Performance investigation of an advanced multi-effect adsorption desalination (MEAD) cycle

Abstract

Energy recovery scheme using multi-effect evaporation is introduced to Adsorption Desalination (AD) cycles. In the proposed cycle, the condensation energy from the desorption process is recovered multiple times for the evaporation/condensation of the sea water hence it provides improved performance in terms of water production and the specific energy consumption. To demonstrate the advanced multi-effect AD cycle, mathematical model is developed using energy, material and concentration balances for each phase along with the isotherm and kinetics modeling of the sorption processes. Hydrophilic mesoporous silica gel with pore surface area of 800m2/g is selected as the adsorbent due to its excellent uptake of water vapor and low regeneration temperature. Condenser - evaporator heat exchanger with three intermediate effects is employed where the top brine temperature (TBT) and the bottom brine temperature (BBT) are about 35 °C and 7 °C, respectively. It is noted that contrary to the conventional multi-effect distillation (MED) cycle, most of the evaporating - condensing effects in the cycle operate at subatmospheric temperature i.e., less than 30 °C owing to the mass transport phenomena from the vapor uptake by the adsorbent. The performance of the AD cycle is investigated at various hot water inlet temperatures ranging from 65 °C to 85 °C since such kind of heat source is abundantly available from process waste heat or renewable energy. Feed water being evaporated at low temperature, thus, lower scaling and fouling potentials, the proposed AD cycle is a strong candidate for handling highly concentrated feed water (TDS > 100,000 ppm) such as produced water, brine rejected from other desalination plants and zero liquid discharge (ZLD) system.