J. ZHANG, Q. XU and K. LI, Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas
R
efrigeration systems are among the most critical operating systems in the chemical processing industry. A refrigeration system generally works by removing heat from low-temper-ature streams and transferring it to higher-temperlow-temper-ature streams through vapor-compression cycles at the expense of mechanical work, magnetism, laser or other means.1Since a refrigeration system can cool down a process stream far below the ambient temperature, it is indispensable to cryogenic cooling and separation operations in many chemical industries, such as the large-scale production of ethylene, oxygen, nitrogen and liquefied natural gas (LNG). Refrigeration systems may employ a single compound as the refrigerant, as long as it is envi-ronmentally safe (e.g., nontoxic), thermodynamically desirable (e.g., having a sufficiently low boiling point, high latent vapor-ization heat and high critical temperature), and operationally feasible (e.g., noncorrosive).
A multi-component mixture can also be used as the refriger-ant.2 From a thermodynamic viewpoint, a mixed-refrigerant
system (MRS) provides refrigeration over a range of tempera-tures, with smaller temperature differences at the lower temper-atures. This leads to a smaller increase in entropy, or a smaller loss of energy.3
The MRS has many inherent advantages over a traditional single-component refrigeration system (SCRS), which has led to the application of MRS in new chemical processes. For example, an ethylene plant may need to process various streams with tem-peratures ranging from +40°C to –140°C. In the conventional refrigeration method, this broad temperature range is accom-plished by a cascade refrigeration system, where three single-component refrigeration subsystems are integrated together. Each refrigeration subsystem will employ a compressor, a set of flash drums, and many other types of auxiliary equipment.
To reduce capital costs and the operational complexity of the refrigeration system, an ethylene plant can employ a single refrigeration system with mixed refrigerants to accomplish the same refrigeration task.4 Thus, the number of compressors is
MIX3
Flowsheet of an MRS.
FIG. 1
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APRIL 2012 HydrocarbonProcessing.comreduced from three to one, and over 25 pieces of equipment are saved. It has been reported that the introduction of a mixed-refrigerant system can reduce the capital cost of the entire eth-ylene plant by 7%.5
This article describes the operating performance of an MRS used in an ethylene plant that was studied through rigorous simu-lation. Insights on the MRS working mechanism are presented.
Based on the simulation, optimization strategies have been devel-oped to improve the MRS operation under the disturbance of cooling-water temperature change.
Process description of an MRS. The studied MRS, which contains a mixed refrigerant of 0.1 wt% H2, 11.7 wt% CH4, 17.6 wt% C2H4, and 70.6 wt% C3H6, is used for an ethylene plant.
As shown in Fig. 1, the refrigeration system has a three-stage compressor. All three compressor stages have suction drums to buffer inlet pressures and knockout liquids if any leak out of the compressor. The first stage (C-1) compresses the refrigerant from a pressure of 0.16 MPa to 0.61 MPa. The outflow of C-1 mixes with the vapor flow from suction drum FD-2 and then goes to the second stage (C-2), which compresses the refrigerant from 0.61 MPa to 1.02 MPa.
The outflow of stage C-2 is partially condensed, by cooling water, to 32°C, and then goes into another suction drum (FD-3) together with a mixed vapor flow from evaporators EE-5, EE-6, EE-7 and EE-9. About 88% of the FD-3 vapor flow goes to
the third stage (C-3). The rest of the vapor flow moves through coolers EC-4, EC-5 and EC-6, which reduces its temperature to
−12°C. Then, the stream is flashed in drum FD-4 at 0.79 MPa to vapor and liquid streams at a temperature of −16°C. The vapor stream is used in evaporator EE-10, while the liquid flow is used in evaporator EE-13.
In the third stage (C-3), the refrigerant is normally compressed from 1.02 MPa to 3.0 MPa, with a flexibility of ±0.2 MPa for the output pressure. The output refrigerant of C-3 is partially condensed to 32°C in EC-2 by the cooling water. The condensate temperature may change from 29°C to 34°C due to the amount of cooling water and the inlet temperature.
The mixed refrigerant is separated into high-pressure light mixed refrigerant (HP-LMR) and high-pressure heavy mixed refrigerant (HP-HMR) in flash drum FD-5. HP-LMR is the vapor output of the flash drum, and HP-HMR is the liquid out-put. The compositions of HP-LMR and HP-HMR vary with EC-2 output temperature and C-3 output pressure. When the temperature is 29°C and the pressure is 2.8 MPa, the composition of HP-LMR is 0.2 wt% H2, 26.7 wt% CH4, 25.4 wt% C2H4 and 47.7 wt% C3H6; while the composition of HP-HMR is 5.7 wt%
CH4, 14.5 wt% C2H4 and 79.8 wt% C3H6.
HP-LMR has the lower boiling point in the refrigeration sys-tem, and can refrigerate process streams to −130°C. HP-LMR is used at evaporators EE-1, EE-2, EE-3 and EE-4 in the chilling train section. It refrigerates the charge gas to −127°C, liquefying most of the C2 and heavier components, while the hydrogen and methane remain in the gas phase. The liquid-phase and gas-phase charge gases are separated by flash drums. The liquid flows to the demethanizer tower, and the gas flows to the Joule-Thompson expansion process to separate hydrogen from methane. After passing through evaporator EE-4, the HP-LMR goes into a pure vapor state, and then travels to the C2 splitter’s overhead condenser EE-13 as the cooling utility.
The HP-HMR has the higher boiling point in the refrig-eration system. About 38% of the HP-HMR is used in EE-5, EE-6, EE-7 and EE-9 to refrigerate the charge gas, hydro-gen and methane flows to 15°C. After that, the HP-HMR goes to the suction drum of C-3. The rest of the HP-HMR is used to condense the overhead stream from the low-pressure depropanizer tower to or under −20°C. After that, it travels to the suction drum of C-2.
TABLE 1. Statuses of process streams in MRS evaporators and coolers
Heat Input Output duty, temp., temp.,
Name Type GJ/hr °C °C Description EE-1 Evaporator 12.4 −102 −127 Charge gas condenser EE-2 Evaporator 4.0 −102 −123 Charge gas condenser EE-3 Evaporator 84.7 −43 −102 Charge gas condenser EE-4 Evaporator 9.0 −21 −40 Charge gas condenser EE-5 Evaporator 17.9 32 14 Charge gas condenser EE-6 Evaporator 22.2 45 14 Caustic tower cooler EE-7 Evaporator 1.0 31 15 Hydrogen cooler EE-8 Evaporator 2.5 23 8 Refrigerant inter-cooler EE-9 Evaporator 28.8 37 11 Methane cooler EE-10 Evaporator 15.7 2 −20 Depropanizer condenser EE-11 Evaporator 0.2 −14 −19 Charge gas condenser EE-12 Evaporator 1.4 23 −14 Refrigerant inter-cooler EE-13 Evaporator 144.1 −35 −36 C2 fractionator condenser EC-1 Cooler 31.9 27 35 Water cooler
EC-6 Cooler 0.7 −20 −19 Refrigerant inter-heater EC-7 Cooler 119.2 −132 30 Charge gas heater EC-8 Cooler 14.9 −15 30 Ethylene product heater EC-9 Cooler 23.3 −18 8 Refrigerant internal heater EC-10 Cooler 1.6 −27 −15 Ethylene product heater
Temperature, °C Theoretical power
needed from
Temperature-enthalpy diagram of the MRS.
FIG. 2
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61HP-LMR finally mixes with the liquid flow from FD-4 and goes to EE-13 to condense the overhead stream of the C2 splitter to under −35°C. Evaporator EE-13 has the largest cooling duty among all of the evaporators in the refrigeration system. The HP-LMR flow is in pure vapor phase, which gives a small amount of cooling duty. Most of the cooling duty of EE-13 is provided by the liquid flow from FD-4.
Modeling and operational optimization. A rigorous simulation model has been developed based on the aforemen-tioned process description. The thermodynamic package used in this simulation is the Peng-Robinson cubic equation of state with the Boston-Mathias alpha function. During the simula-tion, the minimum temperature difference is set at 2°C, and the minimum temperature difference in normal heat exchangers is set at 5°C. A compressor efficiency of 0.72 is used in this case.
To check the performance of the MRS operation, the normal process operation condition has been simulated as the base case.
In the base case, the process stream status in each evaporator and cooler is fixed as input (see Table 1); C-3 outlet pressure is fixed as 2.8 MPa.
Based on the simulation results, Fig. 2 presents the temper-ature-enthalpy diagram to describe the composite hot and cold flows of the entire MRS. The MR hot-flow curve represents the refrigerant as it undergoes condensing operations in various condensers. Thus, the refrigerant functions as the hot stream, and the heat will be removed from it. The released heat will be transferred to cooling water at higher temperature and the cold process stream at lower temperature.
However, contrary to the simulation results, the MR cold-flow curve represents the refrigerant as it undergoes evaporating opera-tions in various evaporators, where the refrigerant funcopera-tions as the cold stream for absorbing heat. The absorbed heat/energy comes from the compressor and the hot process stream at lower tem-perature. Note that, since the minimum temperature difference is set at 2°C, the pinch point lies at a temperature of −20°C. Also note that the horizontal distance of the dashed line represents the theoretical power provided by the compressor. When compressor
24.0 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 2.7 2.8 2.9 3.0 3.1 3.2 3.3
24.5
Compressor output pressure, MPa
Cooling water temperature, °C 38,000
39,000 40,000 41,000 42,000 43,000 44,000 45,000 46,000 47,000
Total compressor work, kW
Profiles of total compressor work and compressor outlet pressure under various cooling-water temperatures.
FIG. 3
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APRIL 2012 HydrocarbonProcessing.comefficiency is known, the total energy consumption of the compres-sor will be identified. Obviously, any operational changes to the process streams or MRS will result in a corresponding energy flow change in the temperature-enthalpy diagram.
Note that, in the simulated base case, a large amount of the cooling-water temperature is set at 27°C. If the cooling-water temperature is changed due to a seasonal temperature difference, it will influence the MRS cycles and cause operational problems.
Therefore, the optimal strategy for operating an MRS under
the disturbance of cooling-water temperature is presented in this article.
Assume the cooling-water temperature ranges from 24°C to 29°C. When the cooling-water temperature decreases, the opera-tional temperature of FD-5 will also decrease. Thus, the amount of HP-LMR will respectively decrease because the vapor fraction of FD-5 will decline with lower temperature. This would make HP-LMR hard to guarantee for the heat duties for evaporators EE-1 and EE-3. To balance it, the compressor output pressure should be decreased to raise the vapor fraction of FD-5. There-fore, the C-3 output pressure should be suitably adjusted within the feasible operating range.
The disturbance of cooling-water temperature also influences the heat duty of EE-13. Note that HP-LMR travels to evaporator EE-13, which has the largest heat duty among the evaporators.
When the cooling-water temperature increases, the temperature of the HP-LMR flowing to EE-13 will also increase. Therefore, the HP-LMR will not be able to provide enough heat duty to EE-13. To handle this problem, the amount of liquid flow from FD-4 should be increased to provide enough heat duty to EE-13.
Based on the developed simulation model, nine case studies have been conducted for a cooling-water temperature change from 24°C to 29°C. Since the main manufacturing process should not be affected, the operating statuses of all process streams in these nine cases are unchanged. This means that the input flowrate, temperature, pressure, composition and output temperature of all process streams are still the same as those shown in Table 1.
Fig. 3 shows simulation results of the nine case studies under var-ious cooling-water temperatures. The related total compressor work
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41,500
Compressor output pressure, MPa
Total compressor work, kW
Simulation results of compressor work consumption and compressor output pressure.
FIG. 4
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APRIL 2012 HydrocarbonProcessing.comand the compressor outlet pressure can be simultaneously obtained from this figure when the cooling-water temperature is given. Thus, Fig. 3 actually provides optimal MRS operation strategies under the disturbance of cooling-water temperature. For instance, if the cooling-water temperature is 28°C, the appropriate compressor out-let pressure should be controlled at 3.15 MPa. Under this scenario, the compressor will consume 45,410 kW of energy.
Fig. 4 provides more insight into the total compressor work and the compressor output pressure. It shows that, when the compressor output pressure increases from 2.8 MPa to 3.2 MPa, the total compressor work increases from 41,836 kW to 46,381 kW. Although lower pressure will reduce compressor work con-sumption and operational cost, it will require lower cooling-water temperature. Therefore, the simulation results provide an effective way to handle this issue. HP
ACKNOWLEDGMENT
This work was supported in part by the Texas Air Research Center (TARC) and the Texas Hazardous Waste Research Center (THWRC).
LITERATURE CITED
1 Vaidyaraman, S. and C. D. Maranas, “Optimal synthesis of refrigeration cycles and selection of refrigerants,” AIChE Journal, Vol. 45, Issue 5, May 1999.
2 Lee, G. C., R. Smith and X. X. Zhu, “Optimal synthesis of mixed-refrigerant systems for low-temperature processes,” Ind. Eng. Chem. Res., Vol. 41, Issue 20, 2002.
3 McKetta, J. J., Encyclopedia of Chemical Processing and Design, Vol. 28, Marcel Dekker Inc., pp. 213-221, New York, New York, 1988.
4 Mafi, M., M. Amidpour and S. M. Mousavi Naeynian, “Development in mixed refrigerant cycles used in olefin plants,” Proceedings of the 1st Annual Gas Processing Symposium, Elsevier, 2009.
5 Stanley, S. J., R. Thakral and J. deBarros, “Changing the ethylene plant process chemistry and flowsheet configuration for improved return on invest-ment,” Petrotech 2009, New Delhi, India, 2009.
Jian Zhang is a research associate at the Dan F. Smith Depart-ment of Chemical Engineering at Lamar University. He has five years of experience in planning and scheduling for the petroleum and petrochemical industries. He holds BS and MS degrees, as well as a PhD, all in chemical engineering, from Tsinghua University in China. Dr. Zhang’s research interests include process planning and scheduling, process simulation, process optimization and synthesis, and industrial waste minimization.
Qiang Xu is an associate professor at the Dan F. Smith Depart-ment of Chemical Engineering at Lamar University. He holds BS and MS degrees, as well as a PhD, all in chemical engineering, from Tsinghua University in China. His research interests include process modeling, scheduling, dynamic simulation and optimiza-tion, industrial pollution prevenoptimiza-tion, waste minimizaoptimiza-tion, and chemical process safety and flexibility analysis. Dr. Xu’s research work on proactive flare minimization and environmentally benign manufacturing has been extensively supported by TCEQ, TARC, THWRC, the US Department of Defense and industry.
Kuyen Li is a professor at the Dan F. Smith Department of Chemical Engineering at Lamar University. He received BS and MS degrees in chemical engineering from Cheng Kung University of Taiwan and a PhD in chemical engineering from Mississippi State University. His research interests include air pollution control by dynamic simulation and advanced oxidation, advanced remediation methods for contaminated soil and sludge, and industrial wastewater treatment by biological and advanced oxidation methods. His research work has been strongly supported by the US Environmental Protection Agency, TCEQ, TARC and industry.
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