The following suggestions may serve as general guidelines for the design and operation of pilot plant or production drying of a variety of hydrocarbons and mixed solvents.
In general, the water content of the liquid to be dried should be 1% or less and preferably 0.1% (1000 ppm) by weight. In this range, the DRIERITE can be considered to be economically feasible as a disposable desiccant and is discarded after a single batch use. Alternate procedures include removal of DRIERITE for regeneration or regeneration in place when the equipment is designed for this purpose.
The viscosity, polarity and solubility of water in the organic liquid will determine how difficult the particular organic liquid is to dry. Alcohols, ketones, and amines are more difficult to dry than ethers and aliphatics. Oils and highly branched molecules are more difficult to dry than short or straight molecules. With highly polar liquids, the desiccant must overcome the attraction of the organic molecule. With highly viscous liquids, the water molecule must "find its way" through the liquid to the desiccant. In both cases the liquid must be in contact with the desiccant long enough for the desiccant to absorb the water. With DRIERITE, compounds like liquid propane and butane may be dried with a contact time of less than one minute. Oils and plasticizers may require a 10 or 15 minute contact time. Chlorinated solvents may be dried with 2 or 3 minute contact times, while alcohols, ketones and amines may require 5 or 10 minutes.
The quantity of DRIERITE necessary to dry a particular batch of liquid must be sufficient to
absorb all of the water available as determined by a sample analysis. DRIERITE has a theoretical
chemical capacity for water of 6.6%; however, in liquid phase drying, we recommend
that 5% be used as a design capacity for the entire column. In other words, 100
pounds of DRIERITE should be used for each 5 pounds of water to be absorbed.
In addition to the above consideration, the size of the drying column or desiccant bed may have to be determined by the flow rate of the liquid to be dried if this flow cannot be adjusted.
Since residence time or contact time between the active desiccant and the liquid is very important, enough DRIERITE must be provided to dry a given flow. Residence time for efficient drying will vary widely depending on the properties of the organic liquid; however, we have found that 3 to 10 minutes should be designed into the operation, if possible, even though contact times of less than a minute have proven successful for some liquids.
The bulk density of 8 and 6 mesh DRIERITE is about 65 pounds per cubic foot, and the granules will occupy roughly 50% of the desiccant bed volume. This has the effect of reducing the residence time to half that of the empty drying column. A liquid flow of one gallon per minute would, therefore, require a desiccant bed volume of 6 gallons (.8 cubic feet) to provide a residence time of 3 minutes. Since this example would require approximately 50 pounds of DRIERITE, it can be stated that for every gallon per minute of flow rate you need at least 50 pounds of desiccant and, perhaps, 100 pounds if a contact time of 6 minutes were desired.
When the two requirements of water absorption capacity and residence time have been met, the quantity of DRIERITE will determine the size of the drying tower.
A tall, small diameter column will give maximum turbulence and, perhaps, improve drying efficiency; however, for convenience in filling and emptying the column with DRIERITE, a height to diameter ratio of 4:1 is good, but this could vary from 1:1 up to 8:1. A total height in excess of 7 feet should be avoided to minimize weight on the granules on the desiccant screen support. A 10 mesh or 20 mesh stainless steel screen backed up by a perforated plate (3/8" holes) makes a good support near the bottom of the tower. The flow in the vertical tower can be in either direction once the unit is filled with liquid. The initial filling with liquid must be done from the bottom, creating an upward flow that will not cause channeling through the granules that occurs from top filling. Driers can be scaled to required size to fit any application. It is necessary to have a drain at the bottom to facilitate removal of liquid before changing desiccant and also a good filter on the outlet pipe line to remove suspended particles of desiccant carried over in the liquid stream. In some cases this filter must be in the 1 to 10 micron range to remove minute particles to meet particular specifications. Since the DRIERITE suspension includes absorbed water, this could cause a moisture determination to show incomplete drying.
Placing the DRIERITE granules in the column should be done carefully by building up uniform
layers of granules. Just pouring them into the center of the vessel will cause a cone shaped
pile of segregated granules with larger granules on the periphery, causing channeling
or non-uniform flow through the drier.
The pressure and flow of the liquid should be regulated to avoid surges that might cause DRIERITE granules to be carried out the outlet pipe. Sometimes a holddown screen on the top of the desiccant bed is necessary. Normally, the flow rate is slow enough that pressure drop in the column is negligible.
After the batch of liquid has passed upward through the column, the liquid remaining in the drier should be drained out and considered not to be dry, because it has not passed through the entire column and, at this point, the desiccant is probably exhausted or saturated with water.
If the results of a final moisture determination on the liquid are not satisfactory, the flow rate and the total quantity of liquid to be dried may have to be decreased until the appropriate degree of dryness is attained. When the drier is used on a tank of liquid in a closed cycle where the liquid is pumped through the drier and back into the tank, the flow may be circulated until the moisture in the entire system gradually drops to the desired level.
DRIERITE can be reused by regenerating it after it is exhausted. This can be done by
removing it from the column and heating it in an oven or it can be reactivated in the column.
If the DRIERITE is removed from the column it should be placed in pans or trays
about one inch deep and heated at 400° to 450° F for two hours. It is recommended that the
organic liquid be vacuumed off before the DRIERITE is removed from the column. This will
make it easier to handle the DRIERITE and reduce any hazards as far as the organic liquid
is concerned. If there are any impurities or resinous materials in the liquid that may be
adsorbed on the DRIERITE and char or decompose at regeneration temperatures, the
DRIERITE should be regenerated under vacuum. With 26" Hg vacuum, the DRIERITE can
be regenerated at 325° F and at 28" Hg at 275° F.
The DRIERITE can be regenerated in the column by several methods. Again, it is recommended that the organic liquid be vacuumed off. Once this is done, the DRIERITE can be regenerated with hot air. It requires approximately 25 scfm of air per 100 pounds of DRIERITE heated to 450° F to regenerate the DRIERITE in four hours. The air should pass downward through the DRIERITE. This prevents condensate from dripping down on the DRIERITE. If vacuuming of the liquid is not feasible, an inert gas can be used instead of the air. With an inert gas, a closed loop with a condenser is normally used. This way, the organic liquid in the DRIERITE may be recovered and the inert gas can be reused. If the organic liquid has a high boiling point or decomposes at the regeneration temperature of DRIERITE, it can be regenerated under vacuum at the temperatures and pressures listed above. The drying column and the regeneration piping should be insulated to reduce heat loss during regeneration. The bed should be cooled to ambient temperature as soon as possible after regeneration. This can be accomplished by using cooling coils, dry air or inert gas on a once through basis or in a closed circuit with a heat exchanger.