Plug Flow Reactors
Plug flow reactor (PFR), sometimes known as continuous tube reactor, is another common reactor design applicable for dilute acid hydrolysis. In this reactor biomass slurry in dilute acid is pumped through a pipe or tube at high temperature. The depolymerization reaction proceeds as the reagents travel through the PFR. Different biomass forms have been tested in plug flow reactors for dilute acid hydrolysis [31-33]. McParland and coworkers developed a small — scale steam injection plug flow reactor to obtain kinetic parameters under experimental conditions of potential commercial interest. During these experiments, dilute H2SO4 was used as the catalyst, and glucose yields were maximized at short residence times. The most favorable operation of the plug flow reactors required very short residence time (6 to 10 seconds) and high operating temperature (around 240°C) [34]. At such short residence times, glucose decomposition reactions become important, and glucose kinetic parameters were therefore determined in a separate study [34].
In small-scale laboratory experiments plug flow reactors have shown fair glucose yields and relatively easy operations. However, a number of research groups have encountered some difficulties upon scaling up the plug flow reactors, even at lower than optimal concentrations. Brennen et al. investigated the dilute acid hydrolysis of lignocellulosic biomass using a 3.8 cm (1.5 in) diameter plug flow reactor, where the system hydrolyzed wood slurries at rates up to 900 kg/h (2000 lb/h) at 200-260°C and 4-20 s residence time [35]. Plug flow systems in the lab and the pilot plant produced yields of glucose of around 50%. These yields are approaching the theoretical limits for such continuous reactor systems.