Sulphuric acid scrubber

Oleum should be stored in thick-walled mild-steel tanks. As there is considerable risk of corrosion at the gas-liquid interface in an oleum tank, care should be taken to admit only dry air to the tank when it is being emptied. During filling, SO3 vapours should preferably be absorbed from displaced air. Therefore, oleum tanks are vented to the atmosphere via a sulphuric acid scrubber. 

The reactor exit gas is air-cooled to 200°C and then passes to a quench scrubber (B) through which an aqueous solution containing ammonium sulphate 30 wt per cent and sulphuric acid 1 wt per cent is circulated. The exit gas temperature is thereby reduced... 

The ammoniacal water in n is distilled in a column with lime in the usual manner for ammonia (see p. 39), and the gaseous ammonia evolved is often directly sent back into the gas stream (between the coolers a and A) to be fixed by the sulphuric acid in g. Since the amount of deposited gas water here only amounts to about 20 per cent, of the washing water formerly needed to extract the ammonia in scrubbers, it is stated that the cost of the distillation of the. ammonia and the quantity of the troublesome waste water is nmeh less than by the ordinary process of the ammonia extraction. 

As shown in Figure 4.21, the salt that is formed from tertiary amine (e.g. DMEA, DMIA and triethanolamine (TEA)) in the acid scrubber, and the scrubbing acid (e g. sulphuric acid) are retransformed by reaction with a strong base (e.g. caustic soda solution), thereby forming free amine and e.g. sodium sulphate. The amine is expelled with water vapour and is then cleaned and concentrated in a column, to an extent that allows it to be re-used. The sodium sulphate solution (together with polluted scrubbing solution) can be recovered or may be disposed of. 

Caesium-promoted catalyst for sulphuric acid plant has a lower ignition temperature and can result in higher overall conversion of SO2 SO3 but is more costly. It can reduce consumption of alkali in tail gas scrubber as compared to conventional potassium-promoted catalyst. 

The oleum boiler is also provided with a good pressure relief valve to release excess pressure if the rate of condensation is less due to any reason. The vapours are released through a scrubber irrigated with 98 % sulphuric acid to prevent atmospheric pollution. 

Manufacture of sulphuric acid by the modified 3 + 2 DCDA process (having three catalyst passes before interpass absorption tower and two passes after it) with caesium-promoted catalyst resulted in better conversion of SO2 to SO3 and negligible escape of SO2 from exit gases. An alkali scrubber for tail gases was provided for taking care of this. 

The exhaust air, saturated with water vapor, is first treated in an absorption unit for removal of HjS by routing it through two successive j et scrubbers working with dilute caustic soda solution. Downstream of these, a centrifugal scrubber is installed as an entrainment separator. The sulphide-containing solution rejected from the scrubbers is used to precipitate zinc in the waste water treatment system of the production plant. By products adsorbed on the activated carbon, such as sulphuric acid and elemental sulphur, are removed periodically by water and alkaline extraction. 

The first method involves SO3 absorption in a scrubber where concentrated sulphuric acid is circulating. A stoichiometric amount of water is dosed to the SO3 absorber to produce about 98% H2SO4. The amount of water dosed is controlled by measuring the in-line H2SO4 conductivity. 

This system is used when the sulphuric acid produced can be utilised locally or easily sold to third parties. The sulphuric acid produced is of an excellent technical grade. The SO3 absorber tower is also used in emergency situations so that the S03/air flow can be diverted immediately to the absorber tower, preventing a complete plant shut-down procedure. The SO2 in the partially converted mixture will be absorbed in the caustic scrubber, which is part of the exhaust gas cleaning system. The design of the SO2 absorber should be adequate to absorb SO2 effectively during the start-up stage. 

Unconverted SOj, SO3 (mainly as sulphuric acid mist) and entrained organic acid are present in the exhaust gas and are the main potential atmospheric pollutants. The recommended practice is to use an electrostatic precipitator (ESP) to trap any particulate material in the form of mist or aerosols and a caustic soda scrubber which absorbs SO2 and small amounts gaseous SO3 that will pass the ESP. 

The exhaust gas leaving the ESP, virtually free from any organic acid and sulphuric acid mist, enters the SO2 scrubber made of acid-proof construction material. The caustic scrubber equipment consists of ... 

Sulphuric acid mist levels are directly related to the dew-point of the dried air (see 5.2.2.). Ihe sulphuric acid mist is effectively removed in the ESP and levels smaller than 10 mg/m ar guaranteed by ESP suppliers (see also 5.9.1. and 5.9.3.). Very small amounts of gaseous SO3 will pass the ESP but will be trapped in the caustic soda scrubber. 

The resulting dilute sulphuric acid solution is transferred by gravity through Moplen (PVC) piping to a pit where the caustic solution of sodium sulphite (see 7.3.3.) from the SO2 caustic scrubber is also discharged, so that neutralisation takes place. The pit contents (pH 8-10) can be used as a liquid component in slurry preparation for detergent powder manufacture. 

Alternatively, the sulphuric acid/oleum may be piped at a low flow-rate directly to the reservoir of the caustic scrubber, providing that there is an interlock between the operation of the acid pump and the operation of the scrubber. 

Due to the excess of caustic soda in the solution leaving the SO2 scrubber (pH 9-12), this liquid can also be used to neutralise liquid acid effluents that cannot be disposed of in an SO3 absorber and oleum/sulphuric acid ex the SO3 cooler system. To avoid SO2 formation due to decomposition of sodium sulphite (Na2S03), the pH should never be lower than 8. 

Similarly, in RA, reactions occur simultaneously with the component transport and absorptive separation, in the same column zone. These processes are used predominantly for the production of basic chemicals, e.g., sulphuric or nitric acids, and for the removal of components from gas and liquid streams. This can be either the cleanup of process gas streams or the removal of toxic or harmful substances in flue gases. Absorbers or scrubbers where RA is performed are often considered gas-liquid reactors (10). If more attention is paid to the mass transport, these apparatuses are instead treated as absorption units. 

As the sulphur is rotated by the scrubber, polysulphides of ammonia are formed, which are in turn decomposed by the hydrocyanic acid, HCN, of the coal gas, with the production of sulphocyanide of ammonia. 

Hence, it is usual to employ mildly acidic conditions to destroy sulphides in solution in order to minimise the amount of H2O2 required. In the gas phase, hydrogen sulphide can be removed by scrubbing the gas stream with a hydrogen peroxide solution. Here, alkaline conditions are more effective owing to better absorption of H2S and avoidance of sulphur precipitation in the scrubber. 

---