Investigation of producing chlorine with electro dialysis method and the effect of operating parameters: a review.
In recent years membrane processes have had important role in industrial isolations and lots of studies have been conducted to increase efficiency of these systems, optimization of selectivity and determining the optimum value of pressure at two sides of membrane or flux density and effective parameters on that. 
Industrial membranes are polymers with small particles and make it possible to transfer mass in two different environments. Their materials in chlor-alkali industry are per fluorinated polymer and they allow passing specific particles with specific size. In chlor-alkali industry some technologies such as mercury cell, diaphragm cell and membrane cell are used, their feed is chlorine sodium and some potassium chloride solution. [1,2]
Chlorine gas was recognized by Carl Wilhelm Scheele in 1774s by means of Hydrochloric acid with metallic compounds. The diaphragm cell process (Griesheim cell, 1885s) and the mercury cell process (Castner-Kellner cell, 1892s) were both introduced in the late 1800s. The membrane cell process was developed much more recently (1970s). Each of these processes represents a different method of keeping the chlorine produced at the anode separate from the caustic soda and hydrogen produced, directly or indirectly, at the cathode. Currently, 95% of world chlorine production is obtained by the chlor-alkali process. The diaphragm process was developed in the 1880s in the USA and was the first commercial process used to produce chlorine and caustic soda from brine. In North America, diaphragm cells are still the primary technology, accounting for roughly 70% of all USA production.
2. Investigating evolutionary procedure of membrane cell process:
Before 1970stwo main processes were being used in order to produce chlorine and sodium hydroxide (except melt saline electrolysis) which included mercury and diaphragm cells. These two processes had commercial importance for about 100 years. The basis of membrane technology goes back to 1967s, when H.E. Beer registered his first invention in anodic coatings containing Ru[O.sub.2]/Ti[O.sub.2] after 30 years of research. Operating steps of this invention were conducted in Diamond Shamrock Company but Vittorio de Nora was the only person who invested million dollars for the progress of this invention and its commercialization, that with his brother Oronzio de Nora and Diamond Shamrock Company did his best in progress of this project.
The beginning of membrane cell goes back to mid 1970s that was as unipolar and dipolar, but this technology was not accepted commercially at that time. First cell designs were Gaps limited between anode and cathode and membranes which functioned with current efficiency higher than 80% between 3.8 and4.2 volts in current density of 3 KA/[m.sup.2]. In this period the main problem of membrane cell was the necessity of improving input saline to cell and reducing consumer energy. Many details of membrane cell process were used such as temperature of discharge current, sodium hydroxide concentration and materials quality, and its difference with mercury cell process is using water without ion. Studies and researches about mentioned items in decades 70 and 80 and consequently developing of technology and technical knowledge led to membrane cells. Primary cells were no like current ion exchange cells, but they were like separators used inside the cell. These separators allowed the exudation of saline water to penetrate into cathode compartment, and consequently obtained sodium hydroxide is polluted with salt. The first ion exchange membrane, which was made by E.I. du Pont de Nemours Company, to some extent allowed passing of saline water to sodium hydroxide solution. 
Between 1970sand 1973 s studies and examinations had more emphasis on evolution of ion exchange membrane and less focus was on equipments and required hardware for commercial electrochemical cell. The performance of these membranes must have been such that it could allow passing of sodium but it resisted against emigration of ion hydroxide to anode compartment. An appropriate and easy solution is to use fluorinated polymers which caused istortion increase of pores inside the membrane and the decrease of membrane resistance against cathodes. Mostly pure polymers are dielectric more than having electrical conductivity, and then ion exchange membranes should have functional groups of acid salts to provide passage of cathodes and ion emigration, that was elected as the first laboratory system and the most effective functional group of Sulfonic acid salts. 
Salts with different molecular mass and film thickness resulted in different electrical resistance, diffusivity and tensile strength. All progresses in this period were involved in successful separation development (membrane) in a small laboratory cell that functioned under controlled circumstances. The first interesting results obtained in late 1970s withion exchange membrane separators which could operate in a life time about one year with sodium hydroxide concentration of 15-28 % in current density 2 KA/[m.sup.2] with Faraday efficiency of 85%. By developing this step, the first pilot cells in large scale (individually) were launched. Unlike laboratory cell with 3 inch diameter, further efforts were conducted on cells with area of 25 [in.sup.2] to 250 [in.sup.2], which had good primary results. Again, in 1973 s some studies were carried out on cells with larger membranes and interesting results were obtained, revealed larger electrolysis cells (with area of 1[m.sup.2]) are completely economic. [3, 4].
In 1974s David Bill presented intrinsic weakness of thin polymer film which included film destruction by fluctuating pressure, sharp edges or because of aggregation of small waste particles on membrane, these factors led in relative small holes on the membrane. These factors were created sometimes, and however resulted in membrane weakness and its total un-functionality and finally its rupture. In laboratory tests, the membrane placed in the Stress Riser Device caused gap and increasing rupture with a small rupture. So thin film of ion exchange membrane, because of mechanical and chemical weakness, resulted in unreliability and inapplicability of the membrane to make big commercial and industrial cells, despite the easy ability to produce and setting. So a cloth woven was required between polymer film layers containing functional groups in order to chemical strength. [3, 4]
Construction of primary types of bipolar electrolysis began since 1978s. Dipolar electric arrangement is simple and current passage is conducted from the back of an anode to the back side of neighbor cathode and then the cell obtains the shape of a filter press. Till 1978 several small bipolar membrane cells (less than 60 ton/day) were set in America, Canada, Sweden and Japan. In first months, industrial owners of this technology showed resistance against these changes that the reason is obvious; to add a process in order to eliminate impurities from saline water required complex actions in industrial scale. On that time, there was no possibility of sedimentation and filtration the impurities of saline water in order to eliminate 99.99% of impurities, and effect of this operation on membrane process was difficult. But further experiments revealed that saline water hardness can reach from 2 ppm to 0.05 ppm that this cause that cell voltage remains low for a long time. 1990s was the peak of progress of this industry. Recapping membranes from both sides of anodes and cathodes with Carboxylate was proposed which resulted in high efficiency and lower voltage, that today it is used to increase efficiency of carboxylate- Sulfonate membrane.
2.1.Future of membrane technology:
Progresses developed in recent 30 years continue likewise and this progress speed is only become smoother in some areas. The most progress relates to membranes which have two recaps with high efficiency and proposed about 1980s. Other progresses are related to mechanical hardware, recognition and understanding more effective parameters on the process. Definitely, depolarized cathodes can be considered as an important innovation in membrane process. Depolarized cathodes were made by air in early 1980 by Diamond Shamrock Company for the first time and this success continued by Eltech. Denaora Company (current Uhde) used its domestic versions with this technology about 2000s. [3, 5]
In depolarized cathodes saving energy rate reduces to 15-20%. Of course, with this perspective it appears an interesting aim but to reach that we require high costs that regarding to current situation is not appropriate. However, the growing process of construction techniques, leads this technology to depolarized cathodes that these changes have their normal path accompanied with other progresses. 
According to predicting the technology of chloralkali industry's progress and innovation in the future, probably we should be expecting a mix of fuel cell and chloralkali cell that in fact the fuel cell is like depolarized cathodes which are out of the cell. Of course, it is difficult to say that influence of fuel cell and its investment cost is lower than depolarized cathode, but apparently depolarized cathodes are more practical. The other issue is under pressure cell that have the ability to produce liquid chlorine gas of electrolysis, the ability to produce sodium hydroxide 50% or solid sodium hydroxide. These issues are prediction of progresses that owners and researchers propose for the future, and some of these issues were proposed since 25 years ago and related researches are being performed at present time, such as electrolyzing sodium bicarbonate to produce sodium hydroxide and Carbone dioxide or to separate chlorine gas from sodium hydroxide solution or a method to produce a determined rate of oxygen and chlorine. This compound is created by tunable anodes which can tune chloride and oxygen in a desired rate. [5, 6]
2.2.Monopolar and Bipolar Designs:
A commercial membrane plant has multiple cell elements combined into a single unit, called the electrolyzer. The electrolyzers follow two basic designs: monopolar and bipolar. In a bipolar arrangement the elements are connected in series with resultant low current and high voltage. The cathode of a cell is connected directly to the anode of the adjacent cell, as shown in Figure 2. The operation of a bipolar electrolyzer can be easily monitored by measurement of element voltages. If element upsets occur, a safety interlock system actuates the breakers (short circuiting switches) and isolates the electrolyzer from the electric circuit. As the influx and efflux of electrolytes for the cells with different electric potential are gathered in common headers, problems of stray current may arise. In the monopolar type all anodes and cathodes are connected in parallel, forming an electrolyzer with high current and low voltage (Figure 3). Due to the long current path, the voltage drop is high and can only be reduced by minimizing the size of cells or introducing internalcopper conductors to lower the resistance. Because of this basic principle, ohmic losses in the monopolar cells are 80-100 kWh per tonne 100 % NaOH, which is much higher than in equivalent bipolar cells. Furthermore, the bipolar safety system is not applicable to the monopolar design, since the cell elements are arranged in parallel, which does not permit the monitoring of deviations in individual cell voltages. 
3. Investigation of processes:
3.1. Diaphragm cell process:
It is the first economic process which is used to produce sodium hydroxide. In this process a diaphragm is used to separate released chlorine in anode and produced sodium hydroxide and hydrogen in cathode. 
If diaphragm is not used in this cell, produced hydrogen will be set in fire with chlorine automatically. Also sodium hydroxide and chlorine react and sodium hypochlorite (NaCIO) and in the next reaction sodium chlorite is produced. Usually this diaphragm is made of Asbestos. Saline water solution saturated which is purified before enters the anode part and by penetrating to all surface of diaphragm enters cathode part. When graphite anodes were used, the diaphragm became inoperable after 90-100 days due to plugging of the diaphragm by particles of graphite. Nowadays, all plants in the European Union use metal anodes and the lifetime of the diaphragm is over one year. At the beginning the diaphragms were made of asbestos only and were rapidly clogged by calcium and magnesium ions coming from the brine. Asbestos was chosen because of its good chemical stability and because it is a relatively inexpensive and abundant material. Beginning in the early1970s, asbestos diaphragms began to be replaced by diaphragms containing 75% asbestos and 25% of fibrous fluorocarbon polymer of high chemical resistance .
They have high resistance. According to environmentalists, asbestos inhalation is dangerous because it causes emergence and strengthens lung diseases especially cancer in human beings .
3.2. Mercury cell process:
The mercury cell process has been in use in Europe since 1892s and accounted in 1999s for 58 % of total production in western Europe. As shown in Figure 4, the mercury cell process involves two "cells". In the primary electrolyser (or brine cell), purified and saturated brine containing approximately 25% sodium chloride flows through an elongated trough that is slightly inclined from the horizontal. In the bottom of this trough a shallow film of mercury (Hg) flows along the brine cell co-currently with the brine. Closely spaced above the cathode, an anode assembly is suspended.
Electric current flowing through the cell decomposes the brine passing through the narrow space between the electrodes, liberating chlorine gas (Cl2) at the anode and metallic sodium (Na) at the cathode. The chlorine gas is accumulated above the anode assembly and discharged to the purification process.
As it is liberated at the surface of the mercury cathode, the sodium immediately forms an amalgam. The concentration of the amalgam is maintained at 0.2-0.4% Na (by weight) so that the amalgam flows freely, 0.3%. The liquid amalgam flows from the electrolytic cell to a separate reactor, called the decomposer or denuder, where it reacts with water in the presence of a graphite catalyst to form sodium hydroxide and hydrogen gas. The sodium-free mercury is fed back into the electrolyser and reused.(Figure 4). [3,7]
History of environmental pollutions caused by mercury and PCDD/FS which are used in diaphragm and mercury cells, are turned to a main difficulty in chloralkali plants. Pollutions today are caused by burying mercury and in the past caused by burying graphite layers used in anodes and other residues around the plant. Difficulties and problems mentioned above and also some special benefits such as eliminating chlorine pollution, reducing power consumption, possibility of increasing manufacture capacity, health of employees caused researches goes toward membrane cells and two necessities of using them become evident more than past and parameters effective on production efficiency increase in membrane cells is present concern of researchers. [6, 7]
Most mercury releases occur as fugitive emissions from the cell room and other locations.
Preventive measures and good management practices can significantly reduce these fugitive emissions The primary specific points of mercury outlets to air are the end box ventilation system and the hydrogen gas vent. Several control techniques may be employed to reduce mercury levels in the hydrogen streams and in the end box ventilation systems. The most common techniques are (1) gas stream cooling, (2) mist eliminators, (3) scrubbers, and (4) adsorption on activated carbon or molecular sieves. Gas stream cooling may be used as the main mercury control technique or as a preliminary step to be followed by a more efficient control device. The proper use of these devices can remove more than 90% of the mercury from the gas streams. Each of the important processes and/or locations where releases may occur are discussed below. 
3.3. Membrane cell process:
The chlor-alkali by membrane cell is one of the most economical and environmentally friendly CA processes. The process utilizes electricity and purified brine as feedstock to produce valuable products such as chlorine, caustic sodaand hydrogen, simultaneously.In this process, the anode and cathode are separated by a water-impermeable ion-conducting membrane. Brine solution flows through the anode compartment where chloride ions are oxidised to chlorine gas. The sodium ions migrate through the membrane to the cathode compartment which contains flowing caustic soda solution. The demineralized water added to the catholyte circuit is hydrolysed, releasing hydrogen gas and hydroxide ions. The sodium and hydroxide ions combine to produce caustic soda which is typically brought to a concentration of 32-35% by recirculating the solution before it is discharged from the cell. The membrane prevents the migration of chloride ions from the anode compartment to the cathode compartment; therefore, the caustic soda solution produced does not contain salt as in the diaphragm cell process. Depleted brine is discharged from the anode compartment and resaturated with salt. If needed, to reach a concentration of 50% caustic soda, the caustic liquor produced has to be concentrated by evaporation (using steam)Figure 5.The caustic soda derived from the membrane process is higher quality than that derived from the diaphragm process. 
The membranes used in the chlor-alkali industry are commonly made of perfluorinated polymers. In Arvand unite membrane is used tetra fluoroethylen. The membranes may have from one up to three layers, but generally consist of two layers(Figure 6). One of these layers consists of perfluorinated polymer with substituted carboxylic groups and is adjacent to the cathodic side. The other layer consists of perfluorinated polymer with substituted sulphonic groups and is adjacent to the anodic side.. 
Sulphonyl group (-S[O.sub.3]H) turns the membrane into a Cation exchange membrane which only the passage of Cations and does not allow Anions. To give the membrane mechanical strength, the membrane is generally reinforced with PTFE fibres. The membranes must remain stable while being exposed to chlorine on one side and a strong caustic solution on the other. The general economic lifetime of chlor-alkali membranes is approximately three years, but ranges between 2-5 years [9,11]
Usually, membrane cells work with saline water cycle and further saturation of saline water. In saline water cycle after diluted saline water exited from the cell, first it is decolorized and then goes to saturation step. If chlorine in saline water is not separated it will harm ion exchange reins in purification step. The cathode material used in membrane cells is either stainless steel or nickel. The cathodes are often coated with a catalyst that is more stable than the substrate and that increases surface area and reduces over-voltage. Coating materials include Ni-S, Ni-Al, and Ni-NiO mixtures, as well as mixtures of nickel and platinum group metals. In Arvand unite nickel metal is used a cathode and platinum oxide is used for the covering. For some reasons nickel is used here; nickel resistance is more than iron and iron will be brittle and fragile in high temperatures, also steel increase iron concentration in the solution, as iron's corrosion cause its entry to the solution and exceeds from permitted limit 3ppm. Anodes used here are usually metallic. In CA Arvand unite anodes are from titanium which has a good resistance. Covers are made from ruthenium oxide. [8,11, 12]
3.3.1. Membrane cell benefits than mercury and diaphragm methods are:
1-Using no mercury and asbestos due to environmental reasons.
2-The decrease in consumer energy to 25-30%
3-Elimination of chlorine and sodium hydroxide and hydrogen pollution to mercury
4-Reduction of cells' repairmen and reduction of hours of chlorine production
5-The possibility of increase in production capacity in similar situations than mercury cells
6-The increase of electrolysis cells' life time
7-The decrease in required area in order to setting the cells
8-Having less need of human force
9-Producing sodium hydroxide with high purity degree
Of course this method has disadvantages too:
1-Input saline water should have high purity that it requires using expensive purifiers
2-To increase sodium hydroxide concentration, using evaporation process is needed
3-Existence of oxygen in produced chlorine. [11,12]
4. Investigating operating parameters effectiveness on membrane cell performance of chloralkali unite (CA):
This investigation is related to Arvand CA unite and it is possible numbers and statistic be different with internal and external similar unites. Some parameters that effect membrane cell performance, chlorine efficient production and increase or decrease of penetration include:
1) Concentration (250-350 gr/lit): saline water concentration has direct effect on penetration rate on membrane. Its normal rate in Arvand unite is 310 gr/lit. Increasing the concentration results in crystallized saline, path congestion and particles deposition on membrane. Decreasing the concentration in systems results in slow, reaction and decrease in system voltage and electrolysis pause.
2) PH (2-5): PH change must be accompanied by temperature change in a definite time. The increase in temperature in PH results in the increase in chlorine production. Its normal rate is 2.
3) Temperature (25-90 degree centigrade): normal rate in chlor-alkali unite in 87 degree. The increase in temperature results in cell destruction and raises consumer voltage, also the decrease in temperature lessens production by having effect on PH and voltage decrease.
4) Current density (2-6 KA/[m.sup.2]): current density is a current which passes of one square meter of membrane. Current density contribution is mostly on NAOH production, however also its increase results in chlorine production. Its standard normal rate is 5.5 KA/m.
5) Electrolyte flow rate (1.34-6.73 ml/s): by flow rate increase in a specified time, voltage of two ends of the cell will decrease which leads to decrease of penetration in membrane. Its normal rate is 3.2.
6) Pressure: its normal rate is 40 milibar. This low pressure difference is between catholyte semi cell cathode and anolyte semi cell anode which causes membrane viscosity to anode and cathode. This pressure difference must be stable and becoming low and high will damage the membrane. [1,8]
There are some other elements could be investigated here but because of high importance of above elements and low importance of other elements we relinquished them.
By carried out investigations and what is exploited from above issues and also by comparing chlorine production methods and evaluation of important parameters the following results are obtained:
The impact of operating parameters including pH, temperature, flow rate, brine concentration and current density on the cell voltage and current efficiency of a chlor-alkali cell was quantitatively. Selectivity: the membrane has an important role in penetration of ions with positivecharge (Na) and not penetration of ions with negative charge (CL), it means its mass transform is highly effective.
Membrane cell is highly resistant and durable and the possibility of rupture is lower, while diaphragm cell life time is sometimes lower than one year. For environment, producing NAOH and chlorine with membrane cell method has a special superiority on mercury and diaphragm methods due to elimination of mercury and asbestos. Membrane cell method is economical. Because of lower consumption of electricity energy, elimination of extra steps such as diffusion on mercury cell, decrease in cells surface and decrease in human forces. Chlorine production capacity in membrane cell method is not comparable to other two methods and it is much higher. The reason is using membrane technologies with high selectivity and penetration, depolarized cathodes, and better and more accurate control and tuning operating parameters which are effective in production. In membrane cell cathode and anodes are used and also their coverings perform the electrolysis with a special accuracy and have specific compatibility with various voltages.
Comparison of membrane, mercury and diaphragm cells processes
Received 22 October 2013
Received in revised form 14
Accepted 20 January 2014
Available online 25 February 2014
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(1) A.R. Zahedipoor, (1) SH.Eslami, (1) M.Deylami, (2) S.S. Mohaghegh, (1) GH.Montazeri
(1) Department of Chemical Engineering, Mahshahr branch, Islamic Azad University, Mahshahr, Iran.
(2) Department of the Environmental Engineering, Payame-Noor University, Dehdasht Branch, Kohgilouyeh & boyerahmad, Iran.
Corresponding Author: A.R. Zahedipoor, Department of Chemical Engineering, Mahshahr branch, Islamic Azad University, Mahshahr, Iran.
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|Author:||Zahedipoor, A.R.; Eslami, S.H.; Deylami, M.; Mohaghegh, S.S.; Montazeri, G.H.|
|Publication:||Advances in Environmental Biology|
|Date:||Dec 1, 2013|
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