Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 66.2.4 Process gas treatmentProcess gases leaving the reactor contain, besides dust and ingressed air, H 2 O, SO 2 , CO 2 ,sulphur gas, SiF 4 and others, with amounts depending on the quality of the fluorspar used. Themain functions of this part of an HF plant are:• to remove CaF 2 and CaSO 4 dust• to condensate HF, and• to remove the low and high boiling impurities from the crude HF.There are different possibilities available to achieve these objectives as shown in Figure 6.3.H 2 SO 4FinalH 2SO 4scrubberExhaust gas tocentral gas treatmentand suction systemH 2 O, (HF)Processgasesfrom kilnorH2 SO 4 scrubberorHF scrubberSulphur trapHF condenserDistillationLow boilersorSiF 4absorptionH 2 SiF 6 solutionfor sale or wasteH 2 SO 4FeedH 2SO 4mixerOleumAnhydrous HFstorageFeed H 2 SO 4to reactorFigure 6.3: Process gas treatment options[22, CEFIC, 2000]In most HF plants, process gases are first scrubbed in a pre-purification column withconcentrated H 2 SO 4 to remove dust and water and to cool the gas below 100 ºC. Secondaryscrubbing and quenching is carried out with liquid HF to remove any remaining dust, H 2 SO 4and water and to cool the gas to approximately 20 ºC. In this HF scrubber, sulphur gas ispartially desublimated. Alternatively, it is possible to avoid using both scrubbers or only thesecond scrubber and pass the gases through a cooled sulphur trap instead. The HF scrubber andthe sulphur trap have to be cleaned periodically in order to remove any desublimated sulphur:the frequency of cleaning depends on the quality of the raw materials.The cooled and purified gases are then passed through condensers, using chilled water or brineas a cooling medium. Here most of the HF is liquified, part of the liquid HF is fed to the HFscrubber, and the other part, representing proper HF production, is passed to the storage or to adistillation column to remove the dissolved low boiling substances, mainly SO 2 and SiF 4 . TheLarge Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 261