Theory to practice
Redhawk operators have been working for almost four years to perfect operation of the ZLD system. Along the way, there have been some successes and some “challenges.” The ZLD system works very well today, but getting to know the peculiarities of the system has taken some time. You can’t go to college and get a degree in ZLD—the only diploma available is from the school of hard knocks.
The Redhawk staff has had much success operating a ZLD system and has elected to share their collective experiences with the industry and those considering similar systems. Here are 10 prescriptive pointers that will either help you operate your existing system more efficiently or make the selection of a system for your new plant less problematic down the road.
1. Include the vapor washer option. A large number of brine concentrators have mesh pads inside the structure just above the sump area. Accessing these pads for cleaning is often difficult. In addition, particulates can sometimes make it past the mesh pads and cause scaling problems on the evaporator fan blades. At Redhawk, evaporated water vapor from the brine concentrator passes through the vapor washer (VW) on its way to the evaporator fan. This provides added fan protection.
Redhawk’s VW is a co-current scrubber design (shown on the right of Figure 10). Water from the VW sump is sprayed into the incoming vapor parallel to the vapor flow. As the large spray droplets combine with the smaller entrained droplets, even larger, coalesced droplets form. As the vapor leaves the VW downcomer, a 180-degree turn causes most of the droplets to leave the vapor steam and fall to the sump area of the VW. The cleansed vapor then flows through a layer of mesh (which removes almost all of the remaining entrained material), leaves the vapor washer, and heads over to the suction of the evaporator fan/compressor. Access to the mesh pads for cleaning in a stand-alone vessel is much easier than it is in a brine concentrator design with self-contained mesh pads.
2. Consider evaporator maintenance. In Redhawk’s falling-film evaporator, two distribution plates sitting above 40-foot-long titanium tubes distribute the brine evenly down the tubes. During mechanical cleaning of the evaporator, these plates are unbolted and set aside to allow access for hydroblasting the tubes. In some other evaporator designs, distributor caps with two small holes distribute the brine down the tubes.
Our experience with the double distribution plate design is that it takes more effort to get to the tubes because the heavy plates must be removed. However, the double-perforated distribution plate design appears to be less prone to plugging with our type of water than the distributor cap design.
At another of our Arizona plants equipped with an evaporator with the distributor cap design we found a buildup of scale between the cap and the inner tube wall. We have experienced instances of through-wall pitting and/or corrosion of the tubes in these locations.
3. Upgrade your crystallizer blower instrumentation. Almost all crystallizer systems have a propensity for foaming due to the nature of the fine particles that develop over time in the system. The original design of Redhawk’s crystallizer blower included local pressure switches on the suction and discharge sides of the vessel. But because the switches were not sensitive enough to pick up small pressure changes, foaming occurred after large load swings. In the end, Redhawk replaced the original pressure switches with more-sensitive pressure transmitters and tied them into the plant’s distributed control system (DCS).
The upgrade enabled the protective trips on the blower to be set to respond to a foam carryover from the crystallizer vapor body. The pressure setpoint was increased from 0.5 to 2.5 psi for two reasons: because small operating pressure swings occur routinely and because the original setpoint could allow a swing into the negative range, creating a vacuum and carrying over foam into the blower. Prior to the upgrade, the blower had to be removed from service and rebuilt several times during the first 18 months of operation because the system response was slow. Finally, the liquor level in the crystallizer vapor body was lowered from 50% to 10%, lowering the foam level below the site glass and making visual inspection easier.
4. Optimize centrifuge performance. The original plant design called for a centrifugal feed pump to control flow from the crystallizer vapor body to the centrifuge. Several different pump designs were tried, but all failed due to the challenging operating environment caused by the boiling crystallizer liquor/salt slurry. The final solution was to remove the pump from the system and use a combination of crystallizer pressure and gravity to feed the crystallizer slurry to the centrifuge.
This solved the centrifuge feed problem but did not offer a method for metering the flow rate. The Redhawk centrifuge is rated for 25 gpm of 25% salt. The centrifuge had to be rebuilt on several occasions due to overfeed flowed by plugging. Plugging causes backup of the slurry, which eventually will find a path across the seals and into the oil system, destroying bearings in the process. The solution: a flow meter and control valve was placed in the feed line upstream of the centrifuge and an automated flushing system controlled by the DCS was installed to keep the centrifuge clean.
5. Test your solids. Redhawk’s operators perform an ASV (apparent settled volume) test twice per shift. In this test, a sample from the evaporator and crystallizer are collected in 1,000-ml graduated cylinders. After the samples are allowed to settle for 30 minutes, the ASV is determined by observing the change in volume between the liquids and solids. For example, in the evaporator there is one salt level. If the settled volume were at 100 ml, then the ASV would be reported as 100/1,000 (10%) or 10.
By experience, the ASV of the evaporator now is maintained in the 5 to 18 range. It is important not to go below 5 to maintain enough seed material to prevent scaling of the evaporator tubes. If the ASV is greater than 18, the evaporator recirculation pump draws too much current, causing an alarm.
ASV is measured at two levels in the crystallizer. The heavier material found at the lower level is called “the salt” or “unders.” The fluffy-looking upper level is called “the fines” or “uppers.” For example, if the fines were at the 800-ml mark and the salt were at the 100-ml mark, the operator would report the crystallizer ASV as “80 over 10.” Operators also perform TDS tests on the evaporator and crystallizer systems twice per shift. The evaporator TDS is maintained below 170,000 mg/l, and the crystallizer is maintained below 375,000 mg/l. It is important not to exceed these limits to avoid exceeding the boiling point rise for each system.
6. You may require a crystallizer purge stream. During the first year of ZLD system operation, the crystallizer system had to be shut down and drained when its performance couldn’t be maintained. The reason was a mystery for some time. Optimizing centrifuge performance (see tip #4) and making the unit more reliable was a big step in the right direction, because the more it ran the easier it was to stay ahead of solids buildup in the evaporator and crystallizer systems. Optimizing centrifuge performance alone, however, did not completely solve the problem.
One important point learned by the Redhawk plant staff while troubleshooting the system was that “the fines” level of the ASV could be misleading, because most centrifuges are set up to remove one type of particle-size range of solids. In our case, the centrifuge favors removal of salts but allows the fines to pass through the centrifuge in the centrate and back to the crystallizer feed tank. The fines then re-enter the crystallizer vapor body and continue in this loop until they build to an “uppers” ASV of 100%. This eventuality should be considered in your plant design.
At one point, a cation polymer was used to help remove the fines. But the problem remained because periodic shutdowns followed by a system drain were still required. A consultant studied the system for several months and concluded that a small portion of the black liquor could not be processed in the ZLD system. He concluded that a small purge of approximately 0.5 gpm from the system would be required to maintain the system solids-liquid balance. A purge line on the centrate return to the crystallizer feed tank was added to minimize the amount of solids contained in the purge (Figure 11). When the dissolved-solids level in the crystallizer and/or the foaming in the vapor body get too high, operators now perform a controlled purge of the system.
7. Plan for ZLD maintenance. Outage planning for the ZLD system has to be taken just as seriously as outage planning for the power blocks. The lead time for certain parts can be weeks or even months. Spare critical parts should be purchased and stored on-site. At Redhawk, a spare crystallizer blower, centrifuge, and evaporator fan blade wheel are kept on-site.
Redhawk plans for spring and fall outages so evaporator tubes, vapor washer mesh pads, and crystallizer heater tubes can be cleaned mechanically by pressure washers. The 400F-rated Teflon crystallizer mesh pads are replaced rather than cleaned. The plant has learned that by performing two mechanical cleanings per year, an expensive chemical cleaning can be avoided longer. Mechanical cleaning can be done for $15,000 to $25,000 per outage, whereas a single chemical cleanings may cost upward of $100,000, depending on the amount of metals in the waste cleaning solution and how that waste has to be processed.
8. Expect water quality changes by season. The constituents of makeup water change during the year, and Redhawk’s experience has been that the operating characteristics of the ZLD system are different in summer and winter. For example, total organic carbon can be much higher during summer months due to algae growth in the holding pond. What’s more, high total organic carbon levels appear to put more demand on the ZLD system. Finally, nitrates also vary seasonally and can increase the boiling point rise in the evaporator and crystallizer systems.
9. Consider adding a brine concentrator surge pond. Redhawk has a 28-acre-foot brine concentrator surge pond that provides for a week’s worth of storage of cooling tower blowdown during ZLD system outages. This pond gives the plant more flexibility than other ZLD plants without surge ponds.
10. Add a hydrocyclone on the seed recycle tank. At Redhawk, the evaporator blowdown is fed to a hydrocyclone which was installed atop the seed recycle tank. It is used to separate the TSS or “seed” from the TDS. If the amount of seed material becomes too high (as measured by the ASV test), operators open the hydrocyclone bypass valve to allow the TSS and TDS to be fed to the crystallizer feed tank. Like the brine concentrator surge pond, the hydrocyclone gives Redhawk added flexibility, in this case, in evaporator operations.
—Mark Yarbrough is the senior chemical control specialist for Redhawk Power Station. He can be reached at 602-407-7805 or mark.yarbrough@aps.com.
Email
Comments
Print
Reuse
