Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


this is a reasonable value of residence time in scrubbers (Lamas, et al, 2016). The droplet velocity and position for these initial diameters are indicated in Figures. 7 and 8 respectively. Figures. 6-8 correspond to 600ppm SO2 concentration, 2,400μmol/kg alkaline concentration, free stream initial velocity 2m/s, 400ºC initial gas temperature and 20ºC initial droplet temperature. As expected, lower diameters correspond to lower velocities. As indicated above, two effects trajectory:


buoyancy and drag. Larger associated to high gravity forces


are responsible for the droplet droplets


are velocities. Nevertheless, as the droplet


and thus to high diameter is


reduced, buoyancy becomes less important and velocity tends to decay. As indicated below, 1 m/s was assumed for the droplet and 1 m/s for the gas. Figure. 7 indicates that all droplets depart from this initial velocity of 1 m/s and the largest droplet increments this velocity due to the importance of buoyancy, while the droplets of 500 and 200μm reduce this initial velocity due to the importance of drag.


Figure 8. Droplet position against time. 600ppm SO2 concentration, 2,400μmol/kg alkaline concentration, 400ºC initial gas temperature and 20ºC initial droplet temperature.


The quantity of sulphur (S) absorbed by each droplet is indicated in Figure. 8. The absorbed S was computed as


the sum of moles of SO , HSO− 2 3 and SO −


2 3


. As can be


seen, larger droplets are associated to higher S absorption rates. Nevertheless small droplets are associated to lower S absorption rates. The reason is simply that the volume is lower. The main advantage of small droplets is that these reach the saturation state in less time due to a more efficient contact with the liquid. In practical applications, this fact promotes small scrubbers.


Figure 6. Droplet diameter against time. 600ppm SO2 concentration, 2,400μmol/kg alkaline concentration, 400ºC initial gas temperature and 20ºC initial droplet temperature.


Figure 9.


S absorbed by a droplet for several initial


diameters. 600ppm SO2 concentration, 2,400μmol/kg alkaline concentration, 400ºC initial gas temperature and 20ºC initial droplet temperature.


3.2


Figure 7. Droplet velocity against time. 600ppm SO2 concentration, 2,400μmol/kg alkaline concentration, 400ºC initial gas temperature and 20ºC initial droplet temperature.


A-340


EFFECT OF THE SO2 INITIAL CONCENTRATION


Figure. 10 indicates the S absorbed by a droplet against time for several initial SO2 concentrations. This figure corresponds


to 200μm initial droplet diameter, 2,400μmol/kg alkaline concentration, free stream initial ©2019: The Royal Institution of Naval Architects


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