In this Tip of the Month (TOTM), we will demonstrate the effect of working fluid impurities on the performance of a simple propane refrigeration system and another one employing a flash economizer. Specifically, we will study the effect on the compressor power, the refrigerant circulation rate, and the condenser duty.

The objective of a refrigeration system is to “pump” low temperature heat from the process fluid to high temperature (ambient) where it is rejected to the environment. Energy is required to pump heat. The amount of energy depends on the quantity of heat to be “pumped’ (chiller duty) and how far the heat has to be pumped (temperature difference between the chiller and the condenser). Compression refrigeration is by far the most common mechanical refrigeration process. It has a wide range of applications in the gas processing industry. It provides chilling for:

1. NGL extraction, LNG production, and LPG product storage
2. Hydrocarbon and water dew point control
3. Reflux in deethanizers/demethanizers

Figure 1. Process flow diagram for the simple and flash economizer refrigeration systems

Let’s consider the case of the previous TOTM in which the objective was to remove 2778 kW from the process gas at -35°C and rejecting it to the environment by the condenser at 35°C. The pressure drop assumptions were: in the line from the compressor discharge to the condenser and in the condenser 50 kPa, in the chiller 5 kPa and in the compressor suction line 30 kPa, between the two stages of flash economizer 20 kPa and between the flash tank and second stage of compressor 20 kPa. Pure propane was used as the base working fluid. An isentropic efficiency of 75 % was used in all cases. For the flash tank economizer, the optimized interstage pressure which minimized the total compressor power was used. In this study, all of the simulations were performed by HYSYS software [2].

The production of propane is through fractionation of NGL; however, achieving a purity of 100% is not economical. Therefore, propane as the working fluid has normally a small fraction of ethane and butanes. We considered these components as impurities and studied their effect on the performance of the propane refrigeration system. The composition and molecular weight of the eleven cases studied are shown in Table 1. The last column represents the ratio of mixture molecular weight to the molecular weight of propane. As in the previous TOTM, we considered the single stage (simple) and the two stage (economizer) refrigeration systems. The summary of simulation results is shown in Table 2.

For graphical representation of the simulation results, the data in Table 2 are plotted in dimensionless form in Figures 2 through 4. Pure propane refrigeration system has been chosen as the base case and the performance of other cases are compared to the base case. Figure 2 represents the effect of ethane and butane impurities on the required circulation rate. Note that in this figure and in the subsequent figures, the y-axis is the ratio of case 2 through 11 variables (circulation rate, condenser duty or compressor power) divided by the corresponding case 1 variable, respectively.  Similarly, the x-axis is the ratio of cases 2 through 11 molecular weights to the molecular weight of case 1. It should be reminded that case 1 is pure propane which was used as the base case.  As shown in this figure, contrary to butanes, the ethane impurity has no practical effect the circulation rate. The increasing butanes impurities cause a decrease in the circulation rate.

Figures 3 and 4 represent the effect of ethane and butanes impurities on the condenser heat duty and the required compressor power requirement, respectively. These two figures indicate that butane impurities don’t have practical effect on the condenser duty and the compressor power requirement. However, ethane impurities increase the condenser duty and the compressor power requirement. In this study, the chiller inlet temperature was kept at -35°C but the chiller outlet temperature varied due to the presence of impurities; therefore, the approach temperature changed. In the next TOTM, we will revisit this case for constant approach temperature.  

On reviewing Figures 2 through 4, the following observation can also be made:
1. The impurities affect the performance of the simple refrigeration systems.
2. The trend of impurity effect is similar for the simple refrigeration system and the refrigeration system employing a flash tank economizer.
3. The effect of impurities on the performance of a flash tank economizer is less pronounced than on a simple refrigeration system.

To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing - Special), and G7 (Process Simulation in Gas Conditioning and Processing) courses.

By Dr. M. Moshfeghian

Reference:
1. Campbell, J.M., “Gas conditioning and Processing, Vol 2: The Equipment Modules”, 8th Edition, Edited by R.A. Hubbard, John M. Campbell & Company, Norman, USA, 2000.
2. ASPENone, Engineering Suite, HYSYS Version 2006, Aspen Technology, Inc., Cambridge, Massachusetts U.S.A., 2006.