♦Courtesy of COGEIM SpA
♦Courtesy of COGEIM SpA
The dryer consists of a vacuum tight cylindrical or rectangular chamber containing a number of heated shelves on which trays are heated. A quick opening permits easy loading, unloading and during maintenance, easy cleaning. Different heating mediums are used, i.e., steam, hot water or hot oil. There are no moving parts inside the vessel, that means no sealing problem and consequently, a good vacuum can easily be maintained. The disadvantages are mainly due to its lower heat transfer rate (long drying time) and impossibility of safely handling toxic products because of the hazards involved in charging and discharging trays.
Tumbler Vacuum Dryer. This dryer is designed for drying of chemical, pharmaceutical products which are not sticky. Its double cone rotating shape ensures direct contact between the material and the heated surface, resulting in uniform heat transfer.
For optimum drying results approximately 50% to 60% of the total volume is required. Any greater percent fill would greatly restrict the product movement and retard the evaporation rate. A frequent condition that occurs with some sticky materials is the formations of balls which can be broken by addition of an intensifier bar with the rotating vessel or by intermittent rotation of the vessel. The unit is completely jacketed and designed for circulation of a heating medium. The tumbler is normally gentle in action and the absence of internal moving parts assures against disintegration of crystals or abrasion.
Generally, it is possible to carry-out some tests in the manufacturer's small scale units, but it is necessary to remember that during shipping the material may have changed its property due to chemical or physical modifications, because the quantity of sample is limited, it is not possible to check long-run performance. It is also impossible to evaluate the behavior of the dried material in the plant's solids-handling equipment. If the pilot test is positive, it is good practice, before designing a production unit, to install a small-scale dryer in the plant and investigate what is possible under actual process conditions. It is essential in this test that a representative sample of the wet feed is used and the test conditions simulate as closely as possible the conditions characteristic of the commercial size dryer. The experimental method for measuring product moisture content should be clearly defined and consistent with that used in the industrial plant. It should be noted that a heat transfer coefficient is the main product of the test and, based on this, a scale-up to the final heating surface can be done.
The heat transfer coefficient combines the surface coefficient for the condensing steam, the resistance of the metal wall and the surface coefficient on the working side. Because conditions vary with the type of material involved, the amount of moisture it contains, the thickness of the layer in contact with the surface, the structure of this layer, and many other factors, it is impossible to construct an overall heat transfer coefficient without experimental data.
Scaleup of laboratory data is a critical step and requires considerable experience. Since scaleup is subject to many factors that are not quantifiable, it is based primarily on experience and is a function of the specific dryer. When the heating surface is known, it is easy to calculate the working volume and the dryer's geometrical volume (Fig. 7).
For a pan dryer, the percent of the total volume occupied by the batch is called the working volume, which is another critical consideration. As the working volume approaches 100% of the total volume, there is less void space available for material movement and contacting of the heated surface.
In the vacuum batch dryer, approximately 60% total volume is required for optimum drying result. A very simple and approximate equation can be used for the scaleup of vertical pan and horizontal paddle dryers:
A = Heat transfer surface, m2
V = Vessel working volume, m3
a = Pilot plant b = Industrial size plant
3.2 Cost Estimation
Capital investment is the total amount of money needed to supply the plant and manufacturing facilities plus the amount of money required as working capital for operation of the facilities. To estimate a fixed-capital investment it is necessary to consider the following costs:
- Purchased equipment
- Service facilities
The cost of purchased equipment is the basis for estimating the capital investment. The various types of equipment can be divided conveniently into:
- Process equipment
- Raw materials handling and storage equipment
- Finished products handling and storage equipment
Of course, the most accurate method of determining process equipment cost is to obtain bids from the supplier. When a dryer unit must be evaluated, the following have to be considered.
Dryer Type and Size. If a vacuum pan dryer is selected, it is then necessary to choose its configuration and size. The size is dependent on the capacity needed and this is based on pilot tests. The configuration is dependent on the property of the material to be dried and the pollution specifications. For instance, the system blades-agitator can be heated or not, and the sealing between agitator and dryer body can be accomplished either by a stuffing-box or a mechanical seal. The latter can either be double-pressurized or simple. The chopper can be installed or not, the agitator rotation can be electrical/frequency converter or hydraulic. All of the hydraulic system has to be considered including all piping connections, etc.
Construction Materials. Normally, the dryers are made of stainless steel. The most common stainless steels used are the type 304 and 316 generally having low carbon content. They contain chromium and nickel at different percents. The addition of molybdenum to the alloy, as in type 316, increases the corrosion resistance at high temperature strength. The presence of chromium increases its resistance to oxidizing agents. The price for the type 304 and 316 is quite similar. If very highly corrosion resistant materials are required then Hastelloy C276 or C22 can be used.
Hastelloy is used where structural strength and good corrosion resistance are necessary under conditions of high temperatures. Compared to stainless steel, the price of a Hastelloy dryer is approximately double. Other less expensive alloys can be used, such as Inconel, 77 percent nickel and 15 percent chromium. Nickel exhibits high corrosion resistance to most alkalies.
Internal Finishing (GMP). For pharmaceutical purpose the internal finishing must be at least 220 grit and the dryer manufactured according to GMP standards. This makes the price of the dryer some 20 to 3 0 percent (%) higher than the standard design.
Installation. The installation involves costs for labor, foundations, platforms, construction expenses, etc. The installation cost may be taken as a percentage of the dryer cost, approximately 20 to 50 percent, depending upon its sophistication.
If no cost data are available for the specific dryer selected, a good estimate can be obtained by using the logarithmic relationship known as the six-tenths-factors rule. A price for a similar one, but having different capacity, is the sole requirement.
Cost dryer A = Cost dryer B
This relation should be only used in the absence of any other information.
3.3 Installation Concerns
The dryer performance is effected by the auxiliary equipment.
Heating System. Depending on the maximum temperature allowed inside the dryer, water <98°C or steam low/medium pressure 3-6 bar can be used as heating medium in the jacket. Due to the relatively low temperature required to dry fermentation products, the heating medium is generally circulating pressurized hot water. The water can be heated by either an electric immersion heater or steam in a shell and tube heat exchanger. The recirculating pump should always be pumping into the heater so that its suction is from the outlet ofthe dryer. In addition, the suction side ofthe pump should always have an air separator to prevent cavitation. The entrapment of air is inevitable in a hot water heating system.
Cooling System. Where cooling of the product is absolutely necessary, a cooling exchanger can be mounted in parallel with the heating exchanger and used at the end of the drying cycle. By turning a couple of valves to direct the flow through the cooling exchanger, the recirculated water will then remove the heat from the product and transfer it to a cooling medium in the cooling exchanger.
Vacuum system. A well designed system should include:
1. Dust collector—which is installed on the top of the dryer and made of a vertical cylindrical casing, complete with an outside jacket. Generally the filter elements are bags fixed on the upper side to a plate and closed on the lower part. The filter bags (Fig. 8) are supported through an internal metal cage. Cleaning of the bags is obtained by a mechanical shaking device or by nitrogen pressure. Design and working conditions are the same as for the vacuum dryer.
2. Condenser—designed according to the scaled-up pilot test evaporation rate. Normally it is a shell and tube unit. It should also be self-draining into a vacuum-receiver, which collects the solvent as well as maintains the vacuum integrity of the entire system. The condensate receiver should have a sight glass so that visual inspection will indicate when it needs emptying. Obviously the receiver should be large enough to contain all of the condensate from one batch of product.
3. Vacuum pump—whose flow-rate depends largely upon the in take of air at the various fittings, connections, etc. Different kinds of vacuum pumps can be used; e.g., rotary—water/oil sealed, reciprocating dry vacuum pump.
If a water sealed-vacuum pump is used, the liquid ring may permit scrubbing the effluents (non-condensed vapors) and removal of the pollution load by controlling the vapor emission. Obviously, when the liquid ring becomes saturated, it must be discharged. This type of device should always be considered where low boiling solvents and hazardous or toxic vapors are involved; better if a closed circuit is considered. This type of pump is simple to operate and requires little maintenance. Depending on service liquid temperature, a single-stage pump will allow a vacuum of 100 to 150 torr. However, it is more usual to employ a two-stage liquid ring pump which will attain 25 torr, and below 10 torr when used in combination with an ejector. The ejectors consist essentially of a steam nozzle, which discharges a highvelocity jet across a suction chamber connected to the equipment to be evacuated. The gas is entrained by the steam and carried into a venturi-shaped diffuser which converts the velocity energy of the steam into pressure energy.
Where it is necessary to operate at the end of the drying cycle below 5 torr, different types of oil sealed rotary pumps can be supplied. Such applications might occur where there is a need to operate at very low drying temperatures.
Hydraulic System. Generally, for vertical pan dryers an hydraulic system is provided for the agitator rotation, the opening-closing of the dryer by a rapid device as bayonet and/or TRI-CLAMP and the lowering of the vessel for maintenance or cleaning purposes. The hydraulic components positioned on the dryer are normally:
1. One hydraulic motor for agitator rotation
2. Three hydraulic cylinders for lowering, raising the vessel
3. Hydraulic cylinder for a rapid opening (the number is dependent upon the dimensions of the vessel)
The hydraulic system consists of: oil reservoir, electric motor, hydraulic pump, heat exchange for oil cooling, oil filters, oil level indicator, electric valves and flow distributors. An hydraulic plant which has been properly installed and care has been taken during the start-up phase, should enjoy long life and not need much maintenance.
A cardinal principle in the operation of a trouble-free hydraulic system, on which all manufacturers agree, is that the operator continuously monitors the quality as well as the condition of the hydraulic fluid to make certain there are no impurities. The reliability of the hydraulic system is directly related to the integrity of the fluid.
The following periodic checks are recommended:
1. Monthly external cleaning and inspection. This will uncover any leaks which can then be repaired.
2. Monthly air filter checking and replacement of the dirty cartridge.
3. Weekly oil filter checking.
4. Weekly oil level check, each time the level falls to the minimum, oil must be added
5. Oil replacement on the average every 2,000-3,000 hours.
6. Heat exchanger must be cleaned semiannually.
Where the handling of materials containing highly flammable solvents is concerned, the dryer must be located in a classified area and the electrical parts designed according to the standards specified for this level.
The mechanical, electrical and instrument specification should also include requirements for:
1. Explosion protection—a vent should be considered for a safe relief of a positive pressure.
2. Avoidance of ignition—potential ignition sources may be electrical equipment, discharge of static electricity or mechanical friction (associated with the agitator). The dryer must be grounded.
3. Facilitating safe operation—ventilation should be provided during loading; a supply of inert gas is required for breaking the vacuum.
Most hazards are listed below:
1. Ignition of dust cloud can occur during unloading of a dusty flammable product from the dryer
2. Ignition of bulk powder can occur if a dryer is opened to atmosphere while still hot
3. Ignition of flammable vapor can occur when loading solvent-wet material into the dryer, and also when unloading the product if the dryer has not previously been purged with nitrogen
4. Exothermic decomposition—some heat-sensitive materials may decompose with evolution of large volumes of gas if they are overheated during drying
The danger from an explosion can be reduced in two different ways:
1. The dryer can be designed according to pressure vessel code and consequently be able to contain any possible explosion
2. The process/operating conditions should be altered to insure a higher level of safety
At the same time, the following start-up and shutdown procedures are recommended: Start-up:
1. Inspect the plant and remove any deposits, check position of valves and settings of temperature and vacuum regulators
2. Purge the dryer with nitrogen
3. Load the wet material in the dryer
4. Start cooling water to the condenser
5. Start the vacuum pump
6. Start the agitator
7. Apply heat to the jacket Shutdown
1. Switch off the heating medium
2. Wait till the product has cooled for sale discharge
3. Close the vacuum line
4. Stop the agitator
5. Fill the vessel with nitrogen to atmospheric pressure
6. Open the dryer and remove the product
7. Clean the dryer
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