3 replies to this topic

[Art Montemayor]

Oman:

You have presented a typical, minor Thermodynamics problem, but you have not told us your background in Thermodynamics (how many courses, hours, etc.). As a result, I am forced to make some basic assumptions in order to try to help you resolve your problem:

• You have had two Thermo courses, but you have slept through most of them;
• You are probably in your 3rd year of Chem. Engrg. University curriculum;
• You have been assigned a homework problem to calculate the size of an anhydrous ammonia storage tank with its contents at 9 barA;
• You have grave difficulty in communicating what your mind knows to be the problem;
• One major cause for this difficulty is your failure to state engineering information in a scientific or engineering manner: pressures should be identified as absolute or gauge;

The answers to your questions are:

1. Are we supposed to use density of liquid or specific volume to get the total volume of ammonia required??? You can use either the density or the specific volume – as long as you know what it is that you are doing.
2. What are the maximum capacity of ammonia storage tanks at these conditions???? Why are you worried about the maximum tank capacity? Find the size of the required tank to contain the given amount of liquid ammonia (I get a 4 meter diameter x 10 meter long cylindrical vessel). The maximum size tank is determined by optimizing what you can build and afford based on the profit you can make.

I hope this resolves your questions and helps you out.

[djack77494]

Oman:
A minor point to be made when designing or using pressurized storage vessels for liquified gases storage - do not design for full vessels. In fact, you should limit the active (useful) liquid storage capacity to not more than 85% of the actual capacity. This allows for some expansion of the fluid and also filling from a tanker that might be at a slightly warmer temperature.

[Art Montemayor]

Doug:

Very good comment and a major, real-life point. I didn't include this valuable piece of practical advice because I was in an Easter weekend rush; but I took it into consideration for the estimated sizing calculation. I simply applied a pure, cylindrical tank, without any 2:1 Ellipsoidal heads as I normally would to come up with the size estimate. I wanted to demonstrate that the required tank is not that big - by today's standards.

I hope Oman takes these hints and makes the appropriate corrections to his calculations after finding out the whys, wheres, and whens of this engineering design challenge.