TECHNOLOGY
Engineers Get First Look at Hydrate Process AIChE meeting hears details of Koppers process for desalinating seawater by forming salt-free propane hydrate crystals Experts on saline water conversion had their first detailed look at the Koppers hydrate process (C&EN, July 25, page 39) last week at the meeting of the American Institute of Chemical Engineers in Washington, D.C. Although the process is still in the early stages of commercial development, it appears to be a major contender in the highly competitive battle to find the most economical way to desalt seawater. In the Koppers process, propane combines with seawater to form saltfree hydrate crystals. The crystals are separated from the mother liquor, washed, and decomposed to yield potable water. Because the crystals form above the freezing point of
water, the method requires less energy input than either freezing or distillation. Energy requirement for the propane hydrate process is 28% lower that that needed for the freezing process, which uses butane as the refrigerant, William Knox of Koppers told the AIChE meeting. The company estimates that its technique can produce potable water at less than 50 cents per 1000 gal. in a 10 million gal.-per-day plant. Mr. Knox adds, "It is believed that this process is feasible and will produce water competitively with any process currently envisioned." In a recent report prepared for the Office of Saline Water by the Syracuse
DESALINATION REACTOR. Koppers" Horace Smith, Jr., inspects the 10-gal. reactor that is part of the company's bench-scale unit for desalting saline water 60
C&EN
DEC.
12, 196 0
University Research Institute, Dr. Allen Barduhn states that the hydrate system has many promising advantages over other methods. Compared with freezing processes, he says, the hydrate method offers about 5% higher coefficient of performance for the main refrigeration cycle and over 50% better performance for the auxiliary refrigeration cycle. Process Conditions. The reactor in the Koppers process operates at about 35° F. and 57 p.s.i.g. Under these conditions, propane combines with water to form insoluble clathrate crystals made up of 1 mole of propane surrounded by 17 moles of water. The washer operates at 35° to 45° F. This is high enough to eliminate the danger of freezing the water and plugging the washer. The decomposition tank runs at about 45° F. and 70 p.s.i.g. A stream of brine is recycled from the washer to the reactor. This keeps the slurry concentration in the reactor at a workable level and holds the over-all conversion at about 40%, the level Koppers has found to be most practical. To conserve energy, product water and the waste brine are used to cool the inlet seawater and the propane recycle system. When the seawater enters the reactor, it is already within a few degrees of the operating temperature. . Many materials form hydrates with water. Koppers chose propane because it is inexpensive, has low water solubility, and reacts with water at pressure-temperature conditions that are economic. Propane also has the key added advantage of being a direct heat transfer medium. In the reactor, some propane is vaporized to remove the heat of formation of the hydrate crystals. Later the propane gas is compressed and condensed on the washed crystals. This supplies the heat for decomposing the hydrate. Product water is easily separated from the propane since the two materials form immiscible layers. If de-
SIMPLIFIED FLOW DIAGRAM
COMMERCIAL PROPANE HYDRATE PROCESS for POTABLE WATER Saline Water 5,000,000 eal./day
1 1
H •
Propane Recycle 11,500,000 gal./day
REACTOR
1
35° F 57 psig
•
1
•
J
• H
Brine Recycle 175,000,000 gal./day
FILTER and III
inn
L
m
DECOMPOSER 45°F 70 psig
H 1• •
Brine Effluent 15,000,000 gal./day
Potable Water 10,000,000 gal./day
Cenco-Petersen
MOLECULAR MODELS can be quickly hand assembled without special tools for three-dimensional study of stereochemical and conformational factors and reaction mechanisms. Designed to overcome many shortcomings of earlier model systems, they • are m a d e up of accurate bond
sired, the small amount of propane dissolved in the water can be removed in a degassing operation. The purified water contains less than 500 p.p.m. solids. The energy advantage of the hydrate process is clear when the melting temperature of ice and the decomposition temperature of propane hydrate are compared. In pure water, for instance, propane hydrate exists at 42.2° F.; ice at 32° F. This spread of roughly 10° F. continues as salt concentration increases, Koppers finds. Other studies made by the company SIIQW that the heat removed in forming the propane hydrate is the same as that required to freeze water. Driving Force. One problem of desalting processes based on freezing is obtaining an economic production rate in the crystal separation and washing steps. The same is true in the hydrate process. Koppers is now working on ways to increase the particle size of the hydrate crystal so that filtering will be more efficient. Particle size is related to the thermal driving force, the difference between actual operating temperature and the equilibrium hydrate formation temperature.
A high driving force, say 6° F., produces small particles that are hard to filter. By increasing the operating temperature, which can easily be controlled by a pressure regulator on the reactor, the driving force is reduced and larger crystals are produced. Koppers says that a driving force of 1° to 2° F. produces crystals that should be practical to handle on a commercial scale. The equipment used by Mr. Knox and co-workers M. Hess, G. E. Jones, Jr., and H. B. Smith, Jr., is centered around a 10-gal. reactor. Admittedly, this is a far cry from a 10 million gal.-per-day commercial plant. However, Dr. Paul Bachman, Koppers' director of research, says, "This process should present no greater obstacles to scale-up than might be expected for any reasonably complex engineering process. The economic relationship between the hydrate process and freezing methods is not expected to be changed by scale-up." Koppers has an agreement with the Office of Saline Water under which Koppers is to obtain engineering data for a pilot plant. If further development goes smoothly Koppers* patent on the hydrate process (U.S. 2,904,511) assures it a strong position.
lengths (1A = 5 cm). • present precise angles of distortion (to 30°). • can be locked or left free to rotate under stress. • return p e r f e c t l y to a p r e s e t angle. The stereochemist can now directly predict inter-atomic distances and bond angle distortion using these trueto-scale models. Polyvalent atoms are colored neoprene balls with threaded aluminum inserts placed at the theoretical bond angles. Monovalent atoms are polystyrene with the appropriate bond permanently attached. The bonds themselves may be disassembled and rebuilt with other lengths to provide different scale factors. The complete No. 71306 CencoPetersen Set contains 55 monovalent atoms, 59 polyvalent atoms, 70 bonds and fittings plus extra parts, all in a partitioned metal case. Send for Booklet 315 for detailed information. $295.00
CENTRAL SCIENTIFIC CO. A Subsidiary of Cenco instruments Corporation 1710 — Irving Park Road*Chicago 13,Illinois Mountainside, N. J. Montreal Santa Clara Somerviile, Mass, Toronto Los Angeles Birmingham, Ala. Ottawa Vancouver Houston Cenco S.A., Breda, The Netherlands Tulsa
DEC.
12,
1960
C&EN
61
