Flocculation

Flocculation is a two-step particle aggregation process in which a large number of small particles form a small number large flocs.

Step 1: Coagulation
Small particles usually carry negative surface charges that hinder aggregation and settling (1a). Coagulant chemicals can adsorb to the particles and balance the charges. The introduction of opposite charges enables particles sticking together to form stable and well suspended submicron flocs (1b). Rapid mixing is required for proper dispersion of coagulant chemicals, to promote particle collisions and submicron floc formation (1c). 

Step 2: Flocculation
Flocculation requires gentle mixing and the use of a high molecular weight polymeric flocculant. The flocculant adsorbs to the submicron flocs and facilitates bridging of gaps between flocs (2a). Bringing particles closer together creates the effective range for Van Der Waals attraction forces to reduce the energy barrier for flocculation and loosely packed flocs form. Aggregation, binding, and strengthening of flocs occurs until visibly suspended macroflocs form (2b). At the right weight, size, and strength sedimentation occurs. Macroflocs are very sensitive to mixing and once torn apart by strong shear it is difficult or impossible for them to reform.

Flocculation happens naturally during the formation of snowflakes or subsea sediments but it is also deliberately applied in the biotechnology, petroleum, pulp and paper, and mining industries.

Biopharmaceuticals
Whole mammalian cells are not often overly burdensome on filtrations as their size and distribution can be reasonably managed. Bacterial and yeast systems in particular have much smaller monomeric cell units. The biomass burden, in addition to the small median particulate size, can create lots of small cell fragments which clog filters and slow down filtration rates. Flocculation is used to induce a sort of affinitive colloidal state among these small cell fragments. Flocculation may also be applied if the cell culture produces multiple products and/or by-products which are expressed within different cellular structures or environments of the fermentation matrix such as membrane bound or intermembrane expressed, supernatant, adsorptive polymer, or even alternative-phase capture such as an emulsion. Flocculation is utilized in such conditions where materials need to be efficiently separated prior to filtration or another method of harvest. Applying flocculation ensures a high flux over filtration units as well as efficient and cost effective separation of cell material from supernatant. 

Oil and Gas Upstream Production
Used water can contain significant amounts of suspended particulate matter, which often takes long to sediment. Flocculation expedites sedimentation and ensures efficient solid/liquid separation. Large volumes of used water can be processed quickly, which minimizes the environmental impact in the sense of land needed for used water storage facilities.

Pulp and Paper Industry
Cellulose fibers is one of the main ingredient in pulp and paper, but it also requires glue, impregnation, and fillers to achieve the required sheet properties for an acceptable paper product. Flocculation is the key processing step to combine fibers, fillers, and other additives in such a way that it dewaters quickly and can be produced in large quantity.

Precious Metal Mining
Product streams often contain a wide range of different metals, which need to be separated in order to obtain a pure product. Selective precipitation of individual metals is usually accompanied by flocculation and sedimentation to ensure quick separation from the remaining liquid.

Flocculation is an important unit operation, which needs to be developed and optimized to run cost effectively. Whether the objective is to flocculate cells fragments, suspended particle matter, fibers and fillers, or precious metals, the steps to develop efficient flocculation processes are very similar.

1. Flocculant type and effective concentration screening

2. Impact of process parameters on flocs

• Mixing intensity, shear stress, and mixing time
• Dosing rate, flocculant concentration, dosing location
• Solids concentration
• Particle size

3. Downstream performance analysis

In the fiber-cement industry, it is key to understand flocculation particle mechanisms along the process. ParticleTrack trends in individual size classes and in situ EasyViewer images show a large number of small particles and a small number of large particles stirred at steady state. When the flocculant is added, the number of fines drops dramatically and the number of large flocs increases significantly. Beyond the point of optimal flocculation the shear of the stirrer starts to destroy the flocs, large counts decrease and small counts increase again.

Understanding flocculation particle mechanisms helps to identify root causes of suboptimal process performance, downstream production challenges, and out-of-specification products. Detailed in situ particle data is the first step to effective process optimization, Quality by Design, and efficient manufacturing.

Flocculation requires gentle stirring. As individual floc types have different strengths, it is particularly important to understand what stirrer rpm is gentle for a specific floc system. ParticleTrack trends for small particles indicate how the small-size class changes when flocs are created and stirred at different rpm. Fast stirring breaks a lot of flocs, and the number of fines almost reaches the same level as before flocculation. Slow stirring leaves most flocs intact and can be considered gentle. In situ EasyViewer images support these findings and show larger flocs at low rpm.

Mixing intensity and shear optimization is a successful strategy to prevent floc breakage. Breaking flocs defeats the purpose of flocculation, increases filtration times, and should be avoided. Filtration rates and cake dewatering are the quickest at the point of optimal flocculation and any deviation from that point impacts operations, product quality, and manufacturing costs.

In a variety of industries, flocculation processes are run in batch or continuous mode with either dynamic or static mixers. One of the key process parameters in both modes is the Mixing Zone Residence Time (MZRT). The point of optimal flocculation is located 1:38 min after flocculant injection. EasyViewer shows that flocs have fully developed and floc breakage is about to become the predominant process. ParticleTrack trends in individual size classes visualize the point of optimal flocculation and define the MZRT-window in which the number of small particles reach an acceptable minimum.

Operating outside the MZRT-window either means that flocs have not yet fully formed or floc breakage is already compromising floc size and optimal process performance. Both cases are suboptimal because of a high number of free small particles and only the MZRT-window ensures the desired product and process performance. The MZRT-window is different for each particle/flocculant system and needs adjustment depending on suspension composition, mixing intensity, and flocculant type.

Chemical companies continuously develop new and innovative flocculants with improve flocculation performance. However, not every flocculant, long established or newly developed, is suitable for every particle system. It is necessary to test and confirm performance with each specific system. ParticleTrack trends reveal that:

• Flocculant A shows a weak flocculation performance and is hardly able to reduce the amount of small particles in the system.
  
  
• Flocculant B captures a considerable quantity of small particles. However, flocculation kinetics are slow and the floc breakage rate is higher than the point of optimal flocculation.
  
  
• Only Flocculant C shows a good overall performance with quick flocculation kinetics, almost complete capture of fines,and an impressive floc stability beyond the point of optimal flocculation.

ParticleTrack and EasyViewer are able to monitor flocculation performance of different flocculants in situ and in real time. This enables scientists and engineers to make fact-based decisions during flocculant selection and flocculation process optimization.

EasyViewer is a probe-based imaging tool that captures high-resolution images of crystals, particles, and droplets as they exist in process.

Exceptional information-gathering capacity combined with excellent usability makes EasyViewer a compelling tool that scientists will enjoy using to accelerate decision making and speed process development.

View a live eDemo from your work or home office on your schedule.

Particle size and count analysis can be studied in real time with the probe-based ParticleTrack with FBRM technology. Obtain a real-time measurement of particle size, count, and shape:

• Direct in-process measurement with no sampling or by-pass loops required
• Study isolated regions (fine and coarse) of the particle population
• Study particles in opaque or translucent slurries and emulsions
• Directly observe how process changes impact particles
• Monitor particles from 0.5- 1000 μm at most process concentrations 

Daniel Burgstaller, Walpurga Krepper, Josselyn Haas, Marine Maszelin, Jure Mohoric, Katja Pajnic, Alois Jungbauer and Peter Satzer, " Continuous cell flocculation for recombinant antibody harvesting." J Chem Technol Biotechnol 2018; 93: 1881–1890 DOI 10.1002/jctb.5500

Efficient, flexible and cost effective methods for continuous cell removal have to be developed before a fully continuous and integrated product train can be realized. The authors describe the development and testing of such an integrated continuous and disposable set-up for cell separation by flocculation combined with depth filtration.

ParticleTrack measurements were performed inline, after the addition of 0.0375% pDADMAC and positioned at the outlet of the tubular reactor (static-mixing). ParticleTrack (FBRM) monitors flocc size and when overlaid with time-resolved pressure, indicates the approach to filter saturation and trends with pressure events. The various methods and flocculant type evaluated resulted in different outcomes for total drug substance yield, purity, and High Molecular Weight Index (HMWI). These outcomes also showed correlation to divergent particle size distributions resulting from the chosen flocc type.

This data builds on previous publications showing correlation to other flocculation parameters such as flocc reagent concentration, mixing force or sheer, mixing resonance time, temperature, and starting condition and composition of whole cell broth. By using flocculation, the required depth filtration areas were successfully reduced by a factor of 4 compared with a conventional filtration train for which ParticleTrack (FBRM) was well suited to enable continuous antibody harvesting.

Below is a selection of flocculation publications.

• Costine et al. "Development of test procedures based on chaotic advection for assessing polymer performance in high solids tailing applications" Processes (2020), 8, 731
• Vajihinejad et al. "Monitoring polymer flocculation in oil sands tailings: A population balance model approach" Chemical Engineering Journal (2018), 447-457
• Burgstaller et al. "Continuous cell flocculation for recombinant antibody harvesting", Journal of Chemical Technology & Biotechnology (2017), 93, 7 
• Senczuk et al. "Evaluation of Predictive Tools for Cell Culture Clarification Performance" Biotechnology and Bioengineering (2016), 113, 3 
• Owen et al. "PDADMAC flocculation of Chinese hamster ovary cells: Enabling a centrifuge-less harvest process for monoclonal antibodies", mAbs, (2015), 7:2, 413-427
• Cobbledick et al. "Demonstration of FBRM as process analytical technology tool for dewatering processes via CST correlation" Water research (2014), 132-140
• Singh et al. "Clarification of Recombinant Proteins from High Cell Density Mammalian Cell Cultures Systems using New Improved Depth Filters" Biotechnology and Bioengineering (2013), 110, 7
• Senaputra et al. "Focused Beam Reflectance Measurement for Monitoring the Extent and Efficiency of Flocculation in Mineral Systems" AIChE Journal (2014), 251-265
• Govoni et al. "Monitoring Flocculation of Newsprint in the Laboratory and on a Paper Machine" (2011), 1-18
• Blanco et al. "Optimal use of flocculants on the manufacture of fibre cement materials by the Hatschek process" Construction and Building Materials (2010), 158-164
• Uncles et al. "Observations of Floc Sizes in a Muddy Estuary" Estuarine, Coastal and Shelf Science (2010), 186-196
• Owen et al. "The preparation and ageing of acrylamide/acrylate copolymer flocculant solutions" International Journal of Mineral Processing (2007), 3-14