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Distillation Concepts

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This booklet contains mainly the concepts of distillation which are required by the plant designers as well as plant operational teams. This will help readers to get more of insight of distillation column understanding.

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Distillation Concepts

  1. 1. By Pankaj Khandelwal Chemical Engineer & Corporate Trainer Koncept Learning Center DISTILLATION CONCEPTS Published By KLC-201
  2. 2. Distillation Concepts Contents a.) Introduction b.) Vapor – Carrier of Energy & High Volatiles c.) Liquid – Carrier of Less Volatiles d.) Contact of Vapor & Liquid e.) Distillation System f.) Available Theory from Literature g.) Experiential Learning – I h.) Experiential Learning – II i.) Experiential Learning – III j.) Experiential Learning - IV P-1Koncept Learning Center klcenter@gmail.com
  3. 3. Preface In the distillation, difference in volatilities is used in the separation of two or more components from their miscible mixtures. The term relative volatility which is worked out from the experiments and available as vapor-liquid equilibrium data. Higher the relative volatility, easier will be the separation of two components from their mixture using distillation process and vice versa. Fluid nature, operational requirements, economics, etc. affects the selection of type of distillation column internals viz. packing or trays. These internals are provided to ensure proper contact of uprising vapors & down flowing liquids. Overall distillation system contains various components like feed pre-heater, distillation column, condenser, reboiler, etc. Knowledge of complete distillation system along with column internals is required to design & operate it more efficiently. Pankaj Khandelwal P-2Koncept Learning Center klcenter@gmail.com Distillation Concepts
  4. 4. Introduction P-3Koncept Learning Center klcenter@gmail.com Distillation is a process for the separation of two or more components of desired composition from their miscible liquid mixtures. Like every separation technique which uses the difference in properties of two components e.g. density difference used in the liquid solid separation from their slurry by gravity settling method, difference in volatility of two components is used for their separation using distillation method. Being one component is more volatile than the other in their binary mixture, on heating liquid boils and generates vapors which are also a mixture of both the components but with relatively higher concentration of more volatile components (as observed experimentally). As shown in the Fig.1, ‘A’ is relatively more volatile than ‘B’. Though vapors generated are enriched with high volatile component in single stage of boiling, to achieve desired concentration it may undergo many more similar kind of stages. Like 70% v/v ‘A’ liquid mixture may generate 85% v/v ‘A’ in vapors. And so on till the desired concentration is not achieved. Separation by distillation becomes easier if the relative volatility between two components is higher and the desired separation is achieved with less number of separation stages. Which reduces down the hardware cost. Increased reflux rate reduces the number of stages required which means lesser hardware cost. While energy consumption which means the operating cost increases with the increase of reflux rate. Thus an optimum design of the distillation system is done by minimizing the total of fixed cost (due to number Distillation Concepts Fig.1Liquid 50 % v/v ‘A’ 50 % v/v ‘B’ Vapor 70 % v/v ‘A’ 30 % v/v ‘B’ Heat
  5. 5. P-4Koncept Learning Center klcenter@gmail.com of stages) and operating cost (due to energy input). Due to high energy and capital cost involved, distillation system is usually the last choice of selected separation technique. In case relative volatility is closer to ‘1’, distillation alone becomes uneconomic. Other alternatives like liquid-liquid extraction followed by the distillation, absorption, etc. are recommended for the desired separation. When relative volatility is exactly ‘1’, separation by distillation technique becomes impossible. As the composition of generated vapor and the liquid remain same, as shown in Fig.2. This is called as Azeotropic mixture and distillation fails to work here as both the components are having same volatility. Distillation Concepts Fig.2Liquid 95 % v/v ‘A’ 5 % v/v ‘B’ Vapor 95 % v/v ‘A’ 5 % v/v ‘B’ Heat
  6. 6. Vapor – Carrier of Energy & High Volatiles P-5Koncept Learning Center klcenter@gmail.com On supply of heat to the liquid, it converts to vapor. For the distillation process to occur, liquid need to be converted to vapor at different stages which are arranged in the vertical column. Vapors are having high thermal energy (heat) as compared to liquid of nearly same composition. For example, 1 kg of liquid water at 100oC carries 100 kcal heat while 1 kg of water vapors carries 640 (=100 + 540) kcal heat. (considering reference temperature for both the cases as 0oC of liquid water). Thus vapors carries 540 kcal additional heat energy which is latent heat of vaporization, than its liquid phase. As heat is required at every stage in the distillation column and can’t be supplied from the external heat energy sources, generated vapors from one stage becomes the heat source for the next higher stage internally. Being vapors are having natural property to move up, so the vapors generated in the lower stages naturally moves up for higher stages as shown in Fig.3. Distillation Concepts Stage-I Stage-II Stage-III Vapors – Heat Source for Stage-I Vapors – Heat Source for Stage-II Vapors – Heat Source for Stage-III Fig.3 As discussed earlier, on conversion of liquid to vapor, concentration of volatile material increases in the vapor phase. This happens at every stage. Thus the vapors generated at the stage-I contain higher volatile materials more than the vapors generated at the stage-II. Same is applicable for other stages too. Thus as the vapors move up, concentration of high volatile materials increase. Vapors naturally moving up carries heat as well as high volatile materials upside in a distillation column.
  7. 7. Liquid – Carrier of Less Volatiles P-6Koncept Learning Center klcenter@gmail.com To carryout the distillation process, liquid inside the distillation column is essentially required as the conversion of liquid mixture to vapor only increases the concentration of more volatile material in the vapor phase. As the vapors moves up along with high concentration of more volatile materials, they leave behind less volatile materials in the liquid phase only or in other words concentration of high volatile materials in the liquid phase reduces. Liquid is having natural property to flow down i.e. under gravity. Liquid flows down from the higher stages to lower stages by the natural gravity, as shown in Fig.4. Thus liquid flows from the higher stage of separation to the lower stage within distillation column. As at every stage, high volatile material is stripped off and leaving behind low volatile material in the liquid phase. Liquid along with downward movement carries less volatile material and finally drawn from the bottom of the distillation column as bottom product stream. Sufficient amount of liquid should be present in the distillation column to ensure its better contact with the vapors required for simultaneous heat and mass transfer within distillation column. Distillation Concepts Stage-I Stage-II Stage-III Fig.4
  8. 8. Contact of Vapor & Liquid P-7Koncept Learning Center klcenter@gmail.com Uprising vapors which carries heat as well as high volatile materials need to be forcibly put in contact with down flowing liquids which carries low volatile materials. Thus inside a distillation column either packing or trays (depending upon the fluid nature, operational requirements, economics, etc.) are provided which ensures sufficient space for the good contact between vapor and liquid. Simultaneous heat transfer (by the condensation of vapors in the liquid phase) and mass transfer (by mixing high volatile material in the vapor phase with low volatile material in the liquid phase followed by flashing of resultant liquid to vapor phase containing higher concentration of more volatile materials as per its equilibrium) takes place in every stage. These equilibrium stages are provided in the form of either packing or trays within the distillation column. a.) Packed Column Different types of packing (see Fig.5) are used in the packed type distillation column are used to provide required surface for the contact of vapors and liquids. Selection of packing type depends upon its efficiency (fixed cost) & pressure drop (operating cost). Being liquid is supplied from the top, it is distributed across the column cross section by the ‘Liquid Distributor’ provided at the top section of the distillation column. Some times, depending upon the higher height of packing, ‘Liquid Redistributors’ are provided in the intermediate sections of the distillation column. Vapors provided at Distillation Concepts Fig.5
  9. 9. P-8Koncept Learning Center klcenter@gmail.com the bottom, naturally spread throughout the column cross section. Flow of liquid is maintained so that the packing is just wet and not flooded. Otherwise upward movement of the vapors through the column will be obstructed. Even less liquid flow rate will dry up part of the packing material resulting less space or wet area available for vapor to carryout simultaneous heat & mass transfer required for the material separation. b.) Tray Column As the name suggests, distillation column is provided with different types of trays (Sieve, Bubble Cap, Valve, Grid, etc.) as shown in Fig.6. Selection of type of tray depends upon the their efficiency & economics. Each tray is provided with the space for contact of vapor and liquid to carryout simultaneous heat and mass transfer. Distillation Concepts Fig.6 Liquid coming from the top gets time over the tray for better contact with the vapor. Each tray is provided with the opening (in the form of sieve, riser, variable valve opening, etc.) for vapors to bubble out through liquid level maintained by the outlet weir provided at the end of each tray. Liquid overflows Fig.7 Outlet Weir Down comer Inlet WeirTray Spacing
  10. 10. P-9Koncept Learning Center klcenter@gmail.com from higher tray to lower through down comer area provided with the help of down comer plate which is sealed with the liquid at the next below tray. This blocks the movement of vapors through the down comer area. Thus vapors are forced to move through the tray opening and the liquid flowing over the tray. As liquid falls from the upper tray to the lower tray through down comer area, high turbulence causes fluctuations in the liquid level over the tray. To minimize it, inlet weir is provided in the trays. All the trays are maintained at certain distance which is called as tray spacing. It is fixed based on the liquid type and tray hydraulic calculations. It shall always be higher than liquid level in the down comer side. Liquid entrainment is also considered while fixing tray spacing. Distillation Concepts
  11. 11. Distillation System P-10Koncept Learning Center klcenter@gmail.com Overall distillation system comprises of various hardware component like equipment (distillation column, pump, heat exchangers, etc.), pipelines (for feed, vapor, reflux, utilities, etc.), instrumentation (for parameters like pressure, temperature, etc. and controls). Overall system is divided into following sub-systems: a.) Feed Preheating Feed condition (cold liquid, saturated liquid, saturated vapor, etc.) required at the feed point of distillation column and the available feed from the upstream process decides the need of its conditioning. Generally feed pre-heater provided with fresh heating utility or any other available process stream required to be cooled, is used for maintaining desired feed condition. b.) Vapor Condensation Top vapors generated in the distillation column are condensed in the condenser with the help of fresh cooling utility or any other process stream require heating. c.) Reflux Flow To ensure sufficient liquid in the distillation column during the start as well as normal plant operation, part of condensed liquid from the condenser is refluxed back at the top most tray of the column. This Distillation Concepts Fig.8
  12. 12. P-11Koncept Learning Center klcenter@gmail.com may be achieved directly by the gravity flow from the condenser (if it is located above the distillation column) or set of reflux tank and pumps are used (if condenser is located below reflux point). d.) Column Heating Throughout the distillation column, heating of liquid is required to generate vapors. As discussed earlier, these vapors generated at one stage becomes the heat source for another. Bottom most tray or the bottom section of the packing in the distillation column requires fresh heat source. This is achieved either by putting direct steam (if allowed) at the bottom of distillation column or steam or any other heating utility indirectly through reboiler. e.) Product Stream Draws As per the composition required, product draws are taken from the top (for high volatile components), from the bottom (for low volatile components) or from the intermediate stage (for desired mixture of low and high volatile components). Being these draws are at high temperatures, they are cooled by top, bottom and intermediate product coolers respectively. Distillation Concepts
  13. 13. Available Theory from Literature P-12Koncept Learning Center klcenter@gmail.com Vapor-Liquid Equilibrium (VLE) It describes the distribution of a chemical species between the vapour phase and a liquid phase. Typical VLE for the binary liquid mixture is shown in Fig.9. Reflux Ratio The Reflux ratio is the ratio between the boil up rate and the take-off rate. Or in other words, it is the ratio between the amount of reflux that goes back down the distillation column and the amount of reflux that is collected in the receiver (distillate). At total reflux, the number of theoretical plates required is a minimum. As the reflux ratio is reduced (by taking off product), the number of plates required increases. The Minimum Reflux Ratio (R min) is the lowest value of reflux at which separation can be achieved even with an infinite number of plates. The total cost, which is the sum of fixed cost and operating cost, must therefore passes through a minimum. The reflux ratio at this minimum total cost is the optimum (or economical) reflux ratio. Distillation Concepts Fig.9
  14. 14. Experiential Learning-I In a binary liquid mixture of n-Heptane & n-Octane at 1 atm pressure, find the equilibrium vapor concentrations for the liquid of following concentrations: Liquid Composition Vapor Composition (Mole Fraction of n-Heptane) (Mole Fraction of n-Heptane) 0.10 --------- 0.20 --------- 0.30 --------- 0.40 --------- 0.50 --------- 0.60 --------- 0.70 --------- 0.80 --------- 0.90 --------- 0.95 --------- P-13Koncept Learning Center klcenter@gmail.com Distillation Concepts
  15. 15. Experiential Learning-II Estimate the pressure inside distillation column at following points for the scheme shown in the figure for three operating conditions: P-14Koncept Learning Center klcenter@gmail.com Distillation Concepts Atmospheric Condition Column in Pressure Column under Vacuum Column Top Pressure Feed Point Pressure Column Bottom Pressure
  16. 16. Experiential Learning-III Estimate the temperature inside distillation column at following points for the scheme shown in figure for three operating conditions: P-15Koncept Learning Center klcenter@gmail.com Distillation Concepts Atmospheric Condition Column in Pressure Column under Vacuum Column Top Temperature Feed Point Temperature Column Bottom Temperature
  17. 17. Experiential Learning-IV Carry out trend analysis for following cases in the distillation Columns. What will happen if: a.) Number of sieves provided in sieve tray column are more or less than required b.) Outlet weir height is lower or higher than required c.) Energy consumption in distillation column if reflux ratio is more or less than required at full capacity plant operation d.) Column diameter if reflux rate is increased or reduced e.) Column diameter if plant capacity is doubled or halved f.) Feed concentration is increased or reduced g.) Tray spacing less or more than required h.) Feed temperature is less or more than required i.) Down comer area provided less or more than required P-16Koncept Learning Center klcenter@gmail.com Distillation Concepts
  18. 18. NOTES P-17Koncept Learning Center klcenter@gmail.com Distillation Concepts
  19. 19. NOTES P-18Koncept Learning Center klcenter@gmail.com Distillation Concepts
  20. 20. Koncept Learning Center (KLC) was established in 2001 with the aim of imparting knowledge and providing training programs from students level to senior management level people of the industry. Our prime objective is to upgrade conceptual knowledge of everyone for attaining excellence in the chemical process industries. Pankaj Khandelwal, a Chemical Engineer (BE-IIT,Roorkee; M.Tech-IIT,Kanpur; DBM-IGNOU) with 14 Years of Industrial Work Experience (Research & Development – Grasim Industries; Basic Engineering – Uhde India; Project Engineering – Atul Products; Process & Commissioning Engineering – Praj Industries) and 17+ Years of Teaching/Training Experience in various Engineering Colleges & Chemical Industries. Author of 35 Technical & Management Articles in International and Indian Journals, book “Present Your Thoughts Effectively” and over 100+ Letters in English Newspapers. About the Trainer Sharing Experiences Since 2001 KLC Conducts Training Programs for o Fresh GETs (Chemical/ Mechanical Engineers) o Plant Operators (B.Sc., M.Sc., Diploma) o Plant Officers (Quality, Safety) o Plant Engineers (Process, Maintenance) o Managers (Technical, Commercial, etc.) o Lab Researchers, Pilot Plant Engineers, etc. o Plant Designers & Drafting Persons 9371121220 9623232500 020-26696107 email: klcenter@gmail.com
  21. 21. 9371121220 9623232500 020-26696107 email: klcenter@gmail.com Soft Skill Programs Communication Skills (Present Your Thoughts Effectively) Time Management Skills (Manage Your Time Effectively) Innovation Skills (Conceptual Approach for Innovative Solutions) Mentoring Skills (Manager to Mentor) Process Plant Concepts • Fluid Flow Concepts • Heat Transfer Concepts • Fluid Mixing Concepts • Reaction Concepts Unit Operation Concepts • Distillation • Evaporation • Filtration • Solids Drying • Crystallization Utility Concepts • Cooling Water/ Cooling Tower • Chilled Water/ Brine System • Steam Generation/ Distribution • Water Treatment Plant • Instrument/ Process Air • Hot Oil/ Thermic Fluids Engineering Calculations • Material & Energy Balance • Pressure Drop Calculations • Pipe Line Sizing • Pump Hydraulic Calculations • Vessel Sizing • Heat Exchanger Design • Distillation Column Sizing Miscellaneous • Steam Jet Ejector • Corrosion/Material Selection • Health, Safety & Environment •Cost Reduction Program • Plant Maintenance of CPI • Technical Auditing of Plant • Design Margins in CPI • Overview of CPI • Distillery Plant Overview Major Training Workshops Scale up of Chemical Plants Process Engineering for Chemical Plants Troubleshooting of Chemical Plant Process Safety Management (PSM) for CPI Maximize Profits by Waste Reduction Basic Training Modules Chemical Engineering Concepts for Non-Chemical Engineers Plant Engineering for Chemical Engineers Chemical Industry Concepts for Plant Operators Koncept Learning Center Since 2001 Sharing Experiences Chemical Process Industries Pharmaceutical & Bio-Pharmaceuticals Pesticides, Specialty Chemicals, Agro-Chemicals, Distillery, Cellulosic Fiber, Process Consultant, EPC Our Training Programs