1. Backhaus, A.A., U.S. Patents 1400849, 1400850, 1400851 (1921); 1403224, 1403225, 1425624, 1425625 (1922); 1454462, 1454463 (1923).
2. Keyes, D.B., Ind. Engng Chem., 24 (1932) 1096.
3. McKeon, T.J., U.S. Patent 2208769 (1940).
4. Brunn, J.H. and Grubb, H.U., U.S. Patent 2384793 (1945).
5. Schniepp, L.E., Ind. Engng Chem., 37 (1945) 872.