A novel two-parts heat integrated dividing wall column with middle vapor recompression

https://doi.org/10.1016/j.seppur.2021.118302Get rights and content

Highlights

  • A novel heat integrated configuration of dividing wall column is proposed.

  • The heat integrated DWCs are further integrated with the pre-separated column.

  • The process with CRS-MVRC-DWC has the largest energy and economic benefits.

Abstract

The intensified heat integrated technology can be applied to the distillation column to improve the utilization of energy resources in the chemical separation processes. For further improving the energy efficiency of the hydrocarbon mixture distillation separation process, a novel two-parts heat integrated dividing wall column (DWC) with middle vapor recompression (MVRC) is proposed in this paper. Differed by the position of vapor recompression, above or beneath the dividing wall in the column, two configurations of MVRC assisted DWC (MVRC-DWC) were developed and comprehensively evaluated by energy, exergy, economic and environmental impact (4E) analysis, with comparison to the existing vapor recompression assisted dividing wall column (VRC-DWC) and a very competitive improved heat integration of VRC-DWC (IHI-VRC-DWC). Moreover, the heat integration between the pre-separated column and different DWC configurations are carefully designed and also evaluated by 4E analysis. Comparatively, the CRS-MVRC-DWC can supply more latent heat of the vapor to the pre-separated column, consequently the process has the largest energy saving and lowest exergy consumption and exergy loss than the other configurations. The TAC saving of process with CRS-MVRC-DWC can reach to the highest value of 25.72%.

Introduction

Distillation is one of the most widely used chemical separation technology in the world, but its energy consumption occupies about 3% in the world [1], or 40–60% in the chemical engineering processes [2]. As an energy intensive unit operation, the major drawback of distillation column is owing to its only 5–10% thermodynamic efficiency [3], [4], which leads to a large energy requirement and also waste. To make full use of energy resources, varied thermal intensification approach has been used for promoting the thermal efficiency of distillation columns, including vapor recompression (VRC) [5], dividing wall column (DWC) [6], internally heat integrated distillation column (HIDiC) [7] and so forth.

As a fully thermal coupled distillation column, DWC has attracted extensive attention, when applied for multi-component liquid mixture separation. It integrates the prefractionator and the main column into one shell, to perform multi-component separation. Studies indicate great potential of DWC in reducing utility consumption, which can achieve up to 30% energy savings as well as 40% installation space reductions over conventional two-column distillation sequence [8], [9]. DWC can also be applied to the reactive, azeotropic and extractive distillation schemes [10]. Nevertheless, the thermodynamic efficiency of DWC is not always high, because it requires high-quality energy in the bottom reboiler, but removes the low-grade heat from the top condenser [11], [12]. For sustainable development, DWC is expected to be transformed as a higher thermal efficiency distillation configuration.

VRC technology, which allows the overhead vapor to be compressed as a higher pressure and temperature, realizes the heat of condensation to be utilized for the evaporation in the reboiler. Furthermore, applying VRC technology to the conventional columns will not change the vapor–liquid flow, temperature and also pressure on each stage, when the heat pump is installed overhead. Shahandeh et al. [13] separated methanol/water mixture and proposed five heat pump-assisted schemes based on this technology. The high energy efficiency of VRC requires a small temperature difference between the top and bottom of the distillation column [14], while there will be little economic advantage when separating the wide-boiling mixtures [15].

Several efforts have been executed to combine the VRC and DWC as a more energy-efficient multi-component separation scheme. However, DWC usually has a wide range of temperature difference across the whole tower. The top to bottom heat pump VRC can’t be used [4] here, as it strongly increases the compressor duty. In recent years, Chew [16] supposed the multistage vapor recompressors assisted DWC but it was limited by the expensive compressor cost. Navarro-Amoros [12] proposed the VRC assisted DWC configuration in which the connected position is at side stage. Xu et al. [17] chose the best side exchanger position by the aid of column grand composite curves (CGCC). Though obtaining some energy saving or economic benefit, the DWC has just been partially thermal integrated. The non-integrated heat energy should be made further use.

HIDiC is another typical internal heat integrated technology [18] for developing more efficient distillation columns. It accurately separates one column into two individual parts at the feed position: one part is used as rectifying section, and the other is served as stripping section. The pressure and temperature in the rectifying section is sufficiently higher than that in the stripping section. This provides enough driving force for encouraging heat to transfer from the rectifying section to the stripping section by a few heat exchangers. Compared with VRC, HIDiC schemes need a lower compression ratio, so that have an advantage in total annual cost (TAC) savings (20%) [19], [20]. Many studies focus on the structural or heat exchange distribution optimization of HIDiC configurations [21], [22]. However, there rarely have articles to report about the combination between HIDiC and DWC. It is because the existence of dividing wall makes DWC a more complex structure and fluid flow pattern inside than conventional distillation column, which is inconvenient for constructing HIDiC coupled DWC.

Cased by optimal design of the hydrocarbon mixture heat integrated distillation separation scheme, we integrated the thermal coupling model of VRC with the two-parts configuration of HIDiC into a novel middle vapor recompression (MVRC) scheme, then it was applied to DWC for its complete heat integration. Two novel MVRC assisted DWC configurations have been carried out and compared to DWC, VRC-DWC and a very competitive improved heat integration of VRC-DWC (IHI-VRC-DWC). Besides, to maximize the energy saving of the whole process, five heat integration schemes between the different DWC configurations and pre-separated column have been carefully designed for the hydrocarbon mixture separation process. By the way, all the developed heat integrated configurations and processes were evaluated by energy, exergy, economic and environmental impact analysis (4E) [23]. Consequently, a heuristic guidance has been developed on the feasibility of integrating a two-parts DWC with the heat integration technology, and this will give the direction in which further development of a DWC with energy-efficiency should be proceed.

Section snippets

Heat integrated configurations design

As shown in Fig. 1(a), DWC performs the basic configuration of intensified heat integration distillation column. It mainly has three sections from top to bottom: common rectifying section (CR), dividing wall section (DW) and common stripping section (CS). A three components mixture feeds into the one side of DW for prefractionation, and the middle boiling component will be extracted out from the other side. Meanwhile, the light and heavy components will be respectively produced at the top and

Energy analysis

The total energy consumption (QT) of each scheme is the sum of the reboiler duty (QR) and 3 times compressor duty (QCOMP) [24]:QT=QR+3QCOMPwhere the coefficient before QCOMP is 3 because the efficiency of heat energy converting to the electrical energy is about 35%.

Exergy analysis

The exergy analysis is the measurement of the energy efficiently used. For the internal distillation column, the potential lost work (irreversibility) can be emerged due to the momentum loss (pressure drop), thermal loss (temperature

Results and discussion

The distillation process for separating hydrocarbon mixture of 2-methylpentane/benzene/toluene/p-xylene was initially designed as a partially heat integrated scheme, where the pre-separated column is used to separate 2-methylpentane, and then the DWC is adopted to separate the benzene/toluene/p-xylene which is a real application in the chemical industry. Commonly, the ease of separation index (ESI) [32] and feed composition will influence the optimization of the column structure. Some

Conclusion

Two different kinds of novel heat integrated DWCs with the middle vapor recompression, CRS-MVRC-DWC and CSS-MVRC-DWC, are proposed in this article. With comparison to the VRC-DWC and IHI-VRC-DWC, the hydrocarbon mixture separation process containing a pre-separated column and a dividing wall column was selected as the representative case to evaluate the performance of different DWC configurations and also the process heat integrated schemes between the pre-separated column and different DWC

CRediT authorship contribution statement

Hao Chen: Conceptualization, Methodology, Investigation, Writing - original draft. Haifeng Cong: Resources, Writing - review & editing. Lin He: Validation, Visualization. Xingang Li: Supervision, Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors acknowledge financial support from National Key Research and Development Program of China (Grant 2018YFB0604903); Key Technology Research and Development Program of Shandong (Grant 2019JZZY020237)

References (35)

Cited by (9)

  • Synthesis of waste heat recovery using solar organic Rankine cycle in the separation of benzene/toluene/p-xylene process

    2022, Energy
    Citation Excerpt :

    The reproduction of the existing VRC-DWC for the separation of benzene, toluene, and p-xylene is illustrated in Fig. 8. The optimal results for the DWC configuration were derived from our previous study [8]. It can be observed that the DWC requires 4067 kW to vaporize its bottom liquid for the internal flow cycle.

  • Energy, exergy, economic and environmental analysis of a novel steam-driven vapor recompression and organic Rankine cycle intensified dividing wall column

    2022, Separation and Purification Technology
    Citation Excerpt :

    In Fig. 2, the feed flow rate is 300 kmol/h and its mole fractions of benzene, toluene, p-xylene are 0.2, 0.6, and 0.2, respectively. The operating pressure of the whole column was set at 1 atm, ignoring pressure drop [40]. According to the accurate calculation for non-polar and low-polar compounds, the SRK method is used for the benzene/toluene/p-xylene system.

  • Energy-efficient synthesis of crude phenol separation process using advanced heat integrated technology

    2021, Energy Reports
    Citation Excerpt :

    The heat integrated technology can be the effective way to enhance the energy performance of distillation columns (Chen et al., 2020) and also is the promising technology for revamp the large and complex distillation process. The conventional heat integrated technologies are commonly like the vapor recompression (VRC), internally heat integrated distillation column (HIDiC), dividing wall column (DWC) (Chen et al., 2021) and multi-effect distillation (MED) (Cui et al., 2017b). The DWC using a dividing wall to integrate two columns into one column to separate a ternary mixture (Aurangzeb and Jana, 2016; Kiss, 2013).

  • Robust solar-energy system design and optimisation for reactive distillation column in methyl acetate hydrolysis process

    2021, Energy Conversion and Management
    Citation Excerpt :

    Thus, to attain a comprehensive solar-energy system design, this design should combine the off-design performance of the ORC part and the design parameters for the entire solar-energy system (both the solar-thermal and ORC parts). Distillation is one of the most energy-intensive processes [22], but it has a low energy efficiency of 5%–10% [23], which has been the impetus for improving distillation energy efficiency using heat-integrated technologies [24]. Vapour recompression (VRC) is a typical heat-integrated technology that is used to increase the pressure of the overhead vapour when heating the bottom liquid with a compressor.

View all citing articles on Scopus
View full text