Multi-effect evaporation – negative pressure in the first effect
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We have built a four-effect evaporation system using steam at 0.4 MPAG; the wastewater to be treated is sodium chloride-based. The temperature in the last effect is 45 degrees. Based on theoretical calculations, the evaporation temperature in the first effect should be around 120 degrees. However, during testing, the actual temperature is only 85 degrees, which seems abnormal. One possible reason for this is that the condensate from the steam enters a condensate tank. This tank has a connection to the shell side of the heater in the second effect, and the non-condensable gases from that section are connected to a vacuum pump, creating a negative pressure. Nevertheless, the amount of steam removed by the vacuum pump is relatively small. Please help analyze this situation
There isn’t enough information! ! Please check what the pressure and temperature are for each effect What is the inlet pressure of the vacuum pump? Four effects – how is the vacuum pump connected to the second effect?
Your analysis is likely correct; it should be caused by a vacuum series connection. When the secondary steam generated in the first evaporation stage is introduced into the shell side of the second evaporator, the non-condensable gases from the shell side of the second evaporator should be vented directly, without connecting them to a condensate return line or a vacuum pump in series.
The steam enters the shell side of the first effect; the condensation tank is connected to the vacuum unit – can this ensure a pressure of 0.4 MPa? Is there an error in the process?
The condensate from the first stage can be discharged directly without the need for buffering in a condensate tank, but a steam trap must be installed. I haven’t seen the entire process, so I’m not sure if there’s a problem somewhere.