The calculated distance (air line) between Frankfurt am Main and Castrop-Rauxel is approximately 116 miles respectively 187 kilometers.
By car or train, the actual journey to Castrop-Rauxel is certainly longer, as only the direct route (as the crow flies) between Frankfurt am Main and Castrop-Rauxel has been calculated here.
Air line (approximately)
187 kilometers
101 nautical miles