A CERTIFICATE COURSE ON CHEMICAL PLANT OPERATOR Distillation Basics Dharmsinh Desai University Mihir P.Shah, Asst Prof. [Pick the date] Anchor Institute for Chemical and Petrochemical Sectors Distillation Basics What is separation Process? According to Thermodynamics every mixing process is a natural process. This will always increase the entropy. So, separation of mixture of two or more components is a reverse procedure of mixing (unmixing). Those operations which transform a mixture of substances into two or more products which differ from one another in composition are called as separation operations. As per Second Law of Thermodynamics, all natural processes take place so as to increase the entropy, or randomness, of the universe. So Mixing causes an increase in entropy. Separation, therefore, into products of different compositions requires a process where equivalent to thermodynamic work must be supplied to cause a separation to occur. Separation operations are inter-phase mass transfer processes because they involve the creation, by the addition of heat as in distillation of a second phase, and the subsequent selective separation of components. This means that, if one would like to have separation, it is required to have creation of second phase to avail separation. Table: 1 Separation based on Inter-phase Mass Transfer Operation Distillation Absorption Crystallization Extraction Drying Leaching Adsoption Feed phase Liquid or Vapor Gas Liquid Liquid Solid+Liquid Solid Gas or Liquid Second Phase Vapor or Liquid Liquid Solvent Solid Crystals Immiscible Liquid Solvent Vapor Liquid Solvent Solid Choice of Separation Process: • If all the components are relatively volatile and required to be separated in pure form then distillation is first choice. • If relative volatility is very high then go for evaporation. • If relative volatility is very low, (close to or equal to 1) then go for special type of distillation like azeotropic or extractive distillation. • 10-30% by mass acetic acid in water is separated by butyl acetate • Azeotropic mixture of 95.6% ethanol in water by benzene. – For last case liquid-liquid extraction or crystallization followed by distillation will be more economical. – For recovery of only component from mixture, it is preferable to go for absorption/stripping/extraction rather then directly doing distillation. Distillation Basics, M.P.Shah Page 2 Fig 1. Choice of separation based on x-y Diagram Distillation: A process in which a liquid or vapor mixture of two or more substances is separated into its component fractions of desired purity based on boiling points (Vapor Pressure) difference, by application or removal of heat. Application – Useful products such as in petroleum refining, beverages, chemical processing, petrochemicals, and natural gas processing. History and Growth of Distillation • • • • • The art of distillation dates back to at least 1st century A.D. By 11th century, it was being used in Italy to produce alcoholic beverages (Batch operation). • Early applications used vaporization and condensation separately in two different chambers. The word Distillation : Latin word “ Destillare” means dripping or trickling down By 16th century, it was improved by providing multiple vapor – liquid stages in a so called rectificatorium which gave born to the word “ Rectification” Rectification : Latin word “recte facere” means to improve. Distillation Basics, M.P.Shah Page 3 • • • • • • By 18th century, it was most widely used industrial method with progressive modifications in process. 1813 Cellier – Blumenthal in France 1820 Clement Germany – Packed tower 1822 Perrier in England – bubble cap tray 1830 Coffey seive tray column 1893 Ernest Soral – La Rectification de l’alcohol Principle of separation Separation in Distillation column is based on difference in boiling points of individual components or vapor pressure characteristics of liquid mixtures. Separation of mixture into two or more products, that have difference in boiling points, preferentially boiling the more volatile compound (MVC) out of the mixture. Distillation is based on the fact that the vapor of a boiling mixture will be richer in the components that have lower boiling points. If vapors are cooled less volatile compound (LVC) will have greater tendency to condense than MVC. Distillation may be carried out by either of 2 principal methods;  Based on production of vapor by boiling liquid mixture to be separated and condensing the vapors without allowing any liquid to return to the still (Reflux);  Based on the return part of the condensate to the still (Reflux) under such conditions that the returning liquid is brought into intimate contact with the vapors on their way to the condenser Boiling Point and Vapor Pressure The Boiling Point is the temperature at which internal vapor pressure of the liquid is equal to the pressure exerted by its surroundings. If the liquid is open to the atmosphere, the boiling point is the temperature at which the internal vapor pressure of the liquid becomes equal to atmospheric pressure (~760 mm Hg). The internal vapor pressure of a pure liquid rises steadily as the temperature is increased until the boiling point is reached. P  e -C/T Vapor pressure is defined as the absolute pressure of vapor at which vapor and liquid are in equilibrium at given temperature. The temperature remains constant throughout the boiling process of a pure liquid. At the boiling point, the liquid and vapor are in equilibrium. If the composition of each phase remains constant, the temperature will remain constant. Fig 2. Vapor pressure changes with temperature Distillation Basics, M.P.Shah Page 4 Why Distillation? • The preeminence of distillation is due to strong fundamental base : Kinetics and Thermodynamics • Inherently lower thermodynamic efficiency (10%) • No other processes are more efficient than Distillation • Inter-condensers or inter-reboilers can enhance the separation. When Distillation will fail? 1. The difference in the volatility between the components are small. 2. A small quantity of LVC is to be recovered. 3. A compound is thermally unstable even under vacuum conditions. 4. The mixture is extremely corrosive or highly fouling. Types of Distillation Operation Based on Mode of operation • BATCH – The feed to the column is introduced batch-wise. That is the column is charged with a batch and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced. – Quality is poor but quantity can be maintained. • CONTINUOUS – The feed to the column is continuous. No interruption occur unless there is a problem with the column or the surrounding process units. They are capable of handling high throughputs and are the most common of the 2 types. Quality of product is appreciable. Fig 3. Batch Distillation Column Fig. 4 Continuous Distillation Column Based on Number of components to be separated • BINARY DISTILLATION – Separation of 2 chemicals. – E.g. separation of ethyl alcohol (ethanol) from water • MULTICOMPONENT DISTILLATION – Separation of a mixture of chemicals – E.g. petroleum refining. Crude oil is a very complex mixture of hydrocarbons with literally thousands of different molecules. Nearly all commercial distillation is multicomponent distillation Some of the Complex Distillation Columns Distillation Basics, M.P.Shah Page 5 1. 2. 3. 4. 5. 6. Crude Distillation Column (Atmospheric Column) Vacuum Distillation Column (Dry and Wet Operations) FCC Fractionators Visbreaker Fractionators Demethanizer Azeotropic/Extractive/Reactive Fig. 5 Complex Distillation column for Petrochemical Sector Basic Components of Distillation Column – A vertical shell where the separation of liquid components is carried out – Column internals such as trays/plates and/or packings which are used to enhance component separations – A reboiler to provide the necessary vaporization for the distillation process – A condenser to cool and condense the vapor leaving the top of the column – A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column. – FEED is the liquid/Vapor mixture that is to be processed – Introduced usually somewhere near the middle of the column to a tray known as the feed tray. – The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Distillation Basics, M.P.Shah Page 6 Fig. 6 Component of Distillation Column Heat Supply in Distillation Column • Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. • In refineries, the heating source may be the output streams of other columns. • The vapor raised in the reboiler is re-introduced into the unit at the bottom of the column. • The liquid removed from the reboiler is known as the bottoms product or simply, bottoms. Heat Removal in Distillation Column • The vapor moves up the column, and as it exits the top of the unit, it is cooled by a condenser. • The condensed liquid is stored in a holding vessel known as the reflux drum. • Some of this liquid is recycled back to the top of the column and this is called the reflux. • The condensed liquid that is removed from the system is known as the distillate or top product. Fig 7. Heat Supply Distillation Basics, M.P.Shah Fig. 8. Heat Removal Page 7 Flash Distillation • Flash distillation is a process typically used to affect separation of crude oil. • The process involves heating a feed stream and then allowing it to expand into a vessel maintained at low pressure. • Partial vaporization then occurs, and a phase equilibrium is (ideally) reached. Fig. 9 Flash Distillation Tray Towers • • • • • • • • • • • • • • • • • • • Tray towers are vertical cylinders. Liquid and gas are contacted in stepwise fashion on trays. Liquid enters at top and flows downward by gravity. It flows across each tray and through a downspout to tray below. Gas flow upward through openings into liquids in the form of bubbles. Overall effect is a multiple countercurrent contact of gas and liquid. Each tray of the tower serves as a stage. No of equilibrium stages depends only upon the difficulty of the separation. No of equilibrium stages are determined by, Material balance calculations Equilibrium considerations Stage efficiency (Real trays) is determined by, Mechanical design The condition of operation Higher stage efficiencies depend upon: Sufficient contact time Large interfacial surface Higher intensity of turbulence The diameter of the tower depends upon the quantities of the liquid and gas flowing through tower per unit time. Tray Tower Internals • Trays : Provide a platform for sufficient contact between liquid and vapor • Downcomers: lead the liquid from one tray to the next • Weirs: maintains the depth of the liquid on the tray • Distribution baffles: Distributes the liquid from downcomer on the tray • Tray drainage : Permit drainage to liquid hold up of column in case of shut down • Feed system : proper distribution of feed on the feed tray Distillation Basics, M.P.Shah Page 8 • • Draw offs : for drainage of liquid from the bottom of the tray Manholes and hand-holes : for proper maintenance and cleaning of the distillation internals. Fig 10. Tray Tower View Fig. 11 Detailed view of Tray Tower Types of tray • Sieve trays • Valve trays • Bubble cap trays Distillation Basics, M.P.Shah Page 9 Sieve Tray Type Columns • Has no separation contacting device. • Large number of holes distributed uniformly over the plate act as passage for vapor, but at the same time the liquid is prevented from draining through the holes. • In some cases, holes are punched with protruding lips. This is known as a jet tray, Fig. 12 Sieve Trays Valve Tray Type Columns • Bubble caps are replaced by poppet type valves which are lifted by rising vapors and act as variable orifices. • These permits effective mixing of liquid and vapor over a wide range of loading. • These valves are circular with domed or flat caps. • Rectangular caps and caps with downward facing cones cannot rotate as the circular ones might do other certain process conditions. Distillation Basics, M.P.Shah Page 10 Fig. 13 Valve Type Trays Bubble Cap Tray Type Columns • Large number of holes over which cylindrical risers or chimneys are placed • Inverted cups are located over the risers • The vapor rises up through the holes and the risers and subsequently through the number of rectangular, triangular or trapezoidal slots provided at the rim of the caps. • The riser diameter is normally 60 – 70% of the cap diameter • Skirt clearance is 13 – 38mm is recommended to prevent plugging of the slots due to residue building up Fig. 14 Bubble Cap tray Distillation Basics, M.P.Shah Page 11 Flow Pattern in Various Type of Trays Fig. 15 Flow Pattern in Various Types of Trays Factors to be considered for Selection of type of Tray • Cost : depends on MOC; for MS BC:ST:VT = 3.0:1.5:1.0 • Capacity : ST>VT>BC • Operating range : vapor and liquid rates; ST<BT<VT; should be flexible for startup and shut down loads • Efficiency : not much change in all trays • Pressure drop on each tray : ST< VT<BT Feed Stage Conditions In determining the operating lines for the rectifying and stripping sections we needed the bottoms and distillate compositions and reflux and reboil ratios. The compositions can be independently specified, but R and VB are related to the vapor to liquid ratio in the feed. Fig. 16 Feed Condition Distillation Basics, M.P.Shah Page 12 Tray Tower Performance • Weeping : – A condition occurring where vapor rate in not large enough to hold all the liquid on the tray so that part of liquid flows over the weir and remaining may rain down through the openings. • Dumping : – A condition when none of the liquid reaches the downcomer, all draining down through the tray holes ( at very low gas rates). • Flooding : – It is excessive accumulation of liquid inside the column due to high pressure drop. – High pressure drop condition would occur when liquid leaves one tray at low pressure and enters other tray at high pressure. – At low liquid flow rates, most of the liquid on the tray is in the form of liquid drops. As the vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. Entire space between two trays is filled with liquid. The liquid accumulates on the tray above instead of flowing to the tray below. The tower is then flooded. – Due to flooding, tray efficiency falls to a lower value. Packed Columns • • • • • • • • Packed towers are used for gas-liq and liq-liq operation. Used for distillation, absorption, • Promote uniform vapor gas flow liq-liq extraction. across the column cross section. Liquid flows downwards over the • strong but without excessive surface of the packing in the form weight of thin films, while gas/vapor rises. The liquid is introduced at the top of the packing by means of a distributing plate and the vapor is introduced beneath a grid which supports the packing, carry out efficient mass transfer operation. It is desire that pressure drop be kept as maximum as possible. Provides large surface area: a high interfacial area between gas and liquid contain adequate passages (void volume) for both streams without excessive liquid hold-up or pressure drop Have an open structure: less resistance to gas flow Perform uniform liquid distribution on the packing surface Fig.17 Packed Column The basic unit consists of: • Shell • Packing Distillation Basics, M.P.Shah Page 13 • • • • Packing support Liquid distributor Intermediate support and redistributors Gas and liquid entrances and nozzles. Table 2. Packing Material Packing material General service application Glazed and unglazed, porcelain or chemical Stoneware Natural and acid conditions, except hydrofluoric, solvent, not good in caustic condition Carbon Hot alkali, all acids except nitric, no oxidizing atmospheres Plastic Alkali, salts, aqueous and acids, depending on resin Steel and other light gauge metals Hot alkalis for steel, other service to suit metals Types of Packing   Random Packing Structured Packing (Regular Packing) Random Packing Random packings are simply dumped into the tower during installation.  Rasching ring: o Hollow cylinder o Diameter: 6 – 100 mm o MOC: Porecelain (except alkalis and HF acid) o MOC: Carbon( except highly oxidizing atm) o MOC: Plastics (except organic solvents)  Lessing rings and other packings with internal partition are less frequently used.  The saddle shaped packings like berl saddle and Intalox are available in the sizes from 6 to 75 mm. Regular Packing Distillation Basics, M.P.Shah • • • • The counter flow trays are a form of regular packing. They offer the great advantage of low pressure drop for the gas and greater possible fluid flow rates. Stacked rasching rings are economically possible only in very large sizes. Wood grids or hurdles are generally inexpensive and frequently used where large void volumes are used. Page 14 Packing Support • The function of support plate is to carry weight of the wet packing, which allowing free passage of the gas and liquid. • Poorly designed support will give high pressure drop and cause local flooding. • Simple grid or perforated plates are used as support. • Made from plastic, ceramic or metals Fig 19 Packing Support Various Other types of Packing Support Fig. 20 Packing Support Liquid Distributor • Performance of packed bed depends on proper distribution of liquid in column. • Central feed pipe, spray nozzle, weir type, orifice type, feed pipe with nozzles on it. • Liquid redistributors • They are used to recollect the liquid that has migrated to the column walls and redistribute it evenly over the packing. • Does dual work packing support and liquid distribution. • Wall wiper type Distillation Basics, M.P.Shah Page 15 Fig. 21 Liquid Distributor Hold Down Plate • At higher gas rates, if surging occurs, the top layer of packing will fluidized. • This may lead to break up of packing or some time blow out. • Also called as bed limiters by avoiding bed expansion. • Hole size on the hold down plates should be less than the diameter of the packing but higher enough to give proper flow of gas and liquid through it. Installing Packing • Ceramic and metal packing are normally dumped to the column “WET”. • The column is partially filled with water and the packing dumped into the water. • The height of the water is always kept higher than the packing length. • In case of dry packing, packing are send into column through the bucket and spread over to the column by maintaining randomness. Characteristics of Packed Column • Low pressure drop / smaller diameter • Random packing scale-up for HETP is difficult; structured packing scale-up is predictable • HETP prediction less well developed than for trays • Low to moderate cost for random packing; high cost for structured packing • Not suitable for fouling service • Feed point flexibility is difficult Selection of equipment for Distillation Column • For high vacuum distillation, a packed column is selected due to small pressure drop requirement. • For very small tower diameter packed tower is preferred. • For handling very high ratio of liquid flow rate, tray towers are preferred. • For corrosive system, packed tower with plastic packings (PP) , glass fiber etc. is more economical than tray made of metals. • If side products are to be drawn from the distillation column, tray tower are preferred. • Packed column are called continuous contact column • Tray columns are called staged contact column Distillation Basics, M.P.Shah Page 16 Deciding Operating Condition in Distillation Column • • • • • • Feed composition and quantity. Reflux ratio (known or unknown) Thermal condition of feed (q line or value) Degree, type or amount of fractionation or condensation of overhead, including composition of overhead or bottoms. Column operating pressure or temperature of condensation of overhead including type of condensation, i.e. total or partial condenser Constant molal overflow from stage to stage for simple ideal systems following Raoult’s law. Total Condenser • • • • Overhead vapor is condensed to liquid state Total vapor pressure of condensate = pressure inside condenser + pressure inside accumulator When heat load on condenser is exactly equal to the latent heat of saturated vapor, condensed liquid will be bubble point liquid. Condensate is divided to two streams one going to top of the column being reflux and second will be distillate. Partial Condenser • • • • Product will be vapor in equilibrium with liquid. Will be designed as top external tray of distillation column. All condensed liquid is returned to column as reflux, while all vapor is withdrawn from the accumulator as product. In this case yc = xD Both liquid and vapor products are withdrawn, with liquid reflux composition being equal to liquid product composition. Fig. 22 Condenser in Distillation Column Distillation Basics, M.P.Shah Page 17 Total Reflux – Minimum Number of Trays • When all overhead vapor from top tray is condensed and returned to the top tray. • Usually column is brought to equilibrium at total reflux for test. • Minimum number of plates are required under this condition. • Called as no feed, no product condition. • Infinite heat requirements, L/V =1. • One of the extreme condition of distillation used for measure of complexity or difficulty of separation. Minimum Reflux – Infinite Number of Trays • As reflux ratio is decreased from infinity for total reflux condition, more theoretical steps or trays are required to complete a given separation. • Another limiting condition for column operation. • Below this ratio the specified separation cannot be made even with infinite number of plates. • Actual reflux ratio is generally considered as 1.2 to 1.5 times minimum reflux ratio. • Graphically it is evaluated as the line with smallest slope from xD intersecting with equilibrium curve at the same point as the “q” line for the mixture following Raoult’s Law. Optimum Reflux Ratio  Should be optimum and the most economical.  Optimum reflux s the value of reflux ratio for which the total cost of distillation is minimum.  Depends upon : Type of system/ feed, specified separation and operating condition.  Change in the desired purity of products will also change the optimum value.  At the minimum reflux : column requires infinite number of trays or packing height so infinite fixed cost, but minimum operating cost (heating, cooling mediums, reflux).  As reflux increases: no of trays and packing decreases, but Dc, size of condenser and reboiler will increases, reflux pump cost will increases so fixed cost decreases and but operating cost increases.  This is not continued as the reflux increases, at certain value, no of trays required will become constant but size of heat exchanger, pump of reflux is still increases.  That means, fixed value will pass through a minimum value at some reflux ratio after that it will again increase.  Operating cost always increases.  Total annual cost = fixed cost + operating cost  So optimum reflux ratio will minimize above cost.  Many hydrocarbon distillations, R = 1.2 to 1.5Rm  Difficult distillations R > 1.5Rm.  Azeotropic mixture R is very high. Distillation Basics, M.P.Shah Page 18 Operating Pressure High Pressure Distillation • • Based on available cheapest cooling medium able to condense top vapor. Bubble point of distillate should be sufficiently greater than cooling water temperature. • In case of partial condenser, bubble point of reflux should be sufficiently greater than temperature of cooling water. • If by this criterion, operating pressure is very high then go for chilled water or brine solution. • Operating pressure is always less than the critical temperature of top product or temperature of cooling medium of overhead condenser cannot be greater than critical temperature of top product. Example 1: • Acetaldehyde – ethanol – water system. • Top product pure acetaldehyde (B.P. 20C) • In country like India cooling water is available at 32C. • Acetaldehyde is stored in bullet type storage tank. To avoid storage cost like refrigeration, insulation etc it is necessary to increase the B.P. of acetaldehyde. • It was required to have B.P. > 50C (To use water as cooling media). • It was decided to keep B.P. = 60C. • This requires to have top product at 3.6atm g pressure (V.P. of acetaldehyde at 60C). Example 2: • Separation of ethane from ethane-propane and butane cooling water can not be used as cooling medium. Why? • Because tc = 32.3C. (not much greater than cooling water temperature of 32C). • Minimum driving force required in STHE is 3-5C. • At 10C V.P. of ethane 30atm. So one can use chilled water as cooling medium but has to work with 30atm pressure. • At 20atm, B.P. -6.4C, one can use proper brine solution, but it will require higher cost of refrigeration and insulation. • If this column is followed by demethanizer, then top product stream of demethanizer can be used as cooling medium. Example 3: • Mixture of chloromethanes  methyl chloride (B.P. -24C) • V.P. of MC at 47.5C is 10atm to avail cooling water facility. Low Pressure Distillation (Vacuum Distillation) • Heat sensitive materials • To facilitate the use of cheaper heating medium in reboiler • Alter vapor-liquid equilibrium data. Advantage of using Vacuum Distillation • Prevents thermal decomposition of heat sensitive materials. – Separation of MCAA – AA. (B.P. of MCAA = 120C) Distillation Basics, M.P.Shah Page 19 • Allows use of more economical heating medium such as steam against hot oil in high capacity plants. – EG – Water (B.P. EG = 197C (bottom), superheated steam = 180C) • Requires fewer stages or lower reflux ratio or both in many cases where relative volatility increases with decrease in the operating pressure. • In some cases vacuum breaks azeotrope and make the separation easier. – Ethanol – water P < 70torr no azeotrope • Reduces undesirable side reactions in reactive distillation. – Ethyl benzene – styrene under vacuum does not polymerize • Provides safety in distillation of toxic and hazardous material. – Separation of HNO3 from azeotrope of HNO3 - water • Increases product recovery when dealing with maximum temperature constraints. - Refinery Vacuum towers Disadvantage of using Vacuum Distillation • Required larger column diameter and larger vapor line diameter • Costlier cooling source is required for condensation. • Presence of air as non-condensables reduces the efficiency of main condenser. • Increases the possibility of contamination of products by air or other components. Operating Temperature Decided based on Thermodynamics study Fig. 23 T-x-y Diagram for Deciding T in Distillation Column Distillation Basics, M.P.Shah Page 20