Conceptual Design and Analysis of a Novel CO2 Hydrate-Based

Subscriber access provided by University of South Dakota

Article

Conceptual design and analysis of a novel CO2 hydratebased refrigeration system with cold energy storage Nan Xie, Chenghua Tan, Sheng Yang, and Zhiqiang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05255 • Publication Date (Web): 09 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Conceptual design and analysis of a novel CO2 hydrate-based refrigeration system with cold energy storage

Nan Xiea, Chenghua Tana, Sheng Yanga, Zhiqiang Liua,b,* a. School of Energy Science and Engineering, Central South University, 932 South Lushan Road, Changsha 410083, China b. Collaborative

Innovation

Center

of

Building

Energy

Conservation

Environmental Control, 88 West Taishan Road, Zhuzhou 412007, China

*Corresponding author: Professor Zhiqiang Liu, Ph.D. School of Energy Science and Engineering Central South University Changsha, 410083, P. R. China. Tel: +86-731-88879863 Email: [email protected] 1 ACS Paragon Plus Environment

&

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract CO2 hydrate can be used as an alternate cooling substance in the air-conditioning system, to minimize the use of traditional refrigerants such as HFCs and HCFCs. A novel CO2 hydrate-based refrigeration system with a function of cold energy storage was designed and investigated, using tetrahydrofuran (THF) as the thermodynamic promoter. Coefficient of performance (COP) of this system was calculated based on the simulation results. Effects of various operation parameters were also studied closely. The cold storage operation was then designed to investigate the energy storage ability of the current system. Results show that the system COP is 6.8, which is the major strength of this novel system. Due to the energy intensive process of CO2 compression, the work imposed on elevating the gas pressure was considerable. Searching for more appropriate additives or alternate guest substances should be further conducted. Besides, compressor efficiency and pump efficiency are both critical for improving the energy efficiency. Enhanced refrigeration performance can also be realized at higher hydrate mass fraction. In addition, two different cold energy storage operation strategies were obtained. This research is of great significance to the in-depth development of hydrate-based refrigeration and cold energy storage system. The proposed system might contribute to minimizing the use of conventional coolants and realizing peak load shifting in the near future.

Keywords: CO2 hydrate; refrigeration; coefficient of performance; Aspen Plus; cold energy storage; DeST

2 ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Gas hydrates, also known as clathrate hydrates, are nonstoichiometric, crystal compounds which have cage-like structures formed by water and gas molecules. Generally, there are three kinds of crystalline structures (sI, sII, sH) depending on the sizes of gas molecules.1 Guest molecules inside the cage interact with water molecules by Van der Waals forces and hence these molecules retain their inherent properties inside the host cage structures.2 Gas hydrate research has progressed over several decades from a mere academic curiosity into various industrial applications.2-4 Gaseous CO2, as a guest substance, forms a sI hydrate with water molecules at certain pressure and temperature. To better understand the nature of CO2 hydrate, structure identification and investigations on thermodynamic properties, phase equilibrium and additives have been carried out by many researchers. Ye and Zhang5 presented their visualization research of CO2+TBAB hydrate using a tetra-n-butyl ammonium bromide (TBAB) aqueous solution. A structural transition indicates that the bromide anion can form a cage structure with water molecules and change the structure of this crystal inclusion compound, moderating the formation condition as a result. Visual observations of pure sI hydrate, pure sII hydrates and a mixture of sI and sII hydrates of CO 2 were presented by Veluswamy et al.6 in the presence of tetrahydrofuran (THF) and sodium dodecyl sulphate (SDS) at different operating conditions. Kumar et al.7 investigated the effects of thermodynamic promoter THF and kinetic promoter SDS on the gas mixture containing CO2. Results showed that in their experimental conditions, gas uptake was increased by almost three folds in the presence of SDS. The formation pressure

was

obviously reduced

by THF.

Different

ternary systems

of

CO2+water+(TBAB/TBAC/TBAF) were reported by Li et al.8, an isochoric



pressure-search method was utilized to measure the equilibrium data of these semi-clathrate hydrates. Yang et al.9 found the mixture of 5% THF and 10% TBAB was the optimal mixture for CO2 hydrate formation from flue gas, considering the phase equilibrium conditions. Thermodynamic equilibrium conditions were also studied by Xia et al.10, they selected TBAB, THF, DMSO and their mixtures as the additives. They pointed that the DMSO can only promote the CO2 solubility. Lin et al.11 measured the equilibrium data of CO2+water+TBAB system and found that the formation pressure was decreased at least 74% in their temperature range. Pivezhani et al.12 studied the effects of various factors such as temperature, impeller speed, initial pressure, volume of water, etc. Li et al.13 reported that the addition of cyclopentane into the pre-combustion carbon capture system will remarkably enhance the process and shorten the induce time simultaneously. Zheng et al.14 also found that the pre-combustion carbon capture can be operated at near ambient temperatures by using TBAF compared to other promoters. Besides, non-surfactant-based methods including hydrate formation in sand packs, silica gels, foams, nanoparticles, etc., 15 might enhance the formation rate by increasing the surface area available for gas hydrate formation. In recent decades, CO2 hydrate has received a growing attention for its various industrial potentials. Hydrate-based CO2 capture and separation technology was thoroughly introduced by Ma et al.16 It is regarded as a promising option for capturing CO2 from gas streaming of fossil fuel power plants.17,18 Some works were made to put CO2 hydrate-based desalination into practice.19,20 CO2 hydrate can also be used as a secondary refrigerant to minimize the usage of traditional greenhouse refrigerants. 21 Besides, it can be formed at positive temperature conditions, which are suitable for the air conditioning applications.22,23 With a dissociation enthalpy higher than ice slurry,


Page 4 of 33

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this hydrate also has a higher thermal capacity than most traditional coolants. 24 With all these strengths mentioned above, research is mainly carried out adopting CO2 hydrate as a secondary refrigerant or cold storage medium in AC system. In addition, CO2 hydrate slurry can be utilized as a refrigerant as well. Heat transfer and removal can be efficiently achieved, relying on the exothermic hydrate formation as well as endothermic hydrate dissociation. A novel hydrate-based refrigeration system was developed for the first time by Ogawa et al. 25 Thermodynamic analysis and a simulation of its operation were carefully performed. The working media were HFC-32/cyclopentane for the simulation, and HCFC-22 for the experiments. Results showed that the coefficient of performance (COP) could be as high as 8.0. The high energy efficiency would be the major advantage of this system. A laboratory-scale model of the novel refrigeration system was also constructed by the authors. Steady operation of two hours was successfully achieved in their experimental efforts. Recently, Zhang et al.26 presented their research on the hydrate-based refrigeration system. Methyl fluoride, cyclopentane/monofluoro cyclopentane and water were utilized as working fluids. Three different systems were designed and investigated by the authors. The system COP was calculated and compared as the major evaluation criterion as well. The obtained COP in their simulation was 2-4 times of the traditional compression refrigeration. Carbon dioxide is a non-toxic, nonflammable, inexpensive gas that can be easily obtained. Utilizing CO2 hydrate as the refrigerant might help reduce the use of traditional coolants. In this study, a hydrate-based refrigeration system was constructed, using CO2 hydrate as the working medium. This system can also achieve an efficient cold energy storage due to the high heat capacity of CO2 hydrate. Thermodynamic promoter THF was employed to alleviate the hydrate formation



pressure. Aspen Plus was utilized to investigate this system. COP of the system was then calculated based on the simulation results. Effects of several factors on the refrigeration system were also studied closely. However, because of the high energy consumption on gas compression, the COP of this system is not as high as those in the literature.25,26 Finally, a case study for cold energy storage operation was preliminarily designed and evaluated, based on the cooling load results calculated from DeST software. The general operation strategies of the present system were then studied considering several parameters such as induction time, hydrate formation rate and flow rate. This study might help to shift the peak load in the years to come. Future studies focused on the hydrate forming substances and appropriate additives should be encouraged, to make this technology mature enough for industrial application.

CO2 hydrate refrigeration system a. Conceptual design Hydrate formation is an exothermic process and the heat released can be removed by environmental fluids like surface water, air, ground water, etc. On the contrary, hydrate dissociation is an endothermic process in which heat absorption from a space to be refrigerated might be achieved.25 The conceptual design of present system was shown in Fig. 1. Gaseous carbon dioxide is charged into the system from a pressurized gas tank. Then, the guest gas is further pressurized to the formation pressure by a gas compressor. After that, the stream of CO2 with high pressure is injected into the cold solution in the hydrate formation reactor. Intensively mixing with the subcooled solution, the guest gas forms a sII-structure hydrate in the presence of the promoter THF. Heat released in this process is taken away by a cooling fluid, driven by a liquid pump to the cold reservoir. A sight glass is installed on the


Page 6 of 33

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formation unit as a watch window to observe the hydrate formation. Flowmeters are also installed in the pipelines to measure the amount of consumed gaseous CO2 and liquid solution. The data is collected by a data acquisition system. The formed amount of hydrate is then calculated by the computer based on the consumed amount of these substances. Once considerable hydrate slurry is formed, a slurry pump will be started automatically to convey the working medium to the user (hydrate dissociation reactor). After heat exchanging with the space to be cooled, the hydrate breaks up into CO2-rich gas and lean solution. The mixture is then separated by a gas/liquid separator to undergo another similar cycle operation. In cold storage operation, the slurry pump will not be started until there is a demand for cold releasing, and the formed hydrate will be stored in the hydrate formation reactor before that. user (hydrate dissociation reactor)

slurry pump

hydrate formation reactor separator sight glass

liquid pump

liquid pump

compressor

gas tank

cold reservoir

Fig. 1 Conceptual diagram of CO2 hydrate refrigeration system. b. Operational parameters Phase equilibrium study for various hydrates has been carried out by many researchers. Thermodynamic promoter tetrahydrofuran was proved to be an efficient additive.27-31 A THF solution of low concentration (around 5 mol%) can dramatically moderate the phase equilibrium condition of CO2 hydrate. To determine the optimal 7 ACS Paragon Plus Environment


operational parameters for current refrigeration system, these available phase equilibrium data (P-T relation) were plotted in Fig. 2, with the saturated vapor curve of CO2.32 It can be observed that the equilibrium pressure of CO2 increases sharply when its temperature being relatively high. Accordingly, T1=280 K, P1=0.25 MPa and T2=292 K, P2=3.0 MPa were chosen as the dissociation condition and formation condition of CO2 hydrate, respectively. Thus, ground water can be used as the cooling fluid for heat removal. Otherwise, heat release can also be realized in cold season when this refrigeration system is in cold energy storage operation. For convenience, the mass flow rate of gaseous CO2 in the present research was specified as 100 kg/h. The molar basis of CO2+ THF+ water for hydrate formation in current system is 2: 1: 17. Accordingly, the flow rates of THF and water were specified to be 82 kg/h and 348 kg/h, respectively, for hydrate formation. Hence, the THF concentration adopted in current research can be calculated as 5.56 mol% for this system. This concentration is assumed to be maintained the same level before the formed hydrate leaves the formation unit. The excess water is then induced into the unit for the maintenance of fluidity. The hydrate mass fraction was assumed to be 60% with the induced excess water. Efficiencies of the gas compressor and the liquid pump were both fixed at 80%.


Page 8 of 33

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2 Equilibrium data of the trinary system of CO2/THF/water in literature27-31 and the saturated vapor pressure curve for CO232. The calorimetric techniques of direct measurements produce the most accurate enthalpy data, but the measurements are always difficult and complicated.27 Based on the available equilibrium data in published works, Clausius–Clapeyron equation could be used to calculate the hydrate enthalpy with a certain accuracy. Sabil et al.27 estimated the enthalpies of dissociation of CO2+THF hydrate by applying this equation. Dissociation enthalpy of mixed CO2 and THF hydrate was estimated to be 145.82 kJ/mol. Molar concentration of THF was 5 mol% according to their research. Similar research was conducted by Delahaye et al.30 Enthalpies of CO2+THF hydrate (concentration of THF being 4.22 mol%) were reported to be 132~163 kJ/mol in the hydrate temperature range from 280 K to 292 K. For simplicity in our research, we assumed that the enthalpy of CO2+THF hydrate to be constant as 150 kJ/mol. c. System simulation Based on the conceptual design of the CO2 hydrate refrigeration system, the flow sheet for simulation was presented in Fig. 3. The simulation model consists of three parts: a hydrate formation/dissociation cycle, a heat removal section and a user section. The hydrate slurry pump was replaced by a valve and the valve controlled the flow rate rather than transporting the hydrate slurry. It is because the pressure difference between formation unit and dissociation unit is much larger than the pressure-drop caused by hydraulic friction in the pipeline.25 Hence, the pressure loss can be neglected in this simulation. Properties of the streams in this novel hydrate-based cooling system are simulated utilizing the PRMHV2 model. Based on the Peng-Robinson equation of state and the modified Huron-Vidal mixing rule, this model has an excellent degree of predictive accuracy for high pressure cases in



current system. Block types and parameters of individual device in current system are shown in Table 1.

Fig. 3 Flow sheet of CO2 hydrate refrigeration system. Table 1. Block types and parameters of individual device. Name

Type

Specification Isentropic; Discharge pressure, 3 MPa; Mechanical

COMPRESSOR Compr efficiency 80% Discharge pressure, 3 MPa; Driver efficiency 1; Pump PUMP

Pump efficiency 80%

SEPARATOR

Sep

Liquid split fraction 1

MIXER

Mixer

/

VALVE

Valve

Outlet pressure, 0.25 MPa

d. Coefficient of performance Coefficient of performance (COP) is commonly adopted as a major evaluation


Page 10 of 33

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


criterion in various refrigeration systems. Normally, it is a ratio of cool energy output to the overall work or power input. Larger value of system COP indicates a higher energy efficiency of the system. In this work, the system COP is defined as a ratio of heat removal in the user section to the overall power consumed by pumps, compressor and so on. The system COP of the novel CO2 hydrate refrigeration system can be defined as: COP=Q0 /Wtotal

(1)

Therefore, the system COP was calculated in this way, based on the simulation results obtained by using Aspen Plus software.

Mathematical model Energy balance and mass balance algorithm were employed in the calculation and evaluation for each individual equipment. Some reasonable assumptions were made in current research. There are no heat dissipation and energy loss in the separator and the mixer because the stream separation and mixing processes were both assumed to be adiabatic. The gaseous stream and liquid solution were separated completely by the separator which cannot happen in practical applications actually. Besides, the gas compression and liquid pumping were both considered as the isentropic processes as well. The variation of the potential energy and the kinetic energy of all the streams was thought to be negligibly insignificant, hence the net work consumed by the pressure changers can be completely contributed to the enthalpy increase of the flowing streams. Furthermore, it was also assumed that the hydrate formation and dissociation can happen immediately once the streams were charged into the reaction units. In addition, the throttling process of the hydrate slurry flowing through the valve was thought to be iso-enthalpy and no hydrate decomposed



Page 12 of 33

during this process. On the basis of all the assumptions mentioned above, the mathematical equations of each device are listed as follows: Separator: mtotal =mgas +(mwater +mTHF )

(2)

QGL-IN =mgas hG-IN +(mwater hL-IN_water +mTHF hL-IN_THF )

(3)

where mtotal, mgas, mwater and mTHF are mass flow rates of the two-phase mixed stream (stream GL-IN), gaseous CO2 (stream G-IN), liquid water and additive THF (both in stream L-IN), respectively; QGL-IN is the energy of the two-phase mixture; and hG-IN, hL-IN_water and hL-IN_THF are the specific enthalpy of gaseous CO2, liquid water and additive THF. Gas compressor: 1

Wcompressor = η mgas (hG-OUT -hG-IN )

(4)

c

where Wcompressor is the net work of the compressor (block COMP) required in the gas compression process; ηc is the overall efficiency of the gas compressor; and hG-OUT is the specific enthalpy of the pressurized CO2 (stream G-OUT). Liquid pump: Wpump =

1 ηp

(

mwater ρwater

+

mTHF ρTHF

)(P2 -P1 )

(5)

where Wpump is the net work of the liquid pump (block PUMP) for liquid pressurizing; ηp is the overall efficiency of the liquid pump; and ρwater and ρTHF are the density of water and THF; and P1 and P2 are phase equilibrium pressure for hydrate formation and dissociation. Mixer: mtotal =mgas +(mwater +mTHF )


(6)

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


QMIX =mgas hG-OUT +(mwater hL-OUT_water +mTHF hL-OUT_THF ) (7) where QMIX is the energy of the pressurized two-phase mixture (stream MIX); and hG-OUT, hL-OUT_water and hL-OUT_THF are the specific enthalpy of pressurized streams of gaseous CO2, liquid water and additive THF (both in stream L-OUT). Formation unit: mwater =mwater_in_hydrate +mexess

(8)

mhydrate =mgas +mTHF +mwater_in_hydrate

(9)

QMIX =mhydrate hHY-IN +mexcess hexcess +Q1

(10)

where mwater_in_hydrate, mexcess and mhydrate are mass flow rates of the water solidified in hydrate, the excess water and the formed hydrate; hHY-IN and hexcess are the specific enthalpy of the formed CO2 hydrate (stream HY-IN) and the excess water; and Q1 is the thermal energy removed by using an environmental fluid. Slurry valve: hHY-IN =hHY-OUT

(11)

where hHY-OUT is the specific enthalpy of the hydrate after isenthalpic throttling (stream HY-OUT). Dissociation unit: mwater =mwater_in_hydrate +mexess

(12)

mhydrate =mgas +mTHF +mwater_in_hydrate

(13)

mhydrate hHY-OUT +mexcess hexcess =QGL-OUT +Q0

(14)

QGL-OUT =QGL-IN

(15)

where QGL-OUT is the energy of the two-phase mixture out of the dissociation reactor (stream GL-OUT); and Q0 is the thermal energy needed for hydrate dissociation, i.e., the cooling output of the current refrigeration system.



Results and discussion As mentioned before, T1=280 K, P1=0.25 MPa and T2=292 K, P2=3.0 MPa were chosen as the dissociation and formation condition of CO2 hydrate in current system. The input energy consumption and the overall system COP were then evaluated and calculated based on the simulated results. Several operation parameters and process factors, such as phase equilibrium condition (i.e. equilibrium temperature as well as equilibrium pressure), gas compressor efficiency, liquid pump efficiency and hydrate mass fraction, were chosen to investigate their effects on the system performance. Hence, sensitivity analysis was also carried out for a better understanding of the current refrigeration system. a. Energy consumption analysis and system COP Simulation results in Fig. 4 show that the net work required by the compressor for pressurizing the gaseous CO2 to the formation pressure is around 5.95 kW. However, the work for the liquid pump is only 0.98 kW, being significantly less than that of the gas compressor. Hence, more than 85% of the total work input was consumed in the compression process of CO2, where a considerable amount of energy might be saved with some energy conservation efforts. The consumed work of liquid pump occupies only 14.1% of the total energy input. The cooling capacity output, i.e., the overall heat taken out of the space to be refrigerated can be evaluated and calculated as 47.4 kW, according to the amount of formed hydrate. Thus, the system COP can be calculated as 6.8, being higher than most traditional refrigeration systems. This indicates that more cooling output can be obtained in the CO2 hydrate-based refrigeration system at the same energy consumption. The high energy efficiency might be the major strength of the present refrigeration system.


Page 14 of 33

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4 Simulation results of present system. Ozone depletion potential (ODP) and global warming potential (GWP) are two most critical evaluation indexes for a refrigeration system in terms of environmental concern. In the CO2 hydrate-based refrigeration system, the value of ODP of CO2 is zero and its GWP is merely 1, being much lower than most working media in conventional air-conditioning systems. Therefore, another obvious advantage of this CO2 hydrate-based refrigeration system is that the usage of traditional refrigerants such as HFCs and HCFCs might be minimized. The study of this environmentally friendly as well as harmless refrigerant in this novel system might offer some insights into the prevention and solution for problems such as global warming and ozone depletion. b. Effect of process equilibrium condition After reviewing all the literature mentioned above, it can be noticed that there are few phase equilibrium data of CO2 hydrate at formation temperature higher than 290 K. Thus, curve fitting is quite necessary to investigate the effect of higher formation temperature on the refrigeration performance. Quadratic curve is currently selected to 15 ACS Paragon Plus Environment


fit the temperature-pressure relation in our range of equilibrium condition (with the specific curve formula y=0.027x2-15.427x+2169.566). Our temperature-pressure fitting results based on the available phase equilibrium data are depicted in Fig. 5. The equilibrium pressure increases with the rising equilibrium temperature, which means higher formation pressure are required for hydrate formation at higher temperature. Much more energy will be consumed in the process of pressure elevating. In Fig. 5, the variation curve of the system COP goes down rapidly with rising formation temperature because pressurizing the gas and liquid substances to a higher pressure will costs more energy. With an increase in formation temperature of only 5 K, COP of the current system was dramatically reduced by around 30%. Although higher formation temperature is advantageous for heat releasing, but the system performance is quite poor in this situation. Therefore, the hydrate formation temperature is another important factor that affects the refrigeration performance.

Fig. 5 Effect of equilibrium condition on system COP. c. Effect of pressure changers efficiency As previously mentioned, the gas compression and liquid pumping are isentropic processes and the net work consumed by the pressure changers can be absolutely 16 ACS Paragon Plus Environment

Page 16 of 33

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transformed into enthalpy increase of the streams. For the same phase equilibrium condition (T1=280 K, P1=0.25 MPa and T2=292 K, P2=3.0 MPa), the total consumption of electric power is determined only by the efficiencies of pressure changers, including the gas compressor and the liquid pump. In brief, it means less electric power are required by pressure changers with higher efficiency for achieving the same pressure increase. Fig. 6 shows the relation between the efficiencies of pressure changers and the system COP in the reasonable range from 50% to 90% with interval of 5%. In these cases, phase equilibrium condition and other operation parameters remain unchanged as specified before. It can be clearly observed that both variation curves are nearly linear functions only with different slopes. With the increase of the compressor and pump efficiencies, the system performance can be enhanced obviously. With the gradual increase of pump efficiency from 50% to 90%, the system COP can be enhanced by around 10%. While the system performance can be greatly improved by nearly 68% when increasing the compressor efficiency in the same range. So, the effect of compressor efficiency is much more significant compared to the other one, which indicates the compressor efficiency can be a key factor in the energy efficiency improvement. Also, it can be concluded that the net work required in the liquid pumping process is much less than that needed in the gas compression. However, extremely high efficiencies of the gas compressor and liquid pump could be both impractical and unrealistic.



Fig. 6 Effect of efficiencies of pressure changers on system COP. d. Effect of hydrate mass fraction The phase transition enthalpy of CO2 hydrate has been proved to be satisfyingly high, even higher than the value of ice slurry. For this reason, the latent heat in hydrate formation and dissociation makes a more significant contribution in the heat removal process than the sensible heat of the liquid solution. Consequently, the heat-carrying capacity of the working medium is clearly determined by the hydrate fraction in the slurry. Therefore, effect of hydrate fraction in the slurry on system performance was investigated and then illustrated in Fig. 7. With the aforementioned equilibrium and operation parameters unchanged, the hydrate mass fraction varies from 50% to 90% with an interval of 5%. It can also be found that the system COP is a positive function of the hydrate mass fraction. The system performance can be enhanced by 9% with the continuous augment of hydrate fraction in weight. Higher mass fraction of CO2 hydrate in the slurry means a better thermal capacity per unit weight of working fluid, improving the system performance as a result. This trend slows down gradually when the value of mass fraction reaches a relatively high level because the formed CO2 hydrate in the reaction unit will deteriorate the gas-liquid 18 ACS Paragon Plus Environment

Page 18 of 33

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contact, preventing further hydrate formation. In addition, excessively high fraction of CO2 hydrate in the slurry might lead to a bad rheological behaviour or even pipeline blockage. Thus, the CO2 hydrate fraction should be limited in a proper range for the continuous operation of this refrigeration system.

Fig. 7 Effect of hydrate mass fraction on system COP.

Case study for cold energy storage operation To study the cold storage operation of the current refrigeration system, a case study of cold storage operation was conducted as well to obtain a proper operation strategy. Obviously, it has been proved that the hydrate will not form immediately in both various experiments and actual practices even when all the formation requirements have been met. Thus, induction time and hydrate formation rate are critical in practical operation. a. Induction time Induction time is an important operation parameter and it is defined as the period of the nucleation process of CO2 hydrate before visible hydrate formation.24 In the research conducted by Sabil et al.33, the induction time for CO2 hydrate formation was 19 ACS Paragon Plus Environment


experimentally measured in the THF solution with a concentration of 5 mol%, which is exactly the same with that in present work. The induction time was found to be from 25.62 minutes to 12.25 minutes in the initial pressure range from 1.53 MPa to 3.76 MPa (the hydrate formation pressure was 2.5 MPa in current work). For simplicity, the induction time for CO2 hydrate formation in present work was estimated as 30 minutes because our equilibrium temperature was higher than those in the literature.33 b. Hydrate formation rate Hydrate formation rate is another parameter that has to be predetermined before the study. It indicates the speed of hydrate growth in the formation process, which is very crucial in present system in the matter of time efficiency. An experimental correlation for the formation rate of CO2 hydrate with the promoter THF was proposed by Sun and Kang24, providing a much more convenient way to estimate the formation rate of this hydrate. However, this correlation can only be utilized at low formation pressure (

Conceptual Design and Analysis of a Novel CO2 Hydrate-Based

Recommend Documents