What percent of biomass solids can be successfully agitated with a turbine agitator? In the early days of the development of biomass hydrolysis reactors, it was common to use rotating barrels (similar to continuous solids dryers), horizontal paddle mixers, and other mixing apparatus intended principally to mix solids as hydrolysis reactors. This is because high solids biomass resembles damp peat moss more than it does a liquid. However, after even a modest degree of hydrolysis, the biomass slurry liquefies. So, if used, the solids mixers would be needed only for the early stages of hydrolysis. The fact that such mixers are expensive and limited in volume to about 40,000 liters or so has led the industry to look into cheaper, scalable alternatives. Today, simple turbine agitators are used for almost all hydrolysis reactors. They cannot handle materials that are basically damp solids but can handle very viscous biomass slurries. The key is to configure the process so that there is at least a couple of percent free water. By that, I mean water that is not bound up within biomass cells but is outside the cells and available to fluidize the mass.
But what % solids does that correspond to? It depends on the kind of biomass and its history of pretreatment and how hydrolyzed it is. Untreated materials such as switchgrass and corn stover are limited to approximately 12-14% solids. The same materials, after pretreatment, may be able to be agitated at concentrations in the 18-22% range. When fully hydrolyzed, they can probably go above 40% solids.
But how does one maximize the % solids in the actual process? A common way is to back mix a portion of already hydrolyzed material with the inlet pretreated material. This can allow the pretreated material to be more than 25% solids, as the turbine agitator does not see the pretreated material alone. In practice, this can work with either a fed-batch reactor or a continuous flow reactor with a back mixed first stage. Lab/pilot testing is always required to ascertain quantitatively what is possible for a given process.
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