1. Chlorine Cooling, Filtration and Drying
Prior to compression, the chlorine gas coming from electrolysis unit must be cooled and the water vapour condensed, the brine mist must be removed by filtration and finally be dried.
(a) Chlorine Cooling and Filtration:
The chlorine gas coming from the electrolysers is water vapour saturated according to the electrolysis temperature of 80-90ºC at about 230-240 mbar cell gas over-pressure and anolyte salt concentration of 200-220 gpl.
The gas is cooled down in two steps with two titanium plate-type heat exchangers, Cl2 Cooler 1, and Cl2 Cooler 2.
Cl2 Cooler 1 is operated with cooling water and the chlorine is cooled down from 90ºC to 40ºC. The cooling water flow rate is adjusted manually from a regulating valve at the water outlet side. The Cl2 temperature is maintained on local thermometer.
In the second stage cooling, chlorine is cooled down to 15ºC by means of chilled water. Chilled water flow rate is manually adjusted with regulating control valve at the water outlet side. Temperature is monitored in the control room through temperature recorder. It is important to keep the chlorine temperature in the range 15-20ºC. At temperature less than 10ºC chlorine hydrates will begin to crystallize out in the cooler and obstruct the gas passage. At temperatures above 20ºC, too much water vapour enters the drying tower and sulphuric acid consumption will be accordingly too high. Both high and low temperature limits are alarmed in the control panel.
A wet chlorine filter is installed between Cl2 Cooler 1 and 2. The function of this filter is mainly to separate the brine mist (aerosols) carried along with the gas. The NaCl from the brine mist would be transformed in Na2SO4 and crystallize in the drying tower if not effectively separated in this filter. The filter unit consist mainly of filter body and cartridge internals of glass fibre. Water is continuously sprayed into the gas stream inlet to the filter to keep the cartridges wet and so avoid undue pressure drop increase. The flow rate of demin. water to be sprayed in the chlorine stream entering the filter is adjusted and monitored through flow meter; the value to be adjusted to about 100l/hr. the pressure drop across the filter can be monitored through PDI 1003, and should normally not be higher than 15 mbar and shall be almost constant during a long time period. If the filter internals are clogged they must be replaced. Chlorine bearing condensate from Cl2 Cooler 1 and 2, and flushing water from wet chlorine filter is sent in to the over-pressure safety vessel from where it flows by gravity in to the brine dechlorination system.
Water vapour mist is effectively separated from the Cl2 gas at Cl2 Cooler 2 outlet in the condensate separator. Undue minimum and maximum pressure in the chlorine system is limited by the under-pressure safety vessel and over-pressure safety vessel correspondingly. They are foreseen to actuate only during an emergency, i.e. in case the pressure control system fails. The high pressure is limited to max. 260 mbar over-pressure. In case the pressure in the system is exceeded, Cl2 is blown through the water seal and absorbed by the waste air dechlorination unit. In the other case, if the pressure is below the atmospheric, air is sucked through the water seal of the under-pressure safety vessel. This means first to protect the system against the vacuum conditions. Both safety vessels are continuously fed with water in order to maintain the correct water flow can be monitored with sight glasses at the water over-flow outlet side of both vessels.
(b) Chlorine Drying:
The moist chlorine coming from the second cooling stage is dried by means of sulphuric acid to a final moisture content of max. 12.5 w/w ppm. The drying is performed in one single Cl2 drying tower. The tower consists mainly of two drying sections. In the first section, Cl2 gas is dried by means of 78% H2SO4, which is circulated over a packed column. Final drying is performed with 98% H2SO4 in the top part of the tower consisting of five tunnel trays.
Concentrated sulphuric acid (98%) is fed to the top tunnel tray and flows down by gravity to the trays packed underneath and finally to the packed column. Chlorine gas is fed at the bottom part of the tower and is dried by streaming in counter-current to the acid flow. Water is mainly absorbed in the first drying section and the corresponding amount of absorption heat is generated there. Best drying performance of the tower is at low acid temperature. For this reason absorption heat from circulating acid is removed with chilled water in the 78% H2SO4 cooler. The 78% H2SO4 is continuously recirculated through the packed column with 78% H2SO4 circulation pump.
A minimum flow rate of acid is necessary to achieve an optimum drying efficiency in packed column. The flow rate is monitored and adjusted with flow meter, the value is to be set to 10m3/hr. Another important parameter for good drying efficiency is the absorption temperature.
The acid temperature is to be adjusted between 15 and 20ºC. It is further important to control the acid concentration, which should be set to 78% H2SO4 by feeding the corresponding amount of fresh 98% H2SO4 to the top tunnel tray. The feed rate of 98% H2SO4 is monitored and controlled through flow meter. The concentration of 78% H2SO4 in circulation is regularly checked by sampling and analyzing the acid from the circulation loop. Concentration (98%) H2SO4 is fed to the tower by gravity flow from 98% H2SO4 Head Tank, via the 98% H2SO4 Cooler.
Fresh acid is cooled with chilled water in order to achieve better tray drying efficiency in the tunnel trays. The temperature should be monitored through thermometer and adjusted between 15 and 20ºC. (At 8ºC the acid will solidify).
The pressure drop across the tower packing and tunnel trays are monitored with pressure differential indicators. The normal pressure drop at 100ºC process load is about 15 mbar across the packing and about 30-40 mbar across the tunnel trays. The chlorine gas leaving the tower is entrained with sulphuric acid droplets. These are effectively separated in the H2SO4 Droplet Separator. The separator is provided ith a packing which retains the acid. The separated acid is returned by gravity flow to the top tunnel tray, the flow can be monitored through the sight glass at the tower outlet.
Downstream the H2SO4 droplet separator, a non-return flap valve is provided to avoid the gas back-flow in case the compressor unit trips. The valve is pneumatically actuated and interlocked with PDISAL. Spent H2SO4 overflows from the tower via a siphon to the 78% H2SO4 Storage Tank. The chlorine absorbed in the acid is blown out with air before it is filled into tank cars.
Air supply is opened some hours before the acid is pumped out, and the chlorine content being checked by sampling and analyzing if necessary.
98% H2SO4 is delivered at limits with tank cars, and filled in to the H2SO4 Storage Tank with H2SO4 filling pump. This pump is provided for three discontinuous operations;
§ To fill 98% H2SO4 in to the Storage Tank.
§ To transfer the acid from the tank up in to the 98% H2SO4 Head Tank.
§ To transfer the 78% H2SO4 from the 78% H2SO4 Storage Tank in to the tank cars. A filling station for tank cars is provided for both purposes.
The 98% H2SO4 Storage Tank is provided with a venting seal pot which avoids the entrance of moist ambient air to come into the tank with subsequent danger of corrosion. The liquid seal is sulphuric acid 98%, which is filled with the filling pump.
2. Chlorine Compression and Liquefaction
For the purpose of chlorine storage, the dried chlorine gas must be compressed and liquefied.
(a) Chlorine Compression:
The dried chlorine gas is compressed from about 0.02 barg to 3 barg with one of two sulphuric acid ring compressors. Each compressor unit consists of;
§ A buffer tank at the suction side,
§ The compressor itself,
§ The gas acid separator,
§ The sulphuric acid cooler.
The chlorine is (nearly isothermally) compressed in a liquid piston type compressor. Inlet and discharge parts are located in the impeller hub. As the vaned impeller rotates, centrifugal force drives the sealing liquid (98% H2SO4) against the walls of the elliptical housing, causing the gas to be successively drawn into the vane cavities, and expelled against discharge pressure. The sealing liquid (98% H2SO4) is externally cooled with chilled water by circulation through the H2SO4 Cooler. The gas separator employed in the discharge line is to minimise carryover of entrained liquid. Final separation of H2SO4 entrained in the gas is performed in the dry chlorine filter. It consists of a filter vessel with cartridge internals. The separated acid flows by gravity into the H2SO4 Collecting Tank.