Mixed-phase Feeds in Mass Transfer Columns and Liquid Separation

The flashing feed sections of crude oil vacuum towers are especially prone to liquid entrainment. This can be detrimental to the performance of the packings/trays above the feed. It is also of disadvantage in case liquid is collected and recirculated back to the furnace. Therefore it is common practice to use special inlet devices that help preventing or minimizing liquid entrainment while maximizing the flash conditions. Such devices are also frequently used in gas/liquid separators. The similarity rules for the mixed-phase feed inlet are discussed and applied to a 1 m diameter air/water simulator that is used as a model for a crude oil vacuum tower of 5.25 m inner diameter. For a given inlet device the turbulent flow field of the dry vapour is fully determined by four essentially geometrical dimensionless parameters. The Froude number and the flow parameter account for the two-phase nature of the flow. Two devices with vanes (a short assembly and the modified Schoepentoeter inlet device) and a standard feed without any device are tested experimentally in the air/ water simulator. Their liquid disengaging capability is assessed by means of the entrainment rate. In the present installation the more complex device reaches virtually zero entrainment below a gas load factor of 0.08 m/s. The entrainment rate of both devices is fairly independent of the flow parameter while the standard feed shows more entrainment when the flow parameter is increased. Compared to the standard feed the short assembly improves only at the higher flow parameter.