Chloro Alkali Plant

7/31/2019 Chloro Alkali Plant

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Chloro alkali plant

The industrial development strategy pursued in the period 1960-1980 led to establishment of variousindustries which were import dependent for basic raw materials. Those industries were vulnerable toforeign exchange scarcity. As a result most of the industries operated at low capacity, particularlythe chemical and allied process industries. The process industries need essentially linkages withlocal production capacities for the sustainable and appreciable level of plant capacityutilisation. Caustic sodaand chlorineare major raw materials used by most process industries. Theprospects for the establishment of the chloro-alkali industry is assessed by the demands for causticsoda, chlorine and related products, the raw material base for electrolysis of brine as well asavailable process technologies.

The chloro alkali plant

The largely import substitution based chemical industry produces paints, varnishes, drugs, soaps,detergents, cosmetics, rubber and plastics as well as insecticides, mosquito coils, candle, glue,industrial starch, etc. The industry is heavily import dependent for raw materials except for localinputs namely palm oil for soaps and natural rubber for rubber products. As a result of import

constraints, the sector is characterised by low capacity utilisation with only 17% of firms operatingbetween 69-79% installed capacity and 33% firms operating under 20% installed capacity. Thesector is hard-hit by the recent trade liberalisation, having to compete with imported products both inquality and price.

In the industrialised countries, it is largely the demand for chlorine for the synthesis of variousorgano-chloro compounds which has stimulated growth and expansion of the chloro alkali industry.


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Figure 1 shows the various industrial uses of chlorine. The other product of electrolysis of brine ishydrogen. It is used in the synthesis of ammonia, hydrogenation of oils production of margarine,production of hydrochloric acid and in the petroleum industry. The use of hydrogen depends on thecapacity of the chloro-alkali plant. For example, a 40,000 ton/year ammonium factory could beintegrated with a 300,000 ton chlorine per year plant.

Technology Assessment

Most investment proposals encounter the problem of choice of process technology for theelectrolysis. This article compares the key parameters of the various technologies applied in thechloro alkali industry. Three different process technologies are applied in the electrolytic productionof caustic soda and chlorine. The technologies are characterised by the type of electrolytic cell used,namely, the diaphragm cell, mercury cell and the membrane cell. In all the processes, saturatedbrine 300-315 g/L is fed into the electrolytic cells.

In the diaphragm cell process, the anolyte is separated from the catholoyte by asbestos diaphragmto prevent contact between chlorine produced at the anode and the caustic soda and hydrogen atthe cathode. in the membrane cell process, cation exchange membranes are used to separate theanode and cathode compartment of the electrolytic cell. At the anode, chlorine ions discharge withevolution of chlorine gas. At the cathode, hydrogen is evolved and caustic soda is formed. Thecation exchange is impermeable to the chloride ions and therefore the caustic soda produced

contains only trace levels of sodium chloride. In mercury cell process, the liquid mercury cathode


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forms amalgam with sodium. The amalgam is removed from the cell and reacted with water indenuders to give caustic soda and hydrogen. Chlorine is liberated at the anode.

Factors which were considered in the choice of process technology include flexibility of scalingcapacity utilisation and environmental impact. Other factors considered were cell materials, powerconsumption and current efficiency. Comparison of the key parameters of the three processes are

shown in Table 2.

Consumption of cell materials

The various cells materials are specified in Table 2. The mercury cell does not use a diaphragm ormembrane. The ion exchange membranes have longer life (over one year) than asbestosdiaphragms. The consumption of asbestos diaphragm is about 800g/ton NaOH. In the mercury cell,mercury cathode is consumed at the rate of 0.15-0.2 kg per ton of caustic soda. Both diaphragmand mercury cells use graphite electrodes, which are consumed at a rate of about 8 kg/ton NaOHfor diaphragm cell and 4 kg/ton NaOH for the mercury cell. The consumption of purified brine, 1.6

ton/ton NaOH is about the same for all three processes.

Electric Energy Consumption

The consumption of electric energy is an important parameter of the electrolytic process. It isdetermined by the equation:

W = (1/K) * (E/n) * 100 (1)

whereW = consumption of electric energy (direct current) per Kg of product, Kwh/Kg.K = electrochemical equivalent of the product, g/a-hr

E = cell voltage, voltn = current efficiency, %.


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The cell voltage, E which determines the energy efficiency of electrolysis, is the sum of thedecomposition voltage of NaCl, E0, anode and cathode over voltage, voltage drops at the electrolyteinter phase Eel, at the anode, Ea, at the cathode, Ec and at the electrical contacts, Eel contacts:

E = E0 + del(phia) + del(phic) + del(Eel) + del(Ea) + del (Ec) + del(Eel contacts) (2)

The theoretical decomposition voltage of sodium chloride E0, 2.3 volts is the same for thediaphragm and membrane cells. The mercury cell his a higher decomposition voltage, 3.17 voltsand therefore from equation 1 the energy requirement is more than that of the diaphragm ormembrane cells. Over voltage of chlorine and hydrogen evolution at 800C in saturated brine on theelectrodes used by various cells are lower on the titanium lined steel, platinised titanium or metal-metal oxide electrodes (del(phia)=26 mV; del(phic)=45 mV for current density- 1000 a/m2) than oncarbon and steel electrodes (del(phia)=251 mV; del(phic)=180 mV for current density - 1000 a/m2).The cell voltage for membrane cell is therefore lower which accounts for the lower powerconsumption according to equation 1.

Quality of caustic soda

The greatest advantage of the mercury cell is the superior quality of caustic soda produced, 50%solution with trace levels of sodium chloride, 0.002-0.003%. The amalgam caustic is widely used inthe rayon industry which requires high purity caustic soda without chlorides. In contrast, thediaphragm cell produces weak 10% caustic soda solution containing 15% sodium chloride. Theweak solution is concentrated to 50% solution with 1% sodium chloride in a triple effect evaporation.The membrane cell produces higher concentration, 30-35% caustic soda containing trace levels of0.001-0.002% sodium chloride. The caustic soda is evaporated to 50% solution but at a lower costof steam usage.

Environmental Impact

Mercury cells have the greatest disadvantage posed by hazardous vapours of mercury at celltemperature, 70-800C. Production floor area for the mercury cell process is about twice the area for

the diaphragm cell process and even larger than for the membrane cell process. Adequateventilation is required to ensure that mercury vapours do not exceed 0.1 g/day/l 000 Kg of product inthe working environment [2]. The electrolytic processes require control of chlorine emission to theatmosphere and water bodies. Sources of pollution include the vent gases from chlorineliquefaction, storage and haulage. In the United Kingdom, Health and safety at work etc., Act 1974permits, with a suitable chimney height, 0.023g/m3 chlorine discharge. Total residual chlorinedischarge into navigable waters in USA is maximum 0.0032 Kg/ 1000 Kg of product for mercury cellprocess and 0.0l3 Kg/1000 Kg of product for diaphragm or membrane cell process, [2].

Choice of process technology

The membrane cell process, compared with diaphragm and the mercury cell processes, hassuperior economic characteristics. The membrane cell process operates in modules. It therefore

provides for easy reduction or expansion of capacity to the demand for caustic soda and chlorine. Ithas low power consumption as a result of low voltage drops at the electrodes and membranes,closer electrode spacing, lower chlorine and hydrogen over voltages. The membrane cell processproduces high purity caustic soda. The cell design does not use hazardous materials such asmercury or asbestos. Operating in modules and combining superior economic characteristics, themembrane cell process is considered most appropriate choice for the electrolysis of brine.

The cation-exchange membrane cell process offers lower production cost and flexibility in adjustingplant capacity according to demand for caustic soda or chlorine. This article underscores thepotential opportunities for investments in integrating the chloro-alkali and petroleum industries.

Caustic Soda Production


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Caustic Soda solutions are produced as a co-product with Chlorine electrolytically by threetechnologies: mercury cells, membrane cells, and diaphragm cells. Each of these processes utilizeNaCl salt as the primary raw material. The salt is electrolytically split using direct current (DC)electricity, resulting in Chlorine and an available sodium ion (Na+) that is reacted with water in thecell to make Caustic Soda and by-product Hydrogen. The hydrogen by-product produced is used asa fuel source, sold to hydrogen customers, or to produce high purity (burner grade) Hydrochloric

Acid. A fourth technology that produces commercially available quantities of Caustic Soda solutionsis a chemical conversion of trona ore. This process produces low quality Caustic Soda, usedprimarily by west coast consumers.

Following is a brief discussion of the three electrochemical processes that are dominant in CausticSoda production in North America. Olin utitlizes all three technologies at various productionlocations.

1. Mercury Cell

Approximately 13% of electrolytically produced Caustic Soda in North America is produced by thisprocess.

The mercury cell operation utilizes mercury as the cathode for the electrolytic reaction, as well as forthe sodium amalgam that is reacted to deionized water to produce the Caustic Soda solution. Thereis essentially no net consumption of mercury in process, as all cells are 'closed loop' with regards tomercury flow across the bottom of the cell and through the cell decomposer, where the amalgam isreacted to Caustic Soda solution. The solution produced by this process is 50 - 52% by weightNaOH, produced directly from the cell without any additional evaporation. This process produces thehighest purity Caustic Soda commercially available.


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The high purity characteristic is descriptive of the very low concentrations of contaminants in theproduct. Salt, or NaCl, is typically less than 10 ppm, with a maximum of 30 ppm. Sodium Chlorates,or NaClO3, are typically 0.5 ppm, with a maximum limit of 1 ppm. Sodium Carbonates, or Na2CO3,

are typically 0.02 weight %, with a maximum limit of 0.06 weight %. Sodium Sulfates, or Na2SO4,are typically 10 ppm, with a maximum of 20 ppm.

The mercury cell produced Caustic Soda is typically referred as Mercury Cell Grade, or morecommonly, Rayon Grade. Most production of rayon fiber is dependent on the availability of the highpurity Rayon Grade Caustic Soda solution. Another very common use of this high purity causticsolution is for DI water exchangers. The DI unit resin literature often specified Rayon Grade CausticSoda only for regeneration. One must remember that this literature was published prior to theavailability of membrane cell produced solutions.

Total equivalent energy, on a DC basis, needed to produced Caustic Soda via the mercury cellprocess is approximately 3600 kWh per metric ton.

2. Diaphragm Cell


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This process utilizes asbestos, or alternate substitutes to asbestos, to separate the co-productsCaustic Soda and Chlorine. The production of 50% Caustic Soda occurs primarily outside of theelectrolytic cell. The diaphragm cell produces a very weak 'cell liquor,' which contains 12-14%, by

weight, NaOH and roughly the same concentration NaCl salt. The 'cell liquor' is subsequentlyevaporated in a three or four 'effect' evaporation process to a final nominal concentration of 50%NaOH by weight (49-52% range). The excess salt is precipitated and filtered through theevaporation process for subsequent reuse/recycle. This process produces the lowest qualityelectrochemical Caustic Soda solutions.

The quality considerations with respect to the diaphragm cell produced Caustic solutions includerelatively high salt, chlorates, carbonates, and sulfates. Salt, as NaCl, concentrations are typically1.0%, with maximums ranging from 1.1 to 1.3 weight %, depending on producer. Sodium Chloratesare typically 0.15 weight %, with a maximum of 0.3 weight %. Sodium Carbonates are typically 0.1weight %, with a maximum of 0.2 weight %. Sodium Sulfates are typically 0.01 weight %, with amaximum of 0.02 weight %.


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The diaphragm cell produced Caustic Soda is often referred to as Diaphragm Cell Grade. It is alsocalled Commercial Grade, Technical Grade, and occasionally Technical Diaphragm or other similarcombinations.

An additional 'grade' of Caustic Soda produced by the diaphragm cell process is the Purified Grade.The production of Purified Grade involves the further evaporation of the 50% Diaphragm Grade


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Caustic Soda solution to reduce the salt concentration. The higher Caustic Soda concentrationforces precipitation of the salts, which are soluble in Caustic Soda solutions in an inverserelationship. The higher concentration solution is then re-diluted to the 50% concentration that iscommercially available as Purified Grade Caustic Soda.

Common uses include process and wastewater neutralization, textiles production, soaps and

detergents and aluminum production. These uses and applications generally will refer to the CausticSoda as any of the various grades previously addressed, dependent on supplier's terminology.

Total equivalent energy, on a DC basis, needed to produce Caustic Soda via the diaphragm cellprocess is approximately 5000 kWh per metric ton.

3. Membrane Cell


The membrane cell process utilizes a selective membrane that separates the Chlorine and Sodiumions. The membrane allows the Sodium ion to 'migrate' across the membrane while keeping theChlorine gas and salt (brine) solution in a compartment on the other side of the membrane. TheSodium ion is reacted with purified water as in the mercury cell to produce the Caustic Soda. Thesolution produced by the membrane cell process is nominally 33-35 weight %. Evaporation isutilized, as in the diaphragm process, to raise the concentration up to the nominal 50 weight %solution suitable for shipments. The salt concentrations are not concentrated as significantly in thisevaporation process due to the selective osmotic nature of the membranes as well as the reducedamount of evaporation required in this process s opposed to the diaphragm evaporation. Minutequantities of salt do migrate across the membrane, concentrating up to the maximum 75 ppm. Notethat other producers employing the membrane cell technology may have a higher maximum limit of100 ppm on the allowable salt concentration in the Caustic Soda solution.


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The high purity characteristic is descriptive of the low concentrations of contaminants in the product.Salt, or NaCl, is typically less than 30 ppm, with a maximum of 75 ppm. Sodium Chlorates, or


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NaClO3, are typically 3 ppm, with a maximum limit of 5 ppm. Sodium Carbonates, or Na2CO3, aretypically 0.03 weight %, with a maximum limit of 0.05 weight %. Sodium Sulfates, or Na2SO4, aretypically 15 ppm, with a maximum of 20 ppm. Note that these limits are Olin specific, with someslight deviation to be expected amongst the various membrane cell operations.

The Caustic Soda produced by the membrane cell process is most commonly referred to as

Membrane Grade. It also has a growing acceptance as a Rayon Grade product in all areas outsideof rayon fiber production. Other terminology that is used include High Purity and High Purity,Technical Grade.

Total equivalent energy, on a DC basis, needed to produce Caustic Soda via the membrane cellprocess is approximately 3360 kWh per metric ton

Refrence:Web Sitehttp://home.att.net/~africantech/GhIE/chloro-alkali.htmhttp://www.olinchloralkali.com/training.asp#cal2000

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Chloro Alkali Plant

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