Separate Hydrolysis and Fermentation (SHF) of Lignocellulosic Biomass

Lignocellulose Hydrolysis

Enzymatic Hydrolysis

SHF Process and its Advantages

• Optimization of Individual Stages - The conditions that favour enzymatic hydrolysis (e.g. temperature, pH, and enzyme concentrations) do not necessarily coincide with those that are best for fermentation. In SHF, each process can be performed under its own optimal conditions. For instance, the hydrolysis stage can be carried out at a higher temperature, which is typically favorable for the enzymes but not for the fermenting microorganisms. This independent optimisation often results in higher yields of fermentable sugars from hydrolysis and better product yields from fermentation.
• Use of Well-Established Industrial Yeasts - SHF allows for the use of traditional, well-established yeast strains in the fermentation stage, at least when the fermentation of pentose sugars is not required (for example when the hemicellulose is removed in the pretreatment stage). These yeasts, such as Saccharomyces cerevisiae, are known for their robustness and high ethanol tolerance, which can result in high ethanol yields (where ethanol is the target of fermentation). These strains are also more resistant to inhibitors that might be present in the hydrolysate.
• Flexibility in Process Design and Operation - Due to the separate nature of the stages, different technologies and reactors can be used for the hydrolysis and fermentation steps, offering greater flexibility in process design. For instance, hydrolysis can be performed in a high-shear reactor to promote enzymatic activity, while fermentation can be conducted in a traditional stirred-tank fermenter.

Disadvantages of SHF

• Longer Processing Times - Since hydrolysis and fermentation occur separately, the total process time for SHF tends to be longer compared to simultaneous processes. This not only increases operational costs, but it also necessitates the use of holding tanks for storing the intermediate products, thus escalating the capital costs and infrastructure requirements.
• Product Inhibition During Hydrolysis - In the SHF process, the hydrolysis and fermentation stages are not simultaneous, which means that the sugars produced during hydrolysis do not immediately undergo fermentation. This means that the process is more likely to be affected by the inhibitory effect that high sugar concentrations can have on the enzymes involved in hydrolysis. This may result in the incomplete hydrolysis of cellulose and hemicellulose.
• Potential for Sugar Degradation - During the hydrolysis step, the produced sugars can degrade if they remain in the reactor for extended periods under acidic or high-temperature conditions, resulting in reduced product yields.

Simultaneous Saccharification and Fermentation (SSF)

• Independent Optimization - The separate nature of the hydrolysis and fermentation stages allows each process to be optimized independently for parameters such as temperature, pH, and enzyme concentration. In contrast, in SSF processes it can be challenging to find a single set of conditions that are optimal for both enzymatic hydrolysis and fermentation. For example, the ideal temperatures for these processes usually differ.
• Use of Well-Established Microorganisms - SHF allows for the use of traditional, robust yeast strains in the fermentation stage, which can lead to high product yields. In contrast, more novel, less-robust and less well established microorganisms tend to be used in SSF.
• Flexibility in Process Design and Operation - Different technologies and reactors can be used for the hydrolysis and fermentation steps, offering greater flexibility in process design.

• Longer Processing Times - Because hydrolysis and fermentation occur separately, the total process time tends to be longer than SSF.
• Increased Risk of Contamination - The longer duration of the process and multiple stages increase the risk of microbial contamination. In contrast, in SSF the shorter process time and single stage reduce the opportunities for contamination.
• Potential for Sugar Degradation - Sugars produced in hydrolysis can degrade if they remain in the reactor for extended periods.
• Possible Product Inhibition - The accumulation of sugars during the hydrolysis phase can potentially inhibit enzyme activity. Whilst in SSF, since the sugars are fermented as soon as they are produced during hydrolysis, there is less risk of high sugar concentrations inhibiting enzyme activity.

Consolidated Bioprocessing (CBP)

• Independent Optimization - The separate nature of the hydrolysis and fermentation stages allows each process to be optimized independently.
• Use of Well-Established Microorganisms - SHF allows for the use of traditional, robust yeast strains in the fermentation stage, which can lead to high product yields.
• Flexibility in Process Design and Operation - Different technologies and reactors can be used for the hydrolysis and fermentation steps.
• More-Established Technology - The development of CBP technologies is still at the R&D stage whilst SHF technologies have been deployed at the commercial scale.

• Longer Processing Times - The total process time for SHF tends to be longer compared to CBP.
• Increased Risk of Contamination - The longer duration of the process increases the risk of microbial contamination.
• Potential for Sugar Degradation - The sugars produced during hydrolysis can degrade if they remain in the reactor for extended periods.
• Cost of Enzymes - In CBP the enzymes are produced directly by the microorganisms inside the reactor. In contrast, SHF processes require that enzymes are added at the hydrolysis stage. Either these SHF enzymes have to be purchased, adding significantly to OPEX, or they need to be manufactured on-site, adding to OPEX and CAPEX.

1. Understanding Your Requirements

2. Detailed Feedstock Analysis

3. Pretreatment (Lab-Scale)

4. SHF Stage 1 (Hydrolysis) Optimisation

5. SHF Stage 2 (Fermentation) Optimisation

6. Product Recovery

7. Valorisation of Remaining Biomass

8. Validation at Higher TRLs

9. Technoeconomic Analysis (TEA)

Bioethanol from Palm Residues