It is mandatory to install Waste Air Dechlorination (WAD) in the Chlor-Alkali Plant in order to comply with the requirements of the applicable environmental legislation. During start up and shut down of the plant, chlorine from the cell house is sent to the sodium hypochlorite plant. In case of any leakage or excess pressure in the process, chlorine will be sucked by the WAD section. The waste chlorine header will be connected to all vulnerable points where there is a chance of any chlorine leakage including vents and seal pots.
The sodium hypochlorite plant consists of the following sections
Chlorine Suction System;
2ndstage Absorption System; and
Chlorine Suction System.
Chlorine gas from chlorine section as well as any leaked chlorine is sucked into the plant through a suction blower. This blower handles only unabsorbed air and water vapors.
1ststage absorption
The 1ststage absorption system shall consist of caustic circulation tank, circulation pump, chilled water PHE and packed column absorber. The 1stabsorber will face concentrated chlorine. The maximum chlorine will get absorbed at this stage. Diluted caustic will be circulated in the absorber from caustic circulation tank and pumped for absorption of chlorine from passing through chilled water PHE to remove the heat of reaction. When the batch is ready the same is transferred to sodium hypochlorite storage tank and fresh diluted caustic is taken from polishing tower / 2ndabsorber tank.
The reaction in the sodium hypochlorite plant can be summarized as below:
2 NaOH + Cl2 →NaOCl + NaCl + H2O
Normally to get a stable product, 10 to 20 gplof excess alkali is maintained in the product and before transferring it is ensured that available chlorine is 150 gpl.
2nd Stage Absorption System
Since some chlorine will escape from 1stabsorber the 2ndstage absorption system will be installed to absorb the remaining chlorine from waste air.
Additionally, this system will consist of one packed absorber with one circulation tank, circulation pumps and plate heat exchanger.
The 2nd stage system consists of:
Chlorine Suction System;
Caustic Dilution and Over Head Tank;
1stStage Absorption System; and
2ndstage Absorption System.
Figure 4-9 2nd Stage Absorption System 4.3.9. Hydrochloric Acid Plant
4.3.9.1. Principle of Operation
HCL is produced as a gas from the combustion of hydrogen and chlorine gases, then absorbed into water and cooled. The combustion process is controlled to provide ultimate safety.
During the combustion process, chlorine and hydrogen react in equal volume to produce HCl gas.
H2 + Cl2 → 2HCl (Gas) (Gas) (Gas)
This exothermic reaction generates a flame temperature of around 2,500°C, and a heat load of approximately 1,000 kCal / kg of HCl when producing a cooled 33% aqueous phase of HCl. The combustion process is operated with a slight excess of H2 over stoichiometric conditions operating the system with this excess of H2 ensures the complete Combustion of chlorine gas, thus preventing it from escaping into atmosphere and producing a pure hydrochloric acid.
4.3.9.2. Synthesis Unit
The heart of the process is synthesis unit (or integrated burner absorber), constructed of impregnated graphite materials encased in a water-cooled shell. The functions / process steps of the synthesis unit include:
Combustion;
Absorption; and
Solution cooling.
Hydrogen and chlorine gas enter the top of the synthesis unit and are burnt into a specific burner designed to ensure the complete combustion. The burner consists of special tubes fitted inside which are easily removable for maintenance.
The addition of absorption water to the HCl gas is carried out at the top of the unit. The gases and absorption water are then cooled down in vertical channels at counter current of service side cooling water circuit. The wetted parts of unit are of the selected material to avoid any corrosion.
At the bottom outlet of absorber, the HCl solution is separated from the gas which is processed in a tail gas tower scrubber to recover the remaining non-absorbed HCl gas.
The synthesis unit is normally operated close to atmospheric pressure, but it is protected against over pressurization with a rupture disc. At the bottom of assembly, and vented to a sump pot beneath the synthesis unit.
4.3.9.3. Tail Gas Tower:
The purpose of the tail gas tower is to absorb or to scrub the gases coming from the bottom of absorber. The gas components are mainly inert with some burnt hydrogen and a little unabsorbed HCl. The absorption water enters the top of the tail gas tower through a distributor and flows downwards over a packing as the gases flow upwards to be exhausted to atmosphere. During this operation, the remaining HCl will be absorbed.
4.3.9.4. Safety:
The overall control philosophy of the HCl synthesis system design is “fail-safe”, which is defined by opening contacts under abnormal conditions. The following safety interlocks are provided to either prevent the system from being started or to cause the system to shut down & the purge itself.
Flame failure;
Low cooling water flow;
Low absorption water flow;
Safety disc breakage;
Very high level in buffer tank;
Low N2 gas pressure while purging the unit during startup; and
Low instrument air pressure gauges.
During operation, if either of the above events occurs, the synthesis unit shuts itself down that a purge with nitrogen can be carried out.