The present invention provides a porous vessel bioreactor apparatus for use in reaction with immobilized enzymes and/or microbial cells, said apparatus consisting of a vertically elongated reaction vessel having at least one liquid reactant inlet; at least one product outlet on the vessel; at least one porous vessel completely submerged in the reactant, said porous vessel having pore size ranging from 5 mm to 0.2 microns and a vertical length less than a level of the reactants to be maintained in the vessel, and immobilized bio-catalyst particles comprising the enzymes and/or microbial cells placed inside the porous vessel such that the liquid reactant is in contact with the bio-catalyst in both radial and axial directions.