Production of nanocrystalline cellulose from rice husk
Creators: name: ISLAM, Md Sakinul
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)
classificationCode: 400499
subject: Chemical engineering not elsewhere classified
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: One of the greatest challenges of the 21th century for world scientific organization is to produce nanocrystalline cellulose (NCC) on a large scale from low cost raw materials. Currently, a significant number of research has been conducted for the production of nanostructured biomaterials, also known as bio-nano-materials, from renewable resources. NCC is one of the best example of bio-nano-materials, which can be produced from cellulose (~ C6H10O5 ~) by acid hydrolysis.
  
NCC is 100% biodegradable and biocompatible.  It is also nontoxic, renewable and eco-friendly nano-material. It has enormous and potential applications in personal care, chemical, food, medical science, pharmaceutical, and engineering materials, etc. However, a lot of investment has been paid for the industrial production of NCC, but no significant outcome has been developed. Hence, the motivation and rationale of the current research is important. 
 
The main problems associated with the NCC production are low production rate, long reaction time, difficulties in lignin separation and cellulose degradation products (CDPs). The common CDPs are glucose, gluconic acid, formic acid, acetic acid, glycolic acid, etc. In addition, NCC production is also associated with other degradation products (DPs), such as arabinose, xylose, furfural, etc., that occur from various chemical modifications of raw biomass. DPs contaminate NCC and significantly reduce its production. However, to separate these DPs from NCC is difficult and expensive. The possible reasons for the formation of DPs are the poor technology and unoptimized process parameters. On the other hand, separation of lignin, which is called delignification, and other components from biomass is time consuming. Therefore, to produce NCC on a large scale the kinetics and process optimization are important. 

For this reason, further research needs to be conducted on the kinetics of delignification, bleaching and acid hydrolysis to optimise the cellulose extraction and minimise cellulose DPs.  Kinetics of delignification, bleaching and acid hydrolysis are also required to explore the engineering aspects associated with potential scale up of NCC production. Delignification plays an important role to produce highly crystalline NCC. In the delignification of biomass, non-cellulosic components and other impurities are generally removed to produce highly pure and crystalline cellulose pulp (CP).  

Process intensification using catalyst is one of the most efficient techniques for biomass pretreatment with faster kinetics. The utilization of a high or low pressure reactor is an excellent alternative to using a random reaction vessel. The rheology of NCC suspension has not been widely explored in every aspect of NCC analysis but this is very important tool to investigate the mass flow rate, viscosity and transport phenomena for large scale production.  

In this research, NCC has been successfully produced from rice husk (RH) by chemical extraction process using low and high pressure delignification processes (LPDP and HPDP) with optimization and kinetic studies.  The three general steps of chemical extraction process for NCC production are delignification, bleaching and acid hydrolysis ¿ all of which have been used in this study. The kinetic studies have been systematically carried out on the alkaline delignification and acid hydrolysis stages of the chemical extraction process. Alkaline delignification of RH using NaOH was mainly performed in high and low pressure reactors with some specific engineering aspects under predetermined experimental conditions. A jacketed glass reactor (JGR) was used as a low pressure reactor (LPR) for LPDP, while a stainless steel reactor (SSR) was used as a high pressure reactor (HPR) for HPDP at 5-10 bars pressurised condition.  The bleaching experiment was performed using NaOCl solution for producing bleached rice husk pulp (B-RHP) from delignified rice husk pulp (D-RHP). Finally, NCC suspension was produced from B-RHP by acid hydrolysis under critical experimental conditions. NCC suspension was then processed by dialysis and sonication to remove dissolved gas and particulates. The dialysed and sonicated sample was then neutralized for characterization. 

For the characterization of NCC, as produced via high and low pressure processes, various tests such as TAPPI, FT-IR, XRD, XRF, CHNO, HPLC, ICP-MS, GC-MS, SEM, EDX, TEM, AFM, TGA and MDSC were performed. The rheological analysis of NCC suspension obtained from LPP was performed using the HR3 and ARES rheometers (TA instruments, USA). The linear viscoelasticity and non-linear viscoelasticity of 10%, 15% and 20 wt.% NCC suspension were investigated at 10 °C, 20 °C and 30 °C. 

The experimental design for optimization was done by response surface method (RSM) using Minitab®2013 software. The Box-Behnken experimental design was adopted during the software operation for optimization. The design was established with respect to four experimental parameters such as time (t), temperature (T), concentration (C) and stirring (rpm). The optimization of delignification, bleaching and acid hydrolysis was conducted in the LPR under atmospheric pressure.

The kinetic modelling of alkaline delignification and acid hydrolysis was extensively studied in various aspects with four experimental parameters such as time, temperature, concentration, and RPM using LPR. The kinetic calculation and data analysis were performed by SigmaPlot and Microsoft Excel softwares. High quality NCC has successfully been produced from RH biomass by chemical extraction process by LPDP and HPDP using LPR and HPR, respectively. The major findings of NCC characterization, process optimization, process development, the kinetics of alkaline delignification and acid hydrolysis are summarised below.

In LPDP, the crystallinity of NCC from XRD was found to be 61%. Particle diameter and length from morphological analysis were found to be 15-50 nm and 100-500 nm, respectively. From the TAPPI and NREL analysis, the purity of NCC was found to be 90-95%. The overall production rate of NCC by LPDP was ~95-97% (Batch: LPDP06) based on bleached rice husk pulp (BRHP).

On the other hand, the crystallinity of NCC produced by HPDP was found to be 72%. Particle diameter was found to be 10-40 nm from morphological analysis. From the TAPPI and NREL analyses the purity of NCC was found to be ~90-93%. The overall production rate of NCC by HPDP was ~90-94% (HPDP05) based on bleached rice husk pulp (BRHP).

The difference between the two processes (LPDP and HPDP), HPDP was faster than the LPDP in relation to producing NCC from RH biomass. The utilization of the LPR and HPR for alkaline delignification of RH was a new approach for converting RH to D-RHP and CP. The catalytic bleaching was found to be better than non-catalytic bleaching as found from the optimization of the bleaching process. The 9-15 hours alkaline delignification of RH was found to be effective at 80 °C with 4M NaOH.

The kinetic modelling of alkaline delignification was performed by LPDP using LPR. From this modelling study, the exponential and empirical delignification models have been developed for the standard pulping process to produce NCC from RH.

The effective acid hydrolysis process has been developed for the mass production of NCC from RH cellulose pulp, illustrating the formation of CDPs (i.e. glucose and xylose). The modelling of glucose and xylose has been performed to investigate the impact of acid on CP production (CP extracted from RH). The sugar formation in the acid hydrolysis of RH cellulose pulp was found to follow first-order kinetics. On the basis of all experimental investigations, it has been concluded that, the current research enables the production of high-quality cellulose pulp which is considered to be a good approach for industrial production of NCC.