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

6.4 Techniques to consider in the determination of BAT6.4.1 Heat transfer designDescriptionChapter 6One of the main heat transfer problems in HF manufacturing comes from the limitation forsupplying enough energy for the main reaction. This limitation is due to:• the properties of the powder products, namely that they become viscous and adherent, andthe corrosive products, thereby limiting the choice of the materials• the large surface area of the equipment, leading to significant heat losses, along with walltemperature limitations resulting from the corrosion and mechanical resistance of theequipment.Table 6.9 shows four options to improve the energy transfer. See also Figure 6.2.Process0 Basic process123Double jacketsand optimisedtemperatureprofile controlPreheating theH 2 SO 4Preheating thefluorspar4 Pre-reactorDescriptionIn the basic process, dried fluorspar and feed sulphuric acid are fed at a givenratio directly to a rotary kiln. All the energy needed for the endothermic reactionis supplied by heating the kiln shell. This indirect heating leads to a higherheating gas temperature resulting from the production levels needed expressed astonnes of HF/m 2 of kiln surface area. Consequently, in some cases, higher kilncorrosion levels, poorer anhydrite and crude HF qualities may occur.The first improvement of this basic process involves the installation of severaldouble jackets around the rotary kiln and feeding the heating gases in atdecreasing temperatures from the inlet to the outlet of the kiln, thereby allowing abetter control of the temperature profile inside the kiln along with animprovement in energy usage. This improvement is only cost effective in newplants as the investment needed to modify the jacket and associated piping of anexisting unit is cost prohibitive.The second improvement is achieved by the preheating of H 2 SO 4 before it is fedinto the kiln. Preheating the sulphuric acid feed is generally easy, but specialconstruction materials are needed if it contains some HF.More complex equipment is needed for indirect preheating of the powderyfluorspar. Dust entrainment (either from the preheater itself or from the kiln)significantly increases when the fluorspar temperature increases, due to thehigher reactivity of the hot reactants, which in turn leads to a limited maximumtemperature. Direct preheating can be achieved for calcining fluorspar by heatingdirectly with gases containing oxygen to approximately 400 – 450 °C. Thisoption has the advantage of removing most of the flotation chemicals, giving aproduct that mixes more easily with the sulphuric acid feed and increasing thereaction rate. Consequently, the kiln workload is considerably reduced. Thedisadvantage is that the fluorspar calcining equipment involves a substantialinvestment.Another improvement involves feeding the raw materials into a pre-reactorbefore feeding them to the kiln. This rotary equipment, which is designed to mixand heat the reactants to give a conversion rate of CaF 2 of around 50 %, isexpensive as it is complex and made of special corrosion resistant alloys. In fact,as it must handle sticky, corrosive and hot reactants, it is subjected to severecorrosion. The substantial conversion of CaF 2 in the pre-reactor has twoimportant beneficial consequences: it lowers the kiln workload and it suppressesthe pasty and sticky state in the kiln almost completely, which gives better heattransfer and avoids many scaling problems. Using a pre-reactor leads to lowerheating gas temperatures, and consequently to lower corrosion risks in the kilnitself, and to a more stable reaction.Table 6.9: Options for improving the energy transfer to the main reactionLarge Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 267