SUMMER TRAINING
DURING 2003
AT
PACL (PUNJAB ALKALIES AND CHEMICALS LTD)
Head Office: SCO 125-127, Sector 17-B, Chandigarh.
Works: PACL, Nangal-Una Road, Naya Nangal, Distt.
Ropar, Punjab. 140126
Before Starting with the training details I would like to
thank Mr. J.P.S. Rawat, G.M. (Works) for giving me
the opportunity to have training at their plant. I would like give my sincere
thanks to the followings for providing their support during my training and
giving me valuable knowledge and information regarding the process:
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1.
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Mr. Sunil Gupta
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Executive Head – Unit I
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2.
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Mr. M.S. Saini
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DGM – HRD
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3.
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Mr. T.S. Mann
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Head – Instrument Department
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4.
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Mr. H.S. Dass
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Quality Control Department
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5.
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Mr. R.C. Arora
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Chemical
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6.
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Mr. S.K. Singh
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Safety Department
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SAFETY DEPARTMENT: Training started
with the Safety department. Any plant is operated with 3 Ms i.e. Man, Machine
and Material and all these are interlinked with each other. 95% of the
accidents in any plant happen because of unsafe act and rest 5% because of
unsafe conditions. Example of this can be that a person is riding a scooter
without break (unsafe condition) but knowing that it has no break becomes
unsafe act.
There are three routes through which the chemicals can
infect body:
1.
Cutaneous: Through Skin
2.
Pulmonary: Through inhalation/breathing
3.
Gastrointestinal: Through injection
For our safety during working in hazardous
environment we need to plug these three.
There are two basic types of safety equipment
we should use for:
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Respiratory – Mask
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Non-Respiratory – Gloves, Helmets,
Safety shoes etc.
For these hazardous chemicals International
numbers have been provided like:
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CAS: Chemical Abstract Service Number
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UNN: United Nation Number
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HAZCHEM: Hazardous Chemical Number
There are a total of 9 classes of Chemicals like
Corrosive, Toxic, Flammable, Non-Flammable,
Radioactive etc.
Pressure Test: Since Chlorine gas
is stored in liquid form and it is in pressurized condition when stored in the
cylinders so pressure test for those containers is required. During the
pressure test pressure 1.5 times more than the actual pressure in maintained
and then container is checked for the leaks. For safety purpose water is used
for the test. Test need to be done once in 2 years.
Work Permit System: For
the purpose of safety if any construction, modification, welding etc. job is
done in the plant then prior to that work permit is needed for the safety
purpose. This ensures that the contractors are aware about the safety measures
so that we can prevent any type of damage to Man or Machine.
Color Coding: Since any plant consists
of many pipelines for the distribution of various liquids and gases so color
coding makes is easier for knowing what is there in any pipeline. Various color
codes are:
Red – HCL
Skyblue
– Brine
Pink – Caustic
Green – D.M.Water
Blue – Air
Yellow – Chlorine
ELECTRICAL DEPARTMENT: It is
necessary to discuss the electrical department here because in the Cell House
(also called the Heart of the plant) DC is used instead of AC. Bhakhra Dam is the source of electricity. From there 66 kV
supply is obtained. We can discuss here that high voltage is used for the
Transmission purpose (66 kV) because we can transfer the electricity with low
current and hence low transmission losses. P = VI. In the plant this High
Voltage is converted to low voltage of 415 V through Transformers. The cell
house is designed to operate at 110 V DC and for doing that we need Rectifiers.
To measure the current and voltage values we use Current Transformers and
Voltage Transformers which convert the current and voltage level to a required
value. (e.g. Current ratio is 600:1 and Votage ratio is 66kV:110V).
For the protection purpose or stopping power during
maintenance, Isolators and Breakers are used.
There are Two Power Transformers of 35 MVA each. The
ratio is 66kV/11kV.
Then there are Auxiliary Transformers
of 2 MVA capacity each. The ratio here is 11kV/415V.
This power is fed to the MCC for running all the machines
of the plant.
For the Cell house there are rectifiers and for that
there are separate Transformers. These converts 11kV to 110V.
Then this 110V AC is fed to the rectifiers and converted to 97 V DC. The
current here is 25kA which goes to each electrolyser.
There are total of 6 electrolysers and total current
of 25x6 = 150 kA is used for the electrolysis purpose.
There are some critical areas where power supply is
always needed and for that there are D.G. Sets.
There are around 400 motors in the plant which contains
pumps, fans blowers, compressors etc.
To improve the power factor Capacitor Banks are
used.
MECHANICAL DEPARTMENT: This
sections contains the following sections:
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Welding Machine Section
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Drilling Machines Section
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Shaper
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Laith machine section (Manual and Automatic)
INSTRUMENTATION DEPARTMENT: This section deals with all the instrumentation e.g.
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Temperature Transmitter
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Pressure Transmitter
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Level Transmitter
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Flow Transmitter
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Density Measurement equipments
Along with this DCS (Distributed Control System) is also
monitored for any fault.
QUALITY CONTROL DEPARTMENT: (QCD):
This section deals with the Sampling of all the Raw Materials, Bye Products and
Final Products and then checking them for their Quality.
e.g. Sampling is done of Salt, Barium Carbonate, Soda ash
(Na2CO3), Calcium Carbonate, Sulphuric Acid, Heat Treatment Salt, Sodium
Bi-sulphate, HDPE bags, LDPE bags etc.
There are various methods for sampling for various
materials. Example for salt sample is collected from top side, back side and
these are mixed to make one sample.
To check the quality of the samples it need to be checked
with various standard processes which includes preparation of the sample and
then titration to know about the %age of the impurities.
BRINE PROCESSING: It consists of
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Brine Saturation
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Brine Preparation
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Brine Precipitation
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Brine Clarification
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Brine Filtration
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Brine Polishing
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Secondary Brine Purification
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Pure Brine Conditioning
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Anolytic dechlorination
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Chlorate destruction
Brine Saturation: It consists of
Saturators. Saturators are charged with salt with the help of bucket elevators
via common feed hoppers. The lean brine containing 220-230 gpl NaCl is fed to
the saturators through dip pipes. In Saturators lean brine rises from bottom to
top through the salt bed and during this passage lean brine gets almost
saturated. Insolubles settles at bottom of saturator. These insolubles are
removed periodically by draining the saturation to sludge pit. The crude brine
leaves the saturator through strainers in the overflow launder at the top of
saturator. The crude brine flows to precipitator tank by gravity. The PH of
brine in saturator shall be kept below 9.0 to avoid the dissolution of silica
in brine.
Brine Precipitation: The
impurities like Sulphate, Calcium, Magnisium present in salt are precipitated
as BaSO4, CaCO3 and Mg(OH)2 in the precipitator tanks.
BaCO3 and Sodium Carbonate solution (10%) are added into
the 1st precipitator tank and caustic soda is added to the 2nd
Tank. As the brine temperature is only about 60 deg C, approx. 400 mgpl Na2CO3
and 200 mgpl NaOH excess is to be maintained in crude brine. The crude brine
containing BaSO4, CaCO3 and Mg (OH)2 precipitate flows to clarifier feed well
by gravity. The precipitation tanks are connected with agitation air to keep
the precipitation in suspension.
Brine Clarification: After
precipitation the brine goes by gravity to Clarifier. Here a centralized raker
keep rotating with slow speed to push the settled sludge towards the center.
This sludge is then pumped out directly through the vacuum sludge filter with
the help of hose pump. In this stage around 99% of the solids are settled down.
The
superlatent overflows into clarified brine tank. For better settling of solids
flocculent is added to brine. This helps in formation of larger flocks out of
very fine precipitates(Particularly of Mg(OH)2).
Brine Filtration: Brine from
Clarifier still contains approx 20 ppm
suspension solids. In order to achieve a reasonable performance from brine
polishing filters these solids must be removed beforehand. This removal is done
in brine filters.
Brine filtration consists of 5 identical filters. Four
are in line and 5th acts as standby.
The filters are filled with anthracite of 5 particle
sizes. The top layer consists of fine particles and acts as the filter media.
The lower layer is meant for supporting the top layer. Distributing headers are
provided with perforation for uniform distribution of brine. Brined enters the
filter housing from top. By means of pump pressure brine is forced through the
filters and solids are retained on top surface of filter bed. The filterate is
collected by the distribution pipes located in the lower part of filter housing
and from there flows to the filtered brine tank. To remove the sludge collected
over the filter media periodic backwashing of filters is necessary. Backwashing
is done once in 6 hours i.e. one filter in line for 24 hours before being taken
out for backwash. The filtered brine consists of approx 2-3 ppm suspended solids flow to filtered brine tank. The brine may
contain some of Al in the dissolved state which can be quite harmful to
membrane.
Al is precipitated by reducing the PH to approx. 7.5 to
8.0. The PH reduction is carried out by adding HCL. The precipitated Al is
removed in polishing filters.
Brine Polishing: Before sending the
brine to Ion-Exchange columns it is send to the polishing units to remove the
final traces of suspended solids. The polishing unit consists of 3 identical
filters two operating and one standby. Brine polishing system consists of three
filters housing containing PP filters candles each. Candles are hung from a
plate bolted to the shell. By means of pump pressure the brine is forced
through the candles and the solids are retained on outer surface. A discharged
channel is installed in the upper part of filters. The pressure drop across filters must not exceed 1.5 bar. Normally one
filter is backwashed after 48 hours of operation depending on quality of brine.
From polished brine taken the brine is pumped to Cl2 recuperator where this
brine is heated by hot Cl2 from cells and then through brine recuperator where
it is further heated to 70-75 deg C before it goes to the ion-exchange column.
Secondary Brine Purification: Pure
brine from polishing filters contains calcium and Mg Cations. These impurities
have to be removed to a concentration of less than 30 ppb to make the brine suitable for membrane cells. The
purification is done in two ion-exchange columns. The columns are arranged in
series. The columns are filled with Duolite C467, a chelating cation exchange
resin with a very hight selectivity for Ca and Mg and Strontium Cations.
Pure Brine Heat Conditioning: For
optimum performance of electrolysers these must be operated at a specified
temperature which means the brine and caustic must be fed to electrolysers at
specified temperatures. The exact temperature however depends on plant load,
condition of membrane and coating. The brine after I.E. passes through brine
H.E. and both operating in parallel. To ensure steady flow of brine to
electrolyser the head tank is allowed to overflow continually. The overflow
should be kept bare min. to reduce the sucking in of air (thereby dust by
overflowing brine). Steam for heating is derived from the steam pressure
reducing station such that saturator steam at 0.5 bar is supplied to H.E.
(Since higher temp. of steam can damage the gasket of brine H.E.)
Chlorate Destruction:
Because of Side reaction, Chlorate is formed in the electrolysis cells and
built up a chlorate level in the brine circuit. In order to maintain a chlorate
concentration of less than 7 gpl measured as NaClO3, a chlorate destruction
unit is provided. The chlorate decomposition takes places with HCL. A good
reaction efficiency(above 80%) is achieved at temperature above 95% and at HCL
concentration not less than 20 gpl in brine.
Anolyte Dechlorination: The depleted brine coming from
cells is saturated with Chlorine. Before the brine is resaturated and purified,
it shall be completely dechlorinated. The brine leaving the cells at a Ph of
4-4.5. For dechlorination the PH must be reduce to approx. 2.0. This is done by
acidified brine from the chlorate destruction tank or by adding 30% HCl from
the head tank to the anolyte. The chlorine saturated anolyte flows by gravity
from cells via anolyste header to anolyte tank. From here anolyte is pumped to
vacuum tank using the existing anolyte pump. In the vacuum tank anolyte is
allowed to boil under vacuum. In the process of boiling Cl2 is desorbed. The
chlorine gas is sent to the chlorine header via chlorate destruction tank. The
lean brine is transferred to the saturator via brine recuperator with the help
of existing brine pump.
CHLORINE TREATMENT UNIT: In
this Unit chlorine gas from the Cell house is first Cooled in the Chlorine
coolers in number of Stages. For initial Stages chilled water, then chilled
brine is used. Cl2 is then compressed in Cl2 compressors and then impurities
are separated in filters and finally sent to liquefier where F-22 is used and
Cl2 converts to liquid Cl2 and sent for the Storage. For storing Cl2 gas tonner
of capacity 900 kg are used. These are tested with 35 kg/cm2 pressure whereas
the pressure of Cl2 is 6-7 kg/cm2 max. to 10 kg/cm2. For the insulation purpose
thermocol or PUF is used.
Chlorine is used by various Segments like:
Water Supply
Paper Industry
Medicine
Fertilizer Industry
Chemical Industries
Refineries
CELL HOUSE: In the Cell house based on membrane process
the basic part is membrane. It acts as a partition between anode and Cathode
compartment of cell. Membrane are currently available to produce caustic conc.
Of up to 35% NaOH with Optimum performance. A minimum of 3 years life of
membrane is expected from economical point of view. The ion-exchange membrane
only allows Na+ to flow across it and blocks every other ion.
Reaction is as follows:
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Anode (+)
Membraneà
NaCl (Brine – 310 gpl)
Na+ + Cl-
Na+-------------------------------------------------à
e- --------------------------------------------------à
2Cl- = Cl2 + 2e-
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Cathode (-)
NaOH (28%)
H+ + OH-
Na+ + OH- = NaOH (35%)
2H+ + e- = H2
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