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10-04-2013, 02:03 AM

Hi all I am a student doing chemical engineering and I am in the process of completing my final year design project. I am currently stuck on my detailed design and any guidance would be greatly appreciated.

Overview of unit:

The unit that I am designing is an existing refrigeration tower used in industry to flash liquid ammonia from inlet feed conditions of 8.3kPa, 19C to AtmP, -33.3C over 5 stages. This allows the ammonia to be cooled to it's bubble point without adding any energy to the system other than into the compressor which is used to re-compress and recycle vapour flash streams to inlet. For the purpose of the design I have treated each stage as an individual flash or knock out vessel for simplicity.

These 5 stages are each incrementally lower in pressure than the previous until atmospheric pressure is reached which provides the flow driving force between stages. I have also done calculations for 1,3,and 7 stage system to compare my design with.

The main reason for the stages is to reduce the duty on the compressor(s), which re-compress the flashed ammonia vapour back to inlet conditions so they can then be recycled.

As a result the compressor is compressing smaller vapour streams which are already at higher pressures and temperatures as opposed compressing the whole vapour stream from atmP back to 8.3kPa which is what would be required if a single knock out vessel where used.

Where I am now stuck:

I have performed an energy balance and calculated the Liquid and vapour flow-rates through the system using thermodynamic data on ammonia that I got from NIST database. I am now trying to size the vessel(s).

My problem now is that I don’t know how to determine the required height and volume of the column. I assumed that I could just set a diameter and determine the height by dividing the volumetric flow-rate by it. That way I get a velocity but this does not shed any light on scale because of the time component. For example if I were to do the calculations in seconds I would get a much smaller height than if I did it using hours which has left me thinking I am missing something, probably obvious. I thought that it would be necessary to incorporate acceleration due to gravity and have been playing around trying to get the desired units to no avail.

I have since used the Souders-Brown equation with a a k value of 0.03m/s however this gives me a diameter of 0.52m and height of 2.8m using a h/d ratio of 4 which seems very small as the unit is processing a total Ammonia flow-rate of 3.5tonnes/hour.

Any Information on designing a flash vessel would be greatly appreciated and thank you for taking the time to read this if I haven't bored you already!!!

ps. I have tried attaching an excel spreadsheet of what I have done so far however the uploader doesn't seem to recognize the file so if you have any guidance on how to upload it I will be happy to share my working. Thanks again!!!!

Overview of unit:

The unit that I am designing is an existing refrigeration tower used in industry to flash liquid ammonia from inlet feed conditions of 8.3kPa, 19C to AtmP, -33.3C over 5 stages. This allows the ammonia to be cooled to it's bubble point without adding any energy to the system other than into the compressor which is used to re-compress and recycle vapour flash streams to inlet. For the purpose of the design I have treated each stage as an individual flash or knock out vessel for simplicity.

These 5 stages are each incrementally lower in pressure than the previous until atmospheric pressure is reached which provides the flow driving force between stages. I have also done calculations for 1,3,and 7 stage system to compare my design with.

The main reason for the stages is to reduce the duty on the compressor(s), which re-compress the flashed ammonia vapour back to inlet conditions so they can then be recycled.

As a result the compressor is compressing smaller vapour streams which are already at higher pressures and temperatures as opposed compressing the whole vapour stream from atmP back to 8.3kPa which is what would be required if a single knock out vessel where used.

Where I am now stuck:

I have performed an energy balance and calculated the Liquid and vapour flow-rates through the system using thermodynamic data on ammonia that I got from NIST database. I am now trying to size the vessel(s).

My problem now is that I don’t know how to determine the required height and volume of the column. I assumed that I could just set a diameter and determine the height by dividing the volumetric flow-rate by it. That way I get a velocity but this does not shed any light on scale because of the time component. For example if I were to do the calculations in seconds I would get a much smaller height than if I did it using hours which has left me thinking I am missing something, probably obvious. I thought that it would be necessary to incorporate acceleration due to gravity and have been playing around trying to get the desired units to no avail.

I have since used the Souders-Brown equation with a a k value of 0.03m/s however this gives me a diameter of 0.52m and height of 2.8m using a h/d ratio of 4 which seems very small as the unit is processing a total Ammonia flow-rate of 3.5tonnes/hour.

Any Information on designing a flash vessel would be greatly appreciated and thank you for taking the time to read this if I haven't bored you already!!!

ps. I have tried attaching an excel spreadsheet of what I have done so far however the uploader doesn't seem to recognize the file so if you have any guidance on how to upload it I will be happy to share my working. Thanks again!!!!