Water. desalination + reuse

PROJECTS | 22 | Desalination & Water Reuse | May-June 2014 for reuse as boiler feedwater. More than 800 m 3 /h of oilfield-produced wastewater is required to feed the plant along with 160 t/h of low-pressure steam. The steam is fully condensed by the evaporation system to return pure steam condensate back to the boiler system. Evaporation blowdown of 97 m 3 /h is produced requiring off-site disposal. The first-effect evaporator condenses the plant steam, while the remaining four effects utilize evaporation steam generated from previous effects. The evaporator accepts produced water as feed, concentrates it about eight times to just below the point of solids precipitation, and produces a very pure distillate. The low-volume concentrated waste- brine discharge is disposed of off-site via deep-well injection. The vapors produced in the last effect are condensed by an air- cooled condenser to recover the distillate. A process flow diagram for the five-effect evaporator system is shown in Figure 1. The evaporator system is designed to condense 160 t/h of plant steam. This is sufficient steam to concentrate more than 800 m 3 /h of produced water to recover clean distillate and produce concentrated brine for disposal. Table 1 shows the expected chemistry of the concentrated produced water. The warm produced-water feed, at 45-85°C, is received by the plant, and hydrochloric acid is added into the feedwater. The hydrochloric acid lowers the pH to convert almost all of the bicarbonate/ carbonate to carbon dioxide gas. Removing the carbonates at this early stage prevents scaling on the evaporators' heat transfer surfaces. The acidified feed is then sent to the fifth-effect evaporator. This multiple-effect evaporation is a countercurrent design: feedwater flows in one direction whereas steam/vapor flows in the opposing direction. Feedwater first enters the fifth effect and plant steam enters the system on the first effect. After the fifth effect, the brine is pumped in series from the fourth effect to the first effect. Therefore the first effect will contain the highest concentrated brine. The evaporator feedwater enters the fifth effect evaporator sump and flows in series toward the first effect. The contents in the sump are pumped by the recirculation pump up to the top of the evaporator, called the floodbox. The brine is evenly distributed into each tube through a twin-spin tube distributor and an uniform thin film is formed on the inside of the tubes. As the thin film flows down the tubes, the brine is heated to its boiling point. Water is driven out in the form of steam vapor and flows down the center of the tube with the concentrated brine flowing into the evaporator sump. The water vapor from the fifth-effect evaporator is drawn from the space above the boiling brine (the steam cavity) through internal mist eliminator pads, to entrap any remaining salts on its way to the condenser. The steam from the steam cavity of the fourth effect is used to drive the fifth effect. The operating sump pressure of the fourth effect is slightly higher than the condenser shell pressure in the fifth effect evaporator. The steam gives up its heat of vaporization in the condenser of the following effect (to heat the thin falling film on the inside of the tube) and condenses on the outside of the tube wall. The feed and vapor flows for the fourth to second effects are similarly configured to the fifth effect. Feed to the fourth effect evaporator sump is drawn off the brine recirculation flow from the fifth effect. The vapor flows through the vapor ducts and traverses through each effect in the opposite direction from the feedwater flows. For the fourth effect, the steam used to drive evaporation originates from the third effect. The steam used to drive evaporation on the third effect comes from the second effect, and so forth. The steam used to drive the first effect evaporator is from low-pressure plant steam. The steam gives up its heat of vaporization in the condenser (to heat the thin falling film on the inside of the tube) and condenses on the outside of the tube wall. The steawm condensate collects at the bottom of the shell side of the condenser in the first effect and returns as high-quality boiler feedwater. EnginEERing SuCCESS Expected to come online at the end of 2015, the multi-effect steam-driven evaporation system provided by GE will allow the Russian oil-producer to recycle its water from the enhanced oil recovery in the treatment of heavy oil produced water. Expected results are shown in Table 2. GE and RGE will continue to develop new technologies and processes to facilitate smarter, more effective, and more environmentally friendly heavy oil recovery applications. l Figure 1: Process Flow Diagram of GE's five-effect evaporator Table 1. Expected produced water chemistry range Table 2. Expected System Performance Data

• Cover