(442d) Thermodynamic Efficiency and Remixing Effect on a Thermally Coupled Extractive Distillation Sequence

Authors: 
Brito, K., Federal University of Campina Grande
Figueirêdo, M., Federal University of Campina Grande
Brandão, W., Federal University of Campina Grande
Brito, R., Federal University of Campina Grande


In the sense of mitigating the high energy consumption in distillation, designing new distillation sequences has been the subject of investigation by many researchers. One of the most promising alternatives is the use of thermally coupled sequences, which are obtained by interconnecting process streams, one in the vapor phase and the liquid phase in another, between two columns in series. Studies have proven the effectiveness of these settings in reducing energy consumption and increasing the thermodynamic efficiency compared to conventional distillation systems. However, some studies still question the thermodynamic efficiency increase that thermally coupled sequences can cause.
In conventional distillation sequences designed to separate mixtures, at least ternary ones, a phenomenon known as remixing effect occurs, which is the "dilution" of the component with intermediate volatility. Indeed, the composition of this component (in liquid phase) reaches a maximum at some point in the column and then decreases (as it is not removed) to a certain value. To purify this component again, more energy is required in the column downstream of the main column. For this reason, the remixing effect is identified as the main source of irreversibility, reducing the thermodynamic efficiency of distillation systems. According to the literature, the use of thermally coupled sequences reduce (or eliminate) this effect, increasing the thermodynamic efficiency. However, studies that relate the remixing effect with thermodynamic efficiency are still incipient. In the literature only two papers studying further the remixing effect, considering both ideal mixtures, were found. The study of the remixing effect in the separation of azeotropic mixtures was not found in the literature.
The fact that azeotropic mixtures present non trivial behavior allied to divergences on thermodynamic efficiency of thermally coupled sequences indicates that further studies should be conducted to increase the understanding of thermally coupled extractive distillation sequences. Thus, from this work, it was aroused the interest in expanding the study of a sequence known as extractive distillation configuration thermally coupled to a side rectifier (TCEDS-SR), considered by some researchers as the configuration able to reduce energy consumption by up to
30% compared to the conventional sequence, besides increasing the thermodynamic efficiency. The focus of this paper is to deepen the understanding of the relationship between thermodynamic efficiency and remixing effect. Thus, TCEDS-SR and conventional sequence were compared, considering the dehydration of aqueous ethanol mixtures as case study, using ethylene glycol as solvent.
Both configurations, conventional and thermally coupled, were evaluated in the steady state using the Aspen Plus® simulator. To simulate each configuration, the RadFrac® block was chosen. Due to the operating pressure of the distillation columns, the vapor phase was considered ideal. For the liquid phase, the NRTL model was used to represent the non-ideality. Binary parameters for each chemical species were obtained from the Aspen Plus® database, which in turn is based on the Dechema database.
The analytical procedure used in this work was recently proposed by a paper published in the literature, which included a new parameter in the analysis of extractive distillation processes: the solvent content in the extractive section; More specifically, at the solvent feed stage. The use of this parameter allowed to find the range of possible solutions that will necessarily contemplate the global optimum operating point, as well as eliminating obstacles intrinsic to the methodologies of analyzes of extractive distillation process previously used.
Figures 1a (conventional sequence, DS) and 1b (TCEDS-SR) present an analysis on the remixing effect. In both cases, this effect is reduced with the increasing of the solvent content. However, the use of thermal coupling did not reduce the remixing effect of the conventional sequence; an outcome contrary to that found in the literature. It is noteworthy that, in the
literature, the remixing effect was not evaluated for extractive distillation sequences.

0.8

0.7

0.6

xEG = 0,3 xEG = 0,4 xEG = 0,5 xEG = 0,6

(a)

0.8

0.7

0.6

(b)

xEG = 0,3 xEG = 0,4 xEG = 0,5

0.5 xEG = 0,7

0.5

xEG = 0,6

xEG = 0,7

0.4

0.3

xEG = 0,8

DS

0.4

0.3

xEG = 0,8

TCEDS-SR

0.2

0.2

0.1

0.1

0.0

0 5 10 15 20 25

Stage

0.0

0 6 12 18 24 30

Stage

Figure 1. Mole fraction profile of water in the liquid phase for the conventional sequence (a) and

TCEDS-SR (b).
The result of the study on the remixing effect was confirmed by evaluating the thermodynamic efficiency of each configuration, as shown in Figure 2. Throughout the space of possible solutions, the efficiency of conventional configuration is greater than in the thermally
coupled sequence. The efficiency of the conventional sequence reaches a maximum and then begins to decrease, showing that the efficiency is very much dependent upon the levels at which the temperatures are provided and rejected in the reboilers and condensers, rather than a possible dilution of the intermediate component (remixing effect). In contrast, the efficiency of the
thermally coupled sequence is an increasing function for the increasing of solvent content.

10

9

8

7

6

5 DS

TCEDS-SR

4

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

xEG

Figure 2. Thermodynamic efficiency of each configuration as a function of solvent content.

Checkout

This paper has an Extended Abstract file available; you must purchase the conference proceedings to access it.

Checkout

Do you already own this?

Pricing


Individuals

AIChE Members $150.00
AIChE Graduate Student Members Free
AIChE Undergraduate Student Members Free
Non-Members $225.00