Page 183 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.19

DISTILLATION

In its basic form, distillation is the boiling of feedwater, condensing the steam produced
from the feedwater, and collecting the condensate which yields a water product that is theo-
retically free of nonvolatile impurities. There are three methods currently used to produce
distilled water: single-stage, vapor compression, and multieffect distillation.

Single-Stage Distillation
This is the most simple still. Feedwater enters the still, then is evaporated and condensed in
a single stage. Cooling water is required to condense the steam produced. This type of still
will produce water of approximately 1 MΩ/cm, with higher purity possible with optional
equipment that removes dissolved gaseous impurities. This still has a small footprint, is not
labor intensive, and will tolerate feedwater with a high level of impurities.

Vapor Compression Distillation
Vapor compression, sometimes called thermocompression distillation, is a method of evap-
oration in which a liquid is boiled inside a bank of tubes. The vapor generated then passes
through a mist eliminator that removes any water droplets. The pure vapor is withdrawn by
a compressor in which the energy imparted results in a compressed steam with increased
pressure and temperature. The high-energy-compressed steam is discharged into an evapo-
rator. At this point, the steam gives up most of its energy (latent heat) to the water inside
the tubes. More vapor is generated and the process is repeated. The condensate (distilled
water) is withdrawn by the distillate pump and is discharged through a two-stream heat
exchanger.
The excess feedwater that did not evaporate is also pumped through an exchanger. Both
the distillate and the blowdown are cooled, and the feedwater is preheated prior to enter-
ing the evaporator. These exchangers minimize the energy consumption of the system and
eliminate the need for additional cooling water. The system operates continuously once it
is started. Additional makeup heat, usually supplied by steam, is required for continuous
operation. Vapor compression is generally considered more economical for large quanti-
ties of water and does not require a high-quality feedwater for proper operation. The vapor
compression still is moderate both in initial and operating costs.
Refer to Fig. 4.3 for a schematic diagram of a vapor compression distillation unit.

Multieffect Distillation
Multieffect distillation units use the principle of staged evaporation and condensation to
generate distilled water. Each stage is called an effect. Distilled water is produced in each
effect by condensing the steam generated by the evaporation of high-purity feedwater in the
previous stage. The initial driving force for the evaporation is power steam applied to the
shell side of the first effect vessel. The multieffect still has the highest initial cost and lowest
operating cost, and it requires the highest quality feedwater of all the stills.
The feedwater enters the vessel and its pressure is boosted by the feed pump. The feed-
water flows through a coil in the condenser, which allows it to pick up heat from the
condensing steam. This preheated feedwater flows through the feed control valve and into
the tube side of the first effect. The first effect level controller senses the feedwater level
and signals the feed control valve to maintain the desired level. Power steam is introduced
into the unit and flows through the steam control valve and into the shell side of the first
effect.

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