Fluidised bed crystalliser and air gap membrane distillation as a solution to geothermal water desalination

Tunisia has five major geothermal districts. Recorded hot spring temperatures range from 294 to 340 K with flow rates 0.1–102 L/s. In southern parts available groundwater resources are mostly hard and brackish (3 g/L). Recently, Tunisia has resorted to this resource for agriculture and potable water uses. Cooling towers have been built to lower down the temperature and hardness. Cooled brackish water irrigates greenhouses and feeds desalination plants. The cooling operation of groundwater rejects an important quantity of thermal energy in the atmosphere. However, energy requirements of thermal desalination plants are too excessive to be supplied by a geothermal resource. Membrane distillation (MD) is an emerged desalination technology, which can be driven by a thermal energy at low enthalpy (less than 363 K) as geothermal energy, and a fluidised bed crystalliser can ensure reduction of an important portion of hardness without significant loss of temperature. MD is realised by means of a microporous hydrophobic membrane separating a warm solution from the cooling chamber, which contains either liquid or gas. A fluidised bed crystalliser permits production of from it of a granular crystal calcium carbonate from the consumption rates of calcium ions, using aragonite or sand as seeding materials. The purpose of this paper is to realise experimental assembly constituted of fluidised bed crystalliser (FBC) and a cell of air gap membrane distillation (AGMD). It presents preliminary results related to characterisation of fluidised bed, growth of seeded particles size of CaCO3 and operating parameters of AGMD. In laboratory scale, the technical feasibility has been shown. More investigation is needed to prove the efficiency and availability of coupled AGMD with FBC.

Tunisian geothermal springs are characterised by high hardness and low-grade temperature. The FBC vortex orifice granulator tool was realised and tested for the precipitation of calcium carbonate under aragonite variety. MD using a low-grade temperature is the suitable desalination process for this case. An experimental setup AGMD has been proposed, and relevant operating parameters are studied. Preliminary results seem to be in favour of coupling the two processes with the aim of geothermal desalting. More investigation is needed to optimise FBC design and operating parameters and to prove the efficiency and the availability of coupled AGMD or other MD configuration, such as SGMD with FBC. However, it seems that MD recovery fraction is not able to reach a high value using only a sensible heat from a geothermal well. In this case, two possibilities can be envisaged. The first one, taking advantage of solar energy potential, the coupled solar plan collectors and geothermal energy can increase considerably MD efficiency. The second way consists of integrating several membrane processes, such as MD+RO where RO uses softening and warm brine of MD. This configuration can give a very high recovery factor widely superior to 80%.