The effect of equipment design and process scale-up on supercritical CO2 extraction: Case study for Silybum marianum seeds
Graphical Abstract
Introduction
The supercritical fluid extraction (SFE) process implies a separation of one compound (extract) from another (matrix) using a supercritical fluid as an extracting solvent. The SFE was shown to be an effective technique for the separation of commercially important extracts from raw plant materials that are rich in valuable bioactive compounds [1], [2], [3]. Besides being biologically active, supercritical extracts are pure and solvent-free, and therefore are eligible for application in food production, medicine, and pharmacy. Due to its high diffusivity, low viscosity, and near-zero surface tension, the most commonly used supercritical solvent is carbon dioxide (CO2) [2]. As a solvent, it has the GRAS (Generally Recognized as Safe) status, good solvation power, and can be easily and completely removed from the final product [2].
Although the SFE technique is widely used for the isolation of valuable plant extracts, a scale-up from laboratory to industry level still presents a challenging step. The Cambridge Dictionary defines scale-up as increasing something in size, amount, or production. If this definition is taken into account, one can find a certain number of studies available in the literature that report the scale-up of SFE processes [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. For example, Paula et al. [8] performed SFE scale-up for B. dracunculifolia leaves by variation in plant material mass from 12.5 g to 70 g and Fernández-Ponce et al. [9] performed SFE scale-up for mango leaves by an increase in extractor vessel volume from 0.1 L to 5.0 L. However, these studies presented scale-up on small-scale systems (due to the high costs of experimental tests at the industrial scale) while prediction models were used for a larger scale. Nevertheless, the resulting data were generally unreliable concerning the differences observed in processes conducted on significantly different equipment scales [3], [9]. The study of intermediate-scale experiments (semi-industrial scale) is a better strategy as it considers the restrictions that may occur on an industrial scale. Therefore, a scale-up of the SFE process proposed in this study was investigated in four high-pressure units, ranging from laboratory to semi-industrial scale, using supercritical CO2 (scCO2) as the working fluid. Silybum marianum (S. marianum) was selected as a model plant in this case study due to its high value as an industrial material. In 2018, S. marianum was placed among the eight top-selling herbal-based dietary supplements in the US natural retail outlets with sales reaching 9.0 million EUR and ranked 20th in the mainstream multi-outlet channel market in the US with total sales of 14.2 million EUR [15]. ScCO2 extracts from S. marianum contain valuable bioactive compounds such as fatty acids [16], [17], [18], α- and γ-tocopherol [1], [19], and silymarin [17]. Although there are several reports available in the literature that compare the effects of S. marianum seeds pretreatment [1], particle size [17], the SFE process pressure/temperature [17], [19], and CO2 flow rate [17] on the extraction yield and extract composition, the present study is the first report that describes the effects of high-pressure equipment design and the process scale-up from 0.28 to 40 L extractor volume. In addition, two literature models were compared for the description of the obtained SFE process kinetics. The first model is based on the adsorption-desorption mechanism that can be described in the following steps: (1) adsorption-desorption equilibrium of extractable component from solid tissue, (2) diffusion of extractable component dissolved in supercritical fluid to the surface, and (3) mass transfer through the external film into the bulk [19], [20], [21]. The second model introduced by Sovova [22] is to date the most used. The main assumption of this model is that, during the pretreatment of plant material, a fraction of oil-containing cells is destroyed, making the oil easily accessible to scCO2. The extraction process is thus divided into two periods. In the first period, easily accessible oil is extracted and an external mass transfer and/or oil solubility in scCO2 govern the process rate. In the second period, the oil that remained within the cells of the plant material is extracted, with internal diffusion governing the overall extraction rate.
It is clear that basic research, involving a literature review of the yields, composition, and bioactivity, shows potential for industrial application of the extract from S. marianum seeds. On the other hand, the analysis taking into account factors such as raw material prices, equipment, and labor costs, as well as prices on the market for a specific product, etc., can provide an approximate evaluation of the process economics and potential for investment. In that sense, this study provides, for the first time, information on the estimated costs for S. marianum extract production based on data obtained using a semi-industrial unit for the SFE process.
Section snippets
Materials
The S. marianum seeds were obtained from the Institute Dr. Josif Pancic (Pancevo, Serbia) and the Prowana (Radzymin, Poland). The seeds were stored in a dark place at room temperature (20 °C) prior to the extraction. Commercial CO2 (purity 99.9 %) purchased from the Messer-Tehnogas (Belgrade, Serbia) was used for the extraction process in laboratory scale units (units I and II, described in detail below) and CO2 (purity 99.9 %) produced by the Grupa Azoty (Zaklady Azotowe “Pulawy” S.A., Pulawy,
Plant material analysis and pretreatment
The moisture content in S. marianum seeds was determined using a laboratory moisture analyzer (MAC 50/1/WH, RADWAG®, Radom, Poland). Before the extraction, seeds were milled using a basic mill (Ika® A11, Warszawa, Poland) and sieved. The average particle size of the plant material used for SFE processes was 0.4 mm. The plant material density was 1192 ± 114 kg/m3. The bed density and porosity were 376 ± 6 kg/m3 and 68.2 ± 2.5 %, respectively.
Supercritical fluid extraction processes
Four high-pressure units were tested for the SFE
The SFE process in the lab-scale unit
The influence of pressure and temperature in the selected range was investigated using S. marianum seeds grown in Serbia and Unit I. The results obtained at 40 °C at various pressures are presented in Fig. 2a. It can be seen that pressure significantly affected the extraction yield. The highest yields obtained at 40 °C and 100, 200, and 300 bar were 1.4 %, 8.9 %, and 11.9 %, respectively. An increase in extraction yield with increasing pressure from 160 to 220 bar at 40 °C was also reported by
Conclusion
This study presented for the first time the scale-up of the SFE process from S. marianum seeds by in situ study (extractor vessels ranging from 0.28 to 40 L) as well as by theoretical model (for two 500 L extractor vessels). First, it was determined at the laboratory scale that the extraction yield increases significantly with increasing pressure from 100 to 300 bar for the SFE process performed at 40 °C and 80 °C. Further, it was shown that the construction of a high-pressure unit on a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Author S. Milovanovic acknowledges a scholarship from the Polish National Agency for Academic Exchange (NAWA), Warsaw, Poland (the agreement number PPN/ULM/2020/1/00023/U/00001). This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Contract No. 451-03-68/2022-14/200135). Work was carried out in the frame of the COST-Action "Green Chemical Engineering Network towards upscaling sustainable processes" (GREENERING ref. CA18224)
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