**2. The basic adsorbent cycle**

The basic adsorbent cycle consists of four major components, namely (i) evaporator, (ii) adsorbent beds, (iii) condenser and (iv) circulation pumps as shown in **Figure 6**. The feedwater is supplied to evaporator and saturation pressure is maintained by the adsorbent uptake capacity to achieve evaporation conditions below ambient level. Once the adsorbent bed is near saturation, adsorption process switched to second bed and regeneration heat is supplied to desorb vapor and prepare adsorbent for next cycle adsorption. The desorbed vapor is condensed in the condenser by circulating the chilled water from evaporator. It can be noticed that electricity

**Figure 6.** Adsorbent cycle 3D model can operate with solar or industrial waste heat from 55 to 85°C. The adsorbent is packed in the beds.


cycle with conventional desalination processes will help to overcome their operational limitations. For example, in first case, RO + AD integration can boost overall recovery to over 80% as compared to 35–40% of RO alone. This will help to protect marine pollution by reducing pretreatment chemical rejection into the sea. In second case, its hybridization with thermal processes such as MED + AD will help to overcome last stage operational temperature limitations of conventional MED system by extending to as low as 7°C as compared to 40°C in conventional processes. It will help to boost water production to almost twofold with same

industrial waste heat or renewable energy. We presented detailed experimentation of both mentioned cases and their economic analysis to show the superiority of hybrid cycles over conventional processes in terms of energy efficiency, marine and environmental impact.

The basic adsorbent cycle consists of four major components, namely (i) evaporator, (ii) adsorbent beds, (iii) condenser and (iv) circulation pumps as shown in **Figure 6**. The feedwater is supplied to evaporator and saturation pressure is maintained by the adsorbent uptake capacity to achieve evaporation conditions below ambient level. Once the adsorbent bed is near saturation, adsorption process switched to second bed and regeneration heat is supplied to desorb vapor and prepare adsorbent for next cycle adsorption. The desorbed vapor is condensed in the condenser by circulating the chilled water from evaporator. It can be noticed that electricity

**Figure 6.** Adsorbent cycle 3D model can operate with solar or industrial waste heat from 55 to 85°C. The adsorbent is

emission will reduce as AD cycle utilized only low-grade

energy input. In both cases, CO2

102 Desalination and Water Treatment

**2. The basic adsorbent cycle**

packed in the beds.

**Table 2.** Summary of major adsorbent, their cost and technological application status. Silica gel is most applied adsorbent because of its stability and reliability [110].

is only supplied for pumping of liquid and major thermal energy is supplied from renewable solar or industrial process waste heat from 55–85°C. The detail of adsorption cycle can be found in published literature [89–105]. The adsorbent selection depends on application temperature. The list of most common adsorbent and their application status are provided in **Table 2**. We developed an advance silica gel with improved uptake by pore opening. We also developed silica gel-coated heat exchanger AD cycle that can achieve heat transfer coefficient twofold higher as compared to conventional packed bed AD cycle [106, 107]. Silica gel has many advantages such as (i) easily available, (ii) lower cost as compared to all available adsorbent, (iii) more stable and reliable and (iv) easy to modify for required application [108–111].

The proposed adsorbent cycle has capability to integrate with conventional desalination process to improve their performance. The detail of integration with SWRO and thermal processes and their advantages are discussed in the following sections.
