Flooding in a Packed Column

Introduction

What is flooding? Before we look into this, we should understand what packing does in a packed column. Packing provides a high surface area, per unit volume of packing, to provide better contact between the gas and liquid phase fluids. In simpler terms, packing allows for better mass and heat transfer between the two mediums to allow for a more efficient separation process. The liquid phase enters the column from the top and flows down the column by gravity. The gas phase is forced up the column through the use of a pump (compressor).

Packings in a column should generally have the following desirable characteristics:

1. High interfacial surface area per unit volume of packing

   A high surface area per unit volume allows for less packing needed for the column, while still maintaining a large enough surface area. The size of the column is limited by a multitude of factors (cost, safety, efficacy, etc.) so the amount of packing needed for a column should generally not exceed the size of the column.

2. High porosity

   A high porosity indicates, per unit volume of packing, there is a high volume of void (empty) spaces making up the packing. A higher porosity provides even more surface area for gas/liquid contact to occur.

3. Uniform distribution of items 1 and 2

   This is needed so that we can accurately model our system of interest. If each individual material making up the packing was different in shape, a more complex system of equations (or use simpler, less accurate model) will need to be used to model the system.

4. Mechanical strength

   The packing will have to withstand not only the weight of the liquid flowing but also the adverse pressure conditions that exists within a distillation tower. Our packing should be able to withstand these conditions without deforming.

5. Chemical inertness

   Packing itself should not chemically interact in the system at all. Since we generally use them in separation processes, reactions will generally only lower product yield and would also require us to replace the packing, incurring more costs on the system.

6. Low cost

   Self-explanatory as to why you want this. We use this as a metric to compare two different types of packings that meet our design specifications and still pick a better product.

Packing in a distillation column is packed in two different ways: structured or random. A hold-down grid is used to hold these packings in place within the tower while operating the column. In a structured packing system, we generally use larger, fixed packing systems which are held in a specific orientation. In a random packing system, smaller packing material is set up randomly in the column with no fixed orientation.

How does flooding occur? Why does this happen? What issues can arise?

In our distillation column, we are using a counter-current flow system between a gas and liquid phase. We essentially want to operate the system by extracting a component in the gas system and have it dissolve and collect in the liquid phase. The liquid phase enters from the top and passes through a liquid distributor to force the fluid to flow through all regions in the column. The gas is forced up the column using a compressor. As these two fluids flow in a counter-current arrangement, they pass through the pores of the packing (void spaces) where most mas transfer is occurring.

Flooding occurs when the weight of the liquid is overpowered by the pressure of the gas flowing up the column. When the liquid enters the column, it begins to flow down the column against a pressure gradient. If the pressure of the gas is high enough to overpower the weight of the liquid phase, you should be able to see that the fluid will stop flowing down the column and will begin accumulating within the column. This accumulation of the liquid phase can be catastrophic to our separation process. This system we are dealing with will be effectively changed from a gas-continuous liquid dispersed system to a liquid-continuous gas dispersed system. What this means is that when the column floods, the gas now bubbles through the liquid instead of passing through the pores of the packing. What does this means?

Essentially, flooding will lower the amount of mass transfer occurring between the two phases. When flooding occurs, the mass transfer does not occur within the high surface area of pores, but through the gaseous bubbles flowing through the liquid. Column are also designed with specific parameters in mind. It is very expensive to build and design a tower, so to cut costs, we can design a column to withstand a maximum weight expected. Flooding can cause for the weight in the column to exceed this maximum (depending on the volume of the tower and density of fluid and gas) and can be very dangerous.

Gas Phase Flow (One-phase Flow)

To examine our system, we will first analyse the column when only gas flows through the column.

When the gas flows through the porous medium up the column, the pressure drop across the column is well modeled by the Ergun equation.

We can see the 2nd equation in this discussion is being represented in the graph above. In the laminar region (Re’ < 1), the first term dominates the equation, so we interpret this term as the laminar flow contribution. In the wholly turbulent region (Re’ > 10,000), the 2nd term dominates the equation, so we interpret this term as the turbulent flow contribution.

Two-phase, Counter-current System

When introducing the liquid phase in to our system, with the gas phase flow still present, the relationship between the pressure drops and flow rates become complicated to model theoretically.

These correlations were found by researchers for engineers to use to design and analyze their systems. There are two lines that indicate two different conditions: A – loading point, B – flooding point. The loading point is when the pressure gradient of the gas phase can significantly counterbalance the weight of the liquid phase. At this point, we begin to see some liquid hold up in the distillation column. The loading point is the beginning to the approach to the flooding point. From the graph, you can see if the superficial velocity is slightly increased from the loading point, we quickly reach the flooding point. The flooding point is characterized by rapidly increasing pressure drop and you can genuinely see some liquid hold up in the tower. We have already discussed the issues that arise when the flooding point is reached.

So if we know the flow of the gas phase, we can see when the tower might flood but generally it can be difficult to measure the superficial velocity of the gas phase to an accurate degree. Thankfully, scientists have found correlations between pressure drop ad the mass flow rates of both phases to determine if the column will flood or not.

By calculating the value of y and x at a certain gas and liquid flow rate, we can the approximate if column will flood or not. If the coordinate you calculated, where the numbers on each line represent the pressure drop in the system, is above or on the gas pressure drop curve, we can say the column has flooded. Coordinates close to the flooding line may be at the loading point as these two points are very close to each other.