4 replies to this topic

[abhilashpkurian]

Hi All,

I had a query regarding vapour pressure.

Our client wants a gas pressure regulator to be attached on top of a H2S cylinder, which is having a pressure of 150 bar, to get an outlet pressure of 10 bar H2S gas.

As far as I know, the vapour pressure of H2S is around 18 barg @ 25 deg C. That means that H2S would turn into liquid at a pressure more than 18 barg at room temperature. Supposedly, the cylinder is only half filled.

If the cylinder is having a pressure of 150 barg, there should not be any H2S gas in it. Only thing present should be H2S in liquid form.

Suppose, we put a regulator on top of the cylinder to get H2S gas. What would be the pressure in the inlet of the regulator, it should be less than 18 bar?

How and where does the liquid turn to gas in the cylinder?

Should the regulator be rated for 150 bar or only 18 bar?

I am confused. Any help?

Regards,
Abhilash Kurian

[Art Montemayor]

Abhilash:

I don’t know if you are a degreed, professional engineer or a layperson, but I am going to assume that you are a professional engineer and that you are aware – or trained – in what I am going to describe and explain to you.

I spent my first 8 years out of university working in the compressed gas industry, so I am familiar with what you are faced with.

Hydrogen Sulfide (H2S) is available in a technical or commercial grade that is 98% volume, minimum. It is also available in a high purity, or C.P. Grade, 99.5% min., volume. H2S is shipped as a flammable gas in compressed gas steel cylinders, in which it exists as a saturated liquid. The steel cylinders are normally filled to only 62.5% of their water weight capacity.

The vapor pressure of H2S varies as follows:

Temperature, oC..........Pressure, barG
16...............................14.56
21...............................17.23
41...............................28.20
46...............................32.00
54.5............................38.40

The Critical Temperature of H2S = 100 oC
The Critical Pressure of H2S = 90 barA

The above data tells us that H2S will exist as a liquid if compressed to a pressure above 90 barA and is cooled to a temperature below 100 oC (which will be the ambient temperature – even in Dubai). This means that if the cylinder of H2S is full and sits in the hot Dubai atmosphere, it should have a pressure of about 28.5 barA – or approximately 30 barG. This does not agree with the 150 bar pressure that you cite (you failed to tell us whether you mean gauge or absolute pressure, but at these levels the point is academic). Something is amiss with the information you supply.

You are correct when you say, “the vapour pressure of H2S is around 18 barg @ 25 deg C”. However, you are wrong when you state, “That means that H2S would turn into liquid at a pressure more than 18 barg at room temperature”.

From all indications, H2S at 150 bar (whether G or A) and ambient temperature is in the supercooled liquid state and not saturated. You are absolutely correct in stating that if the cylinder has a pressure of 150 barg, there should not be any H2S gas in it. Please refer to the detailed thermophysical data I have compiled for your use in the attached Excel Workbook.

I am also confused (as you) by your information giving us the pressure in the cylinder as being 150 barG. I frankly don’t believe it. A cylinder at that condition would have to be 100% liquid full and that would constitute a very serious and dangerous situation in the event the temperature would increase and cause a very high pressure resulting from hydraulic expansion of the liquid and no gas vapor space to absorb the expansion. This situation simply cannot be allowed to endanger the public.

I believe you can forget about installing a pressure regulator on the cylinder and trying to regulate a gas stream from it. There simply cannot be any gas to regulate. You would be expanding a liquid and producing a very cold mixture of liquid and gas.

Please tell us how, where, you obtained the information that the cylinder is at 150 barG of pressure. Is this factual information? Have you verified it? What is the entire story and facts behind this query?

Await your reply.
 H2S_Thermophysical_Properties.xls   139.5KB   198 downloads

[sachindhopade]

hope this helps u
http://www.processca...r_Pressure.aspx

[Art Montemayor]

sachindhopade:

How can your reference be of any help to Abhilash when I've given him DETAILED vapor pressures at temperatures between -50 oC and 99 oC? How much more vapor pressure values do you think he/she needs?

The important point here that I want to make with you and Abhilash is that by not even mentioning the potential danger of an existing excessive hydraulic pressure in a H2S cylinder, you (as well as him) are tacitly allowing the danger to potentially exist. Please read my last post (and study the detailed vapor pressures given in the attached workbook) and advise me if I failed to clearly point out the potential danger of allowing the H2S to get into the supercritical zone without having any vapor expansion volume.

We have not heard anymore from Abhilash in a month - not even a "thank you". I hope he has understood what I tried to convey to him. His data is either flawed - or his communications ability - or he has a very potentially dangerous situation. Have you recognized the same problem?

[djack77494]

Perhaps the cylinder contains a mixture of diluted H2S. While abhilashpkurian or the end user may be interested only in the H2S content, and may therefore just talk about the cylinder as being H2S, they would have provided us with misleading information. I have heard of H2S being sold in cylinders with a gas diluent making up the bulk of the gas, but with H2S being a significant "impurity". As Art has said, you cannot have pure H2S at a pressure above its saturation pressure unless you are refering to a subcooled liquid. For many reasons, especially those already stated, we all hope you are not dealing with a subcooled liquid.