[Home] [APPW 2004] [Journal papers]




S. Craig Van Dyke


HPD, Plainfield, IL USA


forced circulation, black liquor, evaporation, crystallization, case study





HPD has pioneered and currently offers enhanced forced circulation crystallization technology for concentration of  black liquor to high solids. The forced circulation design has provided a non-fouling concentrator option to the industry for over 30 years, and has progressed through an evolution with the use of heat transfer enhancement to minimize capital and operating costs associated with the design. This design has proven to be the technology of choice for the industry, capturing the vast majority of the markets which we serve since its commercial introduction in 1995.

The dominating factors in concentrator design are control of salt crystallization to avoid scale and accommodation of high liquor viscosity. The forced circulation crystallizer is inherently resistant to scale by nature of its design, thereby minimizing capital and operating issues associated with washing. The unit is designed with the same principles as industrial chemical crystallizers, utilizing submerged tubular technology. Evaporation and crystallization of salts within the liquor occur away from the heat transfer surface. The technology can allow processing of liquors with high viscosity without distribution and heat transfer problems – avoiding the need to operate at high temperatures for heat treatment / viscosity reduction . Lower temperature operation reduces corrosion / materials concerns and also minimizes the generation of additional NCG compounds that evolve as lignin begins to break down. Heat transfer enhancement minimizes operating costs by allowing the system to operate with lower power and with lower required steam pressure. Not only has turbulence enhancement been the technology of choice for new forced circulation units but also for retrofit of existing units for the power reduction benefit and improved heat transfer performance.

Three case studies will be presented to demonstrate the advantages and success of the technology.


Concentration of black liquor to high solids has long been one of the biggest challenges for a pulp mill's recovery department. Black liquor evaporation equipment generally falls into three categories indicative of the heat transfer mechanism applied: rising film, falling film, and forced circulation. The viscous properties and fouling nature of heavy liquors have led to adaptations of film evaporation technology historically utilized at lower solids, or application of forced circulation crystallizer technology. Falling film or forced circulation equipment is typically applied for evaporation to high solids, with specialization of the heat transfer surface amongst suppliers. Falling film equipment is available with liquor film inside or outside of tubes or on plates, usually with provisions for on-line washing and scale removal. Forced circulation equipment is characterized by liquor flow inside the tubes of submerged heat exchangers designed with sufficient back pressure to suppress boiling within the tubes. Forced circulation units do not typically require frequent washing to maintain a clean heat transfer surface.

This paper focuses on the advances associated with forced circulation type crystallizers with two case studies to demonstrate the effectiveness of the technology.


Forced Circulation Black Liquor Crystallizers

The enhanced forced circulation design is an evolution to the conventional forced circulation technology that has been used for decades for crystallization of liquor above critical solids. The forced circulation design is very robust, with typically no need for washing or dilution. Above critical solids (typically near 50% TS) sodium salts begin to precipitate from black liquor. To avoid scaling difficulties, evaporation equipment at this point must be designed as crystallizers to allow these salts to form in the bulk liquor and not on heat transfer surfaces. Crystallizer design is based principally upon control of supersaturation. The design parameters include proper recirculation rate, adequate crystallizer active volume and magma density (retention time), and sufficient heating surface.

An alternate strategy to the crystallizer design allows fouling, but provides a means to remove scale faster than it forms and before it negatively impacts capacity or leads to tube plugging. This strategy is employed by the quick "switching" designs in which multiple crystallizer bodies or chambers are continuously moved between product liquor and washing positions. One drawback of constantly switching arises from the fluctuations in product solids occurring every time a switch is performed. In addition, the multiple bodies or chambers are constantly cycling between hotter and colder positions, causing cycling mechanical stresses that can lead to early equipment failure.

Finally, complex piping and valving is required to facilitate the multiple liquor flow configurations.

The forced circulation design is the crystallizer technology of choice when boiling at the heat transfer surface (as occurs with film type designs) is undesirable. This can be the case when the falling film unit requires very high recirculation rates and/or prohibitively large heating surfaces to deal with high developed and residual supersaturation. Also, while a falling film crystallizer can produce very high heat transfer rates at the lower viscosity associated with weaker liquor, film development and resulting heat transfer performance are adversely affected by the high viscosity of heavy black liquor.

As with any crystallizer, the forced circulation unit is designed with sufficient retention volume for crystal growth. However, the developed supersaturation in the tube of a forced circulation unit is lower than in a falling film due to the fact that boiling within the forced circulation tube is suppressed (evaporation in a falling film tube increases developed supersaturation). In addition, temperature rise through the heater is carefully selected to minimize any adverse temperature effects (sodium carbonate and burkeite salts exhibit an inverse solubility). The high tube velocity typically required for the forced circulation design also counters the effects of high viscosity via shear thinning and increased turbulence.

The forced circulation crystallizer can be supplied with a vertical or horizontal heater. Schematics illustrating the vertical and horizontal configurations are shown below:

Figure 1


Figure 2


The vertical heat exchanger design offers a compact footprint, an important consideration for situations when space is limited. The horizontal design provides additional horsepower savings over the vertical design. An orifice plate is required at the entrance nozzle to the vapor body in order to suppress boiling in the vertical heater. The pressure drop created means additional TDH on the recirculation pump and translates directly to increased power requirements. With a horizontal exchanger, no orifice plate is required because the necessary back-pressure to prevent boiling in the tubes is provided by the elevation of liquor above the heat exchanger. In some cases when the elevation of the unit is limited or addition suppression is required, an orifice plate may be added in the horizontal unit for additional suppression.

Enhanced Forced Circulation Black Liquor Crystallizers

While the forced circulation design can be the technology of choice for most situations, it has historically come at the price of high operating power requirement. To successfully employ this resilient technology with today's demand for low utility consumption, an enhancement that provides the aforementioned traditional benefits of forced circulation design at lower power demand was developed. This technology is known as enhanced heat transfer, and employs a proprietary spiral rib type insert inside the heat exchanger tubes. The insert allows for a high degree of turbulence (at high viscosity) at low recirculation rates.

A great deal of research has been performed in the area of heat transfer augmentation. The general goal of heat transfer enhancement is to maximize the heat transfer efficiency (U coefficient) in certain applications for a given energy input. The results of enhanced heat transfer are smaller required heat transfer area and/or a reduction in required pumping power.

In 1990, pilot testing was initiated to study the possibility of applying enhanced heat transfer technology to the high solids crystallizer design. The requirements of the enhanced unit were to increase heat transfer efficiency, decrease power consumption, minimize the possibility of fouling, and still allow for full access to the tubes in case boil out or mechanical cleaning became necessary due to a severe upset which led to pluggage. A proprietary spiral rib type design as shown below was developed and tested. The design has allowed for a 50 -75% increase in heat transfer coefficient and a 50% reduction in power when compared to the conventional forced circulation design.

Figure 3


Initial black liquor work was performed with the proprietary turbulence-enhancing insert in 1993 as part of a study for the Weyerhaeuser, Longview, WA mill. The Weyerhaeuser liquor was tested at concentrations of 68.5% and 75% black liquor solids. Tube loadings were about 60% of those used for conventional forced circulation design, and resulting U coefficients were much higher than ever achieved in an open tube at these solids for either conventional forced circulation or falling film units. The inserts provided a 65% increase in heat transfer efficiency over open tubes. In short, the pilot work demonstrated high heat transfer performance and low horsepower requirements without fouling. The first commercial system was started up in Longview, WA in 1995 and has operated beyond expectations with no fouling.2

Greif Bros. Corporation Riverville, Va Mill

Greif Bros. Corporation paper mill in Riverville, VA is a semi–chem caustic carbonate mill which produces semichemical corrugating medium with hardwood furnish. Historically, the upper limit for concentration of this type of liquor had been 55-60% T.S., with the limitation being high viscosity and subsequent fouling potential. The mill planned an upgrade to their recovery boiler. Due to maintenance issues, the mill replaced the floor of the recovery boiler and pursued higher solids to improve the efficiency of the boiler and thus reduce hydrocarbon emissions. Other benefits include greatly reduced use of auxiliary fuel, reduced air flows and reduced carryover.

Figure 4The mill had existing multiple effect evaporation equipment to concentrate the black liquor to 50% TS. The existing train consisted of a five effect rising film system including an integrated forced circulation concentrator unit. The objective set for the new enhanced forced circulation UHSC (Ultra High Solids Crystallizer), was to concentrate the heavy liquor from 48% to 65% TS. The existing evaporation equipment was already quite taxed such that integrating the new UHSC vapor into the train would not be practical. As such, efficient recovery of the heat from the new UHSC became and additional objective set for the project. The new UHSC system was supplied in 2003 and is described as follows.

The evaporation technology selected for the project was a horizontal enhanced forced circulation crystallizer. Intermediate liquor is pumped from the existing atmospheric storage tank to the UHSC where it is concentrated to 65% T.S. The 65% T.S. Product Liquor from the UHSC flows to the pressurized Product Flash/Storage Tank (PFST) pumped via the recirculation pump, where it is stored at high temperature prior to being pumped directly to the boiler header. The PFST is sized for approximatelysix hours of storage. The intent is to store the high viscosity heavy liquor at high temperature (pressurized) to control viscosity and ability to transfer the liquor to the boiler.

Figure 5

High pressure steam is reduced for use in the UHSC heater. Vapor evolved from the UHSC flow through a pressure reducing station for pressure reduction to just above atmospheric pressure before entering the condensing circuit. This is required to maintain the appropriate liquor temperature in the UHSC irrespective of condensing conditions.

After pressure reduction, the vapor splits to three condensing units. The first is a standby condenser operating with mill water as the condensing medium. This unit is not typically in operation. Second, is a liquor heater which serves to heat inter-effect liquor form the existing multiple effect system. This augments a deficient heating system within that train. Third, is a circulating falling film green liquor heater to heat the liquor to cooking temperature. This augments a problematic heating system which required frequent cleanings.

Residual flash vapors from the system flow to the condensing circuit where they are condensed.

The UHSC Vapor Body is equipped with high efficiency vertical flow euroform type mist eliminator, ensuring low entrainment levels in the condensate The UHSC system started up in January 2004 and has performed very well exceeding the mill's fiscal objectives set for the project. As with all start-ups, initial operating methods are presently being refined to optimize long term system performance. The advantages of the turbulence enhancer proved very attractive for the project. The high viscosity of this liquor at high solids made it an ideal application and assisted with project economics.

Weyerhaeuser Hawesville, KY Mill

Weyerhaeuser (formerly Willamette Industries, Inc) operates a bleached hardwood kraft market pulp mill in Hawesville, Kentucky. In 1996, Willamette Industries (now Weyerhaeuser) had installed a multiple effect falling film crystallization system to concentrate the mill's black liquor to 80% TS. The system started up and was experiencing excessive fouling difficulties in the falling film crystallizer section. The system as a whole, however, was not meeting design capacity or solids due to newer falling film concentrators that required frequent boilouts and made the system very difficult to operate. By early 1998, Willamette desired a workable solution to its evaporation issues which would allow operation at design solids and throughput without the need for constant boilouts. From the previous two years of operation, some key conclusions could be drawn:

  • Figure 6A forced circulation black liquor crystallizer (or any crystallizer for that matter) will typically operate better away from the critical solids point. This results from higher crystal surface area available for release of supersaturation.
  • Forced circulation crystallizers exhibit resistance to calcium scale in part due to an inventory of crystal surface area for salt deposition.
  • It has been documented that maintaining a minimum carbonate to sulfate ratio will provide a positive crystallization chemistry and will minimize scale. This is due to the fact that a higher percentage of the precipitation will occur as burkeite rather than sodium carbonate. One must realize, however, that any saltcake or ash dissolved to affect crystallization chemistry will have to be re-crystallized at some point. The more crystallization to be performed, the more likely it is to occur on heat transfer surfaces. There needs to be a balance between positive solution chemistry, adequate crystal seed availability, and crystallization duty – all of which affect the supersaturation levels within a crystallizer.
  • Finally, because boiling and the resultant development and release of supersaturation (ie crystallization) occurs away from the heat transfer surface in a forced circulation unit, the susceptibility to fouling and chemistry issues is greatly reduced. This superior scaling resistance of the forced circulation design over falling film was exhibited at the Hawesville mill.

In the spring of 1998, Willamette purchased enhanced forced circulation crystallization for the Hawesville Mill.

The Hawesville equipment was modified as shown in the schematic below. HPD supplied enhanced forced circulation crsytallizers to the front of the No. 3 and No. 4 trains. For the No. 3 train HPD supplied a new #1A and #1B high solids crystallizer and a new #6 effect falling film evaporator. In addition, we retrofitted the existing HSC unit with turbulence enhancers. For the No. 4 train, we added a new #1A high solids crystallizer to augment the existing problematic switching falling film crystallizer bodies. The switching falling film units are switched and washed weekly. Both trains finish in enhanced forced circulation bodies. Some key observations to be noted regarding operation of the system at Hawesville are as follows:

  • The switching falling film units must be washed for longer periods of time than originally expected in order to return to clean operation.
  • An NSSC liquor stream is blended with ash and intermediate liquor prior to introduction in the HSC and falling film concentrators. If the solids content of this tank drops to the point that the ash is dissolved, rapid scaling (particularly in the falling film units) is observed.

Figure 7


Boise DeRidder, La Mill

Figure 8As the success of the technology was being demonstrated with new commercial units, many existing conventional Forced Circulation units remain ideal candidates for retrofit with turbulence enhancers. In 1985, HPD supplied the Boise mill in DeRidder, La with a conventional forced circulation concentrator to augment their existing evaporation systems. The Boise DeRidder mill produces kraft linerboard with softwood furnish. The system supplied in 1985 included a high solids crystallizer (two horizontal heaters) and intermediate solids crystallizer (two horizontal heaters). By 2001, the system had been operatational for 16 years when a retrofit project was proposed and evaluated by Boise. The main driving force for the retrofit project was to significantly reduce the power consumption of the existing HSC/ISC system while also increasing the heat transfer performance of the units. The units had been operated at near maximum capacity and the increased heat transfer performance would allow for better operation of the units. In January 2002, the mill had made a decision to proceed with the retrofit project. The overall project schedule was approximately 16 weeks from start to finish. The retrofit of the four forced circulation heaters was to occur during a five day mill outage period which was to also include changing the recirculation pump impellar diameters in order to reduce the recirculation pumping rates. The retrofit was successfully completed and the units have consistently met and exceeded the mill's fiscal and performance objectives set for the project. This project has served as a model for additional retrofit projects completed by HPD.


High solids boiler firing, mill closure and the high level of efficiency required by today's pulp & paper industry demands the use of economical and resilient high solids black liquor crystallization equipment. In March of 1999, Willamette reaffirmed its satisfaction with the enhanced forced circulation technology by purchasing another 80% system for its Red River Mill in Campti, LA and also repeated at numerous other facilities. Forced circulation technology has historically proven to be very effective for black liquor crystallization at high solids. With the further evolution of this technology provided by turbulence enhancement, and the associated reduction in capital and operating costs, forced circulation technology becomes a practical choice for crystallization of high solids liquor.

This statement has been supported by the industry, as more than 30 enhanced forced circulation units have been purchased since the first system was started up in 1995. As well, HPD has performed three retrofits with other planned for future projects. The technology is also being utilized in other industries with similar elevated viscosity challenges.


1 Gore, Christopher, TAPPI 1998 International Chemical Recovery Conference Proceedings, TAPPI Press, Atlanta, pp.33-39.

2 Rieke, J., J. Brinker and S. Bogart, Weyerhaeuser Tries New Crystallizer Design to Reduce Fouling, Cut Power Consumption, Pulp & Paper Magazine, July, 1997.