CHLORINE COMPRESSION AND LIQUEFACTION
Both compressors are arranged in parallel (one as a stand by). Each unit has an individual start-up by-pass line for gas circulation between discharge and suction side. A common by-pass line for both compressors is employed for suction pressure control.
Concentrated Sulphuric Acid necessary for operating the compressors is delivered by a pipe line from the 98% H2SO4 Storage Tank through H2SO4 Filling Pump.
The spent acid from the compressor concentration should never be less than 95% H2SO4 and the acid collected in the collecting tank is padded out by the chlorine gas at 3 barg pressure and sent to the 78% H2SO4 Storage Tank.
For the correct operation of the compression unit, following control and monitoring operations are necessary.
§ Keep suction pressure at a slight positive value. This measure is to avoid moist air sucked into the chlorine system and to detect leakages if any.
§ Monitor discharge pressure, value to be set to about 3 barg.
§ Monitor the sulphuric acid discharge temperature, which should never be higher than 40ºC.
§ Monitor the sulphuric acid discharge temperature from the coolers. Temperature should never be less than 15 ºC (solidification point of 98% H2SO4 at 8ºC).
§ Measure regularly pH value of chilled water outlet from the coolers (acid entrainment possible when cooler tubes are corroded).
§ Check concentration of circulating acid. The value should never be less than 95% (danger of corrosion).
§ Monitor acid flow through the sealing rings.
The compressor acid will slowly be diluted from 98% to 95% H2SO4 due to final (max. 12.5 w/w ppm) concentration of water vapour in the chlorine coming from the drying tower. When the lowest allowable concentration limit is reached, the acid must be replaced. For a safe operation of the compressor, it is recommendable to change over to the stand-by compressor. Then drain a part of the acid with running compressor and replace with fresh acid, circulate the acid for some time and check concentration by sampling and analysis.
For safe operation during compressor change over, proceed as follows;
§ One compressor is in operation, the stand-by compressor is to be started.
§ Open compressor suction side and internal by-pass line valves.
§ Start the compressor; discharge pressure should normally be less than 3 barg.
§ Open discharge valve (back flow is avoided through check valve).
§ Close by-pass valve slowly monitoring discharge pressure. As soon as the discharge pressure is equal to the discharge pressure of the other compressor in line, start change over operation.
§ Slowly close by-pass valve and simultaneously open by-pass valve of compressor in line monitoring the suction pressure, which should be held constant. This mode avoids pressure fluctuations in the chlorine system upstream.
§ When the by-pass line of the first compressor is fully open, close discharge, valve, stop the compressor and close suction valve.
CHLORINE LIQUEFACTION
The dried and compressed chlorine gas is liquefied in one single stage in the Chlorine Liquefier. The liquefied chlorine flows by gravity via a siphon in to the Chlorine Storage Tanks. The siphon must be always vented to the compressor discharge. The sniff gas is sent to the Hydrochloric Acid Synthesis or alternatively to the waste air dechlorination unit.
During normal operating conditions, the tail gas from the liquefier is sent to the HCl Synthesis Unit. If the process load of HCl Unit is decreased, excess chlorine tail gas is sent to the waste air dechlorination unit.
The chlorine liquefier is part of the liquefaction and refrigeration unit.
The refrigeration unit consists essentially of an assembly of two refrigerant compressors (one as stand-by) together with a water cooled refrigerant condenser, a refrigerant collecting flask, a suction super heater and oil separator. The refrigerant (Freon or Freon R22) is evaporated in the chlorine liquefier (a tube bundle heat exchanger) absorbing the corresponding heat to condense the chlorine. The evaporated refrigerant leaves the liquefier, passes the suction super heater and is then compressed with the refrigerant compressor and further liquefied in the refrigerant condenser by means of cooling water. The liquefied refrigerant passes the collecting flask and is transferred to the chlorine liquefier via the suction super heater.
The liquefaction temperature (pressure) of the refrigerant is maintained constant by temperature control valve at cooling water outlet.
The liquefaction capacity of the chlorine liquefier, a function of the liquefaction temperature and pressure, can be varied in the range of 0-100%. For this purpose the chlorine liquefaction temperature is adjusted by means of the refrigerant suction pressure controller. This controller controls the hot refrigerant gas by-pass valve, and so the refrigerant evaporation capacity of the liquefier. The set point of the pressure controller is controlled by the chlorine pressure controller. In addition the compressor unit has four capacity steps, 100%, 75%, 50% and 25% for further capacity control in order to keep the suction pressure in the design range.
When the load on the chlorine liquefier falls from 100%, the refrigerant compressor suction pressure will also start to fall. Variation from design suction pressure will be detected by pressure controller, which will increase the pneumatic signal to the hot gas by-pass control valve and thus allow hot discharge gas ti enter the chlorine liquefier and maintain design pressure.
As the process load continues to fall, the hot gas valve will open fully. When this point is reached and if the process load continues to fall, the suction pressure will start to drop as the hot gas by-pass valve is unable to maintain full load. This further drop in pressure will be detected by pressure transmitter, which will send an electrical signal to a step controller to unload the machine to 75% capacity. If at that stage the process load represents 75% plant capacity, the hot gas by-pass valve will close because the suction pressure will start to rise. Adjustable time delays and differentials are fitted to the step controller to prevent this machine loading back to 100% capacity immediately.
A further decrease in the plant load will again cause the suction pressure to fall which will be detected by the pressure controller which will then start to re-open the hot gas by-pass valve. Further decrease in process loads will cause the compressor to unload to the 50%, 25% capacities. Further decrease to the 0% process load will again open the hot gas valve fully.
At this stage, the hot gas valve will provide enough heat to maintain the compressor running at 25% capacity. If the situation is reached it will then be possible to manually stop the plant.
Any increase in process load will cause the suction pressure to rise and the present controller will act to close the transmitter to load the compressor up through its capacity steps.
The capacity of the liquefaction unit is designed to liquefy at least 95% of the incoming chlorine gas. The tail gas is enriched with the inert gases as O2, N2 and H2. The hydrogen content in the tail gas should be normally low (less than 0.5 v/v %), as well as he H2 contents in the cell gas is depreciable under normal operating conditions (not detectable with the Orsat gas analysis, i.e. less than 0.5 vol. %).
Any appreciable increment in the hydrogen content in the tail gas under same liquefaction conditions should be an indication that hydrogen is passing through any of the several cell element separators (membranes). The source of high hydrogen must be found out as the potential danger exists to exceed the safety limit given at 4 v/v % H2, above which the H2 and Cl2 gas mixture is explosive.
A sample port is provided for analysis purposes at the tail gas outlet in order to analyze (once per shift) the H2 contents.
The liquefaction temperature can be monitored on local and remote thermometers located on the liquid chlorine line outlet of the liquefier. Also the temperature of the sniff gas can be monitored on thermometer.
Under upset conditions, liquid chlorine flow into the waste air dechlorination line. In that case, chlorine will evaporate in the liquid chlorine siphon trap and the low temperature will be detected and alarmed.