Experimental and Theoretical Studies of a Dividing-Wall Column Used

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Ind. Eng. Chem. Res. 2010, 49, 6566–6577

Experimental and Theoretical Studies of a Dividing-Wall Column Used for the Recovery of High-Purity Products Gerit Niggemann,*,† Christoph Hiller, and Georg Fieg Institute of Process and Plant Engineering, Hamburg UniVersity of Technology, Schwarzenbergstrasse 95c, 21073 Hamburg, Germany

Although dividing-wall columns are now well established in industrial practice, their complex hydraulics is not well understood, and open literature does not provide any quantitative information in this respect. This study aims to provide the missing knowledge. Hence, a pilot plant was built to separate a ternary mixture of fatty alcohols into high-purity products of ∼99 wt %. A model was established to simulate this separation process. It could successfully describe the experiments quantitatively and even account for the self-adjustment of the vapor splits. A case study with the validated model highlighted the strong influence of the heat transfer across the vertical partition wall on hydrodynamics and vapor distribution. These aspects are of special interest for the design and scale-up of dividing-wall columns. 1. Introduction Distillation remains the most commonly used separation process in the chemical industry, although it has a major drawback in that it has significant energy requirements. This is especially so for the separation of homogeneous multicomponent mixtures into more than two products, which is traditionally realized in a series of distillation columns. The use of fully thermally coupled distillation arrangements, such as the Petlyuk configuration, leads to significant reductions in both energy and capital costs when compared to conventional two-column arrangements in the separation of ternary mixtures.1 The Petlyuk column is thermodynamically preferable because backmixing of the intermediate product is avoided due to prefractionation of the feed. Theoretical studies have revealed that this column configuration is capable of achieving energy savings of up to 30%.2-4 The most practical implementation of the Petlyuk column results in a dividing-wall column (DWC).5 Some examples of dividing-wall columns in industrial processes have confirmed the economic advantages of this technology for both grass root designs and revamps of conventional columns.6-9 The design of a DWC can be achieved by inserting a vertical partition in the column shell.2 Therefore, the difference between a conventional distillation column and a DWC is found in the column internals. Because the vertical partition separates the column into a feed and a side stream section, dividingwall columns are thermodynamically equivalent to Petlyuk columns. The vertical wall avoids radial mixing of vapor and liquid streams and enables the withdrawal of three products with any thermodynamically feasible purity in one column.10 A DWC is especially favorable in the case that the middle-boiling component represents the main component and a high-purity middle product is required.11 Although theoretical studies have demonstrated the economic advantages of dividing-wall columns, industry has hesitated to build the columns due to a lack of knowledge in operation and control.12 However, this situation has improved over the past few years as several studies about the design, operation, and control of DWC have been published.13-22 Furthermore, a pilot* To whom correspondence should be addressed. Tel: +49 40 428783241. Fax: +49 40 42878-2992. E-mail: [email protected]. † Current address: Evonik Degussa GmbH, DG-TE-VT-F, HanauWolfgang, Germany.

scale column was set up to study operational and control aspects experimentally.23 Another study examined the separation of a quaternary mixture in a lab-scale DWC with two side streams.24 Recent papers have revealed that the combination of reaction and distillation in dividing-wall columns offers further potential in terms of process intensification.25,26 Research and development of dividing-wall columns is not solely restricted to academia; industry has also shown a strong interest. The successful implementation of unfixed wall technology by J. Montz GmbH increased the industrial implementation of dividing-wall columns.27 Currently, more than 70 packed DWCs are operated by BASF worldwide.28 However, an important issue that has not yet been addressed is an extensive model validation for a DWC in combination with a detailed process analysis of specific DWC features, such as the self-adjusting vapor distribution and the heat transfer across the dividing wall. This is exemplified by the quotation of two authors in the field of dividing-wall columns to emphasize this demand: Abdul Mutalib et al.23 mentioned that “exact correspondence between simulation and pilot plant ... would require a more rigorous simulation model and a pilot plant with equipment that allows access to more operating data and with a higher accuracy”. Mueller and Kenig25 agreed that “it is difficult to evaluate the role of the heat transfer with only few experimental results available” and postulated that “significantly more experimental data are necessary to evaluate the model performance and the phenomenon of heat transfer through the dividing wall”. The primary objective of the present work is to close this gap. Thus, for the first time, an integral analysis is presented which provides the basis for better understanding of the process behavior of dividing-wall columns. Here, “integral” means to consider both experimental data and simulation results, which allows for the examination of the complex hydrodynamics of the column and significantly broadens the knowledge about DWC technology. This requires a devoted experimental effort using equipment, measuring devices and control technology that allow flexibility and accuracy in this respect. Such an experimental setup was built at the Hamburg University of Technology, and a ternary mixture of fatty alcohols was chosen to serve as the test system because it represents a mixture of industrial relevance and the demand for high-purity products in fine and commodity chemicals has grown within the past few years.

10.1021/ie1003416  2010 American Chemical Society Published on Web 06/18/2010

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010

Several feed stream compositions were examined while fulfilling the key components’ product specifications of ∼99 wt %. 2. Pilot Plant Recently, a pilot plant dividing-wall distillation column was set up at the Hamburg University of Technology. It was aimed at detailed studies of the entire process behavior of dividingwall distillation columns comprising the steady-state operating points and the process dynamics including the start-up. The gained experimental data were used for a detailed process analysis and an extensive validation of a rigorous process model and also to further the identification of operational problems. These aspects are indispensable for helping to establish an innovative and promising process, such as the DWC. Furthermore, a detailed concept of the entire plant was developed, which consisted of the following components: a P&I flowsheet of the pilot plant, a concept of the process measuring and control technology, and sketches for plant design and setup. 2.1. Setup. Figure 1 shows the P&I diagram of the pilot plant. The pilot plant consists of a stainless steel column shell, a total condenser (HX2), a reboiler (HX1) and a distillate vessel (V2). The DWC has a height of ∼12 m and can be divided into several sections from top to bottom (S1-S7 and LS). A welded wall in the middle part of the column divides the column vertically into two semicylindrical segments, the prefractionating segment and the main column, and avoids radial mixing of the vapor and liquid streams across the entire cross-sectional area of the column. The inner diameter of the distillation column is 68 mm, and the four column sections each contain a 980 mm bed consisting of Montz B1-500 structured packing. Immediately above the partition wall, a funnel is placed, which is moved by two electromagnets to facilitate the liquid distribution to each side of the dividing wall. The magnets are fixed on opposing sides of the outer column jacket. The cycle time of the funnel controls the liquid flow to the prefractionator and the main column section. Combined types of liquid collectors and distributors in the column guarantee both a uniform liquid distribution on each packing section and the addition or removal of liquid streams as feed, distillate, and side stream. The vapor is totally condensed in a shell and tube heat exchanger (HX2) at the top of the column. The condenser is operated with cooling water, and the condensate is collected in the distillate vessel (V2). The liquid in the reboiler (HX1) is heated by electrical flange connection heaters, which have a nominal power of 1.9 kW. The circulation pump (P8) assures proper mixing of the liquid and avoids hot spots in the reboiler. A two-stage rotary vane vacuum pump (P1) is used to obtain the desired operating pressure in the column. Five gear pumps (P2-P6) are used for conveying the liquid inlet and outlet streams of the column. The column jacket, the reboiler, and the flanges are insulated with mineral wool to reduce heat losses. 2.2. Measurement and Control Devices. The distillation process in a DWC is a complex multivariable system with several process variables. Thus, the pilot plant is equipped with elaborate process measuring and control technology to enable the extensive and detailed analysis of the process experiment. The column is operated by the process control system LabVIEW (National Instruments). All measured variables are recorded with a sampling time of 2 s. The measurement and control devices consist of the following elements: All liquid inlet and outlet streams are measured by determining the mass flow. The liquid level in the distillate vessel (V2) and in a bypass tube of the reboiler (V7) are measured by using sensors based on high-frequency microware pulses whereby the

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pulses are reflected by the product surface and the time from emission to reception of the signals is proportional to the liquid level in the corresponding vessel. The reflux stream is chosen to control the liquid level in the distillate vessel and the bottom stream is used as the manipulated variable for the liquid level in the reboiler. The top pressure is measured by using a capacitive pressure sensor. The pressure is controlled with a magnetic valve. Two sensors record the pressure drop during pilot plant operation, where PDIR34 measures the pressure drop between the top and bottom of the column and PDIR33 determines the pressure drop between the top of the column and the feed stage. The pressure drop of the condenser can also be considered due to the valve position. Liquid samples are taken from the feed and from all product streams. The samples are analyzed by using gas chromatography. Finally, the column is equipped with several Pt-100 resistance temperature detectors according to the flowsheet in Figure 1. Three temperature sensors are installed in every packing section to guarantee sufficient process information. 2.3. Component System. In the pilot plant, a model ternary mixture of the linear fatty alcohols n-hexanol (C6), n-octanol (C8), and n-decanol (C10) was chosen for investigation because the high-purity separation of this system is of industrial relevance. Therefore, target purities of around 99 wt % were set for the steady-state operation. In industry, fatty alcohols are produced by using two different feed stocks. The oleochemical route implies the splitting and hydrogenation of vegetable fats to obtain the desired fatty alcohols. Alternatively, fatty alcohols based on a petrochemical feed stock are synthesized (e.g. by using the Ziegler-Alcohol Process, the SHOP-Process or the oxo-Process29). The worldwide production is about 2.5 million t/a, whereby oleochemical and petrochemical feed stocks are used in equal parts.29 Fatty alcohols play an important role in the cosmetics, detergents, coatings, and colorants industries as intermediate products. Usually fatty alcohols are separated under vacuum due to their thermal instability. Thermal decomposition can be avoided at operating temperatures below approximately 230-240 °C. According to their normal boiling points from lowest to highest, n-hexanol is obtained as the top product, n-octanol is obtained as the side stream product, and n-decanol is obtained as the bottom product. 2.4. Experimental Results. The experimental part of this study focused on proving the operability of a DWC for several steady-state operating points with different operating parameters while keeping the desired key components’ purities of ∼99 wt %. Furthermore, the experimental results shown in this section served as a reliable basis for the model validation. In theory, steady-state operating points of chemical engineering plants are characterized by constant process variables without any variations in time when the required product specifications are generally fulfilled. However, in reality, industrial-scale plants are permanently subjected to time-variant changes or disturbances. The necessary indicators and conditions for an experiment being called steady-state are constant column temperatures, constant pressure drops, and constant product qualities. The crucial criterion in this study is provided by successful data reconciliation of component and total mass balances, which determines whether the pilot plant is at steady-state conditions, despite inherent process variations. Data reconciliation is an optimization problem and subject to the corresponding constraints; it can be defined as finding the minimum of the sum of the squared error between the measured data and “reconciled data”.30 Typically, a weighting factor is applied to the measured

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Figure 1. Flowsheet of the pilot plant.

data to define the accuracy of each of the measured data to generate a set of reconciled data. The classical approach to solving such constrained optimization problems, the method of Lagrange multipliers, is employed in these studies.

Table 1 displays several steady-state operating points for different feed stream compositions, feed flow rates, and reboiler heat duties while maintaining on-specification operation. The feed stream scenarios can be classified in categories A-J as

J H G G F E D C C B A feed scenario

C6dC8 > C10 C6 ≈ C10 > C8 C8dC10 > C6 C8dC10 > C6 C8 . C6dC10 C8 . C6dC10 C8 > C10 > C6 C6 ≈ C8 > C10 C8 ≈ C6 > C10 C6dC8dC10 C8dC10 > C6 feed (kg/h) 2.50 3.04 3.00 3.87 3.00 3.06 3.05 2.50 2.50 3.00 2.58 wF,C6 (wt%) 41.1 37.5 24.8 25.0 30.8 25.4 25.4 36.2 35.4 34.0 29.9 wF,C8 (wt%) 39.7 21.5 37.9 38.1 40.0 50.1 45.0 38.9 38.3 32.0 34.7 wF,C10 (wt%) 19.2 41.0 37.3 36.9 29.2 24.5 29.7 24.9 26.3 34.0 35.4 distillate (kg/h) 1.03 1.14 0.74 0.96 0.92 0.77 0.77 0.90 0.88 1.02 0.77 wD,C6 (wt%) 99.5 100 99.5 99.7 100 100 99.9 99.8 100 100 99.8 wD,C8 (wt%) 0.5 0 0.5 0.3 0 0 0.1 0.2 0 0 0.2 side stream (kg/h) 0.99 0.65 1.13 1.46 1.20 1.54 1.37 0.97 0.96 0.96 0.90 wS,C6 (wt%) 0.6 0.6 0.5 0.6 0.6 0.5 0.4 0.4 0.4 0.6 0.4 wS,C8 (wt%) 99.4 99.3 99.1 99.2 99.4 99.4 99.6 99.5 99.6 99.4 99.2 wS,C10 (wt%) 0 0.1 0.4 0.2 0 0.1 0 0.1 0 0 0.4 bottom stream (kg/h) 0.48 1.25 1.13 1.44 0.88 0.75 0.92 0.63 0.66 1.02 0.91 wB,C8 (wt%) 0.5 0.4 1.1 1.2 0.7 0.3 1.3 0.4 0.9 0.4 0.3 wB,C10 (wt%) 99.5 99.6 98.9 98.8 99.3 99.7 98.7 99.6 99.1 99.6 99.7 ptop (kPa) 8 8 8 8 8 8 8 8 8 8 8 liquid split (s:s) 3:3 3:3 3:3 3:3 3:3 3:3 3:3 3:3 3:3 3:3 3:3 reflux ratio (-) 3.3 2.8 3.4 3.5 4.4 4.3 4.7 4.0 4.2 3.1 4.0 Qreboiler (kW) 1.33 1.33 1.14 1.33 1.43 1.33 1.33 1.33 1.33 1.33 1.28 ∆p33 (kPa) 0.58 0.55 0.2 0.46 0.43 0.5 0.6 0.6 0.36 ∆p34 (kPa) 0.83 0.72 0.42 0.74 1.15 0.6 0.78 0.95 0.96 0.72 0.6

9 4 3 2 1 no. of parameter set

Table 1. Operating Parameters and Results for Experimental Steady-State Runs

5

6

7

8

10

11


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Figure 2. Experimental temperature profiles for feed scenario B (left) and E (right).

follows: A, B, C, G, and J have the same fraction of two components in the feed stream and a lower fraction of the third component; D, E, and F represent the preferred feed mixture in a DWC from an energy point of view, with in part high amounts of octanol; and H represents a feed mixture with almost equal mass fractions of all components. The results in Table 1 show clearly that the designed DWC guarantees the desired product purities for various operating conditions. Furthermore, experiments 8 and 9 (representing feed scenario G) point up the reproducibility of the entire set of experiments. Experiments 3 and 4 (representing feed scenario C) show the separation efficiency to be unaffected by feed flow rate changes if the reflux ratio remains constant. The results in Table 1 merely represent a small number of the conducted experiments. For the sake of clarity, further experimental results are not presented in detail here. However, these results are considered when the pressure drops and the condensate stream at the top of the column are discussed. Then, the entire set of experiments is called “all parameter sets” to distinguish them from the parameter sets listed in Table 1. Figure 2 illustrates two typical temperature profiles for feed scenarios B and E. The graphs look very similar when comparing the profiles for both scenarios in that both plots exhibit a maximum temperature difference of around 5 K between the prefractionator and the main column. However, it is noticeable that the temperatures of the prefractionator in B are lower than that of the main column at the top segment (stages 7-9) and are conversely higher at the bottom segment (stages 12-14). In contrast to this, in E, the temperatures of the prefractionator in both the top and bottom segments are always lower than the temperatures found in the main column. Moreover, the vertical double S-shape of the temperature profiles of the main column indicates two regions, where sharp separations occur: the separation of hexanol and octanol in the upper part of the column and the separation of octanol and decanol in the lower part. Figure 3 shows the column pressure drops at steady-state conditions for all experiments performed at the pilot plant, not only for experimental runs listed in Table 1. Usually, the pressure drop is primarily affected by the reboiler duty. Noticeable is the fact that experiments with a fixed reboiler heat duty of 1.33 kW produced pressure drop results for PDIR33 and PDIR34 that varied within a range of ∼0.2 and ∼0.15 kPa,

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Figure 3. Experimental pressure drops of the dividing-wall column operation for different reboiler duties considering all parameter sets.

respectively. An obvious explanation or a conclusion for this phenomenon cannot be drawn, despite the large experimental effort. The vapor distribution has not yet been discussed, although it is a vital process variable of dividing-wall columns. However, the vapor distribution cannot be measured explicitly, and a more detailed analysis based solely on the experiments is not possible. This emphasizes the relevance of the proposed integral approach consisting of experimental and simulations studies to properly understand the DWC process and especially its complex hydrodynamics. 3. Modeling and Validation The identification and interpretation of the pressure drop and the vapor distribution are realized in the validation and simulation studies by means of a rigorous process model. Furthermore, the process model is used to predict the product purities, the concentration, and temperature profiles in a dividing-wall distillation column. The comparison with experimental and simulation data is performed for a wide range of different operating conditions (see Table 1) and is aimed at checking the integrity of the model against the experimental results and at increasing the acceptance of the simulation results. 3.1. Process Model. In earlier studies, the authors have already presented a rigorous process model, which allows for a detailed analysis of the DWC startup.31 The model consists of various elements: equilibrium stages, collectors and distributors, a reboiler, and a condenser. These elements can be arbitrarily combined to simulate any desired column setup. The model is described by ordinary differential equations obtained from mass and energy balances of each element and a set of algebraic equations that are used to predict the physical properties, the vapor-liquid equilibrium and the column hydraulics. The MESH equations for the equilibrium stage model, in which the theoretical stages are numbered from top to bottom, have been set up. Similar model equations are used to describe the other elements (reboiler, condenser, collectors, and distributors). The highly nonlinear differential algebraic equation system is implemented in the commercial software tool Aspen Custom Modeler. The model obtains the required physical and thermodynamic properties of the components via an interface from Aspen Properties. The model is capable of describing the process characteristics of dividing-wall columns, such as the heat transfer across the dividing wall, the liquid distribution above the wall, and the self-adjustment of the vapor distribution below the wall. The heat transfer across the vertical partition may not be neglected due to its impact on the temperature profiles and the required reboiler duty.25,32 The liquid split can be used as a

Figure 4. Experimental and simulated condensate flows at the top of the dividing-wall column considering all parameter sets.

manipulated variable for improving the separation efficiency of the column and for reducing the required reboiler duty. Unlike most published works, in which the vapor split is also considered as a manipulated parameter to simplify the model, here, the selfadjusting vapor split is calculated by the model according to the condition of equal pressure drop on both sides of the dividing wall. 3.2. Model Validation. The basis for a successful model validation is given by the extensive experimental data from the pilot plant. Therefore, the measured operating parameters are used for initializing the process model. Because the pilot plant is equipped with structured packing (Montz B1-500), the column model is discretized into equilibrium stages according to the HETP (height equivalent to a theoretical plate) value provided by the manufacturer, according to which five theoretical stages are contained in each of the packed beds. The separation efficiency of the packing remains constant over a wide range of operation, which justified the assumption of a fixed HETP value without considering the dependency of the vapor load on the separation efficiency. Determining the heat loss of the column is an important aspect for the model validation and is based on accurate analysis of all experiments, not only on those that are shown in Table 1. The first information about the heat loss is given by an energy balance of the DWC. In a second approach, the heat loss is determined on the basis of convective heat-transfer coefficients and the surface temperature profile of the column’s insulation material, which was recorded by a thermographic camera. The second approach resulted in heat losses of about 250 W over the length of the column, which corresponds to ∼20% of the introduced reboiler heat duty. Finally, the overall heat-transfer coefficient is determined, and the heat loss can be described by the following equations, where i denotes an element of the column internals, a theoretical stage, or a collecting and redistributing element Qloss,i ) 5.5 W m-1 K-1(Ti - Tambient) li

(1)

Qloss,reboiler ) 2.45 W K-1(Treboiler - Tambient)

(2)

Equations 1 and 2 allow for an explicit correlation of the heat losses over the entire column height, including the reboiler. As expected, Figure 4 shows a linear relationship between the delivered heat duty and amount of condensate. The prerequisite for this linearity is based on the fact that all evaluated experiments are performed with the same component system


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Table 2. Deviation of the Relative Difference between Experimental and Simulation Results relative error ) (simulation - experiment)/experiment process variable

Figure

mean dev.

standard dev.

min. dev.

max. dev.

condensate stream pressure drop: ∆p33 pressure drop: ∆p34 purity of key components temperatures

4 5 (left) 5 (right) 6a, 6b, 6e 7

0.81 × 10-2 0.073 × 10-2 -0.41 × 10-2 -0.036 × 10-2 0.078 × 10-2

3.01 × 10-2 0.40 × 10-2 1.21 × 10-2 0.13 × 10-2 1.41 × 10-2

-5.89 × 10-2 -0.64 × 10-2 -4.59 × 10-2 -0.31 × 10-2 -4.42 × 10-2

8.08 × 10-2 0.81 × 10-2 1.24 × 10-2 0.25 × 10-2 1.78 × 10-2

and top pressure. Experimental data and simulation results correspond very well to each other (see also Table 2). Furthermore, pressure drop correlations are especially essential for dividing-wall columns to describe the hydraulic profiles in the column properly and, finally, to obtain reliable information about the vapor split. The pressure drop for the packing was calculated according to the correlation given by Mackowiak,33 ∆p ) L

{

[ () [

]

2 3 1/3 a2/3 1 - ε Fvap 1 (ν u )1/3 g ε liq liq ε3 dp 2 -5 a1/3 1 - ε Fvap 1 - CB u2/3 ψ 3 liq dp ε ε

ψ

]

-5

∀Reliq < 2 ∀Reliq g 2

}

(3) with 2 ψ ) K1ReKvap

(4)

Figure 5 illustrates that it is not sufficient just to consider the pressure drop in the irrigated packed bed because the main contribution of the column pressure drop is not caused by the packing. Collection and redistribution units in packed columns definitely affect the overall pressure drop and should not be neglected on the lab scale. This is especially true if the free cross-sectional area available for the vapor flow is dramatically constricted by the liquid redistribution units in comparison with the cross-sectional area of the column.34 Special types of liquid collectors and distributors, which might result in higher pressure drops, are used in the pilot plant; however, pressure drop correlations for these ancillary equipment are not available in the open literature. Therefore, it is necessary to describe these pressure drops empirically. As a promising approach, the following pressure drop

correlation was set up for the nonseparating column internals, the distributor, and collector sections: ∆pcd ) RFβvap,cd

(5)

The parameters R and β were fitted to the experimental data so that eq 5 can be used to describe the entire range of experimental conditions. The parameter set with R ) 0.02 and β ) 3 expresses properly the pressure drop, including the column operation at higher vapor loads. In comparison, values of R ) 0.05 and β ) 2 correspond to the pressure drop only for lower reboiler heat duties. As a result of incorporating this pressure drop correlation for the collectors and distributors into the model, it was possible to achieve good agreement between the experimental and simulation pressure drop data (see Figure 5 and Table 2). The second important process variable of dividing-wall columns is the heat transfer across the dividing wall, especially when operating a small-scale column made of stainless steel. The heat transfer across the dividing wall, Qdw,i, is described by eq 6, where i denotes an element of the column internals, a theoretical stage, or a collecting and redistributing element. The overall heat-transfer coefficient is calculated by fundamental heat-transfer equations for a plane wall and resulted in kdw ) 700 W m-2 K-1. Qdw,i ) kdwAdw,i(Tprefrac,i - Tmain col,i)

(6)

In the following figures, the experiments listed in Table 1 are compared with simulation results in detail. For this purpose, the process model is parametrized with feed stream composition and mass flow rates based on data reconciliation. All operating parameters specified in the model correspond to the data mentioned in Table 1. Further parameter fitting, in addition to the one for the pressure drop correlation of the collectors and distributors, is not carried out. The comparison for the compositions in the product streams shows that simulation results and experimental data are in very good agreement with each another (Figure 6 and Table 2). The experimental and simulated temperature profiles for experiments of feed scenarios B and E are depicted in Figure

Figure 5. Experimental and simulated pressure drops from top to feed (∆p33) and from top to bottom (∆p34) considering all parameter sets.

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Figure 6. Experimental and simulated purities of key components hexanol in the distillate stream (a), octanol in the side stream (b), decanol in the bottom stream (e), and of nonkey components hexanol (c) and decanol (d) in the side stream for the steady-state runs.

7. Again, the results are in good agreement with each other, and they exemplify the congruence of all of the other steadystate runs (see also Table 2). Figure 8 shows the resulting self-adjusting vapor splits for the different experiments. Here, the vapor split ratio is defined as the ratio between the flow rate to the feed stream segment and to the total flow rate in the column. Thus, a split ratio of 1 means that the entire stream is fed to the feed stream segment, whereas a value of 0 means that the total flow enters the side stream section. Figure 8 illustrates noticeable changes of the vapor split values in the range from 0.35 to 0.52, although process parameters that influence the vapor split, such as the cross-sectional area ratio in the dividing-wall section and the liquid split, are kept constant during all experiments. The cross-sectional areas in the prefractionator and the main column were the same because of the central positioning of the dividing wall, and liquid split values of 0.5 should have resulted in constant vapor split values of ∼0.5. This extremely interesting phenomenon is the subject of further examinations in the next section.

Figure 7. Experimental and simulated temperature profiles for feed scenarios B (left) and E (right) from Table 1.


Figure 8. Simulated vapor splits for different steady-state runs. Table 3. Simulation Parameter Sets for the Case Study case no. kdw Rb βb

a

Wm

-2

-1

K

1 (ref)

2

3

4

700 0.02 3

0 0.02 3

700 0 0

0 0 0

a Overall heat-transfer coefficient for heat transfer across the dividing wall; see eq 6. b Parameters for pressure drop correlation in collectors and distributors; see eq 5.

4. Process Analysis Here, a sensitivity analysis of process parameters is carried out by using the validated process model. The main objectives are to systematically identify the influence of both the heat transfer across the dividing wall and the pressure drop in the dividing-wall segment and, finally, to reveal some fundamental aspects of the DWC hydrodynamics. Lestak et al.32 focused on analyzing the influence of the heat transfer across the dividing wall on the required reboiler heat duty while maintaining the product specifications of 95 wt %. The parameter study of Mueller and Kenig25 showed the influence of the heat transfer on the temperature profiles when analyzing an experimental parameter set from Abdul Mutalib et al.23 However, data on the hydrodynamics in a DWC has not yet been published in the open literature. The sensitivity analysis based on the simulation parameters listed in Table 3 will now be presented. The parameter set “1 (ref)” in the first column of Table 3 serves as a reference, and its simulation results have already been extensively and successfully validated with experimental data in the previous section. The other parameter sets neglect either the heat transfer across the dividing wall (case 2), the pressure drop in the nonseparating column internals (case 3), or both (case 4). First, the calculated pressure drops are compared and discussed by means of the sensitivity analysis. Furthermore, the

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simulation results for the self-adjusting vapor split are considered in the discussion. Subsequently, an elaborate and detailed examination of the hydrodynamics in a dividing-wall column is presented by analyzing vapor load, liquid load, and pressure profiles. Two feed scenarios are selected to emphasize the influence of the heat transfer across the dividing wall on the temperature profiles. The amount of heat transferred across the dividing wall is quantified and compared with the net reboiler heat duty. Finally, the significance of the obtained results is evaluated with respect to industrial application. As can be seen in Figure 9, the pressure drop in the pilot plant is primarily affected by the collectors and distributors. Hence, the pressure drop changes significantly with increasing reboiler heat duty for cases 1 and 2, but only slightly for cases 3 and 4. Furthermore, Figure 9 illustrates the effect that one specific reboiler duty (e.g. 1.33 kW) resulted in different pressure drops for cases 1 and 2. Pressure drop ∆p33 varies for the heat duty of 1.33 kW for case 1 within a range of up to 0.2 kPa, and for case 2, within a range of up to 0.1 kPa. This effect is caused by the pressure drop in the nonseparating column internals (case 1 and 2), and it is amplified when considering the heat transfer across the dividing wall (case 1), but it is not observed for cases 3 and 4. The comparison of pressure drops ∆p33 and ∆p34 reveals that the heat transfer across the wall influences the pressure in the dividing-wall segment, but this effect is not observable for the overall column pressure. The absolute pressure values in Figure 10 emphasize this fact. Figure 11 provides a fundamental insight into the vapor split behavior. On one hand, the vapor split results in almost constant values of ∼0.52 when neglecting heat transfer across the wall (cases 2 and 4). On the other hand, the vapor split values are subject to extreme variations when heat transfer is considered (cases 1 and 3). These extreme variations are further amplified when the pressure drops of the collectors and the distributor are considered (case 1). The comparison of cases 1 and 2 shows that the values deviate by up to ∼33%, regardless whether heat transfer across the wall and pressure drop in the nonseparating column internals are considered. The reason for the vapor distribution behavior will be given below, where a closer look is taken into the hydrodynamics. Although the operating parameters of the various experimental runs in Table 1 are different, which obviously implies different hydraulic profiles, the simulated profiles for vapor load, liquid load, and pressure are quite similar from qualitative and quantitative points of view. Feed scenario E is chosen as an example, but the results are representative for all the other runs. Figure 12 illustrates the significantly higher values for vapor and liquid load in the nonseparating column internals. If the heat transfer across the dividing wall is neglected, the vapor

Figure 9. Pressure drops from top to feed (∆p33) and from top to bottom (∆p34) considering all parameter sets for the cases in Table 3.

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Figure 10. Pressure on the feed stage and in the reboiler for the cases in Table 3.

Figure 11. Comparison of different vapor splits for the case study listed in Table 3.

Figure 13. Pressure drop and pressure profile in a dividing-wall column with heat transfer (1 (ref)) and without heat transfer across the dividing wall (2).

Figure 12. Vapor and liquid load in a dividing-wall column with heat transfer (1 (ref)) and without heat transfer across the dividing wall (2).

load on both sides of the vertical partition is almost identical. However, the profile of the vapor load is influenced by the heat transfer across the vertical wall. On the basis of the temperature driving force across the dividing wall, some vaporization effects occur in the upper part of the prefractionator which increase the vapor load. The higher temperatures in the lower part of the prefractionator cause heat transfer across the wall, resulting in vaporization effects in the lower part of the main column and an increase in the vapor load. It should be noted that vaporization effects on one side of the dividing wall due to heat transfer cause condensation effects on the other side of the dividing wall and vice versa.

The liquid load profiles where heat transfer is not considered will be discussed first. The liquid load in the column increases in the direction of the sump. This is expected because a part of the ascending vapor subsequently condenses due to the heat losses of the column and lead to an increasing internal reflux in the lower part of the column. The liquid load in the upper dividing-wall segment is the same on both sides due to a liquid split of 0.5 and a central positioning of the wall. In the lower side of the prefractionator, the load increases because of the feed stream, whereas the load in the main column decreases due to the removal of the side stream. In contrast to these observations, in the case that heat transfer is considered, the liquid load profiles in the prefractionator and main column show no significant deviation from each other. This is because of the aforementioned condensation and vaporization effects due to heat transfer across the dividing wall. It can thus be concluded that the heat transfer across the dividing wall significantly influences hydrodynamics in a DWC. Figure 13 depicts the strong influence of the collectors and distributors on the pressure drop and the column pressure profile. This was previously indicated by the profiles of the vapor load. Unlike the earlier diagrams, the x-axis pressure drops in the left subplots of Figure 13 and Figure 14 do not depict the accumulated pressure drop; rather, they pertain to the pressure drop per element, which is correlated with vapor and liquid load for the corresponding stage or unit in Figure 12. The pressure drop and the pressure profiles in Figure 14, which consider only the pressure drop in the packing section, are depicted for cases 3 and 4; that is, with and without heat


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increases and the purity of the key component decanol in the bottom decreases without heat transfer across the wall. To quantify the amount of heat transferred across the dividing wall, the following variables are considered: Qdw,net )

∑k

dwAdw,i(Tprefrac,i

- Tmain col,i)

(7)

i

Qdw,total )

∑ i

Figure 14. Pressure drop and pressure profile in a dividing-wall column with heat transfer (3) and without heat transfer across the dividing wall (4) but neglecting the pressure drop in collectors and distributors.

Figure 15. Temperature profile in a dividing-wall column with heat transfer (1 (ref)) and without heat transfer across the dividing wall (2) for feed scenarios B (left) and E (right).

transfer. Furthermore, the pressure drop is not equally distributed over each packing section; some theoretical stages dominate the pressure drop profile. It can also be seen in Figure 14 that the pressure drop profiles with regard to heat transfer behave diametrically to those when neglecting heat transfer. Consequently, this explains qualitatively and quantitatively how the heat transfer across the dividing wall affects the hydraulics and ultimately the vapor split in a DWC. The huge impact of heat transfer on the temperature profiles, especially for the prefractionator, in a DWC can be seen in Figure 15. The temperature profiles between the prefractionator and the main column differ significantly from each other, with a maximum temperature difference of ∼20 K for both feed scenarios, in the case that heat transfer is not considered. Additionally, the vertical double S-shaped temperature profiles without heat transfer across the wall are shifted to lower temperatures, especially for feed scenario E, which has an octanol concentration of 50 wt %, as opposed to B, which has only 21 wt %. This shift in the temperature profiles indicates that the purity of the key component hexanol in the distillate

|

kdwAdw,i(Tprefrac,i - Tmain col,i)

|

(8)

The heat transfer from the main column to the prefractionator prevails in the case when Qdw,net < 0; conversely, the heat transfer from the prefractionator to the main column dominates in the case when Qdw,net > 0. Furthermore, Qdw,total represents the amount of heat that is transferred in total when the direction of the heat transfer is neglected. The plots in Figure 16 show the heat transfer results for cases 1 and 3. There are significant variations throughout the parameter sets for both Qdw,net (from -0.15 to -0.32 kW) and Qdw,total (from 0.38 to 0.51 kW). Despite these differences, it can be concluded for these two cases that the heat transfer from the main column to the prefractionator dominates (Qdw,net < 0). For a better understanding of the magnitude of the transferred amount of heat, Qdw,net and Qdw,total are compared with the net reboiler heat duty (reboiler duty less the heat losses in the reboiler). This comparison shows that the values for Qdw,net are in the range of 15-30%, and for Qdw,total, in the range of 35-50% of the net reboiler duty. Figure 17 shows an almost linear relationship between Qdw,net and the vapor split. All steady-state operating points shown in Figure 17 feature product purities in all three key components of at least 98 wt % due to tight control of the product flow rates and sufficient separation efficiency. This relationship is in accord with the previous results and findings. Furthermore, it is illustrated that the higher the heat transfer from the main column to the prefractionator, the stronger the evaporation effects and, thus, the vapor load in the prefractionator. Consequently, more vapor enters the main column, and the vapor split values decrease. To summarize, a successful model validation, which implicitly infers good agreement between simulated and measured temperature profile, must take into account the heat transfer across the dividing wall. This study revealed the tremendous influence of the heat transfer across the dividing wall on the hydraulics within the column and underlines the fact that heat transfer is an extremely important factor of dividing-wall columns. In this study, the influence of the heat transfer across the vertical wall is investigated by varying the overall heat-transfer coefficient, kdw, according to the data in Table 3. Apart from the heat-transfer coefficient and the temperature difference between the prefractionator and the main column, the area of the dividing wall, Adw, also affects the heat transfer across the dividing wall. However, the influence of the heat transfer across the wall on the energy balance depends on the scale of the column because its impact decreases with increasing diameter of the column. Thus, the simulation results without heat transfer are more appropriate for describing a DWC on the production scale. Furthermore, the following has to be considered in case simulation studies for separating a mixture in a DWC reveal a dependency of the product purities on the vapor distribution. It is not only the cross-sectional area ratio of the dividing-wall segment and the choice of the column internals that has to be considered, but also the heat transfer across the dividing wall

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Figure 16. Net heat Qdw,net (left) and total heat Qdw,total (right) transferred across the dividing wall for the cases in Table 3.

the column because of condensation and evaporation effects and, thus, the vapor distribution. These results are especially characteristic for dividing-wall columns on lab scale made of steel and may occur less significantly in dividing-wall columns on production scale. It should be emphasized that dividing-wall column experimental data of such quality and breadth have never before been published in the open literature. Future work will deal with the dynamics of dividing-wall columns, with special focus on the start-up of dividing-wall columns and the development of start-up control approaches. Acknowledgment Figure 17. Influence of the net heat transferred across the dividing wall on the vapor split considering all parameter sets.

due to its influence on the vapor split. Where appropriate, the dividing wall should be fabricated from an insulating material. 5. Conclusion An extensive analysis of dividing-wall columns based on both experimental and simulation studies has been presented for the first time. Several different feed compositions of a ternary mixture of fatty alcohols have been examined in a purposebuilt pilot plant while maintaining product specifications of ∼99 wt % at steady-state conditions. An extensive model validation showed very good agreement between experimental and simulation results for pressure drops, product compositions, and temperature profiles. In addition, the process model can also simulate the self-adjustment of the vapor distribution, an essential aspect of dividing-wall columns. It was found that the heat transfer across the dividing wall and the pressure drop in the nonseparating column internals are extremely important factors and must not be neglected in labscale columns. Furthermore, a case study revealed the effect of the heat transfer across the dividing wall on the column hydrodynamics. When heat transfer across the wall was not considered, the vapor split remained virtually constant at around 0.52 at all operating conditions. This value is expected because the vertical wall bisects the diameter, the column internals are the same on both sides of the wall, and the liquid is distributed equally above the dividing wall. Surprisingly, the vapor split values vary significantly in the range of 0.3-0.5 in the case that heat transfer is considered. This broad distribution of the vapor splits was a surprising and unexpected result, which was not beforehand expected. The detailed case study revealed that the heat transfer across the dividing wall significantly influences the hydraulics of

We gratefully acknowledge the financial support from the Max-Buchner-Forschungsstiftung (MBFSt 2712). Furthermore, we express our thanks to Julius Montz GmbH for generously supporting us with our pilot plant. Nomenclature Symbols a ) specific surface area (m2 m-3) A ) area (m2) CB ) constant used in eq 3 dp ) diameter (m) dev ) deviation f ) feed stream section () prefractionator) Fvap ) vapor load (Pa0.5) g ) gravitational constant (m s-2) HX ) heat exchanger k ) overall heat-transfer coefficient (W m-2 K-1) K1, K2 ) constants used in eq 4 L ) height (m) LS ) liquid split distributor p ) pressure (Pa) P ) pump Q ) heat flow rate (J s-1) Qdw,net ) net heat transferred across the dividing wall (J s-1) Qdw,total ) total heat transferred across the dividing wall (J s-1) Re ) Reynolds number s ) side stream section S ) section T ) temperature (°C) u ) velocity (m s-1) V ) vessel w ) mass fraction (kg kg-1) Greek Symbols R ) pressure drop parameter in eq 5 β ) pressure drop parameter in eq 5

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010 -3

ε ) void fraction (m m ) ∆p ) pressure drop (Pa) ∆p33 ) pressure drop without condenser according to PDIR33 in Figure 1 (Pa) ∆p34 ) pressure drop without condenser according to PDIR34 in Figure 1 (Pa) ν ) kinematic viscosity (m2 s-1) Ψ ) friction factor 3

Superscripts and Subscripts B ) bottom cd ) collector and distributor D ) distillate dw ) dividing wall F ) feed i ) stage index liq ) liquid main col ) main column prefrac ) prefractionator S ) side stream vap ) vapor

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ReceiVed for reView February 13, 2010 ReVised manuscript receiVed May 16, 2010 Accepted May 25, 2010 IE1003416

Experimental and Theoretical Studies of a Dividing-Wall Column Used

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