Thermally Coupled Distillation Systems in Biodiesel Production Technology

Transcription

1 Thermally Coupled Distillation Systems in Biodiesel Production Technology RADU IGNAT*, ALEXANDRU WOINAROSCHY Politehnica University of Bucharest, Dept. of Chemical Engineering, 1-5 Polizu Str., , Bucharest, Romania In this paper the feasibility of using thermally coupled distillation columns, such as the dividing-wall distillation columns (DWC) for the separation of the by-products obtained in the biodiesel production process is analized. The investigated case study for the separation of a methanol-water-glycerol ternary mixture demonstrates the feasibility of using DWC compared to conventional distillation sequences. Simulation models were developed using Aspen PLUS process simulator and for equipment sizing and economic analysis, Aspen Process Economic Analyser software was used. Keywords: thermally coupled distillation, dividing-wall distillation columns, design, simulation, capital cost evaluation Today, most of the used energy comes from fossil fuels: petroleum, coal, and natural gas [1], but, fossil fuels can not be considered as a sustainable energy resource. Because of the increasing price of fossil resources, moreover, the feasibility of their utilization is declining [2]. During the last years extensive research has been conducted to develop renewable and clean fuels that can replace fossil fuels. Fuels obtained from natural sources are known as biofuels [3], e.g., biodiesel, biomass, bioethanol. Biofuels have emerged as clean and renewable alternatives to fossil fuels. Among biofuels, bioethanol and biodiesel have been extensively studied as substitutes for gasoline and petroleum diesel [4]. Biodiesel is one of the most promising alternative fuels, it is renewable, biodegradable, non toxic and has almost very close property to that of diesel fuel [5]. Many methods for biodiesel production were proposed and studied [6-14], but most biodiesel production occurs throughout an alcoholysis reaction known as trans-esterification, were triglycerides with an alcohol and a catalyst react to generate esters of the alcohol called biodiesel, and glycerol as a by-product [15]. The current biodiesel manufacturing processes implies hight energy requirements for the separation of the products resulted from the reaction stage. An alternative solution to reduce the energy demand of conventional distillation sequences is the use of thermally coupled distillation columns. A well known thermally coupled distillation system is the Petlyuk column (fig. 1A). If the Petlyuk arrangement that include the prefractionater and the main column is realized in a single shell, the result is a new thermally coupled system called dividing-wall distillation column (fig. 1B). A DWC column looks like a common distillation column with a side-draw. In reality it is a column that has a vertical separating wall for a given part of its length, which defines the prefractionating region and the main column region. The reflux that comes from the condenser will split on both sides of the separating wall and will realize the reflux in the two parts of the distillation column. The vapour from the reboiler will also split, in the lower part of the column, in accordance with the liquid split and hydrodynamic conditions on both sides of the dividing wall [16]. The purity of the middle product from a DWC is superior to that of a conventional side product column because the middle distillates usually form a strong split above and below the partition and the product is removed from the side of the partition remote from the feed. Compared with a conventional distillation system, DWCs can lead to about 30% energy and capital cost savings. Despite this advantages, there are some product. Column C2 is operated under atmospheric presure. General data resulted from the design procedure are presented in Fig. 1. Petlyuk distillation column (A) and dividing-wall distillation column (B) * rm_ignat@chim.upb.ro; Tel.: REV. CHIM. (Bucharest) 62 No

2 Fig. 2. Conventional column distillation sequence Table 1 GENERAL DATA FOR THE CONVENTIONAL DISTILLATION SYSTEM DESIGN Table 2 PRODUCT STREAMS COMPOSITION FOR CONVENTIONAL DISTILLATION SEQUENCE drawbacks of DWC that lead to limited and slow industrial acceptance due to not yet well-established design procedures compared to conventional distillation column [17]. Commercial simulators, such as HYSYS and Aspen Plus, provide reliable environments for steady-state and dynamic simulation of distillation columns. A special structure as the DWC can also be implemented using thermodynamic equivalent schemes [18]. Triantafyllou and Smith [19] developed a design procedure based on a threecolumn model. In their model, the final structure has to be iteratively adjusted, as the number of trays on both sides of the dividing wall must be the same. This problem also arises when a commercial simulator is used to establish the final column topology. In [20] it is developed a structural design methodology for full thermally coupled distillation columns based on minimum tray structure to give highenergy efficiency. The operational complexity of a DWC derives from the increased number of freedom degrees as compared to a simple column with a side draw. Halvorsen and Skogestad [21] found that the liquid and vapour split ratios, considered as two additional freedom degrees, are important for the thermal efficiency of the DWC. In the researchers [22] found that, for a column with finite number of stages, the energy efficiency of the design depends on the values of the liquid and vapor splits. In this work a feasibility study regarding the use of DWCs in the biodiesel manufacturing process is presented. A case study for the separation of methanol-water-glycerol in a DWC and in a conventional distillation system is analised, the results beeing compared in terms of energy requirements and capital cost investments. The design of the DWC and the conventional distillation system was made in the frame of Aspen Plus process simulator, and the capital cost evaluation was made using Aspen Process Economic Analyzer software. Conventional distillation system design The ternary mixture of methanol-water-glycerol is separated in a conventional two column distillation sequence (fig. 2). The initial design of the simple distillation columns was made using the short-cut distillation design, an option based on Winn-Underwood-Gilliland method avaible in Aspen Plus process simulator. Further, the initial design was adjusted by rigorous simulation. Column C1 is operated under vacuum to keep the temperature low enought in order to prevent the degradation of the glycerol REV. CHIM. (Bucharest) 62 No

3 Fig. 3. Short-cut design model for DWC structure is able to be used also for dynamic simulations. A DWC has seven degrees of freedom: condenser duty (Qc), product rates (distillate D, side stream S, bottom B), reflux rate (R), reboiler duty (Qr) and the liquid split between the two sections of the column (LS). In their work, Mutalib and Smith [24] have introduced the degrees of freedom analysis, showing that the DWCs have a more complex structure compared with side draw columns. Important aspects for the DWC design are the vapour (VS) and liquid (LS) splits between the two sections separated by the internal wall. The vapour split ratio, in practice, is difficult to be manipulated, therefore, can be free to adjust by imposing the same pressure drop on both sides of the DWC. The liquid split ratio in the DWC can be used as a manipulated variable and has a significant effect on the energy consumption as is shown in figure 5. In order to prevent the degradation of the glycerol product, DWC is operated under vacuum, to keep the temperature low enought. NRTL thermodynamic/activity model was used for components property estimation. General data resulted from the design procedure are presented in table 3. Product streams composition is presented in table 4. Equipment sizing In order to evaluate the capital cost of both separation sequnces (conventional and thermally coupled) the equipments implied in the separation process must be Table 3 GENERAL DATA FOR THE DWC DESIGN Fig. 4. Thermodynamic equivalent structure for DWC table 1. Product streams composition for both columns is presented in table 2. DWC design using Aspen Plus The dividing-wall distillation column is not available as a standard unit operation in Aspen Plus process simulator. This structure can be implemented using thermodynamic equivalent schemes. In order to establish the initial configuration of the DWC (number of trays in each section of the DWC, feed tray, side-draw and the position of interlinking vapour and liquid streams), the short-cut distillation design option based on Winn-Underwood- Gilliland method avaible in Aspen Plus process simulator was used. This structure is largely used in the design step, as generally mentioned in the literature [19]. To develop the short-cut design method, the dividing-wall distillation column is shared into three sections (fig. 3). Section 1 corresponds to the prefractionator and sections 2 and 3 to the main column [23]. From this initial design, a four column equivalent structure (fig. 4) was derived and modified by repeated rigorous simulation, in order to achieve purity requirements and minimum energy consumption. Due to its capabilities of reflecting the internal streams and geometrical characteristics of the tray sections [18], the corresponding final design of the DWC Table 4 PRODUCT STREAMS COMPOSITION FOR DWC REV. CHIM. (Bucharest) 62 No

4 Table 5 CONVENTIONAL DISTILLATION SEQUENCE SIZING RESULTS Table 6 DIVIDING-WALL DISTILLATION COLUMN SIZING RESULTS Table 7 COST EVALUATION FOR CONVENTIONAL SEPARATION SEQUENCE sized. Equipment sizing was made using Aspen Economic Process Analyzer databases. The simple distillation columns from the conventional separation sequence, along with the dividing-wall column tray sections (TS1, TS2, TS3, TS4), were sized considering sieve trays with an overall efficiency of 0.78 and a distance between trays of 0.4 m. Other data needed for sizing were taken accordingly to software standard options. For heat transfer equipment, in both cases, were considered Kettle type reboilers, and as condensers, heat exchangers with fixed tubular plate. The required geometric dimensions for further cost evaluation are the tray sections diameter, height for the separation units, and heat transfer area for heat transfer units. In tables 5 and 6 the sizing results for the main equipment are presented. Capital cost evaluation Cost evaluation was realised in the frame of Aspen Economics Process Analyzer software. The software database allows capital cost evaluations for a large variety of process equipment if the characteristic dimensions are known. Cost evaluation for a single column and associeted equipment is relatively simple. However, precise estimates for the DWC conceptual structure are often difficult to obtain. Fortunately, these costs are not necessary to be known with high precision, important being their relative values in order to be compared with conventional distillation systems capital costs. It must be noticed that, for the dividing-wall column, it was used a thermodynamic equivalent structure based on four tray sections, but in reality the DWC is realized under a single shell. For the cost evaluation it was considered a single column with constant diameter and a total height as the sum of the top section (TS2), prefractionator (TS1) and bottom section (TS3). The overall diameter of the DWC was considered as the equivalent diameter of the prefractionator (TS1) and side draw (TS4) sections. The capital cost of the DWC was increased by 10% in order to reflect the internal arrangements required by the presence of the dividing-wall. Cost evaluations for the heat transfer units were made based on the required heat transfer area. The results for the capital cost evaluations for both conventional and thermally coupled distillation sequences are synthesized in tables 7 and 8. Table 8 COST EVALUATION FOR DWC Results and discussions The main goal of this paper is to present a feasibility study of the use of thermally coupled distillation columns, such as DWC column in biodiesel manufacturing technology. In the aim of proving the energy efficiency of the DWC, the case study of separation of a ternary mixture of methanol-water-glycerol in a conventional distillation system and in a DWC was analized and compared in terms of energy requirements. DWC can lead to about 33% energy savings (table 9). The energy efficiency of the DWC depends on the liquid split ratio as shown in figure 5. Beside the energy efficiency of the DWC, another important aspect is the capital cost investment. Because DWCs are complex columns with sophisticated internal arrangements, this can increase the price of the column compared to a two simple colums separation sequence. For this study, the estimated capital cost investments for both separation sequences performed with a specialized software shows a 15.5% reduction of costs in the case of the DWC (table 10). This economy resides from the fact that even if the DWC price is higher, the column needs only one condenser and one reboiler, compared with the conventional distillation sequence where two condensers and reboilers are necessary. The compared results of this case study, indicate that, for the separation of ternary mixtures, the use of thermally coupled distillation columns is more favorable than the use of conventional separating sequences. In this case, the use of the DWC can lead to significant energy and capital cost savings REV. CHIM. (Bucharest) 62 No

5 Fig. 5. Effect of liquid split fraction in DWC on column energy requirement Table 9 COMPARISON OF ENERGY CONSUMPTION Table 10 COMPARISON OF CAPITAL COST EVALUATION Conclusions In this work, the feasibility of the use of thermally coupled distillation columns, such as DWC, was analized in terms of energy efficiency and capital cost investments. The design of the conventional distillation columns and DWC was performed in the frame of Aspen Pus process simulator, which provides a reliable simulation enviroment for steady state simulation. For the DWC design a four column thermodynamic equivalent structure was used. This structure reflects the internal vapour and liquid flows interlinkings of the DWC. For the economic analisys and equipment sizing Aspen Economic Process Analizer was used. As the results demonstrate, DWC is a favorable solution for ternary mixture separations. In a future work it will be presented the dynamic analisys and controllability properties of the DWC. Acknowledgement: Financial support from the Sector Operational Programme Human Resources Development of the Romanian Ministry of Labor, Family and Social Protection through the Financial Agreement POSDRU/88/1.5/S/61178 and from National University Research Council, Ministry of Education, Research, Youth and Sports, Grant ID 1727, Multilevel Optimization of Sustainable Bioprocesses is gratefully acknowledged. References 1. DEMIRBAS, A., Biodiesel. A Realistic Fuel Alternative for Diesel Engines, Springer, KAMM, B., GRUBER, P.R., KAMM, M., Biorefineries Industrial Processes and Products, WILEY-VCH Verlag GmbH & Co., CASTRO, F.I.G., RAMIREZ, V.R., HERNANDEZ, J.G.S., CASTRO, S.H., Chemical Engineering Research and Design, 89, 2011, p CASTRO, F.I.G., RAMIREZ, V.R., HERNANDEZ, J.G.S., HERNANDEZ, S., Chemical Engineering and Processing, 49, 2010, p FAZAL, M.A., HASEEB, A.S.M.A., MASJUKI, H.H., Renewable and Sustainable Energy Reviews, 15, 2011, p SHAKINAZ, A., SHERBINY, E., AHMED, A.R., SHAKINAZ T.E.S., Production of biodiesel using the microwave technique, Journal of Advanced Research, 1, 2010, p CHEN, L., YIN, P., LIU, X., YANG, L., YU, Z., Guo, X., XIN, X., Energy, 36, 2011, p LIU, S., MCDONALD, T., WANG, Y., Fuel, 89, 2010, p PATIL, P.D., GUDE, V.G., MANNARSWAMY, A., DENG, S., COOKE, P., MCGEE, S.M., RHODES, I., LAMMERS, P., NIRMALAKHANDAN, N., Bioresource Technology, 102, 2011, p LEE, J.H., KIM, S.B., KANG, S.W., SONG, Y.S., PARK, C., HAN, S.O., KIM, S.W., Bioresource Technology, 102, 2011, p ZHANG, J., CHEN, S., YANG, R., YAN, Y., Fuel, 89, 2010, p HASHEMINEJAD, M., TABATABEI, M., MANSOURPANAH, Y., KHATAMI, M., JAVANI, A., Bioresource Technology, 102, 2011, p KNOTHE, G., GERPEN, J., JRAHL, J., The Biodiesel Handbook, AOCS Press, DIMIAN, A., BILDEA, S., Chemical Process Design, Wiley-VCF, REYES, J.F., MALVERDE, P.E., MELIN, P.S., DE BRUJIN, J.P., Fuel, 89, 2010, p ISOPESCU, R., WOINAROSCHY, A., DRAGHICIU, L., Rev. Chim. (Bucharest), 59, nr. 7, 2008, p IGNAT, R., WOINAROSCHY, A., Chemical Engineering Transactions, 25, 2011, p WOINAROSCHY, A., ISOPESCU, R., Industrial Engineering Chemical Research, 49, 2010, p TRIANTAFYLLOU, C., SMITH, R., Trans. IChemE. Part A, 70, 1992, p KIM Y.H., Chemical Engineering Journal, 85, 2002, p HALVORSEN I.J., SKOGESTAD, S., Industrial Engineering Chemical Research, 43, 2004, p WANG, S.H., WONG, D.S.H., Chemical Engineering Science, 62, 2007, p LING, H., LUYBEN, W.L., Industrial Engineering Chemical Research, 49, 2010, p RAMIREZ, N.C., GUTIERREZ, J.A., AGUERO, A.C, RAMIREZ, V.R, Chemical Engineering Research and Design, 88, 2010, p.1405 Manuscript received: REV. CHIM. (Bucharest) 62 No