Effect of Organic Solvents on Microalgae Growth, Metabolism and Industrial Bioproduct Extraction: A Review

In this review, the effect of organic solvents on microalgae cultures from molecular to industrial scale is presented. Traditional organic solvents and solvents of new generation-ionic liquids (ILs), are considered. Alterations in microalgal cell metabolism and synthesis of target products (pigments, proteins, lipids), as a result of exposure to organic solvents, are summarized. Applications of organic solvents as a carbon source for microalgal growth and production of target molecules are discussed. Possible implementation of various industrial effluents containing organic solvents into microalgal cultivation media, is evaluated. The effect of organic solvents on extraction of target compounds from microalgae is also considered. Techniques for lipid and carotenoid extraction from viable microalgal biomass (milking methods) and dead microalgal biomass (classical methods) are depicted. Moreover, the economic survey of lipid and carotenoid extraction from microalgae biomass, by means of different techniques and solvents, is conducted.


Calculation Procedure
Fundamental energy requirements and production cost were analysed for isolation of demanded product. The analyses were carried out in simplified form under following assumptions: 1) total solvent recovery, 2) no heat losses, 3) no heat recovery and 4) equipment amortization is not taken into account. Figure S1. Scheme of Calculation Procedure. Figure S1. shows a model for the calculation procedure. All lab-scale technologies are composed of these technological steps -pretreatment, extraction and solvent recovery including its recycling. The specific energy requirement ESEP (J kg -1 ) and the specific production cost CSEP (€ kg -1 ) of separation process used were calculated as follows: where ETOTAL is total energy requirement of separation process (J), CTOTAL is total costs for product separation (€) and mPRODUCT is weight of the product (kg) defined as where mwB is the mass of wet biomass (kg), wdB is mass fraction of dried biomass (-) and yproduct is the yield of product related to dried biomass (-).
The total energy demand of extraction using liquid solvent was calculated: where EPT is the energy needed for pretreatment (J), EEM is the energy needed for mixing during extraction (J), ESSP is the energy needed for solvent separation from an extract (J) and ESC is the energy needed for reverse solvent condensation (J).
The energy requirement needed for pretreatment EPT was calculated: where PPT is the power input of equipment used for pretreatment (W), VPT is the volume of pretreated mixture (m 3 ), tPT is the time of pretreatment (s) and εPT is the specific power requirement of pretreatment (W m -3 ).
The energy requirement needed for mixing during extraction was calculated: where εEM is the specific power input for mixing (W m -3 ), VEM is the volume of mixture during extraction (m 3 ), tEM is the time of mixing during extraction (s). The specific power input for mixing εEM = 300 W m -3 was assumed for calculation.
Assuming that the multi-component solvent is totally separated from an extract by the evaporation the energy needed for separation was calculated in simplified form as follows: where H vap j (T) is the heat of vaporization of j th component of the solvent solution (J kg -1 ) at temperature T (K) and mS-j is the mass of j th component of the solvent solution (kg). The heat of vaporization was calculated using following formula: where A,  and  are parameters overtaken from NIST database for given component, Tr is reduced temperature calculated as ratio of temperature T and critical temperature Tc of given component. The evaporation at normal pressure was assumed. The heats of vaporization were calculated at normal boiling temperature for given component. Assuming that the reverse condensation of solvent components occurs at the same conditions as evaporation the energy needed for condensation ESC equals to ESSP.
The total cost for extraction process was calculated: where CCH is the cost of chemicals (€), CPT is the price of electricity required for pretreatment (€), CEM is the price of electricity required for mixing during extraction (€), CSSP is the price of water steam needed for solvent evaporation (€) and CSC is the price of cooling water needed for reverse solvent condensation (€).
The prices of electricity needed for pretreatment and for mixing during extraction were calculated as follows: where cel is the price of electricity (€ MJ -1 ).
The condensation of saturated water steam was assumed as an energy source for solvent evaporation. The price of water steam needed was calculated: where csteam is the price of water steam (€ kg -1 ) and H cond steam (Tcond) is the heat of condensation of water steam at condensation temperature Tcond. The saturated water steam at temperature of 150°C was assumed for solvent evaporation. The price of cooling water needed for solvent condensation was calculated: where ccw is the price of cooling water (€ kg -1 ), cpcw is the specific heat capacity of cooling water (J kg -1 K -1 ) and Tcw is allowed temperature increase of cooling water. The allowed temperature increase of 15 K and specific heat capacity of cooling water of 4 182 J kg -1 K -1 were assumed and used for calculation.
The error of presented estimations is 20 % in maximum for both energy requirement and production costs.

Supercritical Extraction Technology
The supercritical extraction was calculated under following assumptions: 1) two-stage solvent compression with inter-and after cooling of compressed solvent, 2) reversible adiabatic compression, 3) adiabatic efficiency of 60 % for irreversible compression, 4) mechanical efficiency of 96 % of driving unit, 5) inlet temperature of 20°C and pressure of 101.325 kPa of the solvent before first-stage compression, 6) outlet solvent temperature from coolers equals to extraction temperature reported in the cited article and 7) Poisson constant  = 1.29.
The total energy requirement of supercritical extraction was calculated as