2 replies to this topic

[Nido1]

Hi there,

I am a process engineer and working on a carbon capture and storage project. I wanted to have some input towards venting/depressurization system for the CO2 compression train. I thought someone might be able to help me here.
I am designing a compression train, which will compress 99.5% CO2 from around a 1 bara to 150 bara. After speaking to few vendors we decided on 8 stages compressor with interstage scrubbers and cooling. After about 4^th or 5^th stage we will have oxygen and dehydration unit.

My question is about the venting system. As some of you may know that to depressurize dense phase co2 is not easy as we get solids forming downstream of the PSV or blowdown valve.

Has anyone got CO2 relief and depressurization experience. Please can you share it.

Regards

[Art Montemayor]

Nido1:

For many years I was involved in the design, installation, and management of CO2 plants as well as storage, transportation, and conversion to Dry Ice. I know a little bit about what you are asking.

I am at a loss to understand your reason for entering into the supercritical (SC) Phase zone. I could understand your need if you are transporting by pipeline or storing the SC CO₂ underground. But you state yours is a storage project. If you are storing underground, there are no needs for venting, relief, or depressurization.

I also don’t understand the need for 8 compression stages – presumably with a reciprocating type of compressor. Your pressure range should be easily handled by a 5 stage compressor.

Why are you proposing to dehydrate after the 4^th or 5^th stage? What is normally done in industry today is to dehydrate at 200 to 250 psig – which is the discharge of the 2^nd stage of compression. Additionally, industrial liquid CO₂ is stored at 200 to 250 psig, in the saturated state.

Are you proposing to use pressure vessels to store the SC CO₂ at 150 bara? Are there special reasons or is this a specific need of your scope of work? Please advise by a response post.

You have cause for concern in the special application of a CO₂ relief or depressurization operation. If you clarify in a detailed manner exactly what you are proposing, I can elaborate on how to resolve this issue.

Await your response.

[Technical Bard]

7-8 stage compression, with an integrally-geared centrifugal compressor, can be more economic and energy efficient than doing this route in fewer stages, and for long distance transmission or reinjection, such pressures are attractive.

Dehydration should be done after a pressure of about 50 bar, as this is the minimum saturated water content in CO2. This reduces the load on the dehydration system. Also, if you have H2S removal, that should be done much earlier in the compression path.

Venting/depressuring/relief is likely necessary from the pipeline in the event of a valve trip or failure. The depressuring needs to be rate limited to ensure that the temperature in the piping/pipeline does not drop below the MDMT of the material (usually carbon steel), otherwise brittle fracture could occur. The venting stack should be designed for high velocities and no sharp corners between the valve and the vent. This will help keep any dry ice that forms from coalescing into large pieces before reaching the atmosphere. It must also be vented in a safe location away from personnel.