I highly recommend that you use Aspen HYSYS or Aspen Plus simulation software from Aspen Technologies Corp.
Both software have large electronic libraries of components and thermodynamic models that you could choose from to model this system. Moreover, these software have large number of unit operations and reactor models to simulate the hydrothermal liquefaction of biomass.
It is possible for you to down load test versions of this software to explore their capabilities.
the material of construction suitable for this application could be carbon steel or stainless steel,
If you want to determine the volume of the reactor you should calculate the total volume of the reaction mixture then multiply this value by 1.5 as it is better to let about 1/3 of the reactor empty as a safety factor , then you can calculate the dimensions of the reactor (i.e. diameter and height) using the low of cylinder volume (V=pi*(D^2)/4*H) and you can use the ratio of 1.5 between height and diameter (H=1.5*D)
according to the pressure, temperature and diameter of the reactor you can calculate the thickness of shell and heads as well as the gaskets, bolts, etc...
you can use attached files for the design of this pressure vessel (reactor)