This section of the plant will have flexibility to operate on only 50% caustic soda production (Mode A) and flaking (Mode B) and production of both (Mode C). All the
4.3.7.1. Mode A - Evaporation only
The 32% membrane cell liquor will be fed to the first stage falling film evaporator operating on product side under a vacuum. During a single pass, the caustic solution is evaporated to approximately 38%. The generated vapors are led via duct to a water cooled surface condenser where they are indirectly condensed. Inert gasses are sucked off by a watering vacuum pump. The 38% caustic solution discharged from the bottom part of the first stage evaporator by means of pump is heated to a temperature of 136 °C through heat exchange.
During a single pass through the second stage falling film evaporator operating on product side at atmospheric pressure, the caustic solution is concentrated to 50%. The vapor generated within is used to heat the first effect evaporator.
The second stage evaporator will be heated by steam. The steam condensate will be used to preheat the 38% solution and returns at battery limit. The vapor condensate will be collected in a Tank and leaves the plant at a temperature of approx. 73 0C. The 50%
Caustic solution will be discharged from the bottom part of the second stage by means of pump (P-1201 A/B). It will be cooled down to 45 Chlor-Alkali Plant °C before being sent to Battery Limit through heat exchange.
4.3.7.2. Mode B - Concentration/ Flaking
The 32% membrane cell liquor will be fed to the first stage falling film evaporator operating on product side under a vacuum of about 113 mbar abs. During a single pass through the evaporator, the caustic solution will be evaporated to approximately 46%. The generated vapors will be led via duct to a surface condenser, where they will be indirectly condensed by cooling water. Inert gases are sucked off by watering vacuum pump.
The 46% caustic solution, which will be discharged from the bottom part of the first stage evaporator by means of pump, is heated to a temperature of 128 °C (typical value) through heat exchanger. During a single pass through the second stage falling film evaporator operating on product side at atmospheric pressure, the caustic solution will be concentrated to about 61%. The vapor generated will be used to heat the first effect evaporator along with the vapor coming from the concentration and flaking unit. The second stage evaporator will be heated by steam. The steam condensate will be used to preheat the 46% solution and returns at battery limit. The vapor condensate will be collected in a tank and will leave the plant at a temperature of approx. 74 °C.
The 61% caustic solution from the evaporation will be fed to a specially designed falling film concentrator. During a single pass through this concentrator, the NaOH solution will be dehydrated from 61% up to about 98.5%. The concentrator operates on product side under atmospheric pressure and the generated vapor will be used to heat the first stage evaporator in the evaporation plant.
The heat required for the concentration from 61% up to 98.5% is transferred by molten salt. The heat transfer salt is circulated by pump through natural gas fired heater, where
it is heated to approximately 430 °C. The flue gases pass through a combustion air pre- heater and are then vented through a stack into the atmosphere. The burner placed on top of the molten salt heater, is designed for operation with preheated combustion air. The highly concentrated caustic melt leaves the final concentrator through seal pot and is fed by gravity to a specially designed flaking machine, where it is processed into flakes at low temperature. The flakes are fed by gravity to a semi-automatic bagging scale. The bagging scale is of air tight design in order to avoid moisture pick-up of the hygroscopic flakes. The filled bag will be transported via a conveyor to the bag closing machine. The bag is a double walled bag with a PE inline and a PP woven outer bag. The PE inline is closed manually by a drill binder and the PP woven outer bag will be closed by a sewing machine.
4.3.7.3. Mode C - Evaporation and Concentration/ Flaking
In this mode liquid caustic and flakes can be produced at the same time. The process description is, in principle, the same as for Mode B concentration and flaking.
However, the process parameters are slightly different. The concentration after the first evaporation stage is about 40% NaOH. The concentration after the second evaporation stage is 50% NaOH.
After the second evaporation stage the caustic flow is split into two streams, one feeding the final concentrator and the second one leading to the battery limit. The caustic going to the battery limit is passes through a set of two heat exchangers where the caustic is cooled down to a temperature of 45 °C.
The concentration plant is working with the same principle as described under mode B except that the feed concentration is 50% instead of 61% NaOH. The plant is controlled by instruments to ensure fully automatic operation of the process. Operating personnel are required only at the bagging and bag closing station. However, start up and shut down of the plant requires some manual interactions.