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To reduce calcium scaling in black liquor evaporation systems, many mills resort to chemical additives, redesign of existing evaporation equipment, or the purchase of a new system. However, a less costly way to address this problem is possible with a crystal growth system that effectively alters the black liquor before scaling occurs. In addition, unlike its more costly alternatives, this system remains effective in cases where liquor chemistry changes after the scaling solution is implemented.
Black liquor evaporation systems are inherently prone to scale deposition in an evaporation region known as the critical solids zone. In this zone, the deposition of calcium salts exponentially increases due to various conditions. This article outlines a crystal growth system that is able to avoid the critical solids zone and that is independent of liquor chemistry variations or permanent change to the pulping requirements of a mill.
The crystal growth system described here details a way to influence crystal growth by taking advantage of micro-crystal theory and by reducing the overall viscosity of liquor entering the higher solids effects. This combination results in the same benefits as new technology designs without requiring major capital expenditure for improved performance.
This article also describes technical enhancements to the crystal growth system that can provide further benefits. These enhancements utilize distribution, vapor recirculation, and delta temperature (Delta-T) control technology to improve liquor distribution on the tubesheet, accelerate evaporation, and predict and reduce evaporator washes.
FIGURE 1: The crystal growth system extracts liquor from the evap-oration train, combines it with salts, elevates the liquor temperature, and re-injects it into the evaporator train after treatment in the reactor. 
PREVIOUS APPROACHES TO CALCIUM SCALING. There have been many studies on the cause and effect of calcium scaling during evaporation of kraft black liquor. The information from these studies has been applied to new evaporator designs, which have demonstrated improved performance as a result. However, no one answer could handle all variations in mill operations and the mill-to-mill differences in pulping. Therefore, evaporator operation problems have appeared exclusive to the specific mill.
Black liquor evaporation systems are inherently prone to scale deposition of the double salt “burkeit” that consists of sodium carbonate and sodium. A region of evaporation is reached in the burkeit, known as the critical solids zone. In this zone, the deposition of calcium salts exponentially increases due to various liquor chemistry conditions such as calcium content, sulfate-to-carbonate ratio, soap content, and residual alkali. It is even different within a given pulping process, where changes to alkalinity, re-causticizing efficiency, and calcium levels may swing with planned or unplanned mill operations.
Non-pulping chemicals cannot economically eliminate the critical solids zone. Even the redesign or new design of traditional multi-effect evaporation systems are not able to bypass the region in either long tube vertical (LTV) rising film or falling film evaporators. Instead, evaporator design changes have incorporated selective circulation rates, retention times, and heat flux rates to minimize the effect of the critical solids zone.
One popular method to control scaling through enhanced crystal growth is salting, although it has provided limited benefits. Salting involves adding sodium sulfate, known as saltcake, to the evaporator train. Likewise, recycling of boiler ash laden with carbonate and sulfate crystals has shown limited improvements. These attempts to alter the saturation level of sodium salt crystals so that they attract and precipitate the calcium from the solution before scaling occurs have some merit. However, traditional evaporation processes lack the physical equipment for driving reactions to the highest potential in order to escape the critical solids zone during direct active evaporation.
As with most chemical reactions, time and temperature are key elements in economizing a process. The question becomes how much time and what temperature complements research data and the actual experience of enhancing crystal growth. Beyond the chemistry, the second question is where and how can liquor chemistry modification be changed to produce conditions that remove the critical solids zone from the evaporation process.
Early attempts to alter the traditional black liquor evaporation process included reverse recycling of high solids liquor to fortify or “sweeten” the intermediate solids liquor by using pumping systems alone. The process had little noticeable effect and did not employ any additional chemicals. Evolution of the crystal growth method involved recycling the black liquor into a large retention tank that was used as an intermediate storage vessel. Although benefits were immediately observed by using the tank with no additional chemicals, these diminished over time. After several months of operating with this method, it was noted that the retention tank had filled with solids that resembled wet sand and were identified as precipitated burkeit salts. The presence of the precipitated salt was encouraging, but the accumulation of salt solids could not be tolerated in a standard tank.
CRYSTAL GROWTH SYSTEM CONTROLS SCALING. As described in the previous section, easy approaches for removing the critical solids zones appeared elusive from the start. This required a reconstruction of the formation process. Evidence indicated that real savings were obtainable by eliminating the critical solids zone, but questions about the proper method and its associated cost remained. However, research also showed that there could be a suitable margin for cost recovery.
To address the problems of burkeit scaling in the critical solids zone, a new system that incorporated salt seeding technology was developed. This new crystal growth system treats kraft black liquor by adding pulping makeup salt or recycled salts. In addition, several small vessels are added to the evaporator island and become integral in the liquor flow stream.
To control time and temperature in the system, a new crystal reactor was designed. This new reactor produces crystallized salts, and, as a pumped-through system, it is designed to avoid the accumulation of the salts. Its basic process involves extracting liquor from the evaporation train, combining salts, elevating the temperature and re-injecting liquor into the evaporator train after its treatment in the reactor. The new crystal reactor operates at temperatures between 250°F and 260°F and reaction time is determined by an analysis of the liquor.
Raising black liquor temperature slowly in such a tube and shell exchanger system is critical to avoid plugging, because high heat flux tends to cook the liquor inside the tubes, which results in increased calcium scaling. The system design allows a maximum of 20° delta temperature (Delta-T) change and targets even less for operation. The liquor heaters are in series and use low-pressure steam to avoid scaling.
In the crystal growth system shown in Figure 1, a density control loop controls solids outside the evaporator to avoid upsetting the evaporator train with solids variations. Targeting control of solids to bypass the critical solids zone is key to system performance in conjunction with crystal growth and viscosity stabilization. Due to the nature and range of the soluble/insoluble content of solids produced in this bypass step, density measurement of the reactor feed liquor is best accomplished using nuclear source devices rather than typical refractometers.
FIGURE 2: For falling film evaporators, improved distribution tech-nology can diminish deposition water insoluble scales. 
To handle the increased flows generated by the new system, several pumps will require upgrade and possibly replacement. Although flows will increase, the evaporation rate will remain essentially unchanged, so no additional steam is needed. Improvement in performance will result in a production increase simply from the reduction of wash cycles and re-evaporating wash liquors. The only heat losses generated by the crystal growth system involve radiation losses due to the limits of insulating materials that are used.
Kraft black liquor treated by the crystal growth system does not undergo the critical solids zone in the evaporator where direct heat and vapor flashing occurs. The result is extended runs, along with a Delta-T parameter reduction of 4°F to 6°F, on the effect trailing the crystal growth system. Delta-T is reduced by viscosity stabilization and crystal growth outside the heating zone, since the simple mixing of liquor alone to sweeten input solids does not reduce Delta-T. Overall, the external heating requirements for the new system are offset by the reduced Delta-T, and the extended runs provide the return on capital investment by avoiding costly wash cycles.
The new crystal growth system incorporates all the benefits of a new evaporator design, but allows flexibility for future growth and for a change in pulping conditions. Several variations of the system are adaptable for any evaporator train, depending on the capital available. The new system is installed in steps and requires little downtime. Maximum downtime of one week may be required for full utilization involving recovery ash addition.
Applications of the new system. A crystal growth system as shown in Figure 1 was built at Interstate Paper, Riceboro, Ga., in 1995 and has operated well, reducing wash cycles and running consistent solids up to 70% on the concentrator for which it was implemented. The Riceboro mill operates with 100% southern pine. After installation of the new system, the concentrator operating Delta-T was reduced from 12°F to 8°F on comparable startup conditions.
A similar system is under construction for the APRIL Project’s P.T. Riau Andalan Pulp & Paper mill in Riau, Sumatra, Indonesia. This system uses makeup salt in addition to the boiler ash recycle. The Riau mill operates on 100% tropical hardwoods with high viscosity liquor. There, crystal growth nuclei are integrated into the evaporator train where the critical solids zone would normally occur. In addition, the recovery mix tank is used only for ash recovery and is dedicated to the evaporator system rather than to firing the liquor supply.
Results from the implemented systems. Evapor-ation plants before modification experienced severe tube plugging problems within the critical solids evaporation region (44% to 54% solids). In cases with either 100% softwood or 100% hardwood liquors, plugging was as much as 80% of the tube bundle. In addition, the wash frequency was generally twice per week, and production was gradually limited by the ability of the fouled effects to transfer heat downstream as vapor.
After modifications, washing requirements at the Riceboro mill were stretched out to two weeks or more. Extended runs, however, were limited by fouling of other effects from fiber buildup in the tubes. Where fiber is not a problem, a further decrease in wash frequencies is possible.
Also, improvement in the heat transfer coefficient of the previously mentioned critical solids evaporation body allowed the ability of the entire train to operate longer at a more uniform flow rate. This was because of the steady production of vapor enthalpy delivered upstream for the weaker liquor effects to utilize.
Cost of crystal growth system modifications to a typical evaporation system for production of 1.5 million lb/day solids is $500,000. Metallurgies, instru- mentation requirements, and re-use of existing equipment can affect final cost. The cost is not linear with respect to increasing size, providing even more savings for larger systems as was seen in the Pacific Rim design.
Although savings in steam and re-evaporation of wash liquors can pay for the system in less than one year, equipment design, layout, and a mill’s specific operating procedures affect the payback potential. In addition, mills not fully recovering boil out liquors may re-evaluate their needs in view of Cluster Rule requirements for spill collections, since crystal growth technology can improve evaporator wash frequencies.
IMPROVED DISTRIBUTION TECHNOLOGY. Prior to the above modifications, the Riceboro, Ga., and Riau, Indonesia, evaporators and concentrators mentioned in the previous section were experiencing 80% tube pluggage annually. However, tube pluggage was not totally eliminated with the crystal growth system. This is because the mechanics of evaporators do not maintain continuous hydraulic flow, since hydraulic conditions generated from vapor lock and collapsing vapor still allow some cooking of liquor solids during upset conditions. However, the crystal growth system produced a 15% to 20% reduction in tube plugging that allowed time for cleaning during the annual outages, which had not been previously accomplished.
A falling film evaporator with a crystal growth system can get even better results with improved distribution technology. Low cost distribution systems that exceed original equipment manufacturer (OEM) evaporator system requirements are also available for retrofit and can be installed without pressure part work. Such distribution systems can be used independently or with a crystal growth system.
FIGURE 3: Vapor re-injection works well on evaporation of solids more than 35%, improving vapor take away and design capacity. 
For LTV evaporators, improved distribution technology such as shown in Figure 2 can diminish deposition water insoluble scales, leading to improved operation. However, these evaporators will always have some liquor cooking due to the nature of rising film and unstable flow in each tube. These evaporators also require forced flow to maintain upward velocities that avoid cook outs.
The distribution system shown in Figure 2 accomplishes the same conditions that OEM evaporator system pilot plant units utilize. However, OEM’s often abandon the pilot plant design and provide a different distribution system in the actual falling film production models. A distributor of equivalent cost can be built to replace the OEM tray with holes or slot type units. Spray type distributors are good if properly sized and maintained. However, conversion to spray type distributors is expensive, time consuming, and involves pressure part hot work. In comparison, conversion to a positive flow design to replace trays is accomplished in one shift, with virtually no hot work and no pressure part work.
Interstate Paper, Riceboro, Ga., and the Jefferson Smurfit mill in Mocarpel, Venezuela, have successfully used the positive flow distributors shown in Figure 2 on falling film evaporators since 1995. These distributors consist of tube inserts designed to regulate flow to each tube equally. The inserts do not cause tubes to plug and are easily installed and removed for inspections. The design of the insert allows liquor to flow into the evaporator tube and vents the tubes to prevent vapor burping from the lower liquor flash chamber. The typical cost for positive flow distributors is about the same as replacement cost of the holed-plate type distributors, which is in the range of $30,000 for a 1,000 count tube bundle.
In addition, the combination of improved distribution technology with a crystal growth system’s ability to avoid the critical solids zone, such as at the Riceboro mill, provides extra benefits. This combination can virtually eliminate tube plugging in falling film evaporators. Forced circulation and avoidance of the critical solids zone in LTV evaporators will also result in tube plugging reductions.
VAPOR RECIRCULATION TECHNOLOGY. Conventional falling film evaporators may be retrofitted with capability to recycle 20% or more of the flashed vapor into the upper head. Improving vapor flow, especially in the initial section of the tube, prevents backflow. The additional volume of vapor moving downward in the tube then accelerates the velocities past the film layer and ripples the layer, exposing additional surface that allows more water to escape as vapor. However, velocities are not increased so that they strip off the film layer, since this may cause dry and, subsequently, plugged tubes due to cooking liquor on the surface. Vapor recirculation technology can be used independently or with a crystal growth system.
Recirculation is accomplished using a thermocompressor driven by medium pressure steam similar to non-condensible gas ejectors. Sizing to match the number of tubes in the evaporator body is important. In flooded or potentially flooded distributors such as plate trays, provisions must be made to assure uniform penetration of the recycled vapor into the tubes. This is accomplished with the previously described tube inserts that are designed to allow simultaneous vapor flow and liquor flow at different rates. In spray type distributors, the vapor is injected into the head and randomly passes with the spray droplets into the open tubes.
Vapor injection has been installed using the non-functional view port that exists on some concentrator heads. Otherwise, hot pressure part work is required to provide an entry port. Steam flow into the thermocompressor should be pressure regulated to avoid over pressuring heads that are designed for low pressures. In any case, a relief valve is needed to match the evaporator head design.
Vapor reinjection works well on evaporation of solids more than 35%, improving the vapor take away and improving the original design capacity by lowering the Delta-T required to flash vapor from the liquor film (Figure 3). This system is especially helpful where adequate venting of the lower and upper heads was not provided by the OEM. Vapor re-injection was installed at Interstate Paper’s Riceboro mill in 1995 on two falling film concentrators to improve the evaporation rate by eliminating vapor lock in the tubes.
DELTA-T CONTROL TECHNOLOGY. Evaporators and concentrators are designed and can be reconfigured to achieve the lowest possible Delta-T for each effect. The lower the working Delta-T, the longer the set runs between washes. An evaporator set that produces a uniform rise in Delta-T from body to body over a period of time is referred to as balanced with regard to heat flow and performance. Ultimately, however, the set will dirty over time and require washing to remove soluble deposits. In addition, insoluble deposits derived from various cooking liquors can accumulate over time and require more strenuous removal techniques. In either case, operating with the lowest possible Delta-T will reduce the amount of inevitable soluble or insoluble scale buildups.
Overall, evaporator set Delta-T can be significantly altered in the critical solids zone. Delta-T for an individual evaporator body is controllable when history indicates that the mechanical configuration is not matched for the flow and evaporation rate.
Delta-T is also controllable on a concentrator that is integrally connected to a set of evaporators and where the evaporator performance swings or back pressures the concentrator as in Figure 3. Since Delta-T is measured using the outlet vapor pressure converted to temperature, any change in pressure gives false indication for the calculated working temperature required on that body. Many concentrators have been washed when the trailing evaporator body was the real problem.
Relieving the accruing pressure with Delta-T control technology benefits the concentrator, allowing it to continue under appropriate conditions. The downstream evaporator body is also temporarily relieved, because the inlet pressure is no longer forced, causing deposits on its tubes that produce higher deposition rates.
Eventually, however, the evaporator body will require washing. With the Delta-T control system, the mill can monitor and control the timing of the wash to avoid crashing at an inconvenient time. This system can also be designed for various conditions and to produce specific results. The most economical system is small and allows a minimal pressure reduction on the affected body.
Monitoring of the evaporator system with the Delta-T control systems allows the prediction of an evaporator wash before solids are failing or steam pressure runs out due to performance of a critical effect. The critical effect must be previously identified and set up with the system. This effect cannot be the critical solids zone effect, but can be one where, for example, fiber deposition is a problem.
A study of the evaporator train reveals the potential for correction of Delta-T. Corrections can be made on short outages and do not require pressure part work and are generally low cost. Integration into a DCS system is also possible at the cost of adding loops and I/O points. However, although the system can be configured to work in a captive loop, this causes a loss of predictive monitoring.
Interstate Paper’s Riceboro mill installed a Delta-T control system in 1996. The system allows the pulp mill to schedule production around predictable evaporator washes. Since installation, the mill has seen a minor reduction in total washes required for a given due to the system. *
REFERENCES
1. Harrison, Ray E., Cheng, Peter J., Crowell, Barbara A., Ketcham, Elizabeth A. Tappi Journal " Ultra-high-solids Evaporation of Black Liquor" 71(2):61 1988
2. Osborne, David, 1991 Engineering Conference Proceedings "Falling Film Crystallizing Concentrator Producing 80% Black Liquor Solids" :81 1991
3. Green, Robert P., Hough, Gerald, Chemical Recovery in the Alkaline Pulping Process, 3rd ed., TAPPI PRESS, Atlanta,1992, 24
4. Hedrick, Robert H., Kent, John S., Tappi Journal "Crystallizing Sodium Salts from Black Liquor" 75(12):107, 1992
5.Almond, C.B., Hedrick, R.H., 1985 International Chemical Recovery Conference Proceedings " Sodium Salt Control in Black Liquor Evaporators" 197, 1985
6. Matthias, Kind, Marsmann, Alfons, Chemical Engineering Technology "On Super Saturation during Mass Crystallization from Solution " 13:50 1990
7. Laverly, H.P., Grace, T.M., and Frederick, W.J., Svensik Papperstidning, p. R72-80 (1983)
8. Balch, John T., and Moore, D Kinley, 1995 International Chemical Recovery Conference Proceedings "Liquor Chemistry Studies Contribute to Successful Design of Falling Film Crystallizing Concentrator"
9. Grace, T. M., Sachs, D.G., and Grady, H.J., Tappi Journal " Determination of the Inorganic Composition of Alkaline Black Liquors", TAPPI PRESS, Atlanta, Ga. April 1977, pages 122-125
10. The Institute of Paper Chemistry, " A Study of Evaporator Scaling: Calcium Carbonate Scales", The Institute of Paper Chemistry, Appleton, WI, November 1977, Project 3234
R. Wayne Adams is a power and recovery engineer for Kellogg Brown and Root Engineering, Houston, Texas.
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