# Removing water and nitrogen from reactor effluent

I wish to remove water and nitrogen from my reactor effluent in order to decrease downstream equipment sizes and subsequently capital costs.

Stream composition:

${H_2S}$.................5.6%

${SO_2}$.................3.3%

${S_2}$.................9.2%

${COS}$.................0.9%

${CS_2}$.................0.1%

${CO_2}$.................1.3

${H_2}$.................1.4%

${H_2O}$.................34.5%

${N_2}$.................43.7%

Removing these components would result in a approximately 90% less flow rate.

Restrictions:

• The sulphur MUST stay above melting point, $\pm$130 $^\circ$C at 1 atm.
• ZERO sulphur compounds must leave the water or nitrogen streams. (meaning that if these compounds are in the water or nitrogen streams, they have to be removed at a later stage as well.)

What I have done thus far:

I used the Clausius-Clapeyron equation to solve for a pressure where the dew point of water is higher than the melting point of sulphur. I found that at 3.5 atm of pressure and approximately 130 $^\circ$C, liquid water forms but sulphur stays liquid, and density separation can be performed with the added bonus that sulphur is insoluble in water. However, I cannot think of way to remove the nitrogen or a cheaper,more efficient way to remove the water.

Due to my lack of reputation I can only answer instead of commenting which I would have prefered. Thus my explanation will rather cover some basic ideas and suggestions rather than an elaborate separation process. Also you should not I have not graduated yet and I'm eager to learn myself. So instead of downvoting I would like to receive constructive comments on what I got wrong or what I could improve. Mainly my point is to get a discussion started and see if we could use some ideas to help you.

From what I see you already have a two phase system.

Vapor Phase

• $H_2S$
• $SO_2$
• $COS$
• $CS_2$
• $H_2$
• $N_2$

Liquid Phase

• $S_2$
• $H_2O$

At your given Temperature and Pressure. Please note that I did not have access to all VLE data of all your components, you will have to check that for varying pressures.

1st Separation Step

2nd Separation Step

$N_2$ has the lowest boiling point of all components in your vapor phase. Except for $H_2$. You could lower the temperature to -60,2 °C and then $H_2$ and $N_2$ would stay in vapor state, all other components would be liquified. This would allow for an easy phase separation. Sidenote: I'm no chemist however $H_2$ and $N_2$ react to ammonium, which has a higher boiling point. However you did not consider reactions of the components before so I just assume this might not be a problem.

3rd Separation Step

As you pointed out $S_2$ and $H_2O$ have a significant difference in density. So you could use a mixer/settler setup e.g. to separate them. You would then have to combine the $S_2$ stream with your other $N_2$ free stream.

What I don't really see yet, is how you get your initial reactor effluent. From what I know there is no way $S_2$ would be in gasous state unless you operate under very high temperaturs or under a vacuum. In both cases that would have been a crucial information here to begin with.

I hope this answer helps you a little bit.

• The effluent comes from a thermal furnace of a sulphur recovery plant. It operates at about 1500C. It is then cooled by a waste heat boiler to about 650C before going into a few catalytic reactors to reduce the SO2 and H2S to S2 and H2O. After each catalytic reactor the stream is cooled to allow the sulphur to be extracted. I want to remove the water and nitrogen before they enter the catalytic reactors to decrease the size. – 22134484 Jun 13 '15 at 11:35
• In "1st seperation" step that you proposed, water is still a gas, that is why I thought about pressurising the system to make sure only those two components form liquid. They can be simultaniously removed from the effluent and separated in a single "step". – 22134484 Jun 13 '15 at 12:01
• I was referring to " I found that at 3.5 atm of pressure and approximately 130 ∘C, liquid water forms" at this point. – idkfa Jun 13 '15 at 12:02
• I will look into your 2nd step, I thought about that myself, but the energy required to cool something from 130 to -60 is quite alot. But it is the most used method if you are working with high flow rates. – 22134484 Jun 13 '15 at 12:04
• Obviously, my bad. Also, H2 and N2 wont react at such low temperatures. The main reason for the furnace being 1500C is to thermally disassociate the NH3. The reverse reaction doesnt/barely takes place. I have not run this in my simulation program yet to verify this claim, but literature suggests that it doesnt. – 22134484 Jun 13 '15 at 12:08