Feeding a solid into a high-pressure gas-phase process is substantially more difficult. Rotary valves and screw conveyors can be designed with tight enough tolerances to prevent gas backflow for relatively small pressure differences. If the distance between flights on a screw conveyor is tapered then compression will occur, so a screw conveyor can be designed to act as a solids com-pressor over a small range of pressure increase.
For larger pressure increases, a dual hopper system can be used, as illustrated in Figure 18.20.