In considering the problem of sulphuric acid concentration, it was decided to apply the techniques and materials of construction used so successfully in nitric acid concentration. Although the processes are different, the acid concentrations and temperatures are often very similar. Use of glass lined steel, tantalum, teflon lined steel and zirconium made it possible to realize in practice the concept of a multiple stage vacuum evaporation process. Furthermore, the use of gravity flow systems and natural circulation loops allowed the use of pumps in hot acid services to be minimized while ensuring high heat transfer co-efficients. In the early seventies, Air Products and Chemicals Inc. contracted with C-I-L Inc. to design a sulphuric acid concentration unit (SAC) for their Pasadena, Texas plant. Critical aspects of the design were modelled and studied in pilot plant equipment and the full scale plant was completed in 1975. Since that time 18 more sulphuric acid concentrators have been built by Chemetics, varying in capacity from 70 to 800 tons per day on a 100% basis and concentrating from as low as 7% H2SO4 up to 96% H2SO4.
Process Description
A Chemetics Sulphuric Acid Recovery and Concentration plant consists of a Stripping Column and a two stage, steam heated, sulphuric acid concentrator (SAC) designed to produce 96% H2SO4 Product Acid.
Feed Acid at 70% H2SO4 is pumped to the Feed/Product lnterchanger to preheat the feed acid with heat exchanged from the hot product acid exiting the Stage 2 Separator. The preheated feed acid enters the top of the stripping column. The stripping column is made up of two separate columns arranged in series, the Upper Stripping Column and the Lower Stripping Column. The acid flows down the columns, counter current to rising steam. As the steam rises from the bottom to the top of the two columns, it strips and carries the organic components from the downward flowing liquid stream of spent acid. Steam is fed into the column in two locations; direct steam is fed into the bottom of the Lower Stripping Column and steam is also fed into the Stripper Reboiler located at the bottom of the Lower Stripping Column.
The overhead steam and organic vapours from the column flow to the Stripper Condenser and condense as a two phase distillate. The distillate flows by gravity into the Stripper Decanter and the organic phase is separated from the aqueous phase. The organic phase is recovered and pumped to the client for rework. The aqueous phase flows to the SAC Distillate Tank. The non-condensible gases from the condenser are relatively high in NOx content and are sent to battery limits for treatment by client. The stripped acid from the bottom of the Lower Stripping Column flows by gravity into the Stage-1 Evaporator of the SAC.
The SAC unit will be a two stage plant. Each evaporator/separator stage consists of a Separator with a side mounted, vertical Evaporator, containing a bayonet type reboiler arranged in a natural circulation loop. The acid flows by gravity from the first through to the last stage of the SAC. Heat is supplied to the Evaporators by a controlled flow of steam. The required vacuum in each stage is maintained by steam ejectors followed by shell and tube condensers.
The vapours from the Evaporators flow through the Separators. In the Separators, the vapours pass through a mesh pad Mist Eliminator which removes residual acid droplets. The exit vapours from the Stage 1 Separator is condensed in a shell and tube Process Condenser. A circulating stream of distillate is sprayed on to the tubesheet of the condenser, to desuperheat the vapour and to prevent corrosion damage from strong acid condensing from the vapours. The distillate. then flows via a barometric leg to the Distillate Tank and is pumped to the battery limits. The non-condensible gases from the condenser are vented via the Vacuum System to battery limits for treatment by the client.
The vapours from the Stage 2 Separator enter the Acid Scrubber where a circulating stream of cooled intermediate strength acid absorbs and condenses the water and acid vapours. The remaining non-condensible gases are vented to the Stage 1 Process Condenser through the Primary Ejector, which controls the vacuum level of the Stage 2 Separator.
The hot Product Acid flows by gravity from the Stage 2 Separator through the Feed/Product lnterchanger and into the Product Acid Tank via a barometric leg. The acid is further cooled by recirculation through the Product Acid Cooler and then pumped to battery limits.
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