Method for Regulating the Flow of Liquid Through Mass Transfer Columns

This invention relates to a method for regulating the heights of bubble layers in multistage mass transfer columns having gas-distributor plates arranged at intervals one above the other, in which the compartments formed by two vertically adjacent gas-distributor plates communicate with one another by means of tubes.

In conventional multistage mass transfer columns, the bubble layers on the individual trays are largely constant in height. The countercurrent columns are essentially plate columns, in which the overflow baffles known from the distillation art are elevated in order to obtain tall bubble layers and, hence, prolonged liquid residence times. The columns generally used for co-current operation are plate columns which do not have any overflow baffles or run-off ducts so that the liquid and gas have to flow together through the plate openings. Both countercurrent and co-current operations are characterized by the absence of a continuous column of liquid extending over the entire height of the column.

German Patent Specification No. 335,552 describes a multistage mass transfer column with sieve plates in which the individual compartments communicate with one another by means of run-off tubes in such a way that a continuous column of liquid is formed over the entire height of the column. If the gas load is increased, the height of the bubble layer decreases. Conversely, if the gas load is reduced, the height of the bubble layer increases. This contrary behavior has an adverse effect upon the required mass transfer.

An object of the invention is to provide for the adjustment of the height of the bubble layer in each compartment of a mass transfer column independently of the liquid and gas loads selected. The gas and liquid are intended to be able to flow through the mass transfer column concurrently or counter-currently.

Another object of the invention is to provide a method for regulating mass transfer columns known per se with a continuous column of liquid over the entire height of the column in such a way that the bubble layers in each compartment can be adjusted to heights which are unaffected by the liquid and gas loads selected.

Other objects of the invention will become apparent from the following description of an example of the invention with reference to the accompanying drawing, wherein:

FIG. 1 shows the structure of a mass transfer column for countercurrent operations;

FIG. 2 shows the mass transfer column in counter-current operation for deriving formulae;

FIG. 3 shows the structure of the mass transfer column for parallel-current operation; and

FIG. 4 shows the mass transfer column in concurrent operation for deriving formulae.

The objects of the invention are achieved, generally speaking, by fitting each communicating tube between the compartments with a throttle member which is controlled by the pressure difference between the gas cushions below the gas-distributor plates of two successive compartments.

The particular advantage of the method according to the invention is that, in the event of any increase in load, the bubble layer can be increased in height and accordingly, in the event of reductions in load, the height of the bubble layer can be reduced. This behavior of the mass transfer column is of particular advantage in cases where a certain average residence time has to be maintained in the column for the liquid phase, for example, in cases where the mass transfer column is operated as a chemical reactor.

Referring now to FIG. 1, the structure of the mass transfer column with gas and liquid flowing countercurrently to each other is illustrated. The mass transfer column is divided into vertically spaced compartments by several substantially parallel gas-distributor plates 5 arranged one above the other at certain intervals. The gas-distributor plates 5 advantageously occupy the entire cross-section of the column. In the stationary phase, a liquid-bubble layer 1 and a gas layer 2 are present in each compartment. The liquid is delivered to the uppermost compartment through an inlet 3, while the gas is introduced at the lower end of the mass transfer column through an inlet 4.

The gas, which flows upwardly through the column, is dispersed through each distributor plate 5 and enters into intensive mass transfer with the liquid in each bubble layer 1 before collecting in the gas layer 2 below each distributor plate 5. The gas leaves the mass transfer column through the outlet 6.

Plates 5 are impervious to liquid so the liquid, which flows downwardly through the mass transfer column passes through an outlet 7 from each plate and below a communicating tube 8 into the compartment below until the liquid leaves the mass transfer column through the overflow 9. The gas-distributor plates are designed in such a way that the liquid is unable to flow through the plates, so that the liquid passes from one compartment to the next solely by means of the communicating tubes 8 which are each fitted with a throttle unit 10.

The way in which the method according to the invention works in a countercurrent bubble column is illustrated with reference to FIG. 2. In the stationary phase, an equilibrium prevails between the pressure drop along the column axis 11 between points 12 and 15, and the pressure drop in the communicating tube 8 between points 16 and 17, so that:

according to the equation

the pressure drop along the column axis .DELTA.P.sub.-12-15 is made up of the following constituents:

.DELTA.P.sub.-SP.sbsb.12-13 = hydrostatic pressure of the bubble layer 12-13

.DELTA.P.sub.-BO.sbsb.14 = pressure loss across the distributor plate (point 14)

.DELTA.P.sub.-SP.sbsb.14-15 = hydrostatic pressure of the bubble layer 14-15

The pressure drop in the communicating tube 8 between the points 16 and 17, .DELTA.P.sub.-.sbsb.16-8-17, is the hydrostatic pressure of the ungassed liquid .DELTA.P.sub.-FL.sbsb.16-8-17, less the pressure loss caused by tube friction .DELTA.P.sub.-RE.sbsb.7-8-3 and the pressure loss in the throttle zone .DELTA.P.sub.-DR.sbsb.10 :

accordingly, the pressure equilibrium according to equation 1 is defined as

since the hydrostatic pressure of the bubble layer 18-12 is substantially the same as that of the bubble layer 14-15,

wherein .DELTA.P.sub.-SP is the hydrostatic pressure of the entire bubble layer taken into consideration.

With equation 5, therefore, the basic equation for the pressure equilibrium in countercurrent operation of the multistage mass-transfer column according to the invention, is as follows:

or

in discussing the mode of operation of the column, it is best initially to disregard the pressure loss .DELTA.P.sub.-RE of the flowing liquid in the communicating tubes 8. In addition, it can be negligibly low by adequately dimensioning the diameter of the communicating tubes 8 for all of the liquid loads which occur:

accordingly,

equation 9 shows that, in the event of increases in gas load, i.e. in the event of increases in the pressure loss across the gas-distributor plate .DELTA.P.sub.-BO, the height of the bubble layer or its hydrostatic pressure .DELTA.P.sub.-SP has to be kept lower for a constant setting of the throttle unit (.DELTA.P.sub.-DR = const.), because the hydrostatic pressure of the ungassed liquid in the communicating tube 4 .DELTA.P.sub.-FL represents a constant for a compartment of given height. However, it is also clear that the height of the bubble layer can be prevented from decreasing by compensating for the increase in pressure loss across the gas-distributor plate by reducing the pressure loss in the throttle zone .DELTA.P.sub.-DR. In addition, the height of the bubble layer or its hydrostatic pressure .DELTA.P.sub.-SP can even be increased as required by further reducing the pressure loss in the throttle zone .DELTA.P.sub.-DR.

Conversely, in the event of reductions in gas load, i.e. in the event of a reduction in the pressure loss across the gas distributor plate .DELTA.P.sub.-BO, the hydrostatic pressure of the bubble layer .DELTA.P.sub.-SP, i.e. the height of the bubble layer, can be reduced as required by increasing the pressure loss in the throttle zone .DELTA.P.sub.-DR.

Any change in the liquid load is accompanied by a change in the pressure loss in the throttle zone .DELTA.P.sub.-DR. Unless the throttle unit is readjusted, equation 9 can only be satisfied by changing the term .DELTA.P.sub.-SP, i.e. the height of the bubble layer, which decreases in the event of an increase in liquid load and, conversely, increases in the event of a reduction in the liquid load. This also applies when the pressure loss in the communicating tubes .DELTA.P.sub.-RE is taken into consideration. Since this kind of behavior of the mass transfer column is generally undesirable, it is possible, by suitably setting the throttle unit, to increase or reduce the height of the bubble layer, even in the event of increases or reductions in the liquid load, in such a way that the liquid volumes in each compartment correspond to the residence-time requirements.

The height of the overflow 9 follows the pressure equilibrium.

in which:

.DELTA.P.sub.-19-20 = pressure loss along the column axis 11 between points 19 and 20

P.sub.-20 = pressure at the head of the column at point 20

.DELTA.P.sub.-21-22 = pressure loss in the overflow between points 21 and 22

P.sub.-22 = pressure at the overflow end at point 22

Equation 10 shows that, in the event of changes in the liquid and/or gas loads, the pressure loss in the overflow .DELTA.P.sub.-21-22 has to be adapted to the new situation on the account of the resulting changes in .DELTA.P.sub.-19-20 (P.sub.-20 and P.sub.-22 being constant).

The structure and mode of operation of the mass transfer column according to the invention with the gas and liquid flowing concurrently will be described in the following with reference to FIGS. 3 and 4:

The mass transfer column itself is no different in structure, apart from the inlets and outlets for the liquid, from the mass transfer column already described in detail for countercurrent operation. The gas and liquid both flow upwardly through the column, the gas being introduced at the lower end of the column through an inlet 54 while the liquid is introduced into the lowermost compartment of the column through an inlet 53. The liquid, which passes from one compartment to the next through communicating tubes 58, leaves the uppermost compartment of the column through an outlet 57 and an overflow 59. As in countercurrent operation, the gas only flows along the column axis 61 and leaves the uppermost compartment through an outlet 56.

The mode of operation of the mass transfer column according to the invention with gas and liquid flowing in parallel current is described in the following with reference to FIG. 4. In the stationary phase, the equilibrium

or, disregarding .DELTA.P.sub.-RE,

prevails between the pressure drop along the column axis 61 between points 62 and 65 and the pressure drop in the communicating tube 58 between points 66 and 67. In contrast to counter-current operation, according to equations 7 and 9, the terms .DELTA.P.sub.-RE and .DELTA.P.sub.-DR receive a positive sign in concurrent operation. Since these terms are only influenced by the liquid load, the column operated concurrently behaves in the same way as the countercurrent column in regard to changes in the height of the bubble layer accompanying changes in gas load. By contrast, it behaves oppositely in the event of changes in liquid load. Accordingly, changes to the throttle unit itself in co-current operation affect the changes in the height of the bubble layer oppositely in relation to concurrent operation.

The overflow height can be determined from the pressure equilibrium equation 13:

in which

.DELTA.P.sub.-65-70 = pressure loss along the column axis 61 between points 65 and 70

P.sub.-70 = pressure at the head of the column at point 70

.DELTA.P.sub.-71-72 = pressure loss in the overflow between points 71 and 72

P.sub.-72 = pressure at the overflow end at point 72

Equation 13 shows that, in the case of co-current operation, the overflow height is governed solely by the hydrostatic pressure of the bubble layer (height) in the uppermost compartment of the column where P.sub.-70 and P.sub.-72 are constants.

Although the invention has been described in detail for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.