In this tip of the month (TOTM) we will present the results of several case studies showing the effect of gas molecular weight on the performance and efficiencies of centrifugal compressors. We have considered several “what if” scenarios such as variation of compressor speed as a function of molecular weight, while maintaining the same suction and discharge pressures and mass flow rate. Variation of polytropic head and efficiencies as a function of gas molecular weight for a given compression ratio, and compressor speed has also been studied. In addition, the impact of thermodynamic properties package has been studied.

Compressors can be generally classified in two categories:

  1. Positive displacement; this type of compressor includes reciprocating, rotary screw, sliding vane, liquid ring and rotary lobe. The compression principle is volumetric displacement – reducing the gas volume increases pressure.
  2. Kinetic or Dynamic: this type of compressor includes centrifugal and axial compressors. The compression principle is acceleration and deceleration of the gas – kinetic energy is converted to pressure rise.

Reciprocating and centrifugal compressors are the most popular compressors used in E & P applications. Rotary screw compressors are gaining in popularity in low to moderate pressure gas boosting service, refrigeration systems and fuel gas compression for gas turbines. Further detail may be found in reference [1].
From a calculation viewpoint alone, the power calculation is particularly sensitive to the specification of flow rate, inlet temperature and pressure, and outlet pressure. Gas composition is important but a small error here is less important providing it does not involve the erroneous exclusion of corrosive components. A compressor is going to operate under varying values of the variables affecting its performance. Thus the most difficult part of a compressor calculation is specification of a reasonable range for each variable and not the calculation itself. Maddox and Lilly [2] emphasize that using a single value for each variable is not the correct way to evaluate a compression system.
Normally, the thermodynamic calculations are performed for an ideal (reversible process). The results of a reversible process are then adapted to the real world through the use of an efficiency. In the compression process there are three ideal processes that can be visualized: 1) an isothermal process, 2) an isentropic process and 3) a polytropic process. Any one of these processes can be suitably used as a basis for evaluating compression power requirement by either hand or computer calculation. The isothermal process, however, is seldom used as a basis because the normal industrial compression process is not even approximately carried out at constant temperature.
Due to practical limitation the compression ratio per stage is often in the range between 2 and 6. For large overall compression ratio applications multistage compressors are used. The choice of the interstage pressure is an economic decision and can be estimated by equal compression ratios for each section but may be adjusted to minimize total power requirement.
In order to study the effect of feed gas molecular weight on the performance of centrifugal compressors, several computer simulations using HYSYS [3] were performed. The gas mixtures with the composition shown in Table 1 with molecular weights ranging from 18.2 to 23.17, corresponding to relative density of 0.63 to 0.80, respectively, were used in this study. The characteristics curves for the centrifugal compressors used in this study are shown in Figures 1 and 2. These performance curves were supplied to the simulation software and used in the course of simulations.

Table 1

Case 1: Effect of Molecular Weight on Flow Rate for Fixed ?P (Constant Speed)

For a fixed inlet pressure of 700 kPa, 35 °C, and 15000 RPM, the feed gas relative density was varied from 0.63 to 0.80 with an increment of 0.05. In order to maintain the outlet pressure, the feed flow rate has to vary. We are essentially fixing P1 and P2 and wanting to see the effect on the compressor of varying molecular weight feed. The set up shown in Figure 3 was used to generate the simulation results. The simulation results for compression ratios of 2.0 and 2.5 are shown in Figure 4. The PR EOS [4] is used for thermodynamic properties calculations.

Figure 1

Figure 2

Figure 4 indicates that as the relative density decreased, the flow rate must decrease. Note, for the case of compression ratio of 2.5, no convergence could be achieved for relative density of 0.63 and 0.65 due to the fact the surge limit had been reached. For the same case, the required power as a function of relative density is shown in Figure 5. Since, the flow rate decreased with decreasing relative density, the required power decreased.

Process Flow Diagram

Finally, the variation of polytropic head as a function of inlet actual volumetric flow rate is shown in Figure 6. Note that the relative densities are identified on this diagram to show their influence on the performance of the compressor.

Figure 4

Figure 5 and 6

Case 2: Variable Speed
As in the case 1, for a fixed inlet pressure of 700 kPa, 35 °C, and mass flow rate of 1000 kmol/hr, the feed gas relative density was varied from 0.63 to 0.80 with an increment of 0.05. In this case, the compressor is varying speed to maintain flow rate at the DeltaP speed imposed on it. The schematic setup to generate simulation results is shown in Figure 7. The simulation results for compression ratios of 2.0 and 2.5 are shown in Figures 8 and 9. In addition to the results by the PR EOS, the results obtained by BWRS are shown on these diagrams. The difference between the results of these two EOS for these cases is negligible.

Process Flow Diagram

Figure 8

Figure 9

As shown in Figure 8, as the relative density increases, the compressor speed dropped. However, as relative density or molecular weight increased, the required power increased, see Figure 9.

As shown in Figures 10 and 11, the polytropic efficiency and head decrease with relative density.  More detail of simulation results can be found in Reference [5].

Figures 10 and 11

Conclusions
The impact of relative density (molecular weight) on the performance of a centrifugal compressor was studied by performing a series of computer simulations. Based on the simulation results, it is found that:

  1. For the same feed condition, compression ratio, compressor speed, the flow rates must decrease as the relative density decreases, and will eventually approach a surge condition.
  2. For the same feed condition, compression ratio, compressor speed, as the relative density increases, the flow rate increases which results in more power consumption.
  3. For the same feed condition and rate, and compression ratio, the compressor speed decreases with molecular weight but as expected, the power requirement increases.
  4. The PR EOS and BWRS EOS produced the same simulation results

To learn more about similar cases and how to minimize operational problems, we suggest attending our ME44 (Overview of Pumps and Compressors in Oil and Gas Facilities), ME46 (Compressor Systems – Mechanical Design and Specification), G4 (Gas Conditioning and Processing) and G5 (Gas Conditioning and Processing – Special) courses.

By: Dr. Mahmood Moshfeghian

Reference:

  • Campbell, J. M., “Gas Conditioning and Processing, Vol. 2, the Equipment Modules, 8th Ed., Campbell Petroleum Series, Norman, Oklahoma, 2001
  • Maddox, R. N. and L. L. Lilly, “Gas conditioning and processing, Volume 3: Advanced Techniques and Applications,” John M. Campbell and Company, 2nd Ed., Norman, Oklahoma, USA, 1990.
  • ASPENone, Engineering Suite, HYSYS Version 2006, Aspen Technology, Inc., Cambridge, Massachusetts U.S.A., 2006.
  • Peng, Y. D., Robinson, D. B., “A New Two-Constant Equation of State,” Ind. Eng. Chem. Fund., 15, 59, 1976
  • Moshfeghian, M., Bothamley, M., and Lilly, L.L., “Feed gas molecular weight affects performance of centrifugal efficiency,” Oil and Gas J., May 10, 2008
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