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CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination
DES-13012; No of Pages 12 Desalination xxx (2016) xxx–xxx
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CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination Eman Elsayed a,b,⁎, Raya AL-Dadah a, Saad Mahmoud a, Paul.A. Anderson b, Ahmed Elsayed c, Peter G. Youssef a a b c
School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UK School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK The Institute for Advanced Manufacturing and Engineering (AME), School of Engineering and Computing, Coventry University, Coventry CV6 5LZ, UK
H I G H L I G H T S • • • •
CPO-27(Ni), aluminium fumarate and MIL-101(Cr) were investigated for adsorption desalination application. Optimum desorption temperature for CPO-27(Ni) was higher than 110 °C and for aluminium fumarate was as low as 70 °C. CPO-27(Ni) outperformed other materials at low evaporation temperature (5 °C) and high regeneration temperatures. MIL-101(Cr) and aluminium fumarate outperformed CPO-27(Ni) and silica gel at high evaporation temperature (20 °C).
a r t i c l e
i n f o
Article history: Received 1 April 2016 Received in revised form 18 July 2016 Accepted 21 July 2016 Available online xxxx Keywords: Metal-organic framework XRD Water vapour adsorption Adsorption desalination
a b s t r a c t Adsorption desalination is a promising technology that has recently been investigated. Most of the reported adsorption desalination systems use silica gel as the adsorbent and despite the high stability, it suffers from limited water uptake capabilities leading to a low system performance. Metal-organic frameworks (MOFs) are porous materials with high surface area, pore size, tunable pore geometry and hence providing high adsorption capacity. Currently, limited MOF materials with high water adsorption capabilities and hydrothermal stability are commercially available. CPO-27(Ni) and aluminium fumarate are two commercially available MOFs that have a max−1 imum water uptake of 0.47 gH2O·g−1 ads and 0.53 gH2O·gad , respectively. Another MOF, MIL-101(Cr), exhibits superior water adsorption uptake of 1.47 gH2O·g−1 ad but currently can only be produced in lab-scale. The thermodynamic cycle performance of a two beds adsorption system was evaluated using Simulink software to assess the suitability of those MOFs for adsorption desalination and their performance under different operating conditions. The CPO-27(Ni) was found to produce around 4.3 m3·(ton·day)−1 at an evaporation temperature of 5 °C while aluminium fumarate produced around 6 m3·(ton·day)−1 at an evaporation temperature of 20 °C. As for MIL-101(Cr), the water production rate at 20 °C was 11 m3·(ton·day)−1 highlighting the potential of this material compared to other adsorbents. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Seven hundred million people around the world are suffering from water scarcity, another 500 million are approaching this situation and this problem is expected to worsen by 2025 as shown in Fig. 1 [1]. More than 70% of the earth's surface is covered with water and 97% of this water is salty which can be used to produce fresh water using desalination processes. Desalination is generally defined as the process by which potable water is produced from the seawater or brackish water with high dissolved suspended solids content (N35,000 ppm).
⁎ Corresponding author. E-mail address: [email protected] (E. Elsayed).
The desalination technologies can be categorised into (1) pressure activated systems (the membrane technologies) such as the Reverse Osmosis (RO); (2) thermal energy systems (distillation processes) such as the Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), Mechanical Vapour Compression (MVC) and Solar Distillation and (3) chemical methods such as Ion-Exchange Desalination and Gas Hydrate [3]. Recently, adsorption desalination has been identified with many advantages such as environmentally friendly, driven by low-grade heat sources, low capital cost, low evaporation temperature and hence reduced fouling (formation of scales causing the damage of the evaporation units) effect. The adsorption system consists of an evaporator, a condenser and adsorption/desorption beds. Each bed includes a finned tube heat exchanger with the adsorbent material packed between the fins. The process starts with evaporating the seawater producing
http://dx.doi.org/10.1016/j.desal.2016.07.030 0011-9164/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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Nomenclature Symbols
Description
Unit
A cp E Ea K0 M m n. n P Ps
adsorption potential specific heat at constant pressure adsorption characteristic parameter activation energy LDF model empirical constant mass mass flow rate adsorption/desorption phase, flag exponent fitting parameter pressure saturation pressure of adsorbate at adsorption temperature relative pressure isosteric heat of adsorption ideal gas constant specific cooling power specific daily water production temperature equilibrium uptake maximum uptake no. of cycles per day
J·mol−1 kg·kJ−1·K−1 J·mol−1 J·mol−1 s−1 kg kg·s−1 \ \ \ \ kPa kPa
P/Ps Qst R SCP SDWP T x x0 τ Subscripts a ads cond cf des evap hf HX In Out S t
\ \ kJ·kg−1 J·(mol·K)−1 Rton·ton−1 m3· (ton·day)−1 K gH2O·g−1 ads gH2O·g−1 ads Cycle/day
adsorbent material adsorption condenser cooling fluid desorption evaporator hot fluid heat exchanger inlet outlet seawater time
water vapour which is then adsorbed by the adsorbent material. Then during the desorbing step where the bed is heated, the water vapour is released and then condensed in the condenser producing highgrade distilled water [4]. Wang et al. [5] experimentally investigated the performance and the specific daily water production (SDWP) of a four-bed silica gel adsorption desalination plant. They showed that a SDWP of 4.7 m3·(ton·day)−1 was obtained using a regeneration temperature of 85 °C, an adsorption temperature of 29.4 °C, an evaporation temperature of 12.2 °C and a half cycle time of 180 s with a switching time of 40 s. They proposed that the performance can be further improved by adopting a higher chilled water temperature supplied to the evaporator and a lower cooling water temperature to the designated adsorption beds. Thu et al. [6] developed a numerical model for an advanced adsorption desalination cycle, in
which a higher vapour pressure was achieved through the recovery of latent heat of condensation into the evaporator unit that was built inside the condenser. They used half cycle time of 300 s which was half that used by Wang et al. [5] leading to specific daily water production (SDWP) of around 26 m 3·(ton·day) − 1 which was almost three folds of the conventional cycle. An economic study was held by Ng et al. [7] showing that adsorption desalination required less unit production cost, electrical energy and total primary energy than other conventional desalination systems then they developed a FORTRAN code for a solar-powered adsorption desalination plant with two and four adsorption beds using silica gel as the adsorbent. The two bed system yielded 7.4 m3·(ton·day)− 1 while the four bed system produced 8.9 m3·(ton·day)− 1. The two systems used a regeneration temperature of 85 °C and produced a high grade water with total dissolved solids (TDS) content b 15 ppm. Mitra et al. [8] investigated the effect of cycle time and the condenser temperature on the performance of a four bed silica gel adsorption desalination system. The system was operated at an evaporation temperature of 5.5 °C, a regeneration temperature of 85 °C, an adsorption temperature of 30 °C and a condensation temperature of 24.5–50.5 °C. It was found that increasing the condenser temperature would negatively affect the system performance due to increasing the operating pressure ratio which require higher switching time for bed. An optimum cycle time in the range of 600–900 s was found for maximising specific cooling power (SCP) and SDWP resulting in a coefficient of performance (COP) of 0.41–0.47, a SDWP of 2.4–2.3 m 3·(ton·day)− 1 and a SCP of 18–17 Rton·ton− 1 of silica-gel. A Simulink model for a two bed adsorption system was developed by Youssef et al. [9] comparing the performance of silica gel and silicaaluminophosphates AQSOA-ZO2 at different evaporation temperatures. They showed that at high chilled water temperature (N20 °C), silica-gel outperformed AQSOA-ZO2 producing 8.4 m3·(ton·day)−1 and specific cooling capacity of 62.4 Rton·ton− 1 silica gel. At chilled water temperature b 20 °C, AQSOA-ZO2 outperformed silica-gel as AQSOAZO2 produced 5.8 m3 distilled water per day and 50.1 Rton·ton−1 while silica-gel produced 2.8 m3·(ton·day)− 1 of water and 17.2 Rton·ton−1. Also, Youssef et al. [10] investigated the performance of a four bed adsorption desalination system using the same two materials. The results showed that at low chilled water temperatures (b 20 ° C), AQSOA-Z02 had a SDWP of 6.2 m3·(ton·day)− 1 and SCP of 53.7 Rton·ton−1 compared to only 3.39 m3·(ton·day)− 1 and 15.7 Rton·ton−1 silica gel. At high chilled water temperature (N 20 °C), silica-gel outperformed AQSOA-Z02 with a SDWP of 9.5 m3 (ton·day)−1 and 55 Rton·ton−1 of cooling. All of the above reported work was based on using either silica gel or silica-aluminophosphates as the adsorbent material. Silica gel has the advantage of high stability but suffers from a relatively low hydrophilicity thus high relative pressure value is needed to reach its maximum capacity. This limited capacity highlights the importance of using more hydrophilic porous materials like the metal-organic frameworks
Fig. 1. The growth of global water stress problem from 1995 to 2025 [2].
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
(a)
3
(b)
(c)
Fig. 2. Crystal structure of a. CPO-27(Ni) [2,5 Dihydroxyterephthalic acid as the organic linker and nickel nodes] b. Aluminium fumarate [Fumaric acid as the organic ligand and aluminiumOH-Aluminium chains as metal nodes.] c. MIL-101(Cr) [Terephthalic acid as the organic ligand and chromium nodes].
(a)
(b)
(c)
Fig. 3. Experimental and simulation PXRD patterns of a. CPO-27(Ni), b. Aluminium fumarate and c. MIL-101(Cr).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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Uptake (g H2O. g -1)
(MOFs) investigated in this work. Metal-organic frameworks are porous materials with extraordinary chemical and physical properties including high surface area and porosity, tunable pore geometry and properties leading to high adsorption capacity. MOFs as the name suggests
are composed of organic ligands and metal atoms where the organic ligand and the metallic building units can be changed to control the MOF pore size and geometry to make it suitable for the required applications. Despite these advantages of MOFs, major challenges still exist in the
0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
(a)
Simulated
0.00
0.10
0.20
0.30
0.40
0.50 P/Ps
0.60
Experimental
0.70
0.80
0.90
1.00
(c)
Fig. 4. Water adsorption isotherms of a.CPO-27(Ni), b. aluminium fumarate and c. MIL-101(Cr) at 25 °C. (Solid lines represent adsorption phase and dotted lines represent desorption phase).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
water adsorption applications such as limited water vapour uptake for some MOFs, poor hydrothermal stability for others and finally limited large scale commercial availability of some MOF materials. The authors have identified two MOFs with water adsorption capabilities higher than silica gel and zeolites namely CPO-27(Ni) (known as MOF-74(Ni)) and aluminium fumarate with water vapour uptake 1 −1 of 0.47 gH2O·g− ads and 0.53 gH2O·gads respectively [11,12]. The two MOFs were found to be hydrothermally stable and are mass-produced by Johnson Matthey and MOFs technologies, respectively. In addition, the authors have synthesized another MOF material known as MIL101(Cr) which has very high water adsorption uptake of 1.47gH2O·g−1 ad but currently can only be produced in lab-scale. The main objective of this work is to characterize those MOFs experimentally and through Materials Studio software in terms of their crystal structures and pore geometries. A further assessment was carried out using Simulink modelling to investigate the potential of these MOF materials for adsorption desalination application at various operating conditions.
5
This section describes techniques used to characterize the three MOFs through powder X-Ray Diffraction (XRD) and water adsorption characteristics using Dynamic Vapour Adsorption analyser (DVS). The equilibrium water adsorption characteristics were then used to develop a Simulink model to investigate the potential of the MOF materials for adsorption desalination application at various operating conditions. For aluminium fumarate and CPO-27(Ni), the powder XRD patterns were measured using a Bruker D8 Advance Reflection Diffractometer with Cu Kα radiation (1.5418 Å). The aluminium fumarate sample was scanned from 8 to 45° 2θ, the CPO-27(Ni) sample from 10 to 75°, with a step size of 0.02° and both were spun at 15 rpm. For MIL-101(Cr), Siemens D5005 was used to scan the samples from 2.5 to 30° 2θ with a step size of 0.02°. The dynamic vapour sorption (DVS) gravimetric analyser (Advantage DVS, Surface Measurement Systems, UK) was used to study the water adsorption characteristics. 3. Crystal structure modelling
2. Experimental work CPO-27(Ni) and aluminium fumarate were obtained from commercial sources. The CPO-27(Ni) was supplied by Johnson Matthey, while the aluminium fumarate was supplied by MOF Technologies. MIL-101(Cr) was synthesized according to the published literature [13].
Materials Studio software was used to further characterize the materials through generating the crystal structure, surface area, pore size and pore volume. Fig. 2 shows the crystal structures for the three MOFs showing the organic ligand and the metal nodes which are used to determine the pore volume and surface area. For CPO-27(Ni), the crystal density was 1.2 g·cm−3, the cell volume was 3.8 × 103Å3 and the total
(a)
Water molecules uniformly adsorbed at 25oC and P/Ps=0.9.
Water molecules uniformly adsorbed at 25oC and P/Ps=0.9.
(b)
Fig. 5. water adsorption sites and the adsorbed water molecules field at 25 °C and P/Ps = 0.9 for a.CPO-27(Ni) and b. aluminium fumarate.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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pore volume was 2.3 × 103Å3. The pore volume measured in cm3·g−1 was found to be 0.5cm3·g− 1 compared to 0.36cm3·g−1 [14], 0.41cm3·g−1 [15] and 0.77cm3·g−1 [16]. The pore size was also calculated from the crystal structure and was found to be 12.1 Å compared to 11–12 Å [17] and the calculated surface area was 1225.7m2·g− 1 (1218 m2·g−1 [18]). For aluminium fumarate, the crystal density was 1.087 g·cm− 3, the cell volume was 959.56Å3 and total pore volume was 453.98Å3. The pore volume measured in cm3·g− 1 was found to be 0.436cm3·g− 1 compared to 0.48cm3·g− 1 [12] and the calculated surface area was 1211.5m2·g−1 compared to 1021m2·g−1 [12]). The crystal structure of MIL-101(Cr) showed crystal density of 0.61 g·cm−3, the cell volume was 7 × 105Å3 and the total pore volume was 5.5 × 105Å3. The pore volume measured in cm3·g−1 was found to be 1.3cm3·g− 1 compared to 2cm3·g− 1 [19] and compared to 0.79– 2 cm3·g−1 [20]) and the calculated surface area was 2452m2·g−1 compared to 4100 m2·g− 1 [19], 4100–1560 m2·g−1 [20] and 3400m2·g−1[21]. Fig. 3 shows the powder XRD patterns of the three MOFs and the predicted pattern using the modelled crystal structure. It can be seen that the simulation patterns are in good agreement with the experimental results. The dynamic vapour sorption (DVS) gravimetric analyser was used to study the water adsorption characteristics of the three MOF materials [22,23]. Fig. 4 shows the measured and the simulated water adsorption isotherms at temperature of 25 °C showing close agreement. From Fig. 4a, CPO-27(Ni) exhibited type I adsorption isotherm reaching 81% of its capacity at a very low relative pressure (P/Ps = 0.05) and then a plateau reaching its maximum uptake of 0.47 gH2O·g−1 ads
at a relative pressure of 0.9 (~0.5 gH2O·g−1 ads [24]). This performance is due to the presence of unsaturated metal centres (UMCs) shown in Fig. 5a, also known as open metal sites existing in some MOF structures. These UMCs are metal binding sites formed after the removal of guest molecules of metal atoms attracting water molecules and offering extra binding sites to the guest molecules, especially at low pressures [25]. Also the hydrophilicity of the organic ligand due to the presence of the hydroxyl group is another factor contributing to this phenomenon. On the other hand, the strong interaction between the adsorbed water molecules and those sites would require a very high regeneration (desorption) temperature. Fig. 4b shows that aluminium fumarate, exhibits S shape or type IV isotherm. At low partial pressure (below 0.2), the water uptake was low, but a steep increase in the water uptake took place at the partial pressure range between 0.2 and 0.3 to reach 0.35 gH2O·g−1 ads and then the uptake continued to increase till it reached the maximum value of −1 0.53 gH2O·g−1 ads at a relative pressure of 0.9 (0.5 gH2O·gads at P/Ps = 0.9 [12]). Fig. 4 shows that the adsorption mechanism of aluminium fumarate differs from that of CPO-27(Ni). For aluminium fumarate, the water molecules are uniformly accumulated in the inner pores of the material indicating the hydrophilicity of the inner pore surface Fig. 5b, without the presence of the unsaturated metal sites. In the case of CPO-27(Ni), the presence of unsaturated metal centres caused the formation of strong coordination bonds. The limited water uptake at low relative pressure in aluminium fumarate is due to the hydrophobicity of the organic linker; therefore, higher water vapour pressure is required to induce the pore filling [26].
(a)
(b)
Fig. 6. a. The cyclic performance of MIL-101(Cr) at 25 °C and b. Water adsorption isotherms of MIL-101(Cr) at different adsorption temperatures.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
7
Table 1 Values of Dubinin-Astakhov equation parameters. x0
E
n
0.462
10.014 × 103
4
MIL-101(Cr), is one of the most investigated MOFs due to the high pore volume of 2cm3·g−1, the exceptional high surface area of 4500 m2·g−1, the high water uptake of 1–1.43 gH2O·g−1 ads (at high relative pressure N 0.35), and the high cyclic stability [22,23]. Fig. 4.c shows the measured water adsorption/desorption isotherms for MIL101(Cr) at a temperature of 25 °C which exhibits type IV isotherm (similar to aluminium fumarate). It can be noticed that at low partial pressure (below 0.4), the water vapour adsorption is mainly due to the presence of unsaturated metal centres (UMCs). Nevertheless, the limited water uptake is related to the dominant effect of the hydrophobicity of the organic linker. At higher relative pressure (0.4–0.5), a steep increase in the water uptake took place due to the capillary condensation which takes place in mesoporous materials. Because of this phenomenon, most adsorption isotherms for materials with large pores are always accompanied by a hysteresis loop between the adsorption and desorption branches. At high relative pressure (≥0.5), the pores are almost filled exhibiting a stable uptake. The hydrothermal stability of MIL-101(Cr) was tested over 20 successive adsorption/desorption cycles showing the high performance stability of the material (Fig. 6a). The cyclic performance of CPO27(Ni) and aluminium fumarate and the effect of the adsorption temperature on the performance of the two MOFs can be found in [12,27, 28]. Fig. 6b shows the water vapour uptake of MIL-101(Cr) at different temperatures, 15 °C, 35 °C and 45 °C showing almost the same uptake over the investigated temperature range.
Hot Fluid
Cold Fluid
Chilled Water
Fig. 7. Schematic diagram of the adsorption system.
x ¼ −3:124455E−11A3 þ 1:68302E−07A2 −3:12E−04A þ 0:5948 Ab2900
ð5Þ For MIL-101(Cr), the water uptake at different adsorption temperatures can be predicted through Eqs. (6)–(9): x ¼ 0:42434 expð−0:0002825AÞ
P=Ps ≤0:15
x ¼ 0:4636−0:00024A þ 5:4E−08A2 −4:06E−12A3
x ¼ 1:51−
A 1:35 T
ð6Þ 0:15bP=Ps ≤ 0:4
ð7Þ
0:4bP=Ps ≤0:5
ð8Þ
4. Adsorption isotherms and kinetics models
x ¼ 1:51−0:000266A þ 0:363E−6A2 −0:177E−9A3
The measured adsorption isotherms for water vapour onto CPO27(Ni) were fitted using the Dubinin-Astakhov equation (Eq. 1):
The rate of adsorption or the adsorption kinetics is another crucial parameter determining the residence time required for completion of the adsorption cycle and depending on the interaction between the adsorbent and the adsorbate. The Linear Driving Force model (Eqs. 10–13) has been one of the most used kinetics models as it is applicable to a wide range of adsorbents
n A x ¼ x0 exp − E A ¼ RT ln
ð1Þ
P Ps
ð2Þ
Table 1 gives the values of parameters x0, E and n. In the case of aluminium fumarate and MIL-101(Cr), a number of equations in terms of X and A were used to predict the water uptake at a wide range of operating conditions. For aluminium fumarate, the water uptake at different adsorption temperatures can be predicted through Eqs. (3)–(5): x ¼ 0:111993 expð−0:000258797AÞ
ð9Þ
dx ¼ K s av ðx−x0 Þ dt
ð10Þ
dx Ea ¼ k0 exp − ðx−x0 Þ dt RT
ð11Þ
k0 ¼
F:Dso
ð12Þ
R2p
ð3Þ
AN3987
x ¼ 2:36129−9:93768E−04A þ 1:05709E−07A2
P=Ps N0:5
2900≤A≤3987 ð4Þ
Table 2 Values of LDF equation parameters. Adsorbent
Ea
k0
CPO-27(Ni) Aluminium fumarate
2.5 ∗ 104 1.8 ∗ 104
5.1 1.29
MIL-101(Cr) P/Ps ≤ 0.15 or N 0.5 0.15 b P/Ps ≤ 0.4 0.4 b P/Ps ≤ 0.5
3.6 ∗ 104 1.3 ∗ 104 1.3 ∗ 104
6.36 ∗ 103 0.72 0.033
Table 3 Specification of the 2-bed adsorption desalination system for CPO-27(Ni) and aluminium fumarate. Mass of adsorbent/bed (kg)
36
Half cycle time (s) Switching time (s) Evaporation temp. (°C) Flow rate of chilled water (kg.s-1) Condensation temp. (°C) Flow rate of bed cooling fluid (kg.s-1) Adsorption temp. (°C) Flow rate of condenser cooling fluid (kg.s-1) Regeneration temp. (°C) Flow rate of hot fluid (kg.s-1)
700 70 5, 20 0.8 25 0.8 25 2 70–150 0.8
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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433
Temperature (K)
413 393 373 353 333 313 293 273 0
200
400
Evaporator
600
800
Condenser
1000 1200 1400 Time (s) Adsorption bed 1
1600
1800
2000
Adsorption bed 2
Fig. 8. Temperature profiles of adsorption bed, desorption bed, evaporator and condenser of the CPO-27(Ni) system. (Thot = 150 °C, Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Ea Ds ¼ Dso : exp − RT
ð13Þ
Condenser energy balance: ½Mcond cp ðT cond Þ þ MHX;cond cp;HX
Where F is a constant depending on the shape of the adsorbent particles which is 15 for spherical particles and 8 for cylindrical particles. Table 2 gives the values of parameters Ea and k0. The water adsorption isotherms at different temperatures, the adsorption/desorption stability tests of the two materials and the validity of the proposed kinetics and isotherms models are discussed in Elsayed et al. [28].
n: hfg ðT cond Þ
dT cond ¼ dt
ð15Þ
dcdes M a þ m:cond cp ðT cond Þ T cond;in −T cond;out dt
Adsorption/Desorption bed energy balance: dT ads=des dcads=des ¼ n: Q st M a Ma cp;a þ M HX cp;HX dt dt
m:c f cp T c f ;in −T c f ;out hf
hf
hf
ð16Þ
5. Adsorption system mathematical modelling The isosteric heat of adsorption: A two bed adsorption system was simulated through Simulink to study the performance of the system for water desalination. Fig. 7 shows the system consisting of two adsorption beds packed with MOF material, an evaporator and a condenser. The specifications of the 2bed adsorption desalination system are shown in Table 3. The energy balance equations for the evaporator, the condenser, the adsorption/desorption beds are illustrated in Eqs. (14–17) [8,29,30]: Evaporator energy balance: dT evap M s;evap cp;s T evap ; þ MHX;evap cp;HX ¼ dt dcads : −n: hfg T evap M a þ mchilled cp T evap T chilled;in −T chilled;out dt
Q st ¼ −Rwater
ð17Þ
For assessment of the cycle performance, the specific daily water production (SDWP) is calculated through Eqs. (18–21). Z
t cycle
SDWP ¼ ð14Þ
∂ ln ðP Þ 1 ∂ T
0
Q cond τ dt hfg Ma
ð18Þ
Q cond ¼ m:cond cp ðT cond Þ T cond;out −T cond;in
ð19Þ
4.5 Silica gel
SDWP ( m 3.(ton.day) -1)
4
Aluminium fumarate
CPO-27(Ni)
3.5 3 2.5 2 1.5 1 0.5 0 70
80
90
100 110 120 130 Hot fluid inlet temperature (o C)
140
150
Fig. 9. Effect of hot fluid inlet temperature on the water produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
40
Silica gel
Aluminium fumarate
9
CPO-27(Ni)
35
SCP (Rton.ton -1)
30 25 20 15 10 5 0 70
80
90
100 110 120 Hot fluid inlet temperature (o C)
130
140
150
Fig. 10. Effect of hot fluid inlet temperature on the cooling power produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Z
t cycle
SCP ¼
Q evap dt Ma
ð20Þ
0
Q evap ¼ m:chilled cp T evap T chilled;in −T chilled;out
ð21Þ
These set of energy and mass balance equations were used to build a simulation model where the adsorbent, adsorbate and heat exchangers are assumed to be at the same temperature. 6. Results and discussion The effect of hot fluid and chilled water temperatures on the performance of the adsorption desalination system using various MOF materials was investigated. The hot fluid inlet temperature was in the range of 70 °C–150 °C while the chilled water temperatures were 5 °C and 20 °C. The adsorption bed temperature and the condenser temperature were kept constant at 25 °C with a cycle time of 700 s and a switching time of 70 s. 6.1. The performance of CPO-27(Ni) and aluminium fumarate water desalination systems at a chilled water temperature of 5 °C The adsorption desalination cycle performance of the two MOFs was investigated under variable hot fluid temperature. Fig. 8 shows the temperature-time profile for the adsorption bed, desorption bed, evaporator and the condenser of the CPO-27(Ni) system. Fig. 9 shows the effect of the hot fluid inlet temperature on the SDWP of
Silica gel
7
the MOF materials compared to silica gel. It is evident that the SDWP would increase with increasing the hot fluid temperature. At a hot fluid temperature of 150 °C, the CPO-27(Ni) desalination system produced 4.3 m3/ton·day while the aluminium fumarate system produced 2.66 m3·(ton·day)− 1. At a hot fluid temperature of 90 °C, the silica gel produced 3.7 m3·(ton·day)− 1 outperforming both aluminium fumarate and CPO-27(Ni). The low performance of CPO-27(Ni) is due to that it can only be operated at high regeneration temperature (≥110 °C) while the low performance of aluminium fumarate is due to the low evaporation temperature. Also, it can be noticed that the aluminium fumarate can be regenerated at temperatures as low as 70 °C. The high regeneration temperature required for the desorption of CPO-27(Ni) is due to the high interaction between the adsorbed water molecules and the unsaturated metal sites presented in the CPO-27(Ni). At lower evaporation temperature and lower relative pressure, the CPO-27(Ni) is expected to outperform silica gel and aluminium fumarate due to its low dependency on the relative pressure unlike silica gel. At high regeneration temperatures (higher than 110 °C), the CPO27(Ni) was found to outperform the aluminium fumarate and silica gel producing 4.3 m3·(ton·day)−1 which makes it suitable in applications operated with high temperature waste heat sources. On the other hand, the silica gel and aluminium fumarate were found to outperform CPO-27(Ni) at regeneration temperatures as low as 70 °C making them suitable in application utilizing low temperature waste heat or renewable energy resource as solar energy. As the adsorption desalination system can produce two useful effects namely: high grade distilled water and cooling effect from the evaporation of the seawater in the evaporator unit. Fig. 10 shows the effect of the hot fluid inlet temperature on the SCP of the two systems compared to a silica gel system. The figure shows that increasing the hot fluid
Aluminium fumarate
CPO-27(Ni)
SDWP ( m 3.(ton.day) -1)
6 5 4 3 2 1 0 70
80
90
100 110 120 Hot fluid inlet temperature ( o C)
130
140
150
Fig. 11. Effect of hot fluid inlet temperature on the water produced from the adsorption desalination system. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 700 s and switching time = 70s).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
10
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
Silica gel
60
Aluminium fumarate
CPO-27(Ni)
SCP (Rton.ton -1)
50 40 30 20 10 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
Fig. 12. Effect of hot fluid inlet temperature on the cooling power produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 700 s and switching time = 70s).
temperature will increase the SCP produced from the two systems. The CPO-27(Ni) system produced a cooling effect of 35.3 Rton·ton−1, while the aluminium fumarate system produced 22 Rton·ton−1. This cooling effect temperature is suitable for air conditioning application. 6.2. The performance of CPO-27(Ni), aluminium fumarate and MIL101(Cr)water desalination systems at a chilled water temperature of 20 °C Fig. 11 shows the effect of the hot fluid inlet temperature on the SDWP of the two systems compared to the silica gel system.
The SDWP increased with increasing the hot fluid temperature. At a hot fluid temperature of 150 °C, the aluminium fumarate system produced 6.3 m3·(ton·day)− 1 while the CPO-27(Ni) desalination system produced 4.6 m3·(ton·day)− 1. The silica gel system was found to produce 5.3 m3·(ton·day)− 1 at a hot fluid temperature of 90 °C. The comparison between Figs. 9 and 11 highlights the effect of increasing the evaporation temperature, as it would remarkably affect the water production in case of aluminium fumarate and silica gel while it has almost no effect on the performance of CPO-27(Ni). The increase in the case of aluminium fumarate can be attributed to that the
11 10
(a)
SDWP ( m 3.(ton.day) -1)
9 8 7 6 5 4 3 2 1 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
100 90
(b)
SCP (Rton.ton -1)
80 70 60 50 40 30 20 10 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
Fig. 13. Effect of hot fluid inlet temperature on a. SDWP and b. cooling power produced from the MIL-101(Cr) adsorption desalination system. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 300 s switching time = 30 s), (Heating fluid flow rate=1.6 kg.s−1, bed cooling fluid flow rate = 1.6 kg.s−1, condenser cooling fluid flow rate = 4 kg.s−1 and chilled water flow rate = 1.6 kg.s−1).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx Table 4 Comparison between performances of CPO-27(Ni), aluminium fumarate and silica gel at low evaporation temperature. Adsorbent
CPO-27(Ni)
Aluminium fumarate
Silica gel
Ref. Regeneration temp. (°C) Adsorption temp. (°C) Condenser temp. (°C) Evaporator temp.(°C) SDWP (m3·(ton·day)−1) SCP (Rton·ton−1) Cycle time(s) Switching time (s)
This study 150 25 25 5 4.6 35.3 700 70
This study 90 25 25 5 2.46 21.2 700 70
26, 27 85 29.5 30 10 3.2 19.6 600 40
11
increasing the evaporation temperature from 5 °C to 20 °C did not significantly affect the SDWP produced from the CPO-27(Ni) desalination system which was the opposite in case of silica gel and aluminium fumarate. This is attributed to that the CPO-27(Ni) exhibits type I isotherm adsorbing most of its capacity at low relative pressure. Aluminium fumarate and MIL-101(Cr) exhibit type IV isotherm while silica gel exhibits type III, both isotherms depend profoundly on the evaporation temperature and can have high capacity only at high relative pressure values. At high evaporation temperature of 20 °C, the aluminium fumarate outperformed both the CPO-27(Ni) and silica gel with a SDWP of 6.3 m3·(ton·day)−1. MIL-101(Cr) outperformed all the adsorbents with a SDWP of 11 m3·(ton·day)−1. 7. Conclusion
material exhibits ‘type IV’ where its uptake significantly increases with increasing the evaporation temperature generating higher water vapour pressure on the adsorbent allowing a higher uptake. In case of CPO-27(Ni), the material exhibits' type I′ water adsorption isotherm reaching 81% of its capacity at a very low relative pressure and hence its performance is less dependent on the evaporation temperature. In other words, porous materials with ‘type I′ adsorption isotherm would not be significantly affected by increasing the evaporation temperature as much as materials possessing ‘type IV’ isotherm. Fig. 12 shows the effect of the hot fluid inlet temperature on the SCP of the two systems compared to the silica gel system. The figure shows that increasing the regeneration temperature will increase the SCP produced from the three systems. The CPO-27(Ni) system produced a cooling effect of 38.1 Rton·ton−1, while the aluminium fumarate cycle produced 51.8 Rton·ton−1. The cooling effect temperature in this case is more suitable for processes cooling or moderate air conditioning applications. From Fig. 4 b and c, it is evident that the MIL-101(Cr) exhibits IV isotherm, following the trend as aluminium fumarate and having an exceptional performance at high evaporation temperature. Fig. 13 a and b shows the effect of the hot water temperature on a desalination system with MIL-101(Cr) as the adsorbent at an evaporation temperature of 20 °C. The MIL-101(Cr) system produced 11 m3·(ton·day)− 1 and a cooling power of 90 Rton·ton− 1 at a hot fluid temperature of 150 °C. Another comparative study was held between the CPO-27(Ni), aluminium fumarate and silica gel previously reported for conventional 2-bed adsorption desalination system both at low and high evaporation temperatures. The SDWP in addition to the optimum operating conditions are mentioned in Tables 4 and 5. From Table 4, the silica gel was only reported at an evaporation temperature of 10 °C while the CPO27(Ni) can work at a lower temperature of 5 °C. CPO-27(Ni) has a high performance at such a low evaporation temperature producing 4.6 m3·(ton·day)−1 outperforming both aluminium fumarate and silica gel. This high performance can be only obtained at a high regeneration temperature (N110 °C). Table 5 shows the SDWP of the three adsorbent systems at higher evaporation temperature. It can be noticed that
Table 5 Comparison between performances of CPO-27(Ni), aluminium fumarate, MIL-101(Cr) and silica gel at high evaporation temperature. Adsorbent
CPO-27(Ni)
Aluminium fumarate
MIL-101(Cr)
Silica gel
Ref. Regeneration temp. (°C) Adsorption temp. (°C) Condenser temp. (°C) Evaporator temp.(°C) SDWP (m3·(ton·day)−1) SCP (Rton·ton−1) Cycle time (s) Switching time (s)
This study 150 25 25 20 4.6 38.1 700 70
This study 90 25 25 20 6.3 51.8 700 70
This study 90–150 25 25 20 8–11 66.3–89.7 300 30
26, 27 85 29.5 30 20 5.1 31.1 600 40
Metal-organic frameworks (MOFs) are new porous materials with high surface area, pore size and volume, tunable pore geometry and hence providing high adsorption capacity. Three metal-organic framework materials, namely aluminium fumarate, CPO-27(Ni) and MIL101(Cr), were characterized through XRD and water adsorption measurements. Results showed that MIL-101(Cr) has the highest water uptake of 1.47 kg·kg−1. The performance of these MOFs in a two-bed adsorption desalination system were modelled and compared to silica gel using Matlab/ Simulink at various heat source temperatures. Results showed that the CPO-27(Ni) is more suitable where low evaporation temperature (5 ° C) and high regeneration temperature (≥110 °C) are available achieving a SDWP of 4.6 m3·(ton·day)−1. The aluminium fumarate showed a superior performance at high evaporator temperature values (20 °C) with a SDWP of 6.3 m3·(ton·day)− 1. The aluminium fumarate was also found to require low regeneration temperature (70 °C). As for MIL101(Cr), results showed an exceptional performance of producing 11 m 3·(ton·day) − 1 outperforming any other adsorbent. This work highlights the potential of aluminium fumarate, CPO-27(Ni) and MIL-101(Cr) in adsorption desalination where to the adsorbent can be chosen according to the required SDWP, cooling power and the heat source temperature. References [1] International Decade for Action ‘Water for Life’ 2005–2015, United Nations Office. [2] United Nations Environment Programme, 2008 [3] T. Younos, K.E. Tulou, Overview of desalination techniques, Journal of Contemporary Water Research & Education 132 (2005) 3–10. [4] K.C. Ng, X.L. Wang, L.Z. Gao, A. Chakraborty, B.B. Saha, S. Koyama, Apparatus and Method for Desalination, SG Patent Application Number 200503029-1 (2005) and WO Patent No. 121414A1, 2006. [5] X. Wang, K.C. Ng, Experimental investigation of an adsorption desalination plant using low-temperature waste heat, Appl. Therm. Eng. 25 (2005) 2780–2789. [6] K. Thu, A. Chakraborty, Y. Kim, A. Myat, B.B. Saha, K.C. Ng, Numerical simulation and performance investigation of an advanced adsorption desalination cycle, Desalination 308 (2013) 209–218. [7] K.C. Ng, K. Thu, Y. Kim, A. Chakraborty, G. Amy, Adsorption desalination: an emerging low-cost thermal desalination method, Desalination 308 (2013) 161–179. [8] S. Mitra, K. Srinivasan, P. Kumar, S.S. Murthy, P. Dutta, Solar driven adsorption desalination system, Energy Procedia 49 (2014) 2261–2269. [9] P.G. Youssef, S.M. Mahmoud, R.K. Al-Dadah, Effect of evaporator temperature on the performance of water desalination/refrigeration adsorption system using AQSOAZO2, Int. J. Environ. Chem. Ecol. Geol. Geophys. Eng. 9 (6) (2015). [10] P. G. Youssef, S.M. Mahmoud, R.K. AL-Dadah, Performance analysis of four bed adsorption water desalination/refrigeration system, comparison of AQSOA-Z02 to silica-gel, Desalination 2015;375:100–107. [11] A. Elsayed, R. AL-Dadah, S. Mahmoud, B. Shi, P. Youessef, A. Elshaer, W. Kaialy, Characterisation of CPO-27Ni Metal Organic Framework Material for Water Adsorption, Conference Paper, SusTEM, Newcastle University, UK, 2015. [12] F. Jeremias, D. Frohlich, C. Janiak, S. Henninger, Advancement of sorption-based heat transformation by a metal coating of highly-stable, hydrophilic aluminium fumarate MOF, RSC Adv. 4 (2014) 24073–24082. [13] J. Yang, Q. Zhao, J. Li, J. Dong, Synthesis of metal–organic framework MIL-101 in TMAOH-Cr(NO3)3-H2BDC-H2O and its hydrogen-storage behaviour, Microporous Mesoporous Mater. 130 (2010) 174–179. [14] J. Liu, Y. Wang, A.I. Benin, P. Jakubczak, R.R. Willis, M.D. LeVan, CO2/H2O adsorption equilibrium and rates on metal-organic frameworks: HKUST-1 and Ni/DOBDC, Langmuir 26 (17) (2010) 14301–14307.
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[23] J. Yan, Y. Yu, C. Ma, J. Xiao, Q. Xia, Y. Li, Z. Li, Adsorption isotherms and kinetics of water vapor on novel adsorbents MIL-101(Cr)@GO with super-high capacity, Appl. Therm. Eng. 84 (2015) 118–125. [24] J. Liu, Y. Wang, A. Benin, P. Jakubczak, R. Willis, M. LeVan, CO2/H2O adsorption equilibrium and rates on metal-organic frameworks: HKUST-1 and Ni/DOBDC, Langmuir 26 (2010) 14301–14307. [25] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Water adsorption in MOFs: fundamentals and applications, Chem. Soc. Rev. 2014;43:5594—5617. [26] H. Furukawa, F. Gándara, Y. Zhang, J. Jiang, W. Queen, M. Hudson, O. Yaghi, Water adsorption in porous metal-organic frameworks and related materials, J. Am. Chem. Soc. 136 (2014) 4369–4381. [27] B. Shi, R. AL-Dadah, S. Mahmoud, A. Elsayed, E. Elsayed, CPO-27(Ni) metal–organic framework based adsorption system for automotive air conditioning, Appl. Therm. Eng. 106 (2016) 325–333. [28] E. Elsayed, R. AL-Dadah, S. Mahmoud, A. Elsayed, P.A. Anderson, Aluminium fumarate and CPO-27(Ni) MOFs: characterization and thermodynamic analysis for adsorption heat pump applications, Appl. Therm. Eng. 99 (2016) 802–812. [29] K.C. Ng, K. Thu, A. Chakraborty, B.B. Saha, W.G. Chun, Solar-assisted dual-effect adsorption cycle for the production of cooling effect and potable water, Int. J. Low Carbon Technol. 4 (2009) 61–67. [30] K. Thu, Adsorption Desalination Theory and Experiment, National University of Singapore, 2010.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination Eman Elsayed a,b,⁎, Raya AL-Dadah a, Saad Mahmoud a, Paul.A. Anderson b, Ahmed Elsayed c, Peter G. Youssef a a b c
School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UK School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK The Institute for Advanced Manufacturing and Engineering (AME), School of Engineering and Computing, Coventry University, Coventry CV6 5LZ, UK
H I G H L I G H T S • • • •
CPO-27(Ni), aluminium fumarate and MIL-101(Cr) were investigated for adsorption desalination application. Optimum desorption temperature for CPO-27(Ni) was higher than 110 °C and for aluminium fumarate was as low as 70 °C. CPO-27(Ni) outperformed other materials at low evaporation temperature (5 °C) and high regeneration temperatures. MIL-101(Cr) and aluminium fumarate outperformed CPO-27(Ni) and silica gel at high evaporation temperature (20 °C).
a r t i c l e
i n f o
Article history: Received 1 April 2016 Received in revised form 18 July 2016 Accepted 21 July 2016 Available online xxxx Keywords: Metal-organic framework XRD Water vapour adsorption Adsorption desalination
a b s t r a c t Adsorption desalination is a promising technology that has recently been investigated. Most of the reported adsorption desalination systems use silica gel as the adsorbent and despite the high stability, it suffers from limited water uptake capabilities leading to a low system performance. Metal-organic frameworks (MOFs) are porous materials with high surface area, pore size, tunable pore geometry and hence providing high adsorption capacity. Currently, limited MOF materials with high water adsorption capabilities and hydrothermal stability are commercially available. CPO-27(Ni) and aluminium fumarate are two commercially available MOFs that have a max−1 imum water uptake of 0.47 gH2O·g−1 ads and 0.53 gH2O·gad , respectively. Another MOF, MIL-101(Cr), exhibits superior water adsorption uptake of 1.47 gH2O·g−1 ad but currently can only be produced in lab-scale. The thermodynamic cycle performance of a two beds adsorption system was evaluated using Simulink software to assess the suitability of those MOFs for adsorption desalination and their performance under different operating conditions. The CPO-27(Ni) was found to produce around 4.3 m3·(ton·day)−1 at an evaporation temperature of 5 °C while aluminium fumarate produced around 6 m3·(ton·day)−1 at an evaporation temperature of 20 °C. As for MIL-101(Cr), the water production rate at 20 °C was 11 m3·(ton·day)−1 highlighting the potential of this material compared to other adsorbents. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Seven hundred million people around the world are suffering from water scarcity, another 500 million are approaching this situation and this problem is expected to worsen by 2025 as shown in Fig. 1 [1]. More than 70% of the earth's surface is covered with water and 97% of this water is salty which can be used to produce fresh water using desalination processes. Desalination is generally defined as the process by which potable water is produced from the seawater or brackish water with high dissolved suspended solids content (N35,000 ppm).
⁎ Corresponding author. E-mail address: [email protected] (E. Elsayed).
The desalination technologies can be categorised into (1) pressure activated systems (the membrane technologies) such as the Reverse Osmosis (RO); (2) thermal energy systems (distillation processes) such as the Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), Mechanical Vapour Compression (MVC) and Solar Distillation and (3) chemical methods such as Ion-Exchange Desalination and Gas Hydrate [3]. Recently, adsorption desalination has been identified with many advantages such as environmentally friendly, driven by low-grade heat sources, low capital cost, low evaporation temperature and hence reduced fouling (formation of scales causing the damage of the evaporation units) effect. The adsorption system consists of an evaporator, a condenser and adsorption/desorption beds. Each bed includes a finned tube heat exchanger with the adsorbent material packed between the fins. The process starts with evaporating the seawater producing
http://dx.doi.org/10.1016/j.desal.2016.07.030 0011-9164/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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Nomenclature Symbols
Description
Unit
A cp E Ea K0 M m n. n P Ps
adsorption potential specific heat at constant pressure adsorption characteristic parameter activation energy LDF model empirical constant mass mass flow rate adsorption/desorption phase, flag exponent fitting parameter pressure saturation pressure of adsorbate at adsorption temperature relative pressure isosteric heat of adsorption ideal gas constant specific cooling power specific daily water production temperature equilibrium uptake maximum uptake no. of cycles per day
J·mol−1 kg·kJ−1·K−1 J·mol−1 J·mol−1 s−1 kg kg·s−1 \ \ \ \ kPa kPa
P/Ps Qst R SCP SDWP T x x0 τ Subscripts a ads cond cf des evap hf HX In Out S t
\ \ kJ·kg−1 J·(mol·K)−1 Rton·ton−1 m3· (ton·day)−1 K gH2O·g−1 ads gH2O·g−1 ads Cycle/day
adsorbent material adsorption condenser cooling fluid desorption evaporator hot fluid heat exchanger inlet outlet seawater time
water vapour which is then adsorbed by the adsorbent material. Then during the desorbing step where the bed is heated, the water vapour is released and then condensed in the condenser producing highgrade distilled water [4]. Wang et al. [5] experimentally investigated the performance and the specific daily water production (SDWP) of a four-bed silica gel adsorption desalination plant. They showed that a SDWP of 4.7 m3·(ton·day)−1 was obtained using a regeneration temperature of 85 °C, an adsorption temperature of 29.4 °C, an evaporation temperature of 12.2 °C and a half cycle time of 180 s with a switching time of 40 s. They proposed that the performance can be further improved by adopting a higher chilled water temperature supplied to the evaporator and a lower cooling water temperature to the designated adsorption beds. Thu et al. [6] developed a numerical model for an advanced adsorption desalination cycle, in
which a higher vapour pressure was achieved through the recovery of latent heat of condensation into the evaporator unit that was built inside the condenser. They used half cycle time of 300 s which was half that used by Wang et al. [5] leading to specific daily water production (SDWP) of around 26 m 3·(ton·day) − 1 which was almost three folds of the conventional cycle. An economic study was held by Ng et al. [7] showing that adsorption desalination required less unit production cost, electrical energy and total primary energy than other conventional desalination systems then they developed a FORTRAN code for a solar-powered adsorption desalination plant with two and four adsorption beds using silica gel as the adsorbent. The two bed system yielded 7.4 m3·(ton·day)− 1 while the four bed system produced 8.9 m3·(ton·day)− 1. The two systems used a regeneration temperature of 85 °C and produced a high grade water with total dissolved solids (TDS) content b 15 ppm. Mitra et al. [8] investigated the effect of cycle time and the condenser temperature on the performance of a four bed silica gel adsorption desalination system. The system was operated at an evaporation temperature of 5.5 °C, a regeneration temperature of 85 °C, an adsorption temperature of 30 °C and a condensation temperature of 24.5–50.5 °C. It was found that increasing the condenser temperature would negatively affect the system performance due to increasing the operating pressure ratio which require higher switching time for bed. An optimum cycle time in the range of 600–900 s was found for maximising specific cooling power (SCP) and SDWP resulting in a coefficient of performance (COP) of 0.41–0.47, a SDWP of 2.4–2.3 m 3·(ton·day)− 1 and a SCP of 18–17 Rton·ton− 1 of silica-gel. A Simulink model for a two bed adsorption system was developed by Youssef et al. [9] comparing the performance of silica gel and silicaaluminophosphates AQSOA-ZO2 at different evaporation temperatures. They showed that at high chilled water temperature (N20 °C), silica-gel outperformed AQSOA-ZO2 producing 8.4 m3·(ton·day)−1 and specific cooling capacity of 62.4 Rton·ton− 1 silica gel. At chilled water temperature b 20 °C, AQSOA-ZO2 outperformed silica-gel as AQSOAZO2 produced 5.8 m3 distilled water per day and 50.1 Rton·ton−1 while silica-gel produced 2.8 m3·(ton·day)− 1 of water and 17.2 Rton·ton−1. Also, Youssef et al. [10] investigated the performance of a four bed adsorption desalination system using the same two materials. The results showed that at low chilled water temperatures (b 20 ° C), AQSOA-Z02 had a SDWP of 6.2 m3·(ton·day)− 1 and SCP of 53.7 Rton·ton−1 compared to only 3.39 m3·(ton·day)− 1 and 15.7 Rton·ton−1 silica gel. At high chilled water temperature (N 20 °C), silica-gel outperformed AQSOA-Z02 with a SDWP of 9.5 m3 (ton·day)−1 and 55 Rton·ton−1 of cooling. All of the above reported work was based on using either silica gel or silica-aluminophosphates as the adsorbent material. Silica gel has the advantage of high stability but suffers from a relatively low hydrophilicity thus high relative pressure value is needed to reach its maximum capacity. This limited capacity highlights the importance of using more hydrophilic porous materials like the metal-organic frameworks
Fig. 1. The growth of global water stress problem from 1995 to 2025 [2].
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
(a)
3
(b)
(c)
Fig. 2. Crystal structure of a. CPO-27(Ni) [2,5 Dihydroxyterephthalic acid as the organic linker and nickel nodes] b. Aluminium fumarate [Fumaric acid as the organic ligand and aluminiumOH-Aluminium chains as metal nodes.] c. MIL-101(Cr) [Terephthalic acid as the organic ligand and chromium nodes].
(a)
(b)
(c)
Fig. 3. Experimental and simulation PXRD patterns of a. CPO-27(Ni), b. Aluminium fumarate and c. MIL-101(Cr).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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Uptake (g H2O. g -1)
(MOFs) investigated in this work. Metal-organic frameworks are porous materials with extraordinary chemical and physical properties including high surface area and porosity, tunable pore geometry and properties leading to high adsorption capacity. MOFs as the name suggests
are composed of organic ligands and metal atoms where the organic ligand and the metallic building units can be changed to control the MOF pore size and geometry to make it suitable for the required applications. Despite these advantages of MOFs, major challenges still exist in the
0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
(a)
Simulated
0.00
0.10
0.20
0.30
0.40
0.50 P/Ps
0.60
Experimental
0.70
0.80
0.90
1.00
(c)
Fig. 4. Water adsorption isotherms of a.CPO-27(Ni), b. aluminium fumarate and c. MIL-101(Cr) at 25 °C. (Solid lines represent adsorption phase and dotted lines represent desorption phase).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
water adsorption applications such as limited water vapour uptake for some MOFs, poor hydrothermal stability for others and finally limited large scale commercial availability of some MOF materials. The authors have identified two MOFs with water adsorption capabilities higher than silica gel and zeolites namely CPO-27(Ni) (known as MOF-74(Ni)) and aluminium fumarate with water vapour uptake 1 −1 of 0.47 gH2O·g− ads and 0.53 gH2O·gads respectively [11,12]. The two MOFs were found to be hydrothermally stable and are mass-produced by Johnson Matthey and MOFs technologies, respectively. In addition, the authors have synthesized another MOF material known as MIL101(Cr) which has very high water adsorption uptake of 1.47gH2O·g−1 ad but currently can only be produced in lab-scale. The main objective of this work is to characterize those MOFs experimentally and through Materials Studio software in terms of their crystal structures and pore geometries. A further assessment was carried out using Simulink modelling to investigate the potential of these MOF materials for adsorption desalination application at various operating conditions.
5
This section describes techniques used to characterize the three MOFs through powder X-Ray Diffraction (XRD) and water adsorption characteristics using Dynamic Vapour Adsorption analyser (DVS). The equilibrium water adsorption characteristics were then used to develop a Simulink model to investigate the potential of the MOF materials for adsorption desalination application at various operating conditions. For aluminium fumarate and CPO-27(Ni), the powder XRD patterns were measured using a Bruker D8 Advance Reflection Diffractometer with Cu Kα radiation (1.5418 Å). The aluminium fumarate sample was scanned from 8 to 45° 2θ, the CPO-27(Ni) sample from 10 to 75°, with a step size of 0.02° and both were spun at 15 rpm. For MIL-101(Cr), Siemens D5005 was used to scan the samples from 2.5 to 30° 2θ with a step size of 0.02°. The dynamic vapour sorption (DVS) gravimetric analyser (Advantage DVS, Surface Measurement Systems, UK) was used to study the water adsorption characteristics. 3. Crystal structure modelling
2. Experimental work CPO-27(Ni) and aluminium fumarate were obtained from commercial sources. The CPO-27(Ni) was supplied by Johnson Matthey, while the aluminium fumarate was supplied by MOF Technologies. MIL-101(Cr) was synthesized according to the published literature [13].
Materials Studio software was used to further characterize the materials through generating the crystal structure, surface area, pore size and pore volume. Fig. 2 shows the crystal structures for the three MOFs showing the organic ligand and the metal nodes which are used to determine the pore volume and surface area. For CPO-27(Ni), the crystal density was 1.2 g·cm−3, the cell volume was 3.8 × 103Å3 and the total
(a)
Water molecules uniformly adsorbed at 25oC and P/Ps=0.9.
Water molecules uniformly adsorbed at 25oC and P/Ps=0.9.
(b)
Fig. 5. water adsorption sites and the adsorbed water molecules field at 25 °C and P/Ps = 0.9 for a.CPO-27(Ni) and b. aluminium fumarate.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
pore volume was 2.3 × 103Å3. The pore volume measured in cm3·g−1 was found to be 0.5cm3·g− 1 compared to 0.36cm3·g−1 [14], 0.41cm3·g−1 [15] and 0.77cm3·g−1 [16]. The pore size was also calculated from the crystal structure and was found to be 12.1 Å compared to 11–12 Å [17] and the calculated surface area was 1225.7m2·g− 1 (1218 m2·g−1 [18]). For aluminium fumarate, the crystal density was 1.087 g·cm− 3, the cell volume was 959.56Å3 and total pore volume was 453.98Å3. The pore volume measured in cm3·g− 1 was found to be 0.436cm3·g− 1 compared to 0.48cm3·g− 1 [12] and the calculated surface area was 1211.5m2·g−1 compared to 1021m2·g−1 [12]). The crystal structure of MIL-101(Cr) showed crystal density of 0.61 g·cm−3, the cell volume was 7 × 105Å3 and the total pore volume was 5.5 × 105Å3. The pore volume measured in cm3·g−1 was found to be 1.3cm3·g− 1 compared to 2cm3·g− 1 [19] and compared to 0.79– 2 cm3·g−1 [20]) and the calculated surface area was 2452m2·g−1 compared to 4100 m2·g− 1 [19], 4100–1560 m2·g−1 [20] and 3400m2·g−1[21]. Fig. 3 shows the powder XRD patterns of the three MOFs and the predicted pattern using the modelled crystal structure. It can be seen that the simulation patterns are in good agreement with the experimental results. The dynamic vapour sorption (DVS) gravimetric analyser was used to study the water adsorption characteristics of the three MOF materials [22,23]. Fig. 4 shows the measured and the simulated water adsorption isotherms at temperature of 25 °C showing close agreement. From Fig. 4a, CPO-27(Ni) exhibited type I adsorption isotherm reaching 81% of its capacity at a very low relative pressure (P/Ps = 0.05) and then a plateau reaching its maximum uptake of 0.47 gH2O·g−1 ads
at a relative pressure of 0.9 (~0.5 gH2O·g−1 ads [24]). This performance is due to the presence of unsaturated metal centres (UMCs) shown in Fig. 5a, also known as open metal sites existing in some MOF structures. These UMCs are metal binding sites formed after the removal of guest molecules of metal atoms attracting water molecules and offering extra binding sites to the guest molecules, especially at low pressures [25]. Also the hydrophilicity of the organic ligand due to the presence of the hydroxyl group is another factor contributing to this phenomenon. On the other hand, the strong interaction between the adsorbed water molecules and those sites would require a very high regeneration (desorption) temperature. Fig. 4b shows that aluminium fumarate, exhibits S shape or type IV isotherm. At low partial pressure (below 0.2), the water uptake was low, but a steep increase in the water uptake took place at the partial pressure range between 0.2 and 0.3 to reach 0.35 gH2O·g−1 ads and then the uptake continued to increase till it reached the maximum value of −1 0.53 gH2O·g−1 ads at a relative pressure of 0.9 (0.5 gH2O·gads at P/Ps = 0.9 [12]). Fig. 4 shows that the adsorption mechanism of aluminium fumarate differs from that of CPO-27(Ni). For aluminium fumarate, the water molecules are uniformly accumulated in the inner pores of the material indicating the hydrophilicity of the inner pore surface Fig. 5b, without the presence of the unsaturated metal sites. In the case of CPO-27(Ni), the presence of unsaturated metal centres caused the formation of strong coordination bonds. The limited water uptake at low relative pressure in aluminium fumarate is due to the hydrophobicity of the organic linker; therefore, higher water vapour pressure is required to induce the pore filling [26].
(a)
(b)
Fig. 6. a. The cyclic performance of MIL-101(Cr) at 25 °C and b. Water adsorption isotherms of MIL-101(Cr) at different adsorption temperatures.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
7
Table 1 Values of Dubinin-Astakhov equation parameters. x0
E
n
0.462
10.014 × 103
4
MIL-101(Cr), is one of the most investigated MOFs due to the high pore volume of 2cm3·g−1, the exceptional high surface area of 4500 m2·g−1, the high water uptake of 1–1.43 gH2O·g−1 ads (at high relative pressure N 0.35), and the high cyclic stability [22,23]. Fig. 4.c shows the measured water adsorption/desorption isotherms for MIL101(Cr) at a temperature of 25 °C which exhibits type IV isotherm (similar to aluminium fumarate). It can be noticed that at low partial pressure (below 0.4), the water vapour adsorption is mainly due to the presence of unsaturated metal centres (UMCs). Nevertheless, the limited water uptake is related to the dominant effect of the hydrophobicity of the organic linker. At higher relative pressure (0.4–0.5), a steep increase in the water uptake took place due to the capillary condensation which takes place in mesoporous materials. Because of this phenomenon, most adsorption isotherms for materials with large pores are always accompanied by a hysteresis loop between the adsorption and desorption branches. At high relative pressure (≥0.5), the pores are almost filled exhibiting a stable uptake. The hydrothermal stability of MIL-101(Cr) was tested over 20 successive adsorption/desorption cycles showing the high performance stability of the material (Fig. 6a). The cyclic performance of CPO27(Ni) and aluminium fumarate and the effect of the adsorption temperature on the performance of the two MOFs can be found in [12,27, 28]. Fig. 6b shows the water vapour uptake of MIL-101(Cr) at different temperatures, 15 °C, 35 °C and 45 °C showing almost the same uptake over the investigated temperature range.
Hot Fluid
Cold Fluid
Chilled Water
Fig. 7. Schematic diagram of the adsorption system.
x ¼ −3:124455E−11A3 þ 1:68302E−07A2 −3:12E−04A þ 0:5948 Ab2900
ð5Þ For MIL-101(Cr), the water uptake at different adsorption temperatures can be predicted through Eqs. (6)–(9): x ¼ 0:42434 expð−0:0002825AÞ
P=Ps ≤0:15
x ¼ 0:4636−0:00024A þ 5:4E−08A2 −4:06E−12A3
x ¼ 1:51−
A 1:35 T
ð6Þ 0:15bP=Ps ≤ 0:4
ð7Þ
0:4bP=Ps ≤0:5
ð8Þ
4. Adsorption isotherms and kinetics models
x ¼ 1:51−0:000266A þ 0:363E−6A2 −0:177E−9A3
The measured adsorption isotherms for water vapour onto CPO27(Ni) were fitted using the Dubinin-Astakhov equation (Eq. 1):
The rate of adsorption or the adsorption kinetics is another crucial parameter determining the residence time required for completion of the adsorption cycle and depending on the interaction between the adsorbent and the adsorbate. The Linear Driving Force model (Eqs. 10–13) has been one of the most used kinetics models as it is applicable to a wide range of adsorbents
n A x ¼ x0 exp − E A ¼ RT ln
ð1Þ
P Ps
ð2Þ
Table 1 gives the values of parameters x0, E and n. In the case of aluminium fumarate and MIL-101(Cr), a number of equations in terms of X and A were used to predict the water uptake at a wide range of operating conditions. For aluminium fumarate, the water uptake at different adsorption temperatures can be predicted through Eqs. (3)–(5): x ¼ 0:111993 expð−0:000258797AÞ
ð9Þ
dx ¼ K s av ðx−x0 Þ dt
ð10Þ
dx Ea ¼ k0 exp − ðx−x0 Þ dt RT
ð11Þ
k0 ¼
F:Dso
ð12Þ
R2p
ð3Þ
AN3987
x ¼ 2:36129−9:93768E−04A þ 1:05709E−07A2
P=Ps N0:5
2900≤A≤3987 ð4Þ
Table 2 Values of LDF equation parameters. Adsorbent
Ea
k0
CPO-27(Ni) Aluminium fumarate
2.5 ∗ 104 1.8 ∗ 104
5.1 1.29
MIL-101(Cr) P/Ps ≤ 0.15 or N 0.5 0.15 b P/Ps ≤ 0.4 0.4 b P/Ps ≤ 0.5
3.6 ∗ 104 1.3 ∗ 104 1.3 ∗ 104
6.36 ∗ 103 0.72 0.033
Table 3 Specification of the 2-bed adsorption desalination system for CPO-27(Ni) and aluminium fumarate. Mass of adsorbent/bed (kg)
36
Half cycle time (s) Switching time (s) Evaporation temp. (°C) Flow rate of chilled water (kg.s-1) Condensation temp. (°C) Flow rate of bed cooling fluid (kg.s-1) Adsorption temp. (°C) Flow rate of condenser cooling fluid (kg.s-1) Regeneration temp. (°C) Flow rate of hot fluid (kg.s-1)
700 70 5, 20 0.8 25 0.8 25 2 70–150 0.8
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
433
Temperature (K)
413 393 373 353 333 313 293 273 0
200
400
Evaporator
600
800
Condenser
1000 1200 1400 Time (s) Adsorption bed 1
1600
1800
2000
Adsorption bed 2
Fig. 8. Temperature profiles of adsorption bed, desorption bed, evaporator and condenser of the CPO-27(Ni) system. (Thot = 150 °C, Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Ea Ds ¼ Dso : exp − RT
ð13Þ
Condenser energy balance: ½Mcond cp ðT cond Þ þ MHX;cond cp;HX
Where F is a constant depending on the shape of the adsorbent particles which is 15 for spherical particles and 8 for cylindrical particles. Table 2 gives the values of parameters Ea and k0. The water adsorption isotherms at different temperatures, the adsorption/desorption stability tests of the two materials and the validity of the proposed kinetics and isotherms models are discussed in Elsayed et al. [28].
n: hfg ðT cond Þ
dT cond ¼ dt
ð15Þ
dcdes M a þ m:cond cp ðT cond Þ T cond;in −T cond;out dt
Adsorption/Desorption bed energy balance: dT ads=des dcads=des ¼ n: Q st M a Ma cp;a þ M HX cp;HX dt dt
m:c f cp T c f ;in −T c f ;out hf
hf
hf
ð16Þ
5. Adsorption system mathematical modelling The isosteric heat of adsorption: A two bed adsorption system was simulated through Simulink to study the performance of the system for water desalination. Fig. 7 shows the system consisting of two adsorption beds packed with MOF material, an evaporator and a condenser. The specifications of the 2bed adsorption desalination system are shown in Table 3. The energy balance equations for the evaporator, the condenser, the adsorption/desorption beds are illustrated in Eqs. (14–17) [8,29,30]: Evaporator energy balance: dT evap M s;evap cp;s T evap ; þ MHX;evap cp;HX ¼ dt dcads : −n: hfg T evap M a þ mchilled cp T evap T chilled;in −T chilled;out dt
Q st ¼ −Rwater
ð17Þ
For assessment of the cycle performance, the specific daily water production (SDWP) is calculated through Eqs. (18–21). Z
t cycle
SDWP ¼ ð14Þ
∂ ln ðP Þ 1 ∂ T
0
Q cond τ dt hfg Ma
ð18Þ
Q cond ¼ m:cond cp ðT cond Þ T cond;out −T cond;in
ð19Þ
4.5 Silica gel
SDWP ( m 3.(ton.day) -1)
4
Aluminium fumarate
CPO-27(Ni)
3.5 3 2.5 2 1.5 1 0.5 0 70
80
90
100 110 120 130 Hot fluid inlet temperature (o C)
140
150
Fig. 9. Effect of hot fluid inlet temperature on the water produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
40
Silica gel
Aluminium fumarate
9
CPO-27(Ni)
35
SCP (Rton.ton -1)
30 25 20 15 10 5 0 70
80
90
100 110 120 Hot fluid inlet temperature (o C)
130
140
150
Fig. 10. Effect of hot fluid inlet temperature on the cooling power produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 5 °C, cycle time = 700 s and switching time = 70s).
Z
t cycle
SCP ¼
Q evap dt Ma
ð20Þ
0
Q evap ¼ m:chilled cp T evap T chilled;in −T chilled;out
ð21Þ
These set of energy and mass balance equations were used to build a simulation model where the adsorbent, adsorbate and heat exchangers are assumed to be at the same temperature. 6. Results and discussion The effect of hot fluid and chilled water temperatures on the performance of the adsorption desalination system using various MOF materials was investigated. The hot fluid inlet temperature was in the range of 70 °C–150 °C while the chilled water temperatures were 5 °C and 20 °C. The adsorption bed temperature and the condenser temperature were kept constant at 25 °C with a cycle time of 700 s and a switching time of 70 s. 6.1. The performance of CPO-27(Ni) and aluminium fumarate water desalination systems at a chilled water temperature of 5 °C The adsorption desalination cycle performance of the two MOFs was investigated under variable hot fluid temperature. Fig. 8 shows the temperature-time profile for the adsorption bed, desorption bed, evaporator and the condenser of the CPO-27(Ni) system. Fig. 9 shows the effect of the hot fluid inlet temperature on the SDWP of
Silica gel
7
the MOF materials compared to silica gel. It is evident that the SDWP would increase with increasing the hot fluid temperature. At a hot fluid temperature of 150 °C, the CPO-27(Ni) desalination system produced 4.3 m3/ton·day while the aluminium fumarate system produced 2.66 m3·(ton·day)− 1. At a hot fluid temperature of 90 °C, the silica gel produced 3.7 m3·(ton·day)− 1 outperforming both aluminium fumarate and CPO-27(Ni). The low performance of CPO-27(Ni) is due to that it can only be operated at high regeneration temperature (≥110 °C) while the low performance of aluminium fumarate is due to the low evaporation temperature. Also, it can be noticed that the aluminium fumarate can be regenerated at temperatures as low as 70 °C. The high regeneration temperature required for the desorption of CPO-27(Ni) is due to the high interaction between the adsorbed water molecules and the unsaturated metal sites presented in the CPO-27(Ni). At lower evaporation temperature and lower relative pressure, the CPO-27(Ni) is expected to outperform silica gel and aluminium fumarate due to its low dependency on the relative pressure unlike silica gel. At high regeneration temperatures (higher than 110 °C), the CPO27(Ni) was found to outperform the aluminium fumarate and silica gel producing 4.3 m3·(ton·day)−1 which makes it suitable in applications operated with high temperature waste heat sources. On the other hand, the silica gel and aluminium fumarate were found to outperform CPO-27(Ni) at regeneration temperatures as low as 70 °C making them suitable in application utilizing low temperature waste heat or renewable energy resource as solar energy. As the adsorption desalination system can produce two useful effects namely: high grade distilled water and cooling effect from the evaporation of the seawater in the evaporator unit. Fig. 10 shows the effect of the hot fluid inlet temperature on the SCP of the two systems compared to a silica gel system. The figure shows that increasing the hot fluid
Aluminium fumarate
CPO-27(Ni)
SDWP ( m 3.(ton.day) -1)
6 5 4 3 2 1 0 70
80
90
100 110 120 Hot fluid inlet temperature ( o C)
130
140
150
Fig. 11. Effect of hot fluid inlet temperature on the water produced from the adsorption desalination system. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 700 s and switching time = 70s).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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E. Elsayed et al. / Desalination xxx (2016) xxx–xxx
Silica gel
60
Aluminium fumarate
CPO-27(Ni)
SCP (Rton.ton -1)
50 40 30 20 10 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
Fig. 12. Effect of hot fluid inlet temperature on the cooling power produced from the adsorption desalination systems. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 700 s and switching time = 70s).
temperature will increase the SCP produced from the two systems. The CPO-27(Ni) system produced a cooling effect of 35.3 Rton·ton−1, while the aluminium fumarate system produced 22 Rton·ton−1. This cooling effect temperature is suitable for air conditioning application. 6.2. The performance of CPO-27(Ni), aluminium fumarate and MIL101(Cr)water desalination systems at a chilled water temperature of 20 °C Fig. 11 shows the effect of the hot fluid inlet temperature on the SDWP of the two systems compared to the silica gel system.
The SDWP increased with increasing the hot fluid temperature. At a hot fluid temperature of 150 °C, the aluminium fumarate system produced 6.3 m3·(ton·day)− 1 while the CPO-27(Ni) desalination system produced 4.6 m3·(ton·day)− 1. The silica gel system was found to produce 5.3 m3·(ton·day)− 1 at a hot fluid temperature of 90 °C. The comparison between Figs. 9 and 11 highlights the effect of increasing the evaporation temperature, as it would remarkably affect the water production in case of aluminium fumarate and silica gel while it has almost no effect on the performance of CPO-27(Ni). The increase in the case of aluminium fumarate can be attributed to that the
11 10
(a)
SDWP ( m 3.(ton.day) -1)
9 8 7 6 5 4 3 2 1 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
100 90
(b)
SCP (Rton.ton -1)
80 70 60 50 40 30 20 10 0 70
80
90
100 110 120 Hot fluid inlet temperature (oC)
130
140
150
Fig. 13. Effect of hot fluid inlet temperature on a. SDWP and b. cooling power produced from the MIL-101(Cr) adsorption desalination system. (Tcold = 25 °C, Tchilled = 20 °C, cycle time = 300 s switching time = 30 s), (Heating fluid flow rate=1.6 kg.s−1, bed cooling fluid flow rate = 1.6 kg.s−1, condenser cooling fluid flow rate = 4 kg.s−1 and chilled water flow rate = 1.6 kg.s−1).
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
E. Elsayed et al. / Desalination xxx (2016) xxx–xxx Table 4 Comparison between performances of CPO-27(Ni), aluminium fumarate and silica gel at low evaporation temperature. Adsorbent
CPO-27(Ni)
Aluminium fumarate
Silica gel
Ref. Regeneration temp. (°C) Adsorption temp. (°C) Condenser temp. (°C) Evaporator temp.(°C) SDWP (m3·(ton·day)−1) SCP (Rton·ton−1) Cycle time(s) Switching time (s)
This study 150 25 25 5 4.6 35.3 700 70
This study 90 25 25 5 2.46 21.2 700 70
26, 27 85 29.5 30 10 3.2 19.6 600 40
11
increasing the evaporation temperature from 5 °C to 20 °C did not significantly affect the SDWP produced from the CPO-27(Ni) desalination system which was the opposite in case of silica gel and aluminium fumarate. This is attributed to that the CPO-27(Ni) exhibits type I isotherm adsorbing most of its capacity at low relative pressure. Aluminium fumarate and MIL-101(Cr) exhibit type IV isotherm while silica gel exhibits type III, both isotherms depend profoundly on the evaporation temperature and can have high capacity only at high relative pressure values. At high evaporation temperature of 20 °C, the aluminium fumarate outperformed both the CPO-27(Ni) and silica gel with a SDWP of 6.3 m3·(ton·day)−1. MIL-101(Cr) outperformed all the adsorbents with a SDWP of 11 m3·(ton·day)−1. 7. Conclusion
material exhibits ‘type IV’ where its uptake significantly increases with increasing the evaporation temperature generating higher water vapour pressure on the adsorbent allowing a higher uptake. In case of CPO-27(Ni), the material exhibits' type I′ water adsorption isotherm reaching 81% of its capacity at a very low relative pressure and hence its performance is less dependent on the evaporation temperature. In other words, porous materials with ‘type I′ adsorption isotherm would not be significantly affected by increasing the evaporation temperature as much as materials possessing ‘type IV’ isotherm. Fig. 12 shows the effect of the hot fluid inlet temperature on the SCP of the two systems compared to the silica gel system. The figure shows that increasing the regeneration temperature will increase the SCP produced from the three systems. The CPO-27(Ni) system produced a cooling effect of 38.1 Rton·ton−1, while the aluminium fumarate cycle produced 51.8 Rton·ton−1. The cooling effect temperature in this case is more suitable for processes cooling or moderate air conditioning applications. From Fig. 4 b and c, it is evident that the MIL-101(Cr) exhibits IV isotherm, following the trend as aluminium fumarate and having an exceptional performance at high evaporation temperature. Fig. 13 a and b shows the effect of the hot water temperature on a desalination system with MIL-101(Cr) as the adsorbent at an evaporation temperature of 20 °C. The MIL-101(Cr) system produced 11 m3·(ton·day)− 1 and a cooling power of 90 Rton·ton− 1 at a hot fluid temperature of 150 °C. Another comparative study was held between the CPO-27(Ni), aluminium fumarate and silica gel previously reported for conventional 2-bed adsorption desalination system both at low and high evaporation temperatures. The SDWP in addition to the optimum operating conditions are mentioned in Tables 4 and 5. From Table 4, the silica gel was only reported at an evaporation temperature of 10 °C while the CPO27(Ni) can work at a lower temperature of 5 °C. CPO-27(Ni) has a high performance at such a low evaporation temperature producing 4.6 m3·(ton·day)−1 outperforming both aluminium fumarate and silica gel. This high performance can be only obtained at a high regeneration temperature (N110 °C). Table 5 shows the SDWP of the three adsorbent systems at higher evaporation temperature. It can be noticed that
Table 5 Comparison between performances of CPO-27(Ni), aluminium fumarate, MIL-101(Cr) and silica gel at high evaporation temperature. Adsorbent
CPO-27(Ni)
Aluminium fumarate
MIL-101(Cr)
Silica gel
Ref. Regeneration temp. (°C) Adsorption temp. (°C) Condenser temp. (°C) Evaporator temp.(°C) SDWP (m3·(ton·day)−1) SCP (Rton·ton−1) Cycle time (s) Switching time (s)
This study 150 25 25 20 4.6 38.1 700 70
This study 90 25 25 20 6.3 51.8 700 70
This study 90–150 25 25 20 8–11 66.3–89.7 300 30
26, 27 85 29.5 30 20 5.1 31.1 600 40
Metal-organic frameworks (MOFs) are new porous materials with high surface area, pore size and volume, tunable pore geometry and hence providing high adsorption capacity. Three metal-organic framework materials, namely aluminium fumarate, CPO-27(Ni) and MIL101(Cr), were characterized through XRD and water adsorption measurements. Results showed that MIL-101(Cr) has the highest water uptake of 1.47 kg·kg−1. The performance of these MOFs in a two-bed adsorption desalination system were modelled and compared to silica gel using Matlab/ Simulink at various heat source temperatures. Results showed that the CPO-27(Ni) is more suitable where low evaporation temperature (5 ° C) and high regeneration temperature (≥110 °C) are available achieving a SDWP of 4.6 m3·(ton·day)−1. The aluminium fumarate showed a superior performance at high evaporator temperature values (20 °C) with a SDWP of 6.3 m3·(ton·day)− 1. The aluminium fumarate was also found to require low regeneration temperature (70 °C). As for MIL101(Cr), results showed an exceptional performance of producing 11 m 3·(ton·day) − 1 outperforming any other adsorbent. This work highlights the potential of aluminium fumarate, CPO-27(Ni) and MIL-101(Cr) in adsorption desalination where to the adsorbent can be chosen according to the required SDWP, cooling power and the heat source temperature. References [1] International Decade for Action ‘Water for Life’ 2005–2015, United Nations Office. [2] United Nations Environment Programme, 2008 [3] T. Younos, K.E. Tulou, Overview of desalination techniques, Journal of Contemporary Water Research & Education 132 (2005) 3–10. [4] K.C. Ng, X.L. Wang, L.Z. Gao, A. Chakraborty, B.B. Saha, S. Koyama, Apparatus and Method for Desalination, SG Patent Application Number 200503029-1 (2005) and WO Patent No. 121414A1, 2006. [5] X. Wang, K.C. Ng, Experimental investigation of an adsorption desalination plant using low-temperature waste heat, Appl. Therm. Eng. 25 (2005) 2780–2789. [6] K. Thu, A. Chakraborty, Y. Kim, A. Myat, B.B. Saha, K.C. Ng, Numerical simulation and performance investigation of an advanced adsorption desalination cycle, Desalination 308 (2013) 209–218. [7] K.C. Ng, K. Thu, Y. Kim, A. Chakraborty, G. Amy, Adsorption desalination: an emerging low-cost thermal desalination method, Desalination 308 (2013) 161–179. [8] S. Mitra, K. Srinivasan, P. Kumar, S.S. Murthy, P. Dutta, Solar driven adsorption desalination system, Energy Procedia 49 (2014) 2261–2269. [9] P.G. Youssef, S.M. Mahmoud, R.K. Al-Dadah, Effect of evaporator temperature on the performance of water desalination/refrigeration adsorption system using AQSOAZO2, Int. J. Environ. Chem. Ecol. Geol. Geophys. Eng. 9 (6) (2015). [10] P. G. Youssef, S.M. Mahmoud, R.K. AL-Dadah, Performance analysis of four bed adsorption water desalination/refrigeration system, comparison of AQSOA-Z02 to silica-gel, Desalination 2015;375:100–107. [11] A. Elsayed, R. AL-Dadah, S. Mahmoud, B. Shi, P. Youessef, A. Elshaer, W. Kaialy, Characterisation of CPO-27Ni Metal Organic Framework Material for Water Adsorption, Conference Paper, SusTEM, Newcastle University, UK, 2015. [12] F. Jeremias, D. Frohlich, C. Janiak, S. Henninger, Advancement of sorption-based heat transformation by a metal coating of highly-stable, hydrophilic aluminium fumarate MOF, RSC Adv. 4 (2014) 24073–24082. [13] J. Yang, Q. Zhao, J. Li, J. Dong, Synthesis of metal–organic framework MIL-101 in TMAOH-Cr(NO3)3-H2BDC-H2O and its hydrogen-storage behaviour, Microporous Mesoporous Mater. 130 (2010) 174–179. [14] J. Liu, Y. Wang, A.I. Benin, P. Jakubczak, R.R. Willis, M.D. LeVan, CO2/H2O adsorption equilibrium and rates on metal-organic frameworks: HKUST-1 and Ni/DOBDC, Langmuir 26 (17) (2010) 14301–14307.
Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030
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Please cite this article as: E. Elsayed, et al., CPO-27(Ni), aluminium fumarate and MIL-101(Cr) MOF materials for adsorption water desalination, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.07.030