Question 1 Natural gas typically contains many liquid hydrocarbon components with the heavier components found in smalle

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Question 1 Natural Gas Typically Contains Many Liquid Hydrocarbon Components With The Heavier Components Found In Smalle 1 (110.6 KiB) Viewed 9 times

Question 1 Natural gas typically contains many liquid hydrocarbon components with the heavier components found in smaller amounts than the lighter gaseous ends. It is the heaviest weight components that first condense and define the hydrocarbon dew point temperature of the gas. The dew point temperature also moves in relation to pressure. Consider the process flow diagram shown in Figure 1 for a simple gas plant. The feed composition and conditions are shown in Table 1. The impact of liquid carry over on the sales gas dew point is significant for even a small amount. For 1 % liquid carry over, the dew point offset was about 3.3 °C. Thus, we need to demonstrate the control to maintaining the "spec dew point". It is desired to process the feed gas to produce a sales gas with a dew point of -6.7 °C at 3.723 MPa. The feed gas is compressed to 3.827 MPa by a compressor driven by gas turbine, where the power to the gas turbine originated from fuel gas combustion. The flowrate of the fuel gas was fluctuating due to pressure problem in upstream process. The stream is then air cooled with a fan to 43.3°C. The speed of the fan can be varied depending on the current supplied to the fan. In order to achieve the low temperature needed, the stream is then cooled in the heat exchanger 1, heat exchanger 2 and finally in the refrigeration cooler to -6.7°C before entering the cold separator at 3.723 MPa. Heat Exchanger 1 and 2 are design for full flow to cross exchanging the hot stream with cool stream to recycle the heat without any control. Doing this will allows for lower refrigeration duty and saving cost. To ensure efficient separation, the level of cold separator needs to be maintained at height around 1 to 1.5 m. The chilled sales gas from the cold separator is sent to heat exchanger 1 to warm it up to sales gas spec of 25 °C. The pressure of the liquid LPG from the cold separator is reduced to 0.2 MPa in the JT valve before heated up to 30 °C in heat exchanger 2 and sent for utilization. It is crucial that the pressure of the LPG does not fall below 0.2 MPa at any time as it will impact the downstream process. Refrigerant is used as cooling medium for the refrigeration cooler.
Table 1. Feed composition and condition Component Mole % Nitrogen 1.93 Carbon Dioxide 2.71 Methane 58.67 Ethane 16.17 Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane T (°C) P (MPa) Flow (MMSCFD) 12.86 1.58 0.70 0.84 3.73 0.81 35 1.379 5 a) Based on the process description, draw your design of the control systems for the whole treatment plant on Figure 1. In a control design form, identify the control objective, control configuration, and classify different types of variables involved for each control loop. You will need to add control valves or connections to existing final control elements as the manipulated variable. Justify the need for each control loop. [19 marks] b) Choose the control system for 3 different types of variables. For each of the control system. select the exact suitable type of sensor that should be used for the conditions given. Justify your choice of sensor. [6 marks]
Loop 1 2 3 4 5 Objectives Control Configuration Controlled Measured Variables Manipulated Disturbance
FUEL GAS Feed Gas COMPRESSOR Fast AIR COOLER HEAT EXCHENGER 1 REFRIGERATION. COOLER HEAT EXCHENGER 2 JT VALVE Figure 1. Process flow diagram for simple dew point control gas plant COLD SEPARATOR LPG SALES GAS