Chemical Engineering April 2013 - 39

Sodium Hypochlorite
Chemical Production
By Intratec Solutions
S
odium hypochlorite (NaClO) is the active constituent
in chlorine bleach, a strong oxidizer and
bleaching agent. Increases in household bleach
demand are driven mostly by population growth. In
turn, population growth and its corresponding increases
in water consumption - coupled with limited fresh
water resources - makes water treatment the largest
application for bleach, as well as the fastest-growing
segment of bleach use.
Sodium hypochlorite chemical production is a wellestablished
process in the industry, and the principle
behind its operation is also employed for preventing
chlorine emissions in chlor-alkali plants. The chemical
process relies on the acquisition of chlorine and caustic
soda (sodium hydroxide; NaOH) feedstock from external
sources, in contrast with the electrochemical process
for bleach, which also involves brine electrolysis.
The process
The chlorination of caustic soda to sodium hypochlorite is an exothermic
reaction. The chemical production process depicted below is a
widely used process, similar to the one employed by Solvay Chemicals
(Brussels, Belgium; www.solvay.com), for example. The process is suited
to producing both household bleach (5-6 wt.%) and industrial bleach
(10-15 wt.%), and relies on the chlorination of caustic soda by chlorine
gas within packed columns.
The industrial sodium-hypochlorite plant can be divided into two main
sections: reaction and product discharge; and bleach filtration.
Reaction and product discharge. The chlorine absorption system can
be divided into two parts: in the first, a packed column is operated
with a safe excess of caustic to prevent reduction in pH; the second
part, in turn, receives the liquor from the first column to be postchlorinated,
until the desired bleach concentration is reached. Prior
to reaction, caustic soda is diluted with water in the first buffer tank,
along with the first-column bottom stream. Chlorine gas is diluted
with air and fed into the bottom of both columns. Flow to the second
part of the system is established when the finished product is sent to
the filtration steps.
Bleach filtration. Bleach filtration is necessary to meet product quality requirements
and is often the last step before storage. Usually, the bleach
filter system consists of pressure-leaf filters. After filtration, the product is
sent to storage. Backwash water containing spent filter aid can be taken
to a dry-cake pressure filter for further processing.
Sodium hypochlorite solutions are very sensitive and special operations
must be carried out to prevent its decomposition. Besides pH,
other factors affecting degradation are the initial bleach concentration,
exposure to ultraviolet light, presence of certain metals and elevated
Offgas to
scrubber
6
1
Chlorine
Air
Caustic soda
Water
2
4
5
Bleach
to storage
(5) Bleach filter
system
(6) Backwash
water filter
CW = Cooling
water
CW
3
CW
Backwash
water to
disposal
temperatures. The graph above shows temperature dependency for
bleach degradation, starting from a 12.5 trade-percent solution. Trade
percent is an expression of available chlorine, in units of grams per liter
of available chlorine.
Economic performance
An economic evaluation of the process was conducted for three distinct
locations - the U.S. Gulf Coast region, Germany and Brazil - and is
based on data from the first quarter of 2012. The following assumptions
are made for the analysis:
* A 250,000 ton/yr chemical production unit (NaClO solution, 12.5
wt. %) erected inside a chlor-alkali facility (all equipment is represented
in the simplified flowsheet)
* Storage of products is equal to 20 days of operation, and there is no
storage for feedstock
The estimated capital investment for such a plant on the U.S. Gulf Coast
is about $35 million, the lowest among the regions compared. Germany
presented a higher capital investment, at $40 million, and the highest
operating costs - about $170/ton (compared to $140/ton in the U.S.)
Global perspective
Selling prices for bleach vary substantially, depending on supply and
demand fluctuations of the chlorine/caustic market. Based on the
estimated capital and operating costs, a U.S. Gulf Coast-based bleach
manufacturing venture can reach an internal rate of return (IRR) above
15%, by selling bleach at an average price of $175/ton, a reasonable
value. However, a venture in Germany would not be economically attractive
at the same selling price. ■
Edited by Scott Jenkins
(1) 1st Absorption
column
(2) 1st Buffer
tank
(3) 2nd Absorption
column
(4) 2nd Buffer
tank
Editor's Note: The content
for this column is supplied
by Intratec Solutions LLC
(Houston; www.intratec.
us) and edited by Chemical
Engineering. The analyses
and models presented herein
are prepared on the basis of
publicly available and nonconfidential
information. The
information and analysis are
the opinions of Intratec and
do not represent the point
of view of any third parties.
More information about the
methodology for preparing
this type of analysis can be
found, along with terms of
use, at www.intratec.us/che.
10
11
12
13
5
6
7
8
9
20
15 °C
40
20 °C
60
Time, days
25 °C
30 °C
40 °C
80
100
120
Bleach concentration, trade %
http://www.solvay.com http://www.intratec http://www.intratec.us/che
Chemical Engineering April 2013

Table of Contents for the Digital Edition of Chemical Engineering April 2013
