Application of a fully thermally coupled distillation column for fractionation process in naphtha reforming plant

Application of a fully thermally coupled distillation column for fractionation process in naphtha reforming plant

Chemical Engineering and Processing 43 (2004) 495–501 Application of a fully thermally coupled distillation column for fractionation process in napht...

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Chemical Engineering and Processing 43 (2004) 495–501

Application of a fully thermally coupled distillation column for fractionation process in naphtha reforming plant Ju Yeong Lee a , Young Han Kim b,∗ , Kyu Suk Hwang a a

Department of Chemcial Engineering, Pusan National University, San 30 Jangjeon-dong, Kumjeong-gu, Pusan 609-735, South Korea b Department of Chemical Engineering, Dong-A University, 840 Hadan-dong, Saha-gu, Pusan 604-714, South Korea Received 8 May 2002; received in revised form 22 October 2002; accepted 18 February 2003

Abstract Naphtha reformate is extracted for aromatic compounds and the aromatics are separated into benzene, toluene and xylene in sequence. This separation is conducted using a series of binary-like columns. In this study, the first two columns of the separation process are replaced with a fully thermally coupled distillation column (FTCDC) also known as the Petlyuk column. Though feed of the process contains 18 components, the FTCDC developed for the separation of a ternary system is effectively implemented in the industrial-scale application. A structural design to determine the optimum structure of the column from the minimum tray configuration is utilized, and the minimum tray configuration is found from the stage-to-stage computation with ideal tray efficiency in equilibrium condition. The operating condition to yield a given set of product specifications is found from the HYSYS simulation. The performance of the FTCDC is compared with a conventional two-column process to indicate an energy saving of 13% and the investment cost reduction of 4%. © 2003 Elsevier B.V. All rights reserved. Keywords: Process design; Thermally coupled distillation; Fractionation process; Multi-component distillation

1. Introduction The concept of a fully thermally coupled distillation column (FTCDC) was introduced a half century ago, but its practical utilization has recently been materialized in Europe [1] and Japan [2]. Though the energy saving from the FTCDC is well known, the implementation of the column has been limited owing to the difficulty of design and operation. A conventional two-column system has two reboilers, while an FDCDC in Fig. 1 has one reboiler. For the design of the FTCDC many studies have been conducted to find the minimum reflux flow rate, but lack of structural information of the column incurs some complexity to find the optimal design. Though a short cut design procedure was applied to the design of the Petlyuk column [3], the procedure incurs design error from the empirical design equations as indicated in Amminudin et al. [4]. By

∗ Corresponding author. Tel.: +82-51-200-7723; fax: +82-51-200-7728. E-mail address: [email protected] (Y.H. Kim).

0255-2701/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2003.02.001

employing a staged composition line at any reflux flow, the design procedure is improved [4]. A liquid composition profile following equilibrium distillation line gives the minimum tray structure having ideal thermodynamic efficiency. In case that the assumption of ideal tray efficiency and total reflux operation is applied in a distillation column, the column requires the minimum number of trays for a given separation of feed mixture. The structure of the system is called the minimum tray structure here. When the distillation line of an FTCDC matches the profile, the high efficiency is possible by eliminating the feed tray mixing and remixing of the intermediate component, which are the sources of the efficiency reduction [3]. The structural design using the minimum tray is applied to various systems of ternary [5–8] and quaternary systems [9]. The fractionation process handling aromatic solvents is one of major processes in petrochemical plant. As daily handling amount of the process is very large, the impact from the energy saving in the process is significant. For instance, 135,000 barrels a day is processed through the fractionation process in South Korea alone [10], where about 2.9% of total world crude oil production is consumed [11]. Though

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Fig. 1. Schematic diagram of a fully thermally coupled distillation column.

the whole battery of distillation processes separates many components with a series of binary-like distillation columns, the beginning two columns can be replaced with an FTCDC and the application is conducted here. In this study, the structural design procedure is implemented in the design of the industrial FTCDC to replace the first two columns of the conventional fractionation process in a naphtha reforming plant. A ternary design procedure is extended to handle multi-component mixtures, and practical issues are examined from the performance comparison with the existing conventional distillation system. The design procedure using the commercial software HYSYS is addressed in detail along with the examination of performance and energy saving of the column.

In the development of the minimum tray structure, the thermodynamic efficiency of the distillation is assumed to be ideal. Namely, the assumptions of total reflux, no feed tray mixing and no remixing of the intermediate component are employed. Because the detail of the structural design of an FTCDC has been explained in the previous studies [5–9], the design procedure is not mentioned here except new computation technique for the current application. In this study, 18 components are involved and so the equilibrium computation is complex. Therefore, an ideal process with a simple separator is utilized for the computation of vapor–liquid equilibrium. Taking a small amount of liquid feed (say, 0.01 kmol/h) and a large amount of recycle (say, 1000 kmol/h) gives an ideal equilibrium in the separator for a given pressure. The computed composition of vapor is given to the liquid feed composition of the next stage. This computation is readily carried out with the HYSYS. The molar flow rate of feed is given in Table 1. Though the feed has 18 components, the design procedure handles only three components. Thus, the 18 components are separated into three groups of components as indicated in the table. Because the specifications of three products are given, major components in the products are selected as three main components—one for each product—in the products. Then, from the composition list of feed arranged by the order of volatility, minor components are assigned to one of the groups in which more of the component is included. In Fig. 2, the computed compositions are plotted. The circles are of tray liquid compositions of the prefractionator, and ‘+’ and ‘×’ symbols are of upper and lower sections of the main column, respectively. The minimum tray structure is converted to the actual column with the factor of two and the result is summarized in Table 2. The numbers in parentheses are initially computed ones and are adjusted for the given product specification. The increase of upper section of the main column and the moving upward the upper interlinking tray are to meet the

1.1. Structural design 1

The residue curves of a ternary simple distillation indicate that there are many paths of connection between the heaviest component-rich and the lightest component-rich products. The residue curves denote the composition profile for a packed distillation column in total reflux operation [12]. Though the residue curves represent the liquid composition profile of a packed column, it is generally assumed that they are accurate representations of the profile of a tray column at total reflux [12–14]. Among the connections, two paths including feed and side product compositions are utilized in the design. While the composition of the side product is equal to the liquid composition of the stage of the side draw, the feed composition is different from the feed stage composition. For the design of minimum tray system, however, the composition of the feed tray is taken to be equal to the feed composition because no feed tray mixing is assumed.

S

Intermediate

0.8 0.6

0.4

F

0.2 B 0 0 Heavy

0.2

0.4

0.6

0.8

D 1 Light

Fig. 2. Liquid composition profile of designed FTCDC in minimum trays. The circles and ‘+’ and ‘×’ symbols indicate prefractionator and the main column.

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Table 1 Flow rates of feed and products in kmol/h Component

Feed

FTCDC Overhead

Light Benzene Dimethyl c-pentane

Bottom

Side

Conventional 1st

Conventional 2nd

Overhead

Overhead

Bottom

Bottom

87.85 0.0124

86.830 0.0119

0.0005 0.0000

0.9308 0.0004

85.686 0.0088

2.1640 0.0036

2.1640 0.0036

0.0000 0.0000

Intermediate Methyl c-hexane Toluene n-Octane

0.0075 338.10 0.049

0.0024 0.0033 0.0000

0.0000 3.2667 0.0012

0.0015 335.65 0.0479

0.0015 0.0036 0.0000

0.0060 338.10 0.0490

0.0060 335.28 0.0464

0.0000 2.8180 0.0026

Heavy Ethylbenzene p-Xylene m-Xylene o-Xylene n-Nonane n-Pentyl benzene Methyl-ethyl benzene Tri-methyl benzene Methyl-n-propyl bz Di-ethyl benzene o-Cymen Tetra-methyl benzene Penta-methyl benzene

14.975 57.798 128.55 60.160 0.0057 0.3300 26.010 75.950 0.5700 0.3300 4.1200 4.7500 2.2389

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

14.712 57.379 127.69 60.088 0.0057 0.3299 26.003 75.940 0.5700 0.3300 4.1195 4.7497 2.2389

0.2257 0.3069 0.6129 0.0584 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

14.975 57.798 128.55 60.160 0.0057 0.3300 26.010 75.950 0.5700 0.3300 4.1200 4.7500 2.2389

0.0003 0.0002 0.0003 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

14.975 57.798 128.55 60.160 0.0057 0.3300 26.010 75.950 0.5700 0.3300 4.1200 4.7500 2.2389

specification of overhead product which is highly pure. The original structure does not produce the high concentration of benzene. The last two columns show the tray numbers of a conventional two-column system to produce the same products as the FTCDC system. 1.2. Operational variable design In order to have products of a given specification, one needs a proper set of operation conditions for the distillation system of which the structure is found from the previous section. The conditions are found from trial simulation until Table 2 Tray numbers from structural design and operating conditions for the FTCDC and conventional two-column systems Name

FTCDC Prefractionator

Structural Number of trays Feed/side product Interlinking stages Operating Feed (kmol/h) Overhead (kmol/h) Bottom (kmol/h) Side (kmol/h) Reflux (kmol/h) Vapor boilup (kmol/h) Heat duty (GJ/h)

21 (22) 7 6

Conventional Main 89 (82) 28 74 (58)

801.8

290.1 492.9 9.35

Tray numbers are counted from bottom.

86.8 337.7 377.3 1792 1634 20.71

1st 60 31

801.8 85.70 716.1 401.3 595.2 47.77

2nd 50 26

716.1 337.5 378.6 1106 1315

the computed product composition meets the specification. Because the commercial process design program HYSYS is utilized here, the trial computation with the known distillation structure is relatively simple procedure. The following information of minimum flow is valuable to yield the initial condition for the trial simulation. The initial values of liquid and vapor flow rates are determined from the ratio of the minimum liquid and vapor flows, and the flows are adjusted for a given set of product specification. Increasing flow rates raises the composition of major component in a product, and therefore the flow is changed from a small value until a given composition of the product is yielded. The minimum liquid flow rate [15] in the main column is estimated as   Aφ1 Aφ2 αB B L = max (1) , + αA − φ 1 α A − φ 2 αB − φ 2 where α’s are relative volatilities and φ’s are the solutions of the Underwood equation [16] for a saturated liquid feed. The relative volatility of a group of components is calculated by multiplying the individual volatility and mole fraction of a component and by taking the sum in the group. The result of Eq. (1) is total liquid flow rate of the main column and prefractionator. Also, the minimum liquid flow rate in the prefractionator is given by [15] αC F ¯ = L (2) αA − α C The initial liquid split ratio is obtained from Eqs. (1) and (2), but the ratio may not be optimum [17]. Therefore, it has to be revised until the optimum ratio is yielded from the simulation. The objective of the optimization is the minimization

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of vapor boil-up rate. The variation of the split ratio is in the direction of the minimizing the vapor rate, and the step size of the variation is determined from the gradient of the optimization objective. In the HYSYS simulation, however, this optimization process is included in the package. Therefore, the procedure is not implemented separately. As the commercial software HYSYS is utilized here, obtaining the operating conditions is a relatively simple process compared with the procedure using an in-house developed software. One of the benefits from the commercial program is that the equilibrium calculation for multi-component system is not difficult. The in-house software needs the vapor–liquid equilibrium data that are not easy for the system of a large number of components. A design with the HYSYS requires the structural information of a distillation system, feed composition, flow rates of feed and products and column pressure. Except the structural information, others are given from a design project. Then, reflux flow is iteratively varied until a specific composition of product is yielded. The liquid flow rates derived from Eqs. (1) and (2) are only for reference values which are not necessarily required in the HYSYS simulation. Unless the components in feed formulate an azeotropic mixture, the proposed procedure works. The structural design procedure is based on the residue curves, and the pattern of the curves is similar in various systems except the azeotropic system. In the design of distillation column with the HYSYS, the information of column structure has to be determined at the initial stage. Then, operational variables are given to simulate the process and to examine the specification of products. The variables are adjusted until a predetermined product specification is found. In this regard, the design procedure of this study, which gives the structural information first, is suitable to the HYSYS design application. In a conventional distillation system, two products are drawn from its top and bottom, and its design begins with the reflux flow of the column derived from the minimum reflux. Then the structural information, optimal column height and feed location, is searched. It is the reason why many studies [15,18,19] of the FTCDC are conducted to find the minimum reflux. However, the FTCDC has extra structural variables of interlinking between a prefractionator and a main column, so searching the optimal liquid flow is easier than yielding the optimal structure including many design variables in this case. A structural design technique using the three-column model was proposed by Triantafyllou and Smith [3], and it requires matching the compositions of interlinking streams to result in time-consuming iterative computation. The proposed structural design of this study does not require the matching process. The HYSYS short cut design procedure of a distillation column yields structural information, but it is only good for a conventional distillation system. The FTCDC has interlinking streams of which compositions are not given to prevent from the application of the procedure. Therefore, a separate procedure to determine the structure of the FTCDC

is adopted from Kim and co-workers [5–8]. Once the structure is determined, the formulation of the HYSYS model is simple and the model simulation continues until the desired products of given specification are obtained. The operational variables are found from the simulation result.

2. Results and discussion The FTCDC system is applied to the fractionation process of a naphtha reforming plant as a major petrochemical process application of the energy saving distillation technique. The proposed design procedure is implemented for the processing of a given feed material of which molar flow rates are listed in Tables 1 and 2. The design outcome of the FTCDC system shown in Fig. 3 is summarized in Table 2 along with the result for a conventional two-column system demonstrated in Fig. 4. The molar flow rates of products from both systems are listed in Table 1. For the performance comparison between the FTCDC and the conventional system, the profile of tray liquid composition of the FTCDC is shown in Fig. 5. The circles and the ‘×’ symbols indicate prefractionator and the main column. The profile of the upper section of the prefractionator is different from the profile of the minimum tray design described in Fig. 2. The influence of material balance in a practical distillation is largely responsible for the difference. While the tray composition of an ideal column is determined from equilibrium relation, practical distillation is obtained from both of the equilibrium and material balance. The profile of a two-column conventional system is plotted in Fig. 6. The circles and the ‘×’ symbols indicate the first column and second column. Notice that the composition difference between feed and feed tray. The composition mis-match in the tray leads to a mixing that causes thermodynamic inefficiency. As the irreversible mixing reduces the thermodynamic efficiency of distillation, the large difference shown in Fig. 6 indicates the low efficiency of the conventional system. The reduction of thermodynamic efficiency is caused mainly from the mixing at feed tray and the remixing of intermediate components [3]. The feed tray mixing is indicated by the distance between the feed composition marked with F and the feed tray composition of the circle closest to the feed composition in Figs. 5 and 6. Apparently the conventional two-column system has more mixing. The remixing is shown from the profiles of lower section of the prefractionator of the FTCDC and the lower section of the first column in the conventional system. In the profile the composition of intermediate components increases first and then decreases as the profile moves down the column. Namely, the components are refined and remixed to lower thermodynamic efficiency of the column. More remixing is also shown in the conventional system. From the outcome listed in Table 2, the heat duty of the FTCDC is 13% less than that of the conventional system. For the same products from the same feed with the same number of total trays, less energy is necessary in the FTCDC

J.Y. Lee et al. / Chemical Engineering and Processing 43 (2004) 495–501

Fig. 3. Schematic diagram of a fully thermally coupled distillation column and list of operating conditions.

Fig. 4. Schematic diagram of a conventional two-column system and list of operating conditions.

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1

S Intermediate

1

0.8

0.8

0.6

0.6

D2

Intermediate

B1

0.4

F

0.2

0.2 B 0 0 Heavy

F

0.4

B2 0 0 Heavy

D 0.2

0.4

0.6

0.8

1 Light

D1 0.2

0.4

0.6

0.8

1 Light

Fig. 5. Liquid composition profile of designed FTCDC in actual trays. The circles and ‘×’ symbols indicate prefractionator and the main column.

Fig. 6. Liquid composition profile of conventional two-column system in actual trays. The circles and ‘×’ symbols indicate first column and the second column.

system as previous studies stated [1,3,4]. The temperature of reboiler in the FTCDC is higher than that of the first column in the conventional system, and the steam for the reboiler may cost more. But in this case the same steam of intermediate pressure is utilized in both reboilers leaving no increase of the utility cost from the pressure increase, because both steam pressures are grouped as an intermediate pressure steam which has the same price [20]. While reboiler pressure in the FTCDC is 180 kPa, the pressure of the first column is 170 kPa. Though the total number of trays is same for the conventional system and the FTCDC, the reduction of heat duty lowers the investment cost for the construction of a reboiler and a condenser by 4%. For the examination of economic benefit from the introduction of the FTCDC, utility and investment costs are calculated with the formulas given in Kim and Luyben [21] adopted from Douglas [20]. The steam cost is US$ 5 per one

million Btu, and the column operation is 300 days a year with 7 years of payout period. The detail of cost calculation result is listed in Table 3. Table also contains the variation of the cost with different design factors from the minimum tray structure. As indicated in the last row of the table, the total cost is not much different from the factors between 1.9 and 2.3. Therefore, the design with the factor of 2 utilized as a practical design guideline is acceptable. Though the FTCDC has widely been applied for the separation of ternary mixtures, it can be implemented to multi-component systems. In practical industrial systems, it is not uncommon to have systems of many more components. A successful application of the FTCDC to the fractionation process of this study indicates that the energy conservative system can be applied to many commercial processes not limited from the number of components.

Table 3 Result of cost evaluation with different design factor and of conventional system Factor Tray no. Prefractionator Main Vapor rate (kmol/h) Prefractionator Main Duty (GJ/h) Reboiler Condenser Cost (US$ 1000) Column Tray Heat exchange Total investment Utility (year) Total (year)

1.7

1.8

1.9

2.0

2.1

2.2

2.3

Conv.

18 76

19 81

20 85

21 89

22 93

23 97

24 102

60 50

491 2332

491 2063

491 1673

493 1634

496 1630

487 1625

484 1618

595 1315

84.7 82.5 1915 321 1999 4235 2890 3495

74.9 76.3 1901 313 1877 4091 2556 3141

60.8 58.5 1795 283 1604 3682 2074 2600

59.4 56.9 1843 291 1577 3711 2025 2555

59.2 56.8 1908 304 1575 3787 2020 2561

59.0 56.7 1969 316 1572 3857 2014 2565

58.8 56.5 2044 331 1568 3943 2005 2569

20.7, 47.8 14.8, 47.2 1560 209 2101 3870 2337 2890

J.Y. Lee et al. / Chemical Engineering and Processing 43 (2004) 495–501

3. Conclusions The structural design procedure for a fully thermally coupled distillation system is applied to the industrial-scale fractionation process of a naphtha reforming plant. Because the commercial design software HYSYS requires the structural information in the beginning of the simulation project formulation, the procedure yielding the information helps the HYSYS design process of the FTCDC. The design outcome and performance comparison with a conventional system indicate that the proposed design procedure is useful to implement the FTCDC for industrial applications. The economics study shows the energy saving of 13% and the investment cost reduction of 4%.

Acknowledgements Financial support from the Korea Science and Engineering Foundation (Grant No. ROI-2003-000-10218-0) and partially through the CANSMC is gratefully acknowledged.

Appendix A. Nomenclature A B D F K L NT NT2 S V x y z

amount of component A in feed (kmol/h) amount of component B in feed (kmol/h) or bottom product overhead product feed flow rate (kmol/h) or feed equilibrium constant liquid flow rate (kmol/h) number of trays in a main column number of trays in a prefractionator side product vapor flow rate (kmol/h) liquid composition (mol frac.) vapor composition (mol frac.) feed composition (mol frac.)

Greek letters α relative volatility φ solution of the Underwood equation Subscripts A component A B component B C component C D overhead F feed i component i j component j n tray number from bottom

NF 1 2

501

tray number of feed solution number of Underwood equation solution number of Underwood equation

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