Drizo Iran
Drizo Iran
Drizo Iran
pubs.acs.org/EF
bS Supporting Information
ABSTRACT: Triethylene glycol (TEG) is one of the most important liquid desiccants in the natural gas dehydration industry. In
enhanced TEG regeneration processes, liquid hydrocarbons such as toluene and isooctane are added to the stripping column of
natural gas dehydration unit in order to boost water volatility and regenerate TEG to higher purity. In this study, isooctane and
toluene were selected as liquid hydrocarbon solvents and the eect of these two solvents on TEG purity and the outlet water
concentration from the reboiler of tray column were experimentally investigated and mathematically modeled. The vaporliquid
equilibrium calculations were performed using the NRTL activity coecient model and ideal gas equation of state to represent the
liquid and vapor phases, respectively. Moreover, a comprehensive model was used to determine the liquid molar ow rate on each
tray where it changed with time and tray by tray. The impact of various concentrations of solvents and dierent operating conditions
(total and no reux) on the performance of the tray column was investigated. The modeling results were validated with the
experimental data, and good agreement was observed between them. Results showed that the least water concentration in the
reboiler and the highest TEG purity were achieved by adding 0.15 wt % isooctane under total reux conditions. The achieved results
can provide an initial insight into designing equipments in enhanced TEG regeneration processes with hydrocarbon solvent
injection.
INTRODUCTION
In this section, the main aspects of this study are outlined. As
discussed later, the mathematical modeling was carried out in conjunction with experiments in order to achieve more reliable evaluation of triethylene glycol (TEG) dehydration unit performance.
Natural Gas Dehydration. Natural gas is an important source
of primary energy and it is saturated with water vapor under
normal production conditions.1 The saturated water of natural
gas can cause some operational problems such as hydrate formation, corrosion, etc.2 Among different gas drying processes,
absorption is the most common technique where the water vapor
in the gas stream is absorbed in a liquid solvent stream. Glycols
are the most widely used absorption liquids as they approximate
the properties that meet commercial application criteria. Several
glycols have been found suitable for commercial application.3
Actually, the main reason for glycols popularity is their superior
absorption of water because the hydroxyl groups in glycols form
similar associations with water molecules.4 Triethylene glycol
(TEG) has gained universal acceptance as the most cost-effective
glycol mainly due to more easy regeneration, less vaporization
losses, lower capital and operating costs, higher initial theoretical
decomposition temperature, etc.5 TEG is used in a countercurrent mass transfer operation inside a contractor to establish
the required water content in the outlet gas.6 Bahadori and
Vuthaluru developed a simple-to-use method, by employing
basic algebraic equations, to correlate water removal efficiency
as a function of TEG circulation rate and TEG purity for appropriate sizing of the absorber at a wide range of operating conditions
r 2011 American Chemical Society
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supplier
<0.3%
<0.01%
Merck Company
distillated water
Shiraz University
EXPERIMENTAL SECTION
Materials. The specification of materials used in experiments is
reported in Table 1. Chemicals were used without further purification.
Apparatus and Procedure. In this study, experiments were
conducted in a batch distillation column (see Figure 1). As seen, the
column consists of twenty trays, a total condenser, and reboiler. Samples
were taken from the reboiler in 5 min intervals. Different operating conditions and concentrations of water and solvents were considered to
determine the effect of them on TEG purity. The specification of this
column is reported in Table 2.
In order to gain a deep insight into conducting experiments, some
samples were taken from the feed streamline to the stripping column of
Farashband gas renery (located in Iran) and analyzed by Karl Fischer
Titrator (Mettler Toledo, DL31). The analyzing results showed that the
water content of rich glycol was between 7 and 10 wt %; therefore three
water concentrations of 7, 10, and 15 wt % were selected for the experiments. Furthermore, several experiments were conducted in order to
recognize the best solvent concentration, and 00.05, 0.1, and 0.15 wt %
solvent concentrations were chosen in the end. It was worth mentioning
that a sudden blast was observed at a solvent concentration more than
0.3 wt %, and solvent did not show its eect at concentration less than
0.05%. Thus, the minimum solvent concentration of 0.05 wt % was
considered during the experiments. Moreover, the eect of reux ratio
on TEG purity was investigated, and two reux operating conditions
including total reux and no reux were thoroughly investigated.
outer diameter
8 cm
number of trays
20
pressure (condenser)
101.3 kPa
8 cm
WHS
1.3 cm
WLS
QB
2.36 cm
3587.2 kJ/h
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Figure 2. Karl Fischer Titrator for determining the water content of the
bottom product.
MATHEMATICAL MODELING
A scheme of the batch distillation tray column is shown in
Figure 4. Three basic assumptions are usually made in dynamic
dMB
L1 VB
dt
dMB xB, j
L1 x1, j VB yB, j
dt
1
j 1, :::, Nc
and
dMB HL, B
L1 HL, 1 VB HV, B QB
dt
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A21 = 172.96
12 = 2.3660
(a)
interaction parameters (NRTL)
pair
dM1 x1, j
VB yB, j L2 x2, j V1 y1, j L1 x1, j
dt
j 1, ::::, Nc
watertoluene
A12 = 678.49
A21 = 378.24
12 = 0.1360
waterTEG
tolueneTEG
A13 = 1909.5
A23 = 172.05
A31 = 14.769
A32 = 210.85
13 = 0.8893
23 = 2.2000
and
dM1 HL, 1
VB HV, B V1 HV, B L2 HL, 2 L1 HL, 1
dt
11
dMNt HL, Nt
VNt1 HV, Nt1 VNt HV, Nt
dt
12
dMD xD, j
VNt yNt, j DxD, j L0 xD, j
dt
j 1, :::, Nc
13 = 3.1110
23 = 0.4810
C1
C2
C3
C4
C5
water
73.649
7258.2
7.3037
4.1653 106
toluene
80.877
6902.4
8.7761
5.8034 106
isooctane
87.868
6831.7
9.9783
7.7729 106
TEG
29.368
8897.1
1.4675
2.1263 106
comp
TEG
dMNt xNt, j
VNt1 yNt1, j L0 xD, j LNt xNt, j VNt yNt, j
dt
dMD
VNt L0 D
dt
12 = 1.3430
A31 = 64.723
A32 = 1040.3
comp
dMi HL, 1
Vi1 HV, i1 Vi HV, i Li1 HL, i1 Li HL, i
dt
9
A21 = 407.46
A13 = 145.63
A23 = 1080.1
(a)
and
j 1, ::::, Nc
A12 = 74.268
waterTEG
isooctaneTEG
Table 5. (a) Vapor Pressure Constants for Pure Components24,25 and (b) Physical Properties for Pure Components
j 1, ::::, Nc
dMNt
VNt1 L0 VNt LNt
dt
waterisooctane
dMi xi, j
Vi1 yi1, j Vi yi, j Li1 xi1, j Li xi, j
dt
14
Mw
150.20
(b)
Tc (K) Pc 106 Pa Vc (m3/kmol)
769.50
H2O
18.015 647.13
toluene
92.141 591.80
isooctane 114.23
543.96
3.3200
21.940
Zc
0.5347
0.2462 1.2540
0.0560
0.2280 0.3430
4.1000
0.3140
0.2620 0.2620
2.5600
0.4650
0.2640 0.3010
Modeling Assumptions
(b)
interaction parameters (NRTL)
pair
15
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comp
D
3
E
6
TEG
160.2
1.207
3.064 10
3.242 10
isooctane
446.5
4.343
2.431 102
5.287 105
4.149 108
1.660
3
5
8.356 109
toluene
256.5
22.42
water
7.521 10
1.279 10
3
2.570 10
0.877
2.484 10
6
(b)
comp
TEG
isooctane
A0
75.98
134.9
B0 10
C0 103
D0 106
E0 109
F0 1012
G0 1016
1.169
0. 177
3.852
3.715
1.756
3.284
5.957
4.531
8.503
7.804
3.557
6.417
2.482
4.918
4.604
2.120
toluene
47.37
2.201
water
30.22
1.131
3.850
(c)
comp
TEG
A00
130.9
0.46
isooctane
48.09
0.38
toluene
50.14
0.38
water
54.00
0.34
i1
i1
@ji
A
xk Gki
xk Gkj
xk Gkj
j
17
where
Gij exp ij ij
Aij
ij
T
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xj Tjsat
j1
xj jPjsat
j1
yj
Nc
xj j Pjsat
P
19
water concentration
TOC
TEG bottom
(wt %)
(ppm)
(wt %)
reux
352
97.05
total reux
336
96.37
no reux
10
324
95.62
total reux
10
308
94.67
no reux
15
15
291
273
93.32
92.34
total reux
no reux
QB
HV, B HL, B
22
Rf
Rf 1
23
20
CPL A BT CT 2 DT 3 ET 4
24
CPV A0 B0 T C0 T 2 D0 T 3 E0 T 4 F 0 T 5 G0 T 6
25
21
HVAP A
NUMERICAL SOLUTION
A set of dierential and algebraic equations (DEA) were
solved simultaneously. The developed mathematical model had
00
T n
1
Tc
HL HL TRef
5131
26
Nc
xj CP T TRef
j1
Lj
27
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Figure 7. Variation of TEG concentration in reboiler with time for 15 wt % water and (a) 0.15, (b) 0.1, and (c) 0.05 wt % toluene and isooctane.
HV HV TRef HVAPTRef
Nc
j1
yj CPVj T TRef
28
Mw Ave
FAve
Li
Nc
xj Mwj
j1
Nc
xj Fj
j1
29
30
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Figure 8. Variation of TEG concentration in the reboiler with time for 10 wt % water and (a) 0.15, (b) 0.1, and (c) 0.05 wt % toluene and
isooctane.
total and no reflux operating conditions. As seen, TEG loss decreases considerably under total reflux condition.
Effect of Solvent Concentration on TEG Purity and Its
Loss. The variation of TEG concentration in the reboiler with
time for 15 wt % water and various concentrations of solvents (i.
e., toluene and isooctane) is depicted in Figure 7ac. As seen,
solvent addition can effectively enhance TEG purity in comparison with no solvent injection. In addition, glycol purity is
boosted directly with increasing solvent concentration where
higher TEG concentration is achieved at 0.15 wt % isooctane and
toluene. As obviously recognized from the figures, isooctane can
enhance TEG purity in the bottom product and reduce its loss
from the top product more remarkably than toluene. Moreover,
operating under total reflux conditions in the presence of solvent
is obviously more effective than no reflux conditions on enhancing TEG purity. In fact, the liquid hydrocarbon solvent increases
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Figure 9. Eect of injecting (a) 0.15, (b) 0.1, and (c) 0.05 wt % isooctane and toluene on 15 wt % water in the reboiler.
following figures were depicted under total and no reflux conditions and various concentrations of solvents.
Figure 9ac shows the eect of adding various solvent concentrations on 15 wt % outlet water concentration from the
reboiler. As seen, the least water concentration is achieved under
total reux conditions by adding isooctane. Furthermore, the
least water concentration is achieved at the highest solvent
concentration (0.15 wt %). A remarkable decrease in water concentration in the reboiler in the presence of hydrocarbon solvents
in comparison with no solvent injection implies the impact of
solvent addition which enhances the water volatility in the TEG +
water system and consequently boosts TEG purity. Furthermore,
isooctane can enhance water volatility more eectively than
toluene which can be obviously realized in the gures.
Similarly, Figure 10 illustrates the eect of various solvent
concentrations on 10 wt % water. Investigating dierent operating
conditions and solvent concentrations for 10 wt % water revealed
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Figure 10. Eect of injecting (a) 0.15, (b) 0.1, and (c) 0.05 wt % isooctane and toluene on 10 wt % water in the reboiler.
toluene (wt %)
time (min)
39.71
7147
0.05
36.77
6619
528
0.10
35.91
6464
683
9.56
0.15
35.40
6372
775
10.84
10
43.69
7865
10
0.05
41.01
7382
483
6.14
10
0.10
40.22
7240
625
7.94
10
0.15
39.92
7186
679
15
48.84
8791
15
0.05
46.56
8380
411
5.84
15
0.10
46.11
8300
491
5.59
15
0.15
45.75
8235
556
6.32
5135
0
7.39
8.63
0
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isooctane (wt %)
time (min)
39.71
7147
0.05
36.22
6520
627
8.77
0.10
35.27
6349
798
11.17
12.81
0.15
34.61
6229
918
10
43.69
7865
10
0.05
40.39
7271
594
10
0.10
39.64
7135
730
9.28
10
0.15
39.01
7021
844
10.73
15
48.84
8791
15
0.05
45.99
8278
513
5.84
15
0.10
45.46
8183
608
6.92
15
0.15
45.05
8109
682
7.75
that the solvent addition and total reux condition could eectively
decrease the water concentration in the TEG + water system. Particularly, isooctane was superior to toluene in terms of enhancing
water volatility in the TEG + water system and boosting TEG purity.
Effect of Solvent Concentration on Reboiler Duty. In order
to determine the required reboiler duty under solvent injection
and no solvent injection conditions, the time which the first tray
temperature reached steady state conditions was measured via a
chronometer. These times were multiplied to the reboiler duty so
that the consumed reboiler duty was achieved. Thus, the saving in
reboiler duty was determined by a comparison between the reboiler duty with and without solvent injection. The experimental
results are reported in Tables 8 and 9. As seen, 12.8% and 10.8%
savings in reboiler duty could be achieved by isooctane and
toluene injection, respectively. According to the previous investigations, the azeotropic regeneration process needed a considerably lower energy consumption rate in comparison with other
regeneration processes.27 As seen, the obtained experimental
data justified previously achieved results.
CONCLUSIONS
In this study, the eect of solvent injection on TEG purity and
its loss in the tray column were investigated. Experiments were
conducted in a batch tray column under dierent operating
conditions and solvent concentrations at atmospheric pressure.
The modeling and experimental results showed that the liquid
hydrocarbon solvent addition can remarkably enhance TEG
purity and water volatility in the bottom product and considerably reduce TEG loss in the top product. Furthermore, isooctane
performed better than toluene, and a higher TEG concentration,
lower water concentration, and duty of reboiler were achieved
with isooctane injection. In addition, 0.15 wt % solvent concentration was the ideal solvent concentration in these experiments
because the best results were achieved at this value. In fact, liquid
hydrocarbon solvent vaporized rapidly in the reboiler and
increased the water volatility which enhanced TEG concentration in the reboiler. This modeling and experimental results can
provide a good initial insight into future pilot plant design of
natural gas dehydration columns with solvent injection.
ASSOCIATED CONTENT
bS
0
7.56
AUTHOR INFORMATION
Corresponding Author
*Tel.: +98 711 2303071. Fax: +98 711 6287294. E-mail address:
rahimpor@shirazu.ac.ir.
ACKNOWLEDGMENT
The authors would like to appreciate nancial support of the
South Zagros Oil and Gas Production Company.
NOTATION
CPL = liquid specic heat capacity (J/mol 3 K)
CPv = gas specic heat capacity (J/mol 3 K)
d = column diameter (cm)
densityAve = average density (kg/m3)
D = distillate ow rates (kmol/h)
j = component number
HL = gas enthalpy (J/mol)
HV = liquid enthalpy (J/mol)
HVAP = heat of vaporization (kJ/mol)
L = liquid molar ow rate (kmol/h)
M = molar liquid hold up on tray (kmol)
MV = volumetric liquid holdup on tray (cm3)
MwAve = average molecular weight (kg/kmol)
P = total pressure (kPa)
Psat
j = vapor pressure of j component (kPa)
QR = reboiler heat input (kJ/h)
Rf = reux ratio
V = vapor ow rates (kmol/h)
WHS = Weir height (cm)
WLS = Weir length (cm)
x = liquid weight fraction
y = gas weight fraction
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comp = component
exp = experiment
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