Proceedings of the 1st Annual Gas Processing Symposium H. Alfadala, G.V. Rex Reklaitis and M.M. El-Halwagi (Editors) © 2009 Elsevier B.V. All rights reserved. 1

A Method for Optimal Operation of BOG Compressors in a LNG Gasification Plant Myung Wook Shin,a Namjin Jang,b Dongil Shin,c Chonghun Han,b Soo Hyoung Choi,d and En Sup Yoonb* aIncheon LNG Terminal, Korea Gas Corporation, Incheon 406-130, Korea bSchool of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea cDepartment of Chemical Engineering, Myongji University, Yongin 449-728, Korea dDivision of Chemical Engineering, Chonbuk National University, Jeonju 561-756, Korea

Abstract An algorithm for optimal operation of BOG (boil off gas) compressors in a LNG gasification plant is proposed in this paper, which uses an empirical BOR (boil off rate) model, a MILP (mixed integer linear programming) formulation, and a simplified dynamic tank model. Given the values for a set of process state variables, the proposed algorithm generates an optimal operation schedule for BOG compressors, which minimizes the power consumption while preparing against the potential failure of one of the operating compressors. The performance of the proposed method is compared with that of the conventional method in terms of safety and energy. Keywords: optimal operation, BOG, boil off gas, compressors, LNG gasification plant

1. Introduction A LNG (liquefied natural gas) gasification plant is mainly composed of LNG storage tanks, LNG pumps, LNG vaporizers, and the gas pipelines. The plant receives LNG from a carrier, and stores it in a cryogenic liquid state. The liquid temperature is approximately −160℃ and the pressure is slightly above atmosphere in LNG storage tanks (Lemmers and Fluor Haarlem, 2005; Shin et al., 2005). Because of heat transfer from surroundings, part of LNG in the tank is continuously evaporated as boil off gas (BOG), which should be removed by compressors in order to maintain the tank pressure within a safe range. However, excessive operation of compressors spends excess energy, and even generates excess BOG. Therefore, proper handling of BOG during normal operation is essential for the efficiency of the operation and safety of the LNG terminal. Data obtained from a LNG gasification plant indicate that the rate of BOG generation is greatly influenced by the difference between the vapor pressure of the LNG and the pressure of the gas phase in the tank (Shin et al., 2008). Therefore, in order to minimize the energy consumption of the BOG compressors, the tank pressure should be maintained as high as possible while the safety is guaranteed. An optimal operation algorithm is proposed in this paper, which uses an empirical BOR (boil off rate) model

2 M. W. Shin et al.

and a simplified dynamic tank model. Given the values for a set of process state variables, the proposed algorithm generates an optimal operation schedule for BOG compressors, which minimizes the power consumption while preparing against the potential failure of one of the operating compressors (Shin et al., 2007).

2. Proposed Method

2.1. Load calculation: a BOR model The following empirical equation is used in order to estimate the rate of BOG generation in a LNG storage tank (Shin et al., 2008).

321 KKKVBCF LLSR ρ

=

where CR = rollover coefficient (≥ 1) BS = boil off rate on specification (h–1) ρL = LNG density (kg/m3) VL = LNG volume (m3) K1 = correction factor for the offset of the tank pressure from the LNG vapor pressure K2 = correction factor for the LNG temperature K3 = correction factor for the ambient temperature The BOR on specification BS is to be supplied by the tank manufacturer. For the plant of this study, it is defined as the BOR when the tank pressure equals the LNG vapor pressure, the LNG temperature is −162 °C, and the ambient temperature is 35.3 °C. The coefficient CR represents the rollover effect by the LNG circulation. The correction factors are as follows.

[ ]7941.0)(02135.0)(009568.01089.1)(09781.021 2

1 +−+−++−= vvv PPPPPPK ,

812 ≤−≤− vPP gf/cm2 where P = tank pressure (gf/cm2) Pv = LNG vapor pressure (gf/cm2)

[ ] 1162004542.02 ++= LTK , 157162 −≤≤− LT °C where TL = LNG temperature (°C)

8.1611.197

3 +=

aTK , 3.355 ≤≤− aT °C

where

A Method for Optimal Operation of BOG Compressors in a LNG Gasificaiton Plant 3

Ta = ambient temperature (°C) The LNG vapor pressure correlation is based on the Antoine equation, and the model equation for this study is as follows.

⎟⎟⎠

⎞⎜⎜⎝

⎛+

−=15.273

3.10180934.9exp)/cmkg( 2

Lfv T

P , 158159 −≤≤− LT °C

where TL = LNG temperature (°C) Using the above equations, the BOG generation rate can be calculated from the tank pressure. As the LNG volume is very large, the LNG volume and temperature change very slowly, and thus can be assumed to be constant. Therefore, if a target steady state tank pressure Ps is given, the target compressor load Fo can be calculated using the same equations, which can be summarized as follows. Fo = F(Ps) 2.2. Power minimization: a MILP formulation The following mathematical program determines an optimal operation strategy for multiple compressors in parallel, which minimizes the total average power consumption (Shin et al., 2007).

∑∑∑= ==

+n

i

l

jij

n

ii yW

1 01

min ε

subject to BOG flowrate:

o

n

ii FF =∑

=1

Operation time fractions:

00

=−∑=

i

l

jij xt , ni ,,1L=

Average flowrates:

01

=−∑=

i

l

jijj Ftα , ni ,,1L=

Average power consumptions:

00

=−∑=

i

l

jij

l

j Wtcc

, ni ,,1L=

Operation modes:

4 M. W. Shin et al.

0≥− ijij ty , ni ,,1L= , lj ,,0 L=

Extra constraints: 01 ≥− +ii FF , 1,,1 −= ni L

Integer constraints: ix = 0 or 1, ni ,,1L=

ijy = 0 or 1, ni ,,1L= , lj ,,0 L=

The objective function has two terms. The first one represents the total power consumption, and the second one is a penalty for complicated operations, where ε is a small positive number. Therefore, the minimization of this objective function results in the simplest operation at minimum power consumption. In this problem formulation, the index i ≤ n represents the compressor number, and j ≤ l represents the compressor load level (α0 = 0 < α1 < … < αl−1 < αl = 1). The binary integer variable xi indicates whether compressor i is to be used (xi = 1) or not (xi = 0). The variable tij represents the fraction of the operation period for compressor i to run at level j. The operation period will be determined later. The binary variable yij indicates whether the operation mode of compressor i at level j is used (yij = 1) or not (yij = 0). The target compressor load Fo in the above formulation is normalized by the maximum flowrate of a compressor so that it represents the number of compressors required. The variable Fi represents the average flowrate of compressor i normalized by the maximum flowrate of the compressor so that Fi ≤ 1. The constant cj represents the power consumption of each compressor at load level j, and thus the variable Wi represents the average power consumption of compressor i normalized by its maximum power consumption cl. 2.3. Safety analysis: a dynamic tank model For safety of the LNG gasification plant, we must prepare against failure of BOG compressors during the operation. Generally, BOG compressors require a long time for startup. Therefore, it may be too late to turn on a new compressor just when a compressor fails. The rule proposed in this study is to operate an extra compressor idle as a backup if a failure should result in flare stack ignition within the startup time. Therefore, a dynamic tank model is required which can predict the change in the tank pressure when compressor flowrate changes. Let us apply the ideal gas law to the gas phase in the LNG storage tank. As the tank is large, the gas volume and temperature change slowly. Therefore, the rate of tank pressure change can be expressed as follows.

VRTFF

dtdP o )( −

≈

where F is the molar rate of BOG generation, Fo is the output molar flowrate, R is the gas constant, T is the gas temperature, and V is the gas volume in the tank.

A Method for Optimal Operation of BOG Compressors in a LNG Gasificaiton Plant 5

3. Case Study Consider a plant with ten LNG storage tanks with a volume of 100,000 m3 each and a BOR of 0.1%/day on specification and six BOG compressors with a maximum capacity of 10 tons/h each as shown in Fig. 1. The compressors require 30 min idle operation for startup, and have load levels 0, 50, 75, and 100%, at which the electric currents are 65, 115, 127, and 145 A, respectively, when operated at 6,600 V. The normal operation range of tank pressure is between 50 gf/cm2 and 170 gf/cm2 gauge. If the pressure goes down to 40 gf/cm2 gauge, a vacuum breaker is activated, and if it goes up to 190 gf/cm2 gauge, the BOG is sent to a flare stack.

Figure 1. A BOG compression process (Shin et al., 2008).

Let us consider a high boil off rate situation. All the tanks are 90% filled, and thus the total LNG volume is 900,000 m3. The LNG temperature is −159 °C, and the density is 459.7 kg/m3. The gas temperature is −140 °C, and the ambient temperature is 30 °C. The LNG is pumped out, and 20% is recycled into the tanks. The initial tank pressure is 150 gf/cm2 gauge. The operation history indicates that two and half compressors are necessary in order to maintain the tank pressure in this situation. Note that while the total power consumption should be minimized, a backup compressor is to be on standby if necessary in order to avoid a flare stack even if one of the compressors fails. Determine an optimal operation method. Let us determine the value of the rollover coefficient first. Assume CR = 1, then the initial BOR estimated by the proposed model is 20,750 kg/h. Therefore, CR = 25,000/20,750 ≈ 1.2 is to be used. Changes in the tank pressure for compressor flowrates from 0 to 30 tons/h have been simulated using the proposed model, and the results are presented in Fig. 2. The proposed method starts from a target steady state tank pressure Ps ≤ 170 gf/cm2 gauge. Let Ps = 170 gf/cm2 gauge, and then Fo = 8.978 tons/h. Optimization for this load results in use of one compressor at full level for t13 = 0.8978 and at idle level for t10 = 0.1022. However, as the pressure oscillates, P > Ps occurs unless the operation period is infinitesimally short. Therefore, the time fraction for idle operation should be less than

6 M. W. Shin et al.

0.1022 in practice. Simulation shows that repeating 36 min full and 1.9 min idle operations would result in 167.9 < P < 170 gf/cm2 gauge oscillation, which means t10 = 0.050 is operable. Furthermore, the simulation result indicates that flare stack ignition occurs within 30 min if the compressor fails at a pressure above 166.5 gf/cm2 gauge. This pressure is expected to be reached at t = 41 min when one compressor is operated at full level from P = 150 gf/cm2 gauge. Therefore, the optimal operation schedule is as follows. 1. Operate compressor 1 at full load level Fo = 10 tons/h. 2. Turn on compressor 2 at P = 166.5 gf/cm2 g (t = 41 min), and keep on standby. 3. Idle compressor 1 while P < 170 gf/cm2 g (1.9 min). 4. Switch compressor 1 to full level at P = 170 gf/cm2 g, run until P = 167.9 gf/cm2 g (36 min), and go to step 3.

140

150

160

170

180

190

200

0 50 100t (min)

P (g

f/cm

2g)

00.50.7511.51.7522.52.753

Figure 2. Simulation results of the tank pressure change for numbers of compressors

(Shin et al., 2008).

4. Performance Evaluation In order to test the operation procedure generated by the proposed algorithm, dynamic simulation is performed for the above case using Aspen Dynamics (Alderman, 2005). The literature reports that the composition of methane in LNG varies from 80 to 97% (Woodward, 2007; Eisentrout, 2007), and the boil off gas in the tank is mainly composed of methane and ethane as shown in Table 1. The Peng-Ronbinson equation has been used as the equation of state in order to predict the pressure behavior of BOG. The results of dynamic simulation are shown in Figure 3. At startup, compressor 1 is operated at full load level until the pressure reaches 167.9 gf/cm2 g, which takes 36.5 min. Then compressor 1 is operated idle for 3.6 min until the pressure reaches 170 gf/cm2 g. Next, compressor 1 runs for 42.6 min at full load level and switches to idle level for 2.4 min. The operation procedure is repeated, and the pressure oscillates within the safe range. The backup compressor 2 is turned on at the pressure of 166.5 gf/cm2 g,

A Method for Optimal Operation of BOG Compressors in a LNG Gasificaiton Plant 7

which is 35.8 min after the startup, and keeps on standby. This procedure is represented in Figure 4, and the power consumption of this method is compared with that of the conventional method in Table 2.

Table 1. Compositions of LNG and BOG for example

Component LNG (mol %) BOG at 150 gf/cm2 g (mol %) N2 0.04 0 C1 89.26 98.84 C2 8.64 1.16 C3 1.44 0

n-C4 0 0 i-C4 0.27 0 n-C5 0 0 i-C5 0.35 0

Pressure_tank

Time Minutes

BLO

CK

S("

TAN

K")

.P k

g/cm

2g

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

0.15

0.15

50.

160.

165

0.17

0.17

5

Figure 3. Dynamic simulation result of LNG storage tank pressure.

0

20

40

60

80

100

120

140

160

0 12 24 36 48 60 72 84 96 108 120 132 144 156t(min)

Elec

tric

curre

nt (

A)

Compressor 1Compressor 2

Figure 4. Operation schedule of BOG compressors.

8 M. W. Shin et al.

Table 2. Comparison of operation methods

Method Compressor loads(tons/h)

Target pressure(gf/cm2g)

Power consumption(kW)

Energy save (%)

Conventional 10, 10, 5, 0 150 3,102 - Proposed 10/0, 0 170 1,358 56.22

5. Conclusion An algorithm has been proposed for optimal operation of BOG compressors in a LNG gasification plant. An empirical model is used for estimation of the rate of boil off gas generation in a LNG storage tank, in which a rollover coefficient and correction factors are applied to the boil off rate on specification supplied by the LNG storage tank manufacturer. Based on this model, the total compressor load can be calculated for a given target steady state tank pressure. Next, a MILP formulation is applied for optimal distribution of compressor loads, which minimizes the total average power consumption. Finally, a simple dynamic model based on the ideal gas law is used for prediction of the pressure change in the tank, which can be effectively applied to safety analysis for preparing against the potential failure of one of the operating compressors. The performance evaluation based on a case study indicates that the energy consumption could be reduced by half as compared with the conventional method by operating at a higher pressure within the safe range.

References Alderman, J. A., 2005, Introduction to LNG safety, Process Safety Progress, 24, 3, 144-151. Eisentrout, B., 2007, LNG terminal expansion moves to forefront of contract tender action, LNG

Journal, February 2007, 28-32. Lemmers, S. P. B. and B. V. Fluor Haarlem, 2005, LNG receiving and regasification terminals

operations: Understanding and controlling boil-off gas pressure enhances energy efficiency, Hydrocarbon Processing, 84, 7, 55-62.

Shin, M. W., G. Lee, D. Shin, S. H. Choi, and E. S. Yoon, 2005, Modeling and optimization of the operation of boil-off gas compressors at the LNG gasification plant, Proceedings of the 2nd International Symposium on Advanced Control of Industrial Processes (AdCONIP'05), 166-171.

Shin, M. W., D. Shin, S. H. Choi, E. S. Yoon, and C. Han, 2007, Optimization of the operation of boil-off gas compressors at the LNG gasification plant, Ind. & Eng. Chem. Res., 46, 20, 6540-6545.

Shin, M. W., D. Shin, S. H. Choi, and E. S. Yoon, 2008, Optimal operation of the boil-off gas compression process using a boil-off rate model for LNG storage tanks, Korean J. of Chem. Eng., 25, 1, 7-12.

Woodward, J. L., 2007, Coupling dynamic blow down and pool evaporation model for LNG, Journal of Hazardous Materials, 140, 3, 478-487.