Process Analyzers for Chlor-Alkali Manufacturing

In India at present, there are 42 major Chlor-Alkali plants providing a total turnover of more than Rs. 3000 crores per annum for chlorine and caustic soda production. Such plants always require a highly sophisticated process control and optimization via series of carefully monitored feedback control and recycle loops. Not to mention, the purity of raw material like concentrated brine play an important role in overall safety as well as cost reduction.

Owing to large turn-around times, expensive cost of ownership and human error involved, lab-based analysis tools may not be an ideal choice. At Metrohm, we bring your lab to the physical sampling point in the process line. Such ‘Process Analyzers’ are ruggedly built with a high amount of engineering precision to adapt according to the real-life process conditions while also monitoring multiple control parameters & streams. These systems can be equipped with a wide variety of analytical techniques such as titration, photometry, Ion-chromatography and NIR spectroscopy to analyze organics, in-organics, trace metals and ions in ppm-ppt range. So far, Metrohm Applikon team has successfully installed more than 200 such analyzers in a variety of Chlor-Alkali facilities across the globe.

Process-IC and Process-VA Solutions

Process IC Analyzer: Anions in Caustic

Typically, anionic impurities in 50 wt.-% caustic soda or potash are determined by gravimetric or titration methods which require a variety of reagents with diverse shelf lives and hazards. ASTM method E1787-16 specifies ion chromatography to measure bromide (Br-), chlorate (ClO3-), chloride (Cl-), fluoride (F-), nitrate (NO3-), phosphate (PO43-), and sulphate (SO42-) in concentrated NaOH or KOH solutions.

There are variety of IC-based process analytical solutions in the market that can also offer intelligent sample preconditioning techniques (‘matrix elimination’) to conduct fully automated process stream analysis for multiple anion impurities in a simple yet easy fashion (without any human intervention).

Process VA: Trace Metal Analysis

In the electrolysis cell, iodide (I-) is oxidized to iodate (IO3-), which precipitates in the ion exchange membrane and decreases its lifetime. Its interaction with alkaline earth metals at the membrane surface can cause severe damage to the membrane in case of a blockage. Voltammetry Analysis (VA) is an ideal solution to quantify trace metals in ppb-ppt range. Apart from iodide, such process based VA systems can conduct voltammetry detection of Pb, Fe, Cu, Zn, As, Hg and Cr in industrial process streams via fully automated standard addition. These are equipped with a built-in UV-based digestion mechanism as well.

Process Titration Solutions for Chlor-Alkali : ADI 2045TI

The ion exchange efficiency and its longevity directly depends on purity of the brine fed to the electrolytic cell. For the efficiency to reach its maximum, the hardness of the brine liquor along with any other impurities that may affect membrane performance have to be reduced to their minimal content. Typically, a two-stage filtration process addresses this issue. The initial stage uses caustic soda (NaOH) and sodium carbonate (Na2CO3) to remove excess hardness (Ca2+, Mg2+) via precipitation reactions. In the second-stage, the purified raw brine is send to an ion-exchange resin unit to minimize trace impurities in brine such as iodide, arsenic and mercury. 

Carbonate & Caustic Analysis in Brine
Concentrations of CO32- and OH- in brine can provide important information about the impurity removal process in stage 1 of the brine purification.

Hardness Analysis in Brine
Accurate determination of hardness (before and after the secondary treatment) helps to determine the efficiency of the resin treatment. Before resin treatment, Ca2+ & Mg2+ concentrations are in 0-20 ppm range. Metrohm offers a variety of titration based process solutions for multiple parameter analysis such as hardness, alkalinity and carbonates in brine liquor.

Hypochlorite (ClO-) Analysis in Brine
A certain amount of hydroxide can migrate into the anodic compartment during electrolysis, leading to hypochlorite formation that can result up to 7% loss of efficiency in the caustic soda production.
It is also produced during the purification of waste gases contaminated with Cl2 that are passed through a wet scrubber containing NaOH, forming hypochlorite. If the scrubbers become saturated, chlorine emits to the atmosphere, hence it is important to monitor the scrubbing capacity. For such single parameter solutions, Metrohm offers a variety of low-budget compact solutions to conduct analysis in plug & analyze fashion.

Process Photometry Solutions for Chlor-Alkali: 2035 Process Analyzers

While techniques such as Titration and Ion-Chromatography provide ppm level control at various stages in the process, there is a viable need for more vigorous detection in ppt-ppb range. For example, the recommended concentrations of species such as iodide, calcium and magnesium in electrolysis cell is between 0-20 μg/L. At Metrohm, we offer a customized solution using photometric analysis for low-level detection. Being a compact stand-alone solution, the overall cost of ownership is significantly reduced. 

Trace Level Impurity Analysis in Feed-Brine
After the secondary treatment with an ion-exchange resin, the impurity levels of inlet brine needs to be accurately monitored. For example, in the electrolysis cell, iodide (I-) is oxidized to iodate (IO3-), which precipitates in the ion exchange membrane and decreases its lifetime. Similarly, hardness (Ca2+ and Mg2+) determination in ultrapure brine is necessary to prevent downstream problems in the electrolysis process. Metrohm offers process analyzers equipped to perform photometric evaluation of such impurities in a fully automated fashion. Undesired sample matrix effects such as sample colour or turbidity can be removed with differential measurements, taken before and after the addition of a colour reagent.

Chlorine (Cl2) in Depleted Brine & Waste
Depleted brine can be recycled so that it can be reused in membrane cell electrolysis for improving overall process efficiency. The determination of chlorine (Cl2) in brine is necessary due to the high concentrations, which could be found in the recycled bulk solution of brine. Online monitoring of these levels can lead to more efficient chlorine removal processes, if the analyzer output signals the chemical distribution system in cases of high Cl2 content. A dedicated 2035 Photometric Analyzer can also be used for multiple parameter evaluations.

Process NIR Solutions for Chlor-Alkali: NIRS Analyzer Pro and NIRS XDS Process Analyzer

Near-infrared spectroscopy (NIRS) is a fast, chemical-free, spectroscopic technique in use in many industries for in-process control (IPC) of manufacturing operations and measuring product quality. Measurements are direct and non-destructive. Monitoring with NIRS fits in well with process analytical technology (PAT) initiatives as proposed by the FDA. Metrohm Process Analytics offers two versions of process NIR systems: the NIRS Analyzer Pro and the NIRS XDS Process Analyzer. All Metrohm Process Analytics NIR Analyzers are operated with Vision chemo metric software. This proprietary, easy-to-use software can be used for comprehensive data collection, qualitative and quantitative method development, and diagnostics. 

Moisture in Chlorine Gas

Moisture determination in the produced gases (Cl2, H2) is necessary to overcome corrosion in storage containers and transport pipelines. Vaporization of the gases after storage, without proper removal of moisture beforehand, can also clog the container valves and lead to further handling issues. Determination of moisture content in the gas at different points in the drying process gives information regarding the efficiency of the dryers and the endpoint of the drying process. The NIRS XDS is capable of monitoring nine different process points with the multiplexing capability. Fast, reagent-free analysis is available in a housing suitable for dangerous ATEX environments.

Analysis: Near Infrared Spectroscopy (NIRS)
Typical Range: 0–100% H2O
Detection Limits: Dependent on reference method.
Major absorbance range: 1100–1650 nm
Probe: Micro-transmission probe pair with micro-bundle fibre optics



  • Installed Applications: 200+ Systems Sold


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Application Note - Analysis of calcium, magnesium in brine : Download application

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