Keywords: Energy balance, Ammonium nitrate, Nitric acid, Cooling fertilizer, Bubble-flow, Heat exchanger.

ENERGY BALANCE IN AN AMMONIUM NITRATE-NITRIC ACID PLANT.

INTRODUCTION.
The dual pressure process was chosen for the nitric acid plant essentially because of conversion efficiency (ratio between ammonia converted in acid to ammonia consumed). The catalyst efficiency gives the best yield at low pressure. The pressure chosen at the burner is 5 bar a. and this compromise between catalyst efficiency and compression energy is considered today to be the optimum. The absorption pressure which is also a compromise between investment and compression efficiency, has been fixed at the level of 11 bar a.

The process is split in two parts with the reaction section at medium pressure and the absorption section at higher pressure.

Process air is mixed with the vaporised ammonia and the mixture is fed to the reactor where the oxidation of ammonia takes place on platinum gauze, producing mainly nitrogen monoxide. This reaction takes place at 900^oC and is exothermic. The heat produced is converted in the process boiler to give high pressure, superheated steam and the remaining heat is transferred to the tail gas where the energy is recovered in the tail gas expander. During this cooling of the gas the monoxide is oxidised to nitrogen dioxide.

The gas is then compressed in the nitrous gas compressor at a pressure of 12 bar for the absorption. In the absorption section the nitrogen dioxide is partially dimerised to the peroxide. In the absorption column this peroxide reacts with the water to produce nitric acid and nitrogen monoxide. This oxidation and dimerisation continues along the column in parallel with the reduction of the content of nitrogen oxides.

All these reactions are exothermic and for this reason the absorption column is a relatively complex construction, with water cooled bundles inside to control the temperature profile of the column. This heat is at too low a level to be re-used in the process.

M Saigne, Hydro Agri France, Ambes, France.

17 Pages, 7 Figures, 1 Table.


COOLING WITH A BULK FLOW HEAT EXCHANGER.

SUMMARY.
The paper describes an innovative piece of equipment for cooling prilled or granular fertilizers. Cooling is by conduction with a water cooled plate bank under mass flow conditions. The two main advantages of this method of cooling compared to fluid bed or drum coolers are zero air emissions and lower capital and operating costs.

The use of the equipment is illustrated by means of a case study describing a project recently completed at the Sherritt plant in Alberta, Canada. Two Bulk Flow Heat Exchangers were installed to provide secondary cooling following the fluid bed coolers. Secondary cooling was required to maintain the temperature of product shipped to storage below 40^oC.

N P Jordison, Cominco Engineering Services Ltd, Calgary, Canada.

10 Pages, 5 Figures, 1 Ref.