Effect of Impurities on Propane Refrigeration System – Constant Approach Temperature

In this Tip of the Month (TOTM), we will continue our discussion of the effect of working fluid impurities on the performance of refrigeration systems employing propane as the working fluid. Specifically, we will study the effect on the compressor power, the refrigerant circulation rate, and the condenser duty.

In the previous TOTM, 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 this TOTM, we will revisit the same case for constant approach temperature. In other words, the chiller outlet temperature was kept at -35°C but the chiller inlet temperature/and pressure varied due to the presence of impurities.

The details of a simple single-stage refrigeration system and a two-stage refrigeration system employing one flash tank economizer are given in Chapter 16 of Gas Conditioning and Processing, Volume 2 [1]. Similar to the previous TOTM, the process flow diagrams for the simple and with flash economizer refrigeration systems are shown in Figure 1. Note that pressure drop in different segments of the loops have been considered.

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 Tables 2 A&B.

 

For graphical representation of the simulation results, the data in Tables 2 A&B 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. Recall that case 1 is pure propane which was used as the base case.  As shown in this figure, the ethane impurity increases the circulation rate. The increasing butanes impurities cause a decrease in the circulation rate. The effect of ethane is approximately twice that of butane for a given impurity level.

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 both ethane and butane impurities increase the condenser duty and the compressor power requirement. It is interesting to note that the effect of butane impurities is two times higher than ethane impurities for the same level (mole fraction) of impurities.

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. In order to keep the chiller outlet temperature at the specified value, the incoming refrigerant temperature had to be decreased which resulted in lower chiller pressure (See Tables 2 A&B). This caused an increase in the compression ratio and consequently in higher compressor power consumption. Also, as the ethane impurity increases, the compressor discharge (as well as condenser) pressure increases. In case of simple refrigeration, the compressor discharge pressure increases from 1270 to 1417 kPa when ethane mole fraction is changed from 0 to 5 percent.
4. For the case of pure propane, the compressor power and condenser heat duty are minimum.
5. For the economizer, the feed to the first stage of the compressor is heavier than the feed to the second stage due to the mechanisms of flash separation. The heavier components go with liquid stream and the lighter components go with vapor stream.

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

By Dr. M. Moshfeghian

Reference:

1. Campbell, J.M., “Gas conditioning and Processing, Vol 2: The Equipment Modules”, 8^th 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.