In this work, a heat pump system in the form of vapor recompression (VRC) is introduced in the dividing wall column (DWC) to further improve its thermal efficiency performance. It is a fact that the temperature difference is reasonably large between the top and bottom of a DWC, which typically produces at least a single side product. This may lead to a very large compression ratio (CR), with which, the operation of VRC in the DWC becomes quite complicated and may not be economically so attractive. To improve this situation, the vapor recompression mechanism is further proposed between the side stream and reboiler drum of the DWC column. Utilizing the latent heat of a vapor stream from an intermediate tray in liquid reboiling of the stripper, this side vapor recompressed DWC (SVR-DWC) configuration can reduce the utility consumption and thus improves its energetic and economic potential substantially. This proposed thermally integrated scheme is finally illustrated by a ternary system.

Introduction

As climate changes and resource crisis have emerged as global threats, the capability to deal with environment and energy issues is indeed an important index in determining the future of the national economy [1]. There are a handful of policies framed at the national and international levels targeting to decrease the emissions of greenhouse gases, reduce the dependency on fossil fuels and mitigate the climate change. In this light, the European Union (EU) has set the goals through the 20-20-20 targets in 2007 with a reduction in greenhouse gas emissions of at least 20% below 1990 levels, a consumption of 20% out of renewable energy sources and an increase in energy efficiency by 20% within 2020 [2]. This work is concerned with the thermal integration that is typically used for improving the energy efficiency. Here, a century old chemical unit, namely distillation column, is selected as a potential candidate that shows a maximum thermodynamic efficiency of 20% [3]. Presently, more than 80% of the global energy demand is met by fossil fuels [4]. In the United States, distillation alone accounts for an about 10% of the total industrial energy consumption. Keeping its large energy demand and low thermal efficiency, several heat integration techniques have been scrutinized seeking lower utility consumption and better profitability. The most popular schemes include the vapor recompression (VRC) [5] heat pump system and the dividing wall column (DWC) [6]. It is observed that [7] the former configuration performs well for the separation of close-boiling mixtures because of the requirement of a low compression ratio (CR) in VRC operation. As far as DWC is concerned, it has been known for several decades since the first patent filed in 1949 [8]. Then Petlyuk et al. [9] have developed a fully thermally coupled distillation column (FTCDC) that consists of a prefractionator and a main column, which is popularly known as Petlyuk column. Actually, the DWC column follows the concept of FTCDC by accommodating both the prefractionator and the main tower in a single shell [10]. It should be noted that the first industrial application of DWC was established by BASF in 1985 [11]. Currently, more than 100 DWC units are being used in industry [11]. Compared to a conventional system with direct or indirect sequence of distillation columns, the DWC scheme can achieve up to 30% savings in capital as well as operating cost [12,13]. Interestingly, this configuration requires a single reboiler and a condenser, whereas for example, a conventional two column system (CTCS) used in separating a ternary mixture requires two reboilers and two condensers. Reducing utility consumption as well as number of equipment (i.e., heat exchangers) leads to lower the capital and operating cost of DWC. This apart, the DWC column can also reduce the installation space up to 40% compared to the conventional sequences [14]. This savings in space requirements is owing to the reduced number of heat exchangers and associated equipment such as pumps, their supports etc. Because of these potential benefits, the DWC has emerged as a promising technology in boosting the thermodynamic reversibility of distillation in the current scenario of competition and environmental concerns.