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Water Dew Point


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#1 Ghasem.Bashiri

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Posted 17 October 2007 - 01:54 PM

Dear Sir, Madam
I am going to design a dehydration unit (molecular sieve) in upstream on demethanizer.
During FEED of the project, -110°C as water dew point has been specified by designer.
Minimum temperature in downstream of dehydration unit is -109°C.
Actually, is it required to meet such low water dew temperature?
By knowing that ice will be formed in 0°C, what is the meaning of -110°C as dew point of water?
Vendors such as AMTEK provide instrument for water content measurement up to 0.1 ppm (Attachment). How can I calculate water fraction corresponding to -110°C to check it with instrument vendors? And is it actually possible to meet such low dew point (-110°C) with conventional process (Molecular Sieve dehydration)


Ghasem.Bashiri@gmail.com

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#2 Art Montemayor

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Posted 17 October 2007 - 04:02 PM


Ghasem:

If you have never designed and built an adsorption dryer before, I recommend that you not undertake the attempt you are describing. If you have little or no experience you will not succeed. Allow me to give you some basis for my comment:

  1. I do not believe that you have been specified -110 oC as the product gas dew point. I suspect that what has been specified is actually -110 oF – which is quite a different dew point.
  2. 2. You do not specify the base pressure for the cited dew point. I believe your base pressure is atmospheric pressure – 14.696 psia. You must cite the base pressure associated with any dew point you mention; otherwise, the dew point value is useless because you cannot use it to identify the amount of water associated with the gas in question. Please read the last page of the AMETEK brochure you submitted under “Performance Specifications – Range” and you will notice that it states “requires process pressure as an input”.
  3. The dew point (DP) of a gas is well defined in all circles. It is the temperature at which the gas starts to form the first drop of dew as its temperature is differentially reduced in value. That DP, together with the pressure at which the dew is formed, is related to the actual water content of the gas in ppmw, lb/mmscf, mg/Nm3, etc.


If you check with vendors such as AMETEK, you will discover that at approximately -100 to -125 oF (at atmospheric pressure) you are at the limit of their ability to accurately measure the water content of a gas. And this is the limits also for the Molecular Sieves to dry the gas. These are the usual limits for drying required by cryogenic processes such as the one you are citing. You may be able to achieve even -150 oF, but your measurements will be subject to error at that very low level and the results are more academic than they are practical. At those levels of water moisture you have pretty well reduced the gas to essentially and practically nil water content.

Because you don’t discuss the above and because of the nature of your questions I seriously doubt you have the experience to design and build an adsorption unit of this level and application. I recommend you obtain the services of an experienced design engineer for this effort. You may feel you are qualified to carry this task on, but I can assure you that it will overtax you in technical demands and know-how.



#3 Ghasem.Bashiri

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Posted 18 October 2007 - 01:26 PM

Dear Art Montemayor
Actually I think that people may not answer to me. therefore I breifly stated my problem. Water dew point specification is -110°C at 10 Bara. We know that water will change to ice in 0°C. But water can present in vapor due to VLE. I have some experince in Mol Sieve dehydration unit with -100°C as water dew point in 10 Bara. My question is that is it required to meet such specification?About Calculation, my problem is accuracy of calculation. Becuase with HYSYS or PROII we can calculate this figure. Here you can find my question and answer with some specialist in this subject. let me know your idea in this regard:


In a real project water dew point at outlet of dehydration bed has been specified as "-110°C" (NGL Recovery plant) at 10 Bara. Minimum operating temperature in downstream of dehydration is -109°C. Actually, is it required to meet such low water dew temperature?
[b]Yes, otherwise you will form ice or hydrates.[/b]

By knowing that ice will be formed in 0°C, what is the meaning of -110°C as dew point of water?
Water needs to be in the form of liquid first to form hydrogen bonding and then freeze. Therefore, water molecules could be in the vapour phase (far away from each other) at temperatures far below zero and do not form ice. As you pointed out this is not as such a dew point, but ice point.

Vendors such as AMTEK provide instrument for water content measurement up to 0.1 ppm (Attachment). How can I calculate water fraction corresponding to -110°C to check it with instrument vendors?
This can be achieved using thermodynamic models, though not easy.
And is it actually possible to meet such low dew point (-110°C) with conventional process (Molecular Sieve dehydration)
You need to check this with the vendors and/or experimentally.

#4 hygrometry

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Posted 03 March 2009 - 11:34 PM

You can download a freeware calculator from www.phymetrix.com that will perform all of these conversions.

Hope this helps!


#5 Arvind Iyer

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Posted 04 March 2009 - 01:17 AM

Dear Friend,

Let us not confuse the dew point temperatur with temperature of the process stream.

When it is said that a stream has a water dew-point of -110 deg C, it does not mean that the stream is at that temperature. It is a way of quantifying the amount condensible moisture in that stream.

Pure Water will condense at 0 deg C.
But when small amount of moisture is present in a gas, will it condense at 0 deg C??

So, when will it condense?
It will condense as you reduce the temperature. How low a temperature depends on the amount of moisture.

If the moisure is substantial, it will condense say at -40 C.
If the moisture is still lower, it will condense say at -50 C.
If it is still lower,, it will condense at -110 C.

So, the lower the water dew-point, the lower the moisture content. Hope this helps in understanding the phenomena.

Arvind Iyer

#6 Qalander (Chem)

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Posted 04 March 2009 - 07:38 AM


Dear Arvind;

Probably 'pure water' under normal conditions should condense to liquid state at or around 100 C not necessarily as low as 0 C

I understand there might be a typo in your post, please correct me if wrong?
Best regards
Qalander

#7 ankur2061

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Posted 04 March 2009 - 10:32 AM


Dear Qalander,

Arvind is right. He is not talking of water as a one single component in liquid form. He is talking about air/gas laden with moisture or air/gas saturated with moisture and the separtion of water from air/gas by the reduction in temperature of the air/gas. This post is related to psychrometry.

Please don't take it otherwise but you probably need to either read the post more carefully or go back to the textbooks to understand psychrometry.

Regards,
Ankur.

#8 Qalander (Chem)

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Posted 04 March 2009 - 10:55 AM


Dear ankur, Thanks indeed! for psychometric issues understanding guidance.

Most probably wordings were "Pure Water" Out of that, from where the air with water vapors may enter into the meaning as such.

Or the sentence should be phrased as only laden with pure water vapors since pure water equilibrium conditions may exist for wider range.

However, no further arguments from my end.

Best regards
Qalander

#9 DRS

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Posted 05 March 2009 - 03:43 AM

Is -110C pressure dew point or atmospheric dew point? -110C atmospheric dew point seems to be a bit logical. The requirement of whether to go as low as -110C ADP or not depends upon the process, the initial temperature of the gas and the ambient DBT.

For example, in your case, considering 10 barg and 27C as initial conditions of the gas and supposing that the gas expands to atmospheric pressure adiabatically (a worst case scenario) in the final stage, the gas temperature reduces to -103C (considering ratio of specific heats as 1.31 for natural gas). If your gas is not dryed to -103C, you will have ice formation. So, the gas dew point (atmospheric) should be anywhere below this. If you are drying the gas upto -110C ADP then you can do away with an initial condition of 10barg and 17C.

When you are working in extreme cold conditions, it is safe to dry the gas considering ambient DBT or gas initial temperature, which ever is lower.

If your final conditions are such that the pressure always remians at 10 barg, -110C ADP is redundant.

Secondly, humidity ratio (at saturation) corresponding to -60C is 0.0000067 kg/kg (calculated using Hyland and Wexler equation), which is about 0.067mg/kg of dryair. I don't have any accurate correlation below -100C. Check whether this moisture content is significant with your process flowrates.

#10 DRS

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Posted 05 March 2009 - 04:03 AM

A small clarification to my earlier post. The humidity ratio I mentioned is for air. PhyCalc software mentioned by another member in various threads seems to have an answer for your question. Did some fast checks for air, comparing the data with ASHRAE handbook (based on Hyland and Wexler equation) and the accuracy is pretty good.

#11 Qalander (Chem)

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Posted 05 March 2009 - 04:46 AM

Thanks indeed! DRS and all the other Valuable contributors.

Best Regards
Qalander

#12 ankur2061

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Posted 05 March 2009 - 05:00 AM

Dear,

I didn't find your software very user-friendly. I had downloaded a spreadsheet from the net and which I modified by introducing a new sheet (Definitions,FAQ's) for my understnding which includes certain common definitions and FAQs related to psychrometry.

I am posting the same for the benefit of the readers.

Regards,
Ankur

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#13 Qalander (Chem)

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Posted 05 March 2009 - 05:15 AM

Dear ankur,

<i><b><!--coloro:#008000--><span style="color:#008000"><!--/coloro-->GOOD Helpful Work THIS SHALL PROVE TO BE<!--colorc--></span><!--/colorc-->!</b></i><u></u>

Best Regards
Qalander

#14 tezza

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Posted 23 March 2009 - 05:20 AM

Ghasem,

Firstly I am not an engineer but a chemist, so I cant help you with your design, but I can tell you that a dew point of -110 deg C would be an incredibly dry gas. If the gas was at atmospheric pressure, it would equate to a water concentration of just 1.4 ppbv (yes, parts per <u>billion</u>). If the pressure was 10 bar, it would be equivalent to just 0.15 ppbv.

I work in a polyethylene plant where it is critical to get the ethylene gas as dry as possible since water poisons the catalyst. By using molecular sieve to dry the ethylene the lowest moisture content I have been able to measure is around -75 deg C at atmospheric pressure, which corresponds to around 1 ppmv. I think you have a real challenge on your hands to get to -110 deg C dew point! (if you are interested in my simple Excel application for equating dew point, pressure and water concentration as well as relative humidity and wet and dry bulb temps please email me)

Tezza

#15 boost61

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Posted 06 March 2011 - 07:50 PM

Ankur:

Here is a spreadsheet based on the Bukacek equation for dewpoints in natural gas.

I put this together some years back and it has been useful for both natural gas and ethane systems.

We have often had dewpoints measured in different ways or different pressures or even moistures done via Dräger tubes or Karl Fischer coulometry.

The whole thing is a little messy, but I recommend adapting it to your needs.
The VBA defined function is probably the easiest to use but the tables were done so the various charts could be printed.

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