Bleaching Column

Bleaching column Remove dissolved NOx to give product acid. 

Oxidation Unit The oxidation unit is an empty pressure vessel that takes input reaction gases and blends in additional air from the bleaching column. The extra oxygen provided enables further oxidation to occur and raises the gas mixture temperature to 140°C. At the top of the oxidation unit is a mist eliminator to prevent carry over of acid vapour by entrainment. At the bottom of the vessel is the weak-acid drain. The oxidation unit is constructed from SS304L and has a design pressure of 1200 kPa... 

Bleaching Column The bleaching column is a smaller sieve tray-type column. Impure acid runs down the column from the top tray and air is bubbled up through the liquor to remove dissolved nitrogen oxides. The acid from the base of the column is the final desired 60%(wt.) product. 

Part II of the Design Project (Case Study) for the production of nitric acid concerns the design of two main plant units (the NOx absorption column and the steam superheatei), a pump to deliver red product acid from the absorption column to the product bleaching column, and finally a product storage tank. 

A single-stage single-suction centrifugal pump is recommended to deliver red product nitric acid from the base of the absorption tower to the product bleaching column. 

Absorption column Steam superheater Bleaching column delivery tank Nitric acid storage tank... 

Case Study — Bleaching-Column Feed Pump Specification... 

A PUMP is required to deliver red nitric acid product from the base of the NOx absorption column to the product bleaching column. It must transport 9.5 m3/h of this liquid product against a differential head of 450 kPa. 

This chapter considers the sizing, specification and selection of the pump required to deliver red nitric acid from the NOx absorption column to the bleaching column. The final product acid is prepared in... 

The dynamic losses can only be estimated by first sizing the pipe diameter of the line between the absorption column and the bleaching column. This is performed using recommended liquid velocities (Ref. P1, p. 163) in conjunction with the known volumetric flowrate. The area calculated can be translated into a standard pipe diameter. The dynamic losses are then estimated by two methods. The first employs Genereaux s formula (Ref. P1, p. 160) ... 

The pump selected for the transport of red nitric acid from the absorption column to the product bleaching column is a single-stage single-suction centrifugal-type pump. An AJAX E32-20 pump is recommended for this application. The application requires the pump to satisfy a maximum flowrate of 9.5 m3/h with a maximum differential head of 450 kPa. 

The costs of the two sieve-plate columns (absorption column and product bleaching column) are estimated according to the recommendations presented in Ref. CE9 (p.389). The columns are costed according to their diameter, operating pressure, and the number of sieve trays. 

The product bleaching column also operates at 10 bar, but is only 1 m in diameter, and has 10 trays. The final Australian purchase cost is estimated to be As25 000. 

The mass flowrate of tail gas is known from the overall mass balance, but the tail-gas preheater balance can only be performed if the inlet temperature of the tail gas is known, This must be calculated from the specified design conditions for the absorber, and from the heat exchange at the primary tail-gas warmer. Tail gas leaves the absorber at 10°C (design specification) and it passes through the primary tail-gas warmer where it exchanges heat with the secondary air (for the product-acid bleaching column). The air enters at 232°C and leaves at 80°C. Assuming the tail gas has approximately the same heat capacity as air (a realistic assumption), then ... 

In the bleaching column, warm air strips the nitrogen tetroxide out of the red acid to give a product acid completely free of dissolved nitrogen oxides. The red acid enters the top of the column at 45°C and the secondary bleaching air enters the bottom of the column at 80°C. The final product leaves the bottom of the bleaching column at a concentration of 60% wt. nitric acid (40% water). The same type of... 

Absorption column operating pressure 950 kPa Bleaching column operating pressure 950 kPa... 

Agitator vessel for raw mixture Reactor vessel Bleaching column Final acid cooler... 

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