Column Diameter and Pressure DropIn determining the column diameter, we need to know what is the limiting (maximum) gas velocity that can be used. This is because the higher the gas velocity, the greater the resistance that will be encountered by the down-flowing liquid and the higher the pressure drop across the packings.

Too high a gas velocity will lead to a condition known as flooding whereby the liquid filled the entire column and the operation became difficult to carry out. High pressure will crush and damage the packings in the column.

We will begin our analysis by examining the relationship between the gas pressure drop and gas velocity. Refer to the Figure below that shows a typical gas pressure drop in a packed column.

The horizontal axis is the logarithmic value of the gas velocity G, and the vertical axis is the logarithmic value of pressure drop per height of packing [ pressure drop in a packed bed is the result of fluid friction that is created by the flow of gas and liquid around the individual solid packing materials ].

Note: Each packing has its own characteristics pressure drop chart as reported by the manufacturer - for example, see the Figure above (right).

Analysis of Gas Pressure Drop in Packing

With a dry packing (i.e. no liquid flow, L = 0), pressure drop increases as gas velocity increases according to the linear relationship as shown by line a-a. This is a straight line on a log-log plot.

With liquid flowing in the column, the packings now become wetted (irrigated). Part of void volume in the packings now filled with liquid, thereby reducing the cross-sectional area available for gas flow.

At the same gas velocity, the pressure drop is higher for wetted packings compared to dry packings. For example, compare the case for L = 0 vs. L = 5. The line for DP/L under wetted condition lies to the left of line a-a.

For a constant liquid flow (say L = 5), at low to moderate gas velocity G; the pressure drop characteristics is similar to that of dry packings, i.e. section b-c of the plot is still straight on log-log plot. Up to this point, there is an orderly trickling of the liquid down the packings. There is no observable liquid being trapped among the packings (no liquid hold-up).

As the gas velocity is increased further, the pressure drop increased. Some liquid started to be retained in the packings. When point c is reached, the quantity of liquid retained in the packed bed increases significantly. There is a change in slope of the line at point c as pressure drop increases more rapidly with G. Point c is known as the loading point, as liquid starts to accumulate (load) in the packings.

From point c to d to e, there is a sharp increase in pressure drop at higher G: there is a greater amount of liquid hold-up, a gradual filling of the packing voids with liquid (starting at the bottom of the column), and the column is slowly "drowned" in the liquid.

At point e, there is another sharp change in the slope. At this point the entire column is filled liquid and the gas now has to bubble through the liquid in the packing voids. The gas pressure drop is now very high. Point e is known as the flooding point. The gas velocity at this point is known as the flooding velocity (limiting velocity).

Points to note :

- at constant liquid rate, gas pressure drop increases with gas velocity.

- at constant gas velocity, the gas pressure drop is higher at larger liquid rate.

- each liquid rate has its own loading and flooding points.

- at higher liquid rate, the loading and flooding points occur at lower gas pressure drop.

Operation of a gas absorption column is not practical above the loading point. For optimum design, the recommended gas velocity is 1/2 of the flooding velocity. Alternatively, some design can be based on a specified pressure drop condition, usually well below the pressure drop at which flooding would occur.