Village LW-160, Руководство пользователя и справочное руководство

Страница: 16 / 79

GENERAL THEORY
8
VMT - v. JUNE 2008
3.0 GENERAL THEORY OF OPERATION
3.1 REVERSE OSMOSIS THEORY
Reverse osmosis, like many other practical scientific methods, was developed from processes first
observed in nature. Osmosis is a naturally occurring phenomenon in which a semi-permeable
membrane separates a pure and a concentrated solution (a semi-permeable membrane is defined as
one that preferentially passes a particular substance). Every fluid has an inherent potential that is
directly related to the type and amount of solids in solution. This potential, referred to as osmotic
pressure, increases in proportion to relative concentration of a solution. A concentrated solution,
therefore, has an osmotic pressure that is higher than that of a pure solution.
In an osmotic system, the less concentrated solution will attempt to equalize the concentrations of both
solutions by migrating across the semi-permeable membrane. When enough pure solution migrates
across the membrane such that the inherent potential difference between the solutions in no longer
higher than the osmotic pressure of the membrane, the purer solution will stop flowing. If the pressure
on the concentrated solution is increased to above the osmotic pressure, fluid flow will be reversed. This
condition, called Reverse Osmosis, can be established by artificially pressurizing the more concentrated
solution using a high pressure pump. In this type of system, the concentrated solution (normally referred
to as feedwater) will become more concentrated as pure water flows out of solution and across the
membrane to the permeate side. Discounting the effects of feedwater temperature and salinity, the
operating pressure normally required to produce significant amounts of pure water is at least twice the
osmotic pressure of the membrane being used.
SALINE
SOLUTION
PURE
SOLUTION
SEMI-PERMEABLE MEMBRANE
ATMOSPHERIC
PRESSURE
(14.7 PSI)
SALINE
SOLUTION
PURE
SOLUTION
HIGH
PRESSURE
(800 PSI)
OSMOSIS REVERSE OSMOSIS
Figure 3.1 - Simple (Reverse) Osmosis Process.
3.2 APPLICATION OF REVERSE OSMOSIS
Seawater contains many kinds of solids dissolved in solution. The most prevalent is common table salt
(sodium chloride). Other minerals that may be present in solution are substances that usually contain
various compounds of calcium and sulfate. The sum of all of the solids dissolved in a particular sample
of water is referred to as
Total Dissolved Solids or TDS. Seawater normally averages 32,000 ppm
(parts per million) TDS although variations of 5000 ppm are common in various parts of the world. The
fundamental goal of any desalination process is a significant reduction in the amount of dissolved solids
in water.
In a Reverse Osmosis desalination system, most of the dissolved solids do not pass through the
membrane but are instead carried along the membrane surface. This rejected water, referred to as
brine , becomes increasingly more concentrated as it flows across the surface of the membranes and is
eventually piped to drain. The product water that flows through the membrane is referred to as
permeate . The percentage of feedwater that enters the unit converted to permeate is called the
recovery rate . A higher than optimal recovery rate (which can be obtained by increasing the back