sitara chemical internship

Internship Report (SCIL)

Internship report

Caustic Soda PlantArea 1

Sitara Chemical Industries Limited (SCIL)

Training period (27 July – 7 Sep)(Duration: 6 weeks)

By: Faheem Abbas

BS Chemical Engineering Technology

Signature………………………

Date…………………………..

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Caustic Soda Plant

SCIL was incorporated in 1981 and began producing caustic soda in 1985, initially at a rate of 30 metric tons Caustic a day. The plant’s capacity was gradually increased over years to current level of 545 metric tons a day. In addition, various by-product facilities have been added and expanded from time to time to cope with growing demand. Company entered into Textile Spinning Business in 1995. Its specialty chemicals and export division was established in 2001 and agri chemicals division in 2003.

PRIMARY BRINE SECTION

Primary Brine involves three major Steps:

Saturation of brine

Purification and settling of brine

Filtration of brine

Main purpose of the brine processing is to resaturate the depleted brine coming from the electrolyzers, by adding the quantity of salt which has been electrolyzed. The impurities which has 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.

Process Description

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 (DS-5010 A/B) where it is concentrated to 300-310 g/l. The flow rate of the depleted brine is 80m3/hr. Two saturators are present at BMR one in process and one at standby. Saturators are made of concrete, reinforced 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.

As the rock salt contains many dissolved and suspended impurities, so concentrating the brine imparts various impurities which necessary to be removed, otherwise there

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impurities will damage the membranes of the cell room. The approximate composition of Rock Salt and impurities associated with it are as given below:

NaCl 97.0 % w/w

K+ 0.17 % w/w Mg++ 0.20 % w/w Ca++ 0.60 % w/w SO4

- - 1.50 % w/w I 0.5 ppm

So in order to remove these impurities, brine is treated with Calcium Chloride CaCl2. For this purpose brine coming out of saturator is collected in Brine Collecting Tank (D-510) from where almost 60% of it is sent to CaCl2 pits while 40% overflows from tank to the Settling Pits (D-5610 A/B). Settling pits are working alternatively i.e. one in process and one at standby. Insoluble and suspended impurities are settled down settling pits and brine overflows to the next pit known as Common Pit (D5620). In CaCl2 pits, CaCl2 in the form of solution is infused to maintain 3000-4500 ppm of Ca++ in excess.

Initially concentration of NaSO4 in brine after saturator is 12-14 g/l, when it enters in CaCl2 pits, NaSO4 reacts with CaCl2 to form insoluble CaSO3 which settles down. Now the concentration in CaCl2 pits of NaSO4 is about 3-4 g/l. This brine from CaCl2 pits is also enters into the common pit and hence the overall concentration of NaSO4 in brine would be about 6 - 8.5 g/l.

From common pit brine is pumped to the first Reactor (R-5020 A). The flow rate is 105-110m3/hr. In this reactor brine is treated with 8% Barium Carbonate solution to remove any leftover sulphates. After reactor, brine flows to the first Settler (TH- 5010 A). 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.

Brine over flows from the settler and is collected in a Tank D-515. From D-515, brine is pumped to the second Reactor (R-5020 B) where it is treated with 8% solution of Soda Ash (Na2CO3) and Caustic Soda in order to remove Ca+2and 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. 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 (D-5710 A/B) where any brine which was drained previously with sludge and is sent to Common Pit.

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From second settler brine overflows to the Pre-filtered storage tanks (D-5060 A/B). From tanks D-5060 A/B, brine solution is fed to six Pressure Leaf Filters (F-5070 A-F) in parallel manner. Brine filters are required to remove the suspended solids overflowing with the brine from the settler TH-5010B. There are six leaf filters F5070A-F operating in parallel, five normally in operation and one in standby. Brine from tank D-5060 A/B is pumped and distributed to each filter through individual manual valves, before proceeding 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.

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).

Filtered brine comes out of each filter at a flow rate of 18-20 m3/hr and is fed to two Guard Filters (F-5080 A/B) which stops the Arbocel fibers eventually released by pressure leaf filters. These guard filters are of basket type with a total filtering of 2.78 m2 for each filter. Casing of the filters is made of HRLS (hard rubber lined steel) and filtering cloth is of PP (Polypropylene). Sequence is so that one filters in process and one is at standby. From these guard filters, brine is sent to two Storage Tanks (D-5070 A/B).

From storage tanks, brine is pumped to Secondary Brine section after raising its temperature to 55 C by using a plate type Heat Exchanger (E-5070). Material of construction of the heat exchanger is titanium and its heat duty is 905.0 k Cal/hr. Here low pressure steam and cooling water connections are provided to allow the brine heating or cooling in order to control the temperature of the brine at 50-60 oC.

At the end of Primary section, brine has following proportion of impurities in it.

Ca+2 ----------------------------- 8 ppm (maximum)

Mg+2----------------------------- 1.92-2 ppm (maximum)

Sulphates (SO4) -------------- 10 g/l

Reactions Involved in Primary Section

Sulphates

These are precipitated as barium sulphate by reaction with barium carbonate in 1St reactor R-5020 A.

Na2SO4 + BaCO3 BaSO4 + Na2CO3

Calcium

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This is precipitated as calcium carbonate by reaction with sodium carbonate in 2nd reactor R-5020 B.

CaCl2 + Na2CO3 CaCO3+ 2NaCl

Magnesium

This is precipitated as magnesium hydroxide by reaction with caustic soda in 2nd reactor R-5020 B.

MgCl2 + 2NaOH Mg(OH)2+ 2NaCl

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SECONDARY BRINE

Before feeding brine to electolyzers it is passed through ion exchange resin towers C-50400 A/B/C placed in series in order to absorb Ca++,Mg++ and Sr++ contained in brine.

Brine Secondary PurificationThe brine feeding to the resin tower has following typical specifications

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NaCl 290-310 g/LNa2SO4 7-8 g/LNaClO3 15 g/LCa++ 4-6 ppmMg++ 1-2 ppmSr++ 1 ppmTemperature 50-6 0CPH 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-50700 is fed in the tower through P-50700A/B at a flow rate of 105-110 m3/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 column.

Ion Exchange ResinIon 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 TP-260 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. Divalent transition metals and the main group elements tin and lead form highly stable complexes with the chelating amino methyl phosphonic acid group in the range of pH 2-5.The alkaline earth metals are only chelated in alkaline to strongly alkaline solutions.TP-260 is recommended for use when the Ca is chosen as the break through criterion i-e the exhaustion cycle is ended when the 20ppb of the Ca is detected in the pure brine downstream of the first column of the series.

Regeneration of the Resin BedThe 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+

EQUIPMENT DETAILS

TANK (DIL NaOH)

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Capacity 40 m3

MOC PP-RTANK (DIL HCl)

Capacity 30 m3

MOC FRPTANK (Effluents)

Capacity 75 m3

MOC FRPTANK (ULTRA PURE BRINE)

Capacity 75 m3

MOC FRP

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CELL ROOM

Electrolysis Electrolysis is the Phenomenon of decomposition of an electrolyte by passing electric current through its solution.The process of electrolysis is carried out in the presence of

o A liquid containing mobile ions – an electrolyteo An external source of direct electric currento Two solid rods or plates known as electrodes

Mechanism of electrolysisWhen electric current is passed through a solution of electrolytes, it dissociate into its ions. The captions move towards the cathode and form a neutral atom by accepting electrons while anions move towards anode and form a neutral atom by giving electron, thus oxidation takes place at anode and reduction takes place at cathode.

Process of ElectrolysisThe key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit. The required products of electrolysis are in some different physical state from the electrolyte and can be removed by some physical process. For example, in the electrolysis of brine the products are caustic soda, hydrogen and chlorine. The products, H2 & Cl2 are gaseous while caustic soda is in liquid phase. These gaseous products bubble from the electrolyte and are collected in their respective separators. An electric potential is applied across a pair of electrodes immersed in the electrolyte.Each electrode attracts ions that are of opposite charge. Positively – charged ions move towards the cathode bearing negative charge, whereas negatively – charged ions move towards the anode bearing positive charge. At the electrodes, electrons are absorbed or released by the atoms or ions. Those atoms that gain or lose electrons to become charged ions pass into the electrolyte. Those ions that gain or lose electrons to become uncharged atoms separate from the electrolyte.

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ElectrolyzerThe equipment used to carry the process of electrolysis is called electrolyzer. The cell room comprises of four electrolyzer placed in parallel. Each electrolyzer has capacity of 110 cells. 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 ion exchange membrane is made by an organic polymeric matrix incorporating fixed ionic 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 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 DESCRIPTION

Feed BrineUltra pure brine is stored in tank and pumped into over head feed brine tank. Brine enters in the O/H tank from the top and over flows by gravity from the top side back in tank D-51600. 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 to raise its temperature up to 65-75oC. It is a plate type heat exchanger. Heat exchanger is provided with steam as well as cooling water connections. In the cell room main feed brine line has three connections to feed each of the three electrolysers. These supply lines are underneath of each electrolyser. Brine is fed in the anodic compartment of each cell.

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Cell Room Operations In the cell room four electrolysers are arranged in parallel. In each electrolyser 110 elements are placed in series. 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 Supply11KV 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 electrolysers via bus bars and flexible. There are 2 bus bars and 9 flexible associated with each electrolyser.

Reactions Following reactions takes place in the cellsSodium 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 gas 2H+ + 2e- --------- H2

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

The overall cell reaction is 2NaCl + 2H2O ---------- 2NaOH + Cl2 + H2

Side ReactionsThe 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.

Caustic Soda

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31.5-32.5% caustic soda is produced in cathodic compartment of the cell and is collected in the cathloyte header along with hydrogen gas. It leaves the catholyte header by gravity in the main caustic soda header and passed through hydrogen gas separator. Then it is stored in the cathlyte storage tank to maintain its level about 50% for circulation through the cells. It is also transferred to storage tanks as such 32% caustic soda as finished product.

Depleted Brine The stream leaving the cells, composed of depleted brine and chlorine gas, is discharged in the anolyte header and termed as anolyte. The depleted brine leaves the anolyte header by gravity in the depleted brine header and chlorine gas is separated in the vertical gas header which is connected to main chlorine gas header. The depleted brine is passed through the chlorine gas separator in order to remove maximum chlorine gas contents carried with it. The depleted brine is acidified in the acidification pot and then stored in the depleted brine storage tank.

Chlorine GasIn the anodic compartment of the cell chlorine gas is produced as a result of electrolysis. It is separated from depleted brine in the gas header as well as in the gas separator and is collected in the main chlorine gas header.

Hydrogen Gas Hydrogen gas is produced together with caustic soda in the cathodic compartment of the cells and discharged in the catholyte header. It is disengaged from caustic soda in the gas header as well as in the hydrogen gas separator and is collected in the main hydrogen gas header Hydrogen gas is also cooled before it is transferred to furnaces for HCl production.

Nitrogen GasNitrogen gas is very explosive, makes an explosive mixture with O2/air and on getting spark from any source causes expulsion. So it is very important to make system inert with nitrogen gas. Nitrogen gas, as an inert gas, is introduced in the system to make it inert in case of any abnormality in pressure control system or plant shutdown.

EQUIPMENT DETAILSBRINE O/H TANK

Capacity 28m3

MOC PP+FRP

CAUSTIC O/H TANK Capacity 8m3

MOC Ni

CATHOLYTE TANKCapacity 31m3

MOC Ni

HYDROGEN SEPARATORCapacity 1.6m3

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MOC NiAllowable pressure 0.5bar

CHLORINE SEPARATORCapacity 1.2m3

MOC Ti

ACIDIFICATION POTCapacity 0.54m3

MOC FRP

DEPLETED BRINE TANKCapacity 42m3

MOC FRP

DECHLORINATION

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

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

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

precipitation and potentially adverse chlorate conc. in the caustic soda product.o Chlorates have a strong oxidizing effect which is more evident at high concentrations.Chlorates are formed in the anodic compartment of the cell of electrolyser via chemical or electrochemical reactions. The side reactions taking place between chlorine and hydroxyl ions migrated from cathodic side are as

4OH- + Cl2 = ClO- + Cl- + H2O6ClO- + 3H2O = 2ClO3

- + 4Cl- + 1.5O2 +6H + 6e-

3ClO- = ClO3- + 2Cl-

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 is also favors the chlorates removal, a temperature of 85- 90C is recommended.

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The third parameter affecting chlorates removal is pH. A low pH less than 2.0 is favorable.

Process DescriptionDechlorination is carried out in three successive steps

1. By vacuum stripping2. By Sodium Sulphite treatment3. With activated carbon

Depleted brine leaving the electrolyzers flows into the acidification pot where it is mixed with acidified stream of brine from reactor R-50500. Then it is collected in depleted brine storage tank 07D001, from here the brine is pumped. The discharge of the pump is divided into two streams, one leads to vacuum stripper C-50100 and second leads to acid mixing drum DM-50500. Normally 70 to 75% material goes to C-50100 and 25-30% in DM-50500 for decomposition of chlorates. In DM-50500, 33% HCl is added to lower down its pH almost zero, after acidification the brine flows down by gravity into the reactor R-50500 where it is mixed with chlorinated steam coming from the vacuum stripper C-50100. In R-50500 at low pH and high temperature, chlorates are decomposed, producing free chlorine which is vented in the chlorine sniff line. The over flow of R-50500 entered in the acidification pot and lower the pH of the depleted brine ranging (1.5-2.0)

NaClO3 + 6HCl = NaCl + 3Cl2 + 3H2O

Vacuum StripingBrine containing absorbed chlorine enters from the upper part of the packed tower C-50100 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 heat exchanger E-50100. The condensed chlorinated water is returned in the depleted brine tank while the chlorinated steam is sent to reactor R-50500. The brine flows down by gravity in the dechlorinated brine tank D-50200. The tank is vented to chlorine sniff line.

Sodium Sulphite TreatmentSodium sulphite is added to the depleted brine leaving the tank D-50200 by means of an *in line injection* on suction of the pump P-50200A/B.Sodium sulphite solution is prepared in the tank by absorbing of SO2 in sodium carbonate solution and sent in D-53000 by means of pump P-52500A/B. Sulphur dioxide 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 52500A. During chlorates decomposition following reaction takes place

Na2SO3 + Cl2 + H2O -------- Na2SO4 + 2HCl

Activated CarbonAfter 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

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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 from tank D-51500B. The pH is raised up to (6.0-9.0) before it is transferred in primary brine section.

EVAPORATION UNITEvaporatorAn evaporator is used to evaporate a volatile solvent, usually water, from a solution. An evaporator is essentially a heat exchanger in which a liquid is boiled to give a vapour, so that it is also, simultaneously, a low pressure steam generator. It may be possible to make use of this, to treat an evaporator as a low pressure boiler, and to make use of the steam thus produced for further heating in another following evaporator called another effect. Its purpose is to concentrate non-volatile solutes such as organic compounds, inorganic salts, acids or bases. The recovered end product should have optimum solids content consistent with desired product quality and operating economics. It is a unit operation that is used extensively in processing foods, chemicals, pharmaceuticals, fruit juices, dairy products, paper and pulp, and both malt and grain beverages. Also it is a unit operation which, with the possible exception of distillation, is the most energy intensive.The more common types of evaporators include:

o Batch pano Forced circulationo Natural circulationo Wiped filmo Rising film tubularo Plate equivalents of tubular evaporatorso Falling film tubularo Rising/falling film tubular

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Multi-Effect EvaporationIn multiple effect evaporators the same heat energy is used several times. This is affected by aggressively lowering the temperature from effect to effect. Fresh steam is used to heat the first effect. The vapor generated by boiling solvent from the product at a lower temperature is used as heating medium for the second effect which operates at an even lower temperature. In a similar way, this vapor can be used to heat a further effect; thus two, three or higher multiple effect evaporators can be constructed.With such evaporators the water evaporated per unit mass of steam approaches the theoretical optimum of 2:1 for double effect plants, 3:1 for triple effect plants and so on. Thus with an increasing number of effects the specific steam consumption decreases. The necessary temperature difference per effect is achieved by progressively lowering the operating pressure.However, lower operating costs entail a higher initial investment. The most profitable balance dependsOn the individual application criteria considering concentration, performance, annual production times, length of production, product data, cost and availability of energy.

Plant IntroductionThe 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 three falling film evaporators. At first there was only one unit of capacity 120mt/day, with the passage of time capacity of caustic soda plant was enhanced so another evaporation unit of capacity 250 MT/day was designed to meet industrial and consumer’s needs. 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 DescriptionThe caustic is fed to the first evaporator Ev-2010 by a pump at the discharge of caustic buffer tank. On product side the first evaporator is operated under a vacuum. The caustic leaves the first evaporator effect Ev-2010 at a concentration of 36-37%. The first evaporator is heated with vapors formed in the second evaporator Ev-2020. The shell of the evaporator Ev-2010 is also under vacuum about 250-270 mmHg. The condensate of the Ev-2010 collected in the same tank. The vapors of Ev-2010 are condensed in E-2010. The cooling water is fed in the shell and the vapors are condensed in the tubes. The condensate is collected in the tank TK-2010.The vacuum in Ev-2010 is produced through steam ejectors.After the Ev-2010, pump 2010A/B pumped the caustic soda to the second evaporator Ev-2020 via heat exchangers E-2020 & E-2030. The discharge of the pump P- 2010 is divided into two streams; one admitted to the shell of E-2020 and second is admitted in the tubes of the E-2030. These two streams rejoined before entering the Ev-2020. This evaporator effect is heated with

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the vapors from Ev-2030.the pressure in the shell of EV-2020 is (1.9-2.0) bar. The caustic soda enters Ev-2020 at conc. range 36-37%, and temp. range110 -115oC and leaves at conc. 42-43% and temp.120-125C.This caustic soda is fed to the third evaporator Ev-2030 through pump via plate type heat exchangers E-2040 & E-2050. The condensate is collected in condensate tank TK-2020 and then transferred in TK-2010 from where it is transferred in D-51500B. The discharge of pump P-2020 is again divided into two streams; one admitted to the shell of E-2040 and second is admitted in the tubes of the E-2050. These two streams rejoined before entering the Ev-2030. This evaporator effect is heated with the steam at a pressure of about 14bars.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-2030.The caustic leaves this evaporator at 50% conc. at temp. Of 175-185 oC. Now it is to be cooled before storage. For this purpose it is first passed through tube side of E-2040. Here it’s temp. Is reduced to 155-160 oC. Then it is pumped via P-2030 through the tube side of the E-2020 and E-2060, the final temp is about 55-65 oC. The condensate of the evaporator Ev-2030 also bears a sufficient amount of heat, which is recovered by passing it through the shells of E-2050 & E-2030.The steam condensate at a temp about 97-98 oC is returned to boiler section.

Caustic solidification plant (CSP)

Introduction

Caustic solidification plant is designed in order to concentrate 50 % caustic soda to 97-98% caustic soda in solid form. For this purpose a single effete tube bundle falling film evaporator is used as pre-concentrator. 50% NaOH is concentrated up to 60% NaOH using vapors produced in the final concentrator for heating in this equipment. In the final concentrator the concentration is improved up to 97-98% using outside heating media. Due to increasing demand of the solidified caustic soda three CSP units are designed on same pattern named as CSP-I capacity, CSP-II and CSP-III. Another plant named as CSP-IV is under construction yet.

Process description

a) Pre-concentrator:

50% caustic soda stored in caustic buffer tank is pumped via PBT-1&2 to feed into pre-concentrator. The flow of the feed is controlled by LCV-9.11. There are temperature and pressure gauges to indicate the feed temperature and pressure respectively. Pre-concentrator is heated with vapors formed in final concentrator. The pre-concentrator is designed as tube bundle falling film evaporator. Above the tubes there are vertical slits through which caustic soda feed flows evenly into the tubes and form an unbroken film along the inner wall of the tube.

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The pre-concentrator works at 0.93 bar vacuum on the product side. This vacuum is created in the system by steam ejector. A two stage ejector, 14 bar steam, is used for this purpose. The vapors produced in the pre-concentrator are removed by steam ejector and are condensed in a condenser. Vapors are introduced in the shell side of the condenser while cooling water is introduced in the tubes.

Vapors condensed in the condenser are collected in the condensate tank. The condensate of the pre-concentrator also goes into condensate tank by gravity. From condensate tank it may transfer in M-II plant under level control where it is stored it the D-5150B. The condensate may be overflow into the drain. This condensate is also used for sugar solution preparation and for heating the seal of 60% caustic pump A/B.

b) Final concentrator:

60% caustic soda obtained from pre concentrator is pumped via P8.11A/B to feed it into final concentrator. The feed rate is controlled by FIC-9.09. The final concentrator is basically consisting of 6 individual concentrator tubes and the vapor separator. The caustic soda feed is evaporated in the falling film concentrator at atmospheric pressure by means of heat applied by salt heating medium. The feed liquor is uniformly distributed to each concentrator element. It flows evenly with the inner wall of the tubes from top to bottom. The heat transfer medium moves counter currently from bottom to top inside the heating jackets surrounding the concentrator tubes. The vapor produced in the course of the concentration process reached the horizontal collecting channel together with concentrated caustic side. The collecting channel is provided with conic baffle which separate vapors and molten caustic side. The vapors leads to the separator which retains caustic droplets contained in the vapor. These vapors leave the separator from the top and are used to heat the pre concentrator. The molten caustic soda leaves the separator through its lower part and is directed by the distributing device to the flaker and/or drum filling device.

The heating medium is a mixture of following salts in a particular composition.

KNO3 53%

KNO2 40%

NaNO3 7%

c) Sucrose system:

All the equipment and piping in contact with caustic melt is made of pure nickel are low carbon nickel because of corrosive resistant of nickel. Oxygen

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acts corrosively, particularly when combines with high temperature and form nickel oxide which would contaminate the product i.e. caustic melt, so it is very important to prevent the penetration of atmospheric oxygen into the equipment. For this purpose system is blanketed with nitrogen or steam. In addition by adding 0.15 Kg sugar solution into the system just after the feeding valve to pre concentrator, the nickel pickup of the caustic soda melt is reduced to 2 ppm approximately.

5% aqueous solution of sugar is prepared by using food grade sugar in the water/vapor condensate in the sugar tanks and dosed into the caustic system by pumping via sugar pump at a rate of 3 - 5 lit/hr. Flow rate of sugar dose is very important, low flow may cause increasing nickel ppm and excessive flow may leads to blackish particles in the caustic melt.

d) Flaker:

The flaker consists of a rotating drum/cylinder which dips in the vat containing caustic melt. The cylinder is cooled to 60-55 oC by showering cooling water on its inner walls thus a thin film of caustic melt is formed over the drum surface which is scrapped by a sharp blade/scrapper. The flakes thus formed are directly made to fall into the duct which conveys them to the filling hopper where it is provided with an automated weighing and then filling in PPW bags lined with polyethylene bags which is tied with cable ties and then stacked inserting tag cards in the stitching.

e) Dust collector:

For purpose of operational safety and to facilitate the operator working at filling station, a dust collection system is provided. The system consists of a suction fan .It sucks dry air from the flaker, passed through the duct, weighing system and also from the bags. The suction pipe is dipped into a water tank where the caustic soda is precipitated in water in seal pot when its concentration is about 5 to 10%, it is replaced by another water filling.

Normal operating conditions:

Pre –concentrator:

Caustic feed concentrator 50%

Caustic feed flow rate 1.0 to 1.25 m3/hr

Caustic feed temperature 60 to 65 ‘C

Caustic feed pressure 6.4-6.5 bar

Temperature in the separator 90-110 ‘C

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Vapor inlet temperature 350-360 ‘C

Shell side Vapor outlet temperature 160-165 ‘C product side

Vacuum 0.9-0.95 bar

Caustic outlet temperature 68-72 ‘C

Caustic outlet concentration 60-61 %

Sugar dose flow rate 3-5 lit/ hr

Condenser:

Cooling water inlet temperature 30-32 ‘C

Cooling water outlet temperature 35-40 ‘C

Cooling water flow rate 150-200 m3/hr

Steam Ejector:

Steam pressure 13-14 bar

Water flow rate 900-1000 lit/h

Burner:

Fuel gas flow rate 190-210 Nm3/hr

Salt inlet temperature 390-405 ‘C

Salt outlet temperature 420-430 ‘C

Salt coil temperature 420-435 ‘C

Flue gas temperature 475-500 ‘C

Final concentrator:

Caustic feed concentration 60-61 %

Caustic feed flow rate 1.0-1.25 m3/hr

Caustic feed temperature 68-70 ‘C

Caustic temperature in distributor 360-380 ‘C

Caustic temperature in vat 325-330 ‘C

Salt inlet temperature 420-430 ‘C

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Salt outlet temperature 390-405 ‘C

Vapor outlet temperature 360-370 ‘C

Flaker drum:

Cooling water inlet temperature 30-32 ‘C

Cooling water outlet temperature 35-40 ‘C

Cooling water flow rate 50 m3/hr

Cooling water pressure 1.3-1.5 bar

Air pressure 1 bar

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