18 replies to this topic

[adrina]

Hi

I'm working in a gas refinery ,in environmental dep.

In my refinery sour water are stripped by steam and stripped over head vapours are partially condensed in the air condenser down to 95c ,and the acid gas is seperated in the reflux drum and sent to the LP flare under pressure control.

I want to know is it a good idea to send this stream as a feed for sulphur recovery unit in order to reduction of air SO2 emission or not ? as the flow rate is low .
based on my calculation total SO2 emission per year from this stream is 275 ton. And from 3 sour water stripper unit for 3 seperated gas plant is 825 ton/year.

stream specification are as below:
total stream flow rate :1.139kgmol/hr (MW:28.3)
H2S flow:0.4895 Kg Mol/hr
P:1.8 bar
T:95 C

thanks

[gegio1960]

adrina,
this kind of stream is usually sent to SRU,
Moreover:
- the pressure is enough
- the temperature is ok (usually 90°C)
- you've to check for presence of NH3, and entrained water and hydrocarbons
regards

[adrina]

thanks for your answer
what about flow rate it is enough ?
I"ll check the composition ,as I remember in the material balance for this stream NH3 is zero.How much is the limitation value of NH3 for sending to SRU?

[adrina]

any support? I'm waiting.

[gegio1960]

The flow is very low.
Is your SRU already existing?

[Technical Bard]

The presence of NH3 only requires that you have a combustion temperature in the SRU high enough to reduce NH3 to N2. Contact your SRU licensor for more information, or any number of reference texts on Claus unit design.

[adrina]

yes ,SRU is existing.so what do you suggest?as flow rate is very low, is it better to be flared ?

[kushal.raval]

Yes, usually you can route it to SRU. As it cannot be main contributing scheme to SRU, it will not create any major impact on the SRU configuration. However, licencor intervention may be necessary on this aspect.

[gegio1960]

I confirm kushal point of view.
It could be useful to know the feed flow to SRU in the current/design operating conditions.

[adrina]

merci dear kushal &gegio

feed flow at the moment to SRU is ~15 t/hr and design is 23t/h. but sour water gas over head compare with this flow is very very low.
Whereas in some environmental standard AG flaring is defined" Amine acid gas and Sour water stripper acid gas (SWAG)".
still I have concern about low flow rate .everybody know about the normal flow rate of top of sour water stripper gas in refineries that licensor decide to send it to SRU.

[sheiko]

Adrina,
As far as I know, SWS does not emit SO2 but H2S.
In turn, the SRU will produce SO2 by combustion: H2S + 3/2O2 => SO2 + H2O

Edited by sheiko, 15 November 2012 - 05:14 PM.

[Dacs]

That's what I thought as well, acid gas from SWS will primarily comprise of H2S and NH3.

With that, NH3 poses a unique challenge in Claus units since NH3 has to be burned off at a high temperature (as Technical Bard has mentioned). My concern is as you can customize your Claus furnace configuration (for this particular situation, having a 1st stage chamber to burn off all ammonia in your acid gas), you have to make sure that your SRU unit can handle that much additional NH3 load before sending it there.

On top of that, the associated equipment in your SRU that normally handles NH3 rich streams are usually designed to avoid any ammonium salt formation, I'm not sure if your SRU has that configuration in place.

Yes, usually you can route it to SRU. As it cannot be main contributing scheme to SRU, it will not create any major impact on the SRU configuration. However, licencor intervention may be necessary on this aspect.

While I expect SWS acid gas flow to be lower than from ARU, the NH3 content makes all the difference (as mentioned above).

I guess the best approach is to ask the licensor that has designed your SRU and confirm with them if they can handle the extra load from your SWS unit. They're the best people to answer your question.

As to sending H2S to flare, I believe your plant must meet emission levels and sending your SWS acid gas to flare may upset that.

My 2 cents.

Edited by Dacs, 15 November 2012 - 07:42 PM.

[Pronab]

Normal route to the SWS over head gas is to SRU but there should be another line to Flare system. In case of SRU trip, it will route to flare system. In our plant there is a SWS over head gas normal route is to SRU which was never been used. It is routed to flare system. More over this SWS over head gas contain NH3 and water vapor, which will make problems in SRU operation.
Your SWS overhead gas pressure/temperature are similiar to our system. Is the flow is steady or fluctuation dependeds on upstream feed rate?

[sheiko]

Hi,
Just for info: It might be possible to route the SWS gas to fired-heaters. We do it in my plant for some of them.

[Padmakar Katre]

Hi,
The SRU feed is mainly from an amine regenerator overhead and SWS overhead. I don't understand the logic of not routing the stripper overhead to SRU in in the initial design as the system is existing. The routine as mentioned to LP acid flare is since start or just a start up option?  Why don't you arrange a discussion with SRU process licensor (your existing unit) to check feasibility of routing the SWS overhead gas to SRU?

[RockDock]

You could probably send it to the SRU, but you will need to model it to make sure. Most refineries I come across have major updates they need to perform. You likely stumbled upon a long overdue update. That happens a lot in refineries. Amine Units, SRU, SWS are often neglected and seen as money drains. While in reality, if optimized, I've seen refineries reduce costs anywhere up to about 50 million USD a year, just by optimizing those units.

[neel_avi]

Dear Adrina,

Is 1.8 bar - absolute or gauge pr? If its in gauge then you might have to size the line properly. Two units needs to be nearby. On the basis of general operating conditions of SWS units the pressure at the mentioned temperature will be around 0.9-1.3 barg

SRUs generally have a low turndown/overdesign between 0.15-0.2. The flow being low, routing should not be a problem. The furnace surely will have enough margin to accommodate inerts in the gas if any other than NH3 and H2S. However, I donot require to emphasis that the same needs to be reviewed once.

Ammonia is the major impediment for the claus furnace. Maintenance of temperature profile in the furnace becomes difficult with NH3. If NH3 is not present, I donot see any problem in routing the gas to the system. Generally NH3 has two modes of disposal. Either in the claus furnace if the NH3 content is less than 25 vol% of the total sour gas. If its more than 25% then it can be separated HP and LP strippers and individual NH3 stream can be routed to incinerator of SRU. The same should not be essential in your case.

For flaring You need to check the SO2 emission limit per year.

Licensors send it to SRU, as I understand, on the basis of the composition of sour gas. It is entirely on the operating company who can decide the final destination. For me it is preferably the SRU.

Hope I could put forward my views. Would wait for a feeback fro everyone else.

Regards

Avijit

[josh68]

Dear All

 

Related to the above , I need to know from all  the experienced gúys that in our Stripped water unit  O/H gas to SRU  incinerater High temperatiure alarm ís provided to 95Deg C and the operating temperature is 90DegC . Cound anybody justify it . Is it necessary to control thí to 95 Deg C , the line itself ís steam traced  . If not  we like to remove that alarm

 

please help

[gegio1960]

That alarm is provided to avoid sending water to SRU.