Pressure driven adsorption cycle integrated with thermal desalination

Read from local Zotero full-text PDF (group 5183627 / Joule Heist).

Key findings

• Motivation: desalination demand is rising toward 6900 bcm/year by 2030 (40% supply-demand gap); >19,000 plants in 158 countries produce ~38 bcm/year while consuming 75.3 TWh electricity and emitting 76.2 Mt CO2/year. Conventional adsorption (AD) cycle operates at low heat-source temperature (55-85 C) and achieves >85% recovery via sub-ambient evaporator operation, with high-quality water (TDS < 50 ppm).
• Conventional solar AD pilot at KAUST: 4-bed system, silica gel packed as cake in each bed, 352 m2 rooftop evacuated-tube collectors with 100 kW external radiator. Evaporator held at 10 C; tested at heat-source 85 C and condenser cooling water 25 C, half-cycle time 300 s. COP varied 0.43-0.5 at chilled-water 7 C; full-day cooling capacity tracked hot-water temperature.
• Innovation - pressure-driven AD cycle (PDAD): low-pressure working steam (via a thermal vapor compressor / TVC) regenerates the adsorbent instead of circulating hot water, and steam selectively extracts water vapor from silica-gel pores. Benefits: smaller footprint, eliminates water circulation infrastructure and adsorbent-bed heat exchangers (silica gel dumped in a tank), reduces bed mass and heating requirement, low capital/operating expenditure.
• PDAD pilot at KAUST achieved successful silica-gel regeneration at motive steam pressure of 2-5 bar; multiple TVCs can be staged (parallel for higher desorption rate, series for lower throat pressure / faster desorption).
• Hybrid MED + PDAD: bleed steam at 3-5 bar (tapped from the last LP turbine stage in a cogeneration plant) is used for adsorbent regeneration instead of being throttled, then recompressed PDAD discharge steam at 65 C feeds a multi-effect desalination (MED) system as its heat source, overcoming MED’s bottom-brine-temperature limit.
• Conventional MED is constrained by top-brine temperature ~65 C (salt-scaling from Ca2+, Mg2+, SO4 2-), bottom-brine temperature limited by ambient heat rejection, and a 5-6 C mean temperature difference per effect, limiting effects to 6-8. AD integration lets MED operate from 65 C down to as low as 7 C, doubling the number of effects and water production.
• Reported gains: hybrid MED + PDAD increases water production by up to 22% versus an earlier hybrid MEDAD cycle, and over two-fold versus conventional MED at fixed top-brine temperature; overall thermodynamic efficiency ~20% versus 10-13% for conventional RO, MED, MSF.