Photobioreactors (PBRs) and chemical reactors are often vertical columns with either circular or rectangular cross sections. The reactors are frequently referred to as column reactors and are treated as if they perform in the same manner. The reactors can have two different types of flow established in them regardless of cross sectional shape depending on the saprger/diffuser type and location within the reactor. The flow patterns in the reactors are induced by gas that is bubbled into the reactor volume usually near the bottom of the reactor. When the gas bubbles rise up through the reactor in a plug flow fashion, most of the mixing is in the radial direction which tends to make the reactor liquid and gas more homogeneous across the width of the reactor. The gas bubbles in the reactor may not move up through the reactor in a plug flow fashion, but may instead move vertically up through a portion of the reactor cross-section. This will establish a column of bubbles and liquid rising from the bottom of the reactor up to the surface and, in turn, induce a column(s) of liquid moving downward from the top of the reactor to the bottom. This behavior is similar to an air lift reactor which generally has walls physically dividing the upward (riser) and downward (down comer) flows. Without physical separation of the flows, the percent of cross sectional area of the reactor acting as the riser and down comer is established by the gas flow rate through the reactor, reactor cross sectional area, and the reactor volume. Velocity of flow(s) in the reactors is often based on the superficial gas velocity, which is the incoming gas flow rate divided by the gross cross sectional area of the reactor volume. This parameter may not relate to the two flows in the same manner. The two different flow patterns will be discussed in relation to superficial gas velocity, light in a PBR, chemical reactions in the reactor, and riser and down comer size.

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