Packed Column Experiment (Lab Report)

Packed Column

PRESSURE DROP AND FLOODING

The packed column is used in industry to produce mass transfer, i.e. gas absorption, distillation, and liquid extraction.  This experiment is intended to study the factors affecting the capacity of a packed column to handle liquid and gas flows.  The flow will be counter-current:  gas will move upwards and liquid will move downwards.

As the flow rate of liquid or gas is increased through a packed column of constant diameter, the pressure drop per foot of packing increases.  If there is no liquid, so that the column is dry, we have a case of gas flowing through a packed bed.  In that case we might expect the Ergun equation (Treybal¹, p. 200) to apply.  If liquid is flowing counter-current to the gas, each phase will take some of the room in the column, so each will have an effect on the pressure drop.  We can get some idea of the accuracy of an empirical correlation in the literature by comparing measured pressure drops with values predicted by the correlation for the same conditions.

To have flow of gas upwards through the column, the pressure must be higher at the bottom of the column than at the top.  The liquid flows downward through the packing against the pressure and the flowing gas phase because the liquid is appreciably denser than the gas and so is pulled down by gravity.  The pressure gradient in the column opposes the flow of liquid.  If we keep the flow rate of either liquid or gas constant and increase the flow rate of the other phase, we will eventually come to a limiting condition in which counter-current flow cannot be maintained.  This limiting condition is called flooding.

In practice, the diameter of a packed column is designed for a certain approach to flooding.  The diameter of the column is calculated so that the design gas rate is usually 50 to 70 percent of the flooding rate.  This percentage approach is determined by economics and by the uncertainty of predicting the flooding point.  Decreasing the column diameter for constant mass rates of flow gives higher flow rates of liquid and gas per unit area, and so higher pressure drops and larger pumping costs.  At the same time, increasing the column diameter gives larger equipment costs.  Thus there will be an economic optimum diameter depending upon relative costs and the relation between pressure drop and flow rates.

Experimental Apparatus

The packed column is a 9-foot QVF glass tower with an inside diameter of 5.84 inches.  It contains 5/8-inch pall rings as packing in a bed approximately 5 feet deep (measure more exactly).  It is fitted with a small gas saturator upstream of the tower to minimize evaporation of water in the main column during as absorption

A Roots blower supplies air to the system.  Gas flow can be controlled with the variable speed drive and/or a bypass valve.  Manometers filled with various liquids measure blower exhaust pressure, pressure drop across the orifice, and pressure drop across the column.  The orifice is of diameter 1.501 inches in a pipe of diameter 2.067 inches, and the orifice coefficient can be taken as 0.61

Water flow rates are determined with a rotameter.  The calibration curve is posted near the column.  Adjustable legs are provided to adjust water levels while maintaining liquid seals to prevent leakage of gas.

Procedure

(a)        With dry packing (no water) use four widely different gas flows, with the highest flow giving a pressure drop over the packed column of about 10 cm. of methanol.  The bypass valve should be used to get the smallest flows.  Measure flows and pressure drops.  Remember that gas temperature and absolute pressure will be needed.

(b)        Pressure drops for wet packing should be measured for a minimum of four different gas flows for each of three different liquid flows.  The gas flows should be over about the same range as in part (a).  The largest liquid flow should be close to the maximum water flow posted on the control panel, and the range should be as wide as practical.  Note that water flows above the posted maximum may force water into the manometer containing fluid.

(c)        At the highest liquid flow of part (b), further runs should be done at successively higher gas flows (each one giving a pressure difference across the orifice about 20% larger than the one before).  For each run, besides measurements of flows and pressure drops the appearance of the system at all points should be noted.  The gas flow should be increased until signs of flooding are observed.

Calculations and Technical Report

(a)        From your experimental data, to what power (exponent) of the mass velocity is the pressure drop for dry packing proportional?  Does this indicate that the flow is laminar or turbulent?  Is this result consistent with the relative size of the two terms of the Ergun equation?

(b)        Compare the measured pressure drops for dry packing with the correlation given by the Ergun equation (see Treybal¹).  Note that the pressure difference across the orifice can be related algebraically to G, the superficial mass velocity of gas in the packed column.  Thus it is not necessary to calculate intermediate quantities such as the superficial linear velocity in the column.

(c)        Compare your measured pressure drops for wet packing with the two attached generalized correlations (due to Eckert et al.) which are found in Treybal (Figure 6.34)¹ and Bennett and Myers (p. 613)².

(d)       Plot log (DP) vs log G for each value of L, the mass velocity of the liquid.  Compare the flooding point indicated by this plot with the flooding point observed visually based on the correlation of B. Milne (1994) on the next page. This is a generalized correlation of the form equivalent to the graphic in Treybal(1980) Figure 6.34.

(e)        Design, on the basis of your measurements rather than the correlations in the literature, packed with the same packing as in the laboratory and at the same liquid mass velocity as for your highest liquid flow, a column to treat 5,000 kg/h of gas if the gas rate is to be 65% of the flooding rate?  What pressure drop per foot of packing would be expected?

Technical Letter

Give a Brief comparison between your experiment results and those in the literature and then present the results of your design study.