Reviewer 1 |
Reviewer 1 |
What is novel? There are many papers that use silica from corn cob ash
to make biodiesel, so please clarify what is different about your study.
You should compare results from these similar studies to your
results.
|
Thank you for your comment. The comparison results from the
similar studies and the statement of novelty has now been added
to the Introduction section (paragraph 3 & 4), as follows:
Rice husk is abundantly available as agricultural waste in many
rice-producing countries, including Malaysia. Rice husk has been
reported to have high silica content (90-97%) (Della et al.,
2002), which indicates its potential as silica-based heterogeneous
catalyst for biodiesel production. At the same time, 2.5 to 3 Mt of palm
fatty acid distillate (PFAD), a lower value by-product of palm oil
processing is generated worldwide (Idris et al., 2019). The PFAD is rich
in fatty acids and is readily available as biodiesel feedstock. Rice
husk ash (RHA) has been studied as catalyst precursor for soybean oil
biodiesel (Chen et al., 2013), palm oil biodiesel (Chen et al., 2015;
Roschat et al., 2016), and waste cooking oil to biodiesel (Hazmi et al.,
2020). Hindirawaty et al. (2020) studied RHA as support material for
NiSO4 as a solid acid catalyst in PFAD conversion to
methyl ester at different MeOH to oil molar ratio, catalyst amount and
reusability. PFAD conversion to biodiesel was also studied using kenaf
seed cake-based sulfonated catalyst (Akinfalabi et al., 2019). However,
to our best knowledge, no work has been published on RHA as catalyst
support for H2SO4 as SiA and and NaOH as
solid base catalyst for PFAD conversion to biodiesel.
Thus, this work focused on the synthesis of silica-based catalyst
support from RHA impregnated with H2SO4
(forming Huskcatacid) and
NaOH (forming Huskcatbase), respectively. PFAD was used as fatty
acid source to examine the potential application of the prepared
catalyst for biodiesel production. The formation of biodiesel from PFAD
was performed via esterification using Huskcatacid,
followed by transesterification using Huskcatbase. The biodiesel
produced was evaluated based on several reaction parameters such as oil
to methanol ratio, amount of catalyst, reaction times and temperature
and the optimization of parameters was carried out.
|
It took a few reads to understand the first sentence of the abstract. I
would delete ”using a facile method” for clarity or move it to the end
of the sentence, otherwise it sounds like transesterification was the
facile method.
|
Thank you for your suggestion. The phrase ”using a facile method”
has been deleted and the abstract has been revised, as
follows:
Rice husk ash (RHA) has potential as low-cost catalyst support to
produce biodiesel. In this study, RHA was used as precursor for
heterogeneous silica-based catalyst, Huskcatacid and
Huskcatbase for biodiesel production from palm fatty acid
distillate (PFAD). The formation
of biodiesel from PFAD was performed via esterification using
Huskcatacid, followed by transesterification using
Huskcatbase. The biodiesel produced was evaluated based on
several reaction parameters such as oil to methanol ratio, amount of
catalyst, reaction times and temperature and the optimization of
parameters was carried out. Results showed that Huskcatacid was
efficiently employed in esterification of PFAD to afford 91.6% ester in
1:1 (PFAD:MeOH) and 5 wt% catalyst, followed by transesterification in
1:1 (oil: MeOH) using Huskcatbase (1 wt%) to produce 97.5%
biodiesel in 30 min, with high percentage of methyl palmitate (34.43%)
and methyl oleate (57.86%). Both Huskcatacid and
Huskcatbase have high porosity (2.726 x
10-3
cm3/g and 4.985
x 10-3 cm3/g) and surface
area (7.362 m2/g and 14.493
m2/g), respectively, which offer efficient
esterification, easy glycerol separation and reusability up to three
times to afford biodiesel ranging from 90-97% yield. The biodiesel
obtained was applied on Megatech®-Mark III engine and
demonstrated a proportional increased of torque (ɽ) with the B100
biodiesel loading. Rice husk demonstrated a good potential as solid
support heterogeneous catalysts and feedstock for value-added products,
which also assists to overcome
agricultural waste management issues.
Keywords: rice husk ash; heterogeneous catalysts;
esterification; biodiesel; FFA conversion
|
Delete the sentence saying biodiesel competes with food for vegetable
oil because that’s generally inaccurate and spreads misconceptions about
biodiesel. The vegetable oil most commonly used is excess soybean oil
from growing crops for animal feed. When we grow soybeans for animal
feed, we get more oil than we eat.
|
Thank you for your comment. The sentence implying biodiesel competes
with food has been deleted in the revised manuscript. The whole
Introduction section has now been rewritten to focus on
the catalyst for biodiesel production (previously focused on biodiesel
production). Revised Introduction section is as follows:
Catalyst plays a vital role in the
transesterification reaction of free fatty acids (FFA) to form
biodiesel. Thus, the synthesis of ideal catalyst for biodiesel
production has gained attention of researchers in recent decade.
Sulfuric acid (H2SO4) and hydrochloric
acid (HCl) are common acid catalysts used to accelerate the
transesterification process for a higher yield of biodiesel (Chongkhong
et al., 2007; Canakci and Van
Gerpen, 1999; Fadhil et al., 2012; Zuo et al., 2013). Major issues of
acid catalysts are the corrosive effect on the reactor and high amount
of alcohol required, which incur high production cost (Leung et al.,
(2010). Homogeneous base-catalyst such as sodium hydroxide (NaOH) and
potassium hydroxide (KOH) exhibited faster and non-corrosive reaction
during production of biodiesel (Ma and Hanna (1999). However,
homogeneous base-catalyst suffers disadvantage due to saponification
effect which reduce the yield of biodiesel
(Leung et al., 2010; Romero et
al., 2011). Heterogeneous catalysts like tungstate (Sunita et al.,
2008), ferric alginates (Peng-Lim et al., 2012), silica nanoparticles
(Yang et al., 2017) and titanium dioxide (Carlucci et al., 2019) were
then explored as alternatives available to homogenous biodiesel
catalysts. However, the drawbacks of these catalyst are costly
materials, complicated synthesis routes, and low catalytic activity
(Akinfalabi et al., 2019).
Heterogeneous base-catalyst such as calcium oxide and silica has been
reported to produce a higher yield of biodiesel compared to homogeneous
base catalyst (Leung et al., 2010), but the catalyst has a difficulty to
diffuse into the product mixture to form oil, alcohol and catalyst
(Mbaraka and Shanks, 2006). This limitation has led to the introduction
of catalyst support for heterogenous base-catalyst increase the reaction
with triacylglycerol (Zabeti et al., 2010). Catalyst support is a solid
carrier impregnated with an active
species (Mabena et al., (2011). For instance,
NaOH/Al2O3, NaOH/SiO2
and ZS/Si has been used as catalyst supports in the production of
biodiesel (Taufiq-Yap et al., 2011; Jacobson et al., 2008; Arzamendi et
al., 2007).
The use of biomass-based catalyst from agricultural waste in biodiesel
production reduce the overall cost of biodiesel production. For
instance, silica based catalyst from corn germs and corn cobs gave
optimum production of corn oil biodiesel with catalyst amount of only
19.8% (Milan et al 2018). Silica-based heterogeneous catalysts derived
from Imperata cylindrica offers convenient esterification
(>90 wt% oil) and transesterification to afford 80%
biodiesel (Ngaini et al., 2016). However, Imperata
cylindrica has low silica content (1-8%), which requires a lot of
feedstock for catalyst preparation.
Rice husk is abundantly available as agricultural waste in many
rice-producing countries, including Malaysia. Rice husk has been
reported to have high silica content (90-97%) (Della et al.,
2002), which indicates its potential as silica-based heterogeneous
catalyst for biodiesel production. At the same time, 2.5 to 3 Mt of palm
fatty acid distillate (PFAD), a lower value by-product of palm oil
processing is generated worldwide (Idris et al., 2019). The PFAD is rich
in fatty acids and is readily available as biodiesel feedstock. Rice
husk ash (RHA) has been studied as catalyst precursor for soybean oil
biodiesel (Chen et al., 2013), palm oil biodiesel (Chen et al., 2015;
Roschat et al., 2016), and waste cooking oil to biodiesel (Hazmi et al.,
2020). Hindirawaty et al. (2020) studied RHA as support material for
NiSO4 as a solid acid catalyst in PFAD conversion to
methyl ester at different MeOH to oil molar ratio, catalyst amount and
reusability. PFAD conversion to biodiesel was also studied using kenaf
seed cake-based sulfonated catalyst (Akinfalabi et al., 2019). However,
to our best knowledge, no work has been published on RHA as catalyst
support for H2SO4 as SiA and and NaOH as
solid base catalyst for PFAD conversion to biodiesel.
Thus, this work focused on the synthesis of silica-based catalyst
support from RHA impregnated with H2SO4
(forming Huskcatacid) and NaOH (forming Huskcatbase),
respectively. PFAD was used as fatty acid source to examine the
potential application of the prepared catalyst for biodiesel production.
The formation of biodiesel from PFAD was performed via
esterification using Huskcatacid, followed by transesterification
using Huskcatbase. The biodiesel produced was evaluated based on
several reaction parameters such as oil to methanol ratio, amount of
catalyst, reaction times and temperature and the optimization of
parameters was carried out.
|
Make sure to include all necessary details to repeat your experiments.
For example, how was sulfonation executed? What temperature and volume
was the activated ash soaked in NaOH? Was there mixing during
esterification and transesterification?
|
The details of experimental procedures have been added for clarity, as
follows:
Preparation of activated rice husk ash
Rice husk (1000 g) were cleaned and dried in the oven for 3 days. The
dried rice husk was calcined in a closed furnace at 800°C for 16 h,
cooled to room temperature to obtain rice husk ash RHA (230.0 g). RHA
(50.0 g) was soaked in NaOH solution (50 mL, 20%) and stirred for 2 h
at room temperature. The mixture was filtered, rinsed with distilled
water and dried for 24 h. Concentrated sulphuric acid (99%
H2SO4) was added dropwise, stirred and
washed with distilled water to pH 7 (Wahi and Senghie, 2011). The sample
was dried in oven at 80 °C for 24 h to produce activated rice husk ash
(ARHA) (47.0 g).
Preparation of Huskcatacid
ARHA (20.0 g) was soaked in concentrated H2SO4 (20 mL, 95%) and stirred
for 30 min, decanted heated in a furnace for 12 h at 100°C (Kastner et
al., 2012). The sample was cooled and rinsed with distilled water to pH
2. The sample was dried in the oven at 80 °C for 24 h, cooled to room
temperature to produce Huskcatacid (18.9 g) and stored in desiccator
prior to use.
Preparation of Huskcatbase
ARHA (20.0 g) was stirred in NaOH solution (20 mL, 5M) for 24 h at room
temperature. Excess NaOH solution was decanted and the solid was heated
at 700°C for 10 min (Refaat, 2011). The sample was cooled to room
temperature to yield Huskcatbase (19.2 g) and kept in desiccator prior
to use.
|
Where are the error bars on the % conversion and torque graphs? How
many independent replicates were completed? |
The measurement was
conducted manually, therefor the error bars on the % conversion and
torque graphs was not obtained no replication was
performed. |
The formatting for your equations is inconsistent. Equation 4 is
formatted fine, but equations 1 and 2 should be formatted like equation
4 (2 lines for a fraction). Also equation 4 should be numbered equation
3 because there is no equation 3.
|
Equations 1 and 2 has been formatted as required. Equation 4
has been renumbered as Equation 3 in the revised manuscript, as
follows:
\[Acid\ value,\frac{\text{mgKOH}}{g}=\ \frac{N_{\text{KOH}}\text{\ x\ }\text{Mw}_{\text{KOH}}\text{\ x}\text{\ V}_{t}}{W_{s}}x100\%\ \ (Eq.\ 1)\]
\(FFA,\%=\frac{\text{AV}_{s}}{2}x100\%\ \ (Eq.\ 2)\)
\(Conversion,\ \%=\frac{Weight\ of\ biodiesel\ produced,\ g}{Weight\ of\ feedstock,\ g}x100\%\ \ (Eq.\ 3)\)
|
You say there’s only a small amount of smoke emission, but did you
quantify that? You should have data to support that claim. Just saying
the smoke is normally more black is not adequate.
|
Thank you very much for the comment. The data was obtained during
application of biodiesel via observation study. This could be important
for future work which focusing on biodiesel rather than the solid
support catalyst. We have therefor, revised the sentences in
the following sections, and add statement on the required
quantification of smoke data.
Combustion capability
test
The combustion capability test of PFAD biodiesel (B100) was demonstrated
on the alternative diesel engine (Megatech- Mark III) (Fig. 8). Ethanol
was initially introduced into the combustion chamber to achieve
400-500°C before B100 was introduced. It was observed that, a bright
orange flame with less smoke was released upon combustion of B100 in the
chamber in 10-15 mins. In contrast, CD has been reported to emit black
smoke due to incomplete combustion (Akihama, et al., 2002). A slight,
odourless smoke was produced during combustion of B100 indicated a
sufficient and complete combustion of the engine. However, the smoke
observed through visual and relative smell observation, which require
further quantification for verification.
CONCLUSION
Huskcatacid and Huskcatbase have been successfully
developed from rice husk ash and employed as heterogenous solid support
for the production of biodiesel from PFAD. The catalysts were
conveniently prepared and employed in the esterification and
transesterification reaction to afford high yield of biodiesel
(>97% yield) with reusability up to three times. The
biodiesel performance was demonstrated on alternative diesel engine and
visual observation showed only small amount of smoke
emission compared to the commercial diesel. This study indicated that
rice husk demonstrated a good potential as solid support heterogeneous
catalysts and feedstock for value-added products, which also assists to
overcome agricultural waste management issues.
|
Reviewer 2 |
Reviewer 2 |
This manuscript describes the synthesis and applicatoin of rice husk
derived ’catalysts’ for the synthesis of biodiesel from Palm Fatty Acid
Distillate. The authors describe the synthesis of H2SO4 and NaOH
impregnated rice husk derived silica and their application in
transesterification/esterification of Palm Fatty Acid Distillate. While
the use of such materials may aid production of biodiesel the materials
cannot be described as a heterogeneous catalyst. These are merely
supported reagents and as a consequence of their leaching there is
little improvement in the overall process as a requirement for quenching
and removal of alkali impurities are still required. For this reason I
cannot recommend publication. If however the authors discuss their work
in the context of a supported reagent and benefits over homogeneous
catalysts then the manuscript could be reconsidered.
|
Thank you for the suggestion. The title, abstract and
Introduction section have been revised to discussed
H2SO4 and NaOH impregnated rice husk ash
as catalyst support.
New Title: Rice Husk Ash as Heterogeneous Silica-Based Catalyst Support
for Palm Fatty Acid Distillate Conversion to Biodiesel
The revised abstract and Introduction section is per our
response to Reviewer 1.
|
Specific comments
[2]1.There is no elemental analysis on
the final materials to quantify acid and base site loading.
|
Thank you. Quantitative element composition of the of Huskcatacid
and Huskcatbase was characterized using X-Ray Diffraction (XRD)
and Energy Dispersive X-ray (EDX) analyses (as depited in Table 1).
Other reported, similar studies used the same approach in
characterization of biodiesel catalysts (Fu, et al., 2011; Fu, et al.,
2012; Akinfalabi et al., 2019; Hyndryawati et al., 2020; Idris et al.,
2020).
|