Innovative mapping method for screening reactive distillation designs

https://doi.org/10.1016/B978-0-12-818634-3.50124-7Get rights and content

Abstract

Reactive distillation (RD) technology offers key benefits in many chemical processes, including energy savings and costs reduction. Prior to its application in industry, screening, addressing technical feasibility and economic viability, must be performed at the conceptual design level. But these tasks can be challenging and time-consuming since detailed models are usually needed. To overcome this complexity, we provide a mapping method to quickly assess the applicability of RD. The mapping method overlays key parameters of a real system, i.e. relative volatilities (α) and chemical equilibrium constant (Keq), onto pre-calculated graphs indicating the RD performance, i.e. the reflux ratio (RR) vs number of theoretical stages (NTS) based on generic cases. The mapping method focuses on quaternary systems (A + B ⇌ C + D). A case study (methyl lactate hydrolysis) is used to demonstrate the approach. Three scenarios are presented, applying different characteristic volatility values; each scenario gives rise to a different RD applicability map for equilibrium constants in the range 0.01 to 10. The findings are validated against results of rigorous process simulation and optimisation. The most accurate scenario is that in which α sets are calculated for mixtures with molar compositions 99% C / 1% A, 50% A / 50% B, 1% B / 99% D, respectively. The results show that the mapping approach allows the prediction of number of theoretical stages and reflux ratio to be estimated within 10% of the optimum values.

Introduction

Reactive distillation (RD) is an important intensification technology that offers multiple advantages: 1. an improved chemical process (i.e. higher conversion and selectivity), 2. energy savings, 3. costs reduction, and 4. inherently safer designs (Shah et al., 2012). This technology has received industrial interest for over 30 years; and for example the production of methyl acetate and ethers using RD is commercially well established (Stankiewicz, 2003). Within the same time frame, studies have been performed to intensify operation of a wider range of chemical processes, such as (trans-)esterification, hydrolysis, (de-)hydration and alkylation (Kiss, 2017).

In spite of the promising applications offered by RD, the complexity of designing RD columns has hindered industrial application of the technology. Simpler approaches are needed to guide design engineers and support design decision making as to whether RD is an applicable unit (Segovia-Hernández et al., 2015). Recent work has aimed to provide such guidance, via a mapping approach that facilitates screening of RD application (Muthia, Reijneveld, et al., 2018). The method uses RD applicability graphs which plot reflux ratio (RR) vs number of theoretical stages (NTS) and aid go/no-go screening of proposed RD operations considering the maximum acceptable values for NTS and RR. The mapping method is currently limited to use in quaternary reactions (Muthia, van der Ham and Kiss, 2018). This paper presents an extended development of the method and highlights the importance of using appropriate characterisation of the relative volatilities in the column to predict RD applicability.

Section snippets

Mapping method

Figure 1 (left) shows the RD configuration used in this study. The column is assumed to operate at atmospheric pressure, with negligible pressure drop, and to achieve vapour-liquid and reaction equilibria at each stage. The reactive section is located between the inlets of the lighter and heavier reactants; reaction and separation occur simultaneously in this section. Further separation takes place in the rectifying and stripping sections, to achieve the targeted product purities. It is assumed

Case study

This study focuses on the quaternary reaction (A + B ⇌ C + D), where Tb,C < Tb,A < Tb,B < Tb,D), which has been explored widely for RD applications. Due to space limitations, only the case study of methyl lactate hydrolysis is presented, as shown in Eq. (1).WaterA+Methyl lactateBMethanolC+Lactic acidDTb100°C144.8°C64.7°C216.85°CΔHr=+33.6kJ.mol1

The activity coefficients are calculated by UNIFAC-HOC and a correlation between temperature and chemical equilibrium constant is shown in Eq. (2),

Method development and validation

Table 1 presents three scenarios, applying different definitions for the characteristic α sets. For all scenarios, αAB is for a 50/50 mol% mixture of the reactants, corresponding to the equimolar feed of reactants. In Scenario 1 (S1), (αCA and (αBD are calculated using the compositions of the top and the bottom streams, respectively. Scenario S2 aims to consider volatilities within the column, and not just at its extremities; therefore, the characteristic αCA and αBD are calculated 50/50 mol%

Conclusions

The mapping method aims to help engineers to carry out relatively quick initial evaluation of potential RD applications. The approach uses characteristic relative volatilities and chemical equilibrium constant of a real system to screen the RD designs based on generic RD applicability graphs. This study highlights the importance of having the appropriate characterisation of relative volatilities sets to predict the RD applicability and validates the approach by comparing predictions to rigorous

Acknowledgements

RM gratefully acknowledges full fund support from LPDP (Indonesia Endowment Fund for Education). AAK is thankful for the Royal Society Wolfson Research Merit Award.

References (8)

There are more references available in the full text version of this article.

Cited by (2)

  • A systematic framework for assessing the applicability of reactive distillation for quaternary mixtures using a mapping method

    2020, Computers and Chemical Engineering
    Citation Excerpt :

    This work presents a systematic framework that applies our novel mapping method, for screening and initialization of reactive distillation column designs. The mapping method (Muthia et al., 2018a; Muthia et al., 2019a) has been introduced and demonstrated in near-ideal quaternary systems with the reaction A + B ⇌ C + D. Initially the approach was demonstrated for systems with boiling point order Tb,C < Tb,A < Tb,B < Tb,D (Muthia et al., 2018a), where the two products are the lightest (C) and heaviest (D) components in the system and so are readily separated from each other. A subsequent study demonstrated the approach for systems with other boiling point orders (Muthia et al., 2019b) and provided some insights into the optimal feed locations of reactive distillation columns.

View full text