Exergy analysis and performance enhancement ofisopropanol-acetone-hydrogen chemical heat pump

Abstract

Exergy loss analysis was conducted to identify the irreversibilityin each component of the isopropanol-acetone-hydrogen chemical heatpump (IAH-CHP). The results indicate that the highest irreversibilityon a system basis occurs in the distillation column. Moreover, theeffect of operating parameters on thermodynamic performances of theIAH-CHP was studied and the optimal conditions were obtained. Finally,the potential methods to reduce the irreversibility of the IAH-CHPsystem were investigated. It is found that reactive distillation isapromising alternative. The enthalpy and exergy efficiency of theIAH-CHP with reactive distillation increases by 24.1% and 23.2%, respectively.

Keywords

waste heat reuse / chemicalheat pump / exergy analysis / isopropanol

Introduction

Large amounts of low-temperature(<100°C) waste heat widely exist in industrial processes, whichare usually released to the environment through cooling water [1]. To save energy, it is of importanceto reuse the low temperature waste heat. An effective method to recoverthe waste heat is upgrading it to a degree higher than 200°C,and the upgraded heat can be easily supplied to medium-pressure steamgrid. The isopropanol-acetone-hydrogen chemical heat pump (IAH-CHP)is a competitive alternative in view of its advantages of high upgradingtemperature, possibility of energy storage, and low hazard. Figure1 shows the schematic diagram of the IAH-CHP, which consists of anendothermic reactor, an exothermic reactor, and a distillation column.

The dehydrogenation of isopropanoloccurs at temperature T_L in the endothermic reactor (reboiler in Fig. 1) toyield acetone and hydrogen. The heat Q_L is required to drive this reaction. Theunreactive isopropanol is separated from acetone and hydrogen in thedistillation column. The acetone and hydrogen at condensing temperature T_C is compressed bythe compressor, and then heated in the recuperator.The acetone andhydrogen are then fed into the exothermic reactor where acetone hydrogenationoccurs. A high-temperature heat Q_H in the exothermic reactor is produced andreleased. The effluent is cooled in the recuperator and is then fedback to the distillation column.

Thermodynamic performances were theoreticallyor experimentally studied by many researchers. Gandia and Montes [2 ] established a mathematicalmodel of the IAH-CHP and estimated the optimal design variables byusing enthalpy efficiency analysis. Kim et al. [3] obtained the rate equation of isopropanoldehydrogenation over Raney nickel catalyst, and evaluated the enthalpyefficiency of the system. Chung et al. [4] proposed the IAH-CHP employing the reactive distillationprocess, and found that its energy performances based on the firstlaw of thermodynamics were improved. A demonstration unit was builtand evaluated by Klin Soda and Piumsomboon [5], and the enthalpy and exergy efficiencieswere evaluated.

The above literature review demonstratesthat the IAH-CHP has not been extensively studied, and most worksfocus on the thermodynamics analysis at the system level. The exergyloss method, known as the second law analysis, calculates the exergyloss caused by irreversibility and helps to identify the componentswhere inefficiencies occur. Improvements should be made to these componentsto increase their efficiency. In recent years, exergy loss analysishas been used to design the heat pump system [6,7]. To the best of the authors’ knowledge, however, the optimaldesign of the IAH-CHP has still not yet been conducted by using exergyloss analysis.

Therefore, the objective of thispaper is to calculate the exergy losses of each component in the IAH-CHPsystem, evaluate the effect of the design parameters from the pointof view of exergyanalysis, and propose potential improvement methodsto minimize the exergy loss of the IAH-CHP system.

Mathematical model

The mathematical model of the IAH-CHPis established in this section. The rigorous equilibrium stage (EQ)model RadFrac is established to simulate the distillation column usingthe commercial simulation software Aspen Plus. The reaction is specifiedto occur in the reboiler. The intrinsic kinetic model of the liquid-phasedehydrogenation reaction is needed to incorporate into the mathematicalmodel. The user kinetics subroutine (FORTRAN code) is used to depictthe experimentally-determined reaction kinetic model, and then compiledand linked with the simulation modelin Aspen Plus by using the AspenPlus Simulation Engine.

In this paper, a reaction kineticexpression obtained from Ref. [8] is used:

(1)r_{\mathrm{d}}={\displaystyle \frac{\mathrm{1.55}\times {\mathrm{10}}^{\mathrm{27}}\exp (-\mathrm{198890}/RT)}{\mathrm{1}+\mathrm{6.15}\times {\mathrm{10}}^{-\mathrm{45}}\exp (\mathrm{290420}/RT){(C_{\mathrm{ace}})}^{\mathrm{0.5}}}},

where r_d is the rate of dehydrogenation of isopropanol, R is the gas constant, T is the reaction temperature, and C_ace is acetone concentration.

For the exothermic reactor, the plugflow reactor model with cocurrent coolant is used. The kinetic equationof hydrogenation of acetone is used to incorporate the model, whichis reported by Kato et al. [9] and is shown as

(2)r_{\mathrm{h}}={\displaystyle \frac{k{K}_{\mathrm{a}}{K}_{\mathrm{h}}{P}_{\mathrm{a}}{P}_{\mathrm{h}}}{{(\mathrm{1}+K_{\mathrm{a}}{P}_{\mathrm{a}}+K_{\mathrm{h}}{P}_{\mathrm{h}})}^{\mathrm{2}}}}.

As for an equilibrium reaction, theapparent rate equation can be expressed as

(3)r={\displaystyle \frac{k{K}_{\mathrm{a}}{K}_{\mathrm{h}}(P_{\mathrm{a}}{P}_{\mathrm{h}}-P{P}_{\mathrm{i}}/K_{\mathrm{p}})}{{(\mathrm{1}+K_{\mathrm{a}}{P}_{\mathrm{a}}+K_{\mathrm{h}}{P}_{\mathrm{h}})}^{\mathrm{2}}}},

where k=\mathrm{1.71}\times {\mathrm{10}}^{\mathrm{6}}\exp \left({\displaystyle \frac{-\mathrm{61500}}{RT}}\right), K_{\mathrm{a}}=\mathrm{2.90}\times {\mathrm{10}}^{-\mathrm{11}}\exp \left({\displaystyle \frac{\mathrm{54400}}{RT}}\right), K_{\mathrm{h}}=\mathrm{1.94}\times {\mathrm{10}}^{-\mathrm{15}}\exp \left({\displaystyle \frac{\mathrm{92500}}{RT}}\right), and K_p is the chemicalequilibrium constant, which could be calculated by Gibbs free energychange for the reaction as

(4)K_{\mathrm{p}}=\exp ({\displaystyle \frac{\mathrm{2273.6}}{T}}-\mathrm{13.71}).

Exergy analysis

The irreversibility in a processresults in the exergy loss, which is caused by friction, heat transferunder temperature difference and unrestricted expansion. The commonexergy balance can be expressed as

(5){\psi }_{\mathrm{in}}-{\psi }_{\mathrm{out}}+{\psi }_{\mathrm{heat}}+W=I_{\mathrm{irr}},

where {\psi }_{\mathrm{in}}and {\psi }_{\mathrm{out}} are the inlet and outlet exergies, respectively, of each component, {\psi }_{\mathrm{heat}} is the exergy destroyed throughheat transfer, W is the input workin the process, and I_irr is the exergy loss. Equation (5) can also be writtenas

(6)\sum m_{\mathrm{i}\mathrm{n}}{\psi }_{\mathrm{i}\mathrm{n}}-\sum m_{\mathrm{o}\mathrm{u}\mathrm{t}}{\psi }_{\mathrm{o}\mathrm{u}\mathrm{t}}+\mathrm{\Sigma }Q(1-{\displaystyle \frac{T_0}{T_r}})+W=I_{\mathrm{i}\mathrm{r}\mathrm{r}},

where \psi =(H-H_{\mathrm{0}})-T_{\mathrm{0}}(S-S_{\mathrm{0}})is the flow exergy, T₀ is the environment temperature (298.15 K in this work).Based on Eq. (6), the exergy loss of each component in the IAH-CHPsystem illustrated in Fig. 1 are calculated, respectively.

The coefficient of performance (COP) and exergyefficiency can be expressed as

(7)\mathrm{COP}={\displaystyle \frac{Q_{\mathrm{H}}}{Q_{\mathrm{L}}}},

(8)\eta ={\displaystyle \frac{(m_{\mathrm{6}}{H}_{\mathrm{6}}-m_{\mathrm{7}}{H}_{\mathrm{7}})-T_{\mathrm{0}}(m_{\mathrm{6}}{S}_{\mathrm{6}}-m_{\mathrm{7}}{S}_{\mathrm{7}})}{Q_L(\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}}{T_{\mathrm{L}}}})+W}}.

Results and discussion

The optimized specifications andoperation conditions of the IAH-CHP are listed in Table 1. The propertydata of the five streams of the IAH-CHP and the coolant water steamare summarized in Table 2. Irreversibilities in each of the componentsand the thermodynamic performances for the IAH-CHP system are presentedin Table 3. It is found that the highest irreversibility on a systembasis occurs in the distillation column, which accounts for about85% of the total exergy loss. The enthalpy and rational efficienciesof the IAH-CHP are found to be 0.086 and 0.18, respectively. The resultsindicate that the improvement potential is huge for the IAH-CHP system.

Figure 2 illustrates the variationsof the COP and exergy efficiency with the pressure of the distillationcolumn at different pressures of the exothermic reactor. As shownin Fig. 2, for the fixed pressure of exothermic reactor, the COP andexergy efficiency increases with the increase of pressure in the distillationcolumn, and experiences a maximum at 0.12 MPa. The COP and exergyefficiency varies slightly as the pressure of the endothermic reactorincreases. The maximums of the COP and exergy efficiency are obtainedat P_dis =0.12 MPa and P_exo = 0.12 MPa. The variations of the exergy loss in each of the componentsas the pressure of distillation column increases are depicted in Fig.3. The exergy loss of the distillation column experiences a minimumat 0.12 MPa.

Figure 4 demonstrates the effectsof the isopropanol content of the effluent from the distillation column(stream 1 in Fig. 1) on the COP, exergy efficiency, as well as exergyloss in distillation column and heat released from the exothermicreactor. As shown in Fig. 4(a), the COP and exergy efficiency increasewith the increase of the isopropanol content of the distillate, whichis resulted from the decrease of the irreversibility in the distillationcolumn (see Fig. 4(b)), at the expense of the decrease of the heatreleased (shown in Fig. 4(b)). Therefore, the trade-off between exergyefficiency and the heat released should be made. In this paper, 3%of the content of isopropanol in the distillate is recommended.

The variations of the COP and exergyefficiency with reactant temperature in the inlet of the exothermicreactor at different inlet coolant temperatures are displayed in Fig.5. The COP and exergy efficiency increase as the inlet temperatureof reactant increases and decrease as the inlet temperature of coolantincreases. The big temperature difference between the reactant andcoolant in the inlet of the exothermic reactor results in the smalltemperature change in the exothermic reactor, thereby reduces theexergy input of the distillation column (i.e. the flow exergy of stream5 decreases at the expense of the increase of the heat transfer areaof the recuperator), which leads to the decrease of the exergy lossin the distillation column, as shown in Fig. 6.

From the above parameters studies,the optimal conditions of the IAH-CHP could be P_dis = 0.12 MPa, P_exo = 0.12 MPa, T_in,reac = 473 K, T_in,cool = 433 K,and x_iso,1 = 0.03. It should be noted that the choice of the optimization temperatureof T_in,reac and T_in,cool is based on the experimental results. The byproducts may occur ata higher exothermic temperature and the reaction cannot take placeat a lower endothermic temperature [10, 11]. Table 4 lists the exergyanalysis results under the above optimal condition. It can be seenthat the exergy loss in the distillation column decreases greatly.The reason may be clear if observed from the profiles of temperaturein the distillation column with and without reactive zone as shownin Fig. 7. The temperature in the stages of 8–13 of the columnwith reactive zone is lower than that without reactive zone. However,the exergy loss in the distillation column is still the biggest ofall components. Thereby, further minimizing the irreversibility ofthe distillation column is essential. Vapor recompression distillation(VRD),heat integrated distillation columns(HIDiC), and reactive distillationare usually used to save energy for the distillation process. Forthe VRD and HIDiC case, the heat load decrease at the expense of theextra input power. It is found that, for the two cases, a pressureof the compressor of at least 0.25 MPa is needed to ensure the feasiblilityof the IAH-CHP. This indicates that an additional power of at least0.55 kW is needed while the heat released from the exothermic reactoris 1.59 kW, which may not be economical for a heat pump.

The performances of the IAH-CHP adoptingthe reactive distillation are also investigated through simulationmodeling. For the case of the IAH-CHP with reactive distillation,the reactive distillation column is divided into the lower reactivezone (stage 8–14) and the upper rectification zone (stage 2–7).The exergy analysis results of the IAH-CHP with and without reactivedistillation are also listed in Table 4. It can be seen that the exergyloss with reactive distillation is decreased greatly. Moreover, theCOP and exergy efficiency of the IAH-CHP with reactive distillationincreases by 24.1% and 23.2%, respectively.

Conclusions

The highest irreversibility on asystem basis occurs in the distillation column, which accounts forabout 75%–85% of the total exergy loss.

The optimal thermodynamic performancesof the IAH-CHP system could be obtained at P_dis = 0.12 MPa, P_exo = 0.12 MPa, T_in,reac = 473 K, T_in,cool = 443 K,and x_iso,1 = 0.03.

Reactive distillation is an effectivemethod to enhance the performances of the IAH-CHP system. The COPand exergy efficiency of the IAH-CHP with reactive distillation partincreases by 24.1% and 23.2%, respectively.

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