Operational and Economic Assessment of Distillation Column from the Performance of Tray

Transcription

1 Abstract: Operational and Economic Assessment of Distillation Column from the Performance of Tray Nilesh P. Patil *1, Vilas S. Patil 2 1 Assistant Professor, *2 Professor Department of Chemical Engineering, Currently distillation is the most commonly used separation / purification technique in many industrial applications. Irrespective of its higher energy consumption and limited separation efficiency, distillation remains the first choice of process engineer. Therefore the study of distillation column has unparalleled importance in process design. This study comprises of column internals like trays and packing s. The paper rigorously reviewed the literature published on current operational phenomena and economic feasibilities of trays in distillation columns. The operational features of the column like pressure drop, energy consumption, liquid holdup etc are discussed in brief. The assessment of column based on the challenges in operation like weeping, foaming, entrainment etc is carried out individually. Moreover the collective economical footprint of these problems on the operational performance of whole bioethanol plant is discussed critically. This paper summarizes the operational and economic analysis of state-of-the art column trays. The matrices includes sieve tray, valve tray and bubble cap tray. In most of the cases industrial results are unavailable. Further, the research in this field is too limited to be of value for industrial application. Keywords: Trays; Thermally coupled column; Energy saving; Optimization; Distillation; Twoenthalpy feed; Retrofit. 1. Introduction Regarding distillation, De Koeijer, et. al (1) reported that distillation offers a low thermodynamic efficiency of about 5 20%. Additionally, it has large negative environmental impact of carbon dioxide since the heat is generated through the burning of fossil fuels. In order to bring the significant increase in its thermodynamic efficiency, economy, and decrease in its environmental impact, various technologies like configurations with inter reboiler University Institute of Chemical Technology, North Maharashtra University, Umavinagar, Post Box No. 80, Jalgaon , India and Inter condensers, heat pump systems (2-4), multieffect columns (5-7), thermally coupled sequences (8), and recently the internally heat-integrated distillation columns HIDiC (9-13) have been developed. Specially, HIDiC configurations offers a great potential in energy savings as compared to other technologies, particularly in splitting close boiling mixtures by dividing the column in its corresponding rectifying and stripping sections. It results in the generation of temperature feasible driving forces in the two sections of column. Till date, beside its high energy consumption; distillation remains the most widely used separation technique. Because of its distinct advantages, practically no other separation techniques can compete with distillation. Distillation is the only technique that has proved its applicability on large scale. Olujic et. al. (11) claimed that ever growing application of distillation results in the increase energy consumption since it uses heat in thermodynamically inefficient way. He further quoted that high energy consumption is practically the only weakness of distillation. Therefore there is strong reason for the motivating the research towards improving the energy efficiency of distillation all over the world. Unfortunately, the research interests get shifted largely towards the development of alternative separation techniques with relatively lower energy consumption. These facts collectively lead to the operational and economic assessment of distillation columns. As the energy consumption of the column depends on performance of column internals; assessment of tray performances is selected as one of the major footprint in this direction. The paper in detail summaries the comparative study of column performances based on the different types of trays. The matrices covers sieve tray, valve tray and bubble cap tray etc. Also all the parameters and problems associated with the performance of tray was assessed. Finally the rigorous economic assessment of impacts caused by the various parameters associated with trays is carried out. Particularly the ISSN: Page 500

2 performance of the tray is assessed in terms of their application for ethanol water system in distillery. Also the other options for increasing the energy efficiency of column like direct vapour recompression, lowering compression ratio are checked for their viability. 2. Types of Trays Currently the trays of different geometries are available in the market. However all this tray are classified and belongs to two groups namely sieve trays and valve trays. The types of tray used in distillation columns are as follows: 2.1. Sieve Tray Sieve trays are simply a metal plate having drilled holes known as perforations (sieves) all over the plate through which the vapour emerges and pass into the bulk of liquid flowing over the tray. Typical sieve layout over the tray follows either a square or equilateral triangular hole pitch. Each tray are provided with downcomer which guides the excess liquid over the tray, to its lower tray and outlet weir which maintains the pool of liquid over the tray, thereby facilitating highest vapour liquid contact. Typical sizes of sieves vary based on their applications. Small sieves are not suitable for fouling or corrosive liquids as there are chances of blockage, leading to excessive pressure drop and premature flooding. At the same time small sieves are beneficial in reducing the weeping resulting in the increased tray capacity. Ultimately small sieve trays offers better turndown ratio. As far as the manufacturing cost is concerned, trays with larger sieves are cheaper since the holes can be punched and because of large diameter holes are fewer while the trays with smaller sieves are more expensive since drilling is the only option. In case of varying vapour and liquid loadings in the column, there will be the requirement of varying hole area. Therefore instead of manufacturing different trays to meet the requirement, it is convenient to use the same tray layout, and only the required hole area is adjusted by blanking strips. These trays have an advantage of low pressure drop, high efficiency, throughput capacity, and simplified configuration. Such trays are properly fits for large scale rectification, separation at low pressure and are successfully used in industry. Vapour Figure: 1 Schematic Diagram of Distillation Column with Sieve Trays Characteristics of Sieve Trays 2.2. Valve Tray Sieve tray has simplest design All parts are mechanically immovable There is no liquid seal Susceptible to weeping at low vapour flow or high liquid rates Offers low pressure drop Comparatively cheaper in cost Not Suitable for fouling or corrosive liquids Valve trays are quite similar to that of sieve trays having drilled holes (perforations) all over the plate; the only difference is that each hole is provided with a liftable caps / flapper valve. During normal operation vapour flow lift the caps, thus self creating a flow area for the passage of vapour. These caps directs the vapour to flow horizontally into the liquid over the tray, thus providing better mixing compared to sieve trays. These lifted caps gets down when the vapour velocity fall below it s designed. Particularly these trays find an application in a case where vapour velocity is not constant. The main purpose of cap / valve is to prevent the liquid from dumping through the holes during the fluctuating gas velocities; since low gas velocities are no longer able to sustain the pressure exerted by the liquid over the tray. Each tray is provided with downcomer and outlet weir for the smooth operation of column. Tray layout is either a square or triangular in structure. Sizes of the holes and valves may vary based on their applications. ISSN: Page 501

3 These trays are not suitable for fouling or corrosive liquids as there are chances of valve damage resulting in tray efficiency reduction. Besides this these trays offers a very good package of considerable advantages over a sieve trays. The most remarkable advantage is its capacity to prevent the dumping of liquid at the time of low vapour velocity. This ability empowers these trays in retaining the desired liquid holdup over a tray deck which ultimately results in stable column operation. More precisely the time required for the column to stabilize gets considerably reduced. At the same time reduction in weeping results in increased tray capacity. For valve trays turndown ratio is also better. As far as the manufacturing cost is concerned, valve trays are more expensive as compared to that of sieve trays because of additional cost of flapper valves. In case of requirement of varying hole area, the option of blanking is available. Vapour Characteristics of Valve Trays Figure: 2 Schematic Diagram Distillation Column with Valve Trays Simple design with flapper valve Some parts are mechanically movable Flapper valve offers a liquid seal at low vapor velocity No weeping at low vapour flow or high liquid rates Offers moderate pressure drop Comparatively expensive in cost Not Suitable for fouling or corrosive liquids 2.3. Bubble Cap Tray Bubble cap trays are complex in structure. A tray consists of a metal plate having drilled or punched holes all over the plate. Each hole is provided with riser (small, short pipes set into the tray) or chimney with a cap over it. A cap is mounted over a riser in such a way that, a proper space for the passage of vapour should be maintained in between them. Cap directs the flow of rising vapour to turn through 180 and slots in the cap makes the gas to bubble through the liquid over a tray, thus enabling maximum vapour liquid contact per tray. The level of liquid over a tray is always maintained below the top edge of the riser to prevent dumping of liquid down the tower. Each tray is provided with downcomer and outlet weir. Tray layout for mounting the bubble caps is either a square or triangular. The sizes of bubble caps vary based on their applications. These trays are not suitable for fouling or corrosive liquids. Besides this these trays offers a very good advantages over its rivals. These trays offer better efficiency in terms of prevention of liquid from dumping resulting due to low vapour velocity. It enables them in retaining the desired liquid holdup over a tray ultimately resulting in a stabilized column operation. More precisely the time required for to stabilize the column gets reduced to its minimum. At the same time reduction in weeping results in increased tray capacity. These trays offers better turndown ratio as compared to other trays. As far as duty is concerned, these trays ensure the most efficient separation, but at the same time offer a highest cost among its competitors. Vapour Figure: 3 Schematic Diagram of Distillation Column with Bubble Cap Trays ISSN: Page 502

4 Characteristics of Bubble Cap Trays Complex design with bubble caps All parts are mechanically immovable Riser offers a liquid seal at low vapor velocity No weeping at low vapour flow or high liquid rates Offers higher pressure drop Comparatively expensive in cost amongst all Not Suitable for corrosive liquids 3. Operational and Economic Assessment of Trays The table represents the comparative study of tray parameters and their effects on the performance of column Types of Trays Parameters Sieve Tray Valve Tray Bubble Cap Tray Efficiency High ( 0.9) High ( 0.9) Moderate ( 0.8) Cost Low (X) Moderate (1.5X) High (2X to 3X) Pressure Drop Low Moderate High Maintenance Low Low to Moderate High Capacity High Very High Moderate Turndown Approx 2:1 (Unsuitable for varying loads) Approx 4-5:1 (Suitable for varying loads) Very High (Suitable for low liquid loads) Fouling Tendency Low Low to Moderate High Where X Comparative Parameter Table: 1 Comparative study of tray parameters 4. Parameters Affecting the Performance of Column In this section, the process variables known to have effects on the tray performance, including pressure drop, foaming, tray efficiency and weeping etc are discussed in detail Active Area Active area is the area of the tray either perforated or fitted with valves or bubble caps and available for vapour/liquid contacting. Larger the active area, larger is the capacity of tray Downcomer Area Downcomer area is the area available for the transfer of liquid from one tray to the next tray below it. The main function of downcomer is to allow for vapour disengagement from the liquid. Undersized downcomer will always results in downcomer flooding Hole Area Hole area is the area collectively available for passage of vapors through perforations / valves / bubble cap slots. This is the most critical factor in the design of tray operating range since high vapour velocity will cause heavy liquid entrainment and high pressure drop, whereas low vapour velocity will cause sevier weeping. The open area will also affects the liquid back-up in a downcomer Tray Spacing Tray spacing is a vertical distance between two adjacent trays. It also effects the spray height along with allowable liquid head in the downcomer Pressure Drop Pressure drop is simply the sum of pressure drop caused by the resistance to vapour flow through the tray open area and the head of clear liquid on the tray deck. In most of the cases pressure drop is the major concern as it directly relates the steam consumption. ISSN: Page 503

5 Therefore the efforts will always be in order to maintain the as low pressure drop as possible. Increasing the number of flow paths in high liquid rate services is also one of the way to reduce the pressure drop Entrainment The carryover of liquid droplets due to high vapour velocity is known as entrainment. It is observed that increase in vapour velocity causes a heavy entrainment Weeping The dumping of liquid over the tray due to low vapour velocity is known as weeping. It is observed that decrease in vapour velocity causes a heavy weeping and can be reduced by increase in hole flow factor. 5. Parallel Techniques for Increasing the Energy Efficiency of Column In this section various parallel techniques aiming at the increasing energy efficiency of the column are discussed. Li et. al (14) in his paper studied the hydrodynamic and mass transfer performance of two test flow-guided sieve trays and concluded that flowguided sieve tray, as compared to simple sieve trays shows better characteristics. He further quoted that increased hole size will result in decrease pressure drop and increased efficiency making it suitable for highly viscous mixtures Vapor recompression According to Pribic et. al (15) vapor recompression seems to be the best known arrangement for energy savings in distillation. It consists of condensation of overhead vapors of column to liquid, and using the heat liberated through condensation to reboil the bottom liquid from the same column. The temperature driving force required to enable the heat to flow from cooler top vapors to bottoms hot product liquid is ascertained either by compressing the overhead vapor and condensing them at a higher temperature. The same can be accomplished by reducing the pressure on the reboiler liquid to make it boil at a low temperature followed by compression of bottoms vapor back to column pressure. But from operational point of view later one is not suitable, rather it will be more energy intensive. Therefore the earlier arrangement known as heat pump will be preferred o easy (16). But vapor recompression is not suitable for all separation applications. It is suitable for applications involving close-boiling point products, particularly, for applications with a small temperature difference between the bottom and top of the column Internal Heat Integration in Distillation Column In this technique unlike a conventional vapour recompression system, the vapour leaving the stripping section is compressed to a desired level. Compression of these vapors results in a two-pressure operation in a column ensuring the operation of rectification section at higher pressure. Continuous condensation along the rectification section releases the heat and transferred it to stripping section. In stripping section this heat is utilized for continuous evaporation. In this way the compression ratio is minimized (9, 10, 17). Using this technique a significant reduction in compression ratio can be obtained; but practically this technology was never implemented. It raises the big question on the efficiency and actual viability of this technique Feed Conditions in Distillation Column Since distillation is a most widely used separation method in the chemical process industries and the scale of operation is often a large, relatively small improvements in capital cost or in energy use can make a significant impact on overall production cost. Retrofitting existing equipment will also contribute significantly. Some of the most expensive distillation systems comprise of a high-pressure columns processing vapor feeds. These columns require refrigeration. Wankat et. al. (18) introduced the 2- enthalpy feed (2-F) system in which the feed is divided into two portions that are at the same composition but one part is vapor and the other is liquid. This technique is proved to be most effective for vapor feeds that require refrigeration. The only disadvantage of the process is that columns with vapor feeds typically have larger diameters than columns with liquid feeds, but the same can be countered in having less energy consumption in the reboiler. Further Soave et. al. (19) developed a modified 2-F process for refrigerated distillation systems in which the cold distillate is use to condense part of the feed. Wankat reported that in 2-F system with vapor feed, part of the feed is condensed leaving the remaining as a vapor at same composition. Besides its several advantages, requirement of constant heating and cooling diminishes the usefulness of 2-F system for retrofitting to increase capacity (20). The 2-F system for vapor feeds can be applied to rectifiers, strippers and complete columns with liquid feed (21-24). ISSN: Page 504

6 5.4. Heat Integration in Distillation Column Trailing two techniques are applicable for increasing the energy efficiency of distillation column, whereas this technique is applicable for improvement in energy efficiency of overall process. In this technique the columns are operated based on their pressure difference. More precisely the column operating at vacuum is driven by the top vapors of column operating at pressure. If the columns are operating at same temperature then intentionally a pressure difference is created there in order to ensure the smooth application of the technique. This technique is also known as multi pressure distillation technology. 6. Conclusion From the overall assessment of the various tray types it is observed that every tray has their own merits and demerits. But still movable valve trays are found to be most suitable for various types of applications. A valve tray with movable flapper offers better operating characteristics amongst all. They perform great when the domain of efficiency, capacity, turndown, maintenance etc with a slight higher cost. Various technologies like heat pump systems, multieffect columns, thermally coupled columns etc are developed to bring the significant increase in thermodynamic efficiency and economy of distillation columns. The same techniques will also help in bringing the significant reduction in environmental impact of carbon dioxide emissions. Particularly HIDiC configurations are found to be beneficial in energy savings for splitting close boiling mixtures. 7. Abbreviation 2-F : two-enthalpy feed system 8. References 1. De Koeijer, G.M. and Kjelstrup, S. (2000) Minimizing Entropy Production Rate in Binary Tray Distillation. Int. J. Appl. Thermodyn. 3: Null, H.R. (1976) Heat Pumps in Distillation. Chem. Eng. Prog. 73: Menzies, M.A. and Johnson, A.I. (1981) Steady-State Modeling and Parametric Study of a Vapor Recompression Unit. Can. J. Chem. Eng. 59: Meili, A. (1990) Heat Pumps for Distillation Columns. Chem. Eng. Prog Han, M. and Park, S. (1996) Multivariable Control of Double-Effect Distillation Configurations. J. Process Control. 6: Hasebe, S., Noda, M. and Hashimoto, I. (1999) Optimal Operation Policy for Total Reflux and Multi-Effect Batch Distillation Systems. Comput. Chem. Eng. 23: Ophir, A. and Gendel, A. (1994) Adaptation of the Multi- Effect Distillation (MED) Process to Yield High Purity Distillate for Utilities, Refineries and Chemical Industry. Desalination, 98: Petlyuk, F.B., Platonoy, V.M. and Slavinskii, D.M. (1965) Thermodynamically Optimal Method for Separating Multicomponent Mixtures. Int. Chem. Eng. 5: Nakaiwa, M., Huang, K., Owa, M., Akiya, T., Nakane, T., Sato, M. and Takamatsu, T. (1997) Energy Savings in Heat- Integrated Distillation Columns. Energy, 22: Nakaiwa, M., Huang, K., Naito, K., Endo, A., Owa, M., Akiya, T., Nakane, T. and Takamatsu, T. (2000) A New Configuration of Ideal Heat Integrated Distillation Columns. Computers and Chemical Engineering, 24: Olujic, Z., Fakhri, F., Rijke, A. de., Graauw J. de. and Jansens, P. J. (2003) Internal Heat Integration The Key to An Energy Conserving Distillation Column. Journal of Chemical Technology and Biotechnology, 78: Huang, K., Shan, L., Zhu, Q., Qian, J. (2008) A Totally Heat- Integrated Distillation Column (THIDiC) The Effect of Feed Pre-Heating by Distillate. Appl. Therm. Eng. 28: Jana, A.K. (2009) Heat Integrated Distillation Operation. Appl. Energy, Li, Q.S., Song, C.Y., Wu, H.L., Liu, H. and Qian, Y.Q. (2008) Performance and Applications of Flow-Guided Sieve Trays for Distillation of Highly Viscous Mixtures. Korean J. Chem. Eng., 25(6), Pribic, P., Roza, M. and Zuber L. (2006) How to Improve the Energy Savings in Distillation and Hybrid Distillation- Pervaporation Systems. Separation Science and Technology, 41:11, Freshwater, D.C. (1951) Thermal Economy in Distillation. Trans. I. Chem. E., 29: Jansens, P.J., Fahkri, F., Graauw, J. and Olujic, Z. (2001) Energy Saving Potential of a Heat Integrated Distillation Column. Proceedings of the Topical Distillation Symposium, AIChE 2001 Spring Meeting, Houston, April, pp Wankat, P.C. and Kessler, D.P. (1993) Two-Feed Distillation: Same Composition Feeds with Different Enthalpies. Ind. Eng. Chem. Res., 32, Soave, G.S., Gamba, S., Pellegrini, L.A. and Bonomi, S. (2006) Feed-Splitting Technique in Cryogenic Distillation. Ind. Eng. Chem. Res., 45, Wankat, P.C. (2007) Balancing Diameters of Distillation Column with Vapor Feeds. Ind. Eng. Chem. Res., 46, Wankat, P.C. (2007) Reducing Diameters of Distillation Column with Largest Calculated Diameter at the Bottom. Ind. Eng. Chem. Res., 46, Wankat, P.C. (2014) Improved Rectifying Columns. Ind. Eng. Chem. Res., 53:22, Wankat, P.C. (2015) Economic Analysis for Improved Rectifying Columns. Sep. Sci. Technol., 50:16, Soave, G.S. and Feliu, J.A. (2002) Saving Energy in Distillation Towers by Feed Splitting. Appl. Therm. Eng., 22, 889. ISSN: Page 505