Dual pressure nitric acid technology with high energy recovery

Nitric acid is a colourless strong acid used mostly in the production of nitrogen fertilisers, either in liquid or solid state.

The industrial production of nitric acid process uses the Ostwald method where ammonia (NH₃) is oxidised on platinum-rhodium (Pt/Rh) gauzes to form nitric oxide (NO) that is further converted into nitric dioxide (NO₂) and absorbed in water to form nitric acid (HNO₃). The production process can be divided into two groups depending on the pressure levels applied. The nitric acid production can take place at one pressure level (mono pressure) or two pressure levels (dual pressure). The former requires less equipment and is selected for low capacities (<600MTPD) while for larger capacities (>600MTPD) dual pressure processes are preferred.

In this paper, the dual pressure nitric acid technology of Stamicarbon is described. The process has been designed to guarantee high energy recovery. This is achieved by an appropriate selection of the heat exchanger’s position downstream of the ammonia burner. The main feature of the process is the high tail gas temperature, up to 480°C. This is accomplished by locating the last tail gas heater closer to the Pt/Rh gauzes in the ammonia burner vessel, where higher process temperatures are encountered.

The high tail gas temperature has several advantages. On the one hand, the N₂O decomposition can be performed in a tertiary abatement system without the addition of an external reducing agent such as natural gas. On the other hand, more power can be generated by the expansion turbine, decreasing the amount of work required of the steam turbine. In addition, the expanded tail gas still contains valuable heat that can be exchanged in a tail gas/tail gas heat exchanger, allowing a lower temperature in the stack and therefore, more heat recovered in the system. All these advantages are translated into higher steam export and therefore, more energy savings.

A careful arrangement of the heat exchangers also enables the minimisation of the well-known condensation/re-evaporation effect and hence, corrosion. In turn, this allows the use of standard construction materials which significantly reduces the manufacturing cost of the heat exchangers.

Paz Muñoz, Stamicarbon B.V., Sittard, The Netherlands

15 pages, 3 figures