The volume, and hence mass of gas evolved from a dissociating sample, as well as the rate of gas evolution, can be measured with high precision in the gas flowmeter and collection apparatus shown in Figure 5 that is based on the principles of a Torricelli tube.
The apparatus consists of a closed-ended hollow cylinder made of stainless steel, inverted in a reservoir of distilled water, and suspended from a precision load cell. A column of water is initially drawn up by vacuum into the cylinder, and the weight of the cylinder plus water column is recorded as an initial baseline measurement. Maintenance of a constant drip rate into and out of the reservoir of water in which the cylinder is suspended ensures that the water level in the reservoir remains constant, and hence the buoyancy force on the cylinder remains constant. Methane is bubbled through the water to saturate it prior to dissociation, minimizing further methane solution during testing. Collection of a steady weight baseline is then made on the primed system, after which the hydrate sample can be opened to the flow meter.
As methane gas evolves from the dissociating sample and displaces water in the cylinder, the cylinder weight decreases. The mass of methane collected in the cylinder is then calculated from that weight change by incorporating additional run parameters into the data analysis, including: temperature inside the cylinder, atmospheric pressure, partial pressure of water vapor in methane, and the equation of state of methane gas. Calculation of the number of moles of methane can be simplified by using the ideal gas law, a procedure that accurately predicts the small relative changes of the sample pressure over the range of operation. The capacity of the flow meter shown in Figure 5 is 8 liters, and a typical methane hydrate sample made from 26 g of seed ice will release about 6 liters of gas during dissociation. Samples of the gas can then be collected for subsequent compositional analysis by attaching an evacuated receiving vessel to the vent/vacuum line. The flow meter operates with high accuracy at both high and low rates of gas flow, a result that we have verified over the range 0.01 to 3000 ml/min. Further details are provided by Circone et al., (2000).
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