Project LK0806: Sequential Extraction of Value-Added Products from Wheat Straw

Project LK0806: Sequential Extraction of Value-Added Products from Wheat Straw

Executive summary

Background

A research project entitled ‘Multi-use approach to Cereal Straw Fractionation using Thermomechanical Pulping’ (THERMOPULP) funded by the LINK Crops for Industrial Use programme was completed in 1996. The project demonstrated the potential of wheat straw as a raw material for new markets, through replacement of non-UK materials or less ‘environmentally friendly’ products, with chemical fractions derived from wheat straw. However, the laboratory-based extraction protocols described in the project were considered difficult and costly to implement at scale, mainly because of the use of organic solvents and chlorine-based bleaching. It was proposed that more efficient fractionation could be achieved by developing and optimising chlorine- and solvent-free processes based on aqueous extraction procedures.

Objectives

The overall objective was to demonstrate the industrial viability of a modified laboratory-based process for the extraction of valuable chemical components from straw. The prime aims of this study were therefore:

• The isolation of the triglyceride wax using a solvent-free process
• The elimination of chlorine-based chemicals
• The demonstration of the validity of oxygen-based bleaching for the isolation of hemicellulose and lignin
• The comparison of hemicelluloses derived from the oxygen-and previous chlorine-based processes by structural elucidation and trial in application as paint extenders by ICI
• The transfer of technology from laboratory to pilot scale
• An economic and environmental assessment of the processes

Approaches

The initial consortium comprised The BioComposites Centre, ICI Paints, UCB-Films (now Surface Specialties, UCB), Marlin Chemicals, DuPont and British Sugar. At a crucial stage in the project, two partners (DuPont and British Sugar) withdrew and their roles were ultimately replaced in part by Cambridge Biopolymers for the remainder of the project.

The overall approach was as follows:

• Development of processes at the laboratory scale.
• Chemical characterisation of the products using wet chemical and spectroscopic methods
• Evaluation of laboratory samples by the industrial partners
• Evaluation of the economic and environmental impacts of the processes developed by the scientific performer.

The processes were developed using 1kg batch size. This was found to provide sufficient materials for industrial evaluation of cellulose and hemicellulose. Development of subsequent samples relied on feedback by the industrial partners consisting of performance data with suggestions for improved properties. Towards the final stages of the project, process flow diagrams were constructed and evaluated by the industrial partner.

Main results

Wax: It was found that a hot-water extraction process yielded 53% of the total lipophilic extractives compared with extraction with dichloromethane. This extract contained: free fatty acids (25.8-48.4%), waxes (9.4-27%), sterols (4.1-8.0%), triglycerides (3.3-11.0%) and sterol esters (2.6-5.1%). There were also minor amounts of diglycerides (0.3-0.5%), resin acids (0.5-3.1%) and phenolic compounds (0.9-3.6%). Of the free fatty acids detected, the most abundant were: decanoic acid (C10:0), tetradecanoic acid (C14:0), pentadecanoic acid C15:0), palmitic acid (C16:0), heptadecanoic acid (C17:0) and heneicosanoic acid (C21:0). The most abundant unsaturated free fatty acid was oleic acid (C18:0). The hot water extractives also contained several sterols of which the most interesting was beta-sitosterol which is a commercially significant nutraceutical currently sourced from the exotic plant Saw Palmetto. In addition, extract was a rich source of sterol esters including steryl laurate, steryl myristate, steryl palmitate, steryl heptadecanoate and steryl oleate. The industrial partner concluded that these nutraceutical compounds were more commercially significant than the wax as a whole.

Lignin: The wheat straw lignin was fully chemically characterised and its calorific value was determined. At present the main significance of lignin is as an energy source and this figure will be required when considering a process in which process liquors are combusted in order to provide energy for a ‘closed process’ in which as much energy as possible is recovered.

Hemicellulose: Extraction of wheat straw hemicellulose was based on extraction with alkaline peroxide and TAED followed by precipitation with alcohol. In all cases, the batch size was 1Kg dry weight of chopped straw. The typical yield was 200g dry hemicellulose. Various techniques were assessed including dialysis and ultrafiltration although precipitation by alcohol addition remained the most efficient harvesting technique. Industrial evaluation of the samples provided for paint manufacture indicated that the main barrier to utilisation was an apparent tendency for the hemicellulose to inhibit polymerisation in the early stages of latex synthesis. Several samples were successfully taken through to full paint production and one sample (2586F) gave a paint that satisfied 4 out of the 7 criteria set which represented a significant progress towards a fully conforming sample. Significantly, the sample satisfied the criteria pertaining to physical properties (C&P viscosity; R/T viscosity, scrub resistance and gel strength). The resources of the industrial partner throughout the project were severely limited with the effect of delaying the feedback of results to the research partner so that new samples could be generated on the basis of the industrial evaluation.

Cellulose: The residue remaining from hemicellulose extraction contained >80% cellulose and therefore only required a further bleaching step to give cellulose of good whiteness for viscose production. In these experiments the typical batch size was 300g dry cellulose pulp which was sufficient for industrial evaluation. Two Totally Chlorine-Free (TCF) protocols for producing bleached wheat straw cellulose were developed. These were: alkaline peroxide treatment; and digestion with a mixture of acetic and nitric acids. Both treatments gave good quality cellulose with low hemicellulose and lignin. When these were tested by the industrial partner, the main limitation was the short fibre length of the cellulose, which led to loss of fibre during pressing. It was also found that the ageing times required to reduce the degree of polymerisation (DP) to a suitable value were significantly higher than those required for wood pulps, although this is not necessarily a limitation.

Process Evaluation: The processes were evaluated by Cambridge BioPolymers. The evaluation mainly considered costs and ways of reducing the environmental impact by process stream reclamation. The conclusions were that if the estimated costs are considered commercially viable then the next phase of any development programme should include:

• Refining and optimising the extraction conditions to maximise efficiency and yield.
• Establishing the best phase separation method(s) for the various products for a large-scale process.
• Minimising the usage of chemicals, water, and energy.
• Analysing the composition the effluent streams generated from the process.

Areas for investigation should include recycling as much of the aqueous extraction medium as possible and the conservation of heat within the process. Furthermore, serious consideration should be given to mitigating the environmental impact of any effluents generated. This will be crucial in obtaining Integrated Pollution Prevention and Control (IPPC) permits for the operation of any industrial process.

Conclusions

Straw wax is best regarded as a mixed source of added-value compounds of which the most significant fractions are the triglycerides and phytosterols. At present, the best extraction procedure is solvent extraction although the environmental impact of this technique could be mitigated by using the solvent recovery methods employed by the pharmaceutical and perfume industries.

Straw hemicellulose can be readily produced by chemical methods and there are few barriers to its production. Incorporation into paint formulations is limited by its low solubility and tendency to inhibit the latex process, which is a vital step in paint manufacture. These two issues need to be addressed.

Straw cellulose has a short fibre length which limited its inclusion in the viscose process. The conclusion was that the cellulose could usefully be aimed at products either less dependent on fibre length, such as carboxymethylcellulose, or where short fibre is desirable such as high quality papers.

Implications

Wheat straw wax is a valuable source of triglycerides and phytonutrient compounds, some of which are derived from plants not native to the UK. As this is a growing market area, it is suggested that exploitation should be based on isolating and marketing these compounds.

Wheat straw cellulose has a short fibre length but a high degree of polymerisation. The current viscose process requires cellulose of a longer fibre length than is found in straw. However, there are many other industrial processes that require short fibre length cellulose. Two of these are the paper industry and the carboxymethylcellulose industries. Carboxymethylcellulose has a particularly diverse range of applications from drilling muds and adhesives to pharmaceutical products.

There is a constant demand by industry for water-soluble polymers. The paint industry has investigated both hemicellulose and starch as candidate paint extenders and at present, neither material satisfies all the stringent criteria for inclusion in this quality product. Wheat straw hemicellulose is a potential candidate providing its water solubility can be increased. One method, which was outside the scope of this project, may be to develop enzymatic methods for its production.