FEATURE ARTICLE
Desalination of Produced Water Using Solar Heating
VIJAY K. DHIR Class of 2017–18
Distinguished Professor Samueli School of Engineering University of California, Los Angeles
Almost 2.4 billion gallons of water is produced daily during oil and natural gas excavation. That water contains salts, heavy metals, and traces of naturally occurring radioactive materials, as well as other organic and inorganic materials. Although the composition of the water produced varies with location, salt content can be ten times higher than that found in seawater. The cost of managing this water can be high. Factors include constructing facilities, including equipment; operating the facilities; treating and managing by-products; regulatory considerations; and transportation to the disposal sites, such as deep wells.
A shortage of potable water exists worldwide, especially in the southwestern United States. Millions of people would benefit if a system could transform that wastewater, known as produced water, into potable water.
Working with a research team in Texas A&M University’s College of Engineering, Dhir is developing a high-throughput flash-evaporation and vapor-separation system to generate potable water from produced water. This dynamic method combines vapor production through flashing and phase separation for later condensation of separated water vapor. In the proposed approach, for example, produced water is heated from a solar pond and travels through a set of injector passages. As the fluid flows through the passages, the pressure drops. With the decrease in pressure, the saturation temperature also drops. When the saturation temperature falls below the local fluid temperature, flashing occurs and excess thermal energy is used in producing water vapor. In the presence of vapor, friction and acceleration cause an increase in the rate of pressure drop inside the injector passages. Flash evaporation in the injector passages occurs in a fraction of a second. Later, the two-phase mixture from the injector passages discharges into a larger tube, known as the flow separator. Because of the centrifugal force created by the swirl, liquid containing dissolved solids is pushed outward, whereas lighter phase (vapor) moves into the middle and forms a vapor core. A retrieval tube placed in the middle of the separator tube removes the vapor from the vapor core. Thereafter, vapor is directed to a condenser to produce potable water.
Figure 1
3D VIEW OF THE CONCEPT FOR CREATION OF POTABLE WATER FROM PRODUCED WATER
The condenser can be air-cooled or cooled by produced water or other sources of water available at room temperature. Produced water separated from the two-phase mixture in the separator tube is returned to the solar pond to be reheated and recirculated. Figure 1 shows the injector passages and the flash evaporator/separator. Figure 2 illustrates the complete system layout, including the solar pond and condenser.
Figure 2
SCHEMATIC OF A SOLAR-ASSISTED DESALINATION SYSTEM EMPLOYING THE PROPOSED FLASH EVAPORATOR/SEPARATOR
Solar energy is used in a solar pond to heat the produced water, which is recirculated. With time, the total dissolved-solid concentration in the recirculating water will increase. However, the process should function as long as dissolved solids do not limit the thermal energy that can be extracted from the superheated produced water during flashing. Electric power is needed to run the pumps that will be used to move the water around. That power can be produced from solar panels installed on the site. The concept proposed here is innovative and especially suited for generating potable water from produced water, in which content of total dissolved solids can vary widely and salt content is high. The only limits on the concentration of the solids are their impact on the life of the pumps and the diffusion of heat to the vapor–liquid interface during flash evaporation. The concept is dynamic and compact in comparison with current desalination methods, which are static and require large investments of space and capital. Two methods fall into that category. In multistage flash (MSF) evaporation, seawater evaporation is carried out as heated water progresses through a series of flash stages with decreasing pressure and temperature. At each stage, a portion of the saltwater is evaporated and condensed over the tubes carrying the recirculating brine as the remaining saltwater moves to the next stage. External heating is also applied upstream of the first stage. Simulations with data from a pilot plant (with four-stage heat recovery and a two-stage heat rejection section) have shown that the flash-chamber effectiveness (β) for the six stages of the test plant varied between 0.34 and 0.74. The flash-chamber effectiveness is defined as the ratio of the reduction in brine temperature due to evaporation to the initial superheating. Multieffect distillation (MED) also requires external heat beyond solar heating. MED can be designed as forward feed, backward feed, and parallel feed. In a parallel-feed MED, for example, seawater moves through successive low-pressure chambers. As opposed to MSF, in which latent heat of the vapor is used for efficient heating of seawater, MED uses the latent heat of vapor to vaporize the sea water at a lower pressure.
Figure 3
FORMATION OF GAS CORE IN THE PHASE SEPARATOR TUBE
At Texas A&M, experiments using an air–water system have been conducted to determine the separation efficiency. Figure 3 shows the stable gas core formed in the phase separator tube. A few liquid droplets are collected and transported in the air and retrieved from the gas core. Figure 4 shows dependence of separation efficiency as a function of gas flow rate for a liquid flow rate of 2.5 gallons per minute. The figure also shows that separation efficiency of 95 percent (defined as the mass-flow rate of separated air to total mass-flow rate) can be obtained in an unoptimized case. The goal is a separation efficiency greater than 99 percent.
Figure 4
SEPARATION EFFICIENCY AS A FUNTION OF AIR FLOW RATE
The proposed concept is innovative, dynamic, compact, and modular. It is also high-throughput. Preliminary experiments show that high thermal efficiencies and high effectiveness in terms of the quality of vapor exiting the separator can be achieved. The injectors and separator tube can be made of inexpensive material because of the low pressures and temperatures involved. Existing surface condensers available on the market and used in the power industry can be employed. The only significant unknowns are the cost of the solar pond and its operation. The system is modular and can be easily expanded or shrunk in response to the demand for potable water. The concept can also be used to concentrate total dissolved solids in a small volume of liquid. In a swirling flow, solids with higher density will be pushed to the outer region, whereas less dense fluid with smaller amounts of total dissolved solids will be confined to the middle. The two separate streams could be removed from the phase separator, creating a substantially superior version of this concept. Conservatively, the obtained cost of producing potable water is 26 cents per gallon. About 4,878 gallons of potable water will be generated in one hour while processing 0.1 m3 of produced water per second with initial water temperature of 70ºC from the solar pond. The projected cost is believed to be less than that of creating potable water by using techniques such as reverse osmosis, MED, or MSF. The last two schemes require elaborate infrastructure to implement as well as heat beyond that provided by a solar pond.
IN COLLABORATION WITH
Debjyoti Banerjee, professor and James J. Cain ’51 Faculty Fellow I, J. Mike Walker ’66 Department of Mechanical Engineering, College of Engineering
Ashok Thyagarajan, graduate student, J. Mike Walker ’66 Department of Mechanical Engineering, College of Engineering
