1. Drying
Ashis Kumar Podder

2. What is Drying
It is the removal of a liquid from a solid by evaporation, the necessary heat of vaporization being provided by conduction from a solid surface, convection from a hot gas, or radiation.
Mechanical process of removing liquid from a solid-liquid mixture is not drying. Drying is a thermal method.

3. How Drying Occurs?
Reference: Pharmaceutical Engineering: Principle and Practices by C.V.S. Subrahmanyam; J. Thimma Setty; Sarasija Suresh; V. Kusum Devi.

4. Type of Beds Static Beds Moving Beds Fluidized Beds
Dilute / Pneumatic Beds

5. Type of Hot Air Flow Parallel Flow Perpendicular Flow
Through Circulation Flow
Co-Current Flow
Counter-Current Flow
Cross Flow

6. Classification of Dryers
Based on Heat Transfer
1. Drying of wet solids:
- Convective drying: e.g. Tray Dryer, Tunnel Dryer, Rotary Dryer.
- Conductive drying: e.g. Vacuum Dryer.
- Radiation drying: e.g. Microwave Dryer.
Drying of dilute solutions and suspensions:
- Convective drying: Drum Dryer.
- Conductive drying: Freeze Dryer.

7. Classification of Dryers ……… B. Based on Methods of Solids Handling

8. Tray Dryer Principle Construction Working mechanism Uses Advantages
Disadvantages

9. Tray Dryer ………
A double-layer stainless steel chamber. Insulation between the layers. Inside layer is mirror polished. Door has rubber lining.
Contains a heating device, thermostat and timer.
Wet solid is placed into stainless steel trays (1.2 – 2.4 m2). Trays are placed in the shelf of the drying chamber.
Large dryers can contain one or two trucks that are placed in the chamber directly.
Fresh air enters through an inlet, which passes through a filter, a heater and a fan. The hot air is circulated by means of a few fans usually at 2 to 5 meters per second air velocity.

10. Tray Dryer ………
As water evaporates from the surface of the particle, the inside water diffuses from the interior of the solid by capillary action.
Moist air is partially removed through a filter and outlet. Only 10 – 20 % fresh air is introduced.
The distance between the bottom of upper tray and upper surface of the substance loaded in the subsequent tray must be 40 mm.
Thus constant temperature and uniform airflow over the material can be maintained for achieving uniform drying.
Tray dryers are available in various sizes.

11. Truck Dryer

12. Tray Dryer ……… Advantages: No loss of substances during handling.
It is a batch dryer. Thus small amount of wet-solid mixtures can also be dried separately.
No attrition is observed.
The machine is easier to operate and repair.
Good control to heat and humidity.
It may be operated under vacuum.
Through circulation drying can be provided by modifying it

13. Tray Dryer ……… Disadvantages:
Only a fraction of the solid particles is directly exposed. Heat transfer and mass transfer are comparatively inefficient.
It is not suitable for large scale production.
It requires high labor usually associated with loading and unloading the compartments.
Thermolabile drugs, liquids, slurries, cannot be dried.
The method is time consuming (24 hours).
Uses: Sticky materials, granular mass or crystalline materials, precipitates and paste can be dried in a tray dryer.

14. Fluidized Bed Dryer Consists of a compressed air system, pre-filter, heating device, a perforated platform, a harmonizer, a swollen canvas chamber, a thermostat and a timer. Here wet solid particles are partially suspended in an upward moving forceful hot air stream. In this dryer, wet granules are contained on the perforated platform, compressed hot air is passed through the harmonizer. Compressed hot air stream of varying speed lifts the granules from the bottom. This condition creates a wavelike surface and is called fluidized state. The hot gas surrounds every granule to completely dry them. Thus materials or granules are rapidly and uniformly dried.

15. Fluidized Bed Dryer ……. Principle Construction Working mechanism Uses
Advantages
Disadvantages

16. Fluidized Bed Dryer …….
The air velocity is gradually increased inside the chamber that makes the canvas to swell producing a bigger space for the particles to suspend.
When air velocity is greater ( meter per second) than the settling velocity of granules due to gravitational attraction, the wet granules remain partially suspended in the hot air stream.
This makes the wet granules to dry quickly and fall back in a random boiling motion.
The bags are shaken to remove the entrained particles.
The residence time for drying in this dryer is about 40 minutes. The end product is free flowing.

17. Fluidized Bed Dryer ……. Advantages:
Efficient heat and mass transfer giving high drying rates, thus drying times are shorter than the static bed dryers (15 times faster than tray dryer).
The temperature of a fluidized bed is uniform throughout the system and can be controlled precisely.
The unit has a high output from a small floor space.
Its thermal efficiency is 2 to 6 times than a Tray Dryer.
Can be used as either batch type or continuous type.
Can also be used as a mixer.
Also improves uniformity of mixing the ingredients.
Very quick and economic dryer.
No hotspots are seen like tray dryer.
Higher drying temp. can be used compared to tray dryer.

18. Fluidized Bed Dryer ……. Disadvantages:
Turbulence of the fluidized state may cause excessive attrition of some particles, with damage to some granules and the production of too much dust. So, product loss may occur.
Liquid, too wet, sticky and adhesive materials can’t be dried.
The cleaning of screen and bag filter is troublesome to avoid contamination.
Vigorous movement of particles in hot dry air can lead to the generation of static electricity charges and suitable precautions must be taken.
Uses:
1. This dryer is very suitable for drying of granules in the production of tablets.
2. It can be modified for mixing, granulation and drying operations at a time. (Known as Fluid Bed Processor)

19. Drum Dryer or Roller Dryer
It is a moving bed dryer. It consists of a horizontally mounted hollow steel drum of 0.6 to 3.0 meter diameter and 0.6 to 4.0 meter length, whose external surface is smoothly polished.
Steam or heating coils can be used inside the drum for heat generation. Heat is transferred by conduction to the material that can be controlled with a thermostat.
Drum is rotated with a motor device at 1-10 rpm.
The liquid or paste material present in the feed pan adheres as a thin layer to the external surface of the drum during its rotation.

20. Drum Dryer ………….. Principle Construction Working mechanism Uses
Advantages
Disadvantages

21. Drum Dryer …………..
Wet materials are completely dried during its journey in slightly less than one rotation (from one side to another side of the drum).
Dried materials are scrapped by a knife, which than falls into a storage bin.
Contact time of the material with hot metal is 6 to 15 seconds only.
Therefore, processing conditions such as film thickness and drum temperature are closely controlled.

22. The entire material is continuously exposed to uniform heat.
Drum Dryer …………..
Advantages:
This dryer gives a rapid (few seconds) drying and its mass transfer rate is higher.
The entire material is continuously exposed to uniform heat.
The equipment is compact.
Attrition is not possible.
Disadvantages:
Operating conditions are critical. Skilled operators are needed to control feed rate, film thickness, speed of rotation and temperature.
It is not suitable for solutions or suspensions of low viscosity. of salts with less solubility (solutes will be precipitated at the bottom)

23. 1) Drum dryer is used for drying solutions, slurries, suspensions etc.
Uses:
1) Drum dryer is used for drying solutions, slurries, suspensions etc.
2) The products dried are starch products, ferrous salts, suspensions of zinc oxide, suspension of kaolin, yeasts, pigments, malt extracts, antibiotics, calcium, insecticides, barium carbonates etc.

24. Pneumatic Dryers
Principle: Systems in which the drying particles are entrained and conveyed in a high velocity gas stream.
This systems further improved on fluidized bed. Ex : Spray dryer

26. Operation: Drying of the materials in spray dryer involves 3 stages-
1. Atomization of the liquid from liquid droplets.
2. Drying of the liquid droplets.
3. Recovery of the dried product

27. Atomization of the liquid to form liquid droplets:
The feed is introduced through the atomizer either by gravity or by using suitable pump to form fine droplets. The properties of the final product depend on the droplet form, hence the selection of the type of atomizer is important.
Atomizer of any type-
Pneumatic Atomizer.
Pressure Nozzle.
Spinning Disc Atomizer.

28. The rate of feed is adjusted in such a way that the droplets should be completely dried before reaching the walls of the drying chamber.
At the same time, the product should not be over heated.

29. Drying of the Liquid Droplets
Fine droplets are dried in the drying chamber by supplying hot air through the inlet. The surface of the liquid drop is dried immediately to form a tough shell.
Further, the liquid inside must escape by diffusing through the shell at a particular rate. At the same time, heat transfer from outside to inside takes place at a rate greater than liquid diffusion rate.
As a result, heat inside mounts up which allows the liquid to evaporate at a faster rate.

30. This tendency of a liquid leads to rise in the internal pressure, which causes the droplets to swell. The shell’s thickness decreases where as permeability for vapor increases. If the shell is neither elastic nor permeable, it ruptures and the internal pressure escapes.
The temperature of air is adjusted in such a way that the droplets should be completely dried before reaching the walls of the drying chamber. At the same time, the product should not be over heated.

31. Recovery of the Dried Product
Centrifugal force of atomizer drives the droplets to follow helical path. Particles are dried during their journey and finally fall at the conical bottom.
All these processes are completed in a few seconds. Particles size of the final product ranges from 2 to 500 mm. Particles size depends on solid content in the feed, liquid viscosity, feed rate and disc speed.
Spray dryer of maximum size have got evaporating capacity up to 2000 kg per hour.

32. Advantages:
The method is a continuous process and drying is rapid and efficient. Drying completes within 3 to 30 seconds.
No problem with heat sensitive materials.
Fine droplets provide large surface area resulting in efficient heat and mass transfer.
Using a suitable atomizer, the product of uniform and controllable particle size can be obtained.
Labour costs are low.
It is suitable for the drying of sterile products.
Either the solution or suspension or thin paste can be dried in one step to get the final product ready for package.
Globules of an emulsion can be dried with the dispersed phase inside and continuous phase outside. On reconstitution, the same emulsion will be formed

33. Disadvantages:
Solids can’t be dried.
Only low viscous liquid can be dried.
The method is very bulky and expensive.
The thermal efficiency is low, as much heat is lost in the discharged gases.
Such a huge equipment is not always easy to operate.
Suitable for encapsulation (coating) of solid and liquid particles.

34. Uses: Spray dryers are caused compulsorily, if 1) The product is a better form than that obtained by any other dryer. 2) The quantity of the material to be dried is large. 3) The product is thermo labile, hygroscopic or undergoes chemical decomposition. Example of few products are dried using spray dryer : Acacia, blood, borax, plasma, serum, adrenalin, fruit juice.

35. Vacuum Dryer Principle:
In vacuum dryer, material is dried by the application of vacuum.
When vacuum is created the pressure is lowered so that water boils at a lower temperature. Hence water evaporates faster.
The heat transfer becomes efficient, i.e., rate of drying enhances substantially.

37. Vacuum Dryer
It consists of a cast iron heavy jacketed vessel. It is so strong that it can withstand the vacuum within the oven and steam pressure in the jacket.
The inner space is divided into a number of portions (shelves), which are part of the jacket.
These shelves provide larger surface area for conduction of heat.
Over the shelves, metal trays are kept for keeping the material.
The oven is connected to a vacuum pump by placing condenser in between. Pressure is usually decreased to 30 – 60 kPa.
Steam or hot air is passed through the hollow space of jacket and shelves. As a result, evaporation of water takes place at 25 – 30 °C.

38. Advantages:
Vacuum dryer provides large surface area for heat transfer.
Handling of the material, trays and equipment is easy.
It is easy for switching over to the next materials.
Hot water of desired temperatures can be supplied.
Electrically heated hollow shelves can be used.

39. Disadvantages:
In vacuum dryer, heat transfer coefficients are low.
It has a limited capacity and used for batch process.
It is more expensive than dryer. Labour and running costs are also high.
Sometimes, there is a danger of over heating as the material as the material is in contact with steam heated surface for longer period.

40. 1. Properties of the material being handled :
Preliminary Dryer Selection
The important factors to be considered in the preliminary selection of a dryer are as follows:
1. Properties of the material being handled :
Physical characteristics when wet or dry
Corrosiveness
Toxicity
Flammability
Particle size
Abrasiveness

41. - Type of moisture present (bound, unbound or both).
2. Drying characteristics of the material:
- Type of moisture present (bound, unbound or both).
- Initial moisture content.
- Final moisture content (maximum up to which it is to be dried).
- Permissible drying temperature Probable drying time for different dryers.

42. 3. Flow of material to and from the dryer:
- Quantity to be handled per hour.
- Continuous or batch operation.
- Process prior to drying.
- Process subsequent to drying.
4. Economic consideration:
-Value of the materials.
- Maintenance cost.
- Labour cost.

43. Uniformity of final moisture content. Decomposition of product.
5. Product qualities:
Contamination.
Uniformity of final moisture content.
Decomposition of product.
Bulk density.
6. Recovery problems:
Dust recovery.
Solvent recovery.

44. 7. Facilities available at site of proposed installation:
Temperature, humidity and cleanliness of air.
Available fuels.
Available electric power.
Permissible noise, vibration, dust or heat losses.
Source of wet feed.
Exhaust-gas outlets.

45. Drying Tests :
These tests should establish the optimum operating conditions, the ability of the dryer to handle the material physically, product quality and characteristics, and dryer size.
The principal manufacturers of drying equipment are usually prepared to perform the required tests on dryers simulating their equipment.

46. Based upon the results of the drying tests that establish size and operating characteristics, formal quotations and guarantees should be obtained from dryer manufacturers.
Initial costs, installation costs, operating costs, product quality, dryer operability, and dryer flexibility can then be given proper weight in the final evaluation and selection.

47. FREEZE DRYING
Freeze drying is nothing but a conduction drying method that is operated under vacuum followed by heat transfer in the dryers.
Freeze drying also known as sublimation drying or lyophilization or cold drying but commonly known as “Freeze drying” and “Sublimation drying”.
This method known as freeze drying is based on the state of the material.
This method also known as sublimation drying is based on the mechanism of drying.

48. Basic Principle
Sublimation drying consists simply of reducing both the temperature & pressure below the triple point.
As a result, any heat transferred is used as latent heat and the ice sublimes directly to the vapour state. The vapour so produced is quickly removed by using power exhaust system.
The triple point temperature of water is oK (0.0098°C) and the pressure is 610 N/m2.
Considerations to be taken into account:
The temperature must be below °C / oK. The usual range is between -10 to - 30°C.
Similarly, the pressure must be below 610 N/m2. The usual range is between 10 to 30 N/m2.
Under this conditions, any heat transferred is used as latent heat and ice sublimes directly into vapor.

49. TRIPLE POINT

50. Advantages of the Freeze Drying Process
Characteristics of the freeze dry process are: 1. minimization of chemical decomposition (drying takes place at low temperatures) 2. complete dissolution of dried product (resulting product has a very high surface area) 3. more compatibility with sterile operations (solution is sterile-filtered before filling vials) 4. precise filling weight (fill weight control is more precise for liquid) 5. absence of powder (particulate contamination is minimized).

51. Basic Steps of Freeze Drying Process
Preparation and Pretreatment
Freezing.
Primary drying / Sublimation phase.
Secondary drying / Ordinary vacuum drying
Packaging.

52. Preparation and Pretreatment
Satisfactory freeze drying beyond a certain limit of depth of liquid is not possible.
In this step, the solution is pre-concentrated under normal vacuum tray drying.
This reduces the actual drying by 8 – 10 times.

53. Freezing Operation Pre-Freezing:
The solution can be pre-frozen, that is, before the start of the drying process, and this is usually applied to large containers (also in vials or ampoules) such as blood bottles. This can be done by Shell Freezing.
Shell Freezing: bottles are rotated horizontally and slowly in a refrigerated bath (-50°C).

54. SHELL FREEZING

55. Vacuum System
To reduce the pressure sufficiently, it is necessary to use efficient vacuum pumps.
Usually,
Two stage rotary pump- for the small scale, and
Ejector pump- for the large scale.

56. Primary Drying
Temp = -10 to 30 °C
Press = 3 mm Hg
During the primary drying, the latent heat of sublimation (2900 KJ/Kg) must be provided & the vapour must be removed. This process consist of two things:
Heat Transfer: Heat transfer is critical. Insufficient heat prolongs the process, which is already slow & excess heat will cause melting. So the heat transfer must be controlled.
Vapour Removal: The vapor formed must be removed continually to avoid a pressure rise that would stop sublimation.
On the small scale, vapor is absorbed by a desiccant such as phosphorous pentoxides (P2O5).
On the large scale, vapour is commonly removed by pumping. Ejector pumps are most satisfactory for this purpose.

57. Rate of Drying
The rate of drying in freeze drying is very low, the ice being removed at a rate of about 1 mm depth/hour

58. Secondary Drying
Temp = 60 to 60 °C
Press = 50 mm Hg
The primary drying process leaves products with about 0.5 % moisture in the solid and this is removed by secondary drying process.
The usual method is to raise the temperature often as 50 to 600C. The use of such a high temperature may seem surprising in view of the remarks in the primary drying, but the secondary drying period is ordinary vacuum drying.
The final product is virtually free from moisture & the risk of hydrolysis.

59. Packaging
In view of packaging of the freeze dried products, special care must be done to ensure protection from moisture. After drying, the vacuum is replaced by inert gases.
Containers should be closed without contacting the atmosphere, for example, ampoules are sealed under vacuum.
Or, the closure of containers must be carried out under controlled atmospheric conditions.

60. Packaging…contd.
Freeze dryers are equipped with hydraulic or pneumatic internal stoppering devices designed to push slotted rubber closures into the vials to be sealed while the chamber is still evacuated, the closures having been partially inserted immediately after filling, so that the slots were open to the outside.
If internal stoppering is not available or containers such as ampoules are used, filtered dry air or nitrogen should be introduced into the chamber at the end of the process to establish atmospheric pressure.

61. Freeze Dryers ……. There are essentially 3 categories of freeze dryers:
i. Rotary Freeze Dryers: Rotary freeze dryers are usually used with liquid products, e.g. pharmaceutical solutions, tissue extracts.
ii. Manifold Freeze Dryers: Manifold freeze dryers are usually used when drying a large amount of small containers.
iii. Cabinet Freeze Dryers: Cabinet freeze dryers are the most common, because they can be used to dry a variety of materials.

62. Freeze-Drying Equipment
Fig: Unloading trays of freeze-dried material from a small cabinet-type freeze dryer.

63. A modern Freeze Dryer
LYOPHILIZERS 63
Small Scale
Large Scale

64. Construction of Freeze Dryer
It consists of:
Drying Chamber in which trays are loaded.
Heat supply in the form of radiation source, heating coils.
Vapor condensing or absorption system.
Vacuum pump or steam ejector or both.
The construction of freeze dryer is shown in following figure:

65. Schematic Diagram of Freeze Dryer

66. Application of Freeze Dryer
Pharmaceutical Industry: This method is applied only to biological products- antibiotics (other than penicillin), blood products, vaccines such as BCG, yellow fever, small pox.
* Enzyme preparations – Hyaluronidase.
* Microbiological cultures.
Food Industry: Freeze dried ice cream, instant coffee etc.
Other Industry: Also frequently used in different technological industries. Very recently, some taxidermies have begun use of freeze drying to preserve animals.

67. Advantages of Freeze Dryer
1. Drying takes place at very low temperature, so that enzyme action is inhibited, and decomposition particularly hydrolysis is minimized.
2. The solution is frozen so that the final dry product is a network of solid occupying the same volume as the original solution. Thus the product is light and porous.
3. The porous form of the product gives ready solubility.
4. There is not condensation of the solution prior to drying. Hence, salts do not concentrate or denature proteins, as occurs with other drying methods.
5. Under high vacuum, there is no contact with the air, thus oxidation is minimized.

68. Disadvantages of Freeze Dryer
It is the most expensive method ever invented to transport water just some few inches.
The porosity, ready solubility, and complete dryness yield a very hygroscopic product.
The process is very slow.
This process also uses complicated plant which is very very expensive.
Packaging requires special attention / conditions.

69. Thank You