**4. MED-AD hybrid cycle**

MEDAD is a hybrid of two thermal systems namely multi-effect desalination system and adsorption cycle. The main components of this novel thermal hybrid system are (1) multi-effect distillation (MED) system, (2) adsorption desalination (AD) cycle, and (3) auxiliary equip‐ ments. In this hybridization system, the last stage of the MED is connected to adsorption beds of AD cycle for the direct vapor communication to adsorption beds. Figure 7 shows detailed flow schematic of MED plant combined with AD system.

Adsorption-based desalination is investigated by many researchers [30–45] and reported that optimal specific daily water production (SDWP) for four bed scheme is about 4.7 kg/kg silica gel. The first adsorption desalination plant was installed in the National University of Singapore (NUS) which consists of four silica gel beds. Ng et al. investigated the processes using chilled water at assorted temperature and demonstrated that the specific water produc‐ tion of the system [46, 47]. They also introduced and patented a novel hybrid desalination method "MEDAD cycle" that is a combination of conventional MED and AD cycle [48, 49]. This novel desalination cycle can mitigate the limitations of conventional MED system to increase the system performance. This combination allows the MED last stage to operate below ambient temperature typically at 5<sup>ο</sup> C as compared to traditional MED at 40<sup>ο</sup> C. This not only reduces the corrosion chances but also increases the distillate production to almost 2 – 3 fold as compared to traditional MED systems.

**Figure 7.** Detailed P& ID diagram for an experimental MED facility of nominal heating capacity of 12 kW. The first stage comprises a water-fired steam generator whilst the last stage (3rd stage) is open to a water-cooled cooling tower

#### **4.1. MED-AD simulation**

MEDAD cycle simulation have been conducted [50–53] and presented in Table 5. The simu‐ lation is based on a fully transient model and the predicted results are compared with conventional MED system. It is observed that at same input parameters such as a top-brinetemperature (TBT), water production can achieve up to two fold increase when the hybridized MEDAD is compared with the MED cycle alone.



**Figure 7.** Detailed P& ID diagram for an experimental MED facility of nominal heating capacity of 12 kW. The first stage comprises a water-fired steam generator whilst the last stage (3rd stage) is open to a water-cooled cooling tower

MEDAD cycle simulation have been conducted [50–53] and presented in Table 5. The simu‐ lation is based on a fully transient model and the predicted results are compared with conventional MED system. It is observed that at same input parameters such as a top-brinetemperature (TBT), water production can achieve up to two fold increase when the hybridized

**Modeling equations for steam generator**

*hwh <sup>f</sup>* ,*Thw*,*out*

(*Thw*,*<sup>i</sup>* −*Ttube*,*<sup>i</sup>*

) −*hin*,*o*.*Ain*,*<sup>i</sup>*

.*Aout*,*<sup>i</sup>*

) −*hout*,*<sup>i</sup>*

**Equation Remarks**

(*Thw* −*Ttube*,*<sup>i</sup>*

(*Ttube*,*<sup>i</sup>* −*Tv*,*<sup>i</sup>*

)

Energy balance for the hot water flowing inside the tubes of brine heater.

metal tubes.

Mass balance for the seawater inventory in the

) Energy balance for

**4.1. MED-AD simulation**

198 Desalination Updates

(*Mhw*.*C phw*)

(*MHX* ,*<sup>i</sup>*

.*C phx*,*<sup>i</sup>* ) *dTtube*,*<sup>i</sup> dt* <sup>=</sup>*hin*,*<sup>i</sup>*

MEDAD is compared with the MED cycle alone.

*dThw dt* =(*<sup>m</sup>* •

*hw*,*h <sup>f</sup>* ,*Thw*,*in*

*dMb*,*<sup>i</sup> dt* <sup>=</sup>*mf* ,*<sup>i</sup>* • −*mb*,*<sup>i</sup>* − • *mv*,*<sup>i</sup>* •

.*Ain*,*<sup>i</sup>*

) −(*m* •


**Table 5.** MEDAD modeling equations

Figure 8 shows the transient temperature profiles of a MEDAD cycle. It can be seen that last stages of MED are operating below ambient temperature due to hybridization. It can also be noticed that MED last stages profiles are affected by cyclic AD operation.

**Figure 8.** MEDAD cycle components temporal temperature profiles

Figure 9 mapped the performance parameters of hybrid MEDAD cycle (concentration, GOR, PR, and WPR). It can be observed that the batched-mode water vapor uptake by the coupled AD cycle dominates the performance of the cycle. The performance of the MEDAD cycle with different additional effects with the conventional eight effect MED cycle as baseline cycle is studied in terms of water production rate.

**Figure 9.** MEDAD hybrid cycle performance (concentration, GOR, PR and WPR)

Figure 10 shows the quantum jump in the water production rate of the proposed MEDAD cycle. Another aspect of this hybridization is that the desorbed water vapor from the AD cycle can be recycled back into the MED system for further energy recovery.

#### **4.2. MED-AD experimentation**

**Equation Remarks**

Nusselt film condensation correlation for the calculation of the heat transfer coefficient inside the condenser tubes

Overall heat transfer coefficient equation

Energy balance for the condenser side of the *i* th effect

1/4

3

) −(*Msgh <sup>g</sup>*,*Tv*)

*dqads dt* <sup>+</sup> *Qin*,*<sup>n</sup>*

*Nu* =

200 Desalination Updates

(*Mb*,*n*.*C pb*) + (*MHX* ,*n*.*C pHX* ,*n*)

**Table 5.** MEDAD modeling equations

*hin*,*i*+1*L <sup>i</sup>*+1 *Ktube*,*i*+1

=0.728

*UiAi* <sup>=</sup> <sup>1</sup> 1 *hin*,*<sup>i</sup>Ain*,*<sup>i</sup>*

> *dTn dt* =(*<sup>m</sup>* • *<sup>f</sup>* ,*nh <sup>f</sup>* ,*Tf*

**Figure 8.** MEDAD cycle components temporal temperature profiles

*gh fg*,*Tcondρl*,*Tcond* (*ρ<sup>l</sup>* −*ρv*)*TcondKl*,*Tcond*

(*Tv*,*i*+1 −*Ttube*,*<sup>i</sup>*+1)

*hout*,*<sup>i</sup>Aout*,*<sup>i</sup>*

**MED last stage connected with AD beds**

Figure 8 shows the transient temperature profiles of a MEDAD cycle. It can be seen that last stages of MED are operating below ambient temperature due to hybridization. It can also be

) −(*m* • *<sup>b</sup>*,*nh <sup>f</sup>* ,*Tb*

*Qin*,*<sup>n</sup>* =*hout*,*<sup>n</sup>An*(*Tt*,*<sup>n</sup>* −*Tv*,*n*)

noticed that MED last stages profiles are affected by cyclic AD operation.

*μl*,*Tcond di*

<sup>+</sup> *Rwall*,*<sup>i</sup>* <sup>+</sup> <sup>1</sup>

A three-stage MED system is designed [54], fabricated, and installed in NUS as shown in Figure 11. In MED stages, vapor emanation from feed seawater is achieved by falling film-evaporation process. Evaporation energy is recovered by series of reutilization of vapor condensing energy in successive stages of those produced in preceding stages. Process of vapor production and energy recovery by condensation continues until the last stage of MED. The vapors from the last stage are then directed toward AD beds where they are adsorbed on the adsorbent surface. Adsorbent high affinity for water vapor drops the pressure and hence the saturation temper‐

observed that this drop in pressure and temperature of last stage also affected the operational **Figure 10.** Percentage improvement in water production rate by the MEDAD cycle

ature of last stages falls below ambient, typically up to 5<sup>ο</sup> C. It is observed that this drop in pressure and temperature of last stage also affected the operational parameters of the few preceding stages. parameters of the few preceding stages.

Experiments are conducted in two steps. In first part, system is operated as a conventional

experiments are conducted as a hybrid MEDAD system at assorted heat source temperature

C and results are compared with conventional MED system.

C to 70

C. In second part,

22

Figure 11: MEDAD system installed in NUS **Figure 11.** MEDAD system installed in NUS

C to 70

ranges from 15

MED at assorted heat source temperature ranges from 38

Experiments are conducted in two steps. In first part, system is operated as a conventional MED at assorted heat source temperature ranges from 38<sup>ο</sup> C to 70<sup>ο</sup> C. In second part, experi‐ ments are conducted as a hybrid MEDAD system at assorted heat source temperature ranges from 15<sup>ο</sup> C to 70<sup>ο</sup> C and results are compared with conventional MED system.

**Figure 12.** MEDAD components temperature profiles at 38o C heat source temperature

ature of last stages falls below ambient, typically up to 5<sup>ο</sup>

**Figure 10.** Percentage improvement in water production rate by the MEDAD cycle

MED at assorted heat source temperature ranges from 38

Figure 11: MEDAD system installed in NUS **Figure 11.** MEDAD system installed in NUS

preceding stages.

202 Desalination Updates

ranges from 15

C to 70

parameters of the few preceding stages.

pressure and temperature of last stage also affected the operational parameters of the few

the adsorbent surface. Adsorbent high affinity for water vapor drops the pressure and hence

observed that this drop in pressure and temperature of last stage also affected the operational

Experiments are conducted in two steps. In first part, system is operated as a conventional

experiments are conducted as a hybrid MEDAD system at assorted heat source temperature

C and results are compared with conventional MED system.

the saturation temperature of last stages falls below ambient, typically up to 5

**MED AD**

C to 70

C. In second part,

C. It is observed that this drop in

C. It is

22

Desalination Updates (ISBN 978‐953‐51‐4239‐3)

Figure 12 shows the instantaneous temperatures of MED and MEDAD components at a heat source temperature of 38<sup>ο</sup> C. It can be seen that steady-state events (minimum temperature fluctuations) occur after 1 hour from start-up and experiments for distillate collection are continued for 4 to 5 hours. It is noticed that the inter-stage temperature difference (Δ*T*) is more than twice per stage as compared to the conventional MED stages. This is attributed to the vapor uptake by the adsorbent of AD cycle, resulting in the increase of vapor production. The MEDAD cycle yields a stage Δ*T* from 3<sup>ο</sup> C to 4<sup>ο</sup> C as compared to 1<sup>ο</sup> C or less in the case of MED alone. Table 6 shows the comparison of MEDAD and MED components steady-state temper‐ ature values.

Figure 13 shows the distillate production trace at heat source 38<sup>ο</sup> C from MED stages, AD condenser and combined. The batch operated AD production can be seen clearly. At the start of desorption, the production is higher and it drops with time to zero during the switching period while MED stages production is quite stable. Small fluctuations in MED water pro‐ duction may be due to the fluctuations in the spray of the feed that affect the condensation rate. It can be seen that hybridization boost water production 2 – 3 fold as compared to conventional MED system at same TBT. Water production profiles are similar as explained in simulated results.


**Table 6.** A comparison of conventional MED and hybrid MEDAD systems components temperatures at different heat source temperatures

**Figure 13.** Conventional MED and hybrid MEDAD cycle water production profiles at 38o C heat source temperature

**Figure 14.** MED and MEDAD steady state water production at different heat source temperatures

**Heat source temperature (<sup>ο</sup>C)**

204 Desalination Updates

source temperatures

**MED MEDAD MED MEDAD MED MEDAD**

 63.9 54.4 62.8 50.8 62.3 46.7 56.6 26.1 59.6 50.7 58.7 47.0 58.1 42.5 53.9 25.8 55.4 49.2 54.4 45.7 53.8 41.3 49.4 25.5 51.5 48.5 50.5 44.8 49.9 40.8 45.8 25.1 47.4 44.6 46.7 41.6 46.2 37.9 42.7 23.2 43.9 41.1 43.4 38.6 43.0 34.9 40.3 22.4 38.7 36.6 38.4 34.8 38.1 31.6 36.1 21.0 37.4 35.4 37.2 33.5 37.0 30.3 35.3 18.2 **Operating limit of Conventional MED. The lower operational points are from MEDAD Hybrids** - 30.9 - 27.9 - 23.5 - 16.1 - 26.0 - 23.7 - 19.1 - 12.2 - 22.1 - 19.9 - 16.3 - 10.1 - 18.1 - 14.2 - 11.3 - 7.1 - 13.2 - 11.5 - 9.4 - 5.6

**Table 6.** A comparison of conventional MED and hybrid MEDAD systems components temperatures at different heat

**Figure 13.** Conventional MED and hybrid MEDAD cycle water production profiles at 38o

**SG SG Stage-2 Stage-2 Stage-3 Stage-3 evaporator**

**MED condenser**

C heat source temperature

**AD**

Figure 14 shows the comparison of water production of MED and hybrid MEDAD cycles at assorted heat source temperatures. Quantum increase in water production (two- to threefold) can be observed at all heat source temperatures. These results have good agreement with simulation results. It can also be seen that in conventional MED system last stage temperature is limited to 38<sup>ο</sup> C due to condenser operating with cooling water from cooling tower. While in the case of hybrid MEDAD, the last stage temperature can be as low as 5<sup>ο</sup> C because there is no condenser and last stage is connected to AD beds for vapor adsorption. This higher overall operational gap in proposed hybrid MEDAD cycle helps to insert more number of stages (up to 19 stages) as compared to conventional MED system (about 4–6 stages). More number of stages increases the vapor condensation heat recoveries and hence the water production at same top brine temperatures.

### **5. Exergy analysis for operational cost apportionment**

A computation model is developed for the cogeneration plant where the properties of expanding steam, such as enthalpy (h) and entropy (s) at a given temperature and pressure, are computed for the key states in the schematic diagrams of "PP", "PP+MED," and "PP +MEDAD" cycles. The approach employed here is to calculate the total exergy changes or destruction across the inlet and exit sections of the equipment. For example, the exergy associated with the turbines is the sum of all contributions from (a) the HP-T unit at the same mass flow rate across it, (b) the LP-T unit until the extraction point, and (c) the exergy changes after the point of steam extraction, i.e., *ET* ,1−<sup>2</sup> =*E*1−*<sup>a</sup>* + *Eb*−1' + *E*1'−2. On the basis of total exergy available across whole system, proportion utilized by power plant and desalination system is calculated.

For this comparison study, an assorted range of bled-steam is extracted at low pressure but the mass flow rates are varied to cover the expected practical operational ranges, typically from 10% to 50% of the total flow. At 20% bled-steam from the LP-T, the ratios of power-to-water for exergy and energetic analyses are found to be 95.7%:4.3% (exergy) and 72.2%:27.8% (energetic) methods, respectively. This implies that if the 20% steam were to be continuously consumed by the low pressure turbines (LP-T), its work contribution from LP\_T would be insignificant, i.e., a maximum of 4.3%. However, the energetic value of bled-steam accounts for a disproportionate share of 27.8% due primarily to the high latent heat content of the low pressure steam. This ratio of energy-to-exergy shares of the total working steam is found to be 4 – 7 fold higher, descending with the larger amount of bled-steam as shown in Figure 15. The higher effectiveness of working steam incurred at the MED cycle is attributed to the better thermodynamic matching of steam's latent energy.

**Figure 15.** Exergetic and energetic proportions at different percent of steam extraction

On the basis of the above analysis for primary fuel cost and with data from the published literature [55, 56], the life-cycle cost (LCC) of water production is compared for all capital expenditure (Capex) and operation expenditure (Opex), across all proven industrial processes,

#### Adsorption Cycle and Its Hybrid with Multi-Effect Desalination http://dx.doi.org/10.5772/60400 207

**Figure 16.** A comparison of life-cycle unit water cost for various desalination methods

as shown in Figure 16. Exergy factor calculated above is utilized only for thermal and electricity cost calculations. Energy based analysis has good agreement with GWI [56] data. It can be seen that by LCC, unit water production cost is highest for PP+MSF which amounts to US\$ 1.201/m3 , whilst the lowest unit cost is the PP+MEDAD method which is only at US\$ 0.485/m3 and this unit cost is even lower than the LCC of reverse osmosis (RO) plants.

#### **6. Summary**

For this comparison study, an assorted range of bled-steam is extracted at low pressure but the mass flow rates are varied to cover the expected practical operational ranges, typically from 10% to 50% of the total flow. At 20% bled-steam from the LP-T, the ratios of power-to-water for exergy and energetic analyses are found to be 95.7%:4.3% (exergy) and 72.2%:27.8% (energetic) methods, respectively. This implies that if the 20% steam were to be continuously consumed by the low pressure turbines (LP-T), its work contribution from LP\_T would be insignificant, i.e., a maximum of 4.3%. However, the energetic value of bled-steam accounts for a disproportionate share of 27.8% due primarily to the high latent heat content of the low pressure steam. This ratio of energy-to-exergy shares of the total working steam is found to be 4 – 7 fold higher, descending with the larger amount of bled-steam as shown in Figure 15. The higher effectiveness of working steam incurred at the MED cycle is attributed to the better

thermodynamic matching of steam's latent energy.

206 Desalination Updates

**Figure 15.** Exergetic and energetic proportions at different percent of steam extraction

On the basis of the above analysis for primary fuel cost and with data from the published literature [55, 56], the life-cycle cost (LCC) of water production is compared for all capital expenditure (Capex) and operation expenditure (Opex), across all proven industrial processes,

Recent developments in adsorption theory, adsorption desalination (AD), and conventional MED desalination cycles have been reviewed in this chapter. We highlight the key role of AD cycles which can be hybridized with the proven cycles such as the MED cycle, exploiting the thermodynamic synergy between the thermally driven cycles that significantly improve the water production yields. Experiments were conducted in a lab-scale pilot MEDAD and confirmed the excellent synergetic effects that boosted the water production up to two- to threefold over the conventional MED. We believe that if the hybrid MEDAD cycles are well optimized and operated, it can achieve high GOR and the projected LCC of water production can be lowered to as low as US\$ 0.485/m3 .

### **7. Abbreviation**

MED; Multi-effect desalination

AD; Adsorption desalination

RO; Reverse osmosis

SDWP; Specific daily water production

EDF; Energy distribution function

GOR ; Gain output ratio

SG; Steam generator

PR; Performance ratio

WPR; Water production ratio

TBT; Top brine temperature

PP; Power plant

HP-T; High pressure turbine

LP-T; Low pressure turbine

LBT; Lower brine temperature

LCC; Life cycle costing

PDF; Probability distribution function


