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Pressurised nitrogen gas is stored in a container at a temperature of 290K. A pipeline transfers the gas from the container to a point of use. An isolation valve - installed right at the container - is used to shut off the flow when necessary. On a certain occasion, the operator does not close this valve completely and the flow of gas continues (albeit at a much lower rate) with a pressure drop of 29.5 bar over the valve. a) Do we assume the expansion of gas over a valve to be isenthalpic or isentropic? Why? (2) b) Calculate the temperature of the nitrogen directly after the valve. Assume that the van der Waals equation of state is sufficient to describe non-idealities. The " d " parameter in van der Waals is relatively small for nitrogen, so (only to simplify your calculations here) you may assume that a=0. Furthermore, simply take the Cp value of nitrogen as constant within this temperature range. (8)