8 replies to this topic

[SamSPearson]

Hi,

 

I've been tasked with designing every aspect of a natural gas desulfurization unit which will use zinc oxide as a the sorbent (or reactant) in a fixed bed. I'm struggling to find a place to start in relation to reactor design equations. I'm unsure if packed bed reactor equations apply to this situation, given that the zinc oxide itself is a reactant rather than a catalyst?

 

Manu thanks,

 

Sam

 

[Pilesar]

My suggestion: Start by finding out as much as you can about the ZnO. What is the recommended temperature? What is the recommended velocity through the bed? How much sulfur can be reacted before the ZnO is spent? The catalyst suppliers should be a good source of this type of info. Then size the bed as required by your feed stream. Then size the reactor to contain the bed required.

[SamSPearson]

Hi, 

 

Thank you for your response! Temperature of the intended catalyst is 350 Celsius (from catalyst supplier) and the data provided from BASF - http://www.psbindust... Data Sheet.pdf suggests a capacity of around 28 lbs S / 100 lbs ZnO. My problem is the quantities involved unless I'm doing something elementary wrong. My feed contains 45.63 kmol/hr of Sulphur to be removed. Assuming a bed replacement every 3 months, this works out at 98650.8 kmol (S) / 3 months to be removed. This equates to 3153945.6 kg (S) every 3 months. Therefore, mass of the ZnO required is 11264091.4 kg. Given a density of 1050 kg/m3, this gives a volume of 10727 m^3 for the ZnO - which seems ludicrous. Am I going about sizing this unit the right way?

 

Many thanks in advance, 

 

Sam

[Satyajit]

Hi Sam,

The sulfur content in your feed gas is too high. It seems to be not feasible to remove all the sulfur using ZnO catalyst. You may need a separate sulfur removal unit before this final de-sulfurisation step using ZnO catalyst. The bulk of the sulfur can be removed using a chemical solution like hot potassium carbonate or AMDEA solution together with an activator. The primary sulfur removal unit should be designed by licensor like UOP, BASF or GV. Please contact these licensors with a proper specification of feed and the desired product.

Kind regards,

Satyajit

[SamSPearson]

Hi Satyajit,

 

Thank you so much for your advice! I'm a student trying to size this unit so my knowledge any feel for the right sort of magnitudes isn't great (but obviously I know when something major is up). Do you have any idea what sort of flow rate of sulphur is in a tolerable range for passing through this unit?

 

Many thanks, 

Sam 

[Saml]

Check this link. It has some useful information about possible technologies to get you started

Since the articles are from 2001, some names may have changed.

 

http://www.costain.c...ur-recovery.pdf

[Satyajit]

Hi Sam,

The desulfurization unit comprising of Hydrogenetion reactors followed by ZnO adsorbers are typically designed for 10-40 ppm Sulfur in the feed;it also depends on how much is the feed flow rate. The ZnO reactors are designed with a capacity atleast  for a year.

When sulfur content in the feed is more than 50 ppm and the flowrate is more than 100 Te/hr, it is quite economical to go for a small scrubbing unit as a primary desulfurisation step before the HDS section which works as a final purification step for the feed gas.

When a scrubbing unit is required, it is normally taken as a side stream from the existing CO2 removal unit commonly present in any traditional ammonia plant.

Hope this replies your question.

Kind regards,

Satyajit

[SamSPearson]

Dear All,

 

Thank you for your replies. I'm now well into the chemical & mechanical engineering design, but I'm now wondering how the desulfuriser will be charged, and recharged with new Zinc Oxide once the bed becomes saturated? I was thinking along the lines of there being manholes on every tray that allow the zinc oxide to be taken out and replaced, but I can't find anything in the literature that verifies this, or even suggests another technique?

 

Kind Regards,

 

Sam

[Abnaan_706]

Hey Sam, did you find out how'd you'd replace the zinc oxide beds?

Edited by Abnaan_706, 25 October 2018 - 07:31 AM.