I have a cloth vellum with a grain vellum. An when I try to export from the null it only give me cloth in blender as obj file.

https://i.imgur.com/5Ww06Dt.jpg [i.imgur.com] here the set up.

any help would be great .

You may need to export as alembic, since grains are isolated points, I don't think .obj can store those

or use a sphere geometry as polygon + copy to point on your grain to get polygon from it..