Spray Towers Can Perform Better Than Packed Towers in Capturing CO2

Spray tower is an effective means of capturing condensable or absorbable gases and vapors, and have long been used in a variety of applications. However, the size distribution and spatial distribution of absorbent droplets are very non-uniform in spray towers using small number of spray nozzles, which degrades the capture performance substantially. Thus the capture performance of typical spray towers is much poorer than typical packed towers, and almost all of the CO₂ absorbers in test or operation are packed towers.

Spray towers, however, have several strong points over packed towers: 1) surface area per absorbent mass is much larger and easily controllable via drop size, and 2) internal diffusion which is the determining factor for reaction speed occurs in three principal directions as compared to one direction in liquid films. It implies that the capture performance of a spray tower can be enhanced a lot, if drop-size uniformity is improved and falling speed is controlled via choosing a proper drop size.

In this study, a new type of spray tower was developed, where absorbent droplets were generated with much better size-uniformity (geometric standard deviation ~ 1.2 as compared to > 2.0 with conventional spray nozzles) using a multi-nozzle plate and sprayed vertically and quite uniformly across the gas flow cross-section. (Fig. 1) Through the modification of spatial distribution and size distribution of droplets into a uniform spray, capture performance was enhanced drastically, as compared to the case of non-uniform droplet spray, despite much bigger droplets were used. Effective mass transfer conductance was almost twice as high, and capture efficiency even reached ~95% (Fig. 2), which has hardly been reported to date for CO₂ with a spray tower. Effectiveness of tower length was estimated as ~ L^0.8.

The new spray tower has a number of strong points favorable for scale-up to bigger and longer size: 1) droplet loss to the wall is minimized through vertical injection; 2) fly-back of small droplets is eliminated through the size uniformity; and 3) coagulation is minimized due to very small relative velocity between drops of nearly the same size. Tower height for 90% capture efficiency under typical conditions of large-scale systems (gas velocity 1m/s, liquid-to-gas flow rate ratio 0.002 m³/m³) was estimated to be ~ 10m, which is a far better figure than any other spray or packed towers.

Fig. 1. Photograph of the uniform vertical spray used in this study

Fig. 2. Experimental data of this study (symbols) for towers of different lengths(L) form a single exponential curve when plotted versus liquid-to-gas flow-rate ratio(Q_L/Q_G) modified by the tower length, (Q_L/Q_G)L^0.8. Solid curve represents previous works for conventional spray towers.