Purpose of Sour Water Strippers

Refinery sour water originates largely from delayed coker, hydrode-sulferizer reactor effluents, and catalytic cracker and visbreaker fractionators. The main contaminants are NHj and H S. Sour water stripper bottoms are reused in two places  

When sour water stripper bottoms are used in the crude unit desalter, the NH should be about 10 to 20 ppm. Higher NH levels 

When sour water stripper bottoms are used as a makeup to hydrotreater effluent washwater, an NH3 content of a few hundred ppm is fine. After all, the recirculated washwater has over 10,000 ppm of NH3 anyway. Therefore, the NHj content of the makeup washwater is not critical, as long as 90-i- percent of the NH3 in the sour water stripper tower is stripped out. 

The reflux is generated in the E-2 circulating cooler. The reboiler duty is adjusted so as to control the moisture content (i.e., temperature) of the ammonia-rich gas flowing to the sulfur recovery plant thermal reactor. Too high a temperature will cause too much moisture in the sulfur plant feed. Too cool a temperature will cause NHj salts to plug the vapor feed line to the sulfur plant. 

This design never made much sense to me. Why preheat the feed onto tray 32 with E-1, and then remove this heat in E-2 Wouldn t it have the same effect on the stripping trays to  

---