Measurement of Concentration of Supersaturated NaCl Solution in Salt Dissolvers at Electrolysis Plants


Control of sodium chloride (NaCl) concentration at a salt dissolver where solid salt is dissolved in water, is highly important because of the electrolysis efficiency. A conventional way of measuring the concentration of supersaturated NaCl solution had been performed by using non-contact type sensors (e.g., γ-ray density meter) since NaCl, impurities, and precipitates are in the solution. In this way, there were disadvantages such as troublesome maintenance and therefore, the radiation control is required for γ-ray density meter. Responding to these backgrounds, the FLXA202 2-wire inductive conductivity analyzer provides concentration reading and realizes to improve substantial maintainability, cost performance and measurement accuracy.

Caustic Soda Production Process

Caustic Soda Production Process

Expected Benefits

  • Measures NaCl concentration in salt dissolvers
  • Reduces operating costs

Process Overview

In an electrolysis plant, the solid salt is dissolved in water in a salt dissolver to make brine. This brine contains impurities, such as calcium, magnesium or sulfates, that decrease electrolysis efficiency. The brine is purified by removing these impurities before being fed to an electrolyzer, where the purified brine is electrolyzed to produce caustic soda and chlorine. The caustic soda solution produced at the cathode in the electrolyzer is concentrated in a multiple effect evaporator, as a result the concentration is increased from approximately 32% to 48%.

Solution Details

Measurement system

2-wire inductive conductivity system

Sensor ISC40G(J)-GG-T1-☐☐
Flow-through holder ISC40FF(J)-PJ
Transmitter FLXA202-D-BーD-☐-C5-NN-A-N-LA-☐-NN

2-wire inductive conductivity system

Inductive Conductivity Sensor & Transmitter Installation Diagram


FLXA202 2-wire inductive conductivity analyzer
Power supply voltage: 17 - 40 V DC (from distributor)
Power consumption: 0.9 VA maximum
PH201G distributor
Power supply:
100 V: 20 to130 V DC/80 to 138 V AC, 47 to 63 Hz
220 V: 120 to 340 V DC/138 to 264 V AC, 47 to 63 Hz
Power consumption: 24 V DC: approx. 200 mA
100 V AC: approx. 7 VA
220 V AC: approx. 11 VA


Sampling should be done in the supernatant liquid in a salt dissolver. The supernatant liquid is collected in a settling tank and then the supernatant liquid of the tank is transported to a measurement tank where the sensor is installed.

Field Data

1. Process conditions (salt dissolver)

Temperature: Approx. 65°C
Pressure: Atmospheric pressure
Sample: Supersaturated brine
NaCl concentration control range: 200 to 360 g/L
--> 18.1 to 30 wt%*

* Depends on user's actual data of specific gravity.

2. FLXA202 setting parameters

Relationship between conductivity and concentration is based
on the linear approximation of S = 16.3x that is obtained by actual measurement using SC72 Personal Conductivity Meter.

  • NaCl temperature compensation table is used
  • Reference temperature setting: 25°C
Concentration (%) Conductivity (mS/cm)
18 293
24 391
30 489


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    Chemical plants rely on continuous and batch production processes, each posing different requirements for a control system. A continuous process calls for a robust and stable control system that will not fail and cause the shutdown of a production line, whereas the emphasis with a batch process is on having a control system that allows great flexibility in making adjustments to formulas, procedures, and the like. Both kinds of systems need to be managed in available quality history of product, and to be able to execute non-routine operations. With its extensive product portfolio, experienced systems engineers, and global sales and service network, Yokogawa has a solution for every plant process.

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