An electrolytic chlorination system for pretreatment and post-treatment in desalination systems

DESALINATION Desalination 102 (1995) 321-324

ELSEVIER

An electrolytic chlorination system for pretreatment and post-treatment in desalination systems Donald C. Brandt The ConSeps Group Inc., 908 Fairthorne Avenue, Wilmington, DE 19807, USA Tel.: 302-6561120, Fax: 302-6561126

Abstract

The Uniclor® system is a commercially tested system for on-site chlorine generation from salt. Its characteristics are: Lower cGst than hypochlorite solutions, safe and environmentally clean operation, a way to eliminate transportation and storage of chlorine or chlorine-containing chemicals, a modular in design to meet varying capacity needs, and is electrically operated for quick start/stop operation and regulation of chlorine preduc~

1. I n t r o d u c t i o n

The Uniclor ® system offers a method to replace liquid chlorine or h)]3ochlofites as an on-site source o f chlorine for disinfection of feed and/or product water in desalination systems and to generate chlorine electrol y t i c a l l y f r o m salt brine in a safe, environmentally clean and economical way. Uniclor has more than 35,000 units in service for chlorinating swimnung pools, and is based on state-of-the-art t e c h n o l o g y used in commercial p r o d u c t i o n o f chlorine and caustic soda. Uniclor is a registered trademark o f Sophisticated Systems Inc., and the ConSeps Group Inc. is the exclusive worldwide dealer o f Unic!or ® systems for desalting applications (Fig. 1). The Uniclor ® s y s t e m c a n be used b y r e s i d e n t i a l communities, resorts, military (ROPU's), offshore oil figs, shipboard (pleasure, military 0011-9164/95/$09.50

Fig. 1. Photograph of a Uniclor system unit.

Source: Sophisticated Systems, Inc.

© 1995 Elsevier Science B.V. All rights reserved.

SSDI 0011-9164(95)00069-0

322

D.C. Brandt I Desalination 102 (1995) 321-324

and commercial) and by industries (e.g., utilities, electronics, pharmaceutical) for chlorination and/or neutralization. 2. Chemistry Chlorine is produced by the electrolysis of sodium chloride brine in a membrane cell with a perfluorinated cation exchange membrane which allows sodium ions to pass and retards the passage of chloride and hydroxyl ions. 2 NaCl(s)+ 2H20(I) Energy 2NaOH(1) + Cl2(g) + H2(g)

3. Operation The Uniclor ® system is operated as foUows (Figs. 2 and 3):

DiluterBy-Pass ff

II

,

gallons of water is added to the anode chamber - W a t e r is added to the cathode chamber (start-up only) - DC electric current is applied -Chlorine production is regulated by adjusting the electrical current - Chlorine is removed from the anode vapor space and injected into the desired stream through a venturi - N a O H is removed from the cathode chamber and is combined with the chlorine if chlorination and neutralization is required, or can be removed separately as a 5-15% solution. Output (Model 1200): 1.65 pounds of salt will produce 1 pound of chlorine at 8.5 volts and 20 amps DC 8.5 volts and 20 amps DC will produce 1.2 pounds of chlorine per day (0.05 lbs/h)

I~ Level Diluter/Recharge II Salt Indicator .... Indicatorj.,. Pool.Water ., i :::}(: I ' ~ .,~cL__I Cell _..,,,..~~=u.~ ~..~

Ii,

- 40 lb bag of water conditioner salt plus 9

pH 1111 I ICnlorineI ! I

j

I1_~

\\

T°TFumpl 87DC

~ i ! ~ ~ ~ i I .-- pHBufferedChlorine !~ F t o ~ - To Supply or 8forage " ~ ~ ~ / = V e n iSurpl u r i "usDilution " "

Fig. 2. Process flow diagram. Source: Sophisticated Systems, Inc.

D.C. Brandt / Desalination 102 (1995) 321-324

323

Cost of Uniclor Units. KS 2O Chlorine level In product water r~

1.0 ppm

0.2 isc~n

HAFION~ Perfluorlnated Membrane

Fig. 3. Electrochemicalschematic. Source: DuPont,Nation Products Users Guide.

O

I

i

0.2

0.4

i

i

i

0.6 0.8 idlG Produat Flow. O

i

h

1.2

1.4

1.6

Fig. 4. Capital costs.

- Approximately, 0.8 gallons of 15% caustic solution is produced for every pound of chlorine generated. 4. Benefits The Uniclor ® system offers the following benefits (Figs. 4-6 and Tables 1 and 2): - Produces high purity chlorine gas -Eliminates storage and transportation of chlorine gas -Eliminates storage and transportation of other hazardous chlorination materials such as calcium or sodium hypochlorite - U s e s high quality (99.5%) salt readily available, chemically stable, safe and easily stored - S i m p l e on/off operation, and can be electrically interlocked with the desalting plant - C h l o r i n e production is regulated by adjusting the electrical current - Flexibility to provide chlorine and caustic together or separately

Unicior

Gg(OCI) z Chlorination

NaOCI

method

Fig. 5. Cost to produce chlorine.

-NaOH has economic value for pH adjustment or for other plant uses, e.g. cleamng, neutralization

324

D.C. Brandt / Desalination 102 (1995) 321-324

safe and environmentally clean operation No moving parts, and easy to maintain and operate

-Economical,

-

- Modular: multiple units can be connected in parallel to a c h i e v e the d e s i r e d c h l o r i n e production.

Table 1 Costs of chlorine produced by the Uniclor® system (Electricity: 3.65 kWh/lb chlorine, salt: 1.65 lbs/lb chlorine) Cost component

Coat $avlnga, $11b Chlorine

Typical costs ($1 Utfited States Unit cost Cost/ , ib C! 2

Car;.~be',,:n Unit cost Cost/ lb CI 2

41 i

va N a O C l . . .

v8 Ca(OCI) a ....

s

|

Cost component

°IH |

US

Caribbean

Fig. 6. Uniclor cost savings.

0.08/kWh 0.125/1b

0.29 0.21 0.5

0.2fKWh 0.325/lb

0.73 0.54 1.27

Table 2 Costs of chlorine from hypochlorites (Calcium hypochlorite: 1.9 lb/lb chlorine, sodium hypochlorite (13%): 1.7 gai/lb chlorine)

|

i

Electricity Salt Total

U.S.

Caribbean

Typical costs ($1 United States Unit cost Cost/ lb CI 2

Calcium 2lib hypochlorite (100 lb drums) Sodium l/gal hypochlorite (bulk, >1,000 gall

Caribbean Unit cost Cost/ lb CI2

3.8

2.2/1b

4.18

1.7

2.45/gal

4.17