(GP/GT) for Additional Water Supply in the Lower Rio Grande

EVALUATION OF AUTO DESALINATION
As with all process feasibility evaluations, background information must be obtained
which determines the feasibility of the various process units. In the case of the GP/GT
application, the following wellhead information was provided by the Project Manager,
based upon historical data from the McAllen Ranch wellfield:
Wellhead Temperature: 265°F
Entrained Gas 25 ftl/bbl
Daily Feed 1.0 MGD
Chemical Analysis see attached
Brine Disposal Via Injection Well (others)
Minimum System Recovery 75%
Wellhead Pressure 1000 PSI
• Produce Water Quality SDWA Standards
Based upon the temperature, pressure, and entrained gas information, all membrane
systems can be precluded from further consideration without significant additional
process equipment to cool and degas the streams. The elevated pressure can be
reduced by the utilization of an energy recovery system to both recover the GP energy
and depressurize the stream.
Immediately after depressurization, a single-staged flash unit without a condenser can
be applied to remove the entrained (miscible and immiscible) gases from the stream.
This energy source can be recovered or utilized for other energy production on-site.
Based upon standard thermodynamic design, the heat loss from wellhead,
depressurization, through these units is estimated to be 15-20°, resulting in a flow to
the desalination system at a temperature of approximately 245-250 0.
The basic concept of the autodesalination system has been to utilize the GT aspect of
the flows. Instead of the usual system of heat sources and exchangers throughout the
process, the stream arrives at an elevated temperature, which then provides the
thermal energy to drive the multi-effect unit.
Prior to the actual design of a multi-effect distillation system, attention must be made
to the feedwater chemistry, the brine chemistry, and the number of effects. The
physical chemistry of the flows across the various effects determines the performance,
scale deposition, and corrosivity of the system.
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