In the fluid catalytic cracking process, a standpipe system transfers solids from the low-pressure regenerator to the high-pressure riser bottom. The standpipe system consists of a regenerator withdrawal means, either a hopper or a well where solids can be refluidized and entrained bubbles can escape, a conical transition from the well to a smaller diameter standpipe and a standpipe which builds pressure. The present study is the first to address the withdrawal well vessel and the conical transition to the standpipe. The withdrawal well should be fluidized with a coarse sparger which forms large bubbles which are not easily entrained downward into the conical transition. A theoretical equation predicts the minimum well aeration flowrate. Compression which occurs in the conical transition may cause defluidization. A model quantifies this compression and evaluates the aeration flow,rate required to compensate for it. A conical transition with a small included angle maximizes the range of aeration flowrates over which stable operation without slugs can be achieved.