4 replies to this topic

[peaston]

At university we are in the process of designing the topsides of an oil and gas platform, meeting certain specifications.  having done our heat and mass balances we had to start doing individual tasks, mine being the refrigeration system.

 

So far, we have identified one part in the design that requires the use of refrigeration systems.  the other coolers used are adequate to have seawater being used to provide cooling.

 

The cooler has the following specifications:

 

Inlet Stream coming from component splitter (glycol unit)

Output stream going to a separator

 

Duty of the cooler = 9.896e+6 kJ/h with volume = 0.1m3

 

Temperature feed = 30degC

Temperature output = -5degC - needed to achieve the export cricondenbar 

 

Pressure maintained at 7000kPa

Molar flow kept at 3248.8 kgmole/h

Mass flow = 71,140.7 kg/h

molec. weight = 21.9

 

 

My main concern is deciding how to act in designing this system.

 

So far we have agreed to use propane, R-290, as the refrigerant (readily available in oil and gas platforms, can cool to temperatures as cold as -40degC, no ozone depletion potential, etc).

 

I have a Mollier chart for propane as well to work with.

 

 

we did a refrigeration question at uni that i was struggling to understand.

 

I understand the process involved in refrigeration systems:

• Compressing
• Condensing
• expansing (Joule-Thomson Cooling)
• and evaporating

the cold refrigerant is used to extract heat from the process - as the refrigerant cools the process it absorbs heat, hence it vapourises.

 

So first plan of action i believe would be to establish the process duty - how much heat must be removed from the gas.

 

I need to find a means of condensing propane, which i believe to be either air or cooling water? from that establish what pressure you have to compress the refrigerant to allow it to condense...

 

 

I hope I am on the right track, but some further help would be greatly appreciated.  if you require more data then please ask, I at current dont have access to HYSYS/UNISIM to upload the file but I have a fair bit of data that can be given.

 

Many thanks in advance!

[Art Montemayor]

Peaston:

 

I believe you are going about resolving your assignment in the wrong direction – or without proper direction and experience.  Allow me to explain some important and critical design points that set the basis for offshore platforms operations:

• The offshore platform probably represents the most expensive real estate property on the face of the earth.  Therefore, you never want to incorporate any processing operation that can be avoided – especially any operation that involves risk taking with respect to toxic, flammable, or explosive substances.
• Of course, the obvious fact is that you are already handling and having to process dangerous, flammable, and explosive fluids.  But that can’t be avoided in order to produce, separate, and transport them to shore where they can be safely stored, distributed, and processed further with much more available space, time, and safe resources.  The point here is that if you can avoid doing anything on the platform, then take that alternative – for the sake of SAFETY and CAPITAL EXPENDITURE.  There is an old saying here in the Gulf of Mexico: “There are no parking lots on offshore platforms”.  What is meant here is that should there be a hazardous or deadly event occurring on the platform, there is no place to run to or hide.  I strongly recommend you to accept and apply that philosophy and criteria when doing any design work for offshore facilities.  It is already bad enough that you have to separate the gases from the crude oil and the produced water and then proceed to dry the natural gas and pipeline the crude oil and natural gas to shore while pumping the produced water back into the reservoir.  That, you can’t avoid in order to produce cash flow and justify the whole offshore investment.
• By stating that you are cooling your produced (and separated) gas, “needed to achieve the export cricondenbar” I presume that you are proposing to export the produced hydrocarbon gases as a mixture of saturated fluid in a pipeline.  You also propose to achieve the cricondenbar by cooling the compressed gas to -5 ^oC.  This I believe is rather impractical if you are located on an offshore platform since your only way of exporting the products is normally through a subsea pipeline.  Unless you are located in the artic, the sea water is going to heat up your cooled product and negate all the refrigeration you accomplished on the platform.  Am I right?

With a molecular weight of 21, I suspect that you are producing an associated natural gas stream that must be dried first before exporting to land.  Am I right again?  If so, what is called for is a dehydration process – such as a TEG dehydration unit – and not a refrigeration unit.

 

Refrigeration with propane as a refrigerant is no problem – on land.  I would never attempt to justify using it on a platform installation.  Firstly, you DON’T HAVE PROPANE AVAILABLE on the platform unless you import it from a land facility.  Common sense tells you that even if there is propane in your gas product mixed with other hydrocarbons, it is useless to you as a source for refrigeration.  Does this make any sense to you?  You totally lack the luxury of having propane “readily available in oil and gas platforms”.  Don’t forget: you would have to store and handle the liquid propane on the platform and – as I have tried to explain to you – no worker in his right mind would want to work on that platform for fear of the risks involved in dealing with a propane explosion around the storage facilities (which would have to be justified).  If you are really serious about using a mechanical refrigeration system on board a platform, then at least employ R-134a and condense it with seawater – much as you would using ammonia or propane.  Let us know if you want to continue to design a mechanical refrigeration system and we can certainly help you on that.

 

On a side note:  the free, adiabatic expansion of a liquid refrigerant in a mechanical refrigeration unit IS NOT an example of the Joule-Thomson effect.  Messrs. Joule and Thomson (a.k.a., Lord Kelvin) experimented and formulated the effect using REAL GASES, not liquids.  This is a common misconception shared by many practicing engineers.

[shan]

On an offshore platform, produced gas may be cooled lower than seawater temperature to knock off heavier hydrocarbon components to satisfy gas dew point specifications.  However, -5 C is unusual low temperature.  If the process really needs such a low temperature, you may have to a very good pre-dehydration (mole sieve?) for the produced gas to prevent the form of hydrate and ice.   

[peaston]

Point to note:

• my advisor with  several decades of experience working in industry has advised me that propane is suffice to use as the refrigerant, but obviously there are risks arising from it, which i need to identify in the hazards assessment of the report and give justification as to why the refrigerants pro's outweigh the cons.
• the temperature on needs to be cooled from the glycol unit to -5degC before it undergoes further separation, so with regards to it being passed thru the subsea pipework, this can be ignored.

Having used propane I have managed to compute a number of things:

• since required temperature is -5degC it would be wise to have the refrigerant lower than that, i this case, I chose -10degC
• the condensing temperature i have chosen to be 25degC
• refrigerant flow rate = 3.35x104 kg/h
• Work of compression = 524kJ/s from using a propane compressor with 80% efficiency 
• condensing duty = 3273 kJ/s
• mollier chart for propane used is attached

 

 

file://localhost/Users/andrewpeaston/Desktop/Photo%20on%2027-03-2013%20at%2012.42.jpg

Attached Files

•  Photo on 27-03-2013 at 12.42.jpg   196.83KB   7 downloads

Edited by peaston, 27 March 2013 - 07:34 AM.

[Art Montemayor]

 Peaston:

 

Please take a look at the attached Propane Workbook that I put together in 30 minutes.  It illustrates the engineering manner of presenting basic data.  Using your cellular phone to generate an illustration of a Propane Mollier Diagram really “sucks”.  If you don’t agree with my assessment, try handing that in with your project for grading by your instructor.  I’m afraid if he/she are really have industrial experience, he/she will say the same.  You simply can’t read the graph with accuracy.   My goal here is to help you with your project and enable you to successfully present a viable, credible, safe, engineering design that meets the needs and criteria of society and thereby make a high grade in your course.

 

My points to note are as follows:

• Your academic advisor may have several decades of experience working in industry (I have 53 years), but I seriously doubt that anyone who has worked on an offshore platform would ever consider using a propane mechanical refrigeration cycle on a platform.  It is a very, very serious safety issue and hazard – not only on the normal process leaks and spills that take place from time-to-time, but also on the required storage of the propane and its routine requirement to import it from a shore facility and use the platform cranes and personnel to physically transfer the propane from a barge (or supply ship) and onto the platform.  These are very serious and delicate operations that are always carefully scrutinized for application on an offshore platform – not only by the owner/operator, but also by the local safety regulation agencies that permit, inspect, and supervise such operations.  My point here is that this is a very serious issue that any engineering student should be totally aware of and always take into consideration.  The plain and common truth is that there is NEVER any refrigerant-grade propane normally available on an offshore platform.  You have to import it.  Merely because propane is a hydrocarbon and may be present as a constituent in the crude oil produced, doesn’t mean that it is available as a refrigerant to be used locally on-board.  I have tried to make this clearly known to you in my prior post, but you have opted to avoid discussing this safety issue.
• Although you seem to be located in the UK, you are having trouble writing in English.  I can’t understand your sentence: “The temperature on needs to be cooled from the glycol unit to -5 deg C before it undergoes further separation, so with regards to it being passed thru the subsea pipework, this can be ignored.”  I don’t understand what you propose and what you describe.  Could you furnish a detailed sketch of your proposed process?
• In the same way that you have to have a temperature driving force for your refrigerant evaporator (you choose 5 oC), so also must you supply a driving force between your cooling media and your refrigerant in the condenser.  You have selected 25 oC as your condensing temperature, but you fail to identify what your cooling media is going to be.  Is it seawater? Atmospheric air?  Whatever it is, it must be at least 5 oC BELOW the condensed refrigerant.
• You seem to have already calculated the refrigerant flow rate, work of compression, and condensing duty but you haven’t said how you’ve done it nor furnished your calculations to show that your algorithm is correct.  What are we on the Forum supposed to do with the results?

 For those of us who have worked on an offshore platform and continue to work on such projects the issue of platform safety is our #1 concern.  As an example of what can happen on an offshore platform when a lack of safety design and proper concern exists, read about the Piper Alpha disaster: http://en.wikipedia....iki/Piper_Alpha

 

You have not addressed nor responded to my direct questions on your proposal, so I am left with the understanding that you don’t attach importance to such safety matters.  I am sorry for not successfully passing on serious engineering concerns involving an offshore installation and how safety and serious engineering is done on such installations.  I consider it my personal failing if I don’t tell you the REAL TRUTH about the important safety concerns involving offshore platforms and consequently cannot contribute to this thread applying what I consider to be a very hazardous application.  To do so would be to tacitly convey to you and other students that it is OK to make such designs without considering the dark side of what can tragically happen.  I repeat what I said before: “There are no parking lots on offshore platforms”.  Design carefully and safely.  Your future in engineering will be determined by that criteria.   Propane Mollier Diagram.xlsx   506.78KB   35 downloads