink in thermal contact with the top
Andresen Bjarne Bogeskov
Nulton James Darwin
Salamon Peter
Fish & Richardson P.C.
Manoharan Virginia
San Diego State University Foundation
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