A desulphurisation unit at a high levelLevel measurements in the desulphurisation unit
During the production of cokes approximately 65,000 Nm³/h of coke gas is produced. This crude gas is purified of coal water, tar, benzol, naphthalene, ammonia and hydrogen sulphide. The sulphur in the coke gas mainly occurs in the form of H2S. The incineration of the gas causes dry or wet (“acid rain”) deposition in the region.. To reduce these emissions, ArcelorMittal Gent built a gas desulphurisation unit in 1997. Since then, pure sulphur in liquid form (temperature approximately 140 °C) has been recovered from the gas and SO2 emissions have dropped dramatically.
In a first phase the hydrogen sulphide and ammonia cleaning takes place by pumping water in counterflow with the gas through a series of three washers, being through NH3 washer 2, through NH3 washer 1 and finally through the H2S washer.
The enriched NH3 and H2S washing water is stored in buffer tanks with a content of 635 m³, and from there it is pumped to the deacidification unit and the distillation column of the desulphurisation unit.
In the deacidification unit, the sulphur compounds are distilled out with steam (which is called “stripping” of the H2S gas) with a head temperature of ± 94°C; in the NH3 distillation column the free and fixed ammonia are distilled out with steam and by adding NaOH (“stripping” of the NH3 gas). Because of the aggressive nature of these vapours, this installation is made of special titanium alloys.
The liquid level in the columns is measured during the desulphurisation process.
To this end, the pressure difference P1 – P2 between both extractions at the bottom and at the top of the closed vessel is measured with a pressure transmitter. To protect our measuring equipment against the aggressive vapours, we inject nitrogen in both measuring lines (to make them inert) with a double bubble pipe system We use a flow controller (rota meter) to set the nitrogen flow rate. The flow for both extractions is set at 85 l/h.
The pressure of the nitrogen must be high enough to displace both the total liquid head and the overpressure. Too great a pressure would, however, cause the level in the vessel to fluctuate. For the deacidification unit, we chose 1.5 bar.
Imagine that the vessel is empty, or the fluid level zero. Then the pressure is P1 = P2 = overpressure = 250 mbar, in other words the pressure difference P1 – P2 is 0 mbar.
When the liquid level increases to the maximum, or 900 mm water column, the measured pressure difference is:
0,09 kg/cm² or 0,09 x 0,981 bar or 88,29 mbar
This pressure is a measure for the height of the fluid level.
The transmitter is controlled in such a way that an analogue signal of 4 to 20 mA corresponds to a pressure difference of 0 to 88.29 mbar or a level between 0 and 90 cm. This analogue signal is further processed in the automatic operating system that controls the supply of enriched H2S water.
Each flow controller (rota meter) is fitted with a flow control switch. When the nitrogen flow rate falls below 50 l/h, this switch causes an alarm signal on the computer screen in the control room. This indicates a blockage in one of the measuring lines that results in incorrect level measuring. The measuring equipment is built into a heated synthetic housing. The measuring lines have been wrapped with special electrically heated tracing tape and then thermally insulated.