Report on Sitara Chemicals

MR. MUHAMMAD IMTAIZSENIOR DEPUTY PRODUCTION MANAGER OF AREA-II

SITARA CHEMICAL INDUSTRY (PVT) LTD.

SUBMITTED BY:

MUHAMMAD NORAIZ IQBAL KHANDEPERTMENT OF CHEMICAL & POLYMER ENGINEERING

2008-CPE-25INTERNE ENGINEER (22-07-2011 TO 18-08-2011)

MUHAMMAD NORAIZ IQBAL KHAN [UET FAISALABAD] Page 1

IN THE NAME OF

The Most Beneficent

The Most Merciful

INTERNSHIP REPORT:


(DEPARTMENT OF CHEMICAL & POLYMER Engineering)

(UNIVERSITY OF engineering & technology, Lahore) (FAISALABAD campus)

CAUSTIC SODA PLANT

SUPERVISED BY:

MR. MUHAMMAD IMTIAZ

SENIOR DEPUTY PRODUCTION MANAGER AREA-II

SITARA CHEMICAL INDUSTRIES [PVT.] LTD.

PRESENTED BY:

MUHAMMAD NORAIZ IQBAL KHAN

B.Sc. (Hons.) CHEMICAL ENGINEER

2008-CPE-25

ACKNOWLEDGEMENTSWords are bound and knowledge is limited to praise the ALMIGHTY ALLAH, the Lord of Universe, the


Beneficent, the Merciful, who is the entire source of all complete knowledge and wisdom endowed to

mankind, whose blessing are the cherish fruits of my thoughts and modest effort in the form of is effort

(manuscript). Peace and blessing of ALLAH be upon HAZRAT MUHAMMAD (Peace Be Upon Him), a postal of

Allah, the greatest social reformer, the cause of creation of universe and who is the forever source of

knowledge and torch of guidance for mankind.

I would like to express my sincere gratitude to GM (WORKS) of that industry Mr. EJAZ AHMAD MINHAS SAHB

who gives a chance to me to seek practical knowledge about industry to enhance my wisdom and provided me

necessary guidance and information in the writing of this effort and for providing me a strategic command at

every step.

I would also like to express my sincere gratitude to Mr. MUHAMMAD IMTAIZ (SENIOR DEPUTY PRODUCTION

MANAGER), who always shared me practical knowledge to enhance my wisdom and provided me necessary

guidance and information in the writing of this effort and for providing me a strategic command at every step. I

have no words to thank Mr. ARSLAN SAHB for shearing the basic & important practical information about the

field of caustic soda industry.

It is my pleasure to thanks shift Engineer Mr.SHOIAB SAHB, Mr.DILDAR SAHB & Mr.MEHBOOB SAHB, for

providing me basic guidance during my internship & describes the all about Caustic Soda Plant as well as

Furnace plant. I am very thankful to TRAINING ENGINEERS Mr. Hafiz Muhammad Ali Tayyab, Mr. Zeeshan

Jamil & Adnan Masood for describe the parameter of plants as well as guided me preparation of flow sheets

of plants.

May Allah shower His blessings on them.

C O N T E N T S


PAGE

TITLESr No.

6ABOUT THE ORGANIZATION 01

7HUMAN SAFETY EQUIPMENT 02

8UTILITIES 03

10CAUSTIC SODA PRODUCTION PLANT

04

12PRIMARY BRINE SECTION 05

16SECONDARY BRINE SECTION 06

19,23

CELL ROOM SECTION, DECHLORINATION SECTION

07

25,26

EVAPORATION SECTION, HYDROCHLORIC ACID PLANT(FURNACE)

08

ABOUT ORGANIZATION SITARA CHEMICAL (SCIL):Sitara chemical industry is a well known and famous chemicals manufacturer of Pakistan. It is an organization of very responsible and well trained professionals. At the same time SCIL has well developed and well managed Environment Management System and Quality


Management System which have been certified and audited by SGS under the authority of UKAS.They produced Caustic Soda along with many other chemicals. SCIL produces Caustic Soda in form of either 33% Caustic solution or as 50% Caustic solution or Caustic flakes of 98.5% purity. Four Caustic Soda manufacturing plants are working at SCIL. The Caustic production cycle goes through many steps. These sections include primary brine section, where depleted brine is saturated and purified by settling, chemical treatment and filtration through pressure leaf filters. After primary treatment brine is ultra purified in secondary section bypassing through ion exchange resin beds. When brine meets the required degree of purification and concentration, it is fed to electrolyzers in cell room.In cell room, a current of almost 12.5kA is passed through the brine solution in special type of cells called as membrane cells. Chlorine and Hydrogen are also produced as by products along with Caustic in cell room. They are separated from Caustic and depleted brine and are sent to HCl furnace after necessary treatments.Caustic, thus produced is 33%. It is concentrated up to 50% in three effect, backward feed, falling film, evaporation unit. This 50% caustic is then sent to Caustic Solidification Plant (CSP) to convert it into 98.5% Caustic which is then packed in drums or as flakes in PP bags. Some side reactions also occur in cell room resulting, the formation of chlorates. Therefore brine coming out of cell room contains chlorine and chlorates in it. This brine is sent to the dechlorination section where these contaminates are removed by stripping, treatment with HCl and sodium sulphite and by passing through carbon tower. This dechlorinated brine is then recycled in primary section for saturation. Different utilities such as water, electricity, instrument air and nitrogen for purging H2 and C l lines are also used there.

Although SCIL is manufacturing many important chemicals such as AmmoniumChloride (NH4Cl) Bleaching Powder (Ca(OCl) Chelated Iron Clorinated Parafin Oil Hydrochloric acid (HCl) Ferric Chloride (FeCl) Ferric Sulphate (Fe2(SO4)3) Liquid Carbon dioxide(CO2) Liquid oxygen (O2) etc.

But their main product is Caustic Soda (NaOH) chemically known as Sodium Hydroxide. SCIL is selling caustic Soda in three forms:

(1) 33% liquid Caustic Soda,(2) 50 % liquid Caustic Soda and(3) 98.5 % caustic flakes

HUMAN SAFETY EQUIPMENT(HSE)


WIND SOCKS:Tell us direction of air flow. If any process gases leaks take place on the plant, we cross the path of gas by using the direction shown by wind socks. Never move along the direction of air flow or opposite direction.FIRE FIGHTING:There Are 5 Types of Fire Produced Which May Take Place on the Plant.

Solid typeIt is Due to solid materials i.e. wood, broad.

Liquid typeIt is Due to liquid materials i.e. oil.

Gases typeIt is Due to flame able gases i.e. H2, Cl2.

Chemical spillageIt is Due to chemical materials i.e.

Electrical It is due to spark in electricity circuit.

FIRE EXTANGUISHERS;There are three types of cylinders used in industry to overcome on the fire

DCPDicalcium phosphate

CO2Carbon dioxide cylinders

AFFFAqeous falling foam film(AFFF) cylinders

USAGE FIRE EXTANGUISHERS;For solids:For solids DCP, powder water sand & CO2 is used.For liquids:For liquid AFFF DCP is used.For GasesFor Gases DCP, CO2 is used.Chemical spillage:For chemical spilage DCP, CO2 is used.Electrical:For electrical DCP is used.

UTILITIES AND PROTECTIVE EQUIPMENTS:Different utilities used during process along with their use and required amounts are given in the following lines,WATER:


Uses:Electrolysis feeding, Resin regeneration, Preparation of Brine, Caustic soda and HCl dilution, flocculent preparation.Amount: 15 m3/hSource:Tube well water.Types:-Dematerialized water, R.O. water, cooling Water.Specifications:-PH 6-7, Conductivity 10 micron Siemens /cm (max), Fe 100 pbb (max), SiO2 100pbb (max)COOLING TOWER:Four cooling towers are installed in Area-ii, which cool the required amount of water for process. Turbines are used for pumping the water having flow rate 400 m3/hr. reduction in temperature in this cooling tower is up to 40C(34 to 30).

INDUCED DRAFT COOLING TOWER ZnCrO3 is added to absorb O2 to inactive the algae & fungai. HCl is added to remove the carbonates & Bicarbonates and maintain the PH 6.5-7. Sand filter is used to remove the sand present in water.

INSTRUMENTAL AIR: Uses:For controlling Auto valves etc.


Source:Atmospheric Air.Specifications:-Design pressure 7 bar Temp. 400C Dew point at 7 bar temp -250COil and dust free.N2 GAS:N2 is used for purging to avoid the explosion of H2.CHILLED WATER:Chilled water is used to cool the process stream to a very low temperature. The temperature of chilled water is about 80C. the process of chilling depend upon the absorption refrigeration cycle.

Diagram of Chilled Water used in process

CAUSTIC SODA PRODUCTION PLANT(NaOH)

Sitara Chemical Industries is manufacturing caustic soda (NaOH) by electrolysis of brine solution. Process is carried out in membrane cells equipped with latest technology. At time, SCIL has three caustic


manufacturing plants in working, named as BMR, M-I and M-III. Another plant M-II is being established and is almost in final stages. Overall caustic production of SCIL is 545 MTD with 210 MTD contributed by BMR. Since, my internship program was conducted in Area-II at M-III; therefore in the following lines we will have detailed discussion of all unit operations and unit processes taking place in the cycle of production of Caustic Soda.Very first step in the production of caustic is the preparation of brine solution from Rock Salt. Since this saturated brine solution contains impurities which can scale within pipelines and other equipment and also damage the electrolyzer membrane, therefore various techniques are employed to filter and purify this brine solution. After necessary preliminary treatments, ultra pure brine solution is sent to the cell room where electrolysis is carried out. Two by products Hydrogen and Chlorine are formed along with caustic soda.

The production cycle of NaOH goes through the following series of steps.

1. Primary Brine Section

This section involves saturation of brine solution, its purification by settling after treatment with certain regents and primary filtration by pressure filters.

2. Secondary Brine Section

In this section, brine coming from primary brine section is filtered by resins to remove any remaining impurities which may cause damage to membranes of the electrolyzer.

3. De-chlorination Unit

Here de-chlorination and other preliminary treatments of depleted brine coming from cell room are carried out before sending it to primary brine section.

4. Evaporation Unit

Caustic soda obtained from cell room has a concentration of 32 % w/w. in evaporation unit; this concentration is raised to 50 %.

5. Caustic Solidification Plant( CSP)

In this section, caustic soda from evaporation unit is concentrated up to 98.5% and solidified for packing in bags and drums. In next lines is the detailed discussion on each section along with complete description of all unit operations and unit processes involved.



PRIMARY BRINE SECTION

Primary brine section involves three types of activities, Brine saturation Brine purification and settling Brine filtration

PROCESS EXPLANATION:

Main purpose of the brine processing is to re saturate the depleted brine coming from the electrolyzer, by adding the quantity of salt which has been electrolyzed. The impurities which have been introduced in the brine together with the salt are to be removed first by settling and then by chemical treatment and settling. The suspended impurities are removed by filtration.In this section de-chlorinated depleted brine coming from cell room after electrolysis is concentrated and purified by sedimentation and filtration. After electrolysis, concentration of brine decreases to 180-200 g/l. so it is sent to saturators.SATURATORS:The level of the salt in the brine saturator is kept as constant as possible by feeding salt in the saturator at required rate at the bottom. The raw salt impurities such as Ca and magnesium salt are dissolved in the depleted brine faster as compared to sodium chloride, so that if excessive amount of salt is loaded into the saturator, the impurities content in the saturated brine would be higher than the normal. Where it’s concentrated to 300-310 g/l. The flow rate of the depleted brine is 80m3/hr. Two saturators are present at M-III one in process and one at stand by. Saturators are made of concrete, rein forced with steel and internally epoxy lined. Depleted brine is introduced with the help of five nozzles installed at the bottom of each saturator. Saturator is filled with lumps of Rock Salt with the help of tractor blades from salt yard. Water percolates through the salt lumps dissolving the salt in it and over flows in separate overflow line. Makeup de-mineralized water is also fed through a pipe line to cover water losses and maintain the required flow rate.Follow rate is maintained so that the desired concentration is obtained. If concentration is much higher, salt may crystallize in pipelines, reactors or tanks in cold seasons. When saturator is exhausted (filled with sludge), flow is shifted to second saturator and first one is subjected to cleaning. After concentration brine many suspended and dissolved impurities which are necessary to be remove. Approximate composition of Rock Salt and impurities associated with it are as given below:

NaCl ---------------------------------------------------------- 97% w/wCa+2----------------------------------------------------------021%w/wMg+2--------------------------------------------------------0.06% w/wSO4-----------------------------------------------------------0.9% w/wK+-----------------------------------------------------------0.17% w/wSr---------------------------------------------------------------1.0 ppmSiO2---------------------------------------------------------- --5.0 ppmAl ----------------------------------------------------------------0.2 ppm


Fe ---------------------------------------------------------------1.0 ppmMn --------------------------------------------------------------0.5 ppmNi---------------------------------------------------------------0.05 ppmI -----------------------------------------------------------------0.5 ppmF -----------------------------------------------------------------1.0 ppmCr, Mo, V--------------------------------------------------------0.5 ppmInsolubles ---- ---------------------------------------------0.69% w/wMoisture----------------------------------------------------0.97% w/w

REMOVAL OF IMPURITIES:So in order to remove these impurities, brine is treated with Calcium Chloride CaCl 2. For this purpose brine coming out of saturator is collected in Brine Collecting Tank from where almost 40% of it is sent to CaCl2 pits while 60% overflows from tank to the Settling Pits. Settling pits are working alternatively i.e. one in process and one at stand by. Insoluble and suspended impurities are settled down settling pits and brine overflows to the next pit known as Common Pit. In CaCl2 pits from common pit brine is pumped to the first, CaCl2 in the form of solution is infused to maintain 3000-4500 ppm of Ca++ in excess. The CaCl2 pit is made ready twice in a day and transferred in the common pit. CaCl2 pit is made ready to transfer in 6-8 hrs .CaCl2 which is infused has sp. Gravity 1.1-1.2 and pH in range of 6-9. From the settling pit, the brine over flows into the common pit where it is mixed with the brine coming from the CaCl2 pit.REACTOR 1:The flow rate is 105-110m3/hr. In this reactor brine is treated with 8% Barium Carbonate solution to remove any left over sulphates. SETTLER 1:After reactor, brine flows to the first Settler. Brine over flows from this settler & goes to the other reactor. This settler has a capacity of 1800 m3 and its material of construction (MOC) is RLS (rubber lined steel).Suspended impurities settle down at the conical bottom of settler and are removed through a drain valve and screw pump by a slow moving rack. Rack rotates at a speed of 0.1 RPH. Nalco solution prepared in tank D-5310A/B is dosed at controlled rate. The amount of flocculants, Nalco, is maintained at 1-2 ppm.REACTOR 2:Brine is then goes to the second Reactor where it is treated with 8% solution of Soda Ash (Na2CO3) and Caustic Soda in order to remove Ca+2 and Mg+2. Both reactors are equipped with agitators for intimate mixing to facilitate reaction and to avoid settling of insoluble compounds formed, within the reactor instead of settler. SETTLER 2:After second reactor, brine solution flows by gravity to the second Settler (TH-5010 B). This settler has a capacity of 2630 m3. Flocculent (Nalco) is also added in the settler. This settler is also equipped with a slow moving rake at the bottom to drain settled sludge through a drain valve. This sludge is collected in two Brine Recovery Pits. From second settler brine overflows to the where any brine which was drained previously with sludge and is sent to Common Pit.Brine is then store in Pre-filtered storage tanks (D-5060 A/B). From tanks D-5060 A/B, brine solution is fed to four (4) Pressure Leaf Filters (F-5070 A-D). The end of the filtering cycle is indicated by the pressure drop in the filter .When the filter reaches the maximum operating pressure, the filter must be cleaned. The pressure across the filter should not exceed 3 bars. Filters contain 17 leaves each and total filtering surface of each filter is 38.2 m2.


Casings of all filters are made HRLS (hard rubber lined steel) and filtering cloth is of FRP (fiber reinforced plastic).in parallel manner. Brine filters are required to remove the suspended solids over flowing with the brine from the settler TH-5010B. There are four (4) leaf filters F5070A-D operating in parallel. Brine from tank D-5060 A/B is pumped and distributed to each filter through individual manual valves, before preceding to the filtration the filters must be pre-coated. For pre-coating, the pre-coat slurry is prepared in the tank D-5100.The pre-coat material Arbocel is thoroughly dispersed in the filtered brine.REACTIONS INVOLVED IN PRIMARY SECTION:-SulphatesThese are precipitated as barium sulphate by reaction with barium carbonate in 1st reactor.

Na2SO4+ BaCO3 BaSO4 + Na2CO3

Calcium

This is precipitated as calcium carbonate by reaction with sodium carbonate in 2nd reactor. CaCl2 + Na2CO3 CaCO3 + 2NaCl

MagnesiumThis is precipitated as magnesium hydroxide by reaction with caustic soda in 2nd reactor.

MgCl2 + 2NaOH Mg(OH)2+ 2NaClStrontium,This is precipitated mainly as strontium carbonate

Sr++ + CO3-- SrCO3

Most of the sulphates in the brine are being removed by treating with calcium chloride. Hence the consumption of barium carbonate is controlled. BaCO3 is an imported commodity and is very costly, where as calcium chloride a waste product is produced in HCl neutralization pits. Another advantage is that Calcium sulphate, which is precipitated out, is a good fertilizer where as barium sulphate is dangerous to water bodies and soil.



SECONDARY BRINE SECTIONNature of Ion Exchange Resin:-Ion exchange resins are based on solid insoluble polymers supplied in the form of beads, which have fixed active ionic groups. Mobile ions of opposite charge can be exchanged reversibly and stoichiometercally for ions of the same charge present in the solution. The resin being used has Na ions as mobile ions and exchange with Ca++ & Mg++ present in the brine. When most of the Na+ has been exchanged with Ca++ &Mg++, the bed is exhausted and needs to be regenerated. The exhausted column is taken out of the stream and the other two are remained in operation. TP-260 has weak acidic chelating amino methyl phosphonic acid groups which form stable complexes with a number of transition metals and main group elements.

PROCESS EXPLANATION:Before feeding brine to electolyzers it is passed through ion exchange resin towers C-5040 A/B/C placed in series in order to absorb Ca++, Mg++ and Sr++ contained in brine. The brine feeding to the resin tower has following typical specifications,

NaCl : 290-310g/l, Na2SO4 : 7-8g/l NaClO3 : 15g/l, Ca++ : 4-6ppm, Mg++ : 1-2ppm, Sr++ 1 : ppm max, SiO2 5 : ppm max, Cl2’ : Nil Suspended solids :0.5ppm Temperature :50-6oC PH :9-10

The resin bed in the tower stayed on a grid plate. The spargers are fitted on this plate. The purified brine from tank D-5070 is fed in the tower through P-5070A/B at a flow rate of 105-110m3/hr. As the towers are connected in series, so the brine is fed in the first tower in the series and ultra purified brine is collected from the discharge of the third tower and collected in Ultra Pure Brine Tank (D-5160) . From D-5160 brine is pumped to Brine Head Tank (06D-001). Then it is sent to the cell room after controlling its temperature through two Heat Exchangers (06E-001) and (06E-002).

Regeneration of the Resin Bed:-The chemical nature of the resin functional group is suitable to form complex compounds with a lot of metal ions. The resin at the beginning of the operating cycle is in the sodic form. During the operation it fixes Ca++ & Mg++ to which it has a high affinity, and releases Na+. When the resin is exhausted, means when it has absorb max. Ca & Mg ions, it must be reconverted in the sodic form. This process is referred to as “resin regeneration”.

The regeneration process requires a sequence of different operations. The most significant of which are a treatment with diluted HCl which displaces the metal ions previously fixed by the resin functional groups followed by a treatment with diluted NaOH solution which makes the resin to change form the acidic form to the sodic form i-e H+ replaced with Na+


STEPS INVOLVE IN THE REGENERATION PROCESS

1) Brine drainage (half empty) 2) Washing with demi water 3) Counter washing with demi water4) Drainage5) Washing with 4% HCl solution 6) Drainage7) Washing with demi water8) Drainage9) Counter washing with 5% NaOH solution10) Drainage11) Washing with demi water12) Drainage13) Water replacement with brine14) Filling with brine

After regeneration resetting is done almost in 60 minutes and brine is circulated between resin towers and brine tanks D-5070. The circulation of brine remains on till the required results of Ca & Mg are achieved. Then the column is taken in operation and placed in the last in series.Chemical Reaction Involved:- Chemical reactions involved in secondary purification and regeneration of resin towers are as given below.BRINE PURIFICATION

Na2 (Resin) + Ca++ Ca (Resin) + 2Na-Na2 (Resin) + Mg++ Mg (Resin) + 2Na+

RESIN REGENERATION

(Washing by HCl)Ca (Resin) + HCl H2 (Resin) + Ca++

Mg (Resin) + HCl H2 (Resin) + Mg++

(Washing by NaOH)H2 (Resin) + NaOH Na2 (Resin) + H2O



CELL ROOM SECTIONElectrolyzer:-The equipment used to carry the process of electrolysis is called electrolyzer. The cell room comprises of four (4) electrolyzers A/B/C/D placed in parallel fashion. Each electrolyzer has capacity of 114 cells. At present electrolyzer A has 114, electrolyzer B has 114 and electrolyzer C has 114 elements. Each element acts as an electrolyzer individually. Each element consists of anode and cathode compartments separated by a membrane. Anodic side is made of titanium while cathodic side is made of nickel. The ion exchange membrane is clamped between the half shells with interposed PTFE gaskets. The half shells are bolted together at the flange, thus constituting a cell element.ION EXCHANGE MEMBRANE:The ion exchange membrane is made by an organic polymeric matrix incorporating fixedionic groups neutralized by mobile ions of opposite charge. Fixed group are sulphonic type or carboxylic type, while mobile counter ion is sodium ion. Only the cation exchange selectivity of the membrane can prevent migration of OH-. The flow of Cl- across the membrane is about negligible, because membrane selectivity and electric field across the membrane opposes the chloride ions transport. The raw material feeding to the electrolyzer is brine. The brine nearly saturated, is introduced into the anode compartment of the cell. The aqueous solution in this compartment is called anolyte. The membrane separates the anolyte from the catholyte compartment. 32% caustic soda from catholyte tank plus Demineralized water to make a 29-30% caustic solution is admitted to catholyte compartment where sodium hydroxide is formed by the combination of hydroxyl ions and sodium ions, which migrate through the membrane to the cathode. Chlorine gas, usually called cell gas, is formed at the anode while Hydrogen gas and sodium hydroxide are formed at the cathode.

PROCESS EXPLANATION:Ultra pure brine is stored in tank D-5160 and pumped vi P-5160A/B into over head feed brine tank. 06D001. Brine enters in the overhead tank from the top and over flows by gravity from the top side back in D-5160. The outlet is from the bottom of the tank and leads to cell room by gravity. The brine is heated in the way before entering cell room through heat exchanger 06E001 to raise its temperature up to 65-75 C. It is a plate type heat exchanger. Heat exchanger is provided with steam as well as cooling water connections. There is also a stand by heat exchanger 06E002 which is either used to fill a single electrolyzer or in case of emergency. In the cell room main feed brine line has four connections to feed each of the four electrolyzers. These supply lines are underneath of each electrolyzer. Brine is fed in the anodic compartment of each cell.Catholyte (33% NaOH) is stored in the storage tank 31D002. Its temperature is about 80-85 0C. It is cooled in 31E001 before transferred in overhead tank 31D001. A temperature of 70-75C is usually required to feed in cells. 31E001 is a plate type heat exchanger which is provided steam as well as cooling water connections. There is also a stand by heat exchanger/cooler 31E003 which is used in case of emergency or for single electrolyzer filling. This heat exchanger can be used only for cooling purpose as there is no steam connections associated with it. Catholyte enters into catholyte overhead tank 31D001 from the top and over flows from top side. Outlet is from the bottom and leads to cell room by gravity.


CELL ROOM In the cell room four (4)electrolyzers are arranged in parallel. In each electrolyzer 114 elements are placed in series. At present Electrolyzer A has 114, B has 114, C has 114 & has 114 elements. Brine is introduced in the anodic compartment of the cell while caustic soda is introduced in cathodic compartment of the cell. When current is introduced, electrolysis takes place. As a result of it caustic soda is produced in the cathodic compartment together with hydrogen gas where as chlorine gas is produced in the anodic compartment.Electric Supply Circuit:-11KV power is supplied from grid station. It is passed through transformer to step it down to 440 V. Then it is passed through rectifiers to convert it into DC which is then supplied to electrolyzers via bus bars and flexibles. The current passing through the electrolyzer is about 12KA.REACTIONS:-Following reactions takes place in the cells Sodium chloride and water are dissociated in the brine solution according to the equations

NaCl Na+ + Cl-

H2O H+ + OH-

The principal anode reaction involves the oxidation of the anion Cl- to produce chlorine gas 2Cl- Cl2 + 2e-

The primary cathode reaction is the reduction of the cation H+ to produce hydrogen gas2H+ + 2e- H2

The sodium cation Na+ then combines with the OH- ions to form third overall product NaOH

Na+ + OH- NaOHThe overall cell reaction is

2NaCl + 2H2O 2NaOH + Cl2+ H2

SIDE REACTIONS:The predominant side reactions in the anodic compartment of the cell are

4OH- O2 + 2H2O + 4e-

2OH- + Cl2 ClO- + Cl- + H2O

6ClO- + 3H2O 2ClO3- + 4Cl- + 3/2 O2 + 6H+ + 6e-

3ClO- ClO3- + 2Cl-

The source of the OH- ions in these reactions is the migration of OH- ions through the membrane from the catholyte solution, caused by attraction to the positively charged anode.

PROCESS LINE CONNECTIONS AT THE CELL :

The process line connections to the electrolyzer can be individually isolated by means of valves on both the supply and product sides.

The following cell headers are installed in the cell room.


Anolyte Ultra pure brine filling Ultra pure brine feed Catholyte Catholyte filling Catholyte feed Anolyte draining Catholyte draining Catholyte sample Anolyte sample Chlorine Hydrogen Ultra pure brine vent Catholyte vent Catholyte by pass Anolyte by pass Nitrogen to anolyte header Nitrogen to catholyte header

CHLORINE GAS HANDLING : In the anodic compartment of the cell chlorine gas is produced as a result of electrolysis. It is separated from depleted brine in the vertical gas header as well as in the gas separator and is collected in the main chlorine gas header.



DECHLORINATION SECTIONIntroduction:-Depleted brine from electrolyzer usually contains 0.3 g/lit dissolved chlorine at a Ph about 3.5-4.0 and 1.0 g/lit as available chlorine in the form of hypochlorite ions. Depleted brine also carries 2000-2500 ppm of free chlorine. The process of removal of chlorine from the depleted brine is termed as “Dechlorination” Chlorine must be removed from brine because of the following facts.

The removal of impurities in the primary brine purification becomes difficult. In the presence of high contents of chlorine, the impurities in the rock salt are more

easily dissolved during brine saturation. Chlorine oxidizes ion exchange resins, hence increasing its consumption. The

chlorates are necessary to remove because It decreases the sodium chloride solubility resulting in decreased efficiency, possible

salt precipitation and potentially adverse chlorate conc. in the caustic soda product. Chlorates have a strong oxidizing effect which is more evident at high

concentrations.

Chlorates are formed in the anodic compartment of the cell of electrolyzer via chemical or electrochemical reactions as mentioned before. Chlorates removal rate is a function of the chlorate concentration so that it is convenient to operate at a relatively high chlorate concentration in the feed brine. High temperature also favors the chlorates removal, so a temperature of 85- 90C is recommended. The third parameter affecting chlorates removal is pH. A low pH less than 2.0 is favorable.

PROCESS EXPLANATION:

Dechlorination is carried out in three successive steps By vacuum stripping By Sodium Sulphite treatment With activated carbon

Depleted brine leaving the electrolyzers flows into the acidification pot where it is mixed with acidified stream of brine from reactor. Then it is collected in depleted brine storage tank, from here the brine is pumped. The discharge of the pump is divided into two streams, one leads to vacuum stripper and second leads to acid mixing drum. Normally 70 to 75% material goes to vacuum stripper and 25-30% in mixing drum for decomposition of chlorates. In mixing drum, 33% HCl is added to lower down its pH almost zero, after acidification the brine flows down by gravity into the reactor where it is mixed with chlorinated steam coming from the vacuum stripper. In reactor at low pH and high temperature, chlorates are decomposed, producing free chlorine which is vented in the chlorine sniff line. The over flow of reactor entered in the acidification pot and lower the pH of the depleted brine ranging 1.5-2.0

NaClO3+ 6HCl NaCl + 3Cl2 + 3H2

By Vacuum StripingBrine containing absorbed chlorine enters from the upper part of the packed tower where vacuum is generated by means of steam ejector. In such condition brine leaving the tower has 20-50ppm of chlorine. The wet chlorine stripped from the brine leaves the tower and is


cooled in the shell and tube type heat exchanger. The condensed chlorinated water is returned in the depleted brine tank while the chlorinated steam is sent to reactorR-5050. The brine flows down by gravity in the dechlorinated brine tank. The tank is vented to chlorine sniff line.By Sodium Sulphite Treatment:Sodium sulphite is added to the depleted brine leaving the tank by means of an inline injection on suction of the pump. Sodium sulphite solution is prepared in the tank by absorbing of SO2 in sodium carbonate solution and sent in mixer by means of pump. Sulphur dioxide (SO2) is prepared by burning sulphur mud in the sulphur furnace which is sucked through an absorber by a suction fan. The gas is entered from the bottom of the absorber and left from the top. The sodium carbonate solution is fed from the upper side of the absorber and flows down back to the same tank. When the solution is ready it is transferred in tank for storage.During chlorates decomposition following reaction takes place

Na2SO3+ Cl2+ H2O Na2SO4+ 2HClWith Activated Carbon:After treatment with sodium sulphite, brine is passed through an activated carbon bed in tower C-50200. Care must be taken that the carbon should work with acidic brine pH less than 2 and chlorine less than 50ppm. High chlorine contents in brine feeding the tower causes to violent.Reaction with the carbon, relatively high pH in the feed brine to tower causes carbon degradation in the form of fine particles which can plug the tower. The brine passing out of the tower is made alkaline by the addition of dilute caustic effluent. The pH is raised up to 6.0-9.0 before it is transferred in primary brine section. When the carbon is packed down it is necessary to counter wash the carbon bed tower. When the pressure drop across the carbon bed became more frequent, then it is packed with fresh activated carbon. Normally this situation occurs after 40-45 days


EVAPORATION SECTIONIntroduction:-The plant is designed for continuous concentration of caustic soda 31-32% to 50%. The concentration is improved by evaporation in a triple effect counter flow falling film evaporator. The plant consists of two plate type evaporators & one falling film evaporator. Falling film evaporator is a tube bundle heat exchanger with a separator at the bottom. In the separator vapors and thick liquor are separated. The feed liquor is uniformly distributed in the tubes by a distribution device, thus the liquid moves down forming a thin film along inside wall of the tube. That is why it is called a falling film evaporator.

PROCESS EXPLANATION:The caustic is fed to the first evaporator D-2010 through a heat exchanger by a pump installed at the discharge of caustic buffer tank D-20100. On product side the first evaporator is operated under a vacuum. The caustic leaves the first evaporator effect D-2010 at a concentration of 36-37%. The first evaporator is heated with vapors formed in the second evaporator D-2020. The condensate of the D-2010 collected in the tank TK 2040. This vacuum is produced by connecting the condenser, duct D-2010 and 1st evaporatorD-2010 to TK-2040. The vapors of D-2010 are condensed in E-2030. The cooling water is fed in the shell and the vapors are condensed in the tubes. The condensate is collected in the tank TK-2040.The vacuum in D-2010 is produced through steam ejectors. After the D-2010, pump pumped the caustic soda to the second evaporator D-2020 via heat exchangers. The discharge of the pump is divided into two streams; one admitted to the D-2020 and second is admitted of the D-2030.These two streams rejoined before entering the EV-202. This evaporator effect is heated with the vapors from EV-203.the pressure in the shell of EV-202is about 1.9-2.0.The caustic soda entersEV-202 at conc. range 36-37%, and temp. range110 -115C and leaves at conc. 42-43% andtemp.120-125C.This caustic soda is fed to the third evaporator EV-203 through pump P-202 viaE-204 & E205. The condensate is collected in condensate tank TK-202 and then transferred inTK-201 from where it is transferred in D-5150B.The discharge of pump P-202 is again divided into two streams; one admitted to the shell of E-204 and second is admitted in the tubes of the E-205. These two streams rejoined before entering the EV-203. This evaporator effect is heated with the steam at a pressure of about14bars.The caustic side is under a pressure of 2 bars. The vapors produced here are used to heat the second evaporator and the condensate is collected in the tank TK-203.The caustic leaves thisevaporator at 50% conc. at temp. of 175-185CThe discharge of pump P-202 is again divided into two streams; one admitted to the shell of E-204 and second is admitted in the tubes of the E-205. These two streams rejoined beforeentering the EV-203. This evaporator effect is heated with the steam at a pressure of about14bars.The caustic side is under a pressure of 2 bars. The vapors produced here are used to heatthe second evaporator and the condensate is collected in the tank TK-203.The caustic leaves thisevaporator at 50% conc. at temp. of 175-185C.Now it is to be cooled before storage. For this purpose it is first passed through tube sideof E-204. Here it’s temp. is reduced to 155-160C. Then it is pumped via P-203 through the tubeside of the E-202 and E-206, the final temp is about 55-65C. The condensate of the evaporatorEV-203 also bears a sufficient amount of heat, which is recovered by passing it through the shellsof E-205 & E-203.The steam condensate at a temp about 97-98C is returned to boiler section.


HYDROCHLORIC ACID FURNACESINTRODUCTION:Chlorine and hydrogen gases are produced together with caustic soda in the electrolyzers. These gases are transferred in the furnaces area for synthesis of hydrochloric acid. Following reaction takes place

H2 + Cl2 → 2HCl

During the reaction a lot of heat is generated and temperature inside the furnace is about 2500-3000C. HCl gas produced in the furnace is absorbed in water/weak acid. As a lot of heat is generated during HCl synthesis, plenty of cooling water is circulated in the exchanger block to cool down HCl gas as well as acid produced and also to improve absorption.Unabsorbed gas termed as tail gas is separated from the acid is passed through the absorber and absorbed in the water thus producing weak acid of 4-8% conc., which is fed to the furnace and 33-35% HCl is produced.CONSTRUCTION OF FURNACE:Furnace is consists of three main parts

Burner Furnace exchanger assembly Separator

BURNER:The burner consists of two silicon/glass tubes fitted in side the Graphilor components. The tubes are properly packed and fitted so that could not move freely. These are easily replaceable. There are two inlet points, one for chlorine gas and second for hydrogen gas. Chlorine gas enters from the top in inner tube while hydrogen gas enters from the side in outer tube.FURNACE EXCHANGER ASSEMBLY:The furnace exchanger assembly consists of a combustion chamber fitted with two portholes which are covered with sliding plates containing sight glass. The sight glass permits both lance introductions in the furnace as well as inspection of flame.The furnace exchanger assembly composed of a wear/distribution plate, two hollow block and eighteen exchanger blocks for furnace A-G and twelve for H and I.The exchanger block has vertical as well as horizontal openings/tubes. HCl gas and acid passes through vertical tubes where as cooling water is circulated in the horizontal tubes.SEPARATOR: The separator is a Graphilor component separating the acid produced from the tail gas. The bottom of this unit contains a safety disc which damages when the pressure inside the furnace reaches 0.5bars to avoid explosions in the furnace.PROCESS DESCRIPTION:Chlorine gas obtained from the electrolyzer is introduced in inner tube from the top of the furnace. Inner tube is blocked from the front and openings from the side. This arrangement facilitates mixing of chlorine and hydrogen gas. Hydrogen gas is introduced from the side of the furnace in outer tube. Lance is introduced in the middle of the outer tube through one of the two portholes and flame is observed from the other porthole covered with sight glass.


Hydrogen and chlorine gases burn and combustion takes place in the combustion chamber. A lot of heat generated and temperature inside the furnace is about 2300-3000C, so a plenty of cooling water is circulated through furnace exchanger block to cool down HCl gas and HCl acid produced. Outlet temperature ranges 40-45C.Tail gas is admitted in the bottom of the absorber and moves upward. The water/Demi water is admitted from the top side and moves downward thus producing 4-8% weak acid. Unabsorbed gas is vented out high in the atmosphere.The weak acid formed is introduced in the furnace from top side moves co-currently with HCl gas and further absorption takes place producing 33-35% hydrochloric acid. The acid and HCl gas is separated in the separator .Hydrochloric acid is transferred in the storage tanks and HCl gas is diverted to absorber.

VALVE TYPES:A valve is a mechanical device that controls the flow of fluid and pressure within a system or process. A valve controls system or process fluid flow and pressure by performing any of the following functions:

Stopping and starting fluid flow Varying (throttling) the amount of fluid flow Controlling the direction of fluid flow Regulating downstream system or process pressure Relieving component or piping over pressure

There are many valve designs and types that satisfy one or more of the functions identified above. A multitude of valve types and designs safely accommodate a wide variety of industrial applications.Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing.TYPES OF VALVES:

a. Globeb. Gatec. Plugd. Balle. Needlef. Butterflyg. Diaphragmh. Pinchi. Checkj. Safety/reliefk. Reducing

STEAM TYPESBasically Steam Are Of Two Types

1. Saturated steam2. Super heated steam.

STEAM TRAPESThree types are Mechanical steam trapes, Temperature sensor steams trapes, Magnetic steam trapes.


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