WSP10809

• File Number: 8273.500
• Description: Colorado River Basin Salinity Control - Federal Agency Reports - EPA
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 4/1/1978
• Title: Integrating Desalination and Agricultural Salinity Control Alternatives
• Water Supply Pro - Doc Type: Report/Study

Vacuum Freezing - Vapor Compression

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A second basic desalting approach involving a phase change
to separate salt and water is freezing. This process operates
on the pri~cip1e that saline water at the freezing point will
simultaneo~sly form pure water vapor and salt-free ice crystals.
The ice crystals are then collected and melted by compressing
the water ~apor to a slightly higher pressure and temperature
(U. S. Department of the Interior, 1972).

Although the prototype VF-VC systems are still small and
some of the important technology remains to be completely
developed, the process has a number of advantages over the
previously: discussed distillation processes. For instance, the
VF-VC syst~m operates at lower temperatures which minimizes
corrosion and scaling and the heat transfer requirements are
much lower:. A schematic diagram of the VF-VC process is shown
in Figure '6.

After: a pretreatment step including deaeration and filtra-
tion, feedwater flow is divided and passed through two heat
exchangers; with brine and product water to cool the flow to
almost the freezing point. The flow then enters the hydro-
converter having a low atmospheric pressure where part of the
water flashes into vapor and part is crystallized into ice.
The brine ~nd ice mixture is then pumped to the counterwash
vessel. Since there is a differential in the density of the
ice and brine, the ice aggragates at the top of the brine where
it is washed by about 5% of the product water before being
scraped from the top of the ice surface and returned to the
hydroconvexter vessel. The final process step in the hydro-
converter ;involves compression of the water vapor until it
condenses un the ice crystals. The ice also melts in this
process anu with the condensed water vapor is pumped from the
system as the product water.

The V>F-VC desalting process is most economically applied
to cold feedwater sources having TDS concentrations of 5000-
50,000 mg/~. Product water is usually 300-500 mg/~. The BPR
is computed by:

TDS. -
~

BPR =

TDS
P
- TDS.
~

.................................................................... (59)

60,000

and since no cooling water is required, the total intake rate
is:

C. = C + Cb .....,.................................... (60)
~ . p
where Cb is determined from Eq. 48.
A summary of the costing model is given in Table 4,

39

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