Process Monitoring Solutions for Chlor-Alkali Industry
Ritesh N. Vyas
Product Manager (Asst.) for Electrochemistry
Metrohm India Limited

For a global production capacity of chlorine at 37.0 Million Tons per Annum, India's contribution is less than 3% today. Even at this capacity , India's Chlor-Alkali industrial sector makes an annual turnover of more than 3000 Crores. Over the past decade, significant efforts been made to find technological solutions that can provide sustainable growth to this market. This review highlights the recent developments in Process Monitoring Solutions for Chlor-Alkali Industry.

Chlorine is one of the 15 major elements that constitutes human body. Apart from being the most commonly used bleaching and water disinfection agent around, it is also an important raw material in 80% of modern day pharmaceuticals and drugs. In addition, chlorine is a vital ingredient in manufacturing a variety of products in petroleum as well as paper & pulp industry.

On a global scale at present, about 95% of the chlorine is manufactured using an electrochemistry based Chloro-Alkali Process. Chlorine is produced simultaneously with caustic soda (NaOH) via controlled electrolysis of a sodium(or potassium) chloride solution ('Brine'). Caustic soda is a useful by-product in industries producing aluminium, cellulose products, and textiles. The other by-products include hydrogen gas, which has a variety of applications in alternative energy sector and hypochlorite, which is extensively used in modern day bleach solutions.

In India, there are 42 major Chlor-Alkali manufacturers providing a total turnover of more than Rs.3000 crores per the major contributions of AMAI in the past annum. By 2018, the overall production capacity of India will reach more than 13 lakhs tons per annum for caustic soda and 11.5 lakh tons per annum for chlorine. The Alkali Manufacturer's Association of India (AMAI) established in 1960 is an Apex body representing Indian Alkali Industry for entities such as Govt. of India and Department of Ministries. One of decade has been the transition of 99% of these facilities towards an environmental friendly "Membrane Cell Process" from an age-old "Mercury Diaphragm Process" . Such an adoption of 3rd to 5th generation electrolyzers was a huge step towards process sustainability, cost-optimization and energy efficiency in this market. This has allowed the minimization in the overall mercury waste by almost 98%

Membrane Cell Process

highly advanced ion-exchange membrane based electrochemical reactor for process efficiency. Such a membrane stands between the anodic and cathodic compartments of the electrolysis cell. It will allow sodium ions (Na+) and water molecules to pass through, but limit chloride and other anions. As a highly purified solution of 'Brine' is electrolyzed to Cl2 gas on the anodic side, H2 gas and caustic soda are simultaneously produced in the cathodic compartment.

Such ion-exchange membranes are commonly made of per-fluorinated polymers and are generally two layers thick. The membranes must remain stable while being exposed to such aggressive solutions, and have an average lifetime of 3–5 years. The ion exchange efficiency is vital in order to limit impurities in the concentrated caustic product. Partial membrane blockage from precipitation reactions will lead to high operational and/or replacement costs, which are not good for process efficiency and cost optimization.

Brine Liquor Purification

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 twostage 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 .

A vigorous real-time process control is required to monitor species such as carbonates and caustics in brine filtered out from stage 1 followed by calcium, iodide & magnesium in the brine from stage 2. The Cl2 and H2 gas produced also needs to be analysed for moisture content.

Trace Anion Impurities 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).



















Trace Metal Analysis in Process Streams

In the electrolysis cell, iodide (I-) is oxidized to iodate (IO3-), which precipitates in the ion exchange membrane and decreases its lifetime. The interaction of alkaline earth metals (Ca2+, Mg2+, others) with iodide 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 selected 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 process streams via fully automated standard addition technique .

Hypochlorite (ClO-) in brine

A certain amount of the formed hydroxide can migrate into the anodic compartment during electrolysis, leading to formation of hypochlorite and chlorate. These impurities can result in a loss of current efficiency of up to 7% in the production of caustic soda. A process-based system capable of conducting basic potentiometric titration is typically an answer to the issue.

Moisture Analysis in Chlorine Gas Streams
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.

Chlorine (Cl2) in brine (Depleted brine)

Depleted brine can be recycled so that it can be reused in membrane cell electrolysis, which reduces costs incurred during the multiple purification steps of the raw brine. The determination of chlorine (Cl2) in brine is necessary due to the high concentrations, which can be found in the recycled bulk solution. 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. Online photometric solutions provide single or multiple stream analysis in low ppb levels.

Summary

Compared to an expensive yet bench top based ICP-OES or GC-MS systems, such real-time based process analyzers are costeffective and efficient solutions to every other analytical need in Chlor-Alkali industry. Not only are they capable of providing 'real' data for control parameters, they also can also be recognized as the future guiding platforms for overall process sustainability.