The Utilization of Industrial Waste Heat for the Production of Fresh Water by Membrane Distillation: Industrial Case Studies in the Petrochemical and Gas Industry in Qatar

Abstract

Membrane distillation differs from other membrane technologies in that the driving force for desalination is the difference in vapor pressure of water across the membrane, rather than total pressure. The membranes for MD are hydrophobic, which allows water vapor (but not liquid water) to pass. The vapor pressure gradient is created by heating the source water thereby elevating its vapor pressure. The major energy requirement is for low-grade thermal energy. Moreover, the Qatari economy is based on its massive hydrocarbon industry. In such industries water is routinely used in a number of applications in the form of process or cooling water. In a number of cases the water used can be seawater but with certain restrictions due to corrosion, fouling and water composition, large volumes of fresh water are required around the chemical plants. It is well known that many processes produce large amounts of excess heat i.e., heat beyond what can be efficiently used in the process. Industrial waste heat recovery methods attempt to extract some of the energy as work that otherwise would be wasted. Typical methods of recovering heat in industrial applications include direct heat recovery to the process itself, economizers, regenerators, and waste heat boilers. An investigation into the potential of using industrial low-grade waste heat in desalination using membrane distillation has been carried out. Three well-known chemical processes were considered: LNG, ethylene and VCM. Using an approach based on pinch technology for heat integration, process streams in the three processes were screened to eliminate unsuitable sources of low-grade heat. Consequently, the LNG and ethylene processes were eliminated because of their unsuitable cooling curves that tended to highlight extreme temperatures. The VCM process on the other hand showed a promising outlook, in particular in the direct chlorination section where a major vapor stream is condensed through the temperature range 118 to 460C. This is precisely the ideal range for low -grade heat recovery. Exploiting literature data and modeling concepts, a flowsheet for a potential MD plant was designed with relevant terminal temperatures.