This paper has applied thermodynamics principles to evaluate the reliability of 390 m3/hr natural gas processing plant. The thermodynamics equations were utilized in the evaluation, characterization, and numerical simulation of key process parameters in natural gas liquid extraction plant. The results obtained show the comparison of the coefficient of performance, compression ratio, isentropic work, actual work, electrical power requirements, cooling water consumption in intercoolers, compressor power output, compressor capacity, and isentropic, volumetric, and mechanical efficiency of the two-stage refrigeration unit with a flash gas economizer and these were compared with the designed specifications. The second law of thermodynamics was applied in analyzing the refrigeration unit and the result shows that exergetic losses or lost work due to irreversibility falls within operating limit that is less than 1.0%. Similarly, the performance of expansion turbine (expander) parameters was monitored and the results indicate a considerable decrease in turbine efficiencies as the inlet gas pressure increases resulting in an increased power output of the turbine leading to a higher liquefaction rate.
The production and availability of natural gas liquid depend largely on the supply of raw natural gas from production wellhead and the operating conditions of the process unit that make up the extraction plant. Most gas processing plants are faced with problems ranging from inadequate supply, poor facility performance, and human factors. These problems can lead to low productivity of natural gas liquids and reduction in gas quality which could result in shutting down of the plant. Poor facilities such as inadequate electricity supply and processed water supply used in process equipment also lead to intermittent operations and malfunction of process equipment such as pumps, compressors, and valves if they are not adequately checked.
Human factors may also result from the inability of gas plant operators to monitor thermodynamics parameters such as the pressure, flow rates, and temperature on process equipment which could result in loss of data in control room and unintended activation or deactivation of process devices and reduce the plant efficiency. In spite of the fact that these thermodynamics parameters are monitored daily in a gas plant, there are problems of low gas feed inlet pressure and insufficient gas flow rate. These have resulted in low volume of natural gas liquids produced, the extracted natural gas liquids not attaining the expected cryogenic temperature requirement, and variation in gas quality discharging from the outlet of natural gas liquid extraction unit bottom product.
If the available inlet gas pressure is low, it can result in compressor system suction pressure falling below atmospheric pressure. This can also lead to air leakages into the compressor system, contributing to pulsation, corrosion, and low heating value of natural gas. In order to solve these problems, performance evaluation of the process units in natural gas liquids extraction using thermodynamics principles is necessary to ensure that these problems are minimized.
It is seen from literature survey that several papers have been published focusing on thermodynamic analysis of a gas turbine power plant.
Rahman et al. [
Keith and Kenneth [
This paper examined the reliability of a gas plant which is the ability of a plant to maintain a stable efficiency with respect to time using thermodynamics equations.
The simulation of process units in propane refrigeration cycle involves the applications of thermodynamics principles to the following. Calculate the amount of heat added to or removed from process streams. Estimate the power requirements for process equipment such as pumps, compressors, and turbine. Evaluate the performance of a flash separator at various temperatures and pressures. Determine the bubble and dew point temperature associated with distillation and bottom products.
A systematic diagram of propane refrigeration cycle is shown in Figure
Process flow diagram of propane refrigeration of natural gas with application of Hysys software.
A turbo expander or an expansion turbine is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce useful work that is used to drive a compressor. Because work is extracted from the expanding high pressure gas, the expansion is an approximation by an isentropic process (a constant entropy process).
In the process of producing work, the expander lowers the bulk stream temperature which could result in partial liquefaction of the bulk stream (natural gas).
A systematic diagram of an expansion turbine (expander) is presented in Figure
Flow diagram of expander process.
The simulation of process units in propane refrigeration resulted in loss of energy on the system. The second availability balance was applied to calculate the energy losses or lost work due to irreversibility of the process. The thermodynamics equations applied on each of the process units are as follows [
Compressor unit:
condenser unit:
expansion valve:
Refrigerant chiller: the chiller is the unit where the process gas (natural gas) gives up its energy to the liquid refrigerant. The refrigerating effect which is the amount of heat absorbed by refrigerant or removed from the refrigerated space is expressed as
The fraction of refrigerant vaporized in the chiller is determined from enthalpy balance as follows:
The results obtained from the evaluation with the above equations are shown in graphical form in Figures
Pressure enthalpy diagram of a propane refrigeration cycle.
Temperature entropy diagram of a propane refrigeration cycle.
The results on the refrigeration unit of a gas plant were obtained using the
The two-stage compression refrigeration system in Figures
Summary of two-stage compression refrigeration.
Parameters | Units | 1st stage compressor | 2nd stage compressor |
---|---|---|---|
Suction pressure | bar | 1.9 | 2.5 |
Discharge pressure | bar | 2.5 | 5.0 |
Compression ratio | — | 1.316 | 1.414 |
Isentropic work of compression | J/kg | 20,644.25 | 22,094.91 |
Actual work of compression | J/kg | 20,644.25 | 21,872.85 |
Isentropic efficiency | % | 48.56 | 51.98 |
Mechanical efficiency | % | 20.71 | 46.51 |
Volumetric efficiency | % | 55.0 | 46.7 |
Actual power requirement | KW | 0.0969 | 0.0960 |
Cooling water consumption in intercoolers | m3/s | 7.2612 | 19.282 |
Compressor capacity | m3/s | 2.1566 | 2.1096 |
Work input in comp 1 and comp 2 | J/kg | 273.14 | 273.14 |
Work output | J/kg | 1197.89 | 2414.11 |
The process parameters of other auxiliary units in propane refrigeration plant are shown in Table
Process parameters of other auxilliary units.
Parameters | Unit | Condenser | Economizer | Expansion valve 1 | Expansion valve 2 | Refrigerant chiller |
---|---|---|---|---|---|---|
Suction pressure | bar | 5.0 | 3.3 | 4.7 | 3.3 | 2.3 |
Discharge pressure | bar | 4.7 | — | 3.3 | 1.9 | 1.9 |
Heat load | J/kg | −17,386.7 | 5733.78 | — | — | 17,036.04 |
Mass flow rate of vapor—economizer | kg/s | — | 0.0034 | — | — | — |
Mass flow rate of liquid—economizer | kg/s | — | 0.1305 | — | — | — |
Mass flow rate of cooling water—condenser | kg/s | 6.72 | — | — | — | — |
Joule Thompson coefficient | — | — | — | 8.57 | 12.86 | — |
Mole fraction of liquid remaining in chiller | — | 0.035 | 0.00014 | |||
Mole fraction of vapor evaporated in chiller | — | 0.025 | 0.025 | — | 0.045 | |
Mass flow rate of liquid—condenser accumulator | kg/s | 0.1339 |
The energy balance of the propane refrigeration unit obtained from the evaluation based on the equations highlighted for the analysis is shown in Table
Energy balance propane refrigeration unit.
Process units | Energy gain (KJ/kg) | Energy lost (KJ/kg) | % of lost work |
---|---|---|---|
Compressor 1 | 12.30 | −334.28 | −0.015 |
Compressor 2 | 32.11 | −501.45 | −0.023 |
Condenser | — | 12483.89 | 0.06 |
Expansion valve JT-1 | — | 12495.54 | 0.06 |
Expansion valve JT-2 | — | −1435547.8 | 6.62 |
Economizer drum | 5733.78 | — | — |
Refrigerant chiller | 17036.04 | −1242710.43 | −5.73 |
|
|||
Total | 22814.23 | 216981.07 | 0.972 |
By the design conditions, the centrifugal compressors used in compressing the refrigerant should operate with a pressure ratio of 1.2 : 1 and 1.4 : 1, isentropic efficiency of 70–80%, volumetric efficiency of 60–89%, and mechanical efficiency of 20–50%. By analytical method, the pressure ratio was found to be 1.316 and 1.414; the isentropic efficiency was found to be 48.56% and 51.97%; the volumetric efficiency was found to be 55.0% and 46.7% for the first and second stage compressors, respectively, while the mechanical efficiencies are 57.7% and 46.51%, respectively. The refrigeration cycle was operating with an overall coefficient of performance of 62.37 with a refrigerating capacity of 4,922.2 tons after examining the performance of other auxiliary units within the systems. Tables
The thermodynamics equations were applied to construct Figures
The following recommendations are highlighted to ensure optimum efficiency and reliability of NGL plant. The feed gas must be free from CO2 and water. This affects plant efficiency and operations if not properly checked by freezing the fittings, valves, and other associated equipment. The inlet strainer differential pressure must not be high; otherwise the expander will trip off on high differential pressure. The refrigeration unit must be operated within the operating and design conditions to avoid overfreezing or warming of the demethanizer column. There is need for proper insulation of piping system within the NGL extraction plant. The reason is not to allow the surrounding heat to enter the system, thereby warming the system, and there may be freezing out of the plant. Proper sizing of the process line using the various line balance methods to compare the amount of natural gas entering the process plant with the amount put in should be adopted at the design stage of the process plant.
The specific heat capacity of water
The enthalpies of refrigerant at compressor inlet (KJ/kg), outlet (KJ/kg), vapor mixture coming from compressor and economizer units (KJ/kg), at the condenser inlet (KJ/kg), and at the inlet of the refrigerant chiller (KJ/kg), respectively
The enthalpies of the gas expander inlet, gas expander outlet, and at the exit pressure but at the inlet entropy (KJ/kg), respectively
The specific heat capacity ratio of gas to expander
The lost work or rate of irreversibility of refrigerant (KJ/kg)
The mass flow rate of the gas to expander (kg/s)
The refrigerant circulation rate
The mass flow rate of cooling water (kg/s)
The inlet and outlet gas pressure to expander (bar), respectively
The amount of heat removed in the intercoolers
The entropies of refrigerant at compressor inlet (KJ/kg·K), compressor outlet (KJ/kg·K), leaving the economizer (KJ/kg·K), and vapor mixture (KJ/kg·K), respectively
The entropies of inlet cooling water and outlet cooling water, respectively
The dead state temperature
The inlet gas temperature to expander (°C)
The actual gas temperature at expander outlet (°C)
The theoretical gas temperature at expander outlet (°C)
The electrical power requirement to compressor (KW)
The mole fraction of vapor leaving the economizer unit
The temperature difference within the system boundary
The expander efficiency
The mechanical efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.