HYDROCHLORIC ACID SYNTHESIS, STORAGE AND FILLING
The present operating instructions describe the production of 33% hydrochloric acid, from chlorine and hydrogen gas produced in the membrane electrolysis plant.
HCl Synthesis:
The unit consists of the HCl synthesis unit with Tail Gas Scrubber, Steam Ejector, HCl Collecting Tank and integrated Absorber.
The synthesis of HCl is performed by combustion of H2 and Cl2 gas. Both the reaction and absorption heat in water are highly exothermic. The reaction is;
½ H2 + ½ Cl2 ------------> HCl (g) + 1667 kJ/mol
For optimum combustion and low free chlorine content in acid, the gases are burnt with about 10% excess volume flow of hydrogen. The content of free chlorine can so be reduced to max. 10 w/w ppm Cl2.
The HCl gas which contains inerts and the previously mentioned excess of hydrogen is absorbed in demineralised water in the falling film absorber to produce hydrochloric acid of max. 33% w/w. The acid flows by gravity to the HCl Collecting Tank. The tail gas from the oven is sent to the HCl Scrubber where the remaining HCl gas is absorbed in counter flow to the demineralised water.
The heat generated during the combustion and absorption is eliminated from the jacket cooled graphite blocks by cooling water.
The cooling water pressure had been reduced according to the maximum design pressure of the unit.
The tail gas jet is used for start-up of the synthesis to suck in sufficient air as a replacement for chlorine.
The demineralised water which is used for the absorption of the HCl gas is fed to the top of the Tail Gas Scrubber from where it flows by gravity via the packings to the falling film absorber of the synthesis. This quantity is controlled according to the concentration of the produced hydrochloric acid and assures constant quality over the full load range of the unit.
The acid from the HCl Collecting Tank is pumped with HCl pump to the HCl Storage Tank and to different internal consumers.
The hydrogen gas sent to synthesis come from the H2 Cooler at a temperature of about 40ºC. Before entering the burner it passes through the H2 Flame Arrestor, function is to avoid occasional flame back flow in to the H2 pipeline.
The dry chlorine used for synthesis comes from the compression and liquefaction unit. A branch connection with moist chlorine gas from the bleaching powder unit also supplies chlorine to the synthesis. Nitrogen necessary for start-up and shut-down operations is provided by N2 flasks battery arranged nearby the furnace unit.
Design Features:
The construction material of the HCl Furnace is Diabon (an impregnated graphite material). The furnace consists of a combustion chamber and a long life quartz/graphite burner, and a isothermal falling film absorber, built as an integral part of the wall of the combustion chamber. Combustion chamber and absorber are incorporated in a steel shell which acts as a cooling jacket. This unit is protected with a diabon bursting disc due to over-pressure (max. 0.3 bar) initiates discharge of the hydrogen chloride vapours at the top of the unit. The high response accuracy rupture disc absorbs explosions and the steel shell will resist to even more violent explosions, which are practically only possible in case of mal-operations. The unit is connected to the cooling water circulation system; the maximum allowable cooling water pressure is 4 barg.
The tail gas scrubber is a packed tower with incorporated demister.
Operation of the HCl Synthesis Unit:
The unit is operated at a slight over-pressure. The use of suction with the steam jet is only necessary for starting up to take up the required atmospheric oxygen. Ignition of the burner is effected manually with a H2 torch.
An emergency cut-off system is provided to avoid damage by overheating or explosion.
An automatic monitoring and control cabinet is locally provided for the safe operation of the unit. This monitoring unit is connected to the main control room.
Control and Interlocking System:
Control Instrumentation
1. Capacity Control
The production capacity of the HCl synthesis is proportional to the flow of hydrogen (plus 10% excess). The flow of the chlorine gas is controlled by FRCAHL-2202 which acts on control valve FV-2202 in the dry chlorine gas line. Moist chlorine stream from the bleaching powder unit is discontinuous and flow rate relatively small compared with the main Cl2 stream. For start-up operation FV-2202 is by-passed with flow meter FI-2205 to adjust the Cl2 flow manually at site. The flow of hydrogen is controlled by the ratio controller FFIC-2201, which signalizes the set value to controller FRCAHL-2201. Last one acts on control valve FV-2201 on the hydrogen line to the burner. For start-up FV-2201 is by-passed by flow meter FI-2204 to adjust the H2 flow manually at site.
2. Demin. Water Control
The flow rate of demineralised water is a function of the acid concentration at the synthesis outlet. The acid concentration is determinate in function of the density measured in both density meters DI-2202 and DIAHL-2201. Density measurement is temperature compensated by TIAH-2204 in the acid exit line. The flow rate of water is controlled by FICSAHL-2203, which acts on control valve FV-2203 on the feed line to the Tail Gas Scrubber.
3. Level and HCl Pressure Control
The level in the HCl Collecting Tank 22-D002A is monitored and controlled by the local controller LICAHL-2201, which acts on control valve LV-2201 on the acid transfer line to the storage tank. In order to establish a constant pressure in the acid supply line to the several internal consumers, a pressure controller PIC-2206 is provided on the pump discharge side, which acts of pressure control valve PV-2206 in the by-pass line to the collecting tank.
Safety Interlocking of HCl Synthesis:
A cut-off of HCl synthesis is initiated by the following faulty conditions giving alarm in the main control panel.
FFRSAHL-2201 Low flow ratio of H2 to Cl2
HS-2204 Overrides low flow ratio
FICSHAL-2203 Low flow of absorption water
FISAL-2206 Low flow of cooling water
PISAL-2201 Low H2 header pressure
PISAHL-2202 Low or high Cl2 header pressure
HS-2201 Overrides low or high Cl2 header pressure
BSAL-2201 Flame failure
HS-2205 Overrides flame failure
Operation of the HCl Synthesis:
§ Concentration
The HCl synthesis furnace is designed to produce hydrochloric acid with 33% w/w. at higher concentration, the HCl contents in the tail gas increases. At a lower acid concentration than 33%, the capacity of the storage tank in terms of HCl 100% is reduced correspondingly. For the above-stated it is therefore recommended to adjust the acid concentration in the range from 30 to 33%.
§ Temperature
The HCl synthesis is designed for an outlet temperature of the produced acid of maximum 43ºC at the maximum cooling water supply temperature max. 35ºC. A higher acid temperature results in a higher HCl concentration in the tail gas. It is therefore recommended that the maximum temperature difference for the cooling water shall not exceed 10ºC.
§ Hydrogen Excess
For a safe operation of the furnace, the hydrogen by chlorine ratio should be adjusted to 1.05 (FFIC-2201) at higher loads. Below 50% this ratio should be increased to max. 1.1. An indication for a satisfactory ratio is the chlorine content in the acid, which should be below 10w/w ppm. The higher the hydrogen excess, the lower the chlorine contents and vice versa. The upper limit is determined by the efficient absorption of HCl gas to approx. 1.15.
