What Is A Wet Scrubber And How To Make It The Most Efficient?

What is a wet scrubber?

A scrubber is basically a liquid and particle contactor. Just like when it rains in nature, the fine particles in the air get washed away."

 


Video Transcript:

So what is a scrubber? A scrubber is basically a liquid and particle contactor. So if you look outside when it rains, all the pollution we generate and fine particles in the air get washed out by water droplets. So that's a physical contact between particle and water droplet.

There are various throat designs as you saw in the previous slide with the four scrubbers and I will explain it as we move on. So the various throat design's main purpose is to have contact between gas, water droplets, and solid particles. And that is what creates pressure drop in the scrubber. So scrubbers basically work on impaction, interception, and diffusion theory.

So imagine a piece of pipe, you have air, water droplets, and solid going through it. If particles are big enough because of its own mass, so say particles generally speaking, 3 micron and larger are because of its own mass when flowing in the straight path with water droplets, air, and solid going in the same direction. The solid particle because of its own weight hits the water droplet and gets collected.

When particles are between 1 to 3 micron the particle tries to go around the water droplet it falls within a half diameter if the water droplet gets collected, that's called interception In diffusion the particles are so fine they behave like air molecules. So we all are standing or sitting and air molecules constantly going around us but we don't feel it because air molecules are so light they change their path and go around.

In scrubbers, because of the constraint of the throat, when particles are fine it creates a random zig zag motion, but it does not have more area to go anywhere and that's how it collides with the water droplets and get collected. So in general finer particles need more pressure drop to have impaction, interception, diffusion. Heavier and larger particles need less pressure drop to collect those particles.

So particle collection depends on the particle size distribution and the particle's specific gravity, among a lot of other factors. Pressure drop, liquid rate (liquid to gas ratio), and percentage of solid in the recycled loop are very very important for efficiency. To obtain impaction, interception, and diffusion you can create more pressure drop to bring them together or you can use more liquid rate or you can use a lower percentage of solid in the recycled loop, but means discharge of more water.

These three parameters you can change so you can have lower pressure drop, higher liquid rate, or lower percentage in the recycled loop. You can have higher pressure drop, lower liquid rate, and different percentages of solid in the recycled loop. So these three variables can be adjusted to achieve the efficiency goal we want. When we talk about the percentage of solid in the recycled loop, there are two types of solid - dissolved solid and undissolved solid.

When particulates are soluble in the water we can design for higher percentage in the recycled loop. So many different things we can do with scrubber design as long as we know the design data and the goal. Cyclone scrubber vs pulse jet filter, the difference for the cyclone scrubber part is the design data. Your design data has to be very very precise and many times when I meet with customers and engineering groups.

I say to them spending some money to gather the data will save you a lot of money throughout the life of the equipment because if your data is not correct you will either use more horsepower, more liquid, or a combination of both so design data is a key part in the scrubber.