16 replies to this topic

[Anshul619]

Dear all,

 

While sizing contactor,I came across the terms.

 

Yin=

Yout=

 

These are the water content in natural gas at inlet and outlet of process gas.

 

Through excel sheet(BY Mr. Ankur Shrivastav) i have found Yin= but Yout= is not coming correctly.

 

These are the process conditions.

 

Wet gas=

 

Temp:  60 degree C

press  :42.1 barg

mass flowrate: 9474 kg/h

density :48.97

composition;H2S-0.49%

                   CO2- 2.47%

                    sweet gas-97.03

 

dry gas=

 

Temp 63.76 degree C

press  41.93 barg

mass flowrate 9426.448 kg/h

density 47.87

composition;same as above..

 

the answer of both are

Yin= 4068 mg/m3(st)

Yout= 62.649 mg/m3(st)

 

Yout is not coming correct but Yin i correct.

 

Please advise........

 

 

 

PS: If I have made a mistake by giving incomplete process data.....please correct me.

Edited by Anshul619, 17 June 2013 - 01:26 AM.

[ankur2061]

Anshul,

 

What is your contactor doing? Is it an amine contactor for treatment of sour gas or is it a Glycol dehydration contactor? You do not mention this in your post. If it is an amine contactor then the composition of the inlet and outlet from the contactor cannot be same. H2S and CO2 have to decrease in the outlet gas. Normally the outlet gas from an amine system is water saturated which could even mean that the water content of the outlet gas is than the inlet gas. That is the reason that gas dehydration systems are installed downstream of the amine sour gas treatment units. 

 

When I use the spreadsheet I have posted (New Correlation for Water Content of Sour NG) with the process conditions and gas composition you have provided, I get the following results

 

Water Content of Inlet gas = 4059 mg/Sm³

 

Water Content of Outlet Gas =  4807 mg/Sm³

 

If your contactor is a Glycol dehydration contactor, the water content of the outlet gas has to decrease and the above process conditions and composition provided by you for the oulet gas are not correct. Also Glycol Dehydration systems are generally fed with sweet gas and not sour gas. Sour gas feed to glycol dehydration systems is detrimental to the performance of glycol dehydration systems.

 

Hope this helps.

 

Regards,

Ankur.

[Anshul619]

 

Sir,

 

The contactor I am sizing is of Glycol.

 

In this contactor,I am getting answers as you are getting from your excel sheet.

 

Yin is coming correct but Yout is very high according to sheet.It should be 62.649 mg/m3(st).

 

So in Glycol contactor composition of water is also necessary????????

 

here is data:

Wet gas: 0.37%

Dry Gas: 0.01%

 

Please help..........

[ankur2061]

Anshul,

 

The spreadsheet uses saturation pressure and saturation temperature of the gas as input for determining the water content of NG.

 

In your data for the outlet natural gas you are mentioning the operating temperature of the gas (63.67°C), which is incorrect data that you are providing as an input in the spreadsheet. What is required is the water dewpoint temperature of the outlet gas to be entered at the corresponding outlet pressure to know the saturation water content of the outlet gas. Apparently for a pressure corresponding to 41.93 barg the corresponding water dewpoint of your gas should be -12.4°C. Using the goalseek function I arrived at this temperature for a water content of 62.65 mg/Sm³.

 

If you have actual dewpoint data at the outlet of the contactor at the corresponding saturation pressure (41.93 barg), you can use that to determine the water content of the outlet gas.

 

Hope this helps.

 

Regards,

Ankur.

Edited by ankur2061, 17 June 2013 - 06:40 AM.

[Anshul619]

Sir

 

How did reached the conclusion that dew point is -12.4 degree C.

 

Plz clarify..

 

Thank you for reply.

[ankur2061]

Anshul,

 

Did you read my reply carefuly? I clearly mentioned that if you are looking to have a water content of 62.65 mg/Sm³ in your outlet gas at 41.93 barg, the corresponding dewpoint of the gas would be -12.4°C. This can be obtained using the goal seek function in the spreadsheet by setting the water content to 62.65 mg/Sm³ as a goal and let excel provide a solution for the temperature of the gas (which is in fact the dewpoint of the gas). In your case that temperature gets calculated as -12.4°C. 

 

Another way of doing the calculation is to define the water dewpoint of your outlet gas, which is what is needed to be defined for any TEG dehydration unit and input it in the spreadsheet I have provided to obtain the water content in mg/Sm³ at the given pressure at the outlet of the TEG contactor.

 

Regards,

Ankur.  

[Anshul619]

Sir,

 

Thank you for reply.

 

For inlet gas(wet gas) I simply put the temp given in HMB.

 

But for outlet gas(dry gas) Yout= is not given to me..

 

the value I told you came from already sized data of TEG Contactor from which I am practicing and learning.

 

I have to find it out from shell dep 20.04.10.10.

 

my question is how can i calculate dew point when only temp and press are given......

 

Pls help.

[demank]

Dear Anshul,

When you have a composition from the gas stream,

I think you can predict dew point condition (P, T) by using HYSYS.

[ankur2061]

Anshul,

 

Dewpoint is temperature.

 

Composition and Pressure of the outlet gas as mentioned by demank will provide you the dewpoint.

 

Howver, dewpoint is the defining parameter for any Gas Dehydration unit to be engineered. Sales Gas specifications clearly specify quality parameters such as Water Dewpoint, Hydrocarbon Dewpoint, Wobbe Index etc.

 

The water dewpoint is an input for doing any engineering related to a Gas Dehydration unit others being P&T of the inlet gas, inlet gas composition, solid contaminant loading of the gas etc.

 

Regards,

Ankur.

[Anshul619]

Ankur,

 

Thank you sir for sharing your experience with me.

 

But Can you provide me an excel sheet for the calculation of the dew point.

 

I do not have access to Aspen Hysys in my workplace.

 

I would be grateful to you if you can provide me calculations.

[Anshul619]

 

Thanks demank for your reply.

 

But I do not have any access to aspen hysys in my workplace.

[ankur2061]

Anshul,

 

Pipeline sales gas specification allows a maximum of 7 lb/MMScf (110 mg/Sm³) of water in North America and many other countries. That means the water content of your outlet gas from the TEG contactor should not exceed this value.

 

Now if you use the goal seek method in my spreadsheet the water dewpoint comes out to be -4.5°C corresponding to a pressure of 41.93 barg and 110 mg/Sm³.

 

Please note that as the pressure increases the dewpoint increases for a given gas composition and vice-versa. In other words the water carrying capacity of natural gas increases with decreasing pressure for a given temperature and decreases with decreasing temperature for a given pressure.

 

Regards,

Ankur.

Edited by ankur2061, 18 June 2013 - 05:30 AM.

[Anshul619]

Ankur,

 

I cannot find the goal seek function in your excel sheet.

 

I am uploading the xcel sheet which I have & plz convey if I have same sheet as you have.

 

Thanks

Attached Files

•  New_Correlation_for_Water_Content_of_Sour_NG.xls   55KB   27 downloads

Edited by Anshul619, 18 June 2013 - 05:18 AM.

[ankur2061]

Anshul,

 

If you are using Excel 2007 / 2010, click "Data" on the taskbar, you will see the tab "Data Tools". In "Data Tools", click What-if-Analysis" and you will find "Goal Seek". Set value of water content as 110 mg/Sm3 and ask to vary the temperature cell. Excel will do the iterations to give you the temperature value corresponding to 110 mg/Sm3 and 41.93 barg.

 

Goal seek can be used to manipulate pressure or temperature corresponding to the desired water content in the natural gas.

 

Regards,

Ankur.

[Anshul619]

Thank you sir

 

For your help....

 

I didnt knew this function of xcel.

[serra]

Anshul,

Excel includes two tools which can be useful to find a numerical solution when a analytic solution is not available,

the first is the goal seek function which allows to find a root of a function,

the second  is a minimizer which allow to find the minimum of a function of several variables.

While goal seek has a single target cell and changing cell, the minimizer (solver) can work out solutions to problems involving several variables, changing several cells.

You'll find a description of these tools in Excel help,

 

there are many applications of these tools,

in addition to the example provided by Ankur,

I would mention that you can calculate a dew point by solving 

 

SUM(Zi / Ki) = 1

 

where Zi is the composition  of component i

and Ki is the Kvalue of component i

this can be solved with the goal seek function in Excel 

 

you can calculate Kvalues in different ways, for high pressures preferably with a EOS,

with Google you'll be able to find examples of Excel pages for solving these problems.

Edited by serra, 19 June 2013 - 10:36 AM.

[Anshul619]

Thank you serra