December 17, 2012 9:21 pm
After completing a 12-month demonstration trial, a unique humidification-dehumidification (HDH) process is about to move from the laboratory to the oilfield. According to Prakash Narayan, an MIT researcher and co-developer of the process, the 1 m3/d (264 GPD) prototype system has proven its ability to produce distillate-quality water from a 75,000 mg/L TDS feed while achieving an 85 percent recovery.
Dr Narayan said that the process is particularly well suited for volume reduction in brine management applications. “Unlike most HDH systems, the system employs air as carrier gas operating in a closed loop, and its performance is not dependent on ambient humidity or other climatic conditions. And, by employing a multistage bubble column, we get direct-contact dehumidification at a much higher heat transfer coefficient than other HDH systems,” he said.
The system was developed and tested by Narayan and his colleagues in Professor John Lienhard’s MIT research group, with funding and collaboration from Saudi Arabia’s King Fahd University of Petroleum & Minerals. It has been demonstrated to concentrate high salinity feedwater beyond saturation levels at a top temperature of 70°C (158°F), an electricity consumption of 1 kWh/m3 (3.8 kWh/kgal) and a 5.0 GOR.
Each individual system consists of two, one-meter diameter columns. Feedwater first enters the top of a two-meter tall dehumidifier column, flowing downwards in a coil tube that passes through each stage. Upon exiting the dehumidifier, the temperature of the pre-heated feedwater must be heated an additional 10-15°C (18-27°F). It is then introduced at the top of the five-meter tall humidifier column where it is sprayed over a bed filled with plastic packing material to increase the column’s liquid surface area. Some of the feedwater evaporates, humidifying the air, while the non-evaporated portion of the feed is removed from the bottom of the column as saturated brine.
The humidified air exits the column and is sparged successively into the dehumidifier column from the bottom-most (first) stage to the top-most (last) stage, to continue the process. Distillate formed during the dehumidification of humid air is collected from each stage and discharged as product water.
The system’s relatively low temperatures and pressures enable the vessels and humidifier packing to be constructed of nonmetallic materials. In fact, the low cost of the packing means that if scaling should occur, it is more economical to replace it than to clean it.
Initially, the technology will target brine concentration applications in the upstream oil and gas industry. The size of the columns has been economically optimized so that each two-column module has a footprint of approximately 2.5 m2 (27 ft2) and can treat up to 20 bbl/d of brine. Multiple modules can be combined as shown in the diagram, see previous page, to meet site-specific flow requirements.
Narayan told WDR that a company has been formed to commercialize the system, adding, “We hope to deploy field pilots by mid-next year and are looking to expand our technical team to support that effort.”
WDR’s current CDR rating for this technology is 7.1.