Distillation Handbook CONTENTS Introduction ......................................................3 Distillation .........................................................4 APV in Distillation ...........................................5 Basic Principles of Distillation .....................6 Distillation Terminology ..................................9 System Components ...................................14 Steam Stripping ............................................23 Solvent Recovery ..........................................26 Distillation Column Control ........................31 Modular Systems ..........................................35 Applications ....................................................37 Case Study.....................................................44 Major APV Distillation Customers.............49 2 INTRODUCTION While the use of distillation dates back in recorded history to about 50 B.C., the first truly industrial exploitation of this separation process did not occur until the 12th century when it was used in the production of alcoholic beverages. By the 16th century, distillation was also being used in the manufacture of vinegar, perfumes, oils and other products. As recently as two hundred years ago, distillation stills were small, of the batch type, and usually operated with little or no reflux. With experience, however, came new developments. Tray columns appeared on the scene in the 1820s along with feed preheating and the use of internal reflux. By the latter part of that century, considerable progress had been made. Germany’s Hausbrand and France’s Sorel developed mathematical relations that turned distillation from an art into a well defined technology. Today, distillation is a widely used operation in the petroleum, chemical, petrochemical, beverage and pharmaceutical industries. It is important not only for the development of new products, but also for the recovery and reuse of volatile liquids. For example, pharmaceutical manufacturers use large quantities of solvents, most of which can be recovered by distillation with substantial savings in cost and pollution reduction. While distillation is one of the most important unit operations, it is also one of the most energy intensive operations. It is easily the largest consumer of energy in petroleum and petrochemical processing, and so, must be approached with conservation in mind. Distillation is a specialized technology, and the correct design of equipment is not always a simple task. This handbook describes APV’s role in developing distillation systems, details different types of duties, discusses terminology and calculation techniques, and offers a selection of case studies covering a variety of successful installations. 3 Distillation Distillation, sometimes referred to as fractionation or rectification, is a process for the separating of two or more liquids. The process utilizes the difference of the vapor pressures to produce the separation. Distillation is one of the oldest unit operations. While the first technical publication was developed in 1597, distillation already had been practiced for many centuries – specifically, for the concentration of ethyl alcohol for beverages. Today, distillation is one of the most used unit operations and is the largest consumer of energy in the process industries. APV has been conducting business in the field of distillation since 1929. A brief history of APV in distillation is shown in Figure 1. Today, APV mainly concentrates its marketing efforts in the area of solvent recovery, waste water stripping, chemical production and specialized systems, such as high vacuum systems for oils. A HISTORY OF APV IN DISTILLATION 1929 First Distillation Columns Manufactured 1933 West Tray License Obtained 1935 First Major APV Designed and Manufactured Distillation System 1935 Distillation Laboratory Established 1939 First Fuel Ethanol Distillation System 1939-45 Many Toluene/Benzene Systems Produced 1946 Acetic Acid Recovery System The Largest Order APV Had Ever Received 1969 Acquired L.A. Mitchell Group and Glitsch License for Valve Trays 1971 First Distillation System in USA 1990 The 100th U.S. Distillation System Figure 1. Brief history of APV distillation. 4 APV in Distillation Complete Solutions for Your Distillation Requirements Process Technology Conceptual Design Process Simulation Pilot Plant Testing 80 Years of Experience Process Guarantee Control Systems Integration with Process Technology Functional Design Specification Project Management Project Engineering Equipment Fabrication Installation Training Start Up After Sales Service Customer Service Troubleshooting Spare Parts 5 Basic Principles Of Distillation When a mixture of two or more liquids is heated and boiled, the vapor has a different composition than the liquid. For example, if a10% mixture of ethanol in water is boiled, the vapor will contain over 50% ethanol. The vapor can be condensed and boiled again, which will result in an even higher concentration of ethanol. Distillation operates on this principle. Clearly, repeated boiling and condensing is a clumsy process, however, this can be done as a continuous process in a distillation column. In the column, rising vapors will strip out the more volatile component, which will be gradually concentrated as the vapor climbs up the column. The vapor/liquid equilibrium (VLE) relationship between ethanol and water is shown in Figure 2. A similar relationship exists between all compounds. From this type of data, it is a relatively simple task to calculate the design parameters using one of the classical methods, such as McCabe-Thiele. The key to this separation is the relative volatility between the compounds to be separated. The higher the relative volatility, the easier the separation and vice versa. For a binary system, the mole fraction y of component a in the vapor in equilibrium with the mole fraction x in the liquid is calculated from the following equation. ya = α.xa 1 + (α-1).xa Where xa is the mole fraction of a in the liquid and α is the relative volatility. The larger the relative volatility, the more easily the compound will strip out of water. For ideal systems which follow Raoult’s law, the relative volatility is calculated by α = Pa/Pb Where P and P are the vapor pressures of components a and b at a a b given temperature. 6 The partial pressure p of component a above a binary ideal solution can be calculated by pa = Pa.xa Where x is the mole fraction of component a in the liquid. a Similarly in a binary mixture, for component b. p = P .x b b b Notice that the sum of the partial pressures must equal the total system pressure: P=p +p . For non ideal mixtures (usually the case with steam b a stripping duties), the partial pressure is calculated from Pa = γaPaxa Pb = γbPbxb Where γ is the activity coefficient of the compound. The activity coefficient essentially quantifies the deviation from ideality. For multicomponent mixtures, the mathematical representation of the VLE becomes more complex. It is necessary to use complex equations to predict the performance. The simplification commonly used as a substitute for the rigorous equations is K value. y =Kx . The ratio of the a a K value of different components reflects the relative volatilities between those components. It is not the intention of this publication to discuss methods for calculating a distillation system. Classical graphical calculations have been the McCabe- Thiele method, using the data shown in Figure 2, and the Ponchon Savarit method, which is more accurate and uses an enthalpy diagram, as shown in Figure 3, as well as the VLE data. All these graphical methods have been rendered obsolete by the various process simulation programs, such as SimSci. Even with these highly sophisticated programs, there is still a need for test work on many systems. For ideal mixtures, which are rare, the program will provide a theoretically correct solution. For non ideal mixtures, the program can only make estimates by using thermodynamic equations such as UNIFAC. Experimental data can be used for more precise solutions. A considerable amount of experimental data, however, is in the program database. 7 1~ (1) ETHANOL C2H6O 0.80 (2) WATER H2O +(1+) + + + + A8N.1T1O22IN0E C 1O5N9S2T.8A6N4T S 226.104REGI2O0N- + 9+3+ + +C+ METHCOODN 1S I S T E N-CY 0.60 (2) 8.07131 1730.630 233.426 20- 100 C METHOD 2 + γ 1 PRESSURE- 760.00 MM HG ( 1.013 B ) 0.40 LT: DALAGER P.J. CHEM. ENG. DATA 14,298 (1969). WILSON VCMWNUAONRAINLNTIRQSL SGLTUO AAU ANANLC RET SS : - 24 - 921 11429..7.7..64185823A579781079702 1 459 2 51 07710 A.....75218221939147407 33728 A0L.2P8H6A212 00..0200 γγ ∞∞ 12 == 26..6021 0.00 0.20 0.40 0.60 0.80 1.00 Figure 2. X1 CALCULATIONS BY ENTHALPY COMPOSITION DIAGRAM 1200 1100 1000 1.00 F Saturated Vapor 2º 3 at 900 0.90 Liquids 800 1at0 1% a Ltmiquid220 ºF220 ºF 0.80 o Pure 700 20% 0.70 Vapor d t 30% m e Enthalpy, Btu/lb. Solution Referre 654320000000000 210.1 ºF 208.590%206.98020%4.872003.%460%50%14907.2% 189.2 184.5 181.7 179.6 177.8 176.2 174.3174.0173.7173.4173.2173.0172.9172.8 172.8173.000000.....2345600000 Mass Fraction Ethanol in Equilibriu ativ 100 118600ºF Saturated Liquid 0.10 el 140 R 120 100 80 0 32 6040 0 Freez2in0g Line 0 -100 0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Mass Fraction Ethanol Water Mixtures Figure 3. Calculations by enthalpy-composition diagram. 8 Distillation Terminology To provide a better understanding of the distillation process, the following briefly explains the terminology most often encountered. Solvent Recovery The term “solvent recovery” often has been a somewhat vague label applied to the many different ways in which solvents can be reclaimed by industry. One approach employed in the printing and coatings industries is merely to take impure solvents containing both soluble and insoluble particles and evaporate the solvent from the solids. For a duty of this type, APV offers the Forced Circulation Evaporator, a compact unit which combines a Paraflow plate heat exchanger and a small separator. As the solvent laden liquid is recirculated through the heat exchanger, it is evaporated and the vapor and liquid are separated. This will recover a solvent, but it will not separate solvents if two or more are present. Another technique is available to handle an air stream that carries solvents. By chilling the air by means of vent condensers or refrigeration equipment, the solvents can be removed from the air stream. Solvents also can be recovered by using extraction, adsorption, absorption and distillation methods. Solvent Extraction Essentially a liquid/liquid process where one liquid is used to extract another from a secondary stream, solvent extraction generally is performed in a column somewhat similar to a normal distillation column. The primary difference is that the process involves the mass transfer between two liquids instead of a liquid and a vapor. During the process, the lighter (i.e., less dense) liquid is charged to the base of the column and rises through packing or trays while the more dense liquid descends. Mass transfer occurs and one or more components is extracted from one stream and passed to the other. Liquid/liquid extraction sometimes is used when the breaking of an azeotrope is difficult or impossible by distillation techniques. 9 Carbon Adsorption The carbon adsorption technique is used primarily to recover solvents from dilute air or gas streams. In principle, a solvent laden air stream is passed over activated carbon and the solvent is adsorbed into the carbon bed. When the bed becomes saturated, steam is used to desorb the solvent and carry it to a condenser. In such cases as toluene, for example, recovery of the solvent can be achieved simply by decanting the water/solvent two phase mixture which forms in the condensate. Carbon adsorption beds normally are used in pairs so that the air flow can be diverted to the secondary bed when required. On occasion, the condensate is in the form of a moderately dilute miscible mixture. In this case, the solvent must be recovered by distillation. This would apply especially to water miscible solvents such as acetone. Absorption When carbon adsorption cannot be used because certain solvents either poison the activated carbon bed or create so much heat that the bed can ignite, absorption offers an alternate technique. Solvent is recovered by pumping the solvent laden air stream through a column countercurrently to a water stream, which absorbs the solvent. The air from the top of the column essentially is solvent free, while the dilute water/solvent stream discharged from the column bottom usually is concentrated in a distillation column. Absorption also can be applied in cases where an oil rather than water is used to absorb certain organic solvents from the air stream. Azeotropes During distillation, some components form an azeotrope at a certain stage of the fractionation, requiring a third component to break the azeotrope and achieve a higher percentage of concentration. In the case of ethyl alcohol and water, for example, a boiling mixture containing less than 96% by weight ethyl alcohol produces a vapor richer in alcohol than in water and is readily distilled. At the 96% by weight point, however, the ethyl alcohol composition in the vapor remains constant (i.e., the same composition as the boiling liquid). This is known as the azeotrope composition and further concentration requires use of a process known as azeotropic distillation. Other common fluid mixtures which form azeotropes are formic acid/water, isopropyl alcohol/ water, and iso butanol/water. 10