Application Notes

pH/ORP Control In NaOH Production
Using Membrane Cell Process


Caustic soda has traditionally been produced by the Mercury (Hg) Cell process. Today, however, industries worldwide are switching to advanced Membrance Cell Technology. This technology produces energy savings of approximately 30% and eliminates mercury contamination. The average capacity of a plant using the Mercury Cell Process is about 200 tons per day. In contrast, the Membrance Cell Process allows smaller users to produce caustic on-site for captive consumption. pH measurement is critical in the production of caustic, but difficult to measure under adverse process conditions such as :

. Presence of stray voltage
. high salt cocentration
. high alkalinity
. high variable temperature
. presence of corrosive chemicals

Grounding the process stream helps, but pH fluctuations still persist. This application note shows the superiority of Bela Instruments' Differential Electrode Technique for pH and ORP measurement.



A typical Membrance Cell Process is shown in Figure 1. Pure brine is fed to the anode chamber of the membrance cell. Part of the 32% caustic (NaOH) beingused is diluted to 31 % caustic and is then fed to the cathode chamber of the membrance cell. Anolyte, impure brine with chlorine and chlorine compounds, is produced by the anode chamber. The anolyte is further processed to eliminate chlorine and precipitates, make up brine strength and remove calcium (Ca +2) and magnesium (Mg +2) ions.

FIGURE 1 Typical Membrane Cell Process Flow Chart

Measurinq Point
Stream Type
Process Composition

pH/ORP Ranqe

(for pH)
Pure brine
NaCL (310 grams/liter)

4 to 14 pH

(for pH)
Anolyte (from Cell)
NaCL (200 grams/liter) NaCIO 3

4 t0 14pH

(for pH)
Anolyte (acidified)
NaCL (200 grams/liter), NaCIO 3, HCI, Cl 2, NaOCI

0 to 6 pH

(for pH/ORP)
Lean Brine
NaCL (200 grams/litter), NaCIO 3
-500 to +500 mV
(for pH)
Polished brine
NaCL (310 grams/liter)

4 t0 14pH


As process streams come in contact with the electrolytic cell, stray voltages are created in the brine feed as well as the downstreams. Traditionally, a "current interrupter system", which involves grounding the process stream, has been used with conventional pH sensor installation (Figure 2). This, however, has been generally unsatisfactory.

FIGURE 2  Conventional pH/ORP Sensor Installation
    with Current Interrupter System


FIGURE 3  BI'3 Differential Electrods Technique
         LCP pH/ORP Sensor Installation


The Differential Electrode Technique pH and GAP sensors (With 1-1/2" NPT tee) are easy to mount in the process piping as shown in Figure 3. The PVDF sensor body resists virtually all chemicals and is well suited for this process. Also, the DEMT eliminates the need for a current interrupter system. Figure 1 shows five critical measuring PVDF points throughout the Membrance Cell Process where Bl's PVDF pH and ORP sensors have proven to be successfully applied.