Suggested Response - Q5

** Nitrogen can be supplied as a gas in pressurised containers. If you only need the nitrogen as gas, why do you think most users still use liquid nitrogen? (Hint – Most nitrogen gas canisters are at 150 bar gauge pressure). Assume that both liquid and gas cylinders have a volume of 100 L. Nitrogen has an expansion ratio of 694.

Response:

Transportation costs need to be considered. If we have 100 L of liquid nitrogen, we can calculate how much gas this will give us. The expansion ratio is given at 694 for nitrogen. Thus:

V subscript 1 equals 100 cross times 694 equals 69 comma 400 L

Next, we need to use Boyle’s law:

P subscript 1 V subscript 1 equals P subscript 2 V subscript 2

Rearrange the equation to solve for V1 and enter the values: 

V subscript 1 equals fraction numerator P subscript 2 V subscript 2 over denominator P subscript 1 end fraction equals fraction numerator 151 cross times 100 over denominator 1 end fraction equals 15 comma 100 L

Now let’s calculate the number of gas cylinders we would need for every liquid cylinder:

N equals fraction numerator 69 comma 400 over denominator 15 comma 100 end fraction equals 4.59

Clearly the transport costs would be a lot higher if we didn’t use liquid nitrogen. You would also have to exchange cylinders more frequently.

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