Auta, HA; (2017) Ultra scale-down process synthesis of microalgae primary recovery operations. Doctoral thesis , UCL (University College London).

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Abstract

The majority of products made by microalgae and requiring extraction before use are restricted, commercially, by the high cost of the harvesting methods employed. Although considerable progress has been made in biofuel development there are still relatively few studies on the initial downstream processing stages. Ultra-scale down (USD) approaches have previously been established to study the impact of the engineering environment on biopharmaceuticals; they are valuable because they enable study of a wide range of operating parameters using minimal quantities of material and resources. The aim of this project is therefore to establish a USD platform for the rapid evaluation of pre-treatment and recovery operations for microalgae downstream processes. The first objective was to explore flocculation as a pre-treatment step. Flocculation is a difficult process to operate reproducibly, hence standardization of flocculation conditions becomes vital in order to characterize and quantify process performance. A series of scale-down flocculation reactors were designed, characterised and scaled-up using Chitosan to flocculate heterotrophically grown Chlorella sorokiniana. These enabled flocs with defined particle size distributions to be consistently and reproducibly produced. An optimal Chitosan concentration of 9.9 ± 0.4 mg.g-1 of algal dry cell weight was determined. Scale-up of the flocculation process from the scale-down reactor (120 mL) to a 7.5L STR stirred tank reactor was achieved at a fixed impeller tip speed during flocculant addition and ageing (0.29 and 0.07 ms-1 respectively). Due to the complex nature of unit operations, it is generally difficult to obtain data at laboratory scale that closely reflects the performance of operations at pilot scale or beyond. For the second objective of this work, a USD model of a cross-flow filtration process for microalgal biomass recovery was established. This could accurately reproduce the flux-TMP (transmembrane pressure) profiles of lab-scale hollow fibre cartridges when operated at a defined shear rate. The benefits of flocculation on filtration performance include a reduction in membrane cleaning cycles and a 20% reduction in filtration time. Filtration results were also in good agreement between the two scales for both unflocculated and flocculated feeds. The USD method enabled a 14-fold decrease in the volume of material required. It also demonstrated the benefits of flocculation. And lastly, USD was achieved using a 14.5–fold reduction in membrane area at matched volume: surface area ratio. The third objective was to establish a USD centrifugation method using a rotating disk shear device to expose particles to hydrodynamic shear before centrifugal separation. Evaluation of the influence of flocculation on centrifugation efficiency showed the benefits of increased particle size on clarification. Clarification efficiencies exceeding 99% was obtained even at low centrifugal forces using an optimal Chitosan dosage. The USD findings were validated at pilot scale using a CARR PowerfugeTM centrifuge. Similar clarification performance was predicted using 2000-fold less broth volume than was required for the pilot scale study. Sonication and homogenization as small scale cell disruption options for lipid release of heterotrophically grown C.sorokiniana were explored. Comparison of the optimal conditions of the two methods showed cell disruption and lipid release were similar in both cases on a g.g-1 basis. Finally comparison of the transesterified material produced using either USD microfiltration or USD centrifugation steps for harvesting showed major differences in terms of yield of fatty acid methyl esters (FAME). USD methods for evaluation of primary recovery operations and their interactions appear particularly useful in microalgae bioprocess synthesis. This work is the first to evaluate the use of USD technologies with microalgal cells. It illustrates the power of small-scale mimics to enable rapid selection and optimisation of different process options and thereby rational selection of the overall bioprocess sequence.

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