Grains can be conveyed in dense phase but the major drawback is degradation. Rice grains loose their shine and break if conveyed, wheat grains break open. Velocities can be kept very low to keep the degradation to a minimum. If you are interested in keeping the quality then pneumatic conveying in not a good idea. It can be used if the grains are going into mill / process. Try contacting Prof. Mike Bradley or Richard Farnish @ University of Greenwich. They have done substantial research on degradation / conveying of grains. ■