Article pubs.acs.org/IECR
Equilibrium Model and Performances of an Isopropanol−Acetone− Hydrogen Chemical Heat Pump with a Reactive Distillation Column Min Xu,† Fang Xin,†,‡ Xunfeng Li,† Xiulan Huai,*,† Jiangfeng Guo,† and Hui Liu§ †
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡
ABSTRACT: The intrinsic kinetic of the liquid-phase dehydrogenation of isopropanol over an amorphous Raney nickel catalyst was determined to be in the temperature range of 348 to 355 K. The kinetic model based on the Langmuir−Hinshelwood mechanism was adopted for experimental data fitting. An isopropanol−acetone−hydrogen chemical heat pump (IAH−CHP) with a reactive distillation column was proposed in this paper. A mathematical model was established to evaluate the energy performance of the IAH−CHP in terms of coefficient of performance (COP) and exergy efficiency, in which the obtained intrinsic kinetic was incorporated. The optimal feed location and operating pressure of the distillation column were determined first. Moreover, the thermodynamic performance of the IAH−CHP improved with the increase of hydrogen−acetone molar ratio in the entrance of the exothermic reactor and the decrease of feed temperature of the distillation column. The operating performances between IAH−CHP with and without reactive distillation part were compared. The results indicate that the COP and exergy efficiency of an IAH−CHP system with reactive distillation part increase by 19.2% and 16.7% at most, respectively, compared with an IAH−CHP without reactive distillation part.
1. INTRODUCTION
acetone and hydrogen. The reaction equation is shown as follows:
Large amounts of low-temperature waste heat ( 82.4 °C) or liquid phase (TL ≤ 82.4 °C) for different temperatures of heat source. The IAH−CHP system implementing gas-phase dehydrogenation of isopropanol, which was first proposed by Prevost and Bugare,4 has been theoretically studied by some researchers.5−9 A demonstration unit was also built and evaluated by KlinSoda and Piumsomboon.10 The liquid-phase dehydrogenation of isopropanol has remarkable advantages: (i) lower heat source temperature and (ii) absence of the limitation of chemical Received: Revised: Accepted: Published:
Figure 1. Schematic diagram of the isopropanol−acetone−hydrogen chemical heat pump. © 2013 American Chemical Society
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4040
October 22, 2012 February 4, 2013 February 24, 2013 February 25, 2013 dx.doi.org/10.1021/ie3028872 | Ind. Eng. Chem. Res. 2013, 52, 4040−4048
Industrial & Engineering Chemistry Research
Article
equilibrium in endothermic reactors. For the heat source temperature lower than 100 °C, the IAH−CHP based on the liquid-phase dehydrogenation can be implemented easily. Saito et al.11 evaluated the energy efficiency of continuous and storage-type IAH−CHP implementing gas-phase dehydrogenation of isopropanol. Gandia and Montes12 established a mathematical model and estimated the optimal design variables. Kim et al.3 obtained the rate equation of isopropanol dehydrogenation at 82.4 °C over Raney nickel catalyst and evaluated the energy efficiency of the system. Some studies used reactive distillation technology to improve the performances of the IAH−CHP system. Reactive distillation is a new process intensification technology in the chemical industry. This technology combines chemical reaction and distillation separation in one single processing unit. Catalyst bags are loaded on the plates of the distillation column in which chemical reaction occurs and the product is simultaneously separated from the reactant. Gaspillo et al.13 experimentally investigated the performance of reactive distillation columns for isopropanol dehydrogenation with Ru−Pt catalysts using a gasphase dehydrogenation reactor as a reboiler. Chung et al.14 investigated the performance of the IAH−CHP adopting the reactive distillation process through modeling and simulation, and the optimal designs were obtained at the condition of an endothermic temperature of 90 °C and an exothermic temperature of 200 °C. The intrinsic kinetic model of the liquid-phase dehydrogenation reaction is required to be incorporated into the mathematical model. Many isopropanol dehydrogenation kinetic experiments were performed at boiling point (82.4 °C) over Raney nickel,3 Ni2B,15 and Ru−Pt,16 but these data are not available for the reactive zone of the distillation column in which temperature gradient exists. Although the above works have been done, a rigorous mathematical model for an IAH−CHP implementing liquidphase isopropanol dehydrogenation with reactive distillation has not been reported yet, and optimal parameters related to this model are not available in the literature. To address the above issues, the objectives of this work are to: (i) identify the suitable intrinsic kinetic model for catalytic isopropanol dehydrogenation over an amorphous Raney nickel catalyst; (ii) establish a mathematical model for an IAH−CHP system implementing liquid-phase isopropanol dehydrogenation with reactive distillation; (iii) evaluate the effect of the design parameters of the IAH−CHP system on thermodynamic performances, and (iv) compare the operating performance between IAH−CHP systems with and without reactive distillation part.
Figure 2. Experimental setup of isopropanol liquid-phase dehydrogenation. 1: Condenser; 2: oil bath; 3: 4-necked-flask; 4: ice trap; 5: stirrer.
the flask and heated up to the selected temperature. The reaction is started by adding 0.3 to 0.8 g of amorphous Raney nickel catalyst (ZL-N211, Anshan Zhongli Catalysts Factory), which is prepared by alkali leaching of rapidly quenched Ni−Al alloys,21 to the reactor through a funnel. Table 1 shows the Table 1. Specifications of the Raney Ni Catalyst Used in Our Work bulk composition (wt %)
SBET (m2/g)a
SNi (m2/g)b
Vp (cm3/g)a
dp (nm)a
Ni94.5Al5.5
104.4
22.5
0.1652
10.01
a
Determined by N2 adsorption at 77 K using a Micromeritics TriStar3000 apparatus. bDetermined by means of volumetric hydrogen chemisorption under the assumption that one Ni surface atom chemisorbs one hydrogen atom.
characterization of the catalyst including bulk composition, BET surface area, pore volume, mean pore diameter, and active nickel surface area. During the reaction, the hydrogen flow rate is recorded online every second. Four runs are performed at 348, 351, 353, and 355 K. All of the experimental runs are performed at the temperature lower than or close to the bubble point of the mixture in the flask due to the little conversion of isopropanol (
