Assignment task:
To produce pipeline quality dry natural gas, sourced gas must undergo processing to remove impurities. These impurities include water, carbon-dioxide and the heavier hydrocarbons (essentially reducing every component except the methane and ethane to trace amounts). Water must be removed at the wellhead to prevent the formation of methane hydrates within the gas processing plant and the gas transmission pipelines.
Usually the removal of water vapour from natural gas is performed using either absorption (e.g. glycol dehydration) or adsorption using a solid adsorbent (e.g. solid-desiccant dehydration). Heavier hydrocarbon compounds may then be removed using absorption while lighter hydrocarbons such as ethane require a cryogenic expansion process to extract a significant portion.
In the present processing plant, the reduction of the water content to an acceptable level is to be achieved by dehumidification using a mechanical refrigeration plant. Natural gas water content of less than 0.1% by weight will be considered an acceptable level.
The natural gas is supplied to the refrigeration plant near the wellhead at 42oC and 1.5 bar. Under these conditions it is found to consist of:
Methane (CH4) - 80.0%wt
Ethane (C2H6) - 9.0%wt
Propane (C3H8) - 3.0%wt
Carbon Dioxide (CO2) - 2.0%wt
Nitrogen - 1.0%wt
Water vapour (H2O) - 5.0%wt
You are to specify the requirements of the refrigeration system capable of reducing the water vapour content from the natural gas source to acceptable levels. As part of this process, you should:
Find suitable phase and material property data for the constituent gases and the water vapour.
Determine the properties of the gas-vapour mixture using suitable models.
Derive psychrometric relationships for the gas-vapour mixture and create the associated
psychrometric chart (the chart should include at least the dry bulb temperature, specific and relative humidity and enthalpy).
Plot the water removal process on your psychrometric chart.
Determine necessary operational requirements of the refrigeration system:
o the minimum temperature of the natural gas required for the process and
o the heat to be removed per unit mass of natural gas
State all assumptions made and discuss the limitations of your modelling and hence the accuracy of your results.
Your solution should be communicated in a short report. The report (not including the cover page) must be no more than eight (8) pages long.