A thermal conversion vessel (133; 200) for converting an hydrolysis mixture of α-hydroxyisobutyramide (HIBAM), α-sulfatoisobutyramide (SIBAM), 2-methacrylamide (MACRYDE) and methacrylic acid, into a mixture comprising 2-methacrylamide (MACRYDE), wherein said vessel (133; 200) comprises: - at least three compartments (C1, C2, C3,..Ci) comprising an inner wall (206a, 206b,... 206i) separating said compartments into two communicating parts (C1a, C1b) by a passage provided between bottom of said vessel (133; 200) and said inner wall, - said compartments having a space above said inner wall, for separating gas phase from liquid phase during thermal conversion, - each of said compartments being connected to an outlet valve (204a, 204b,...204i) and each of said outlet valve (204a, 204b,...204i) is at the bottom of said vessel (133; 200), - an opening for feeding with said hydrolysis mixture, said opening being at the top of the vessel (133; 200), wherein the at least three compartments (C1, C2, C3,..Ci) are separated from each other by an overflow wall (205a, 205b,..., 205i), over which the hydrolysis mixture to be converted flows to pass from one compartment to the other, wherein the inner wall (206a, 206b,... 206i) is fixed to the side wall of the vessel (133; 200), characterized in that the height (h) of either overflow walls (205a, 205b,..., 205i) or inner walls (206a, 206b,... 206i) is comprised between 2/3 and 3/4 of the height of the vessel (133; 200), wherein the vessel (133; 200) does not contain, any agitator or mixing element, meaning devices or elements that disturb the laminar flow in order to create turbulences as an agitator with rotatable axis or stirring elements or baffles or introduction of gas bubbles. - at least three compartments (C1, C2, C3,..Ci) comprising an inner wall (206a, 206b,... 206i) separating said compartments into two communicating parts (C1a, C1b) by a passage provided between bottom of said vessel (133; 200) and said inner wall, - said compartments having a space above said inner wall, for separating gas phase from liquid phase during thermal conversion, - each of said compartments being connected to an outlet valve (204a, 204b,...204i) and each of said outlet valve (204a, 204b,...204i) is at the bottom of said vessel (133; 200), - an opening for feeding with said hydrolysis mixture, said opening being at the top of the vessel (133; 200), wherein the at least three compartments (C1, C2, C3,..Ci) are separated from each other by an overflow wall (205a, 205b,..., 205i), over which the hydrolysis mixture to be converted flows to pass from one compartment to the other, wherein the inner wall (206a, 206b,... 206i) is fixed to the side wall of the vessel (133; 200), characterized in that the height (h) of either overflow walls (205a, 205b,..., 205i) or inner walls (206a, 206b,... 206i) is comprised between 2/3 and 3/4 of the height of the vessel (133; 200), wherein the vessel (133; 200) does not contain, any agitator or mixing element, meaning devices or elements that disturb the laminar flow in order to create turbulences as an agitator with rotatable axis or stirring elements or baffles or introduction of gas bubbles.