EVAPORATION OF CAUSTIC SODA
INTRODUCTION:
Evaporation is a thermal separation process, widely used for concentration of liquids in the form of solution, suspension and emulsion. Concentration is done by boiling out a solvent (water) from the liquid. Caustic soda produced from the electrolysis of brine is not concentrated solution but contains water. This evaporation is done by using industrial evaporators. Different types of evaporators use for the removal of water from caustic soda are:
Ø
Natural\Forced circulation
evaporator
Ø
Falling and
Rising film evaporator
Ø
Plate evaporator
Ø Multiple-effect evaporator
Mostly Multiple-effect evaporator used for the concentration of caustic soda solution because putting together evaporators saves heat and thus less energy. Plate and Frame heat exchanger are used to evaporate water from caustic soda. Each plate and frame exchanger is arranged in series with separator. The main components of this process contain the following:
Ø Plate and Frame heat exchanger
Ø Separator
Ø shell and tube heat exchanger
Ø condensate
Ø piping and instrument
Plate and Frame Heat Exchanger:
Plate heat exchanger is type of heat exchanger use metal plates to transfer heat between two fluids. It is preferred to other heat exchanger due to large surface area and temperature change. In this way fluid easily spread out over the plates. It consists of pressed and corrugated metal plates fitted between thick carbon-steel frame. Caustic solution passes through adjacent layers between the plates, either in series. The gaskets confine the solution and prevent them from escaping into the atmosphere. The heating medium (hot steam) flows between every other plate. The counter flow of fluids over the plates produces high level of turbulence which increases the heat transfer rate.
Separator:
A vapor-liquid separator is a vertical vessel used to separate water vapors from caustic soda solution. Gravity causes the caustic solution to settle to the bottom of the vessel. The vapors travel upward to the condenser. In multiple effect evaporator, two or three separator also used with plate and frame heat exchanger.
Condensate:
Condensate is use during the process of evaporation of caustic soda solution to condense the water vapors. The water vapors remove from the solution is converted into water by using condenser.
CONSTRUCITON AND WORKING:
When
Caustic soda or sodium hydroxide is produce from electrolysis, it is not 100%
pure but contains water. From ion exchange membrane cell it is produced 30%
concentrated solution. Mostly Industrial requirement is 50% concentrated
caustic solution, so evaporator is used for the concentration of solution. Three
plate and frame heat exchange are arranged in series with separators. First of
all feed caustic solution of 30% send to plate and frame heat exchanger to
evaporate the water. Plate heat exchanger use hot steam to evaporate the
caustic solution. Vapors and boil caustic solution send to separator to
separate the water vapors and sodium hydroxide on the basis of gravity. After
this caustic soda is converted to 35% concentrated solution. Which is then pump
to plate and shell tube heat exchanger by centrifugal pump. Out let of both
exchanger is sent to next plate and frame heat exchanger and then it is
provided heat by using steam come from the third separator. It is then send to
Second separator for the separation of water vapors and 35% caustic solution.
At this stage 42% concentrated solution is prepared and again pumps to next
plate and shell tube heat exchanger. Now its output is send to two shell and
tube heat exchanger attach with third separator. This all process of
evaporation is recirculation of caustic soda solution for the preparation of
50% concentrated solution. At the third
PFD (PROCESS FLOW DIAGRAM)
Separator
water vapors move upward and caustic solution move downward by gravity. All
water vapors from the three separators are condensed by using condensate. Now
50% solution is pump to cooler and then sends to storage tank. In all these
process heat is recovered by moving steam from third separator to first and
caustic solution moves to first separator to shell and tube heat exchanger.
Complete process is evaporation is done in three stage effects of evaporation.
Stable operation:
To check the concenteration of caustic production notes the temperature and density of solution. Then from the stem table note down the concentration along the given temperature and density.
Reference:
http://www.niroinc.com/news_archives/evaporation_process_principles.asp
http://en.wikipedia.org/wiki/Evaporator#Falling_film_evaporator
http://en.wikipedia.org/wiki/Vapor-liquid_separator
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