Hydrogen Vent line design for pharma batch reactor of 6000 Litres; Steam dilution.

[plantprowler]

I've a couple of questions re. venting of hydrogen from a 6000 L batch reactor at the end of a batch.

I was doing a safety review and this is a specialty chemical facility and there's no flare just a vent stack. The amount of residual hydrogen is about 20% of the reactor volume and at low pressure of approx 6 bar. H2 alarms, rupture disks, IR flame detectors etc. are all in place.

What are the best practices? I could not find any guidelines for this sort of situation. e.g. I see steam dilution as one recommendation but where exactly does one add the steam? Close to the vent release point? Also, what kind of design is recommended to deal with the condensate that will drain back down the line? I'm trying to think of a safe sketch.

Finally, is there a reccomeded dilution ratio, I am aware that H2 has very wide flammability limits.

In another source I read that hydrogen vent lines should NOT have flame arrestors? Is this still a best practice?

 

[jari001]

You could check out ICCSafe.org, I believe they have some minimum design standards about hazardous chemicals like hydrogen you can review. I work with some processes that evolve small amounts of H2, and the design engineers for those looked at explosion venting scenarios, blast panels for the surrounding structure elements, and piping layout/instrumentation/equipment to prevent ignition form rapid venting. We use the International Building Code for most code reviews around structural issues that is affected by handling H2.

When I was an operator at a plant that did hydrogenation reactions, we'd vent H2 into an N2 purged header that was connected to all the hydrogenators that fed into a thermal oxidizer; we'd have recovered and recycled the H2 if we could have, but the trace amounts of other chemicals in it would invalidate its use as a raw material in different products.

This article discusses some recommended codes to follow for Hydrogen handling -

https://www.chemengonline.com/how-to-handle-hydrogen-in-process-plants/?pagenum=2.

I would look at NACE for materials of construction questions to avoid H2 embrittlement and/or H2 assisted cracking if that's a point of concern.

 

[georgeverghese]

Table 27-18 in Perry Chem Engg Handbook 7th ed summarises some relevant properties for H2:
Upper / lower flammability limit : 74.2 / 4.0 %v/v
Flame velocity : 2.83m/sec
Suggest the following, taking oil/gas practice as an example:
Install an inert gas injection line at the reactor end of the vent line to clear the line of any O2. Use this line prior to restart after maintenance.
Install another injection point not far from the vent release point. This is the continous safety purge which should also maintain exit gas velocity at above 2.8m/sec at the vent exit nozzles even when there is notionally no H2 going through the vent stack. This prevents flame flashback into the vent header. Enable a low flow alarm (at the least) on this purge gas velocity.
Whether you trip the plant on loss of purge will depend on the flashback / deflagration pressures created in the vent header as a result of loss of purge: in hydrocarbon systems, 7barg is considered to be the min required mechanical design pressure for all units and vent headers to withstand deflagration pressures. Ask a process safety expert / consultant what the corresponding min design pressure should be for H2 systems so you can avoid tripping the plant on accidental loss of purge leading to deflagration / flashback.
Ideally, use N2 or inert gas. Agreed, if you use steam, the vent header will have to be piped such that you can collect condensate / drain out on a continous basis. Corrosion may also be an issue if you use plain carbon steel for this vent line.
Presume you dont have O2 in the reactor at any time during normal operation.

 

[plantprowler]

Thanks @george

Re. "if you use steam, the vent header will have to be piped such that you can collect condensate / drain out on a continuous basis. "

This is where I am stuck. What sort of arrangement would you suggest / have you seen?

I don't want to create additional hazards while incorporating such an arrangement.

Also, would the static generation risks be lesser if using steam instead of N2?

 

[georgeverghese]

Install low point condensate collection drums or piping drip legs on this vent header and have them drained out regularly. Dont know about static generation risk reduction advantages with steam - a process safety or electrical engineer would know. If you could choose, continous purge with purge gas velocity in excess of flame velocity is a much better flashback protection mechanism than a flame or detonation arrestor, if you ask me.

 

[georgeverghese]

Max deflagration pressure with H2-air mix starting at atmospheric conditions appears to be 8.8bar abs from this maplesoft - mathcad calc sheet found on the net; Process safety engineer to verify

https://www.maplesoft.com/applications/view.aspx?SID=154404&view=html