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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Creating Fuel from Algae

Problem

Green tides are overgrowths of algae that occur in response to many factors, including excess nutrients in marine water. Up to 10start superscript, 6, end superscript tons of a particular algae, E. prolifera, can be collected from a single green tide event. Rather than waste this biomass, researchers attempt to convert the algae into a renewable source of fuel called bio-oil using a process known as hydrothermal liquefaction (HTL). In HTL, water is added to dried algae; the mixture brought to sufficiently elevated temperature and pressure conditions to allow the water to decompose the algal biomass. After the reaction mixture is cooled, it can be separated according to the scheme shown in Figure 1.
Figure 1: Schematic diagram for bio-oil production and separation
Figure 2: Constituents of Bio-Oil. Analysis of the bio-oil indicate that it is composed of over 180 compounds. The top five most common constituent molecules are shown below.
CompoundNameFormula
1hexadecanoic acidCHstart subscript, 3, end subscript(CHstart subscript, 2, end subscript)start subscript, 13, end subscriptCOOH
2oleic acidCHstart subscript, 3, end subscript(CHstart subscript, 2, end subscript)start subscript, 7, end subscriptCHCH(CHstart subscript, 2, end subscript)start subscript, 7, end subscriptCOOH
32-methyl-2-cyclopenten-1-oneCstart subscript, 6, end subscriptHstart subscript, 8, end subscriptO
43-methyl-2-cyclopenten-1-oneCstart subscript, 6, end subscriptHstart subscript, 8, end subscriptO
52,3-dimethyl-2-cyclopenten-1-oneCstart subscript, 7, end subscriptHstart subscript, 10, end subscriptO
Adapted from: Zhou, D., Zhang, L., Zhang, S., Fu, H., & Chen, J. (2010). Hydrothermal liquefaction of macroalgae Enteromorpha prolifera to bio-oil. Energy & Fuels, 24(7), 4054-4061; Zhou, D., Zhang, S., Fu, H., & Chen, J. (2012). Liquefaction of macroalgae Enteromorpha prolifera in sub-/supercritical alcohols: direct production of ester compounds. Energy & Fuels, 26(4), 2342-2351.
The researchers could have used many solvents in extracting the oil from the aqueous reaction mixture. What characteristic of CHstart subscript, 2, end subscriptClstart subscript, 2, end subscript makes it a better solvent than hexane for this procedure?
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