Design Considerations for Tubular Reactors

1.3.1 Airlift Pump

For the algae to grow optimally, the mass transfer characteristics of the photobiore­actor must be optimised to suit the specific strain of algae. Mass transfer is achieved by pumping compressed air and carbon dioxide into the reactor, thus creating flow through the culture that is dispersed within the solar tubing [22]. It is preferred that the aeration and mixing of the cultures in tubular photobioreactors are carried out by an airlift pump [35]. The airlift pump must provide an adequate velocity to circulate the culture through the solar receiver so that the dissolved oxygen build up within the culture can be stripped by the degassing section before it accumulates. Airlift pumps are typically used in algal cultivation instead of standard mechanical pumps. Airlift pumps have been found to cause less damage to algal cells and are less expensive to instal than mechanical pumps [5] .

Pump

Fig. 1 Different configurations of microalgae cultivation system. (a) Raceway pond (RP) [5]. (b) External loop tubular reactor (ELR) [21]. (c) Horizontal tubular reactor (HTR) [5]

1.3.2 Degassing Column

The degassing section of the reactor plays an integral role in the success of the cul­tivation. The degasser is designed to remove accumulated dissolved oxygen and extricate gas bubbles from the culture. An excessive amount of bubbling within the tubing can hinder light absorption. Therefore, gas-liquid separators are employed [5]. It is required that the culture spends the least amount of time in the dark regions of the reactor. The degassing zone is considered to be optically deep in comparison with the solar receiver. It is a dark zone and hence it is unsuitable for growth. It is a necessity that the degassing section’s volume is much lower than the volume of the solar receiver to allow the culture to spend longer periods in the section of the reac­tor that is optimal for biomass proliferation. The airlift pump controls the liquid
velocity in the solar tubing. The velocity required by the system will depend on the configuration of the tubing and gas holdup in the riser and downcomer regions within the airlift operation. Once the tubing geometrics are selected, the height of the airlift section and the appropriate areas for the downcomer and riser portions of the airlift can be determined.

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