Air pollution control using laser analysis – Case Study

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Courtesy of Siemens Industry, Inc. - Process Analytics

An LDS 6 analyzer monitors HF emissions during the production of hydrofluoric acid in accordance with the German Technical Instructions on Air Quality Control (TA Luft)

The customer

Fine chemicals and inorganic basic chemicals have been developed and produced at the Seelze chemical site in northern Germany for more than 100 years.

Such a basic chemical is hydrofluoric acid (HF), an extremely corrosive, colorless, and pungent smelling liquid. It even attacks glass and has a strong caustic effect on skin, mucous membranes, and the corneas of the eyes. The production of hydrofluoric acid causes emissions which must be monitored. The use of analyzing equipment approved for this purpose is mandatory. The limit values for production equipment in Germany are defined by the factory inspectorate in accordance with TA Luft.

The challenge

HF emissions occur at various points during the production of hydrofluoric acid (HF) from fluorite (CaF2) and sulfuric acid (H2SO4):

During the drying of fluorite

The moist fluorite (moisture content approx. 10%) is dried in a continuous flow drum at 800 °C. This drum is heated indirectly. The drum exhaust gases are collected, purified and discharged via a stack. The end product is dry CaF2 (moisture content < 0.1%).

During the production of HF

Fluorite and sulfuric acid are premixed and transferred to a heated rotary kiln. The conversion to hydrogen fluoride (HF) and anhydride (CaSO4) takes place here. The kiln exhaust gases are passed through a heat exchanger and then discharged via the stack. The end products are hydrogen fluoride (HF) and calcium sulfate (CaSO4).

During the milling of anhydride

The anhydride produced along with the hydrofluoric acid is neutralized using calcium hydrate and subsequently milled. The resulting emissions are collected, purified and likewise discharged through the exhaust stack. The end product is calcium sulfate (CaSO4).

In the bunker exhaust

The CaF2 produced during the fluorite drying process is transferred by a chain conveyor to the fluorite bunker. The displaced air is collected, purified and discharged via the stack.

The HF limit value to be monitored in the plant is 3 mg/m³.

To reduce the emissions, the collected exhaust gas is routed through and purified in a six-stage wet scrubber.

The exhaust gas following the wet scrubber is routed into the environment via a GRP stack (30 m; diam. = 0.49 m). Two ventilators support the natural draft effect of the stack. The volumetric flow is 6 000 m³/h.

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