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


et al (1988), Oikawa et al. (2003) and Williams (1999) analyzed seawater wet scrubbers used in power plants. Sun et al. (2008) showed that the mass transfer coefficient in seawater is about twice as great as that of the NaOH solution with pH 8.35. Darake et al. (2014) carried out an experimental study and mathematical modeling about SO2 removal by seawater in a packed- bed tower. Caiazzo et al. (2013) analyzed a seawater spray scrubber operating under marine diesel exhaust conditions and compared seawater to distilled water. Sukheon and Nishida (2003) analyzed the effect of seawater under SOx, particulate matter, CO2 and NOx.


The complexity of the SO2 absorption process makes it difficult to analyze through theoretical or experimental analyses. In


computer technology has become numerical procedures an important


the recent years, the development tool


to predict process. Most of droplet provides the droplet the numerical work available in the


literature refers to simulations of the whole geometry of scrubbers. Instead the whole scrubber, the analysis of a single


information about diffusion,


convection inside the droplet, or evaporation. There are several papers that analyze single liquid droplets in the literature, most of them focused on SO2 scavenging by rainwater in the atmosphere. For instance, Babolal et al. (1981) studied atmospheric SO2 scavenged by water drops by means of a simple numerical method. Huckaby and Ray (1989) analyzed numerically the absorption of SO2 by a stationary water droplet under condensation or evaporation and, more recently, Chen et al. (Chen, 2001 & 2006, Chen, et al, 2011 and Chen, et al, 2012) developed a CFD model to analyze SO2 reduction by water droplets in the atmosphere analyzing both liquid droplets and gas medium.


The purpose of this paper is to develop a numerical model to analyze SO2 absorption by seawater and continue the work developed elsewhere (Lamas et al, 2016). To this end, a moving water droplet


immersed in a gas medium was


analyzed. Both liquid and gas phases were included in the model. The proposed CFD model takes in consideration the fluid motion inside and outside the droplet, heat transfer, chemical reactions and mass diffusion.


2. NUMERICAL IMPLEMENTATION


The model proposed in the present paper analyzes the amount of SO2 absorbed by a single droplet which falls counter flow with respect to an exhaust gas stream with SO2. The computational domain is indicated in Figure. 2. As can be seen, the domain size is 15 x 6 droplet radii from the droplet center since it was verified that these dimensions eliminate any potential effect of the outer boundaries on the flow close to the droplet. Several tests were performed in order to set


of absorption


When viewed in a fixed coordinate system, the droplet and gas move relatively with each other. Modeling this movement


requires a large domain imposed as and thus high


computational time. An alternative approach is to work using a coordinate system which moves with the droplet. In this case, the droplet is fixed in space and the relative droplet-gas velocity is


inlet and outlet


boundary conditions, i.e., the gas flow enters and leaves the computational domain at the relative velocity. This velocity is variable due to the effects of buoyancy and drag. Buoyancy tends to accelerate the droplet, while drag tends to decelerate it.


2.1 CHEMICAL MODEL


The principle of operation of a seawater scrubber is based on a spray of droplets which fall counter flow to an exhaust gas containing SO2. When the gas is in touch with a droplet, the concentration at the interface follows the Henry’s law. A concentration gradient is induced in the interface and SO2 is transported into the droplet by mass diffusion according to the Fick’s law. Chemical dissociation reactions also take place inside the droplet.


While several alkaline species are presented in seawater (


HCO− HCO− 3 , CO −


2 3


, B(OH)− 4 , OH− , HPO −


2 4


, etc), the main


contribution to alkalinity is by far the bicarbonate ion 3


, and typical concentrations are 2,400μmol/kg of


H2O (Sverdrup, et al, 1942; Dickson & Goyet, 1994 & Andreason & Mayer, 2007).


Regarding chemical the more appropriate


dimension of the computational domain. The problem was simulated as axisymmetric.


SO (aq) 2H O(l) HSO (aq) H O (aq)−+ 2−+


SO (g) SO (aq) +


22 ↔ 2


HSO (aq) H O(l) SO (aq) H O (aq) −


32 3+↔ + 33 2 CO (aq) CO (g)↔ 22


2 33↔ + 3


HCO (aq) H O (aq) CO (aq) 2H O(l) −+


+ ↔+ 2 reactions, the model proposed by


Andreasen and Mayer (2007) was adopted. This assumes the following chemical reactions:


(1) (2) (3) (4) (5)


Figure 2. Computational domain.


A-336


©2019: The Royal Institution of Naval Architects


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