DE2718129A1 - Improving countercurrent diffusion by material pulsation - using unequal half periods from a cam operated piston, e.g. for extn. or gas-liq. contact process - Google Patents

Abstract

A method for intensifying mass exchange in diffusion processes and of increasing the speed of movement of the continuous phases in counter-current processes involves pulsating the materials involved. With a pulsation frequency between 5-150 Hz and an amplitude determined by the process, the time in which the compression, upward stroke or half period (lambda/2), takes place is varied from the time for the decompression, downward stroke or second half period (lambda/2)2. The ratio of such half periods (lambda/2)1/(lambda/2)2 is 0.1-20. The half periods are so chosen that the first half period promotes the mass exchange and the second half period accelerates the counter-current speed of the dispersed phases. Used in counter-current diffusion processes, such as extraction processes or gas-liquid contact. The contact surfaces can be regulated by the gas bubble size according to requirements; column diameter can be increased; the phases are dispersed uniformly across the column.

Description

Process for intensifying the exchange of substances

in diffusion processes The present invention relates to a process for intensifying the exchange of substances in diffusion processes unequal oscillatory (pulsatory) movement of the mixtures with one for the process optimal frequency and amplitude, the half-periods (upward thrust (A / 2) 1 and Downward thrust (> / 2) 9) are different. The values of (/ 2) 1 and (X / 2) 3 become chosen so that on the one hand the mass transfer and on the other hand the movement of the discontinuous phase (gas bubbles or liquid droplets) are accelerated.

It is known that to intensify mass transfer processes, especially in extraction processes, currently more and more frequent pulsation with the same Half-periods is used, which is due to the following advantages of the pulsation columns is based: - It finds a more uniform distribution of the disperse phase based on the column cross-section instead.

- The contact surface can be adjusted according to the leaf size, which depends on the frequency and the amplitude of the pulsation.

- Due to the fact that they remain unchanged for longer periods of time Surface of the disperse phase can use the countercurrent principle even with several theoretical Transport units are applied.

- The scale factor is equal to one, so when enlarged the column diameter from, for example, laboratory scale to pilot and industrial scale, while maintaining the separation effect in question, the height of the column remain the same.

It is also known that as a function of frequency and amplitude a mixing effect occurs, which reduces the extraction effect. By the pulsatile movement to which the continuous phase is exposed is reduced With increasing amplitude and frequency the movement in the countercurrent of the dispersed ones and continuous phase until the "gold up" occurs.

These disadvantages are not only eliminated by the present invention Rather, through the correct choice of the ratio (a / 2): / 2) 2 a Increase in the countercurrent velocity of the two phases can be achieved also the countercurrent velocity with a unidirectional flow (without pulsation ) exceeds. By applying the inventive concept, the extraction effect the previously known pulsation columns as well as the non-pulsed extraction columns are largely exceeded.

In absorption and / desorption processes, the use of found from pulsation columns that the mass transfer in the liquid phase, expressed in the product Kl.a of the mass transfer number and the related phase interface in the examined ranges of frequency (0 to 30 Hz) and amplitude (O up to 25 mm) with increasing frequency and increasing amplitude up to 8 times The value increases compared to the non-pulsed gas flow. The mass transfer number Kl only becomes positive in the area of small liquid loads, i.e. small film thicknesses influenced by the pulsations.

As with the liquid-side mass transfer, there was an increase of the product Kg.a from mass transfer number and related phase interface with increasing Frequency and increasing amplitude of the vibrations impressed on the gas phase. However, this increase is considerably less than in the case of the liquid-side mass transfer.

Backmixing is considered to be a possible cause for this event considered to be the gas phase caused by the pulsation. Through this back mixing becomes the improvement possible due to the higher turbulence in the case of a pulsating gas flow the mass transfer is not fully effective. (S. Sicardi, Chim & Ind. Suppl (1975) 2) According to the invention, these negative effects are reduced by lengthening the half-cycle - Compression and shortening of the half-period expansion in the absorption process and vice versa largely prevented in the desorption process. By extending the half-period - Compression with positive effects on the transport of electricity in the direction of the gas c fluid and the shortening of the half-period expansion, being a mixing takes place, which, although less effective, still has a positive effect on the change in the concentrations at the interface of the two phases.

According to the invention, the absorption process takes place among advantageous Conditions instead of when the liquid phase is the continuous in the column used Phase represents and the disperse phase consists of gas bubbles which are involved in the exchange of substances involved components included.

It is also known that pulsation columns are also used for rectificative separation can be used. Such processes can be found under varying within narrow limits Pressure instead, because the compression ratio is relative due to the large volume of steam must be kept small.

Due to the pressure fluctuations caused by the pulsation the efficiency of the individual trays increased, or in the case of packed columns the equivalent height of the filling is reduced; the speed increase on the However, the interface and the intensification of the phase contact cause this The effect is still below the values of a theoretical soil based on a real ground remains. Thus the efficiency of the separation is also for pulsation columns primarily depending on the reflux ratio and that by the design the number of theoretical plates reached in the column.

The inventive method applied to rectification eliminated partly these disadvantages by a new rectificative separation system, thereby characterized in that the column is completely filled with boiling liquid, in which, according to the amplitude, frequency, the difference between (J / 2) 1 and (1/2) 2, and the nature and composition of the components pulsatory pressure fluctuations form.

Due to the boiling liquid due to the supply of heat to the lower part of the column, upward moving gas bubbles arise with the adjacent liquid are in a lively exchange of substances, caused due to the compressions taking place in different time units and subsequent Expansions, causing alternating condensation and evaporation in the gas bubbles take place and the turbulence inside the gas bubbles as well as at the interface increases sharply.

These conditions are in addition to the effect of the relationship the partial vapor pressures also evaluate other material properties of the components; For example, the molecular volume and intermolecular cohesive forces involved in the turbulent movement, generated by the pressure fluctuations, is a weighty one Influence on the movement of the molecules and their accumulation in one of the have both phases. This enables rectificative separations to be made, which by ordinary rectification, for example because of the very close boiling points of the components is not possible.

Based on the figures, the method according to the invention is intended in detail to be discribed. Fig. 1 shows the device for generating unequal pulsations and their attachment to columns in which diffusion processes take place.

With the help of cam 1, which describes a rotational movement ( In Fig. 1 the direction of rotation is indicated by an arrow), unequal oscillative (pulsatory) movements are generated. In the example shown takes the half-cycle (> / 2) 1 on the figure with 2 denoted 3/4 of the duration of one whole period, and the half-period (# / 2) 2, which corresponds to the extension (in 1 denoted by 3) 1/4 of a whole period.

By increasing the speed of the cam, you can get the desired Reach the frequency of the pulsation. The difference between the largest and the smallest The radius of the cam represents the amplitude. By choosing a suitable profile the cam the desired ratio between (1/2) 1 and (1/2) 2 can be achieved.

onno The connecting rod is on the cam 1 via the fields 5 4 pressed, which is aligned by the bearing 6.

In cylinder 7, the diameter of which is adapted to the column volume, the piston 8, which is connected to the connecting rod 4 by the bolt 9, moves is. The cylinder 7 is through the opening 10 with the interior of the column in which the exchange of substances takes place, connected. To prevent solids from getting over the opening 10 get into the cylinder 7, this is provided with a sieve 12.

The cylinder 7 and piston 8 can be through an elastic membrane be replaced.

Fig. 2 shows a diagram in which the fading of the braking effect the particle (ordonate) is plotted in relation to the amplitude (abscysse). The braking effect is defined as the ratio of the mean rate of descent over time in the oscillating flow field to the rate of descent in a liquid at rest Vo Wg The curves represent approximate analytical solutions of the quasi-static equation of motion for the unidirectional flow (curve 1), the alternating flow (curve 2) (hiit pulsation) (H.W.Ho. Dissertation Iowa-University 1964) and the unequal alternating outflow (unequal pulsation), in which (X / 2) 1 = 3/4 and (X / 2) 2 = 1/4 occupies the duration of a period (curve 3).

The hatched horizontal area delimits the types of movement of rectified and alternating flow against each other.

The representations in Fig. 2 clearly demonstrate the advantages of the dissimilar Pulsation (curve 3) the undesirable braking effect of the disperse phase, which becomes noticeable in processes without pulsation as well as with the usual pulsation (especially in extraction processes in which the movement of the disperse Phase due to the small differences in density between the two liquid phases is difficult) is omitted. To counteract the mixing effect, the Columns are provided with vertically arranged thin-walled tubes, the diameter of which is of the same order of magnitude as that of the ascending gas bubbles. Through this Measure, the transverse mixing becomes perfect and the longitudinal mixing the greatest Part canceled and an increased efficiency in terms of the equivalent length an HTU (height of a transition unit) is achieved.

In the laboratory tests, a column with an internal diameter of 80 mm was used and a filling height of 600 mm is used. The filling consisted of glass tubes with an inside diameter of 3 - 5 mm and a wall of 0.5 mm.

The mixture to be separated consisted of a solution of 50% ethyl alcohol in water. The mixture was placed halfway up the filling. After a Time of about 30 minutes, necessary to reach a steady state, could by varying the frequency between 2 - 30 Hz and using 3 different ones Cams that guaranteed a change in amplitude from a single to a double value and at the same time it made possible the ratio # 1 between 5/6 / 1/6 = 5 and 2/3 / 1/3 = 2, set a-2.

Optimal values were achieved at a frequency of 25 Hz.

The compression ratio was around 0.2: 1 and 0/2) 1 / (s / 2) = 4.5.

Claims (3)

PATENT CLAIMS 1. Method for intensifying the exchange of substances in diffusion processes and the increase in the speed of movement of the phases in countercurrent processes by pulsation, characterized in that at a frequency of 5 - 150 Hz and an amplitude corresponding to the process, the time in which the Compression, up thrust, or half cycle (A / 2) 1 taking place is different of the time corresponding to the expansion, downward thrust or half-period (A / 2) 2, the ratio A / 2) (x / 2) 2 takes values from 0.1 to 20, and that the half-periods be chosen so that the one half-period supports the exchange of substances and the second half cycle accelerates the countercurrent velocity of the disperse phase. 2. The method according to claim 1, characterized in that the dissimilar Pulsations can be generated with the help of a cam that the desired ratio between (h / 2) 1 and (A / 2) 2 is reached across the profile of the cam and that the unequal Pulsations via a connecting rod and a piston moving in a cylinder or transferred into the column via an elastic membrane. 3. The method according to claim 1 and 2, characterized in that at its application in absorption and rectification processes, the liquid phase represents the continuous phase and the discontinuous phase is gaseous, that the column filling made of thin-walled glass tubes whose diameter is different from the same It is of the same order of magnitude as that of the gas bubbles and that the said tubes are parallel stand at the column axis.