Sour Water Stripper Bottom Temperature 2

[sheiko]

Hi,

We use superheated steam (+150 degC superheat) as live steam in a SWS.

I was wondering if using saturated steam would significantly help reducing the bottom temperature?

My theory is that steam traffic will decrease, thus decreasing the pressure drop in the tower, the bottom pressure and, as a result, the bottom temperature (boiling point at bottom pressure).

May I know your opinion?

"We don't believe things because they are true, things are true because we believe them."

 

[TD2K]

What are you trying to accomplish in reducing the bottoms temperature?

What is your current bottoms pressure and pressure profile across the tower? Without the sensible heat from the superheat (assuming the same lb/hr is used), more of the steam has to condense traveling up the tower to heat the down-flowing liquid but I'm having a hard time seeing the tower dP is going to drop such that the bottoms' temperature is a lot reduced.

 

[sheiko]

Thanks

We have a cooler at the bottom of the SWS, and the objective is to reduce the temperature downstream the cooler due to environmental constraints. Given a scenario in which the cooler duty couldn't be increased, I was trying to guess the effect of steam temperature on the tower bottom temperature.

The tower has 10 trays, the top pressure is 1 barg and the bottom pressure 1.2 barg.

"We don't believe things because they are true, things are true because we believe them."

 

[TD2K]

If I look at the steam tables 1.2 barg should give a bottoms temperature of about 255F, is that about where you run? Even if you reduced the bottoms pressure to 1 barg the temperature would only drop by about 5F so that's not going to have a major effect.

Where do the overhead gases go? These can be routed to a sulfur plant so you likely have limited abilities to reduce the overhead pressure. Sometimes they go to a firebox for incineration which may give you more ability to reduce the pressure (assuming the tower doesn't flood with the lower operating pressure).

Is the downstream cooler a shell and tube or air cooler? What is the cold side fluid? Have you looked at the dT on it and whether the desired reduction is relatively small or significant? If it's a significant reduction (say you currently cool the sour water to 150F and you want to get it down to 100F), you likely need a new cooler or second trim cooler.

Have you looked at the feeds streams to this tower to see if any of them can be processed in other systems? The less water to the stripper, the less bottoms stream to be cooled and the cooler the hot stream will be cooled to.

Can you simply add cold water to the outlet stream and cool it down that way? That of course increases the volume of total waste water to be processed.

 

[sheiko]

Thanks again TD2K!

Well, you made it indeed clear that the benefit of desuperheating the steam is very small.

The tower acid gases can go to acid flare (0.5 barg) or to the burners of a fired-heater (1.5 barg). The tower top is not pressure controlled.

Concerning the cooling of the bottom outlet, we can reuse an old cooler (confirmed by HTRI simulation) so we have a cheap solution.

"We don't believe things because they are true, things are true because we believe them."