A new unified model to simulate columns with multiple phase divisions and their impact on energy savings

https://doi.org/10.1016/j.compchemeng.2020.106937Get rights and content

Highlights

  • New method for simulation of distillation columns with any number of phase divisions.

  • Unified set of MESH equations for any distillation process with phase division.

  • Unified set of MESH equations for parastillation, metastillation and DWC.

  • Case study using para- and metastillation processes in alcoholic distillation.

  • Decreases of 15% in total annual costs of distillation processes.

Abstract

This paper presents a new unified model that allows the simulation of distillation columns with any integer number of phase divisions. Such columns have the potential for substantial savings in energy and capital costs. However, conventional simulators do not cover columns with phase divisions and previous simulations required a tailored algorithm for each type of column. The proposed model uses a unique set of MESH-equations for parastillation, metastillation, and conventional distillation. With small modifications in the stage indexing, the model allows the simulation of conventional, top and bottom-DWCs. This generalization was possible by introducing two new variables: the number of liquid (θ) and vapor (β) phase divisions. The positive impact of increasing the number of phase divisions was demonstrated in the bioethanol distillation, by analyzing parastillation and metastillation columns. These columns reduce the operational and total annual costs in 19% and 15%, when compared to conventional columns.

Introduction

Distillation operations consume a significant amount of energy. The U.S. Energy Information Administration predicts an increase in energy consumption by U.S. industries from 26 quadrillion British thermal units (Btu) to 36 quadrillion Btu between 2019 and 2050 (U. S. Energy Information Administration, 2020). The bulk chemical and refining industries are the most energy intensive, representing respectively 29% and 18% of total U.S. industrial energy consumption (U. S. Energy Information Administration, 2020). In both sectors, separation processes represent approximately 60% of the energy usage, and about 95% of these separation processes are distillation operations (U.S. Department of Energy, 2005). This significant energy demand has made distillation the focus of many studies aiming to improve its energy efficiency. Among these studies, the division of internal column streams has been shown to be an efficient mechanism for energy improvement. This technique is known as distillation with parallel streams, and it includes the parastillation, with division of the vapor phase; metastillation, with liquid division, and the best-known dividing-wall column (DWC), with liquid and vapor divisions.

The DWC integrates two conventional columns of the Petlyuk system into a unique column shell. This configuration, in comparison to the conventional one, reduces the number of condensers and reboilers, and thereby decreases the capital and operational costs by up to 30% and 40%, respectively (Kiss and Suszwalak, 2012). Reports point to 100 DWCs operating industrially (Luyben, 2013). However, DWC configurations have been used for specific processes, where two conventional columns are replaced by one DWC, mainly in the context of ternary mixture separations. Single distillation columns, as the ones used in binary separations, can be replaced by parastillation and metastillation columns, with energy and capital savings. These are less-known techniques that consider the vapor (parastillation) or the liquid (metastillation) internal flow divisions, into two or more parallel streams.

Two-liquid divisions, in metastillation, can decrease the stage area up to 30%, in comparison to a corresponding conventional distillation column (Mizsey et al., 1993). Additionally, metastillation columns may present Murphree efficiencies greater than those reported for conventional columns (Gouvêa, 1999). On the other hand, the two-vapor divisions, in parastillation columns, lead to reductions in column height of up to 50%, when compared with conventional distillation columns (Biasi, 2016; Gouvêa et al., 2000; Heucke, 1987; Meirelles et al., 2017; Mizsey et al., 1993). Furthermore, replacing a conventional distillation column by a parastillation column can lead to a decrease in energy consumption of about 30% (Biasi, 2016; Canfield and Jenkins, 1986; Gouvêa, 1999; Meszaros and Fonyo, 1990; Moraes, 2006).

The construction of lab-scale parastillation columns were reported by Belincanta et al. (2006, 2005). The authors investigated the separation of the ethanol-water mixture and did not report operational issues related to the phase division. Even with the important advantages presented by para- and metastillation, there is only one report about the industrial installation of three parastillation columns in England, (Canfield and Jenkins, 1986) and none about metastillation. The report by Canfield and Jenkins (1986) mentions that one of the parastillation columns presented a reduction in the reflux ratio of 41%, over conventional distillation. However, the authors do not provide further information, stating proprietary issues. They concluded that industrial parastillation columns were completely satisfactory, improving separation and reducing energy consumption over conventional distillation (Canfield and Jenkins, 1986). There is no further report about the industrial use of para- and metastillation columns. This can be attributed to the lack of scientific and technological information about these columns in comparison with conventional distillation equipment, and the specific difficulties of simulating para- and metastillation processes. Most of the few academic works on para- and metastillation columns have investigated simplified processes, but did not consider potential industrial cases. Except for the works of Meszaros and Fonyo (1990) and Mizsey et al. (1993), that consider hydrocarbon multicomponent mixtures, all other works consider simple binary separations. Furthermore, there is no work concerning the control structure for para- and metastillation columns. Some information about the hydrodynamics of parastillation columns is provided in Belincanta et al. (2006, 2005). Given the lack of information on these new types of columns, greater industrial acceptance requires more robust research on the subject, since current distillation processes require equipment with optimized performance that is designed on the basis of well-established methodologies.

This paper presents a generic tool for simulation of any type of distillation equipment, including conventional columns, para- and metastillation columns and the three different DWC configurations. Up to now, Aspen Plus® and other commercial simulators do not cover para- and metastillation columns. Although studies have shown that it is possible to simulate the DWC equipment using commercial simulators (Kiss, 2013), this alternative is not straightforward to execute. A DWC is represented, in the simulators, by the integration of more than one column. However, the resulting system of equations is not solved as a fully coupled system using a Newton-type method, but instead by a nonlinear Gauss-Seidel-type iteration which alternatingly solves the equations of one column given values for the other. This solution approach has poorer convergence properties than those based on the fully coupled system.

In this work, we present a new unified model that allows the simulation of distillation columns with any integer number of phase divisions. Our model is based on the so-called MESH equations (Seader et al., 2011a) and avoids the simplifying assumptions made in short-cut methods for columns with divided streams in the vapor or liquid phases (Heucke, 1987; Meirelles et al., 2017). The proposed model uses a unique set of MESH equations for all columns, including parastillation, metastillation, dividing-wall column (DWC), and conventional distillation. By small adjustments of the indices of the MESH equations, it is also possible to adapt the model for simulation of other DWC configurations, as the bottom and top-DWC. The model is simply adjusted, to a specific column, by specifying few parameters and, therefore, can be used to analyze capital or operational cost savings that may be obtained by using columns with phase divisions, in comparison to conventional distillation columns. Newton's method is used to solve the model equations and does not suffer from the convergence problems observed with previous approaches. Previous simulations (Biasi, 2016; Canfield, 1984; Canfield and Jenkins, 1986; Gouvêa, 1999; Meszaros and Fonyo, 1990; Mizsey et al., 1993) required a tailored algorithm for each type of column, which is time consuming and therefore costly. Using a unique set of equations, our approach reduces the implementation time and facilitates the comparison of different column configurations. This procedure allows systematic analyses and comparisons of such different configurations and, therefore, supports the design of new, energy efficient and capital cost reducing columns.

We generalize the mathematical model of the MESH equations by introducing two new variables: θ and β. These two variables represent the number of phase divisions in the liquid (θ) and vapor (β) phases. It enables the simulation of columns with multiple phase divisions, using a unique set of equations. The suggested mathematical model was implemented and validated. Conventional distillation and DWC were validated comparing our results with those from Aspen Plus® commercial simulator and from Luyben (2013). The validation for metastillation and parastillation was done by comparing with results from the literature (Gouvêa, 1999; Gouvêa et al., 2000). We also provide some details about the convergence and some tips used to decrease the computational time required by the simulation.

A case study illustrates the use of para- and metastillation columns applied to the production of hydrous bioethanol (Batista et al., 2012), which is a case of industrial interest. Results demonstrate that para- and metastillation techniques lead to savings in equipment and/or operational costs. It is possible to reduce operational costs by about 19% through the replacement of conventional distillation by parastillation. Additionally, the height of parastillation columns may be reduced by 33%, when compared to the height of conventional columns. These reductions can decrease the total annual cost of distillation processes in 15%. These results, associated with the unified model for simulating columns with multiple phase divisions, may increase the industrial interest in para- and metastillation columns. In addition, the unified model simplifies the simulation of the DWC equipment and allows a faster comparison of the performance of different column configurations.

Section snippets

Simulation of distillation processes considering mesh equations

The methodologies for simulating distillation columns can be classified as short-cut methods or rigorous simulation procedures. The short-cut methodologies are based on simplified models, for example, models that assume constant molar overflow along the column and/or constant relative volatility. One well known method is the MacCabe-Thiele approach. Meirelles et al. (2017) presented a general approach for the calculation of parastillation and metastillation columns based on the adaptation of

General implementation of the generic model

The mass balance (M), enthalpy balance (H) and equilibrium relations (E) correspond to a total of (2C+1)·N equations and (2C+1)·N variables (Tables 1 and 2). The variables are the molar liquid and vapor flows (ln, i and vn, i) and the temperatures (Tn). The system of nonlinear equations can be solved by any method suitable for nonlinear relations. For conventional distillation, Newton`s method was suggested by Naphtali and Sandholm (1971) and described in detail by Seader et al. (2011a) and

Case study: the influence of vapor (parastillation) or liquid (metastillation) phase divisions

We illustrate the use of our proposed model via the simulation of parastillation and metastillation of alcoholic mixtures. This case study illustrates the use of the mathematical model presented here, but it also expands understanding of the para- and metastillation processes. The first example considers the binary ethanol/water mixture. Although we selected this system as a generic case study, it should be noted that this mixture is a simplified version of the system involved in the production

Conclusion

In this work, we propose a generic mathematical model for simulation of columns with parallel streams, including parastillation, metastillation and divided-wall columns. The initial challenge was the formulation of a unique set of MESH equations able to describe, with no further implementation, those different distillation processes. The model was implemented and validated considering different processes reported in the literature. Results showed good convergence and were in great agreement

CRediT author statement

Lilian C. K. Biasi: Conceptualization, Methodology, Software, Validation, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization

Matthias Heinkenschloss: Methodology, Software, Writing - Review & Editing, Supervision

Fabio R. M. Batista: Conceptualization, Writing - Review & Editing, Supervision

Roger J. Zemp: Validation, Investigation, Writing - Review & Editing, Supervision

Ana L. R. Romano: Validation, Investigation

Antonio J. A. Meirelles:

Acknowledgments

This study was financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001, by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) – Financial Codes 140212/2017-5, 307398/2019-6, 406963/2016-9, and 406856/2013-3, by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) – Financial Codes 2014/21252-0, 2016/10636-8, and by NSF DMS 1819144.

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