- Email: [email protected]
Simulation study of the dynamic performance of a MRC plant with refrigerant charged or leaked
Cryogenics 52 (2012) 8–12
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Simulation study of the dynamic performance of a MRC plant with refrigerant charged or leaked Heng Sun a,⇑, Dan Shu a, Zhihua Jiang b a b
China University of Petroleum, Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, Beijing 102249, China CNOOC Research, Beijing 100000, China
a r t i c l e
i n f o
Article history: Received 6 January 2010 Received in revised form 27 September 2011 Accepted 5 October 2011 Available online 10 October 2011 Keywords: A. Processing B. LNG B. Gas mixtures C. Thermodynamics
a b s t r a c t The running condition of a MRC plant is affected by the charge or leakage of the refrigerant. It is significant for the design and operation of the plant. A new model which is established based on the process simulation, mass conservation and characteristics of the system was employed to study the dynamic performance in these cases. The results show that the light composition mainly affects the pressure and the heavy composition affects the liquid level of vessel more obviously. This is due to the fact that the light composition mainly stays in the vapor phase and the heavies stay in the liquid phase mostly. The case when leakages occur at different location was also studied. The results can provide useful information for the adjustment of mixture refrigerant and operation of a MRC plant. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mixture refrigerant cycle is most widely used in LNG plants. Composition of the refrigerant is an important factor which affects the energy efficient of a LNG plant. Many process simulations and optimizations of MRC cycle were reported [1–4], which concern on how to design of a LNG plant. However, the operation of a MRC plant is also a significant problem [5–7]. Jørgen and Sigurd have studied the mixed fluid casade (MFC) LNG process developed by The Statoil Linde Technology Alliance, and how to adjust these to achieve optimal steady-state operation [5]. They also carried out the optimization of operation of a relatively simple LNG process, namely the PRICO process [6,7]. Arjun and Morten developed a simplified model which is applicable for control structure design without compromising [8]. Hasan et al. presented a generalized model for the compressor operations in multiple interacting refrigerant cycles in LNG and other cryogenic applications and applied the model on the AP-XTM LNG process [9]. Wilco et al. discussed the potential application of modern control technology in LNG production [10]. The running composition is very important parameter which should be controlled to ensure the high efficient operation of a MRC plant. However, the running composition is different with the composition of refrigerant charged into the plant, and can not be directed controlled or changed during a plant is running. Besides, the running condition of a MRC plant will be ⇑ Corresponding author. Tel./fax: +86 10 69726893. E-mail address: [email protected] (H. Sun). 0011-2275/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2011.10.001
changed when a pure composition of refrigerant is charged into the system or leakage of refrigerant occurs. Therefore, the relations between charged composition and charge composition are keys to know about the running condition of a MRC plant, which is also affected by other running conditions of the system. Few literatures about this problem were public reported in detail before. A dynamic simulation model for a small-scale MRC plant is established to study how the running composition and work condition of the system changes when refrigerant is charged into the plant or leaked in this paper.
2. MRC process The process flow diagram of a MRC plant studied here which employ a simply SMR (Single Mixture Refrigerant) cycle is shown in Fig. 1. The HYSYS software is used to simulate the liquefaction process. The LKP EOS was used to calculate both the phase-equilibrium and the Enthalpy. There are totally 17 state points in the process, which is marked in Fig. 1. The volume that each point represented is listed in Table 1. The volumes of the two vessels are 10 m3.
3. Mathematic models The relation of running composition and charged composition can be obtained based on the mass conservation, which is as follows:
9
H. Sun et al. / Cryogenics 52 (2012) 8–12
Air cooler
Air cooler
Seperator vessel
Seperator vessel Cold-box
Compressor of 1st stage
J-T valve
Compressor of 2nd stage
J-T valve
LNG Nature gas
Production valve
Pre-treating unit
Fig. 1. The process flow diagram of MRC plant.
Table 1 Calculation volume at different location. Locations
Volumes
Locations
Volumes
Locations
Volumes
Locations
Volumes
1 2 3 4 5
6.6375 2.913 1.4565 1.913 1.628
6 6v 6l 7
0.814 0.942 0.2355 1.157
8 9 11 12
2.157 1.157 3.1195 0.98125
13 21 22 23
0.98125 1.98125 2.98125 2.98125
ni ¼
X
nm ¼
qn;i;j zi;j V p;j þ
X
ni
ni ci ¼ P ni
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h
ð1Þ ð2Þ ð3Þ
n is the mole quantity, mol; z is the running molar fraction; c is the charged molar fraction; x is the running molar fraction of liquid refrigerant; y is the running molar fraction of vapor refrigerant; M is the molar weight, kg/kmol; V is the volume, m3; qn is the molar density, mol/m3; h is the relative liquid level of the inter-stage separator, %; p is the pressure; k is the pressure ratio; j is the thermal state at j location; k is the the k seperator; m is the mass of mixture refrigerant; e is the phase separator; p is the pipe and equipment; l is the liquid phase; v is the vapor phase; i is the i composition; and s is the s stage compressor. There are independent i 1 variables if i pure materials are used in the mixture refrigerant whilst the number of equations is i. How to choose additional variables and equations is the key to build up the model. The suction pressure of the first stage of the refrigerant compressor is the i variable. The assumption is made that the compression ratios of each stages keep constant while the suction pressure is changed:
pout;s ¼ ks pin;s
ð4Þ
Thus, the number of variables and equations are identical. However, the non-linear equations are ill-conditioned because that it does not describe the true performance of the MRC facility. Sometimes very irrational solution could be obtained. Therefore, addition one variable and additional one equation need to be added. The flow rates of liquid and vapor stream after the outlet of the first stage of refrigerant compressor are respectively determined by the flow resistance Sl and Sv:
Q m;l
sffiffiffiffiffiffi Dp ¼ Sl
sffiffiffiffiffiffi Dp Q m;v ¼ Sv
ð5Þ
It can be obtained by dividing Eq. (5), (6):
pffiffiffiffi Q n;v M l Sl pffiffiffiffiffi ¼ Constant f¼ ¼ Q n;l M v Sv
ð7Þ
The liquid level h1 of inter-stage separator vessel is selected as the last variable. The running composition, suction pressure and liquid level are i + 1 variables and i + 1 equations consist of Eqs. (1) and (7). Thus, a mathematical model which can predict the change of the running conditions of a MRC plant based on the charged composition of refrigerant is set up. It is simply to solve the equations if the running composition is known. However, complex nonlinear equations are needed to be solved if running composition is unknown. This problem can be converted to an optimization problem with constraints. The variables are running composition, suction pressure and liquid level. The optimal objective is as follows:
f ¼
X X
qn;i;j zi;j V p;j þ
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h ni
2
ð8Þ The constraint is Eq. (7). It can be converted into a optimization problem without constraint with penalty method, which is as follows:
f ¼
X X
qn;i;j zi;j V p;j þ
pffiffiffiffi M l Sl pffiffiffiffiffi þA f M v Sv
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h ni
2
ð9Þ where A is a penalty coefficient which could be set as 10–100 in this case. The optimization problem can be solved using BOX method, which generate an original complex including n + 1 points for n variables. The worst point is substituted with the reflection point after calculating the values of all the points. The process is iterated until the optimized point is obtained. 4. Running performance when refrigerants charged into a plant
ð6Þ
The effect of charge of pure C1, C3 and C5 into the plant on the work condition of a MRC facility is studied using the model
10
H. Sun et al. / Cryogenics 52 (2012) 8–12
Fig. 4. The effect of charge of i-penthane on the running composition.
Fig. 2. The effect of charge of methane on the running composition.
consequently. Meanwhile, the fraction of N2 is almost kept constant and those of other compositions are decreased. When C5 is charged, the result is rather different with the case when light refrigerant is charged in. The fraction of C5 is increased slightly, and the fraction of C3 is rapidly decreased. Besides, the fraction of C1 and N2 is increased slightly. The change of fractions of C2 is ignorable. This is caused by the change of vapor–liquid equilibrium at the vessels. The variations of suction pressure and liquid level of the vessel is shown in Figs. 5 and 6. It can be seen that the suction pressure is increased rapidly while C1 is charged into the plant whilst only minor increase of suction pressure occurs when C3 is charged. While C5 is charged into the system, the suction pressure is decreased obviously. All the liquid levels of the separators of three cases are increased while the amplitude is different. Effect of C5 is the most notable and the level is increased most slightly when C1 is added into the plant. It can be conducted that the increased of light composition mainly cause the pressure through the cycle increased and the charge of heavies mainly cause the liquid level increased. This can be explained that most of the light composition stayed in the vapor phase meanwhile most of the heavies are liquefied and enter the liquid phase of the vessels.
Fig. 3. The effect of charge of propane on the running composition.
established. The original composition of the refrigerant is listed in Table 2 and the composition and quantity of the refrigerant charged into the plant is calculated according to Eqs. (1)–(3). Once a pure C1, C3 or C5 is charged into the plant, the running performances, such as running composition, suction pressure and liquid level of vessels will be changed accordingly. The change of running composition is illustrated in Figs. 2–4 respectively when C1, C3 and C5 are charged into the system. It can be drawn from Fig. 1 that the C1 fraction in running composition is increased when C1 is charged into the system, whilst fractions of all the other compositions are decreased. However, the results are a bit different when C3 is charged. In this case, the fraction of C3 is increased
5. Running performances when leakage of refrigerants occurs It is hard to avoid the leakage of the refrigerant from a MRC plant absolutely. Therefore it is valuable to research the effect of leakage at different locations on the running condition of the system. The change of running compositions of when leakages occur at suction pipe and inter-stage pipe of compressor is illustrated in Figs. 7 and 8, respectively. It can be drawn that leakages at these two locations are very similar because the compositions at the two positions are very near. The fractions of the light composition in running compositions are decreased and those of heavies are increased. The variations of suction pressure and liquid level of
Table 2 Running composition, charge composition and charge quantity of refrigerant in original state. Running composition (%)
Charged composition (%)
Charged refrigerant (kmol)
C1 C2 C3 C5 N2
24 27.4 18.7 22.3 7.5
13.4 17.8 22.9 42.3 3.6
32.86 43.75 56.34 103.99 8.78
Total
100
100
245.72
H. Sun et al. / Cryogenics 52 (2012) 8–12
11
Fig. 5. The effect of charge of methane, propane and i-penthane on suction pressure.
Fig. 8. The effect of leakage at inter-stage pipe on the running composition.
Fig. 6. The effect of charge of methane, propane and i-penthane on vessel level. Fig. 9. The effect of leakage at different locations on suction pressure.
Fig. 10. The effect of leakage at different locations on vessel level.
6. Conclusions Fig. 7. The effect of leakage at suction pipe on the running composition.
inter-stage vessel when leakages occur are also very similar, as shown in Figs. 9 and 10. In Figs. 9 and 10, L1 denotes location of suction pipes and L2 denotes inter-stage locations. The effect on the suction pressure when leakage occurs at suction pipe is a little more obviously than that at inter-stages pipe whilst leakage at inter-stages pipe affects liquid level of inter-stage separator a little more notable.
A mathematic model is set up on the basis of process simulation, mass conservation and the inherent performance of a MRC plant. It can be used to predict the change of the running conditions of the plant when the refrigerant charged into the plant is changed, such as charge or leakage of refrigerant. The cases that C1 C3 and C5 are charged into the plant are studied and analyses based on this model. The results show that when light composition is charged, the fraction of this composition is increased and that of all the other compositions are decreased. This also causes the increase of the suction pressure rapidly and affects the liquid level slightly. The results are different when heavies are charged into
12
H. Sun et al. / Cryogenics 52 (2012) 8–12
the system. Sometimes, the fractions of another light composition may be increased when heavy refrigerant is charged into the system. Meanwhile, the liquid level will be increased rapidly and the effect on the suction pressure is ignorable. This is because that the light compositions mainly stayed in the vapor phase and the heavies mainly liquefied and accumulated in the inter-stage vessel due to the change of the vapor–liquid phase equilibrium. The effects of leakage at different locations are also studied. The results reveal how the running composition and other parameters of the system changes under different condition. The information is useful for the control and adjustment of the running composition of a MRC plant. This is also helpful for the operation of the plant when running condition changes. Acknowledgments The authors are grateful for funding by the Research Fund for the Doctoral Program of Higher Education for New Teacher (No. 200804251006: Research on the liquefaction flow process of coalbed gas with oxygen using MRC-distillation method).
References [1] Cao Wen-sheng, Lu Xue-sheng, Lin Wen-sheng, Gu An-zhong. Parameter comparison of two small-scale natural gas liquefaction processes in skidmounted packages. Appl Therm Eng 2006;26(8-9):898–904. [2] Remeljeja CW, Hoadley AFA. An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes. Energy 2006;31(12):2005–19. [3] Wang BQ. LNG processing technologies and comparison analysis. Natural Gas Ind 2009;29(1):111–3. [4] Pu L, Sun SX, Chen XH, Li YZ. Calculations and exergy analysis on several kinds of liquefaction processes of natural gas. Chem Eng (China) 2008;36(2):54–8. [5] Jórgen BJ, Sigurd S. Optimal operation of a mixed fluid cascade LNG plant. Comput Aided Chem Eng 2006;21:1569–74. [6] Jørgen BJ, Sigurd S. Single-cycle mixed-fluid LNG process Part I: Optimal design. In: Proceedings of the 1st annual gas processing symposium; 2009. p. 211–8. [7] Jørgen BJ, Sigurd S, Single-cycle mixed-fluid LNG process Part II: Optimal operation. In: Proceedings of the 1st annual gas processing symposium; 2009. p. 211–8. [8] Arjun S, Morten H. Model requirement for control design of an LNG process. Comput Aided Chem Eng 2007;24:533–8. [9] Hasan MMF, Shamsuzzaman MR, Karimi IA. Optimization of compressor networks in LNG operations. Comput Aided Chem Eng 2009;27:1767–72. [10] Wilco S, Maria PC, Francois C, Jan O, Unlocking the potential of modern control and optimization strategies in LNG production. In: Proceedings of the 1st annual gas processing symposium; 2009. p. 227–38.
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Simulation study of the dynamic performance of a MRC plant with refrigerant charged or leaked Heng Sun a,⇑, Dan Shu a, Zhihua Jiang b a b
China University of Petroleum, Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, Beijing 102249, China CNOOC Research, Beijing 100000, China
a r t i c l e
i n f o
Article history: Received 6 January 2010 Received in revised form 27 September 2011 Accepted 5 October 2011 Available online 10 October 2011 Keywords: A. Processing B. LNG B. Gas mixtures C. Thermodynamics
a b s t r a c t The running condition of a MRC plant is affected by the charge or leakage of the refrigerant. It is significant for the design and operation of the plant. A new model which is established based on the process simulation, mass conservation and characteristics of the system was employed to study the dynamic performance in these cases. The results show that the light composition mainly affects the pressure and the heavy composition affects the liquid level of vessel more obviously. This is due to the fact that the light composition mainly stays in the vapor phase and the heavies stay in the liquid phase mostly. The case when leakages occur at different location was also studied. The results can provide useful information for the adjustment of mixture refrigerant and operation of a MRC plant. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mixture refrigerant cycle is most widely used in LNG plants. Composition of the refrigerant is an important factor which affects the energy efficient of a LNG plant. Many process simulations and optimizations of MRC cycle were reported [1–4], which concern on how to design of a LNG plant. However, the operation of a MRC plant is also a significant problem [5–7]. Jørgen and Sigurd have studied the mixed fluid casade (MFC) LNG process developed by The Statoil Linde Technology Alliance, and how to adjust these to achieve optimal steady-state operation [5]. They also carried out the optimization of operation of a relatively simple LNG process, namely the PRICO process [6,7]. Arjun and Morten developed a simplified model which is applicable for control structure design without compromising [8]. Hasan et al. presented a generalized model for the compressor operations in multiple interacting refrigerant cycles in LNG and other cryogenic applications and applied the model on the AP-XTM LNG process [9]. Wilco et al. discussed the potential application of modern control technology in LNG production [10]. The running composition is very important parameter which should be controlled to ensure the high efficient operation of a MRC plant. However, the running composition is different with the composition of refrigerant charged into the plant, and can not be directed controlled or changed during a plant is running. Besides, the running condition of a MRC plant will be ⇑ Corresponding author. Tel./fax: +86 10 69726893. E-mail address: [email protected] (H. Sun). 0011-2275/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2011.10.001
changed when a pure composition of refrigerant is charged into the system or leakage of refrigerant occurs. Therefore, the relations between charged composition and charge composition are keys to know about the running condition of a MRC plant, which is also affected by other running conditions of the system. Few literatures about this problem were public reported in detail before. A dynamic simulation model for a small-scale MRC plant is established to study how the running composition and work condition of the system changes when refrigerant is charged into the plant or leaked in this paper.
2. MRC process The process flow diagram of a MRC plant studied here which employ a simply SMR (Single Mixture Refrigerant) cycle is shown in Fig. 1. The HYSYS software is used to simulate the liquefaction process. The LKP EOS was used to calculate both the phase-equilibrium and the Enthalpy. There are totally 17 state points in the process, which is marked in Fig. 1. The volume that each point represented is listed in Table 1. The volumes of the two vessels are 10 m3.
3. Mathematic models The relation of running composition and charged composition can be obtained based on the mass conservation, which is as follows:
9
H. Sun et al. / Cryogenics 52 (2012) 8–12
Air cooler
Air cooler
Seperator vessel
Seperator vessel Cold-box
Compressor of 1st stage
J-T valve
Compressor of 2nd stage
J-T valve
LNG Nature gas
Production valve
Pre-treating unit
Fig. 1. The process flow diagram of MRC plant.
Table 1 Calculation volume at different location. Locations
Volumes
Locations
Volumes
Locations
Volumes
Locations
Volumes
1 2 3 4 5
6.6375 2.913 1.4565 1.913 1.628
6 6v 6l 7
0.814 0.942 0.2355 1.157
8 9 11 12
2.157 1.157 3.1195 0.98125
13 21 22 23
0.98125 1.98125 2.98125 2.98125
ni ¼
X
nm ¼
qn;i;j zi;j V p;j þ
X
ni
ni ci ¼ P ni
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h
ð1Þ ð2Þ ð3Þ
n is the mole quantity, mol; z is the running molar fraction; c is the charged molar fraction; x is the running molar fraction of liquid refrigerant; y is the running molar fraction of vapor refrigerant; M is the molar weight, kg/kmol; V is the volume, m3; qn is the molar density, mol/m3; h is the relative liquid level of the inter-stage separator, %; p is the pressure; k is the pressure ratio; j is the thermal state at j location; k is the the k seperator; m is the mass of mixture refrigerant; e is the phase separator; p is the pipe and equipment; l is the liquid phase; v is the vapor phase; i is the i composition; and s is the s stage compressor. There are independent i 1 variables if i pure materials are used in the mixture refrigerant whilst the number of equations is i. How to choose additional variables and equations is the key to build up the model. The suction pressure of the first stage of the refrigerant compressor is the i variable. The assumption is made that the compression ratios of each stages keep constant while the suction pressure is changed:
pout;s ¼ ks pin;s
ð4Þ
Thus, the number of variables and equations are identical. However, the non-linear equations are ill-conditioned because that it does not describe the true performance of the MRC facility. Sometimes very irrational solution could be obtained. Therefore, addition one variable and additional one equation need to be added. The flow rates of liquid and vapor stream after the outlet of the first stage of refrigerant compressor are respectively determined by the flow resistance Sl and Sv:
Q m;l
sffiffiffiffiffiffi Dp ¼ Sl
sffiffiffiffiffiffi Dp Q m;v ¼ Sv
ð5Þ
It can be obtained by dividing Eq. (5), (6):
pffiffiffiffi Q n;v M l Sl pffiffiffiffiffi ¼ Constant f¼ ¼ Q n;l M v Sv
ð7Þ
The liquid level h1 of inter-stage separator vessel is selected as the last variable. The running composition, suction pressure and liquid level are i + 1 variables and i + 1 equations consist of Eqs. (1) and (7). Thus, a mathematical model which can predict the change of the running conditions of a MRC plant based on the charged composition of refrigerant is set up. It is simply to solve the equations if the running composition is known. However, complex nonlinear equations are needed to be solved if running composition is unknown. This problem can be converted to an optimization problem with constraints. The variables are running composition, suction pressure and liquid level. The optimal objective is as follows:
f ¼
X X
qn;i;j zi;j V p;j þ
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h ni
2
ð8Þ The constraint is Eq. (7). It can be converted into a optimization problem without constraint with penalty method, which is as follows:
f ¼
X X
qn;i;j zi;j V p;j þ
pffiffiffiffi M l Sl pffiffiffiffiffi þA f M v Sv
X
qv ;i;k yi;k V k ð1 hÞ þ
X
ql;i;k xi;k V k h ni
2
ð9Þ where A is a penalty coefficient which could be set as 10–100 in this case. The optimization problem can be solved using BOX method, which generate an original complex including n + 1 points for n variables. The worst point is substituted with the reflection point after calculating the values of all the points. The process is iterated until the optimized point is obtained. 4. Running performance when refrigerants charged into a plant
ð6Þ
The effect of charge of pure C1, C3 and C5 into the plant on the work condition of a MRC facility is studied using the model
10
H. Sun et al. / Cryogenics 52 (2012) 8–12
Fig. 4. The effect of charge of i-penthane on the running composition.
Fig. 2. The effect of charge of methane on the running composition.
consequently. Meanwhile, the fraction of N2 is almost kept constant and those of other compositions are decreased. When C5 is charged, the result is rather different with the case when light refrigerant is charged in. The fraction of C5 is increased slightly, and the fraction of C3 is rapidly decreased. Besides, the fraction of C1 and N2 is increased slightly. The change of fractions of C2 is ignorable. This is caused by the change of vapor–liquid equilibrium at the vessels. The variations of suction pressure and liquid level of the vessel is shown in Figs. 5 and 6. It can be seen that the suction pressure is increased rapidly while C1 is charged into the plant whilst only minor increase of suction pressure occurs when C3 is charged. While C5 is charged into the system, the suction pressure is decreased obviously. All the liquid levels of the separators of three cases are increased while the amplitude is different. Effect of C5 is the most notable and the level is increased most slightly when C1 is added into the plant. It can be conducted that the increased of light composition mainly cause the pressure through the cycle increased and the charge of heavies mainly cause the liquid level increased. This can be explained that most of the light composition stayed in the vapor phase meanwhile most of the heavies are liquefied and enter the liquid phase of the vessels.
Fig. 3. The effect of charge of propane on the running composition.
established. The original composition of the refrigerant is listed in Table 2 and the composition and quantity of the refrigerant charged into the plant is calculated according to Eqs. (1)–(3). Once a pure C1, C3 or C5 is charged into the plant, the running performances, such as running composition, suction pressure and liquid level of vessels will be changed accordingly. The change of running composition is illustrated in Figs. 2–4 respectively when C1, C3 and C5 are charged into the system. It can be drawn from Fig. 1 that the C1 fraction in running composition is increased when C1 is charged into the system, whilst fractions of all the other compositions are decreased. However, the results are a bit different when C3 is charged. In this case, the fraction of C3 is increased
5. Running performances when leakage of refrigerants occurs It is hard to avoid the leakage of the refrigerant from a MRC plant absolutely. Therefore it is valuable to research the effect of leakage at different locations on the running condition of the system. The change of running compositions of when leakages occur at suction pipe and inter-stage pipe of compressor is illustrated in Figs. 7 and 8, respectively. It can be drawn that leakages at these two locations are very similar because the compositions at the two positions are very near. The fractions of the light composition in running compositions are decreased and those of heavies are increased. The variations of suction pressure and liquid level of
Table 2 Running composition, charge composition and charge quantity of refrigerant in original state. Running composition (%)
Charged composition (%)
Charged refrigerant (kmol)
C1 C2 C3 C5 N2
24 27.4 18.7 22.3 7.5
13.4 17.8 22.9 42.3 3.6
32.86 43.75 56.34 103.99 8.78
Total
100
100
245.72
H. Sun et al. / Cryogenics 52 (2012) 8–12
11
Fig. 5. The effect of charge of methane, propane and i-penthane on suction pressure.
Fig. 8. The effect of leakage at inter-stage pipe on the running composition.
Fig. 6. The effect of charge of methane, propane and i-penthane on vessel level. Fig. 9. The effect of leakage at different locations on suction pressure.
Fig. 10. The effect of leakage at different locations on vessel level.
6. Conclusions Fig. 7. The effect of leakage at suction pipe on the running composition.
inter-stage vessel when leakages occur are also very similar, as shown in Figs. 9 and 10. In Figs. 9 and 10, L1 denotes location of suction pipes and L2 denotes inter-stage locations. The effect on the suction pressure when leakage occurs at suction pipe is a little more obviously than that at inter-stages pipe whilst leakage at inter-stages pipe affects liquid level of inter-stage separator a little more notable.
A mathematic model is set up on the basis of process simulation, mass conservation and the inherent performance of a MRC plant. It can be used to predict the change of the running conditions of the plant when the refrigerant charged into the plant is changed, such as charge or leakage of refrigerant. The cases that C1 C3 and C5 are charged into the plant are studied and analyses based on this model. The results show that when light composition is charged, the fraction of this composition is increased and that of all the other compositions are decreased. This also causes the increase of the suction pressure rapidly and affects the liquid level slightly. The results are different when heavies are charged into
12
H. Sun et al. / Cryogenics 52 (2012) 8–12
the system. Sometimes, the fractions of another light composition may be increased when heavy refrigerant is charged into the system. Meanwhile, the liquid level will be increased rapidly and the effect on the suction pressure is ignorable. This is because that the light compositions mainly stayed in the vapor phase and the heavies mainly liquefied and accumulated in the inter-stage vessel due to the change of the vapor–liquid phase equilibrium. The effects of leakage at different locations are also studied. The results reveal how the running composition and other parameters of the system changes under different condition. The information is useful for the control and adjustment of the running composition of a MRC plant. This is also helpful for the operation of the plant when running condition changes. Acknowledgments The authors are grateful for funding by the Research Fund for the Doctoral Program of Higher Education for New Teacher (No. 200804251006: Research on the liquefaction flow process of coalbed gas with oxygen using MRC-distillation method).
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