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Side-rectifier divided wall column for offshore LNG plant
Separation and Purification Technology 139 (2015) 25–35
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Side-rectifier divided wall column for offshore LNG plant Young Han Kim ⇑ Dept. of Chemical Engineering, Dong-A University, Busan 604-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2014 Received in revised form 28 October 2014 Accepted 1 November 2014 Available online 7 November 2014 Keywords: Side-rectifier DWC Compact distillation column Floating liquefied natural gas plant Low pressure processing
a b s t r a c t A side-rectifier DWC (divided wall column) is proposed to replace the whole FLNG (floating liquefied natural gas) plant, which comprises of three distillation columns in the conventional system. The proposed column is compact equipment suitable to harsh offshore operation of the gas plant, which is operated at low pressure to reduce the investment and electricity costs. The economics, thermodynamic efficiency and operational difficulty of the side-rectifier DWC are evaluated and compared with the conventional system. The heating and cooling duties of the side-rectifier DWC are 5.9% and 5.1% less than those of the conventional system, respectively. The economic evaluation indicates that the side-rectifier DWC requires 57% less investment and 10% more utility cost than the conventional system. However, low feed pressure used in the side-rectifier DWC saves the compressor cost of 5.1 times the investment cost and the electricity cost of 1.8 times the total utility cost of the high-pressure conventional system. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Two conventional distillation columns in sequence are combined into a single Petlyuk distillation column, implemented as a DWC (divided wall column) in practice [1], and three products are yielded from the column. When the DWC is employed in an offshore LNG (liquefied natural gas) plant utilizing three conventional distillation columns as shown in Fig. 1, the DWC replaces last two columns [2–4]. If a rectifying column is connected to the prefractionator of the Petlyuk column, four products are processed from the single-reboiler column as illustrated in Fig. 2 and the three columns in the conventional system are replaced with it. In this configuration most of the lightest component is separated in the rectifier, which significantly reduces the separation load of the next column. For the practical implementation of the proposed Petlyuk column, a side-rectifier DWC as demonstrated in Fig. 3 can be constructed. Considering the space limitation and the harsh condition of operation of the offshore LNG plant, the single-reboiler distillation column is suitable to the LNG plant. Though new separation techniques were developed recently [5], distillation has been the most reliable process for the natural gas separation. In addition the side-rectifier DWC has high distillation efficiency, because it has the same column structure as the common DWC. The extended DWC having three walled sections in the middle is known to produce four products [6,7], but the control of product specification is ⇑ Tel./fax: +82 51 200 7723. E-mail address: [email protected] http://dx.doi.org/10.1016/j.seppur.2014.11.002 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
more difficult than the common DWC of two sections. The difficulty explains why the extended DWC has not been commercialized, while the common DWC is being utilized in many field operations [8–10]. The integration of multiple binary columns into the DWC raises the distillation efficiency for less energy demand, but the column operating pressure has to be equal for the all sections of the DWC in spite that the binary columns are operated at different pressures. When the pressure difference is not large, the integration can be applied to the complex distillation processes [11,12]. However, the pressure limitation makes the extended DWC more difficult to implement than the common DWC. The former needs a single pressure from the three column pressures of the conventional system, while the latter does from two pressures. In addition of the difficulty of the selection of column operating pressure in the extended DWC, the column operating temperature is unfavorable to result in high utility cost. The difference of condenser temperature and reboiler temperature in the DWC is very large due to the single operating pressure. The separate column pressures in the conventional distillation system are manipulated for the low-cost utilities used in the condenser and reboiler. Because the gas separation process in the LNG (liquefied natural gas) plant handles the feed having the components of large difference of boiling points, the separation is relatively easy but cryogenic cooling and high temperature boiling are necessary. In the conventional distillation system of the LNG plant the operating pressures of the multiple columns are adjusted between 0.3 MPa and 3 MPa to make the column operating temperature close to ambient temperature [13,14]. The adjustment reduces the
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Nomenclature Ac AR B bp Ccol Ccomp Ccond Cf Cp Creb Ctray D DC E F Fc f H Hc L P Q R
condenser heat transfer area (m2) reboiler heat transfer area (m2) bottom product rate (kmol h1) compressor power (kW) column cost ($) compressor cost ($) condenser cost ($) fabrication factor () pressure factor () reboiler cost ($) column internal cost ($) overhead product rate (kmol h1) column diameter (m) Exergy (kJ kmol1) feed flow rate (kmol h1) fabrication coefficient () Carnot factor () enthalpy (kJ kmol1) column height (m) liquid flow rate (kmol h1) pressure (Pa) energy stream (kJ h1) gas constant ()
consumption of cryogenic refrigerant and high temperature steam. However, the number of columns is limited in the offshore operation of the LNG plant, where the DWC is a good alternative for the operation due to its small column number in spite of the requirement of high-cost utility [2–4]. For the reduction of the utility cost in the DWC, diabatic distillation can be applied, which has in-tray heat exchangers to
S T V W
entropy (kJ kmol1 K1) or side draw rate (kmol h1) absolute temperature (K) vapor flow rate (kmol h1) work (kJ)
Greek
c g
ratio of specific heats () thermodynamic efficiency ()
Superscripts B bottom product D overhead product F feed L liquid S side draw V vapor Subscripts j tray number m minimum o ambient Q energy stream
disperse the high-cost heat duties of the reboiler and condenser [4]. The duty of cryogenic cooling was partially reduced by applying two inter-coolers to the DWC [3]. The heat transfer at the intray heat exchangers is driven by lower temperature difference than the adiabatic distillation. The heat transfer not only lowers the heat duty at the condenser and reboiler, but also reduces the exergy loss in the distillation process [4,15]. But in practice, the
Fig. 1. Schematic diagram of the conventional offshore FLNG plant.
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
procedure of the side-rectifier DWC are addressed, and its cost and thermodynamic efficiency are examined. 2. Process description
Fig. 2. Schematic diagram of the side-rectifier Petlyuk column.
installation and maintenance of the in-tray heat exchangers in every tray are difficult. In this study a single reboiler distillation column, side-rectifier DWC, producing four products is proposed, and its application to the floating LNG plant is evaluated in the separation performance and heat duty requirements. The characteristics and design
The raw natural gas from gas well contains liquid components of oil and water, which are separated through the multiple separators having successively reduced pressures up to 20 kPa to maximize gas recovery and to stabilize the crude oil [13]. The processed gas is compressed to high pressure to minimize the number of processing equipments in the next stage. A typical gas mixture contains various components as listed in Table 1. The feed components are of wide variation of volatility, but methane is more than 86%. The first column in the distillation system processes the most of the feed because of the large amount of methane, and therefore a specially designed distillation system can be applied in the processing. Though the onshore separation system utilizes five columns as demonstrated in Lee et al. [2], the number of the columns is limited in the offshore operation. A three-column offshore process is conventionally utilized as shown in Fig. 1. The first two columns with recycle produce the mixture of methane and ethane as a typical LNG fuel. The last column produces LPG and C5 plus heavier components to be fed to hydrocarbon chemical processes. When the prefractionator of the Petlyuk column accommodates a small column on its top as described in Fig. 2, the pretreatment column for the common DWC used in the previous studies [2–4] can be eliminated to make the whole distillation system a singlereboiler column. In practice the proposed column becomes a side-rectifier DWC as illustrated in Fig. 3. Because the offshore operation requires the compactness of equipment due to limited space and harsh environment, the side-rectifier DWC is a good candidate for the LNG plant if the column operating pressure is properly selected. Due to the single operating pressure in the DWC unlike the conventional system of multiple column pressures, the condenser temperature is lower than that of the conventional system and its reboiler temperature is higher. Therefore, the in-tray heat exchangers are installed to reduce the consumption of high cost utilities for the condenser and reboiler. The distribution of the in-tray heat exchangers is explained in the next section. 3. Design of the side-rectifier DWC 3.1. Column design In the design of a conventional distillation column using a commercial program the operating pressure is determined first, and the number of trays and operating conditions are optimized for Table 1 List of feed conditions.
Fig. 3. Schematic diagram of the side-rectifier DWC (divided wall column) equivalent to the Petlyuk column.
Name
Value
Temperature (°C) Pressure (MPa) Flow rate (kmol/h)
35 1.0 15,120
Composition (% mol fraction) Nitrogen Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane n-Heptane
1.54 86.39 6.47 2.87 0.72 0.82 0.41 0.31 0.31 0.15
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
the minimum of total annual cost. The design of the side-rectifier DWC limits the number of trays due to the harsh condition of offshore operation. The numbers of trays were set to 20 and 25 for the rectifier-prefractionator and main column shown in Fig. 2, respectively, considering the results of previous studies [2,3]. Because the number of trays was limited in the offshore operation of distillation column due to the harsh environment at sea, the maximum number was set at 25 and the number was not optimized. Instead the location of feed and interlinking trays was iteratively optimized in the simulation by checking the product composition. The difference of tray numbers between the rectifier DWC and the conventional system is five, of which the difference of the pressure drop is 5 kPa. The difference was not significant to affect the column performance, because the column operating pressure was 0.8 MPa. The location of feed and side draw and the interlinking stages between the prefractionator and main column are found from the iterative computation for the minimum heat duty at the given product specification. The feed condition and composition are found in Table 1. Though the conventional offshore LNG plant operates three columns having different pressures between 0.8 MPa and 7 MPa, the operating pressure of the side-rectifier DWC has to be unique. The pressure was set at the lowest pressure of 0.8 MPa, because low pressure operation gives high thermodynamic efficiency and requires no extra cost for high pressure equipment [13]. In addition, the compression of the raw gas from the well is not necessary to feed to the column. The design of a distillation column using the commercial software requires two input data: column operating pressure and the number of total trays, and they are determined as explained above. Because the HYSYS was used in this study, the locations of feed, side draw and interlinking streams were iteratively computed for the minimum use of utilities. The operating condition for a given set of product specification from a feed was also found from the iterative computation. The product specifications are 0.89, 0.98 and 0.99 for the methane, LPG and C5+ product, respectively. The design results of the side-rectifier DWC, structural design and operating condition, are listed in Table 2 along with those of the conventional LNG plant. The nominal amount of overhead product
from the main column is mixed to the overhead product of the rectifier. 3.2. In-tray heat exchanger design Because three columns used in the conventional LNG plant are combined to the single-reboiler column, the temperature difference of condenser and reboiler is very large almost equal to the boiling point difference between the lightest and heaviest components in feed, methane and C5+ in this study. Therefore, high-cost utility is required, and the operating cost is more than the conventional system. The same problem has been indicated in the previous studies of the DWC application [3,4]. For the reduction of the utility cost, the heat duty in the condenser and reboiler is minimized by implementing the inter-coolers and inter-heaters, in-tray heat exchangers. The heat duty of the in-tray heat exchangers can be optimally allocated by the theorem of equipartition of entropy production [16]. The heat exchangers have to be installed in every tray due to the theorem, but the practical installation in all trays is difficult. Therefore, the in-tray heat exchangers are placed in every third tray in this study, and the heat duty is adjusted to provide minimum liquid flow in the trays [17]. The sections of divided wall are not included in the installation of the in-tray heat exchangers, because the space does not allow placing them. The location and heat duty distribution of the in-tray heat exchangers are listed in Table 3. The overall heat transfer coefficient was assumed as 0.8 kW/m2 K and 1 kW/m2 K for in-tray cooler and in-tray heater, respectively. Their heat transfer areas are listed in Table 4. Vertical panels installed on the wall of a distillation column significantly increase the heat transfer area [18]. As indicated above the heat duties at the condenser and reboiler requiring high cost utility are significantly reduced by employing the in-tray heat exchangers. The detail of the design results including feed and product specifications is given in Fig. 4. The stream mass and heat flow rates are listed in Table 5, and the stream names are shown in Figs. 1, 3 and 4. Primary reason to install the in-tray heat exchangers is to reduce the cost of utility by using less-cost refrigerant and steam.
Table 2 Structural information, operating conditions and feed and product compositions in the conventional and side-rectifier DWC systems. Tray numbers are counted from top. Name
Structural Number of trays Feed/side draw tray Interlinking trays Operating Pressure (MPa)-top Temperature (°C) Overhead Bottom Feed (kmol/h) Product (kmol h) Overhead Bottom Side Reflux (kmol/h) Vapor boilup (kmol/h) Cooling duty (MW) Reboiler duty (MW) Precooler/Comp. (MW) Composition (mol frac.) Feed Product-C1,2 -LPG -C5+
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rectifier
Prefract.
Main
10 8
10 7
20 13
12
8 3
25 5 4/14
7.0
1.1
0.8
0.8
0.8
0.8
34.6 17.6 15,120/349
55.1 62.8 718
30.0 118.4 369
62.2 27.7
26.4 34.8 15,120
90.2 115.7
14,750 718
349 369
215.5 153.5
14,739
101.5 1.14 4.7 0.01 14.6
178 526.5 1.04 2.85 0.52
183.2 352.6 2.02 2.24
33 15,620 0.42
0.93/0.04/0.01 0.95 0.32
8.0 0/0.58/0.42
0.46/0.32/0.18 0.95 0.58
100 180
21 177 183 21 47.9 0.59 0.30
0.93/0.04/0.01 0.95
0.99 0.99
1.00 0.98 0.99
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35 Table 3 Design results of heat exchangers in the rectifier and main column of the side-rectifier DWC. Tray numbers are counted from top. Rectifier Tray No. Condenser
Main column Temp. (°C)
Duty (MW)
62.2
0.42
60.6 52.8 43.6 35.1
2.0 2.5 2.5 3.0
Tray No
Temp. (°C)
Duty (MW)
90.2
0.59
2 4
33.0 34.2
1.2 1.0
17 20 23
83.9 99.9 112.3 115.7
1.5 1.5 1.5 0.3
Tray cooling 2 5 8 11 Tray heating
Reboiler
The in-tray heat exchangers near to the condenser and reboiler have less temperature difference with them than others. Because the theorem of equipartition of entropy production [16] requires even distribution of heat transfer through the trays, the in-tray heat exchangers are installed including near to the condenser and reboiler. In addition, the diabatic operation of a distillation column improves thermodynamic efficiency [15,19]. 4. Results and discussion The design results of the side-rectifier DWC are analyzed and compared with the conventional system of three distillation columns. The investment and operating costs are evaluated, and
the thermodynamic efficiency of the side-rectifier DWC is compared with the conventional system. 4.1. Design results The results of structural and operating condition of the siderectifier DWC are listed in Table 2. The detail of equipment design results is included in Table 4. Because the main column of the DWC produces three products, five more trays are necessary than the tallest debutanizer in the conventional system. As indicated above the column pressure is set to the lowest pressure among the conventional three columns, which gives the condenser temperature much lower than the conventional system. However, its heat duty
Table 4 Equipment size and economic evaluation of the conventional and side-rectifier DWC systems. Units are in million U.S. dollars, and the utility cost is annual. Name
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rectifier
Prefract.
Main
Investment Column Diameter (m) Height (m) Cost
10.1 6.1 2.707
1.97 6.1 0.218
1.66 12.2 0.294
10.39 7.32 1.377
0.51 4.88 0.04
2.38 15.24 0.516
Tray Number () Cost
10 0.165
10 0.013
20 0.02
12 0.206
8 0.001
25 0.044
Condenser H. T. Area (m2) Cost
195.8 0.682
43.3 0.118
84.3 0.169
17.5 0.061
Reboiler H. T. Area (m2) Cost
0.7 0.019
142.6 0.281
112.2 0.224
In-tray Heat Ex. H. T. Area (m2) Cost Preheater H. T. Area (m2) Cost
24.6 0.076 15 0.061
83,104x2, 125 0.477
50,42 (cooling), 75x3 (heating) 0.178, 0.352
608.3 1.424
Compressor Power (kW) Cost Total Utility Steam Cooling water Refrigerant 4.538 Electricity Total Feed compression (1 MPa to 7 MPa) Investment Electricity
412 1.566
0.086
7.9
3.389
0.166 0.091
0.289
0.417 0.249
5.12 5.547
40.32 10.01
0.708 6.117
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Fig. 4. Schematic diagram of the side-rectifier DWC with in-tray heat exchangers.
Table 5 List of stream mass and heat flow rates. Stream names are shown in Figs. 1, 3 and 4. Stream
Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW) Energy stream Heat flow (MW)
Conventional system Feed
D1
F2
D2
R2
F3
D3
B3
35 7.1 1.51 104 329.2 CE1 14.6
34.6 7.0 1.48 104 330.5 CE2 4.7
17.6 7.0 718 25.4 CE3 1.04
55 1.1 349 7.9 CE4 2.0
93.4 6.6 349 7.4 RE1 0.01
62.8 1.1 369 15.6 RE2 2.9
30.0 0.8 215.5 8.1 RE3 2.2
118.4 0.8 153.5 7.3 PE1 0.52
DWC-adiabatic Stream Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW) Energy stream Heat flow (MW)
Feed 35 1.0 1.51 104 323 CE1 8
D1 62.8 0.8 1.47 104 323.2 CE2 10.5
LP 26.7 0.8 869 3.6 CE3 2.7
VP 26.1 0.8 1.56 104 348 RE1 4.7
D2 78.3 0.8 20 0.5
Side 48.9 0.8 183 7.2
B 114.7 0.8 181 8.5
D1 62.3 0.8 1.47 104 323.3
LP 27.7 0.8 878 3.6
VP 26.4 0.8 1.56 104 349
D2 90.6 0.8 21 0.5
S 49.2 0.8 183 7.3
B 115.7 0.8 177 8.4
DWC-diabatic Stream Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW)
Feed 35 1.0 1.51 104 323
is minimized for the reduced use of high-cost refrigerant by employing in-tray heat exchangers. The amount of C5+ product
was 15% more than the conventional system, while LPG was produced 15% less. Note that the price of C5+ product is higher than
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
that of LPG product. The feed pressure in the DWC is much lower than the conventional system, and therefore the cooling load is about half of the conventional system due to less temperature elevation from the feed compression. In addition the compression of recycle stream from the depropanizer is not necessary giving less investment and utility costs in the DWC. The small amount of overhead product from the main column is mixed to the overhead product of the rectifier. Due to the improved thermodynamic efficiency of the DWC, the heating and cooling duties of the side-rectifier DWC including the in-tray heat exchangers are 5.9% and 5.1% less than the conventional system, respectively. The profiles of LPG composition in the main column of the rectifier DWC and the debutanizer of the conventional system are shown in Fig. 5. The composition variation of the lower 20 stages in the DWC is similar to that of the debutanizer. It indicates that the addition of 5 stages to the debutanzer is sufficient to separate the mixture of methane and ethane from the top of the DWC. Because of the limitation of column height in the offshore operation and large boiling point difference between the mixture and LPG products, the small number of stages is added to the DWC. The tray vapor composition in the demethanizer of the conventional system is plotted in Fig. 6(a), and those of the rectifier DWC with and without in-tray heat exchangers are also shown in the figure. The DWC without in-tray heat exchanger as described in Fig. 3 is labeled as adiabatic DWC in Fig. 6, while the DWC with in-tray heat exchangers is given as diabatic DWC. Because a partial condenser is used in the adiabatic DWC, its vapor composition at top tray is lower than others. The liquid compositions of LPG and C5+ components are plotted in Fig. 6(b) and (c), respectively. The LPG is produced as side cut in the DWC, and their composition profiles are different from the debutanizer of the conventional system. The tray temperature, vapor flow rate and liquid flow rate are shown in Fig. 7(a–c), respectively. The mixture of methane and ethane is produced from the top of the DWC, and therefore the top temperature of the DWC is lower than that of the debutanizer producing the LPG. The in-tray heat exchangers in the diabatic DWC lead to the stepwise variation of vapor and liquid flow rates.
31
Fig. 6. Vapor compositions in the demethanizer of the conventional system and the DWCs with and without in-tray heat exchangers (a). LPG composition (b) and C5+ composition (c) in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers.
4.2. Economic evaluation The economics between the conventional distillation system and the side-rectifier DWC is compared in terms of investment and utility costs [20]. The investment cost includes the costs of column, trays, and heat exchangers, which are calculated from the cost equations given in Appendix A. The utility costs of steam
Fig. 5. Variation of tray liquid composition in the main column of rectifier DWC (a) and debutanizer in the conventional system (b).
Fig. 7. Tray temperature in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers (a). Vapor flow rate (b) and liquid flow rate (c) in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers.
and refrigerant depend on temperature. The correlation of the costs and temperature was derived from the reference data [21,22] and given in Kim [4]. The results of cost evaluation are listed in Table 4. The investment of the rectifier DWC is less than 43% of the conventional system. The large difference comes from the high pressure operation at the demethanizer in the conventional system. The pressure requires additional cost in the construction of column and heat exchangers, and the investment of the compressor [23,24] used for recycling the overhead product of the depropanizer is added. The utility cost of the DWC is 10% more than that of the conventional system because of the lower cooling temperature requiring high-cost refrigerant. The significant reduction of the investment cost compensates the utility cost increase. The critical issue in the economics between the conventional system and the rectifier DWC is the column operating pressure. Employing high
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Fig. 8. Enthalpy-Carnot factor diagrams; first column (a), second column (b) and third column (c) of the conventional system, and rectifier-prefractionator (d) and main column (e) of the side-rectifier DWC.
column pressure of 7 MPa in the conventional system requires a feed compressor of the cost more than 5.1 times the total investment of other equipment in the conventional system and the electricity cost of 1.8 times the total utility cost as listed in Table 4. While the feed pressure in the conventional system is 7 MPa, the pressure of the rectifier DWC is 1 MPa. As the production of raw natural gas from the well continues, the gas pressure drops significantly and the compression of the raw gas is required in the conventional system [13]. The compression cost is listed in Table 4.
When a payback time of 5 years is applied, the TAC (total annual cost) of the compressor is 2.5 times that of the conventional system. The offshore operation requires compact equipment due to the severe condition of environment. The side-rectifier DWC can be constructed in a single reboiler column by combining the rectifier and DWC like a top DWC having two condensers and one reboiler [2]. Though there are two dividing walls, one at top and the other in the middle, the whole system is placed in a column. The system
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
of demethanizer and a DWC [3] has two condensers and two reboliers making two complete column system. Compared with the system of demethanizer and DWC, the costs of investment and utility of the rectifier DWC are 57% less and 3% more, respectively. The thermodynamic efficiency of the system of demethanizer and DWC is 0.9% point higher than that of the rectifier DWC. For the in-tray cooling the log-mean temperature difference was taken as 30° and that of heating was 20°. The cost of heat exchangers is proportional to the heat transfer area to the power of 0.65. When the temperature difference becomes a half, the heat transfer area doubles and the cost increases by 57%. However, the additional cost of the in-tray heat exchanger is 5% of the investment of the conventional system, which is small compared with the 57% reduction of the total investment cost benefited from adopting the rectifier DWC. 4.3. Exergy loss and thermodynamic efficiency The exergy loss indicates the thermodynamic efficiency of chemical processes. Distillation is a large energy-consuming process, and its efficiency is important to reduce the energy consumption. In this section the thermodynamic efficiency of the side-rectifier DWC is calculated using the exergy loss, and its efficiency is compared with that of the conventional system. The maximum available work from a thermodynamic process is represented as exergy calculated from the enthalpy and entropy of the process at its pressure and temperature.
E ¼ ðH Ho Þ T o ðS So Þ
ð1Þ
where the subscript o indicates atmospheric pressure and ambient temperature. In a tray of the distillation column the rate of exergy loss is calculated from the exergy balance.
E_ loss ¼ Lj1 ELj1 þ V jþ1 EVjþ1 þ F j EFj Lj ELj V j EVj Sj ESj E_ Q;j
ð2Þ
The last term, thermal exergy of an energy stream and equipment, is found from
To E_ Q;j ¼ Q j 1 T
ð3Þ
The exergy loss in the distillation column includes the losses in trays, which is difficult to evaluate. Instead, a diagram of the Carnot factor and stream enthalpy flow of the feed and products and the external heat flow is useful to calculate the exergy loss in the distillation column [25,26]. The Carnot factor is defined as
To f ¼ 1 T
ð4Þ
The Eq. (4) indicates that the exergy loss in energy stream is the multiplication of the Carnot factor and energy flow. Practically the
exergy loss is computed as the area between the feed and products as demonstrated in Fig. 8. The thermodynamic efficiency of the distillation column is defined as [27,28]
_ W
m g¼ _ W m þ E_ loss
ð5Þ
and the minimum work is calculated from the exergy of feed and products in an adiabatic distillation column [29].
_ m ¼ DED þ BEB FEF W
ð6Þ
The in-tray heat transfer of the DWC was added to the minimum work as thermal exergy. The itemized exergy losses and the exergy flow of streams are listed in Table 6. The reduction of heat duty in the DWC leads to less exergy loss, but the low pressure and low temperature of the overhead product, mostly methane, give less minimum work than the conventional system. Because the minimum work is only a portion of the exergy loss, the thermodynamic efficiency is very low in both systems. 4.4. Low pressure operation The compact structure of distillation column for offshore LNG processing is one advantage of the proposed side-rectifier DWC. The other benefit is low pressure operation. The gas pressure at the well decreases up to 20 kPa as the production proceeds, and the pressure is boosted to 7 MPa before feeding to the conventional gas plant [13]. High pressure and low temperature operation generates gas hydrates in the transportation lines, and hydrate inhibitors are used to prevent plugging pipes due to the solid hydrates [13]. Moreover the well gas is almost always saturated with water. However, the plugging is less likely in the low pressure operation, and no heating is necessary before entering the separation process. The low pressure operation requires less investment cost because of thin vessel wall and small gas compressor used for the feed compression. In addition, less electricity is consumed resulting in low utility cost. The pressure of overhead product from the demethanizer in the conventional system has to be lowered for shipping to the LNG carrier. The product, mostly methane, is liquefied at a temperature about – 160 °C at atmospheric pressure for the transportation by the LNG carrier [30]. Therefore, the compression in the LNG processing is not necessary. Though the transportation and storage require smaller volume in the high pressure operation, the investment and electricity costs are greatly saved in the low pressure operation. 4.5. Process operation The application of the common DWC in field operation has been successful for many years [9,31], but the operation of the DWC is
Table 6 Exergy loss and thermal efficiency in the conventional and side-rectifier DWC systems. Units are in MW. Name
Trays Cooling Heating Total Feed Overhead Bottom Side Diabatic Min. work Thermodynamic efficiency (%)
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rect.–Pref.
Main
92.59 1.986 0
5.281 0.202 0.771
93.85 0.118
3.520 0.240 0.059 97.79
39.03 42.90 3.757
3.757 0.617 1.771
1.208 0.188 0.274 102.50 1.771 1.010 0.382
3.095 2.9
23.58 23.30
0.037 0.438 0.909 3.96 2.37 2.4
34
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Table 7 Changes of product mole fraction with the percent change of manipulated variables. Product
Rectifier DWC
Pre-column DWC
Conventional
C1
Manipul. variable % change Mole frac. change
Reflux flow 12.1 6 106
Reflux flow 6.8 1 106
Reflux flow 3.2 4.6 104
LPG
Manipul. variable % change Mole frac. change
Prefract. vapor flow 12.5 6 104
Prefract. vapor flow 16.7 6 104
Reflux flow 1.2 4 104
C5+
Manipul. variable % change Mole frac. change
Boilup ratio 2.2 4.1 103
Boilup ratio 2.1 2 103
Boilup ratio 4.3 1.2 102
more difficult than the conventional distillation column. It is the main concern of the field operators considering the DWC adoption. The operation of the side-rectifier DWC in this study is expected to be similar to the DWC operation. The overhead product from the rectifier can be separately adjusted using the reflux flow to the rectifier. As listed in Table 7, the gain of product mole fraction from the applied change of manipulated variables indicates that the conventional system shows the best control performance as explained in Wolff and Skogestad [32]. However, the performance of the rectifier DWC is better than that of the existing pre-column DWC [3]. In the control of C1 product specification the ratio of the change of methane mole fraction to the variation of reflux flow rate is the gain, which indicates the ease of the process operation. The larger the gain is, the easier the process manipulation is. Though the conventional system has much larger gain than the DWCs, the rectifier DWC has about three times the gain of the pre-column DWC. For LPG production the rectifier DWC shows the improvement by one third over the pre-column DWC. In case of C5+ production the rectifier DWC gives about twice better performance than the pre-column DWC. The difficulty of products control in the proposed DWC is alleviated by applying a sufficient margin of specification in the column design [32]. In addition the adjustment of heat duty at the in-tray heat exchangers manipulates the internal vapor and liquid flows in the column, which provides more degrees of freedom in the column control. 5. Conclusions The offshore LNG (liquefied natural gas) plant requires a compact gas separation system due to the harsh environment, and a side-rectifier DWC (divided wall column) is proposed for the purpose. The characteristic and design procedure of the column are addressed here. The economics and thermal efficiency of the proposed DWC are compared with those of the conventional system. The side-rectifier DWC is compact, and its low pressure operation gives many benefits, such as low investment due to thin vessel wall, no formation of hydrates and lower compression of feed. The heating and cooling duties of the side-rectifier DWC are 5.9% and 5.1% less than those of the conventional system, respectively. The economic evaluation indicates that the side-rectifier DWC requires 57% less investment and 10% more utility cost than the conventional system without counting the cost increase of feed compression. The low compression of feed saves the compressor cost of 5.1 times the total plant investment and the electricity cost of 1.8 times the total utility cost in the conventional system. The thermodynamic efficiency of the DWC is 2.4%, while that of the conventional system is 2.9%. Acknowledgments Financial support from the Basic Research Program (2011-0021819) through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and Technology is gratefully acknowledged. Appendix A The cost of distillation column includes the column shell and internal, and the shell cost is given as
C col ¼
M&S C f D1:066 H0:802 Cp C C 280
ðA1Þ
where the M & S is the Marshall and Swift index and the value of 4th quarter of 2011 of 1536.5 was used here. The coefficient Cf is given as 3919.32 from Olujic et al. [33], and the pressure related correction Cp is from Douglas [21]. The column diameter Dc is calculated from the maximum vapor flow rate.
pffiffiffiffi Dc ¼ 0:08318 V
ðA2Þ
where V is the rate of vapor in kg-mol/h. The column height Hc is calculated from two-feet tray spacing and the tray number. The cost of column internal is calculated from the equation.
C tray ¼
M&S 97:243D1:55 HC F c C 280
ðA3Þ
where the fabrication coefficient Fc is given in Olujic et al. [33]. From the reference the heat exchanger cost equation was adopted as
C cond ¼
M&S 1609:13A0:65 C 280
ðA4Þ
where Ac is the heat transfer area of condenser in the unit of square meters. Similarly the reboiler cost is found as
C reb ¼
M&S 1775:26A0:65 R 280
ðA5Þ
where AR is the heat transfer area of reboiler. The investment cost of compressor is calculated from
C comp ¼
M&S 0:82 2047:24bP 280
ðA6Þ
where the compressor power bP is
cRT in bp ¼ c1
" # , ðc1Þ=c Pout 1 V 3600 Pin
ðA7Þ
The coefficient c is the ratio of specific heats, and c = 1.4 is used here [34]. The R is the ideal gas constant, and T and P are absolute temperature and pressure, respectively. The compressor efficiency was assumed as ideal, and the suction temperature was 35 °C. The discharge temperature was not used in the cost computation. The process operation is assumed to be 330 days per year and 24 h a day.
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Appendix B. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2014. 11.002. References [1] O. Yidirim, A.A. Kiss, E.Y. Kenig, Dividing wall columns in chemical process industry: a review on current activities, Sep. Purif. Technol. 80 (2011) 403– 417. [2] S. Lee, N.V.C. Long, M. Lee, Design and optimization of natural gas liquefaction and recovery processes for offshore floating liquefied natural gas plants, Ind. Eng. Chem. Res. 51 (2012) 10021–10030. [3] H.W. Chun, Y.H. Kim, Application of a divided wall column for gas separation in floating liquefied natural gas plant, Korean J. Chem. Eng. 30 (2013) 1473–1479. [4] Y.H. Kim, Application of partially diabatic divided wall column to floating liquefied natural gas plant, Energy 70 (2014) 435–443. [5] S.H.R. Shooshtari, A. Shahsavand, Reliable prediction of condensation rates for purification of natural gas via supersonic separators, Sep. Purif. Technol. 116 (2013) 458–470. [6] G. Kaibel, Distillation columns with vertical partitions, Chem. Eng. Tech. 10 (1987) 92–98. [7] Y.H. Kim, Structural design of extended fully thermally coupled distillation columns, Ind. Eng. Chem. Res. 40 (2001) 2460–2466. [8] P.B. Shah, Squeeze more out of complex columns, Chem. Eng. Prog. 98 (2002) 46–55. [9] R. Premkumar, G.P. Rangaiah, Retrofitting conventional column systems to dividing-wall columns, Chem. Eng. Res. Des. 87 (2009) 47–60. [10] A.K. Jana, Heat integrated distillation operation, Appl. Energy 87 (2010) 1477– 1494. [11] A.A. Kiss, D.J.P.C. Suszwalak, Enhanced bioethanol dehydration by extractive and azeotropic distillation in dividing-wall columns, Sep. Purif. Technol. 86 (2012) 70–78. [12] A.A. Kiss, R.M. Ignat, Innovative single step bioethanol dehydration in an extractive dividing-wall column, Sep. Purif. Technol. 98 (2012) 290–297. [13] A.J. Kidnay, W.R. Parrish, D.G. McCartney, Fundamentals of Natural Gas Processing, second ed., CRC Press, Boca Raton, FL, USA, 2011. [14] S. Mokhatab, S.M. Poe, Handbook of Natural Gas Transmission and Processing, second ed., Gulp Professional Pub, Waltham, MA, USA, 2012. [15] G.M. de Koeijer, S. Kjelstrup, P. Salamon, G. Siragusa, M. Schaller, K.H. Hoffmann, Comparison of entropy production rate minimization methods for binary diabatic distillation, Ind. Eng. Chem. Res. 41 (2002) 5826–5834.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Side-rectifier divided wall column for offshore LNG plant Young Han Kim ⇑ Dept. of Chemical Engineering, Dong-A University, Busan 604-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2014 Received in revised form 28 October 2014 Accepted 1 November 2014 Available online 7 November 2014 Keywords: Side-rectifier DWC Compact distillation column Floating liquefied natural gas plant Low pressure processing
a b s t r a c t A side-rectifier DWC (divided wall column) is proposed to replace the whole FLNG (floating liquefied natural gas) plant, which comprises of three distillation columns in the conventional system. The proposed column is compact equipment suitable to harsh offshore operation of the gas plant, which is operated at low pressure to reduce the investment and electricity costs. The economics, thermodynamic efficiency and operational difficulty of the side-rectifier DWC are evaluated and compared with the conventional system. The heating and cooling duties of the side-rectifier DWC are 5.9% and 5.1% less than those of the conventional system, respectively. The economic evaluation indicates that the side-rectifier DWC requires 57% less investment and 10% more utility cost than the conventional system. However, low feed pressure used in the side-rectifier DWC saves the compressor cost of 5.1 times the investment cost and the electricity cost of 1.8 times the total utility cost of the high-pressure conventional system. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Two conventional distillation columns in sequence are combined into a single Petlyuk distillation column, implemented as a DWC (divided wall column) in practice [1], and three products are yielded from the column. When the DWC is employed in an offshore LNG (liquefied natural gas) plant utilizing three conventional distillation columns as shown in Fig. 1, the DWC replaces last two columns [2–4]. If a rectifying column is connected to the prefractionator of the Petlyuk column, four products are processed from the single-reboiler column as illustrated in Fig. 2 and the three columns in the conventional system are replaced with it. In this configuration most of the lightest component is separated in the rectifier, which significantly reduces the separation load of the next column. For the practical implementation of the proposed Petlyuk column, a side-rectifier DWC as demonstrated in Fig. 3 can be constructed. Considering the space limitation and the harsh condition of operation of the offshore LNG plant, the single-reboiler distillation column is suitable to the LNG plant. Though new separation techniques were developed recently [5], distillation has been the most reliable process for the natural gas separation. In addition the side-rectifier DWC has high distillation efficiency, because it has the same column structure as the common DWC. The extended DWC having three walled sections in the middle is known to produce four products [6,7], but the control of product specification is ⇑ Tel./fax: +82 51 200 7723. E-mail address: [email protected] http://dx.doi.org/10.1016/j.seppur.2014.11.002 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
more difficult than the common DWC of two sections. The difficulty explains why the extended DWC has not been commercialized, while the common DWC is being utilized in many field operations [8–10]. The integration of multiple binary columns into the DWC raises the distillation efficiency for less energy demand, but the column operating pressure has to be equal for the all sections of the DWC in spite that the binary columns are operated at different pressures. When the pressure difference is not large, the integration can be applied to the complex distillation processes [11,12]. However, the pressure limitation makes the extended DWC more difficult to implement than the common DWC. The former needs a single pressure from the three column pressures of the conventional system, while the latter does from two pressures. In addition of the difficulty of the selection of column operating pressure in the extended DWC, the column operating temperature is unfavorable to result in high utility cost. The difference of condenser temperature and reboiler temperature in the DWC is very large due to the single operating pressure. The separate column pressures in the conventional distillation system are manipulated for the low-cost utilities used in the condenser and reboiler. Because the gas separation process in the LNG (liquefied natural gas) plant handles the feed having the components of large difference of boiling points, the separation is relatively easy but cryogenic cooling and high temperature boiling are necessary. In the conventional distillation system of the LNG plant the operating pressures of the multiple columns are adjusted between 0.3 MPa and 3 MPa to make the column operating temperature close to ambient temperature [13,14]. The adjustment reduces the
26
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Nomenclature Ac AR B bp Ccol Ccomp Ccond Cf Cp Creb Ctray D DC E F Fc f H Hc L P Q R
condenser heat transfer area (m2) reboiler heat transfer area (m2) bottom product rate (kmol h1) compressor power (kW) column cost ($) compressor cost ($) condenser cost ($) fabrication factor () pressure factor () reboiler cost ($) column internal cost ($) overhead product rate (kmol h1) column diameter (m) Exergy (kJ kmol1) feed flow rate (kmol h1) fabrication coefficient () Carnot factor () enthalpy (kJ kmol1) column height (m) liquid flow rate (kmol h1) pressure (Pa) energy stream (kJ h1) gas constant ()
consumption of cryogenic refrigerant and high temperature steam. However, the number of columns is limited in the offshore operation of the LNG plant, where the DWC is a good alternative for the operation due to its small column number in spite of the requirement of high-cost utility [2–4]. For the reduction of the utility cost in the DWC, diabatic distillation can be applied, which has in-tray heat exchangers to
S T V W
entropy (kJ kmol1 K1) or side draw rate (kmol h1) absolute temperature (K) vapor flow rate (kmol h1) work (kJ)
Greek
c g
ratio of specific heats () thermodynamic efficiency ()
Superscripts B bottom product D overhead product F feed L liquid S side draw V vapor Subscripts j tray number m minimum o ambient Q energy stream
disperse the high-cost heat duties of the reboiler and condenser [4]. The duty of cryogenic cooling was partially reduced by applying two inter-coolers to the DWC [3]. The heat transfer at the intray heat exchangers is driven by lower temperature difference than the adiabatic distillation. The heat transfer not only lowers the heat duty at the condenser and reboiler, but also reduces the exergy loss in the distillation process [4,15]. But in practice, the
Fig. 1. Schematic diagram of the conventional offshore FLNG plant.
27
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
procedure of the side-rectifier DWC are addressed, and its cost and thermodynamic efficiency are examined. 2. Process description
Fig. 2. Schematic diagram of the side-rectifier Petlyuk column.
installation and maintenance of the in-tray heat exchangers in every tray are difficult. In this study a single reboiler distillation column, side-rectifier DWC, producing four products is proposed, and its application to the floating LNG plant is evaluated in the separation performance and heat duty requirements. The characteristics and design
The raw natural gas from gas well contains liquid components of oil and water, which are separated through the multiple separators having successively reduced pressures up to 20 kPa to maximize gas recovery and to stabilize the crude oil [13]. The processed gas is compressed to high pressure to minimize the number of processing equipments in the next stage. A typical gas mixture contains various components as listed in Table 1. The feed components are of wide variation of volatility, but methane is more than 86%. The first column in the distillation system processes the most of the feed because of the large amount of methane, and therefore a specially designed distillation system can be applied in the processing. Though the onshore separation system utilizes five columns as demonstrated in Lee et al. [2], the number of the columns is limited in the offshore operation. A three-column offshore process is conventionally utilized as shown in Fig. 1. The first two columns with recycle produce the mixture of methane and ethane as a typical LNG fuel. The last column produces LPG and C5 plus heavier components to be fed to hydrocarbon chemical processes. When the prefractionator of the Petlyuk column accommodates a small column on its top as described in Fig. 2, the pretreatment column for the common DWC used in the previous studies [2–4] can be eliminated to make the whole distillation system a singlereboiler column. In practice the proposed column becomes a side-rectifier DWC as illustrated in Fig. 3. Because the offshore operation requires the compactness of equipment due to limited space and harsh environment, the side-rectifier DWC is a good candidate for the LNG plant if the column operating pressure is properly selected. Due to the single operating pressure in the DWC unlike the conventional system of multiple column pressures, the condenser temperature is lower than that of the conventional system and its reboiler temperature is higher. Therefore, the in-tray heat exchangers are installed to reduce the consumption of high cost utilities for the condenser and reboiler. The distribution of the in-tray heat exchangers is explained in the next section. 3. Design of the side-rectifier DWC 3.1. Column design In the design of a conventional distillation column using a commercial program the operating pressure is determined first, and the number of trays and operating conditions are optimized for Table 1 List of feed conditions.
Fig. 3. Schematic diagram of the side-rectifier DWC (divided wall column) equivalent to the Petlyuk column.
Name
Value
Temperature (°C) Pressure (MPa) Flow rate (kmol/h)
35 1.0 15,120
Composition (% mol fraction) Nitrogen Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane n-Heptane
1.54 86.39 6.47 2.87 0.72 0.82 0.41 0.31 0.31 0.15
28
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
the minimum of total annual cost. The design of the side-rectifier DWC limits the number of trays due to the harsh condition of offshore operation. The numbers of trays were set to 20 and 25 for the rectifier-prefractionator and main column shown in Fig. 2, respectively, considering the results of previous studies [2,3]. Because the number of trays was limited in the offshore operation of distillation column due to the harsh environment at sea, the maximum number was set at 25 and the number was not optimized. Instead the location of feed and interlinking trays was iteratively optimized in the simulation by checking the product composition. The difference of tray numbers between the rectifier DWC and the conventional system is five, of which the difference of the pressure drop is 5 kPa. The difference was not significant to affect the column performance, because the column operating pressure was 0.8 MPa. The location of feed and side draw and the interlinking stages between the prefractionator and main column are found from the iterative computation for the minimum heat duty at the given product specification. The feed condition and composition are found in Table 1. Though the conventional offshore LNG plant operates three columns having different pressures between 0.8 MPa and 7 MPa, the operating pressure of the side-rectifier DWC has to be unique. The pressure was set at the lowest pressure of 0.8 MPa, because low pressure operation gives high thermodynamic efficiency and requires no extra cost for high pressure equipment [13]. In addition, the compression of the raw gas from the well is not necessary to feed to the column. The design of a distillation column using the commercial software requires two input data: column operating pressure and the number of total trays, and they are determined as explained above. Because the HYSYS was used in this study, the locations of feed, side draw and interlinking streams were iteratively computed for the minimum use of utilities. The operating condition for a given set of product specification from a feed was also found from the iterative computation. The product specifications are 0.89, 0.98 and 0.99 for the methane, LPG and C5+ product, respectively. The design results of the side-rectifier DWC, structural design and operating condition, are listed in Table 2 along with those of the conventional LNG plant. The nominal amount of overhead product
from the main column is mixed to the overhead product of the rectifier. 3.2. In-tray heat exchanger design Because three columns used in the conventional LNG plant are combined to the single-reboiler column, the temperature difference of condenser and reboiler is very large almost equal to the boiling point difference between the lightest and heaviest components in feed, methane and C5+ in this study. Therefore, high-cost utility is required, and the operating cost is more than the conventional system. The same problem has been indicated in the previous studies of the DWC application [3,4]. For the reduction of the utility cost, the heat duty in the condenser and reboiler is minimized by implementing the inter-coolers and inter-heaters, in-tray heat exchangers. The heat duty of the in-tray heat exchangers can be optimally allocated by the theorem of equipartition of entropy production [16]. The heat exchangers have to be installed in every tray due to the theorem, but the practical installation in all trays is difficult. Therefore, the in-tray heat exchangers are placed in every third tray in this study, and the heat duty is adjusted to provide minimum liquid flow in the trays [17]. The sections of divided wall are not included in the installation of the in-tray heat exchangers, because the space does not allow placing them. The location and heat duty distribution of the in-tray heat exchangers are listed in Table 3. The overall heat transfer coefficient was assumed as 0.8 kW/m2 K and 1 kW/m2 K for in-tray cooler and in-tray heater, respectively. Their heat transfer areas are listed in Table 4. Vertical panels installed on the wall of a distillation column significantly increase the heat transfer area [18]. As indicated above the heat duties at the condenser and reboiler requiring high cost utility are significantly reduced by employing the in-tray heat exchangers. The detail of the design results including feed and product specifications is given in Fig. 4. The stream mass and heat flow rates are listed in Table 5, and the stream names are shown in Figs. 1, 3 and 4. Primary reason to install the in-tray heat exchangers is to reduce the cost of utility by using less-cost refrigerant and steam.
Table 2 Structural information, operating conditions and feed and product compositions in the conventional and side-rectifier DWC systems. Tray numbers are counted from top. Name
Structural Number of trays Feed/side draw tray Interlinking trays Operating Pressure (MPa)-top Temperature (°C) Overhead Bottom Feed (kmol/h) Product (kmol h) Overhead Bottom Side Reflux (kmol/h) Vapor boilup (kmol/h) Cooling duty (MW) Reboiler duty (MW) Precooler/Comp. (MW) Composition (mol frac.) Feed Product-C1,2 -LPG -C5+
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rectifier
Prefract.
Main
10 8
10 7
20 13
12
8 3
25 5 4/14
7.0
1.1
0.8
0.8
0.8
0.8
34.6 17.6 15,120/349
55.1 62.8 718
30.0 118.4 369
62.2 27.7
26.4 34.8 15,120
90.2 115.7
14,750 718
349 369
215.5 153.5
14,739
101.5 1.14 4.7 0.01 14.6
178 526.5 1.04 2.85 0.52
183.2 352.6 2.02 2.24
33 15,620 0.42
0.93/0.04/0.01 0.95 0.32
8.0 0/0.58/0.42
0.46/0.32/0.18 0.95 0.58
100 180
21 177 183 21 47.9 0.59 0.30
0.93/0.04/0.01 0.95
0.99 0.99
1.00 0.98 0.99
29
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35 Table 3 Design results of heat exchangers in the rectifier and main column of the side-rectifier DWC. Tray numbers are counted from top. Rectifier Tray No. Condenser
Main column Temp. (°C)
Duty (MW)
62.2
0.42
60.6 52.8 43.6 35.1
2.0 2.5 2.5 3.0
Tray No
Temp. (°C)
Duty (MW)
90.2
0.59
2 4
33.0 34.2
1.2 1.0
17 20 23
83.9 99.9 112.3 115.7
1.5 1.5 1.5 0.3
Tray cooling 2 5 8 11 Tray heating
Reboiler
The in-tray heat exchangers near to the condenser and reboiler have less temperature difference with them than others. Because the theorem of equipartition of entropy production [16] requires even distribution of heat transfer through the trays, the in-tray heat exchangers are installed including near to the condenser and reboiler. In addition, the diabatic operation of a distillation column improves thermodynamic efficiency [15,19]. 4. Results and discussion The design results of the side-rectifier DWC are analyzed and compared with the conventional system of three distillation columns. The investment and operating costs are evaluated, and
the thermodynamic efficiency of the side-rectifier DWC is compared with the conventional system. 4.1. Design results The results of structural and operating condition of the siderectifier DWC are listed in Table 2. The detail of equipment design results is included in Table 4. Because the main column of the DWC produces three products, five more trays are necessary than the tallest debutanizer in the conventional system. As indicated above the column pressure is set to the lowest pressure among the conventional three columns, which gives the condenser temperature much lower than the conventional system. However, its heat duty
Table 4 Equipment size and economic evaluation of the conventional and side-rectifier DWC systems. Units are in million U.S. dollars, and the utility cost is annual. Name
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rectifier
Prefract.
Main
Investment Column Diameter (m) Height (m) Cost
10.1 6.1 2.707
1.97 6.1 0.218
1.66 12.2 0.294
10.39 7.32 1.377
0.51 4.88 0.04
2.38 15.24 0.516
Tray Number () Cost
10 0.165
10 0.013
20 0.02
12 0.206
8 0.001
25 0.044
Condenser H. T. Area (m2) Cost
195.8 0.682
43.3 0.118
84.3 0.169
17.5 0.061
Reboiler H. T. Area (m2) Cost
0.7 0.019
142.6 0.281
112.2 0.224
In-tray Heat Ex. H. T. Area (m2) Cost Preheater H. T. Area (m2) Cost
24.6 0.076 15 0.061
83,104x2, 125 0.477
50,42 (cooling), 75x3 (heating) 0.178, 0.352
608.3 1.424
Compressor Power (kW) Cost Total Utility Steam Cooling water Refrigerant 4.538 Electricity Total Feed compression (1 MPa to 7 MPa) Investment Electricity
412 1.566
0.086
7.9
3.389
0.166 0.091
0.289
0.417 0.249
5.12 5.547
40.32 10.01
0.708 6.117
30
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Fig. 4. Schematic diagram of the side-rectifier DWC with in-tray heat exchangers.
Table 5 List of stream mass and heat flow rates. Stream names are shown in Figs. 1, 3 and 4. Stream
Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW) Energy stream Heat flow (MW)
Conventional system Feed
D1
F2
D2
R2
F3
D3
B3
35 7.1 1.51 104 329.2 CE1 14.6
34.6 7.0 1.48 104 330.5 CE2 4.7
17.6 7.0 718 25.4 CE3 1.04
55 1.1 349 7.9 CE4 2.0
93.4 6.6 349 7.4 RE1 0.01
62.8 1.1 369 15.6 RE2 2.9
30.0 0.8 215.5 8.1 RE3 2.2
118.4 0.8 153.5 7.3 PE1 0.52
DWC-adiabatic Stream Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW) Energy stream Heat flow (MW)
Feed 35 1.0 1.51 104 323 CE1 8
D1 62.8 0.8 1.47 104 323.2 CE2 10.5
LP 26.7 0.8 869 3.6 CE3 2.7
VP 26.1 0.8 1.56 104 348 RE1 4.7
D2 78.3 0.8 20 0.5
Side 48.9 0.8 183 7.2
B 114.7 0.8 181 8.5
D1 62.3 0.8 1.47 104 323.3
LP 27.7 0.8 878 3.6
VP 26.4 0.8 1.56 104 349
D2 90.6 0.8 21 0.5
S 49.2 0.8 183 7.3
B 115.7 0.8 177 8.4
DWC-diabatic Stream Temperature (°C) Pressure (MPa) Molar flow (kmol/h) Heat flow (MW)
Feed 35 1.0 1.51 104 323
is minimized for the reduced use of high-cost refrigerant by employing in-tray heat exchangers. The amount of C5+ product
was 15% more than the conventional system, while LPG was produced 15% less. Note that the price of C5+ product is higher than
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
that of LPG product. The feed pressure in the DWC is much lower than the conventional system, and therefore the cooling load is about half of the conventional system due to less temperature elevation from the feed compression. In addition the compression of recycle stream from the depropanizer is not necessary giving less investment and utility costs in the DWC. The small amount of overhead product from the main column is mixed to the overhead product of the rectifier. Due to the improved thermodynamic efficiency of the DWC, the heating and cooling duties of the side-rectifier DWC including the in-tray heat exchangers are 5.9% and 5.1% less than the conventional system, respectively. The profiles of LPG composition in the main column of the rectifier DWC and the debutanizer of the conventional system are shown in Fig. 5. The composition variation of the lower 20 stages in the DWC is similar to that of the debutanizer. It indicates that the addition of 5 stages to the debutanzer is sufficient to separate the mixture of methane and ethane from the top of the DWC. Because of the limitation of column height in the offshore operation and large boiling point difference between the mixture and LPG products, the small number of stages is added to the DWC. The tray vapor composition in the demethanizer of the conventional system is plotted in Fig. 6(a), and those of the rectifier DWC with and without in-tray heat exchangers are also shown in the figure. The DWC without in-tray heat exchanger as described in Fig. 3 is labeled as adiabatic DWC in Fig. 6, while the DWC with in-tray heat exchangers is given as diabatic DWC. Because a partial condenser is used in the adiabatic DWC, its vapor composition at top tray is lower than others. The liquid compositions of LPG and C5+ components are plotted in Fig. 6(b) and (c), respectively. The LPG is produced as side cut in the DWC, and their composition profiles are different from the debutanizer of the conventional system. The tray temperature, vapor flow rate and liquid flow rate are shown in Fig. 7(a–c), respectively. The mixture of methane and ethane is produced from the top of the DWC, and therefore the top temperature of the DWC is lower than that of the debutanizer producing the LPG. The in-tray heat exchangers in the diabatic DWC lead to the stepwise variation of vapor and liquid flow rates.
31
Fig. 6. Vapor compositions in the demethanizer of the conventional system and the DWCs with and without in-tray heat exchangers (a). LPG composition (b) and C5+ composition (c) in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers.
4.2. Economic evaluation The economics between the conventional distillation system and the side-rectifier DWC is compared in terms of investment and utility costs [20]. The investment cost includes the costs of column, trays, and heat exchangers, which are calculated from the cost equations given in Appendix A. The utility costs of steam
Fig. 5. Variation of tray liquid composition in the main column of rectifier DWC (a) and debutanizer in the conventional system (b).
Fig. 7. Tray temperature in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers (a). Vapor flow rate (b) and liquid flow rate (c) in the debutanizer of the conventional system and the DWCs with and without in-tray heat exchangers.
and refrigerant depend on temperature. The correlation of the costs and temperature was derived from the reference data [21,22] and given in Kim [4]. The results of cost evaluation are listed in Table 4. The investment of the rectifier DWC is less than 43% of the conventional system. The large difference comes from the high pressure operation at the demethanizer in the conventional system. The pressure requires additional cost in the construction of column and heat exchangers, and the investment of the compressor [23,24] used for recycling the overhead product of the depropanizer is added. The utility cost of the DWC is 10% more than that of the conventional system because of the lower cooling temperature requiring high-cost refrigerant. The significant reduction of the investment cost compensates the utility cost increase. The critical issue in the economics between the conventional system and the rectifier DWC is the column operating pressure. Employing high
32
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Fig. 8. Enthalpy-Carnot factor diagrams; first column (a), second column (b) and third column (c) of the conventional system, and rectifier-prefractionator (d) and main column (e) of the side-rectifier DWC.
column pressure of 7 MPa in the conventional system requires a feed compressor of the cost more than 5.1 times the total investment of other equipment in the conventional system and the electricity cost of 1.8 times the total utility cost as listed in Table 4. While the feed pressure in the conventional system is 7 MPa, the pressure of the rectifier DWC is 1 MPa. As the production of raw natural gas from the well continues, the gas pressure drops significantly and the compression of the raw gas is required in the conventional system [13]. The compression cost is listed in Table 4.
When a payback time of 5 years is applied, the TAC (total annual cost) of the compressor is 2.5 times that of the conventional system. The offshore operation requires compact equipment due to the severe condition of environment. The side-rectifier DWC can be constructed in a single reboiler column by combining the rectifier and DWC like a top DWC having two condensers and one reboiler [2]. Though there are two dividing walls, one at top and the other in the middle, the whole system is placed in a column. The system
33
Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
of demethanizer and a DWC [3] has two condensers and two reboliers making two complete column system. Compared with the system of demethanizer and DWC, the costs of investment and utility of the rectifier DWC are 57% less and 3% more, respectively. The thermodynamic efficiency of the system of demethanizer and DWC is 0.9% point higher than that of the rectifier DWC. For the in-tray cooling the log-mean temperature difference was taken as 30° and that of heating was 20°. The cost of heat exchangers is proportional to the heat transfer area to the power of 0.65. When the temperature difference becomes a half, the heat transfer area doubles and the cost increases by 57%. However, the additional cost of the in-tray heat exchanger is 5% of the investment of the conventional system, which is small compared with the 57% reduction of the total investment cost benefited from adopting the rectifier DWC. 4.3. Exergy loss and thermodynamic efficiency The exergy loss indicates the thermodynamic efficiency of chemical processes. Distillation is a large energy-consuming process, and its efficiency is important to reduce the energy consumption. In this section the thermodynamic efficiency of the side-rectifier DWC is calculated using the exergy loss, and its efficiency is compared with that of the conventional system. The maximum available work from a thermodynamic process is represented as exergy calculated from the enthalpy and entropy of the process at its pressure and temperature.
E ¼ ðH Ho Þ T o ðS So Þ
ð1Þ
where the subscript o indicates atmospheric pressure and ambient temperature. In a tray of the distillation column the rate of exergy loss is calculated from the exergy balance.
E_ loss ¼ Lj1 ELj1 þ V jþ1 EVjþ1 þ F j EFj Lj ELj V j EVj Sj ESj E_ Q;j
ð2Þ
The last term, thermal exergy of an energy stream and equipment, is found from
To E_ Q;j ¼ Q j 1 T
ð3Þ
The exergy loss in the distillation column includes the losses in trays, which is difficult to evaluate. Instead, a diagram of the Carnot factor and stream enthalpy flow of the feed and products and the external heat flow is useful to calculate the exergy loss in the distillation column [25,26]. The Carnot factor is defined as
To f ¼ 1 T
ð4Þ
The Eq. (4) indicates that the exergy loss in energy stream is the multiplication of the Carnot factor and energy flow. Practically the
exergy loss is computed as the area between the feed and products as demonstrated in Fig. 8. The thermodynamic efficiency of the distillation column is defined as [27,28]
_ W
m g¼ _ W m þ E_ loss
ð5Þ
and the minimum work is calculated from the exergy of feed and products in an adiabatic distillation column [29].
_ m ¼ DED þ BEB FEF W
ð6Þ
The in-tray heat transfer of the DWC was added to the minimum work as thermal exergy. The itemized exergy losses and the exergy flow of streams are listed in Table 6. The reduction of heat duty in the DWC leads to less exergy loss, but the low pressure and low temperature of the overhead product, mostly methane, give less minimum work than the conventional system. Because the minimum work is only a portion of the exergy loss, the thermodynamic efficiency is very low in both systems. 4.4. Low pressure operation The compact structure of distillation column for offshore LNG processing is one advantage of the proposed side-rectifier DWC. The other benefit is low pressure operation. The gas pressure at the well decreases up to 20 kPa as the production proceeds, and the pressure is boosted to 7 MPa before feeding to the conventional gas plant [13]. High pressure and low temperature operation generates gas hydrates in the transportation lines, and hydrate inhibitors are used to prevent plugging pipes due to the solid hydrates [13]. Moreover the well gas is almost always saturated with water. However, the plugging is less likely in the low pressure operation, and no heating is necessary before entering the separation process. The low pressure operation requires less investment cost because of thin vessel wall and small gas compressor used for the feed compression. In addition, less electricity is consumed resulting in low utility cost. The pressure of overhead product from the demethanizer in the conventional system has to be lowered for shipping to the LNG carrier. The product, mostly methane, is liquefied at a temperature about – 160 °C at atmospheric pressure for the transportation by the LNG carrier [30]. Therefore, the compression in the LNG processing is not necessary. Though the transportation and storage require smaller volume in the high pressure operation, the investment and electricity costs are greatly saved in the low pressure operation. 4.5. Process operation The application of the common DWC in field operation has been successful for many years [9,31], but the operation of the DWC is
Table 6 Exergy loss and thermal efficiency in the conventional and side-rectifier DWC systems. Units are in MW. Name
Trays Cooling Heating Total Feed Overhead Bottom Side Diabatic Min. work Thermodynamic efficiency (%)
Conventional
Rectifier DWC
DeC1
DeC3
DeC4
Rect.–Pref.
Main
92.59 1.986 0
5.281 0.202 0.771
93.85 0.118
3.520 0.240 0.059 97.79
39.03 42.90 3.757
3.757 0.617 1.771
1.208 0.188 0.274 102.50 1.771 1.010 0.382
3.095 2.9
23.58 23.30
0.037 0.438 0.909 3.96 2.37 2.4
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Y.H. Kim / Separation and Purification Technology 139 (2015) 25–35
Table 7 Changes of product mole fraction with the percent change of manipulated variables. Product
Rectifier DWC
Pre-column DWC
Conventional
C1
Manipul. variable % change Mole frac. change
Reflux flow 12.1 6 106
Reflux flow 6.8 1 106
Reflux flow 3.2 4.6 104
LPG
Manipul. variable % change Mole frac. change
Prefract. vapor flow 12.5 6 104
Prefract. vapor flow 16.7 6 104
Reflux flow 1.2 4 104
C5+
Manipul. variable % change Mole frac. change
Boilup ratio 2.2 4.1 103
Boilup ratio 2.1 2 103
Boilup ratio 4.3 1.2 102
more difficult than the conventional distillation column. It is the main concern of the field operators considering the DWC adoption. The operation of the side-rectifier DWC in this study is expected to be similar to the DWC operation. The overhead product from the rectifier can be separately adjusted using the reflux flow to the rectifier. As listed in Table 7, the gain of product mole fraction from the applied change of manipulated variables indicates that the conventional system shows the best control performance as explained in Wolff and Skogestad [32]. However, the performance of the rectifier DWC is better than that of the existing pre-column DWC [3]. In the control of C1 product specification the ratio of the change of methane mole fraction to the variation of reflux flow rate is the gain, which indicates the ease of the process operation. The larger the gain is, the easier the process manipulation is. Though the conventional system has much larger gain than the DWCs, the rectifier DWC has about three times the gain of the pre-column DWC. For LPG production the rectifier DWC shows the improvement by one third over the pre-column DWC. In case of C5+ production the rectifier DWC gives about twice better performance than the pre-column DWC. The difficulty of products control in the proposed DWC is alleviated by applying a sufficient margin of specification in the column design [32]. In addition the adjustment of heat duty at the in-tray heat exchangers manipulates the internal vapor and liquid flows in the column, which provides more degrees of freedom in the column control. 5. Conclusions The offshore LNG (liquefied natural gas) plant requires a compact gas separation system due to the harsh environment, and a side-rectifier DWC (divided wall column) is proposed for the purpose. The characteristic and design procedure of the column are addressed here. The economics and thermal efficiency of the proposed DWC are compared with those of the conventional system. The side-rectifier DWC is compact, and its low pressure operation gives many benefits, such as low investment due to thin vessel wall, no formation of hydrates and lower compression of feed. The heating and cooling duties of the side-rectifier DWC are 5.9% and 5.1% less than those of the conventional system, respectively. The economic evaluation indicates that the side-rectifier DWC requires 57% less investment and 10% more utility cost than the conventional system without counting the cost increase of feed compression. The low compression of feed saves the compressor cost of 5.1 times the total plant investment and the electricity cost of 1.8 times the total utility cost in the conventional system. The thermodynamic efficiency of the DWC is 2.4%, while that of the conventional system is 2.9%. Acknowledgments Financial support from the Basic Research Program (2011-0021819) through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and Technology is gratefully acknowledged. Appendix A The cost of distillation column includes the column shell and internal, and the shell cost is given as
C col ¼
M&S C f D1:066 H0:802 Cp C C 280
ðA1Þ
where the M & S is the Marshall and Swift index and the value of 4th quarter of 2011 of 1536.5 was used here. The coefficient Cf is given as 3919.32 from Olujic et al. [33], and the pressure related correction Cp is from Douglas [21]. The column diameter Dc is calculated from the maximum vapor flow rate.
pffiffiffiffi Dc ¼ 0:08318 V
ðA2Þ
where V is the rate of vapor in kg-mol/h. The column height Hc is calculated from two-feet tray spacing and the tray number. The cost of column internal is calculated from the equation.
C tray ¼
M&S 97:243D1:55 HC F c C 280
ðA3Þ
where the fabrication coefficient Fc is given in Olujic et al. [33]. From the reference the heat exchanger cost equation was adopted as
C cond ¼
M&S 1609:13A0:65 C 280
ðA4Þ
where Ac is the heat transfer area of condenser in the unit of square meters. Similarly the reboiler cost is found as
C reb ¼
M&S 1775:26A0:65 R 280
ðA5Þ
where AR is the heat transfer area of reboiler. The investment cost of compressor is calculated from
C comp ¼
M&S 0:82 2047:24bP 280
ðA6Þ
where the compressor power bP is
cRT in bp ¼ c1
" # , ðc1Þ=c Pout 1 V 3600 Pin
ðA7Þ
The coefficient c is the ratio of specific heats, and c = 1.4 is used here [34]. The R is the ideal gas constant, and T and P are absolute temperature and pressure, respectively. The compressor efficiency was assumed as ideal, and the suction temperature was 35 °C. The discharge temperature was not used in the cost computation. The process operation is assumed to be 330 days per year and 24 h a day.
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