jamesbury
APPLICATION REPORT
Chlorine
Caustic Evaporation
Vapor to
Condenser
6
3
2
First
Effect
Second
Effect
2
5
Steam
7
7
4
Caustic
6
3
3
2
Steam
Condensate
6
1
3
2
Process
Condensate
1
Caustic
Feed
2
Product
Caustic Evaporation
(Membrane Cell)
Manual Isolation
Rotary Control
2
4" - 10" (100 – 250DN) 815L 11 3600 XZ
w/ Gear Actuator
1
3" - 8" (80 – 200DN) 815L 11 3600 XZ
w/ QPX Actuator w/ Positioner *
3
3" - 6" (80 – 150DN) 815L 11 7200 XZ
w/ Gear Actuator
4
3" - 6" (80 – 150DN) 815L 11 7200 XZ
w/ QPX Actuator w/ Positioner *
5
8" - 16" (200 – 400DN) 815L or W 11 2236 X
w/ QPX Actuator w/ Positioner *
6
7
3" - 4" (80 – 100DN) 815L or W 11 3600 XZ
w/ Gear Actuator
Alt.
2" - 3" (50 – 80DN) 7150 31 3600 XT
10" - 18" (250 – 450DN) 815L or W 11 2236 XZ
w/ Gear Actuator
* Positioner can be:
NE Style Electro Pneumatic
NP Style Pneumatic
ND Style Digital
Caustic Processing
There is a significant difference between the processes
required to prepare commercial caustic solutions from
chlorine cells. Mercury cells produce marketable 50%
NaOH directly from the decomposers. Diaphragm cells
produce a much lower concentration of NaOH (10 – 12%)
and it contains nearly equal amounts of salt. Membrane
cells, like diaphragm cells, produce caustic (32% NaOH)
that is not up to commercial concentration, but the
liquid contains very little dissolved salt and requires
significantly less processing than diaphragm cell liquor.
Chlo8 • 8/2011
APPLICATION REPORT
Evaporation
Diaphragm and membrane cell liquor requires concentration
to obtain commercial grade of NaOH. Because of the unique
difference in liquor characteristics, processing of diaphragm
cell liquor is more extensive. About 6 lbs. (2,721 grams) of
H2O per 1 lb. (453 grams) NaOH needs to be removed from
diaphragm cell liquor. Besides the evaporation of large
amounts of H2O, precipitated salt is removed, cooled and
recycled through brine saturation and treatment. About 1 lb.
(453 grams) of H2O per 1 lb. (453 grams) of NaOH needs to be
removed from membrane cell liquor. The amount of H2O to
be evaporated from the cell liquor means that diaphragm cell
liquor requires a 3 – 4 stage evaporation process.
Temperatures tend to be higher in the processing of the
liquor. This may require valves of higher pressure class and
more noble material. Two or three stages are normal in a
membrane cell plant. A two effect membrane cell evaporator
with backward flow is shown.
Valve Requirements
The evaporation process doesn’t present any challenges from
a pressure prospective. Temperature, however, has an affect
on material selection. Attention is important regarding this
detail. Chlorine Institute Pamphlet 94 Table 5.4.3.1 is a good
source for this data.
There is extensive use of modulation control and isolation
valves in the evaporation process. First and second effect
control valves modulate from either liquid level or
temperature control from the evaporators (1 ). Caustic
discharge control is modulated by caustic density or
temperature (4 ). First effect heat exchanger temperature is
controlled by modulating 100 psi (6 bar) steam flow (5 ).
There are a substantial number of manual isolation valves on
pumps and evaporation tanks (2 , 3 ), and heat
exchangers (6 , 7 ).
The information provided in this bulletin is advisory in nature, and is intended as a guideline only.
For specific circumstances and more detailed information, please consult with Metso.
Metso Automation Inc.
Europe, Vanha Porvoontie 229, P.O. Box 304, FI-11301 Vantaa, Finland.
Tel. +358 20 483 150. Fax +358 20 483 151
North America, 44 Bowditch Drive, P.O. Box 8044, Shrewsbury, MA 01545, USA.
Tel. +1 508 852 0200. Fax +1 508 852 8172
South America, Av. Independéncia, 2500- Iporanga, 18087-101, Sorocaba-São Paulo
Brazil. Tel. +55 15 2102 9700. Fax +55 15 2102 9748/49
Asia Pacific, 20 Kallang Avenue, Lobby B, #06-00, PICO Creative Centre, Singapore 339411, Singapore.
Tel. +65 6511 1011. Fax +65 6250 0830
China, 19/F, the Exchange Beijing, No. 118, Jianguo Lu Yi, Chaoyang Dist, 100022 Beijing, China.
Tel. +86-10-6566-6600. Fax +86-10-6566-2575
Middle East, Roundabout 8, Unit AB-07, P.O. Box 17175, Jebel Ali Freezone, Dubai,
United Arab Emirates. Tel. +971 4 883 6974. Fax +971 4 883 6836
www.metso.com/automation