This contribution deals with the application of numerical methods in CPC-Systems' microreaction development process and demonstrates the feasibility and significant benefits for the suggested design method. It is focused on the design of capillary residence tubes, which is necessary if the residence time provided by the original microreactor is not sufficient to complete the reaction. The residence time distribution is calculated from a straightforward numerical model based on the common assumption that axial gradients can be neglected. The results can be adapted easily to other capillary diameters or reaction conditions. As an example, the method is applied to the case of sequential synthesis.