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Evaluation of combined absorption heat pump–methanol steam reforming system: Feasibility criterion as a measure of system performance
Energy Conversion and Management 52 (2011) 1974–1982
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Evaluation of combined absorption heat pump–methanol steam reforming system: Feasibility criterion as a measure of system performance Willy Yanto Wijaya ⇑, Shunsuke Kawasaki, Hirotatsu Watanabe, Ken Okazaki Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 7 November 2010 Accepted 21 November 2010 Available online 8 January 2011 Keywords: Methanol steam reforming Absorption heat pump Feasibility criterion
a b s t r a c t This paper presents an evaluation of combined absorption heat pump (AHP) and methanol steam reforming (MSR) system. To measure the effectiveness of this combined system, a feasibility criterion was proposed, which measured the ratio between net energy gain obtained by MSR reaction over energy required by AHP system. By using the proposed feasibility criterion, optimum AHP step number could be determined. Other parameters pertaining to both AHP and MSR system were also determined and calculated. In particular, discussions would focus on the effects of steam–carbon molar ratio (S/C) and gas hourly space velocity (GHSV) of experimental MSR upon feasibility criterion of combined system. It was shown that the decrease of GHSV caused the increase of feasibility criterion up to the AHP step number 3; meanwhile, the increase of S/C resulted in the shifting of feasibility criterion peak from AHP step number 3 to 2. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
CH3 OHðlÞ þ H2 OðlÞ $ CO2ðgÞ þ 3H2ðgÞ ;
ð1Þ
Global warming and the rapidly depleted fossil fuels have accelerated hydrogen as one of promising fuels in the future [1]. In the far future ahead, the production of hydrogen will rely totally on the contribution of renewable energy. However, for the short and middle-term scenario, the fraction of renewable energy contribution will be still significantly small [2]. Therefore, hydrogen production still relies heavily on the hydrocarbon processing as the main feedstock. The hydrocarbon processing to produce hydrogen mainly comprises of the reforming reaction, partial oxidation, and combination of both (autothermal reaction) [3], with reforming reaction gives the highest yield of hydrogen concentration, as shown by Ahmed and Krumpelt [4]. Since hydrogen production methods from hydrocarbon still leave some carbon footprint, short to middle-term efforts to mitigate the CO2 emission is still required, as proposed by Damm and Fedorov [5]. Meanwhile, huge amount of waste heat is being dissipated each year. In Japan, more than 800 PJ waste heat is being discarded by various industrial sectors annually [6]. Most of this waste heat is low-quality heat, less than 100 °C, which is inefficient to be recovered into electricity/other useful works. Methanol steam reforming (MSR) is one of the most favorable ways for hydrocarbon-based hydrogen production. The chemical reaction of MSR can be written as:
⇑ Corresponding author. Tel./fax: +81 3 57342179. E-mail address: [email protected] (W.Y. Wijaya). 0196-8904/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.11.013
DHMSR ¼ þ130:97 kJ=mol CH3 OH;
DGMSR ¼ þ9:18 kJ=mol CH3 OH: The products of this reaction (three moles of hydrogen) have higher value of enthalpy and Gibbs free energy compared with one mole of methanol, which is due to the endothermic nature of MSR. Besides, by MSR process, exergy rate of the thermal energy (waste heat) could be increased significantly and stored into hydrogen energy, as shown in Fig. 1. Exergy rate (e) is defined as the ratio of Gibbs free energy (potential useful work) over the enthalpy (energy amount), which can be expressed as:
e¼
DG : DH
ð2Þ
Researches in the field of MSR have been extensively carried out during this past decade. One of the topics most intensively studied is the catalyst development and investigation for the MSR process. Copper-based catalysts are so far the most commonly employed catalysts, showing high effectiveness in the MSR reaction as reported by Lindstrom and Pettersson [7]. The preparation of the Cu/ZnO/Al2O3 catalysts can be carried out through homogenous precipitation method as demonstrated by Shishido et al. [8], and also sol–gel method as studied by Matsumura and Ishibe [9]. Shishido et al. [8] found out that the catalytic activity was well correlated with the surface area of Cu metal and the maximum activity was obtained with the Cu/Zn ratio of 1/1. On the other hand, using sol–gel method, Matsumura and Ishibe [9] discovered that the catalytic activity increases with the copper content up to 40 wt.%. Microstructural characterization of various real Cu/ZnO/
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Nomenclature cp,m cp,s D GHSV N n n_ m n_ s Q_ A S Sf S/C TA TC TD TE TG Tmix x _ AHP W y
specific heat capacity of methanol, J mol1 K1 specific heat capacity of steam, J mol1 K1 coefficient to adjust gradient of sigmoid function gas hourly space velocity, h1 number of measurement data step number of absorption heat pump molar flow rate of methanol, mol s1 molar flow rate of steam, mol s1 heat supplied by AHP absorber to the flowing steam, J s1 standard error of measurement standard error of fitting steam–carbon (methanol) molar ratio temperature of AHP absorber, K temperature of AHP condensor, K temperature of waste heat, K temperature of AHP evaporator, K temperature of AHP generator, K mixing temperature of steam and methanol, K coefficient to adjust dislocation of sigmoid function internal work of absorption heat pump, J s1 average of measurement data
Fig. 1. Concept of waste heat exergy enhancement by MSR.
Al2O3 catalysts had been investigated by Kurr et al. [10] using in situ X-ray diffraction, in situ X-ray absorption spectroscopy, temperature programmed reduction and tunneling electron microscopy. They found out that the catalytic activity and thermal stability under MSR reaction conditions was also influenced by structural defects in the bulk of the materials. Alternative support material for the Cu/ZnO nanoparticles, such as employing high porous carbon, recently has also been studied by Kudo et al. [11]. Their preparation method included the carbonization of an ion exchange resin loaded with metal cations. Their results indicated that the ratio of Cu/Zn in the resin did not influence the micropore structure of carbon support, but significantly affected the form of Cu particles. Although many researchers have developed new catalysts for MSR, the waste heat temperature (<100 °C) is still not sufficient to convert methanol into hydrogen employing those catalysts. Another possible approach is by developing the reactor design or process engineering. Nagano et al. [12] analyzed the heat trans-
yi DG DH
/
single measurement data Gibbs free energy change of reaction, kJ mol1 enthalpy change of reaction, kJ mol1 exergy rate efficiency of absorption heat pump efficiency of methanol conversion into product species feasibility criterion
Subscript AHP MSR A C D E G m mix s
absorption heat pump methanol steam reforming absorber condenser waste heat evaporator generator methanol mixing steam
e gAHP gMSR
Superscript ° standard reference state (298.15 K, 1 atm)
fer enhancement for the methanol steam reformer. By applying internal corrugated metal heater and external catalytic combustion heater, they could maintain uniform temperature distribution up to the methanol liquid hourly space velocity 3 h1. The transient behavior of a small methanol reformer after cold start was also studied by Horng [13]. He constructed a small methanol reformer and attempted to compromise several parameters for shortening the start-up time. Combination of MSR with other energy system, however, has been less considered by research groups working in the MSR field. In a recent paper by Chen and Lin [14], the MSR reaction was exposed to an environment with microwave irradiation. They reported that the MSR could be heated and triggered rapidly within a short time due to the double absorption of microwaves by both the reagents and the catalyst. These numerous researches have pushed significant progress and unveiled understanding of many aspects of the MSR characteristics. Nevertheless, most of these researches did not put any emphasis on the endothermic nature of MSR and the potential of using the waste heat as the endothermic energy supply to enhance the total energy level. As shown in Fig. 1, huge amount of industrial low-quality waste heat has potential to be recovered as MSR endothermic heat. However, this waste heat temperature (<100 °C) is not sufficient for converting methanol into hydrogen in the actual MSR experimental process. In the actual process, MSR requires temperature level at least 200 °C for achieving good conversion rate into hydrogen [15]. Therefore, we propose utilizing an absorption heat pump (AHP) to enhance the temperature of the waste heat to the degree favorable for MSR reaction. To date, none of the pertinent literatures in the MSR-related field, which evaluate the combined system of MSR and AHP, could be found. However, we believe that integration of AHP into MSR system, for recovering the waste heat, has a promising applicability in the industrial sectors. In this paper, we present a basis to measure the effectiveness of the combined AHP–MSR system using a proposed feasibility criterion. Theoretical approach is applied to analyze the AHP system, coupled with the experimental results of MSR, aimed to evaluate the feasibility of the combined AHP–MSR system.
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2. Experiments of MSR 2.1. Experimental details The experimental set-up of MSR consists of a reactor module, heating system, and gas analysis system as shown in Fig. 2. Inside the reactor module, a packed-bed reaction chamber with dimension 11 69 3 mm3 was filled with 1.5 g of Cu/Zn/Al2O3 MDC commercial catalyst. The density of this catalyst was 1.2 kg l1 with composition CuO 42%, ZnO 47%, and Al2O3 10% as specified by its manufacturer. This catalyst was crushed into grains, and then sieved to obtain the particle size 500 lm to 1.18 mm in diameter. The reactor module was encapsulated with a ceramic heater, equipped with a temperature controller (FKC-11™ Tokyo Karasu Kikai) to provide heating and keep a constant reaction temperature during the experiments. Temperature measurement was conducted by inserting a K-type thermocouple (1 mm diameter) at the middle of the reaction chamber. To confirm the profile of temperature distribution, two additional thermocouples were inserted as a test case at the locations of 15 mm from the inlet and outlet of the reaction chamber. The temperature variation between these two locations and the middle point of the chamber was within the range of ±3 °C. In addition, a ribbon heater, set at temperature about 110 °C, was used to preheat and vaporize the methanol solution before entering the reaction chamber. To ensure the measurement quality, prior to performing the experiments, a leakage check was carried out by measuring the flow rate of a nitrogen gas feed before entering the reactor and
comparing the outlet flow rate with a bubble flow meter. The measurement devices were also carefully calibrated, with the accuracy and relative errors are shown in Table 1. Before performing the MSR reaction, the catalyst bed was pre-heated and reduced using mixture of nitrogen and hydrogen, 50 ml min1 each, regulated by a flow meter (Horiba Stec SEC-B40™) at 250 °C for 2 h. This reduction process was intended to remove the oxygen molecules attached on the catalyst due to oxidation. Methanol solution was fed into the reactor module using a micro-feeder. The methanol solution flew through the evaporator (ribbon heater) to be vaporized into the gas phase before entering the reaction chamber. In the reaction chamber, methanol and water vapor reacted catalytically, in the endothermic manner, with enthalpy (energy) enhancement about 49.5 kJ for each mole of converted methanol:
CH3 OHðgÞ þ H2 OðgÞ $ CO2ðgÞ þ 3H2ðgÞ
DH298K ¼ 49:5 kJ=mol:
ð3Þ
The operating conditions of MSR experiments were performed at temperature range 160–225 °C measured by thermocouple inserted to the center of the reaction chamber, with variation of four different gas hourly space velocity (GHSV = 4000 h1, 2666 h1, 2000 h1, 1333 h1). GHSV is defined as the ratio of the gas flow (methanol and water vapor) into the reaction chamber over the volume of the catalyst: 1
1
GHSV½h ¼
Flow rate of CH3 OH and H2 O at inlet ½m3 h : Volume of catalyst ½m3 ð4Þ
Fig. 2. Schematic of the methanol steam reforming experimental set-up.
Table 1 Measurement range, accuracy, and relative error of measurement devices. Measurement
Equipment
Range
Accuracy
Relative error (%)
Gas flow Reaction temperature Temperature control Methanol solution feed
Horiba Stec SEC-B40™ K-type 1.0 mm Sakaguchi FKC-11™ Fine Tokyo Karasu Kikai Micro-feeder Furue Science
0–100 ml/min 0–650 °C 0–1093 °C 22.7 ll/h to 3.3 ml/s
±0.1 ml/min ±0.1 °C ±0.1 °C ±0.1 ll/h
±1 ±0.5 ±0.5 ±0.5
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Besides GHSV, two different conditions of steam to carbon (methanol) molar ratio (S/C = 1 and 2) were also performed in this experiment. In the steady flow system, S/C is defined as:
S=C ¼
n_ s ; n_ m
ð5Þ
where n_ s is molar flow rate of steam at the inlet of reactor, and n_ m is molar flow rate of methanol vapor at the inlet of the reactor. After undergoing the catalytic reforming reaction, the product stream from the reactor outlet was then cooled by passing over a cold-trap in order to condensate the un-reacted water and methanol. Constant flow of nitrogen (50 ml/min) was supplied as the reference gas for the gas chromatography analysis. The product gases and the supplied nitrogen were analyzed using gas chromatograph (GC-8APT, Shimadzu Corp., TCD with Molecular-sieve 13 X column, N2 carrier gas). Prior to the MSR experiments, the gas chromatograph was calibrated by means of standard gases and atmospheric air to ensure that the measurement results of the product gases were reliable. 2.2. Experimental results The results of methanol conversion at different operating conditions are shown in Fig. 3. The methanol conversion (gMSR) was determined from:
gMSR ½% ¼
n_ m ðat inletÞ n_ m ðat outletÞ 100; n_ m ðat inletÞ
ð6Þ
where n_ m is the molar flow rate of methanol vapor at the inlet and outlet of MSR reactor respectively. To ensure the consistency of measurement results, three times measurements were conducted for each experimental point obtained. Errors in the experiments can arise from the instruments condition, calibration, as well as observation and measurement uncertainties. The accuracy of these experimental results was validated with the standard error, described as:
S¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP 2 u N yi y t i¼1 y N
;
ð7Þ
is the average of the where yi is the single measurement data, y measurement data, and N is the total number of the measurements. The mean value of the standard error of the methanol conversion was 5.94%. From Fig. 3, it is obvious that due to the endothermic nature of MSR, the conversion of methanol is enhanced along with the increasing temperature. This enhancement pervades for all different GHSV and S/C, and might be attributed to endothermic reaction equilibrium proportional dependency on temperature as well as
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the reaction kinetics of the catalytic reforming. On the other hand, the increase of GHSV results in lower conversion of methanol, where this could be attributed to the shorter residence time of methanol solution having contact with the catalyst bed inside the reactor chamber. For our experimental conditions, with GHSV smaller than 2000 h1, S/C = 1, and reaction temperature of 225 °C, the conversion of methanol could achieve level as high as 90%. Meanwhile, the steam–carbon molar ratio (S/C) also contributed positively on the conversion of methanol. This is due to the equilibrium shift of MSR reaction towards products formation, as governed by Le Chatelier’s principle. Besides, the proportion of methanol in the solution also decreases, thus the chance of each CH3OH molecule to have contact with catalyst is becoming higher. For the same GHSV of 4000 h1, reaction with S/C = 2 shows better performance in term of methanol conversion than the one with S/ C = 1, and it seems that the positive effect of S/C is becoming more emphasized at the higher temperature. The results of our MSR experiments, with variation of temperature, GHSV, and S/C, show relatively good agreement with results as reported by Purnama et al. [16]. In their MSR experiments, they investigated the profile of methanol conversion as a function of catalyst weight over the flow rate of methanol (W/Fm), where they achieved methanol conversion as high as 90% at 250 °C with the condition of S/C = 1 and W/Fm = 0.013 g s mol1. Some differences when comparing to our experimental results can be attributed to the different amount and size of the Cu/ZnO/Al2O3 catalyst they used, as well as the different flow rate. However, the profile of their hydrogen yield indicated similar trends when compared with the trend of our methanol conversion. The experimental works of Basile et al. [17,18] also demonstrated the increasing characteristics of methanol conversion when the GHSV decreased. They obtained the methanol conversion about 60% at 300 °C and S/C = 3 with weight hourly space velocity from 0.36 h1 to 1.82 h1. Their unfavorable results in term of methanol conversion might be due to the quality of their own-fabricated Cu/Zn/Mg-based catalysts and the re-use of the same catalysts for several cycles of MSR experiments. However, their results showed interesting correlations regarding the effects of membrane and reaction pressure on the methanol conversion and hydrogen recovery. As shown in Fig. 3, methanol conversion in MSR exhibits a nature of sigmoid-curve-like phenomena. The different conditions of GHSV and S/C will particularly affect the gradient of this conversion curve growth. Taking these into account, the experimental results can be curve-fitted into sigmoid function, interpolated, and then be used in the calculation for the combined system of MSR and AHP. 3. Calculation method Fig. 4 shows the schematic of combined AHP and MSR. Waste heat contained in the 100 °C steam is boosted by AHP system to become high temperature steam, and then fed to the MSR reactor. Simultaneously, liquid methanol is evaporated and heated up to the temperature level of waste heat (TD). The methanol gas is mixed with high temperature steam in the MSR reactor, and reacted with the help of catalysts, producing output hydrogen. The calculation method proposed in this paper comprises of thermodynamic-based theoretical calculation of AHP and interpolation results of empirical MSR experimental data. The calculation scheme is shown in Fig. 5, and details will be discussed in Section 3.2. 3.1. Description of AHP system
Fig. 3. Experimental results of methanol conversion as a function of temperature in several GHSV and S/C conditions.
AHP system consists of four main parts: absorber, evaporator, generator, and condenser, as shown in Fig. 6. The waste heat in
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Fig. 4. Schematic of combined AHP and MSR.
the form of hot water or steam is transported into generator and evaporator, heating those two parts, and changing the phase of
the circulating refrigerant. From evaporator, the refrigerant gas enters the absorber, having exothermic reaction, releasing heat that is to be used for generating steam in the desired temperature level. Meanwhile, the refrigerant gas from generator will be condensed into liquid form, requiring less energy to be pumped to the evaporator. The temperature of generator (TG) and evaporator (TE) is relatively similar, and thus will be simplified as one uniform temperature, denoted by TD. Cooling water in room temperature will be used to cool the condenser, maintaining its temperature of TC. The heat released in absorber will be used to heat up steam (containing waste heat with temperature TD) to become desired output temperature of AHP, denoted by TA, before eventually being mixed with methanol. The main advantage of AHP system compared to vapor-compression heat pump is in the term of its driving input power. While vapor-compression heat pump generally requires electricity (high quality form of energy) and consumes huge amount of this energy for its compression processes, AHP can make use of low-quality energy (such as waste heat) for its main driving forces. When lowquality or wasted energy is available in abundance, AHP can achieve the break-even point despite its high initial investment cost [19,20].
Fig. 5. Scheme of calculation to evaluate the combined absorption heat pump–methanol steam reforming system.
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Fig. 6. Schematic of absorption heat pump system, with temperature conditions of generator, evaporator, condensor and absorber.
Q_
3.2. Calculation scheme of combined AHP–MSR system As shown in Fig. 5, the calculation is divided into four steps. First step is the determination of the AHP operating parameters, which include TC, TD, and n (AHP step number). Step number is the number of the stage/step in AHP necessary for further boosting of output temperature of absorber (TA). In this research, the value of TA is adjusted by n (AHP step number). The relation between n and TA can be derived from Dühring chart of H2O/LiBr solution that shows linear proportionality between refrigerant temperature and solution temperature in the AHP system [19,20], and can be expressed as:
T A ¼ T D ðT D =T C Þn :
ð8Þ
In this calculation, a fixed temperature of waste heat TD = 373 K, is used to heat up evaporator and generator of AHP. Meanwhile, the condenser is cooled by water in room temperature, assumed to reach equilibrium at TC = 298 K. By increasing the step number while fixing TD and TC constant, higher TA can be obtained. Besides TA, the second step to calculate AHP parameters also in_ AHP ) and coefficient of cludes internal work of AHP (denoted by W _ performance (COP). W AHP is the ideal work of AHP internal system, based on thermodynamics, which can be derived from equations:
Fig. 7. Temperature of AHP absorber as a function of AHP step number.
T
A gAHP ¼ _ A ¼ ; and W AHP T A T D
Q_ A ¼ n_ s cp;s ðT A T D Þ;
ð9Þ ð10Þ
therefore:
_ AHP ¼ n_ s cp;s ðT A T D Þ2 =T A ; W
ð11Þ
where n_ s is the molar flow rate of steam passing through AHP absorber to be heated up from TD to TA, Q_ A is the heat supplied by absorber to the passing steam, gAHP is the efficiency of AHP, and cp,s is the specific heat of steam (regarded to be constant at 36.5 J mol1 K1). Accordingly, the performance of AHP can also be described conveniently with COP which is described by following equation:
COP ¼
TA TA TC
TD TC : TD
ð12Þ
_ AHP , COP) After obtaining the performance parameters of AHP (W and output temperature (TA), the next step considers parameters involved in the combined system of AHP and MSR. Output steam from AHP with temperature TA is to be mixed with methanol gas, yielding a mixing temperature (Tmix) which can be expressed by:
Fig. 8. Coefficient of performance (COP) as a function of AHP step number.
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n_ s cp;s ðT A T mix Þ ¼ n_ m cp;m T mix T CH3 OH ;
ð13Þ
where n_ s is the molar flow rate of steam to be mixed with methanol gas, cp,s is the specific heat of steam (36.5 J mol1 K1), n_ m is the molar flow rate of methanol gas to be mixed, cp,m is the specific heat of methanol gas (61.4 J mol1 K1), Tmix is the equilibrium temperature of mixing, and T CH3 OH is the temperature of methanol gas before being mixed. Since methanol can be pre-heated by waste heat, T CH3 OH is regarded to reach TD (373 K) before the mixing process. After the methanol gas and steam are mixed at temperature Tmix, with the help of catalyst, steam reforming of methanol occurs. Therefore, Tmix corresponds to temperature of reaction in experimental MSR. The value of (n_ s /n_ m ) in Eq. (13) corresponds to steam–carbon ratio (S/C) in the MSR experiments. Experiment results of methanol conversion at certain GHSV and S/C are curve-fitted with sigmoid function described by:
gMSR ðT mix Þ ¼
1 ; 1 þ expðDðT mix xÞÞ
Fig. 11. Feasibility criterion as a function of AHP step number, at S/C = 1, GHSV 4000 h1 and 1333 h1.
ð14Þ
where Tmix is the reaction temperature, D and x are coefficients which determine the gradient and dislocation of the sigmoid curve to be fitted to the experimental data. The fitting results of methanol conversion in several GHSV and S/C condition are then to be used as one of the numerators to calculate feasibility criterion. The errors associated with the fitting were validated using standard error. The maximum value of the fitting standard error of the methanol conversion was 2.23%.
The final step of calculation scheme is to determine feasibility criterion (/), which is defined as:
/¼
Energy gain by MSR n_ m gMSR ðT mix Þ DHMSR ¼ ; _ AHP Work required for AHP W
ð15Þ
where gMSR(Tmix) is the interpolated results of methanol conversion expressed in Eq. (14), DHMSR is the enthalpy (energy) enhancement of MSR reaction described in Eq. (3), n_ m is the molar flow rate of _ AHP is the internal work required in AHP system, methanol, and W which is expressed in Eq. (11). The feasibility criterion will serve as a parameter to determine the effectiveness of this combined system by comparing net energy gain as its numerator and work required as its denominator. As the step number changes, while fixing the TD and TC, output temperature (TA) will also change, which subsequently affect the mixing temperature, conversion effi_ AHP , and consequently the feasibility criterion. ciency, W 4. Results and discussion 4.1. Characteristic of AHP
Fig. 9. Mixing temperature of methanol gas and steam as a function of AHP step number, at S/C = 1 and 2.
Fig. 10. Calculation results of methanol conversion as a function of AHP step number, at S/C = 1, GHSV 4000 h1 and 1333 h1.
The characteristic of AHP is indicated by three main parameters i.e. temperature of absorber (TA), coefficient of performance (COP), _ AHP Þ. The result of TA is shown in and internal work of AHP ðW Fig. 7. If the temperature of condenser (TC) and generator/evaporator (TD) are fixed, the achievable TA will solely depend on number of steps in the AHP system. The increasing step number will be
Fig. 12. Calculation results of methanol conversion as a function of AHP step number, at S/C = 1 and 2, GHSV 4000 h1.
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Fig. 13. Feasibility criterion as a function of AHP step number, at S/C = 1 and 2, GHSV 4000 h1.
proportional to the exponential increase of TA. On the other hand, as shown in Fig. 8, the increase of step number will be inversely proportional to the COP. The COP value reaches 0.55 at one step number, 0.41 at two step number and further decrease to 0.33 at three step number. COP as a measure of thermodynamic performance of AHP depends only on the temperature of AHP parts, as expressed in Eq. (12). Accordingly, the internal work of AHP required to heat up n_ s moles of steam flowing in the absorber from TD to TA will increase along with the increasing step number. This is in accordance with AHP condition that at higher step number, AHP set-up complexity will increase and higher TA will be achieved, and therefore more work is required to achieve such condition. Besides, the increase of steam–carbon ratio (S/C) required for the mixing of methanol gas and steam in the combined system will also increase the _ AHP . W 4.2. Mixing temperature of steam and methanol After the steam is enhanced thermally by AHP system to the temperature level of TA, it is mixed with methanol gas pre-heated by waste heat. The mixing temperature with respect to the AHP step number is shown in Fig. 9. At one step number, increase of S/C from 1 to 2 has slight effect of mixing temperature increase of about 16 K. At higher step number, the effect of S/C on Tmix will become increasingly significant. This can be attributed to the much higher TA achievable by higher step number, as well as higher heat content of higher S/C value. The mixing temperature corresponds to the reaction temperature of the experimental MSR. 4.3. The effect of GHSV on methanol conversion and feasibility criterion Two different values i.e. GHSV 4000 h1 and 1333 h1 will be picked as representation to show the effect of GHSV on interpolated methanol conversion and feasibility criterion (/). As shown in Fig. 10, at the same S/C, GHSV 1333 h1 results in higher conversion at step number 2 and 3 compared to those of GHSV 4000 h1. This is in accordance with experimental results discussed in Section 2.2. On the other hand, at AHP step number higher than 3, methanol conversion has achieved its optimum point for both GHSV, since Tmix at this step number already reaches temperature more than 500 K which enables the methanol conversion level almost 1. Feasibility criterion with respect to the AHP step number is shown in Fig. 11. The results show that up to step number 3, lower GHSV results in higher feasibility criterion (/). The optimum / is achieved at step number 3. At step number higher than 3, / for
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both GHSV shows similar results since at this condition, the conversion of methanol has achieved its optimum condition, and _ AHP and n_ m diminishes each other. It is the effect of GHSV on W interesting to note that at step number 2 and 3, the relatively similar increase of methanol conversion by lowering GHSV from 4000 h1 to 1333 h1 will give different effect on feasibility criterion increase at those step numbers. This is caused by the fact that _ AHP also increases, therefore at higher step number; the required W making the ratio of the methanol conversion as the numerator over _ AHP as the denominator becomes decreasing. W Decreasing GHSV brings positive influence to the increase of feasibility criterion at step numbers up to 3. However, it should be pointed out that in the actual implementation, a lower GHSV means the slower production rate of hydrogen, which might be insufficient for achieving economics of scale in industries. Therefore, a tradeoff between feasibility criterion and production rate has to be considered in the real implementation of this combined AHP–MSR system.
4.4. The effect of S/C on methanol conversion and feasibility criterion The result of S/C (steam–carbon molar ratio) on the interpolated methanol conversion is shown in Fig. 12. It shows that at higher S/ C, higher conversion of methanol can be achieved. The trend of conversion at AHP step number higher than 3 is similar to that of GHSV. On the other hand, the increase of S/C results in interesting changes of feasibility criterion (/). Since different value of S/C will _ AHP , at higher value of step number, cause the change of required W / of different S/C will not overlap each other, as shown in Fig. 13. Besides, increasing S/C will result in the shifting of / peak value from step number 3 to 2. This shifting is due to two main reasons. One is the difference of Tmix which subsequently affect the conversion level of methanol. Another reason is the rapid increase of _ AHP at higher step number and higher S/C. The shifting required W of feasibility criterion (/) peak to lower AHP step number bring positive indication since lower step number of AHP corresponds to less investment cost of building the AHP system.
5. Conclusion Combined system of MSR and AHP was evaluated, and its effectiveness was determined by investigating the feasibility criterion proposed in this study. Related parameters pertaining to the combined system, such as characteristic parameters of AHP, experimental MSR, and mixing temperature were calculated and discussed. Some results revealed that the decrease of GHSV caused the increase of feasibility criterion up to the step number 3. On the other hand, the increase of S/C resulted in the shifting of feasibility criterion peak from the step number 3 to 2. At the optimum condition, with constant TD 373 K and TC 298 K, our calculation results showed that feasibility criterion value of about 4 to 6 could be achieved at step number 2 with the condition of S/C = 2 and GHSV 4000 h1 or S/C = 1 and GHSV 1333 h1 respectively.
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[6] Sumitomo H, Kado S, Nozaki T, Fushinobu K, Okazaki K. Exergy enhancement of low temperature waste heat by methanol steam reforming for hydrogen production. In: 8th Asian hydrogen energy conference, Beijing, China; 2005. [7] Lindstrom B, Pettersson LJ. Hydrogen generation by steam reforming of methanol over copper-based catalysts for fuel cell applications. Int J Hydrogen Energy 2001;26:923–33. [8] Shishido T, Yamamoto Y, Morioka H, Takehira K. Production of hydrogen from methanol over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation: steam reforming and oxidative steam reforming. J Mol Catal A: Chem 2007;268:185–94. [9] Matsumura Y, Ishibe H. Selective steam reforming of methanol over silicasupported copper catalyst prepared by sol–gel method. Appl Catal B: Environ 2009;86:114–20. [10] Kurr P, Kasatkin I, Girgsdies F, Trunschke A, Schlogl R, Ressler T. Microstructural characterization of Cu/ZnO/Al2O3 catalysts for methanol steam reforming – a comparative study. Appl Catal A: Gen 2008;348:153–64. [11] Kudo S, Maki T, Miura K, Mae K. High porous carbon with Cu/ZnO nanoparticles made by the pyrolysis of carbon material as a catalyst for steam reforming of methanol and dimethyl ether. Carbon 2010;48:1186–95. [12] Nagano S, Miyagawa H, Azegami O, Ohsawa K. Heat transfer enhancement in methanol steam reforming for a fuel cell. Energy Convers Manage 2001;42: 1817–29. [13] Horng RF. Transient behaviour of a small methanol reformer for fuel cell during hydrogen production after cold start. Energy Convers Manage 2005;46:1193–207.
[14] Chen WH, Lin BJ. Effect of microwave double absorption on hydrogen generation from methanol steam reforming. Int J Hydrogen Energy 2010;35:1987–97. [15] Wijaya WY, Watanabe H, Okazaki K. Methanol steam reforming for hydrogen production: the GHSV and temperature effects on methanol conversion and selectivity of H2, CO, and CO2. In: Proceedings of MISW, Tokyo, Japan; 2009. p. 22. [16] Purnama H, Ressler T, Jentoft RE, Soerijanto H, Schlogl R, Schomacker R. CO formation/selectivity for steam reforming of methanol with a commercial CuO/ZnO/Al2O3 catalyst. Appl Catal A: Gen 2004;259:83–94. [17] Iulianelli A, Longo T, Basile A. Methanol steam reforming in a dense Pd–Ag membrane reactor: the pressure and WHSV effects on CO-free H2 production. J Membrane Sci 2008;323:235–40. [18] Iulianelli A, Longo T, Basile A. Methanol steam reforming reaction in a Pd–Ag membrane reactor for CO-free hydrogen production. Int J Hydrogen Energy 2008;33:5583–8. [19] Inoue N, Irie K, Fukusumi Y. Analysis on characteristics of a boosted temperature type absorption heat pump. Ebara Report 2006;210–1:12–9 [in Japanese]. [20] Inoue N, Irie K, Fukusumi Y. Analysis on static characteristics of heat transformer. Trans JSRAE 2005;22–2:173–84 [in Japanese].
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Evaluation of combined absorption heat pump–methanol steam reforming system: Feasibility criterion as a measure of system performance Willy Yanto Wijaya ⇑, Shunsuke Kawasaki, Hirotatsu Watanabe, Ken Okazaki Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 7 November 2010 Accepted 21 November 2010 Available online 8 January 2011 Keywords: Methanol steam reforming Absorption heat pump Feasibility criterion
a b s t r a c t This paper presents an evaluation of combined absorption heat pump (AHP) and methanol steam reforming (MSR) system. To measure the effectiveness of this combined system, a feasibility criterion was proposed, which measured the ratio between net energy gain obtained by MSR reaction over energy required by AHP system. By using the proposed feasibility criterion, optimum AHP step number could be determined. Other parameters pertaining to both AHP and MSR system were also determined and calculated. In particular, discussions would focus on the effects of steam–carbon molar ratio (S/C) and gas hourly space velocity (GHSV) of experimental MSR upon feasibility criterion of combined system. It was shown that the decrease of GHSV caused the increase of feasibility criterion up to the AHP step number 3; meanwhile, the increase of S/C resulted in the shifting of feasibility criterion peak from AHP step number 3 to 2. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
CH3 OHðlÞ þ H2 OðlÞ $ CO2ðgÞ þ 3H2ðgÞ ;
ð1Þ
Global warming and the rapidly depleted fossil fuels have accelerated hydrogen as one of promising fuels in the future [1]. In the far future ahead, the production of hydrogen will rely totally on the contribution of renewable energy. However, for the short and middle-term scenario, the fraction of renewable energy contribution will be still significantly small [2]. Therefore, hydrogen production still relies heavily on the hydrocarbon processing as the main feedstock. The hydrocarbon processing to produce hydrogen mainly comprises of the reforming reaction, partial oxidation, and combination of both (autothermal reaction) [3], with reforming reaction gives the highest yield of hydrogen concentration, as shown by Ahmed and Krumpelt [4]. Since hydrogen production methods from hydrocarbon still leave some carbon footprint, short to middle-term efforts to mitigate the CO2 emission is still required, as proposed by Damm and Fedorov [5]. Meanwhile, huge amount of waste heat is being dissipated each year. In Japan, more than 800 PJ waste heat is being discarded by various industrial sectors annually [6]. Most of this waste heat is low-quality heat, less than 100 °C, which is inefficient to be recovered into electricity/other useful works. Methanol steam reforming (MSR) is one of the most favorable ways for hydrocarbon-based hydrogen production. The chemical reaction of MSR can be written as:
⇑ Corresponding author. Tel./fax: +81 3 57342179. E-mail address: [email protected] (W.Y. Wijaya). 0196-8904/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.11.013
DHMSR ¼ þ130:97 kJ=mol CH3 OH;
DGMSR ¼ þ9:18 kJ=mol CH3 OH: The products of this reaction (three moles of hydrogen) have higher value of enthalpy and Gibbs free energy compared with one mole of methanol, which is due to the endothermic nature of MSR. Besides, by MSR process, exergy rate of the thermal energy (waste heat) could be increased significantly and stored into hydrogen energy, as shown in Fig. 1. Exergy rate (e) is defined as the ratio of Gibbs free energy (potential useful work) over the enthalpy (energy amount), which can be expressed as:
e¼
DG : DH
ð2Þ
Researches in the field of MSR have been extensively carried out during this past decade. One of the topics most intensively studied is the catalyst development and investigation for the MSR process. Copper-based catalysts are so far the most commonly employed catalysts, showing high effectiveness in the MSR reaction as reported by Lindstrom and Pettersson [7]. The preparation of the Cu/ZnO/Al2O3 catalysts can be carried out through homogenous precipitation method as demonstrated by Shishido et al. [8], and also sol–gel method as studied by Matsumura and Ishibe [9]. Shishido et al. [8] found out that the catalytic activity was well correlated with the surface area of Cu metal and the maximum activity was obtained with the Cu/Zn ratio of 1/1. On the other hand, using sol–gel method, Matsumura and Ishibe [9] discovered that the catalytic activity increases with the copper content up to 40 wt.%. Microstructural characterization of various real Cu/ZnO/
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Nomenclature cp,m cp,s D GHSV N n n_ m n_ s Q_ A S Sf S/C TA TC TD TE TG Tmix x _ AHP W y
specific heat capacity of methanol, J mol1 K1 specific heat capacity of steam, J mol1 K1 coefficient to adjust gradient of sigmoid function gas hourly space velocity, h1 number of measurement data step number of absorption heat pump molar flow rate of methanol, mol s1 molar flow rate of steam, mol s1 heat supplied by AHP absorber to the flowing steam, J s1 standard error of measurement standard error of fitting steam–carbon (methanol) molar ratio temperature of AHP absorber, K temperature of AHP condensor, K temperature of waste heat, K temperature of AHP evaporator, K temperature of AHP generator, K mixing temperature of steam and methanol, K coefficient to adjust dislocation of sigmoid function internal work of absorption heat pump, J s1 average of measurement data
Fig. 1. Concept of waste heat exergy enhancement by MSR.
Al2O3 catalysts had been investigated by Kurr et al. [10] using in situ X-ray diffraction, in situ X-ray absorption spectroscopy, temperature programmed reduction and tunneling electron microscopy. They found out that the catalytic activity and thermal stability under MSR reaction conditions was also influenced by structural defects in the bulk of the materials. Alternative support material for the Cu/ZnO nanoparticles, such as employing high porous carbon, recently has also been studied by Kudo et al. [11]. Their preparation method included the carbonization of an ion exchange resin loaded with metal cations. Their results indicated that the ratio of Cu/Zn in the resin did not influence the micropore structure of carbon support, but significantly affected the form of Cu particles. Although many researchers have developed new catalysts for MSR, the waste heat temperature (<100 °C) is still not sufficient to convert methanol into hydrogen employing those catalysts. Another possible approach is by developing the reactor design or process engineering. Nagano et al. [12] analyzed the heat trans-
yi DG DH
/
single measurement data Gibbs free energy change of reaction, kJ mol1 enthalpy change of reaction, kJ mol1 exergy rate efficiency of absorption heat pump efficiency of methanol conversion into product species feasibility criterion
Subscript AHP MSR A C D E G m mix s
absorption heat pump methanol steam reforming absorber condenser waste heat evaporator generator methanol mixing steam
e gAHP gMSR
Superscript ° standard reference state (298.15 K, 1 atm)
fer enhancement for the methanol steam reformer. By applying internal corrugated metal heater and external catalytic combustion heater, they could maintain uniform temperature distribution up to the methanol liquid hourly space velocity 3 h1. The transient behavior of a small methanol reformer after cold start was also studied by Horng [13]. He constructed a small methanol reformer and attempted to compromise several parameters for shortening the start-up time. Combination of MSR with other energy system, however, has been less considered by research groups working in the MSR field. In a recent paper by Chen and Lin [14], the MSR reaction was exposed to an environment with microwave irradiation. They reported that the MSR could be heated and triggered rapidly within a short time due to the double absorption of microwaves by both the reagents and the catalyst. These numerous researches have pushed significant progress and unveiled understanding of many aspects of the MSR characteristics. Nevertheless, most of these researches did not put any emphasis on the endothermic nature of MSR and the potential of using the waste heat as the endothermic energy supply to enhance the total energy level. As shown in Fig. 1, huge amount of industrial low-quality waste heat has potential to be recovered as MSR endothermic heat. However, this waste heat temperature (<100 °C) is not sufficient for converting methanol into hydrogen in the actual MSR experimental process. In the actual process, MSR requires temperature level at least 200 °C for achieving good conversion rate into hydrogen [15]. Therefore, we propose utilizing an absorption heat pump (AHP) to enhance the temperature of the waste heat to the degree favorable for MSR reaction. To date, none of the pertinent literatures in the MSR-related field, which evaluate the combined system of MSR and AHP, could be found. However, we believe that integration of AHP into MSR system, for recovering the waste heat, has a promising applicability in the industrial sectors. In this paper, we present a basis to measure the effectiveness of the combined AHP–MSR system using a proposed feasibility criterion. Theoretical approach is applied to analyze the AHP system, coupled with the experimental results of MSR, aimed to evaluate the feasibility of the combined AHP–MSR system.
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2. Experiments of MSR 2.1. Experimental details The experimental set-up of MSR consists of a reactor module, heating system, and gas analysis system as shown in Fig. 2. Inside the reactor module, a packed-bed reaction chamber with dimension 11 69 3 mm3 was filled with 1.5 g of Cu/Zn/Al2O3 MDC commercial catalyst. The density of this catalyst was 1.2 kg l1 with composition CuO 42%, ZnO 47%, and Al2O3 10% as specified by its manufacturer. This catalyst was crushed into grains, and then sieved to obtain the particle size 500 lm to 1.18 mm in diameter. The reactor module was encapsulated with a ceramic heater, equipped with a temperature controller (FKC-11™ Tokyo Karasu Kikai) to provide heating and keep a constant reaction temperature during the experiments. Temperature measurement was conducted by inserting a K-type thermocouple (1 mm diameter) at the middle of the reaction chamber. To confirm the profile of temperature distribution, two additional thermocouples were inserted as a test case at the locations of 15 mm from the inlet and outlet of the reaction chamber. The temperature variation between these two locations and the middle point of the chamber was within the range of ±3 °C. In addition, a ribbon heater, set at temperature about 110 °C, was used to preheat and vaporize the methanol solution before entering the reaction chamber. To ensure the measurement quality, prior to performing the experiments, a leakage check was carried out by measuring the flow rate of a nitrogen gas feed before entering the reactor and
comparing the outlet flow rate with a bubble flow meter. The measurement devices were also carefully calibrated, with the accuracy and relative errors are shown in Table 1. Before performing the MSR reaction, the catalyst bed was pre-heated and reduced using mixture of nitrogen and hydrogen, 50 ml min1 each, regulated by a flow meter (Horiba Stec SEC-B40™) at 250 °C for 2 h. This reduction process was intended to remove the oxygen molecules attached on the catalyst due to oxidation. Methanol solution was fed into the reactor module using a micro-feeder. The methanol solution flew through the evaporator (ribbon heater) to be vaporized into the gas phase before entering the reaction chamber. In the reaction chamber, methanol and water vapor reacted catalytically, in the endothermic manner, with enthalpy (energy) enhancement about 49.5 kJ for each mole of converted methanol:
CH3 OHðgÞ þ H2 OðgÞ $ CO2ðgÞ þ 3H2ðgÞ
DH298K ¼ 49:5 kJ=mol:
ð3Þ
The operating conditions of MSR experiments were performed at temperature range 160–225 °C measured by thermocouple inserted to the center of the reaction chamber, with variation of four different gas hourly space velocity (GHSV = 4000 h1, 2666 h1, 2000 h1, 1333 h1). GHSV is defined as the ratio of the gas flow (methanol and water vapor) into the reaction chamber over the volume of the catalyst: 1
1
GHSV½h ¼
Flow rate of CH3 OH and H2 O at inlet ½m3 h : Volume of catalyst ½m3 ð4Þ
Fig. 2. Schematic of the methanol steam reforming experimental set-up.
Table 1 Measurement range, accuracy, and relative error of measurement devices. Measurement
Equipment
Range
Accuracy
Relative error (%)
Gas flow Reaction temperature Temperature control Methanol solution feed
Horiba Stec SEC-B40™ K-type 1.0 mm Sakaguchi FKC-11™ Fine Tokyo Karasu Kikai Micro-feeder Furue Science
0–100 ml/min 0–650 °C 0–1093 °C 22.7 ll/h to 3.3 ml/s
±0.1 ml/min ±0.1 °C ±0.1 °C ±0.1 ll/h
±1 ±0.5 ±0.5 ±0.5
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Besides GHSV, two different conditions of steam to carbon (methanol) molar ratio (S/C = 1 and 2) were also performed in this experiment. In the steady flow system, S/C is defined as:
S=C ¼
n_ s ; n_ m
ð5Þ
where n_ s is molar flow rate of steam at the inlet of reactor, and n_ m is molar flow rate of methanol vapor at the inlet of the reactor. After undergoing the catalytic reforming reaction, the product stream from the reactor outlet was then cooled by passing over a cold-trap in order to condensate the un-reacted water and methanol. Constant flow of nitrogen (50 ml/min) was supplied as the reference gas for the gas chromatography analysis. The product gases and the supplied nitrogen were analyzed using gas chromatograph (GC-8APT, Shimadzu Corp., TCD with Molecular-sieve 13 X column, N2 carrier gas). Prior to the MSR experiments, the gas chromatograph was calibrated by means of standard gases and atmospheric air to ensure that the measurement results of the product gases were reliable. 2.2. Experimental results The results of methanol conversion at different operating conditions are shown in Fig. 3. The methanol conversion (gMSR) was determined from:
gMSR ½% ¼
n_ m ðat inletÞ n_ m ðat outletÞ 100; n_ m ðat inletÞ
ð6Þ
where n_ m is the molar flow rate of methanol vapor at the inlet and outlet of MSR reactor respectively. To ensure the consistency of measurement results, three times measurements were conducted for each experimental point obtained. Errors in the experiments can arise from the instruments condition, calibration, as well as observation and measurement uncertainties. The accuracy of these experimental results was validated with the standard error, described as:
S¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP 2 u N yi y t i¼1 y N
;
ð7Þ
is the average of the where yi is the single measurement data, y measurement data, and N is the total number of the measurements. The mean value of the standard error of the methanol conversion was 5.94%. From Fig. 3, it is obvious that due to the endothermic nature of MSR, the conversion of methanol is enhanced along with the increasing temperature. This enhancement pervades for all different GHSV and S/C, and might be attributed to endothermic reaction equilibrium proportional dependency on temperature as well as
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the reaction kinetics of the catalytic reforming. On the other hand, the increase of GHSV results in lower conversion of methanol, where this could be attributed to the shorter residence time of methanol solution having contact with the catalyst bed inside the reactor chamber. For our experimental conditions, with GHSV smaller than 2000 h1, S/C = 1, and reaction temperature of 225 °C, the conversion of methanol could achieve level as high as 90%. Meanwhile, the steam–carbon molar ratio (S/C) also contributed positively on the conversion of methanol. This is due to the equilibrium shift of MSR reaction towards products formation, as governed by Le Chatelier’s principle. Besides, the proportion of methanol in the solution also decreases, thus the chance of each CH3OH molecule to have contact with catalyst is becoming higher. For the same GHSV of 4000 h1, reaction with S/C = 2 shows better performance in term of methanol conversion than the one with S/ C = 1, and it seems that the positive effect of S/C is becoming more emphasized at the higher temperature. The results of our MSR experiments, with variation of temperature, GHSV, and S/C, show relatively good agreement with results as reported by Purnama et al. [16]. In their MSR experiments, they investigated the profile of methanol conversion as a function of catalyst weight over the flow rate of methanol (W/Fm), where they achieved methanol conversion as high as 90% at 250 °C with the condition of S/C = 1 and W/Fm = 0.013 g s mol1. Some differences when comparing to our experimental results can be attributed to the different amount and size of the Cu/ZnO/Al2O3 catalyst they used, as well as the different flow rate. However, the profile of their hydrogen yield indicated similar trends when compared with the trend of our methanol conversion. The experimental works of Basile et al. [17,18] also demonstrated the increasing characteristics of methanol conversion when the GHSV decreased. They obtained the methanol conversion about 60% at 300 °C and S/C = 3 with weight hourly space velocity from 0.36 h1 to 1.82 h1. Their unfavorable results in term of methanol conversion might be due to the quality of their own-fabricated Cu/Zn/Mg-based catalysts and the re-use of the same catalysts for several cycles of MSR experiments. However, their results showed interesting correlations regarding the effects of membrane and reaction pressure on the methanol conversion and hydrogen recovery. As shown in Fig. 3, methanol conversion in MSR exhibits a nature of sigmoid-curve-like phenomena. The different conditions of GHSV and S/C will particularly affect the gradient of this conversion curve growth. Taking these into account, the experimental results can be curve-fitted into sigmoid function, interpolated, and then be used in the calculation for the combined system of MSR and AHP. 3. Calculation method Fig. 4 shows the schematic of combined AHP and MSR. Waste heat contained in the 100 °C steam is boosted by AHP system to become high temperature steam, and then fed to the MSR reactor. Simultaneously, liquid methanol is evaporated and heated up to the temperature level of waste heat (TD). The methanol gas is mixed with high temperature steam in the MSR reactor, and reacted with the help of catalysts, producing output hydrogen. The calculation method proposed in this paper comprises of thermodynamic-based theoretical calculation of AHP and interpolation results of empirical MSR experimental data. The calculation scheme is shown in Fig. 5, and details will be discussed in Section 3.2. 3.1. Description of AHP system
Fig. 3. Experimental results of methanol conversion as a function of temperature in several GHSV and S/C conditions.
AHP system consists of four main parts: absorber, evaporator, generator, and condenser, as shown in Fig. 6. The waste heat in
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Fig. 4. Schematic of combined AHP and MSR.
the form of hot water or steam is transported into generator and evaporator, heating those two parts, and changing the phase of
the circulating refrigerant. From evaporator, the refrigerant gas enters the absorber, having exothermic reaction, releasing heat that is to be used for generating steam in the desired temperature level. Meanwhile, the refrigerant gas from generator will be condensed into liquid form, requiring less energy to be pumped to the evaporator. The temperature of generator (TG) and evaporator (TE) is relatively similar, and thus will be simplified as one uniform temperature, denoted by TD. Cooling water in room temperature will be used to cool the condenser, maintaining its temperature of TC. The heat released in absorber will be used to heat up steam (containing waste heat with temperature TD) to become desired output temperature of AHP, denoted by TA, before eventually being mixed with methanol. The main advantage of AHP system compared to vapor-compression heat pump is in the term of its driving input power. While vapor-compression heat pump generally requires electricity (high quality form of energy) and consumes huge amount of this energy for its compression processes, AHP can make use of low-quality energy (such as waste heat) for its main driving forces. When lowquality or wasted energy is available in abundance, AHP can achieve the break-even point despite its high initial investment cost [19,20].
Fig. 5. Scheme of calculation to evaluate the combined absorption heat pump–methanol steam reforming system.
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Fig. 6. Schematic of absorption heat pump system, with temperature conditions of generator, evaporator, condensor and absorber.
Q_
3.2. Calculation scheme of combined AHP–MSR system As shown in Fig. 5, the calculation is divided into four steps. First step is the determination of the AHP operating parameters, which include TC, TD, and n (AHP step number). Step number is the number of the stage/step in AHP necessary for further boosting of output temperature of absorber (TA). In this research, the value of TA is adjusted by n (AHP step number). The relation between n and TA can be derived from Dühring chart of H2O/LiBr solution that shows linear proportionality between refrigerant temperature and solution temperature in the AHP system [19,20], and can be expressed as:
T A ¼ T D ðT D =T C Þn :
ð8Þ
In this calculation, a fixed temperature of waste heat TD = 373 K, is used to heat up evaporator and generator of AHP. Meanwhile, the condenser is cooled by water in room temperature, assumed to reach equilibrium at TC = 298 K. By increasing the step number while fixing TD and TC constant, higher TA can be obtained. Besides TA, the second step to calculate AHP parameters also in_ AHP ) and coefficient of cludes internal work of AHP (denoted by W _ performance (COP). W AHP is the ideal work of AHP internal system, based on thermodynamics, which can be derived from equations:
Fig. 7. Temperature of AHP absorber as a function of AHP step number.
T
A gAHP ¼ _ A ¼ ; and W AHP T A T D
Q_ A ¼ n_ s cp;s ðT A T D Þ;
ð9Þ ð10Þ
therefore:
_ AHP ¼ n_ s cp;s ðT A T D Þ2 =T A ; W
ð11Þ
where n_ s is the molar flow rate of steam passing through AHP absorber to be heated up from TD to TA, Q_ A is the heat supplied by absorber to the passing steam, gAHP is the efficiency of AHP, and cp,s is the specific heat of steam (regarded to be constant at 36.5 J mol1 K1). Accordingly, the performance of AHP can also be described conveniently with COP which is described by following equation:
COP ¼
TA TA TC
TD TC : TD
ð12Þ
_ AHP , COP) After obtaining the performance parameters of AHP (W and output temperature (TA), the next step considers parameters involved in the combined system of AHP and MSR. Output steam from AHP with temperature TA is to be mixed with methanol gas, yielding a mixing temperature (Tmix) which can be expressed by:
Fig. 8. Coefficient of performance (COP) as a function of AHP step number.
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n_ s cp;s ðT A T mix Þ ¼ n_ m cp;m T mix T CH3 OH ;
ð13Þ
where n_ s is the molar flow rate of steam to be mixed with methanol gas, cp,s is the specific heat of steam (36.5 J mol1 K1), n_ m is the molar flow rate of methanol gas to be mixed, cp,m is the specific heat of methanol gas (61.4 J mol1 K1), Tmix is the equilibrium temperature of mixing, and T CH3 OH is the temperature of methanol gas before being mixed. Since methanol can be pre-heated by waste heat, T CH3 OH is regarded to reach TD (373 K) before the mixing process. After the methanol gas and steam are mixed at temperature Tmix, with the help of catalyst, steam reforming of methanol occurs. Therefore, Tmix corresponds to temperature of reaction in experimental MSR. The value of (n_ s /n_ m ) in Eq. (13) corresponds to steam–carbon ratio (S/C) in the MSR experiments. Experiment results of methanol conversion at certain GHSV and S/C are curve-fitted with sigmoid function described by:
gMSR ðT mix Þ ¼
1 ; 1 þ expðDðT mix xÞÞ
Fig. 11. Feasibility criterion as a function of AHP step number, at S/C = 1, GHSV 4000 h1 and 1333 h1.
ð14Þ
where Tmix is the reaction temperature, D and x are coefficients which determine the gradient and dislocation of the sigmoid curve to be fitted to the experimental data. The fitting results of methanol conversion in several GHSV and S/C condition are then to be used as one of the numerators to calculate feasibility criterion. The errors associated with the fitting were validated using standard error. The maximum value of the fitting standard error of the methanol conversion was 2.23%.
The final step of calculation scheme is to determine feasibility criterion (/), which is defined as:
/¼
Energy gain by MSR n_ m gMSR ðT mix Þ DHMSR ¼ ; _ AHP Work required for AHP W
ð15Þ
where gMSR(Tmix) is the interpolated results of methanol conversion expressed in Eq. (14), DHMSR is the enthalpy (energy) enhancement of MSR reaction described in Eq. (3), n_ m is the molar flow rate of _ AHP is the internal work required in AHP system, methanol, and W which is expressed in Eq. (11). The feasibility criterion will serve as a parameter to determine the effectiveness of this combined system by comparing net energy gain as its numerator and work required as its denominator. As the step number changes, while fixing the TD and TC, output temperature (TA) will also change, which subsequently affect the mixing temperature, conversion effi_ AHP , and consequently the feasibility criterion. ciency, W 4. Results and discussion 4.1. Characteristic of AHP
Fig. 9. Mixing temperature of methanol gas and steam as a function of AHP step number, at S/C = 1 and 2.
Fig. 10. Calculation results of methanol conversion as a function of AHP step number, at S/C = 1, GHSV 4000 h1 and 1333 h1.
The characteristic of AHP is indicated by three main parameters i.e. temperature of absorber (TA), coefficient of performance (COP), _ AHP Þ. The result of TA is shown in and internal work of AHP ðW Fig. 7. If the temperature of condenser (TC) and generator/evaporator (TD) are fixed, the achievable TA will solely depend on number of steps in the AHP system. The increasing step number will be
Fig. 12. Calculation results of methanol conversion as a function of AHP step number, at S/C = 1 and 2, GHSV 4000 h1.
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Fig. 13. Feasibility criterion as a function of AHP step number, at S/C = 1 and 2, GHSV 4000 h1.
proportional to the exponential increase of TA. On the other hand, as shown in Fig. 8, the increase of step number will be inversely proportional to the COP. The COP value reaches 0.55 at one step number, 0.41 at two step number and further decrease to 0.33 at three step number. COP as a measure of thermodynamic performance of AHP depends only on the temperature of AHP parts, as expressed in Eq. (12). Accordingly, the internal work of AHP required to heat up n_ s moles of steam flowing in the absorber from TD to TA will increase along with the increasing step number. This is in accordance with AHP condition that at higher step number, AHP set-up complexity will increase and higher TA will be achieved, and therefore more work is required to achieve such condition. Besides, the increase of steam–carbon ratio (S/C) required for the mixing of methanol gas and steam in the combined system will also increase the _ AHP . W 4.2. Mixing temperature of steam and methanol After the steam is enhanced thermally by AHP system to the temperature level of TA, it is mixed with methanol gas pre-heated by waste heat. The mixing temperature with respect to the AHP step number is shown in Fig. 9. At one step number, increase of S/C from 1 to 2 has slight effect of mixing temperature increase of about 16 K. At higher step number, the effect of S/C on Tmix will become increasingly significant. This can be attributed to the much higher TA achievable by higher step number, as well as higher heat content of higher S/C value. The mixing temperature corresponds to the reaction temperature of the experimental MSR. 4.3. The effect of GHSV on methanol conversion and feasibility criterion Two different values i.e. GHSV 4000 h1 and 1333 h1 will be picked as representation to show the effect of GHSV on interpolated methanol conversion and feasibility criterion (/). As shown in Fig. 10, at the same S/C, GHSV 1333 h1 results in higher conversion at step number 2 and 3 compared to those of GHSV 4000 h1. This is in accordance with experimental results discussed in Section 2.2. On the other hand, at AHP step number higher than 3, methanol conversion has achieved its optimum point for both GHSV, since Tmix at this step number already reaches temperature more than 500 K which enables the methanol conversion level almost 1. Feasibility criterion with respect to the AHP step number is shown in Fig. 11. The results show that up to step number 3, lower GHSV results in higher feasibility criterion (/). The optimum / is achieved at step number 3. At step number higher than 3, / for
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both GHSV shows similar results since at this condition, the conversion of methanol has achieved its optimum condition, and _ AHP and n_ m diminishes each other. It is the effect of GHSV on W interesting to note that at step number 2 and 3, the relatively similar increase of methanol conversion by lowering GHSV from 4000 h1 to 1333 h1 will give different effect on feasibility criterion increase at those step numbers. This is caused by the fact that _ AHP also increases, therefore at higher step number; the required W making the ratio of the methanol conversion as the numerator over _ AHP as the denominator becomes decreasing. W Decreasing GHSV brings positive influence to the increase of feasibility criterion at step numbers up to 3. However, it should be pointed out that in the actual implementation, a lower GHSV means the slower production rate of hydrogen, which might be insufficient for achieving economics of scale in industries. Therefore, a tradeoff between feasibility criterion and production rate has to be considered in the real implementation of this combined AHP–MSR system.
4.4. The effect of S/C on methanol conversion and feasibility criterion The result of S/C (steam–carbon molar ratio) on the interpolated methanol conversion is shown in Fig. 12. It shows that at higher S/ C, higher conversion of methanol can be achieved. The trend of conversion at AHP step number higher than 3 is similar to that of GHSV. On the other hand, the increase of S/C results in interesting changes of feasibility criterion (/). Since different value of S/C will _ AHP , at higher value of step number, cause the change of required W / of different S/C will not overlap each other, as shown in Fig. 13. Besides, increasing S/C will result in the shifting of / peak value from step number 3 to 2. This shifting is due to two main reasons. One is the difference of Tmix which subsequently affect the conversion level of methanol. Another reason is the rapid increase of _ AHP at higher step number and higher S/C. The shifting required W of feasibility criterion (/) peak to lower AHP step number bring positive indication since lower step number of AHP corresponds to less investment cost of building the AHP system.
5. Conclusion Combined system of MSR and AHP was evaluated, and its effectiveness was determined by investigating the feasibility criterion proposed in this study. Related parameters pertaining to the combined system, such as characteristic parameters of AHP, experimental MSR, and mixing temperature were calculated and discussed. Some results revealed that the decrease of GHSV caused the increase of feasibility criterion up to the step number 3. On the other hand, the increase of S/C resulted in the shifting of feasibility criterion peak from the step number 3 to 2. At the optimum condition, with constant TD 373 K and TC 298 K, our calculation results showed that feasibility criterion value of about 4 to 6 could be achieved at step number 2 with the condition of S/C = 2 and GHSV 4000 h1 or S/C = 1 and GHSV 1333 h1 respectively.
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