A flow reactor comprising: a module having a process fluid passage therein, the process fluid passage comprising an interior surface, the process fluid passage further comprising a portion thereof which portion further comprises: an input end at which process fluid is to flow into the portion during use and an output end at which process fluid is to flow out of the portion during use, and a cross section along the portion delimited by the interior surface of the passage along the portion, the cross section having a cross-sectional area and a cross-sectional shape, the cross-sectional area having multiple minima along the passage between the input end and the output end, the passage characterized in that the cross-sectional shape of the portion varies continually along the portion, the interior surface of the portion includes either no pairs of opposing flat parallel sides or only pairs of opposing flat parallel sides which extend for a length of no more than 4 times a distance between said opposing flat parallel sides along the portion, and the portion contains a plurality of obstacles positioned along the portion between the input end and the output end; wherein the portion further comprises successive chambers each with a nozzle-like entrance and a narrowing exit; wherein at least one of the plurality of obstacles is located within a first chamber and intersects a straight line having a first endpoint located at a center of the entrance of the first chamber and a second endpoint locate at a center of the exit of the first chamber; wherein the flow reactor further comprises two or more obstacles in a single chamber; and, wherein the obstacles extend only partially across the height of the chamber. a module having a process fluid passage therein, the process fluid passage comprising an interior surface, the process fluid passage further comprising a portion thereof which portion further comprises: an input end at which process fluid is to flow into the portion during use and an output end at which process fluid is to flow out of the portion during use, and a cross section along the portion delimited by the interior surface of the passage along the portion, the cross section having a cross-sectional area and a cross-sectional shape, the cross-sectional area having multiple minima along the passage between the input end and the output end, the passage characterized in that the cross-sectional shape of the portion varies continually along the portion, the interior surface of the portion includes either no pairs of opposing flat parallel sides or only pairs of opposing flat parallel sides which extend for a length of no more than 4 times a distance between said opposing flat parallel sides along the portion, and the portion contains a plurality of obstacles positioned along the portion between the input end and the output end; wherein the portion further comprises successive chambers each with a nozzle-like entrance and a narrowing exit; wherein at least one of the plurality of obstacles is located within a first chamber and intersects a straight line having a first endpoint located at a center of the entrance of the first chamber and a second endpoint locate at a center of the exit of the first chamber; an input end at which process fluid is to flow into the portion during use and an output end at which process fluid is to flow out of the portion during use, and a cross section along the portion delimited by the interior surface of the passage along the portion, the cross section having a cross-sectional area and a cross-sectional shape, the cross-sectional area having multiple minima along the passage between the input end and the output end, the passage characterized in that the cross-sectional shape of the portion varies continually along the portion, the interior surface of the portion includes either no pairs of opposing flat parallel sides or only pairs of opposing flat parallel sides which extend for a length of no more than 4 times a distance between said opposing flat parallel sides along the portion, and the portion contains a plurality of obstacles positioned along the portion between the input end and the output end; wherein the portion further comprises successive chambers each with a nozzle-like entrance and a narrowing exit; wherein at least one of the plurality of obstacles is located within a first chamber and intersects a straight line having a first endpoint located at a center of the entrance of the first chamber and a second endpoint locate at a center of the exit of the first chamber; wherein the portion further comprises successive chambers each with a nozzle-like entrance and a narrowing exit; wherein at least one of the plurality of obstacles is located within a first chamber and intersects a straight line having a first endpoint located at a center of the entrance of the first chamber and a second endpoint locate at a center of the exit of the first chamber;