Dampeners (See Pulsation Dampeners) Desalination
Dampeners (See Pulsation Dampeners) Desalination
Dampeners (See Pulsation Dampeners) Desalination
Desalination
D-1
D-2 Dialysis; Electrodialysis
Dialysis; Electrodialysis
liquids from a parent mixture by using the differences in their boiling points.
Both processes may be used for purification or separation. A typical example of
distillation is extracting pure solvent (such as water) from a mixture of solute and
solvent (such as brine).
A typical example of fractional distillation is in the separation of various
hydrocarbons, for instance, butane, pentane from a hydrocarbon mixture that
results from some process in the overall refining process. In this example, a refinery
fractional distillation apparatus is commonly termed a fractional distillation
column or a fractionating column and can be several hundred feet high, depending
on the refinery throughput. This column then consists basically of an outer shell,
several metallic trays arranged through the columns height to enable the drawing
off of various liquids. Pumps and pipes inside and outside the column conduct the
fluid in required directions. Pressure and temperature in the column is controlled
to the most suitable values for the process in question. Degrees of vacuum are used
to accelerate separation rates.
A vessel of this nature is custom designed, generally by the design contractor for
the overall plant. Besides all the relevant physical property tables, designers will
use past experience to a considerable extent to determine the final parameters and
dimensions for the column. In prototype applications some estimation or guesswork
is unavoidable, which may have consequences not necessarily within the column
itself, but with higher-precision elements of the process downstream. For example,
a fractionating column-handling process oil in an oil sands plant had various
molecular weights of hydrocarbon taken off at various points along its length. Based
on flash point, one of these streams was designated as the source supply for the
purged seal oil system in the plant. That stream contained more colloidal coke than
was expected, making it unsuitable for the application. The problem was solved by
taking the required supply from higher up the column (a lighter end of lower
molecular weight).
Diverter commonly describes a flat-plate-type valve hinged at one end that is moved
to divert flow from one stack or passage to the other. There is normally no
appreciable degree of speed or mobility attached to this type of device, as there
might be with a butterfly valve in a control system. In a 150,000-barrel/day refinery,
for instance, a flapper valve or flapper diverter valve can be made of concrete and
weigh up to 5 tons. The absolute closure of the closure seal may not be critical, and
the allowable gap between the flapper surface and the mating flange it sits on may
be as large as 0.010 in.
Doctor
A doctor is a paper industry device. It is used to lead the paper sheet and keep the
paper roll(s) clean.
Drying
These are many types of drying media and methods. A variety of methods to
produce heat for drying (ovens) or air for drying (fans) or moisture absorption
chemical (desiccant) are used and generally customized for a specific application.
There are fluid-bed dryers, where solid particulates disperse heat to a gas.
Sophistication may be added by using different stages for the dryer with dust
collection (cyclone separators or otherwise) at each phase. In a pneumatic conveying
system, an agitator supplied with warm air can be used to accelerate drying. With
vacuum dryers, moisture removal is completed below atmospheric pressure. Drying
temperatures, however, may vary considerably. Vacuum drying is particularly
popular in pharmaceutical manufacturing, electronics, metallurgical, and food
industries. See Some Commonly Used Specifications, Codes, Standards, and Texts.
Drying, Freeze*
Freeze drying is sometimes what is meant by the term drying in the process
engineers terms. The following information is based on the solvent Tebol 99. It
indicates what freeze drying is and the properties sought and the performance
parameters measured in a typical freeze-drying agent.
Freeze drying or lyophilization is a process that removes a solvent, typically
water, from a frozen solution by sublimation. Studies in the 1930s and 1940s were
done on blood serum and foods. More recently, research has focused on using freeze
drying for pharmaceuticals, cosmetics, and chemicals. An increasing number of
parenteral products have been prepared by freeze-drying techniques. The method
reduces particulate contamination, improves product quality and stability, and
enhances the dissolution rate on reconstitution.
The pharmaceutical industry takes advantage of the freeze-drying process to
maintain the activity and viability of various delicate biological materials. These
materials include antibiotics, peptides, proteins, vaccines, and microbial cells.
While freeze drying with water has proven useful, it has several inherent
limitations:
Uneven moisture distribution in the freeze-dried product
Uneven stability or unpredictability of the final product
Useless for water-insoluble or hydrolyzable products
High energy costs
Long process cycles
Perhaps most important, freeze drying with water is restricted to those materials
that are soluble and stable in a water system.
Much attention has been devoted to optimizing freeze-drying cycles. Recent
studies have shown that addition of tertiary butyl alcohol (TBA) can markedly
improve the freeze-drying process.
TBA as a processing aid:
Helps dissolve products that are difficult to dissolve in water
Gives a product with a high specific surface area
Accelerates the drying process by reducing dried product resistance
Prevents the product from reaching the collapse temperature
Produces a pharmaceutically elegant product that can be reconsitituted easily
Rates of sublimation of TBA/water solution. The relative rates of sublimation for each
component depend on its concentration. Figure D-2 compares sublimation rates by
plotting the molar ratio of TBA remaining to water remaining versus time. During
the sublimation of a 20 percent TBA solution, the TBA/water ratio remains
constant, which shows that both sublime at the same rate. Higher TBA
concentration solutions have negative slopes, indicating that TBA is subliming
faster than ice. Lower TBA concentration solutions show that ice was subliming
faster than TBA. Figures D-3 and D-4 show appropriate clothing for operator
handling of this product.
FIG. D-1 Phase diagram for TBA-water system. (l.s. = liq. state.) (Kasraian, K., DeLuca, P.,
Pharmaceutical Research, Vol. 12, No. 4, 1995; permitted by the Plenum Publishing Corporation.)
Porous resistance of the dry product layer. Research has shown that adding 5% w/v
TBA into a 5 percent w/v sucrose solution considerably shortens the primary drying
stage by lowering the resistance of the dried cake. Figure D-6 shows that the frozen
solution without TBA initially had a high resistance, approximately 60 cm2 torr
hr/gm, due to the formation of a skin. Once the skin cracked, the resistance
improved to 10 cm2 torr hr/gm. The solution containing TBA had a resistance of
0.53 cm2 torr hr/gm. Figure D-7 shows the result. Without TBA, the sucrose solution
dried in 100 hours. Adding 5 percent w/v TBA lowered the drying time to 10 hours.
Drying D-7
FIG. D-2 Ratio of TBA to water as a function of time for 10%, 20%, 44%, and 80% TBA solutions.
(Source: Kasraian, K., DeLuca, P., Pharmaceutical Research, Vol. 12, No. 4, 1995; permitted by the
Plenum Publishing Corporation.)
FIG. D-3 Suitable clothing for operator handling. (Source: ARCO Chemical.)
D-8 Drying
FIG. D-4 Suitable clothing for operator handling. (Source: ARCO Chemical.)
Applications
TBA as mass transfer accelerator. Beecham Pharmaceuticals has extensively studied
the effect of organic solvents, especially TBA, on freeze-drying efficiency and
product properties. (See Figs. D-8 and D-9.)
The use of TBA for freeze drying the common antibiotic gentamicin, in the
presence of maltose, has been reported. Adding TBA reduced the drying time from
39 hours to 28 hours and maintained the porous structure of the product.
FIG. D-5 Polaroid photographs of frozen TBA-water mixtures with different concentrations of TBA: (a) frozen deionized
water, (b) 10% w/w TBA aqueous solution, (c) 50% w/w TBA aqueous solution, (d) 70% w/w TBA aqueous solution. (Source:
Kasraian, K., DeLuca, P., Pharmaceutical Research, Vol. 12, No. 4, 1995; permitted by the Plenum Publishing Corporation.)
D-10 Drying
FIG. D-6 Normalized dried product resistance versus thickness of dried product: (a) 5% w/v
sucrose freeze dried in a microbalance at a temperature of -35 C, (b) 5% w/v sucrose containing
5% w/v TBA freeze dried in a microbalance at -35 C. (Source: Kastaian, K., DeLuca, P.,
Pharmaceutical Research, Vol. 12, No. 4, 1995; permitted by the Plenum Publishing Corporation.)
Drying D-11
FIG. D-7 Mass loss of water from: (a) 5% w/v sucrose solution, (b) 5% w/v sucrose solution containing 5% w/v TBA. Drying
temperature -35 C in the microbalance. (Source: Kasraian, K., DeLuca, P., Pharmaceutical Research, Vol. 12, No. 4, 1995;
permitted by the Plenum Publishing Corporation.)
D-12 Drying
elevate the collapse temperature or prevent the frozen product from reaching the
collapse temperature due to faster rate of sublimation.
AKZO Pharma Division of Organon International B.V. also reported how TBA
concentrations affect the stability of freeze-dried sucrose formulations. Adding 5
percent TBA to a 180 mg/ml sucrose solution resulted in a pharmaceutically
acceptable, stable freeze-dried cake with no collapse. Therefore, while TBA addition
did not change the collapse temperature of the sucrose solution, it increased the
rate of sublimation. The increase thereby prevented the product from ever rising
to the collapse temperature.
Drying D-13
Shelf
System Temperature () Remarks
Taken from DeLuca, P. P., Kamat, M. S., Koida, Y. Congr. Int. Technol.
Pharm., 5th, 1989, 1, 439, permitted by the publisher, Rue J.-B. Clement.
TBA in freeze drying of lipids and liposomes. Use of liposomal formulations is rapidly
gaining popularity in pharmaceutical research and development. Liposomes are
increasingly serving as carriers for antigens and/or drugs for different routes of
administration. The physical stability of the liposomes during long-term storage
has been a matter of intense investigation for some time. To ensure its therapeutic
properties, a liposomal dosage form must be stable with respect to its drug-carrier
characteristics. While several approaches can stabilize liposomes, lyophilization is
one of the best available methods to extend shelf-life.
D-14 Drying
Taken from DeLuca, P. P., Kamat, M. S., Koida, Y. Congr. Int. Technol. Pharm., 5th, 1989, 1, 439, permitted
by the publisher, Rue J.-B. Clement.
Taken from DeLuca, P. P., Kamat, M. S., Koida, Y. Congr. Int. Technol.
Pharm., 5th, 1989, 1, 439, permitted by the publisher, Rue J.-B. Clement.
(b)
FIG. D-10 (a) Polaroid photographs of sucrose during freeze drying without TBA. Region I: dried
material. Region II: collapse. Region III: frozen matrix. (b) SEM of freeze-dried sucrose (10% w/v).
(Source: Kasraian, K., DeLuca, P., Pharmaceutical Research, Vol. 12, No. 4, 1995; permitted by the
Plenum Publishing Corporation.)
D-15
(a)
(b)
FIG. D-11 (a) Polaroid photographs of sucrose during freeze drying in the presence of TBA. (b)
SEM of sucrose (10% w/v) freeze dried with TBA. (Source: Kasraian, K., DeLuca, P., Pharmaceutical
Research, Vol. 12, No. 4, 1995; permitted by the Plenum Publishing Corporation.)
D-16
Drying D-17
Freeze drying of water unstable drugs. The Upjohn Company reported a process
to manufacture a stable, lyophilized formulation of prostaglandin E1 (PGE-1) for
use in the treatment of erectile dysfunction. Lyophilization of a buffered lactose
formulation of PGE-1 from a TBA/water mixture provides superior product stability
than when freeze drying from a 100 percent aqueous system. The level of TBA that
afforded the product maximum stability appeared to be when the TBA amount
ranged from 1725 percent (v/v). The unique kinetics of the degradation pathway
of PGE-1 indicates that it is imperative to keep PGE-1 molecules as far apart as
possible in order to minimize the interaction of two PGE-1 molecules. TBA is most
likely enabling the PGE-1 molecules to be kept further apart during the freezing
and lyophilization phases of manufacture.
Bristol-Myers Company has reported on the use of TBA as a solvent for the
in-vial deposition of 7 (dimethylaminomethylene) amino-9a-methoxymitosane in
sterile unit dosage form. This compound is not stable in water. It is introduced into
a sterile vial in a TBA solution. Then the TBA is removed by lyophilization. The
deposited material contains up to 0.5 mole equivalent of TBA as a hemi-solvate and
is very stable to heat.
FIG. D-12 Temperature-time profile for freeze-drying cycle of macromonomer. (Source: P. DeLuca,
PharmTech Conference Proceeding, 1994, p. 375. Copyright by Advanstar Communications, Inc.)
Figure D-12 illustrates the temperature profile for the samples and the water
content at various stages of drying. During the primary drying stage, the TBA
solution remained at a lower temperature showing faster drying and the
temperature increased after 13 hours showing evidence for lower water content.
After 17.5 hours of cycle time, the TBA solution sample reached 1 C while the water
sample remained at 4 C. The freeze-dried material with TBA showed very low
moisture content (0.12 percent) compared to the material freeze dried in water that
showed 0.22 percent moisture. The residual TBA was 65 ppm.
Schott Glaswerke has a patent on using TBA to prepare a high purity glass
powder with a mean particle size of less than 10 mm. Glass powders having a
particle size up to 300 mm are ground to the desired particle size in the presence of
a grinding liquid comprising water and TBA. The slurry is then frozen, and the
solvent is subsequently removed from the frozen slurry by freeze drying. The
resultant glass powder is particularly suitable as a filler for synthetic resins in the
dental sector.
Ducting, such as that provided with another major accessorya gas turbine intake
filter system, for examplemay be provided by the vendor of the major accessory.
If it refers to the gas passageway from the exhaust end of a gas turbine to an HRSG
(see Cogeneration), the entire package is likely to be provided by the gas turbine
vendor. At any rate, ducting of major consequence is generally custom designed for
a plant. If well designed in terms of supports and seals and if not subject
to fluctuating temperatures, it could well remain a low-maintenance item through
the life of a plant. Expansion joints, however, are often subjected to fluctuating
temperatures.