Reducing the Environmental Impact of Oil Refining Processes - Sour Water Stripping Unit

Just like any other chemical processes industry, the oil refining presents a great environmental impact. Through decades engineers, scientists and researchers have dedicated efforts to minimize the environmental footprint from petroleum refining.    

Some of the major impacts produced by crude oil processing are the water and atmospheric emissions. 

The petroleum derivates production needs a large amount of water for cooling fluid, steam generation or to direct use in the process like in the crude oil desalting step. Water has become an increasingly scarce resource and any effort dedicated to reducing the volume applied in the process is welcome. 

One of the most important environmental process units in a petroleum refinery is the so-called sour water stripping unit. Sour water is the water that had contact with the petroleum or his derivates during some step in the process, this contact can be like rectification steam in distillations columns or in contact with hydrocarbon phases. Contaminants like NH3 and H2S tends to concentrate in the aqueous phase, so the sour water commonly has a high concentration of these compounds.

The Sour Water Stripping Unit apply the concept of fluid rectification with steam and the partial pressure reduction to move the phase equilibrium to the vapor phase, releasing the contaminants from the liquid, like presented in Figure 1. 

Figure 1 – Phase Equilibrium in Sour Water Stripping Process

Like any other process technology, the sour water stripping unities was developed and improved along the time, mainly to reduce atmospheric emissions and to raise the water reuse in the refineries.

The initial design concept for sour water stripping units had one rectifying tower, in this tower both contaminants (NH3 and H2S) were removed and form the stream called sour gas like described in Figure 2.

Figure 2 – Typical Arrangement for Sour Water Stripping Unit with Single Tower 

In these cases, the tower operates with relatively low pressure (about 1,0 kgf/cm2).

Initially, the designs predict to send the sour gas to burn in fired heaters, like in distillation units. Nowadays, with the environmental restrictions and the necessity to reduce SOx and NOx emissions the project concept were changed and the sour gases are directed to sulfur recovery units with a chamber to convert the NH3 to N2, this is necessary to avoid that the NH3 prejudice the H2S conversion in elemental sulfur through Claus process. 

The modern designs rely upon the installation of two towers, one for the H2S removal and the second for the NH3 removal like described in Figure 3.

Figure 3 – Typical Arrangement for Sour Water Stripping Unit with Two Towers 

For units with two towers, the H2S rectifier operates under pressures about 5 to 11 kgf/cm2, while the ammonia rectifier operates under pressures about 1 to 2 kgf/cm2.

The arrangement with two towers show some advantages in relation to the project with a single tower, once that allows higher recovery of H2S like elemental sulfur, reducing the SOx emissions. Furthermore, the design with two towers allows recovering the ammonia present in the sour water or converts this stream to N2.

As a disadvantage in comparison with the single tower design, we can mention the higher initial investment, higher energy consumption and increased operational complexity.

Read original by Dr. Marcio Wagner da Silva

Process Engineer and Project Manager at Petrobras