2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 89 -
linear, quadratic and interaction effects of the parameters
are given in
Table III
. A posit
ive sign in the ‘t’ value
indicated a synergistic effect, while a negative sign
represented an antagonistic effect of the parameters on
the ethanol concentration.
TABLE
II
E
XPERIMENTAL DESIGN FOR ETHANOL PRODUCTION USING RSM
Run X
1
X
2
X
3
X
4
Ethanol production
(g/l)
1
0.5
2
2
20
31
2
2
0.5
2
20
48
3
1.25 1.25 1.15 12.5
79
4
2
2
2
5
31.78
5
1.25 1.25 1.15
20
69.94
6
2
0.5
0.3
20
56.75
7
0.5
0.5
0.3
5
33.56
8
1.25 1.25 1.15 12.5
79.45
9
0.5
2
0.3
20
38
10
1.25 1.25 1.15 12.5
79.45
11
1.25 1.25 1.15 12.5
79.45
12
1.25
0.5
1.15 12.5
76.02
13
1.25 1.25 1.15 12.5
79.45
14
2
2
0.3
5
22.7
15
2
1.25 1.15 12.5
64.97
16
1.25 1.25
2
12.5
67.67
17
0.5
0.5
2
5
20.43
18
1.25 1.25 1.15
5
60.86
19
1.25 1.25
0.3 12.5
74.48
20
0.5
1.25 1.15 12.5
65.4
21
1.25
2
1.15 12.5
74.48
T
ABLE
III
S
UMARY OF MODEL COEFFICIENT ESTIMATE BY MULTIPLES LINEAR
REGRESSION
Relationship Factor
Coefficient
‘t’ Value
‘P’ Value
Intercept
-48.2
-
-
Main effects
(Linear)
X
1
62.5
-0.34
0.7445
X
2
34.9
-1.22
0.2674
X
3
18.4
-9.45
<0.0001
X
4
7.8
7.21
0.0004
Interactions
(pure
quadratic)
X
1
2
-27
-27.21
<0.0001
X
2
2
-9.1
-9.16
<0.0001
X
3
2
-12.9
-16.65
<0.0001
X
4
2
-0.3
-26.8
<0.0001
Interactions
(Cross
product)
X
1
X
2
-6.4
-5.14
0.0021
X
1
X
3
4
8.12
0.0002
X
1
X
4
0.7
5.17
0.0021
X
2
X
3
4.7
9.51
<0.0001
X
2
X
4
-0.8
-6.74
0.0005
X
3
X
4
0.2
-4.64
0.0035
Ethanol concentration was significantly affected by the
synergistic effect of the linear terms of inoculums size.
This result suggested that the inoculums size had a direct
relationship with the production of ethanol from FWH,
i.e. any minor change in this variable may cause a great
positive or negative change in ethanol concentration.
Besides, the quadratic terms (X
12
, X
22,
X
13,
X
14
, X
23
and
X
34
) showed the synergistic effects on the ethanol
concentration, typically (X
12,
X
14
and X
23
) (P < 0.001).
A. Optimization Conditions for Ethanol Production
Surface and contour plots demonstrating the effects of
different parameters, two parameters varied while
keeping the third parameters at middle level, on the
ethanol concentration were shown in
Figs. 1
. The
stationary points were examined by analyzing these plots.
Generally, circular contour plots indicate that the
interactions between parameters are negligible. On the
contrary, elliptical ones indicate the evidence of the
interactions
[34]
. From the plots, it was easy and
convenient to understand the interactions between two
nutrients and also to locate the optimum levels.
Fig. 1A
showed the effect of KH
2
PO
4
and (NH
4
)
2
SO
4
on the
ethanol concentration. The convex response surface
suggested well-defined optimum variables (KH
2
PO
4
and
(NH
4
)
2
SO
4
) and that the ethanol concentration increased
to the peak with the increase of KH
2
PO
4
and (NH
4
)
2
SO
4
up to 0.83 and 1.44 g/l, respectively; then declined with
the further increase of these two parameters. This result
demonstrated that the response surface had a maximum
point for ethanol production.
Fig. 1B
showed the effect of
yeast extract and (NH
4
)
2
SO
4
on the ethanol concentration.
The convex response surface suggested well-defined
optimum variables (yeast extract and (NH
4
)
2
SO
4
) and that
the ethanol concentration increased to the peak with the
increase of yeast extract and (NH
4
)
2
SO
4
up to 0.9 and
1.44 g/l, respectively; then declined with the further
increase of these two parameters.
Fig. 1C
showed the
effect of inoculum size and (NH
4
)
2
SO
4
on the ethanol
concentration. The convex response surface suggested
well-defined optimum variables (inoculum size and
(NH
4
)
2
SO
4
) and that the ethanol concentration increased
to the peak with the increase of inoculum size and
(NH
4
)
2
SO
4
up to 15% and 1.44 g/l, respectively; then
declined with the further increase of these two
parameters. This result demonstrated that the response
surface had a maximum point for ethanol production
(79.5 g/l).
Fig. 1D
shows the effect of yeast extract and
K
2
HPO
4
on ethanol production. The equation
demonstrated that interaction between inoculum size and
KH
2
PO
4
showed highly significance. At the middle
concentration of yeast extract (0.9 g/l) and middle
concentration of K
2
HPO
4
(0.93 g/l)
gave maximum
ethanol production (79.5 g/l), a further increase in
concentration of yeast extract and K
2
HPO
4
the trend was
reversed. In a relative low concentration yeast extract and
K
2
HPO
4
, optimum ethanol production could be attained.
Fig. 1E
showed the effect of inoculum size and KH
2
PO
4
on the ethanol concentration. An increase in
concentration of inoculum size with KH
2
PO
4
concentration increased the ethanol production, but at
high KH
2
PO
4
(higher than 0.93 g/l), ethanol production
decreased.
