DRYING

1. DRYING • removal of relatively small amount of water or organic liquids • final processing step before packaging • as a preservative technique esp. food • freeze-dried for biological & pharmaceutical materials 2 methods of drying: 1. batch - material put into dryer & drying proceeds for a given period of time 2. continuous - material continously added to dryer & continously removed 3 catagories of drying: 1. Direct contact with heated air at atmospheric pressure 2. Vacuum drying - heated indirectly either by contact with a metal wall or by radiation 3. freeze drying - water is sublimed from the frozen material

2. What is the Drying Process … • Drying - water liquid vaporization; not as efficient as centrifuge, 1050 BTU/lb of water removed.  • Energy intensive • Frequently over dried at added costs, dusting, product loss • Drying accounts for ~12% manuf. costs • Final moisture varies “dried” table salt contains 0.5 % water, dried coal 4%. • Solids can have many different forms, flakes, granules, crystals, powders, etc. The liquid can be on the surface, within the surface in cellular structures, such as wood. Consider the method of handling, dusting, rough or gentle treatment.

3. What is the Drying Process … • Generally, the materials being dried are solids that have been isolated by means of filtration following Xon from water or from one or more organic solvents. • As with most operations, drying is best viewed as part of an integrated process that includes these crystallization and isolation steps; changes in these operations can affect PSD, crystal form and moisture content, and they can have a significant impact on the drying efficiency. • Product drying is not a particularly energy-efficient process. Consider for example, that it can take 5-10 times the amount of energy to remove a kg of solvent in a drying operation than in a distillation operation. Thus, it is important to remove as much solvent or moisture from the cake as possible beforehand, and the selection of isolation equipment (pressure filter, centrifuge, etc..) has a major impact on accomplishing this goal. • The PSD obtained affects the efficiency of down-stream operations such as formulation and product effectiveness such as bio-availability and shelf life. • We will see later on in the ppt, the different types of dryers used in the Pharma industry and the parameters to consider when selecting the equipment for a particular API process. • It also is important to establish realistic specifications for these materials during product development – for example, drying to “zero” moisture content is not practical – which will, of course, depend on process requirements and material characteristics. Likewise, having robust analytical methods for monitoring drying or for product release is critical, as is establishing safe drying temperature ranges that will maximize drying efficiency without risking product decomposition, melting or agglomeration

4. In general terms, drying can be described by 3 processes operating simultaneously. The first process is energy transfer from an external source to the water or organic solvent in the material. There are two methods of transferring heat to the wet solids: direct and indirect. When using direct dryers, Heat transfer for drying is accomplished by direct contact between the wet solid and hot gases. The vaporized liquid is carried away by the drying medium. Indirect dryers: Heat for drying is transferred to the wet solid through a retaining wall. The vaporized liquid is removed independently of the heating medium. Rate of drying depends on the effectiveness of the contact between wet material with the hot surfaces. • The second process is the phase transformation of the water or organic solvent from a liquid or liquid-like state to a vapour state. Mass transfer in the solid as a liquid and a vapour and as a vapour from the exposed surfaces. Movement within the solid results from a concentration gradient which is dependent on the characteristics of the solid. A solid to be dried my be porous or no porous. It can also be hygroscopic or non hygroscopic. • The third process is the transfer of the vapour generated away from the pharmaceutical material and out of the drying equipment (pressure/vacuum) Analysis of the 3 processes is complicated by the fact the 3 processes are coupled to each other, and all 3 need to be considered simultaneously. • Conducting a drying study can help you determine where the major resistances to drying occur with your product, and the best types of drying systems to deliver the characteristics desired in pharmaceutical products.

5. Difference between drying and evaporation • 1- In drying processes, the main operation usually carried out on solid materials, e.g. powders, or products. • 2- Drying in most of the cases means the removal of relatively small amounts of water from solids .Evaporation include the removal of large amounts of water from solutions. • 3- In most cases, drying involves the removal of water at temperatures below its boiling point, whereas evaporation means the removal of water by boiling a solution. • 4- In drying , water is usually removed by circulating air over the material in order to carry away the water vapour , while in evaporation , water is removed from the material as pure water vapour mixed with other gases.

6. What is the Drying Process … Removal of small amount of liquid, usually water – Large amounts of water normally removed by press or centrifuges. Thermal methods employed. Heat and Mass transfer

7. Adiabatic dryers, solids are exposed to the heated gasses in various methods: • Blown across the surface cross circulation • Blown through a bed of solids, through-circulation; solids stationary; wood, corn etc • Dropped slowly through a slow moving gas stream, rotary dryer • Blown through a bed of solids that fluidize the particles; solids moving; frequently called fluidized bed dryer • Solids enter a high velocity hot gas stream and conveyed pneumatically to a collector Flash Dryer

9. Table 1 - Classification of dryers

10. Table 1 - Classification of dryers

11. Table 1 - Classification of dryers

12. Solid drying process is very complexwith micro and nano mechanisms Liquid movement due to capillary forces Diffusion due to concentration gradients Liquid vapor flow due to pressure differences Vapor diffusion due to vapor pressure differences, concentration differences Osmotic pressure created by colloidal bodies has soluble and insoluble fractions Vapor Effusion – A relationship of vapor flow to pore diameter Thermodiffusion Vaporization-condensation mechanism

13. The Drying Process can be classified as: Classifications Adiabatic Dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-Adiabatic Dryers When a dryer does not use heated air or other gasses to provide the energy required the drying process is considered a non-adiabatic. In the case of Adiabatic Dryers The process can be considered to be two related processes: Solids Drying Air Humidification We will view dryer control from the air humidification process

17. How is the moisture reported? Moisture content can be expressed as: wet / (wet + dry) wet / dry

19. Measuring the moisture content Many different techniques are available for measuring the moisture content of amaterial. The technique used in a given instance depends upon the material being studied,equipment available, and the time available for the measurement. The moststraightforward method of moisture measurement is to use a drying oven. A sample ofthe product is heated at a specified temperature and pressure (usually atmosphericpressure or a specified vacuum) for a specified time to remove all moisture (i.e., dryuntil there is no further weight loss). The loss in mass of the sample represents themoisture removed from the product. The temperature, drying time, and pressure aredependent upon the product being analyzed. Microwave drying ovens and chemicalanalysis are also used for some moisture measurement applications. An extensive listof standards for moisture measurement is provided by AOAC (1990).

20. EQUILIBRIUM MOISTURE CONTENT, X* • lowest moisture content obtainable at equilibrium • on dry basis (kg of water/ kg of moisture-free solid) • depends on structure of solid, temperature & moisture content of gas • varies greatly with type of material for any given % relative humidity • decreases with increase in temperature • assumed constant for moderate temperature ranges The solid’s moisture content is a function of the humidity of the drying air. The moisture cannot be lower than the equilibrium moisture content corresponding the humidity of the incoming air. 50% RH air equilibrium moisture Wool 12.5 % Newspaper 5.5%

21. Equilibrium relative humidity (ERH)

24. EQUILIBRIUM MOISTURE CONTENT, X* • bound water - the minimum moisture a material can carry - intersection of 100% humidity line in equillibrium water content vs relative humidity • Unbound water = excess water held primarily in the voids of the solid • free moisture content, X - moisture above the equilibrium moisture content • - can be removed by drying Moisture amount that can be release/free during drying X = Xt - X* where Xt = total moisture content X* = equilibrium-moisture content – cannot be predicted

25. Humidity, H - kg of water vapour in 1 kg of dry air • Saturation humidity, HS • Percentage humidity, HP • Percentage relative humidity, HR ( HR HP) HUMIDITY & HUMIDITY CHART where pA = partial pressure of water vapour in air pAS = saturated partial pressure of water vapour in air H = humidity of air HS = humidity of saturated air • Dew point - temp. at which a mixture of air-water would be saturated

26. HUMIDITY CHART

27. HUMIDITY & HUMIDITY CHART • Humid heat, cS - amount of heat required to raise the temp. of 1 kg dry air plus water vapour present by 1K cS (kJ/kg dry air.K) = 1.005 + 1.88H • Humid volume, H - total volume of 1 kg dry air plus water vapour present at 1 atm & given gas temperature H (m3/kg dry air) = (2.83 x 10-3 + 4.56 x10-3H)T T is in Kelvin • H –Humidity found from chart • Total enthalpy of 1 kg of air plus its water vapour, Hy Hy (kJ/kg dry air) = (1.005 + 1.88H)(ToC- 0) + 2501.4H

28. wet cloth/wick Air flow DRY & WET BULB TEMPERATURE Wet-bulb temperature : decreases in temperature below the dry-bulb temperature until the rate of heat transfer from the warmer air to the wick is just equal to the rate of heat transfer needed to provide for the evaporation of water from the wick into the air stream. Dry bulb temperature: the ordinary temperature you measure with a thermometer

29. Dry Bulb Temperature - Tdb The Dry Bulb temperature, usually referred to as "air temperature", is the air property that is most commonly used. When people refer to the temperature of the air they are normally referring to the dry bulb temperature. The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air. Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. The temperature is usually given in degrees Celsius (oC) or degrees Fahrenheit (oF). The SI unit is Kelvin (K). Zero Kelvin equals to -273oC. The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart or along the left side of the Mollier diagram. Constant dry bulb temperatures appear as vertical lines in the psychrometric chart or horizontal lines in the Mollier diagram.

30. Wet Bulb Temperature - Twb The Wet Bulb temperature is the adiabatic saturation temperature. Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air. The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation from the wet muslin is reduced when air contains more water vapor. The Wet Bulb temperature is always between the Dry Bulb temperature and the Dew Point. For the wet bulb, there is a dynamic equilibrium between heat gained because the wet bulb is cooler than the surrounding air and heat lost because of evaporation. The wet bulb temperature is the temperature of an object that can be achieved through evaporative cooling, assuming good air flow and that the ambient air temperature remains the same. By combining the dry bulb and wet bulb temperature in a psychrometric chart or Mollier diagram the state of the humid air can be determined. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric chart.

31. Humidity HUMIDITY CHART Wet bulb temp. =20oC, dry bulb temp. = 30oC,humidity = ? 0.0115

32. Air: T2, H2 Air: T1, H1 Humidity HUMIDITY CHART Importance of pyschrometric analysis for drying Assumption: evaporation surface is a liquid film H2 Constant wet-bulb temperature process H1 T2 T1

33. Batch Drying If air is passed over a moist solid, air temperature will be reduced as the water is evaporated. Calculated through an enthalpy balance: Ti = Inlet Dry Bulb Temperature To = Outlet Dry Bulb Temperature G = Air Mass Flow C = Air Heat Capacity Fw = Mass rate of water evaporation Hv = Heat of vaporization

34. Batch Drying The outlet temperature value will be between the inlet and the wet bulb temperature. The rate of evaporation dFw is equal to: Ti Inlet Dry Bulb Temperature Tw Wet Bulb Temperature a Mass transfer coefficient R Rate coefficient dA Surface Area

36. Drying Curve

37. Drying Curve

40. total moisture content , Xt = RATE OF DRYING CURVES • batch drying • experimental determination data : WS = weight of dry solid W = total weight of wet solid vs time t To obtain as free moisture content X vs time t: free moisture content, X = Xt - X* Measure sample every minutes

41. RATE OF DRYING CURVES • batch drying • experimental determination To obtain as rate of drying, R : Get slopes of tangents at different values of t : where A = exposed surface area for drying

42. RATE OF DRYING CURVES X = (0.35 + 0.325)/2 = 0.338 R = -21.5 (-0.07) = 1.493

43. RATE OF DRYING CURVES Point AB : Warming up (unsteady) period where the solid surface conditions come into equilibrium with the drying air. Point A’ : hot solid Point B-C: constant-rate drying period in which surface of the solid remains saturated with liquid because the movement of water vapour to the surface equals the evaporation rate.  Thus the drying rate depends on the rate of heat transfer to the drying surface and temperature remains constant. Surface temperature  TW Point C : critical free moisture content, XC , where the drying rate starts falling and surface temperature rises. Insufficient water on surface

44. RATE OF DRYING CURVES Point C-D : first falling-rate drying period which surface is drying out. Rate of water to surface is less that rate of evaporation from surface Point D : surface completely dry Point D-E : second falling-rate period in which evaporation is from inside of solid. Point E : equilibrium moisture content, X*, where no further drying occur

45. CONSTANT RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: Experimental drying curve Predicted mass-and-heat coefficients Experimental drying curve: Under similar conditions to actual process Drying curve X vs. t Rate-of-drying curve R vs. X where RC = constant rate of drying WS = kg of dry solid used A = exposed surface area for drying

48. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 1. Graphical integration Most accurate

49. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 2. Special cases a) Rate is linear function of X

50. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 2. Special cases b) Rate is a linear function thru’ origin (a straight line from C to E at the origin) or and