ABSTRACT
This chapter deals with integrated processes using carbon dioxide in supercritical conditions for the development of functionalized materials. The basics of the involved process units are presented to allow a general understanding, and for enabling to extend the design to a variety of applications. A conceptual design is proposed by integrating three steps for producing scaffolds with antibacterial properties. A setup for an integrated scCO2 extraction unit of natural compounds with posterior impregnation on solid matrices is described in detail including a final step of foaming, as a way of
product formulation, by making use of the same supercritical fluid as being used for the prior extraction and impregnation steps. The strategy followed is based on minimizing the loss of extract matter in the tubes, vessels, and heat exchangers of the equipment by directly using the scCO2-extract solution for impregnation, avoiding the low efficiency of extract recovery and energy consumption occurring in a separation step forced by pressure reduction. Results of production of functionalized poly(caprolactone) scaffolds with natural compounds extracted from Patagonian Usnea lichen are presented as an example. To establish appropriate operating conditions for each of the processing steps, supercritical extraction of Usnea as well as sorption kinetics and resulting material properties have been studied in detail before arriving at an optimized overall process. 13.1 IntroductionSince the beginning of the eighties of the past century, supercritical extraction from solid natural resources is finding a growing number of industrial applications. Starting from food industry and stimulants (coffee, hops), a large variety of almost any lipid-containing seeds, spices or herbs has been extracted to obtain products of high purity and quality. Most of these high-price products are applied in cosmetics or pharmaceutical industry [1-3]. A distinction is made depending on whether the main aim is to extract a valuable component or to retrieve some undesired substance in order to increase the value of the source material, like in the case of extraction of aflotoxines from cork. Apart from extraction, high pressure can also be applied for modifying the original properties of the (natural) source material or to formulate an intermediate product. Treatment of rice with compressed or supercritical carbon dioxide (scCO2) generates certain properties that have advantages for posterior processing [4]. It has been proved that carbon dioxide induces swelling of plant cell structures, which in its turn improves fluid accessibility to extractable substances and, therefore, enhances the yield during the extraction process [5]. One important drawback is the energetic balance of high pressure extraction [6]. Principally, a high electrical input is required for condensing the CO2 after the product separation step. To overcome this important disadvantage, closed solvent loops should be considered and big pressure steps
avoided. Consecutive processing steps have been designed in a way for improving the energetic balance in industrial processes, like decaffeination of green coffee beans, especially when the pressure level can be maintained at isobaric conditions throughout the process. To separate the extractable from the compressed solvent, an adsorption process can be included. However, this usually implies a further separation step for regenerating the adsorbent. Hence, the separation of the extract from the supercritical solvent needs to be envisaged in a smart way. One possibility is bonding the extracted substance to a substrate, guided in a certain way that the formulation of an end product is included in the process. A number of challenges need to be overcome: (i) Different kinetics of the materials in the different process steps (ii) Alteration of substrate properties (iii) Technical challenges, such as isobaric pumps (hermetic pumps) (iv) Product retrieval This chapter shows how different processing mechanisms may fit together to an integrated process, concentrating it on three steps: extraction, sorption, and formulation (Fig. 13.1). Different examples are given for research, pilot, and industrial-scale processes, focusing on a novel process for manufacturing antibacterial scaffolds or devices based on phytoextracts and biopolymers.
