Abstract: A benzene polycarboxylic acid represented by the formula: ##STR1## wherein n is an integer of 2 or 3, can be obtained under mild conditions with a high yield and with scarce formation of by-product by reacting a monochlorobenzene polycarboxylic acid represented by the formula: ##STR2## wherein n is an integer of 2 or 3, or its alkali metal salt with carbon monoxide in an aqueous solution of an alkali metal compound in the presence of a carbonylation catalyst with irradiation of ray of 340-400 nm, and subsequently acidifying the reaction solution.

Abstract: An improved method and apparatus for mixing fluid and/or granular substances, whereby greater mutual dispersion of at least two substances is achieved by attaining greater turbulence while minimizing the required amount of mechanical structure. Each of at least two substances is fed at a different angle in a belt-shaped stream to a nozzle, under pressures according to the viscosity of each substance. After initial mutual dispersion is achieved in the nozzle, the mixture passes to a mixing tube having a varying cross-section along its length, the varying cross-section creating further turbulence of the mixture passing through the mixing tube, causing further dispersion of the substances within the mixture.