a0	specific surface, m2·m−3
A	function of time in desorption model
B	function of time in desorption model
dp	particle size, m
D12	binary diffusion coefficient, m2·s−1
e	extraction yield, kg (kg biomass)−1
kf	external mass transfer coefficient, m s−1
K	partition coefficient, kg biomass (kg CO2)−1
M	CO2 in extraction bed, kg
n	number of mixers
N	biomass feed, kg
p1,2	roots of the desorption model, s−1
P	pressure, MPa
q	(=q’t), solvent-to-feed, kg CO2 (kg biomass)−1
q’	(=Q’/N), specific flow rate, kg CO2 (kg biomass)−1·s−1
Q’	flow rate, kg·s−1
R	constant in desorption model
Re	(=Q’dp/(μSE)), Reynolds number
S	constant in desorption model
SE	cross section of extractor, m2
Sc	(=μ/(D12ρf), Schmidt number
Sh	(kfdp/D12), Sherwood number
t	time, s
tc	(=tf + ti), characteristic mass transfer resistance time, s
tf	(=ε/(kfa0)), external mass transfer resistance, s
ti	mass transfer resistance in particle pores, s
tr	(=M/Q’), residence time, s
T	temperature, °C
T1	estimate of time constant in desorption model, s
V	volume of extraction bed, m3
x	solid phase concentration, kg (kg biomass)−1
xt	transition solid phase concentration, kg (kg biomass)−1
xu	(=x0 + γ y0), content of extractable lipids in biomass, kg (kg biomass)−1
y	fluid phase concentration, kg (kg CO2)−1
ys	solubility, kg (kg CO2)−1
z	dimensionless axial co-ordinate
Greek letters
γ	(=M/N = q’tr), CO2 in extraction bed-to-feed ratio, kg kg−1
ε	void fraction
μ	viscosity, kg·m−1·s−1
ρf	CO2 density, kg·m−3
ρs	density of particles, kg·m−3
Subscripts
∞	asymptotic value
+	equilibrium
0-Jan-1900	initial value
1-Jan-1900	end of extraction of free solute
2-Jan-1900	start of desorption
a,b	two sets of adjusted model parameters
j	order of mixer