ITTEHAD CHEMICAL INTERNSHIP REPORT

Hamidmalik Aftab 8/16/14 Internship

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Caustic Soda Production Unit

http://www.ineris.fr/ippc/sites/default/interactive/brefca/bref_gb_ptmo.htm

Sodium hydroxide (NaOH), also known as lye and caustic soda, is an inorganic compound. Sodium

hydroxide is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner.

Sodium hydroxide is industrially produced as a 50% solution by variations of the electrolytic

chloralkali process. Chlorine gas is also produced in this process. Solid sodium hydroxide is

obtained from this solution by the evaporation of water. Solid sodium hydroxide is most commonly

sold as flakes, prills, and cast blocks.

Steps For Caustic Soda Production:

Process of Caustic Soda is divided in following plants:

1. Brine Purification (I.E.M) 2. Production Cell

3. Evaporation Unit 4. Flacks Formation

5. Packing Unit.

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Brine Purification for I.E.M:

Brine is a solution of salt (usually sodium chloride) in water up to about 26% (a typical saturated solution, depending on temperature).

I.E.M brine purification unit provides feed of brine solution to ion exchange membrane cell room. The brine solution feed must be ultra-pure. The ideal concentration of brine is 280g/l.

Following are the main components of I.E.M brine unit.

1. Saturator

2. Purifier

3. Settler

4. Primary and Secondary Filters

5. Ion Exchange Unit

1.Saturator:

Saturator consists of large storage of rock salt. At the rock salt, depleted and dechlorinated brine, also called anolite is showered in order to concentrate it. The anolite at a temperature

of 65-70 oC. The brine solution is stored in 3 pits from where it is sent to first purifier.

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2.Purifier:

Removal of Sulphates:

Solution of barium carbonate in which some quantity of hydrochloric acid has been added, is introduced into first purifier. Capacity of first purifier is 70m3. Excess carbonates are

maintained in the solution to insure removal of sulphate ions.

(0.2-0.4% excess in brine feed to DSA cell room and 0.4-0.5% excess in brine feed to I.E.M cell room). Sludge formed is periodically removed from bottom of the first puri fier.

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Removal of Carbonates:

Overflow from first purifier goes to second purifier where caustic solution is added into it.

Addition of Accofloq:

From purifier the brine is transferred to settler tank before the introduction of brine into the

settler, a flocculent named “Accofloq” is added.

Purifier is made of mild steel and lined with fiber glass.

Settler:

Settler has capacity of 1000m3, brine overflows from settler after settling. A scraper is also

installed in the center, near the bottom of the settler to push the sludge settled, towards the bottom of the tank where an automatic gate valve opens for 15 sec. After every 15 minutes.

From settler, the brine moves to a storage tank. Settler is made of mild steel and coated with

a type of resin which is corrosion resistant.

Pr imary and Secondary Filters:

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Primary Filter:

From settler, the brine moves to a series of filters where 5 different layers of pebbles are

used to filter the brine solution. The layer at the top is of anthracite and remaining layer is of pebbles whose size increases from top to bottom.

Secondary Fi l ters:

In secondary filters, 140 tubes of CPVC are installed, sleeves and a sock is mounted over it.

Alpha cellulose is filled into the filter o wet the sock with it and subsequently drained before the purifier start operation. And then the brine is passed through the tubes.

Impurities stick with alpha cellulose and are washed later during regeneration. From secondary filters, brine is pumped to feed tank for ion exchange unit.

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Ion Exchange Unit:

From secondary filters, brine is transferred to ion exchange unit, where an ion exchange resin of general formula R-Na2 is used to completely remove all impurities out of it. The

purified brine is then sent to I.E.M cell room.

I.E.M unit:

The cells are not only the most important but also the most component of cell rooms.

Construction:

A cell consists of following parts.

1. Bulk Heads 2. Bulk Head Insulators 3. Inter Cells Spacers

4. Current Distributers (made of coppers with mesh of nickel sticking over it) 5. Membranes g

6. Electrode Plates Cathode is made of Nickel Anode is made of titanium

Electrodes are activated with coating of noble metals to minimize voltage drop. Life of electrodes depends on chlorine formed. Greater the chlorine lesser will be the life.

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Brine is injected from bottom right side of the cells and caustic (28%) is injected from

bottom left side of the tank. While product i.e. 31% caustic is recovered from top right side of the tank and hydrogen is

also recovered from the same side. While depleted brine, also called anolite is recovered from top left side of the cell and

chlorine is also recovered from the same side.

Drain line are provided at bottom side while overflow lines are provided at top side of the tank, both brine and caustic.

Working/Chemical Reactions:

Sodium ions travel towards the cathode through semi-permeable membrane.

Sodium ions are absorbed by the membrane and transferred to other side when more ions approach it.

Chloride ions travel towards anode and are oxidized to form chlorine gas.

Hence chlorine is collected from anodes and hydrogen and caustic of required concentration i.e. 31% is collected at cathode.

Each cell consumes approximately 9 volts. From the power plant 13KV are received and stepped down to 370 volts.

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Assembling and Disassembling of cell:

For the process disassembling to start the cell is taken out of circuit by attaching a jumper in parallel to the cell to avoid any disturbance in operation of other cells. Cell is the drained to empty it from brine and caustic solution is sent to caustic circulation tank. A part of drain line is transparent to make visible the flow of draining liquid from the cell. Than the cell is brought to ground level where it is washed and then cell rotation frame is used to change its position from horizontal to vertical in order to disassemble it. Both side rods and nuts of top bulk head are removed. After removing the bulk head insulator, the whole assembly is disassembled. All the components are washed and checked for any leakage using

pressurize air frame.

The assembling process is the reverse operation of disassembling operation.

Purging operation:

Air is used for purging of hydrogen from the whole system or the cell when they are

taken out of the circuit. Nitrogen is used only when chlorine is required to be taken out of the whole system.

De-chlorination Unit:

Depleted brine is sent to depleted brine tank at ground level, from where it is pumped to an overhead de-chlorination tower. The tower is randomly packed with rashing rings.

Brine enters from the top and both dechlorinated brine as well as chlorine is removed from bottom of the tower from two outlets at same level.

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The chlorine is sent to heat exchanger at higher level where it temperature reduces from

89oC to approximately 50oC. This chlorine is pumped with the help of liquid ring type rotary compressor and passed through depleted brine tank and finally sent to hydrochloric

acid furnaces. The liquid used to lubricate pump is also sent to depleted brine tank. The de-chlorinated brine is sent to chlorinated brine tank, from where it is sent to saturators

of I.E.M. brine unit.

Multiple Effect is an evaporation:

From I.E.M. Cell room we obtain caustic soda at concentration of 31% which is not the demand of customers. So to achieve 50% concentration multiple effect heat exchangers are used for evaporation. In ICL there is two units for evaporation Unit-1 contain Plate & Shell type heat exchanger while Unit-2 only contain Shell Type heat exchangers. Feed is Pre-Heated by utilizing the leaving stream (50% caustic Soda), before entering to main heat exchangers. Unit-1 id forward feed heat exchangers and Unit-2 is backward feed heat exchangers.

A Multiple Effect is an evaporation unit which consists of several evaporators thermally

linked and thus reducing considerably energy consumption.

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Evaporators is operating at reduced pressure (vacuum). Consequently, they are linked up so that the vapour given off in an evaporator can be used to heat a subsequent one, which operates at a

lower pressure, and so on. In such arrangement effects can be evaporators and crystallizers, of any type and also stripping column and flash vessels. We have flash vessels.The first effect is

heated directly using shell and tube heat exchange and in next effects are heated by process vapours from the previous effects in plate and frame heat exchanger. The last effect process

vapours are condensed on a heat sink. Heating is achieved commonly with live steam, but can also be done with a heat transfer fluid such as thermal oil or a molten salt, or waste heat from other parts of the plant like dryer vapour for example. In considering the heat balance for a single effect evaporator, the enthalpy of the evaporated vapour is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1T/hr of vapor will be produced by 1T/h of live steam.

In a case of a triple effect evaporation plant 1T/h of live steam will be able to evaporate 3T/h of water, so only 33% of the energy requirement of a single effect. Say of a triple effect, with the use of maximum only 33% of live steam in the triple effect evaporation plant compared to the single effect evaporation plant you are still able to do the same evaporation.

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We are using counter current arrangement in heat exchange with flash vessel 2 and 3 and mix (

co and counter ) arrangement with 1.Co-current, counter current, parallel or mixed flow of process solution with respect to live steam are possible based on process requirements and

energy optimization. In case of a co-current multiple effect the concentrated process solution leaves the unit at the cold end, in case of a counter-current flow, the concentrated process

solution leaves at the hot end of the evaporation plant.

For large evaporation capacity the number of effect can also be governed by maximum construction size of an evaporator as well as profitability of energy recovery.

Alumina refineries are used to concentrate Sodium aluminate liquor know as spent liquor (mainly 5 or 6 effects falling film plants with or without salting out and associated flash tanks, but also 3 or 4 effects).

They are also used for black liquor concentration in the pulp and paper industry (7 or 8 effects),

Caustic soda concentration (3 effects is almost standard).

The main feature of multiple effect evaporators are as follow:

• Live steam consumption decreases as number of effects increases

• Cooling water consumption in condenser decreases as number of effects increases • Co-current, counter-current, parallel or mixed flow

• Any type of evaporator or crystallizers, stripping column or flash vessels can be used