Separation and control of gas-liquid flows at horizontal T-junctions
The separation of gas-liquid flows is an integral part of many industrial processes. Traditionally, such separations are performed in large vessels under the effect of gravity. However, such vessels can contain inherently large inventories of potentially flammable and/or toxic material. The main objective of this thesis is to combine the knowledge of partial phase separation at T-junctions with control strategies to enhance the development of continuous compact partial phase separators. Such applications would form an integral part of more intensive phase separation systems that allow for smaller downstream separator vessels. This would be especially beneficial to the petroleum industry where safety, space, weight and cost are all issues related to off-shore oil platforms. For such applications a simple definition for a partial phase separator would be one that produced two streams, one rich in gas and the other rich in liquid, each containing less than 10% v/v of the unwanted phase. A series of optimisation experiments produced the final T-junction configuration. This comprised of two horizontal T-junctions placed in series, the first with a vertically upwards side-arm, the second with a vertically downwards one. The addition of control valves on the exit streams of the T-junctions extended previous fundamental studies, incorporating the concept of control and flexibility. An automatic liquid level control on the down leg provided a physical barrier against gas entrainment by maintaining a constant liquid presence within that pipe. A further control valve beyond the second junction then optimised the liquid hold-up above this down leg. Experiments showed that the run valve setting was only dependent on the approaching flow regime and independent of the inlet phase flowrates. A simple active control strategy was developed based around these control valves such that for stratified flows the run arm control valve was set at 20% open, while for slug flows the valve was required to be 55% open. Under this control scheme it was possible to obtain a liquid only stream and a gas-rich stream which always satisfied the simple separation criterion of less than 10% v/v liquid-in-gas. Within industrial situations it is rare to operate under steady-state flow conditions continuously and there will be at least one time dependent variable. Examples of general transient situations involve plant shutdown and start-up, changes in flowrates in response to planned operating conditions and emergency situations. Even more relevant to the petroleum industry however, is bringing an additional well on line. Within the petroleum industry the problem of multiphase transient flows has lead to the development of many commercially available prediction packages but none that handle branched pipe networks. A series of experiments were performed to compare the outlet phase mass flowrate responses for a straight pipe and the T-junction separator. The results indicate that in general the T-junction responses are analogous to those observed in a pipe. However, the existence of pipe branches adds another level of complexity as the flow splits exhibit a very non-linear nature.