Table 4. Results of the central composite design response surface regression analysis for benzoic acid production

Parameter	DF	Estimate	Standard error	T value	Significant level
Intercept	1	74.411029	5.621842	13.24	< 0.0001
a	1	−3.043319	0.278768	−10.92	< 0.0001
b	1	116.262240	36.399545	3.19	< 0.0001
c	1	−428.248725	36.399545	−11.77	< 0.0001
a × a	1	0.037506	0.003514	10.67	< 0.0001
b × a	1	−5.486842	0.884150	−6.21	< 0.0001
b × b	1	4816.854099	180.319220	26.71	< 0.0001
c × a	1	8.936404	0.884150	10.11	< 0.0001
c × b	1	−214.750000	222.506215	−9.63	< 0.0001
c × c	1	2050.187433	180.319220	11.37	< 0.0001
d × a	1	−2.807018	0.884150	−3.17	< 0.005
d × b	1	−1443.750000	222.506215	−6.49	< 0.0001
d × c	1	1585.416667	222.506215	7.13	< 0.0001
d × d	1	2471.020766	180.319220	13.70	< 0.0001