Pharmaceutical Technology Evaporation PDF

9/23/2024 Pharmaceutical Technology Evaporation and Evaporators Mina Mehanny, Ph.D. 1  9/23/2024 Lecture Outline Definition of Evaporation Applications of Evaporators in Pha...

9/23/2024 Pharmaceutical Technology Evaporation and Evaporators Mina Mehanny, Ph.D. 1  9/23/2024 Lecture Outline Definition of Evaporation Applications of Evaporators in Pharmaceutical Industry Evaporation Processing Factors Heat Transfer to Boiling Liquids in an Evaporator Evaporator Types (Uses/Advantages/Disadvantages) Mina Mehanny, Ph.D. 3 Definition Evaporation may be defined as the removal of a solvent from a solution by vaporization for the concentration of solutions by boiling. Evaporation is used to increase the concentration of relatively non-volatile dissolved or suspended components in a solution or slurry by evaporating portions of the liquid phase using energy supplied by a medium, often steam. Mina Mehanny, Ph.D. 4 2  9/23/2024 The dissolved or suspended components do NOT appear in the vapor phase (If they do, the process is referred to as distillation). Example of applications: 1. concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice. Mina Mehanny, Ph.D. 5 2. Evaporation processes to evaporate seawater to provide drinking water have been developed and used (Desalination). 3. The important product in evaporation can be either the more concentrated mixture left behind or the overhead vapor (which is often, but not necessarily, subsequently condensed). Mina Mehanny, Ph.D. 6 3  9/23/2024 Some processes of evaporation can be accompanied by: 1. Crystallization, as the residual liquor grows more and more concentrated. 2. If continued further, evaporation evolves to drying (or dehydration, if the constituent removed is water), as the bottoms product obtained becomes primarily solid rather than liquid. Mina Mehanny, Ph.D. 7 Applications of Evaporators in Pharmaceutical Industry 1. Prepare for a next unit operation, crystallization, drying, etc. 2. Reduce deteriorative chemical reactions (e.g. hydrolysis). 3. Solvent recovery 4. Better microbiological stability 5. Reduce transportation and storage costs Mina Mehanny, Ph.D. 8 4  9/23/2024 Processing Factors 1. Concentration in the liquid - Evapoartion is a surface process..i.e mass transfer occurs at the surface: low viscosity: high mass transfer coefficient high viscosity: low mass transfer coefficient Adequate circulation and/or turbulence must be present, Why?? to keep the coefficient from becoming too low 2. Solubility solubility increases with temperature crystallization may occur when a hot concentrated solution is cooled to room temperature Mina Mehanny, Ph.D. 9 Processing Factors 3. Temperature sensitivity of materials Food and biological materials may be temperature sensitive and degrade at higher temperature or after prolonged heating. 4. Foaming or frothing food solution such as skimmed milk and some fatty-acid solution form a foam or froth during boiling. Mina Mehanny, Ph.D. 10 5  9/23/2024 Processing Factors 5. Pressure and temperature High operating pressure: high boiling point 6. Scale deposition and materials of construction Some solutions deposit solid materials called scale on the heating surfaces. results in the overall heat-transfer coefficient decreases and evaporator must be cleaned. Mina Mehanny, Ph.D. 11 HEAT TRANSFER TO BOILING LIQUIDS IN AN EVAPORATOR The heat required to boil a liquid in an evaporator is usually transferred from a heating fluid, such as steam or hot water, across the wall of a jacket or tube in or around which the liquid boils. Mina Mehanny, Ph.D. 12 6  9/23/2024 HEAT TRANSFER TO BOILING LIQUIDS IN AN EVAPORATOR A qualitative discussion of the methods used to secure high rates of heat flow can be based on the equation: Q = U.A.ΔT where Q is the rate of heat flow, U is the overall heat transfer coefficient, A is the area over which heat is transferred, and ΔT is the difference in temperature between the fluids (liquid to be evaporated and steam for example). Mina Mehanny, Ph.D. 13 HEAT TRANSFER TO BOILING LIQUIDS IN AN EVAPORATOR If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor (if the feed entering has TF near the boiling point) The concept of an overall heat-transfer coefficient is used in the calculation of the rate of heat transfer in an evaporator. Mina Mehanny, Ph.D. 14 7  9/23/2024 The general equation can be written Where: Q is the rate of heat transfer in W (btu/h), U is the overall heat-transfer coefficient in W/m2. K (btu/h ft. °F), A is the heat-transfer area in m2 (ft2), TS is the temperature of the condensing steam in K (°F), Tl is the boiling point of the liquid in K (°F). Mina Mehanny, Ph.D. 15 The overall heat transfer coefficient is derived from a series of individual coefficients that characterize the thermal barriers opposing heat transfer. Thus, for the heating fluid, the film coefficient for a condensing vapor, such as steam, is high provided that permanent gases and condensate are removed by venting and draining. Other factors are, the area of the heat transfer surfaces, which should be as large as possible, and the temperature difference between the heating surface and the boiling liquid. Mina Mehanny, Ph.D. 16 8  9/23/2024 EVAPORATORS Types A) Based on heating media B) Based on Effect 1. Single effect evaporators 2. Multiple effect evaporators C) Based on circulation of liquor over heating surface 1. Natural Circulation evaporators 2. Forced Circulation evaporators 3. Film type (wiped film) evaporators Mina Mehanny, Ph.D. 17 Evaporation Under Reduced Pressure Description: It is also known as vacuum evaporation Vacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Mina Mehanny, Ph.D. 18 9  9/23/2024 Evaporation Under Reduced Pressure Description: When the process is applied to a product and the water is evaporated and removed, the product can be stored for long periods of time without spoiling. It is also used when boiling or heating a substance at normal temperatures would chemically change the consistency of the product. Vacuum evaporation is used whenever possible because there is a saving of time, heat and labour. Mina Mehanny, Ph.D. 19 1.Single-effect Vacuum Evaporator A single effect evaporator is utilized using water as feed and condensed water and liquid water as product. This evaporator may be run with and without a vacuum at different steam pressures. Running the evaporator under a vacuum proved to be more efficient with respect to steam usage for evaporation. Mina Mehanny, Ph.D. 20 10  9/23/2024 In this type of evaporator the vapor from boiling liquid is condensed and collected or discarded. Therefore, it can be used for solvent collection. Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost. However, for large-capacity operation, using more than one effect will markedly reduce steam costs Mina Mehanny, Ph.D. 21 Diagram: Mina Mehanny, Ph.D. 22 11  9/23/2024 1. Single-effect evaporators Simplified diagram of single-effect evaporator Mina Mehanny, Ph.D. 23 1. The feed enters at TF 2. Saturated steam at TS enters the heat- exchange section. 3. Condensed steam leaves as condensate or drips. 4. The solution in the evaporator is assumed to be completely mixed. Hence, the concentrated product and the solution in the evaporator have the same composition. Mina Mehanny, Ph.D. 24 12  9/23/2024 5. Temperature T1 is the boiling point of the solution. 6. The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution. 7. The pressure is P1, which is the vapor pressure of the solution at T1. Mina Mehanny, Ph.D. 25 Disadvantage: It uses steam inefficiently Mina Mehanny, Ph.D. 26 13  9/23/2024 2.Triple-effect Vacuum Evaporator This equipment adopts the principle of multitubular circulation out-heating working principle to keep the short heating time of materials, rapid evaporation speed, large concentration ratio, so that to keep the raw-effect of materials effectively. Energy will be well saved 70% than single-effect evaporator. Materials are evaporated & concentrated under sealed state which is clean and comfortable. Besides, it is equipped with special foam-removing device to prevent the material running phenomenon. Mina Mehanny, Ph.D. 27 2.Triple-effect Vacuum Evaporator 2.Triple-effect Vacuum Evaporator All the parts touched with materials are made of stainless steel and be polished. The equipment has the properties of good corrosion- proof, easy of cleaning in accordance with requirements of Pharmacy, Food Hygiene. Mina Mehanny, Ph.D. 28 14  9/23/2024 Simplified diagram of forward -feed triple-effect evaporator. Mina Mehanny, Ph.D. 29 If the feed to the first effect is near the boiling point at the pressure in the first effect, 1kg of steam will evaporate almost 1 kg of water. The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect. Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect. As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator. Mina Mehanny, Ph.D. 30 15  9/23/2024 Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased. This also holds approximately for more than three effects. However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators. Mina Mehanny, Ph.D. 31 In forward-feed operation, the fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow. This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures. The boiling temperatures decrease from effect to effect. This means that if the first effect is at P1 = 1 atm abs pressure, the last effect wilt be under vacuum at a pressure P3. Mina Mehanny, Ph.D. 32 16  9/23/2024 Backward-feed multiple-effect evaporators In the backward-feed operation shown in Fig. 8.2-4 for a triple- effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. Simplified diagram of backward-feed triple-effect evaporator. Mina Mehanny, Ph.D. 33 Forward-feed operation Backward-feed operation Mina Mehanny, Ph.D. 34 17  9/23/2024 This method of reverse feed is advantageous when the fresh feed is cold, since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects. However, liquid pumps must be used in each effect, since the flow is from low to high pressure. This reverse-feed method is also used when the concentrated product is highly viscous. Mina Mehanny, Ph.D. 35 Parallel-feed multiple-effect evaporators Parallel-feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt. Mina Mehanny, Ph.D. 36 18  9/23/2024 Advantages of Multiple Effect Evaporators 1. Suitable for large scale and for continues operations. 2. Highly economical when compared to single- effect evaporator. Mina Mehanny, Ph.D. 37 1.Natural Circulation Evaporators Circulation is obtained by convection currents arising from heating surface Subtypes: A) The evaporating pan B) Horizontal Tube evaporator C) Vertical tube evaporators with steam outside (Calandria or short-tube evaporators) Mina Mehanny, Ph.D. 38 19  9/23/2024 A) The Evaporating pan Simplest and oldest type evaporator Hemispherical or shallow open pans made of copper, stainless steel, aluminum or other metal surrounded by a steam jacket. Used for concentration of Jams and Jellies, also for some pharmaceutical products Steam enters the jacket and gives its heat content, condenses and then leave as a condensate Mina Mehanny, Ph.D. 39 Advantages: Simple, easy and cheap Easy to clean Stirring is possible Disadvantages: Not suitable for heat sensitive materials. Heat transfer decreases significantly when the conc. Increased (viscosity). Mina Mehanny, Ph.D. 40 20  9/23/2024 Different types of evaporators: (a) horizontal-tube type (b) vertical-tube type Mina Mehanny, Ph.D. 41 B) Horizontal Tubes evaporators Steam is passed through tubes which are immersed in the pool of liquid to be evaporated. Feed is introduced into the evaporator until the steam compartment is immersed (Heat transfer occurs and the solvent is evaporated and steam condenses) These tubes interfere with natural circulation and minimize liquid agitation. Overall heat transfer coefficients are low. Mina Mehanny, Ph.D. 42 21  9/23/2024 Mina Mehanny, Ph.D. 43 Advantages 1. Cheap 2. Easy to install 3. Require less space for installation 4. Suitable for liquids that not crystallize 5. Can be used for batch/continuous operation Disadvantages 1. Not suitable for viscous liquids because of poor circulation Mina Mehanny, Ph.D. 44 22  9/23/2024 C) Vertical Tube evaporators Tubes are vertical with steam outside (Calandria or short tube evaporators) Solution boils inside vertical tubes with heating media, usually steam, held in steam chamber, through which tubes pass. i.e: liquid passes inside tubes and steam is outside the tubes Boiling of liquid in tubes, causes liquid flow upward through tubes and un- evaporated liquid flows downward through central hole. Mina Mehanny, Ph.D. 45 Mina Mehanny, Ph.D. 46 23  9/23/2024 C) Vertical Tube evaporators These overcome disadvantages of horizontal tube evaporators 1. Natural circulation is promoted 2. Heat transfer coefficients are higher 3. Solid built inside tubes is removed by mechanical cleaning 4. Viscous liquids can be used, but circulation is slow 5. Used in sugar and salt industries Disadvantage 1. These are impractical when solution is very viscous or form foams or is heat sensitive 2. Not suitable for thermolabile products Mina Mehanny, Ph.D. 47 2. Forced circulation evaporators The natural circulation induced by boiling in a vertical tube may be supplemented by an axial impeller mounted in the down comer of the calandria. This modification is used when viscous liquids or liquids containing suspended solids are evaporated. Such units are employed in evaporative crystallization. Mina Mehanny, Ph.D. 48 24  9/23/2024 3. Film evaporators The feature of this kind of evaporator is that the required concentration of solution can be reached when the solution flows through the heating chamber once, and the solution flows like a liquid film along the heating tube to undertake the heat transfer and evaporation. Mina Mehanny, Ph.D. 49 3. Film evaporators The main features of single-pass film evaporator are that the heat transfer efficiency is high, the evaporation speed is high, and the residence time of the solution inside the evaporator is short. Therefore, single pass type evaporator is especially applicable for dealing with the evaporation of heat- sensitive solution. Mina Mehanny, Ph.D. 50 25  9/23/2024 3. Film evaporators According to different flowing directions of materials in evaporator and different reasons for film formation, film evaporators can be classified into the following types: A. Climbing film Evaporator B. Falling film evaporator Mina Mehanny, Ph.D. 51 A. Climbing film Evaporator The heating chamber of climbing film evaporator is composed of one vertical long tube or more. The ratio of the tube length and tube diameter is 100 to 150. Mina Mehanny, Ph.D. 52 26  9/23/2024 A- Climbing film Evaporator Feed solution enters from the bottom of the evaporator after preheated, and the heating vapor is condensed outside the tube. After heated to boil, the solution is vaporized quickly and the generated secondary vapor rises at high speed inside the tube. Mina Mehanny, Ph.D. 53 The solution is driven by the rising vapor at high speed. The solution rises like a liquid film along the tube inner wall and at the same time the vapor evaporates the solution continuously. After the vapor-liquid mixture enters the separator and the liquid product and secondary vapor are separated, the mixture is discharged from the bottom of the separator. Mina Mehanny, Ph.D. 54 27  9/23/2024 Advantages: 1. Relatively high heat transfer coefficient 2. Ability to handle foamy and viscous liquids. Mina Mehanny, Ph.D. 55 B. Falling film evaporator Mina Mehanny, Ph.D. 56 28  9/23/2024 The difference between it and climbing film evaporator is that its feed solution enters from the top of the heating chamber, drops in the form of film along the tube inner wall under the action of gravity, and is evaporated to increase the concentration during the dropping process. Mina Mehanny, Ph.D. 57 The vapor-liquid mixture flows to the bottom to enter the separator, and the liquid product is discharged from the bottom of separator. Mina Mehanny, Ph.D. 58 29  9/23/2024 The falling-film distributors must be installed on the top of each heating tube, to ensure that the solution drops in the form of film along the inner wall. Falling film evaporator can be used to evaporate the solution with higher concentration and viscosity, but is not applicable to the solution easy to crystallize or scale. Mina Mehanny, Ph.D. 59 C. Rising-falling film Evaporator Rising and falling film evaporators are some time combined in to a same unit. When a high ratio of evaporation to feed is required and concentrated liquid is viscous, a tube bundle can be divided in to two sections with the first section functions as a rising film evaporator and the second section serves as falling film evaporator. Mina Mehanny, Ph.D. 60 30  9/23/2024 Further reading Chapter 9: Evaporation and Distillation Original lectures were kindly provided by Prof. Mona Abdel-Mottaleb Videos links: https://www.youtube.com/watch?v=3vmr4b3vWFE&t=2s https://www.youtube.com/watch?v=hCCLhwyodS4 https://www.youtube.com/watch?v=COxnCz1WOXw https://www.youtube.com/watch?v=COxnCz1WOXw https://www.youtube.com/watch?v=vG-ze17ADPI https://www.youtube.com/watch?v=bpDTcpLUoYI Mina Mehanny, Ph.D. 61 31