Optimizing Performance of Refrigeration System with an External Sub-Cool Economizer

Continuing the January 2008 [1], May 2008 [2], May 2014 [3], and December 2017 [4] Tips of The Month (TOTM), this tip demonstrates the application of an external sub-cooler to optimize the performance of a mechanical refrigeration system. Specifically, by utilizing a cold process stream we will minimize the compressor power and condenser duty. The details of three typical refrigeration systems are given in Chapter 15 of Gas Conditioning and Processing, Volume 2 [5]. They are referred to as follows:

1. A simple refrigeration system (Fig 15.1);
2. A refrigeration system employing one flash tank economizer and two stages of compression (Fig 15.7), see also the May 2014 [3] and December 2017 [4] TOTM
3. A Simple Refrigeration System with a sub-cool heat exchange economizer (Fig 15.9), see also the May 2008 [2] and May 2014 [3] TOTM.

 

Figure 1 presents the process flow diagrams for a simple system and the system with a sub-cool heat exchange economizer. As part of a hydrocarbon dew point control plant, this tip will evaluate and compare these two refrigeration systems.

 

Figure 1. Process flow diagrams for a simple refrigeration system and with a sub-cool economizer

 

 

Let’s consider cooling the process gas to -20°C [-4°F] by removing 2733 kW (9.325 MMBtu/hr) in a propane chiller and rejecting it to the environment by a propane condenser at 37.8°C [100°F]. Pure propane is used as the working fluid in the simulation. In this tip, all simulations were performed with UniSim Design software [6] using the Peng-Robinson equation of state. Assuming an approach temperature of 5°C [9°F] and a 6.9 kPa (1 psi) pressure drop in the propane chiller, the pressure of saturated propane vapor leaving the chiller is 203.3 kPa (29.5 psia), and at a temperature of -25°C     [-13°F] . Assuming no frictional losses in the suction line to the propane compressor, the resulting suction pressure is 203.3 kPa (29.5 psia).

 

The condensing propane pressure at the specified condenser temperature of 37.8 °C (100 °F) is 1303 kPa (189 psi). The condenser frictional losses, plus the frictional losses in the piping from the compressor discharge to the condenser was assumed to be 34.5 kPa (5 psi); therefore, the compressor discharge pressure is 1338 kPa (194 psia). The propane compressor adiabatic efficiency was assumed to be 75%.

 

 

 

External Sub-Cool, Economizer

 

The process streams 9A and 9B are part of a hydrocarbon dew point control plant and are shown on the top of Figure 2. This stream is the extracted NGL stream from the refrigeration plant, combined with the plant inlet condensate. The stream properties are shown in Table 1.  To prepare the liquids to be fed to the deethanizer, the process specification is to raise the temperature of the NGL product stream 9A from -3.9°C (25°F) to 20°C (68°F) in HEX E-102. The resulting duty is 713.6 kW (2.435 MMBtu/hr). This heat will be supplied from a propane refrigerant sub-cool economizer heat exchanger. The process duty and the temperature of the NGL product stream is set by the stabilization process requirements, thus the sub-cool economizer duty is fixed.

 

The sub-cool economizer cools the condensed propane (refrigerant stream R4) from 37.8°C (100 °F) at 1303 kPa (189 psia) to a cooler temperature at 1269 kPa (184 psia), depending upon the specified propane refrigerant flow rate (stream R5). The pressure drops in HEX E-102 and HEX E-104 are 35 kPa (5 psi) respectively. The heat removed by the sub-cool economizer is fixed by the process duty required to heat the NGL process stream 9A.

 

 

Figure 2. Cold process stream 9A part of the hydrocarbon dew point control plant is utilized to sub-cool refrigerant stream R5.

 

 

Table 1. Process conditions for streams 9A and 9B

Process Stream	9A	9B
Temperature, °C	-3.87	20
Pressure, kPa	1535	1500
Rate, kmol/h	666	666
Temperature, °F	25.03	68
Pressure, psia	222.6	217.5
Rate, lbmol/hr	1468.3	1468.3

 

 

 

Determination of Refrigerant Circulation Rate of Sub-Cool Economizer System

 

The refrigerant circulation rate has a considerable impact on the compressor power and consequently on the condenser duty. Figure 2 presents the variation of compressor power as a function of the refrigerant circulation rate. This figure indicates that the power is minimum at 995.5 kW (1335 hp) for a circulation rate of 690 kmol/h (1521.5 lbmol/hr), with fixed propane chiller and sub-cool exchanger duties.

 

 

Figure 3. Impact of refrigerant circulation rate on the compressor power

 

 

  

Summary

 

For the same chiller duty, chiller and condenser temperatures, adiabatic compression efficiency, and pressure drops, the results of the sub-cool exchange economizer system are compared with the results of a simple refrigeration system in Table 2. This table indicates that by utilizing an external sub-cool exchange economizer for this case study with optimized propane circulation rate, the compressor power and condenser duty are reduced by 25 % and 25.4%, respectively.

 

Table 2. Comparison of the key parameters of two refrigeration systems

Refrigerant Rate, kmol/h		Simple System, kW		Sub-Cool System, kW
Simple	Sub-Cooler	Comp	Cond	Comp	Cond	Sub-Cooler
920.5	690	1328	4061	995.5	3029	713.6
Refrigerant Rate, lbmol/hr		Simple System, MMBtu/hr (hp)		Sub-Cool System, MMBtu/hr (hp)
Simple	Sub-Cool	Comp	Cond	Comp	Cond	Sub-Cooler
2029.7	1521.5	4.532 (1781)	13.86	3.446 (1335)	10.34	2.435
Reduction, %	25.04			25.04	25.41

 

 

To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Practical Computer Simulation Applications in Gas Processing), and G6 (Gas Treating and Sulfur Recovery) courses.

 

By: Dr. Mahmood Moshfeghian

 

References:

1. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2008/01/refrigeration-with-flash-economizer-vs-simple-refrigeration-system/, John M. Campbell Tip of the Month, January 2008.
2. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2008/05/flash-tank-vs-hex-economizer-refrigeration-system/, John M. Campbell Tip of the Month, May 2008.
3. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2014/05/refrigeration-with-heat-exchanger-economizer-vs-simple-refrigeration-system/, PetroSkills Tip of the Month, May 2014.
4. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2017/12/optimizing-performance-of-refrigeration-system-with-flash-tank-economizer/, PetroSkills Tip of the Month, December 2017.
5. Campbell, J.M., “Gas Conditioning and Processing, Volume 2: The Equipment Modules,” 9^th Edition, 3^rd Printing, Editors Hubbard, R. and Snow–McGregor, K., Campbell Petroleum Series, Norman, Oklahoma, PetroSkills 2018.
6. UniSim Design R443, Build 19153, Honeywell International Inc., 2017.