I have one product for solid oral development. The process is top spray granulation. In one trial I have used Isopropyl alcohol(IPA)/Dicholromethane(DCM) mixture for granulation and polymer coating. After 60 min Air drying and 90 min drying at 55°C, the granules failed in Residual limits for IPA (`22000 ppm obtained). In another trial, I have used DCM/Methanol mixture for granulation and polymer coating. After 60 min Air drying and 90 min drying at 55°C, the granules failed in Residual limits for DCM (`5000 ppm obtained).

What are the possible reasons?

How can I remove residual solvents?

Note: I can't use any other solvent other than stated above.