Evaporation section of H3PO4 plant

Evaporation section:

Filter acid stored in a weak acid storage tank is pumped to evaporation section by evaporator feed pumps. The line is provided with an isolation valve to disconnect the entire evaporation section if required a magnetic flow meter to measure the feed acid going to evaporator and a control valve to maintain constant flow of acid as desired.

The evaporator circuit consists of a flash chamber where vapor generated due to boiling is separated with minimum entrainment,  a down corner which forms the suction for forced circulation pump,  a high capacity low head axial flow pump, a heat exchanger to supply the necessary heat for affective boiling. As the acid is continuously charged into the system, the level in the flash chamber builds up, this is taken out through overflow valve. The acid that overflows goes to the suction of strong acid pump where it is pumped to strong acid storage tanks. All the vessels and pipelines are of mild steel rubber lined and since there is a limitation on the operating temperature of these vessels and in view of boiling under vacuum to ensure that the temperature is below 95ºC. The vapors coming out of flash chamber are led to barometric condenser where most of the water vapor evaporated is condensed. The non condensable gases then go to vacuum pump which maintains the required vacuum to separate vapor by gravity. In addition, the vapor separator design allows for a large surface of the liquid to be exposed to the vapor space to allow the heat absorbed in the heat exchanger to be flashed off into the vapor space. The circulating liquid is introduced tangentially into the vapor separator so that the spinning liquid forms a vortex to expose a considerably large surface to the vacuum.

Since the evaporation is to be carried out under vacuum and limitations of materials of construction, the vapors generated are to be continuously condensed as they are generated to reduce the load on the vacuum producing device. Accordingly, the vapors are routed to barometric condenser, which uses seawater as a cooling medium. The condenser is of mild steel rubber lined and disc and doughnut type. As the water cascades down, the vapors come in contact with them and get cooled and condensed. The water after condensing vapors gets heated and then through a barometric leg goes to a sump from where outflows to effluent trench.

The non condensable from the barometric condenser, mainly consisting of air and dissolved gases, are removed by means of a vacuum pump to maintain the required vacuum. The three evaporators are operated in a forward feed manner and the product acid, through line is routed to strong acid storage tanks from where it is exported to fertilizer plant by means of strong acid export pumps.