PRO/II Process Simulation

Natural Gas Liquefaction

by

Mixed Refrigerant (MR) Cycle in Brazed Aluminum Exchangers (Mal-distribution)

by Greenwood


The Purpose of  simulation Brazed aluminum heat exchanger (BAHX) is sensitive to mal-distribution of liquid. Therefore, it is necessary to investigate an effect of liquid mal-distribution to BAHXs. Most critical part is low pressure mixed refrigerant (LPMR) inlet to brazed aluminum heat exchanger. In this case, vapor and liquid were separated in a vessel and separately distributed to each channel of BAHX. But question is whether such provision is needed for high pressure mixed refrigerant (HPMR) inlet to BAHXs.

 PRO/II was used for this objective. For study purpose, simple mixed refrigerant cycle (MR cycle) was selected as an example and all operation condition were taken exactly same as the case exhibited in

Natural Gas Liquefaction by Mixed Refrigerant (MR) in Core Exchanger (Distillation Effect)

 

Feed Gas Condition

Total Feed gas 6,000kgmol/h (roughly 100ton/h)

Composition:

Component

 mol %
Nitrogen 1
Methane 93
Ethane 3
Propane 2
Butane 1

Feed pressure: 60 Bar G

Feed temperature: 40oC


Mixed Refrigerant Condition

MR flow rate: 19,000kgmol/h

Pressure of  HPMR at exchanger inlet: 49.5BarG

Pressure of LPMR at exchanger inlet: 3 BarG

Temperature: 40oC

MR Composition:

Component

 mol %
Nitrogen 12
Methane 27
Ethane 26
Propane 20
Butane 15

Simulation Model

It was assumed that liquid tends to flow into 1/3 of BAEX installed in parallel. This liquid rich group is named E-1. The other group is named E-2. Under even distribution, E-1 receives 33.33% of HPMRL1 and HPMRV1. But under mal distribution, E-1 receives more HPMRL1 and less HPMRV1. It was assumed that the quantity of HPMRV1 may be determined to keep same moral flow of HPMR1. Reason of this is volumetric flow of HPMR1 is kept fairly constant due to pressure balance.

Pressure drop of each E-1 and E-2 were taken as 1Bar. Heat loss of exchanger was taken as 3%.

To avoid convergence calculation, MR stream was cut at LPMR exchanger inlet.

In modeling PRO/II, E-1 and E-2 were split into 30 zones, and each heat duty, weighted average LMTD, MITA and UA were viewed after each trial run.


PRO/II model

 

Summary of Simulation

Liquid Distribution unit total flow even mal mal mal mal
HPMRL1/HPML mol ratio % 100 33.33 40.00 45 50 55
HPMRV1/HPMV mol ratio % 100 33.33 29.76 27.09 24.41 21.74
LPMR1/LPMR mol ratio % 100 33.33 33.33 33.33 33.33 33.33
NG1/NG mol ratio % 100 33.33 33.33 33.33 33.33 33.33
HPMRV1 kgmol/h 12,375 4,125 3,683 3,352 3,021 2,690
HPMRL1 kgmol/h 6,625 2,208 2,650 2,981 3,313 3,644
HPMR1=HPMRV1+HPMRL1 (keep constant) kgmol/h 19,000 6,333 6,333 6,333 6,333 6,333
LPMRINT1 kgmol/h 19,000 6,333 6,333 6,333 6,333 6,333
NG1/NG kgmol/h 6,000 2,000 2,000 2,000 2,000 2,000
E-1






Duty1 Tcal/h NA 37.58 37.53 37.5 37.47 37.43
weighted LMTD1 C NA 7.35 6.89 6.51 6.09 5.63
MITA1 C NA 2.40 2.28 2.19 2.1 2.02
UA1 Mcal/h C NA 5,114 5,545 5,756 6,152 6,650
E-2






Duty2 Tcal/h NA 75.17 75.22 75.26 75.29 75.33
weighted LMTD2 C NA 7.35 7.56 7.7 7.85 7.99
MITA2 C NA 2.40 2.47 2.52 2.57 2.58
UA2 Mcal/h C NA 10,230 9,956 9,769 9,590 9,429
mal-distribution HPMRL1/even distiribution HPMRL1 - NA 1 1.20 1.35 1.50 1.65
mal-distribution UA1/even distribution UA1 % NA 100 108.43 112.55 120.30 130.04
mal-distribution UA2/even distribution UA2 % NA 100 97.32 95.49 93.74 92.17


Conclusion

When design margin of E-1 is around 20%, and ratio of mal-distribution HPMRL1/even distribution HPMRL1 is over 1.5, or when 50% of total liquid flows into E-1,  E-1 could not perform design capacity.


Acknowledgment

Author is grateful to Invensys Systems Japan, Inc. for letting author to use PRO/II v9.

 

August 22, 2011

Rev. August 24, 2011

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