DE2552803A1 - METHOD AND DEVICE FOR DRYING BY MOLECULAR Sieves - Google Patents

Description

UEXKÜLLi STCLB RG PATENTANWÄLTE

2 HAMBURG 52

BESELERSTRASSE 4

DR. J.-D. FRHR. by UEXKÜLL

DR.ULRICH GRAF STOLBERG fj JT Γ O p Γ) O DIPL-ING. JÜRGEN SUCHANTKE

W. R. Grace & Co. (Priority: November 27, 1974

and December 23, 1974, US 527 832 and US 535 282

Grace Plaza - 12719 -)

1114, Avenue of the Americas

New York, NY / V.St.A. Hamburg, November 24, 1975

Method and device for drying by means of molecular sieves

The invention relates to a method and a device for drying by means of molecular sieves, in particular a method for removing moisture from fluid streams and for regenerating or reactivating moisture-laden ones Alkali aluminum silicate zeolite particles.

Made from natural or synthetic crystalline alkali metal aluminum silicate zeolites manufactured molecular sieves have been found to be particularly effective in cleaning fluid streams. They are used in particular for drying air, petroleum feedstocks or industrial gases.

Gases can be generated with the help of molecular sieves or zeolites a few ppm water content can be dried. In cryogenic systems gases must be dried to a fraction of 1 ppm in order to defrost heat exchangers even at up to

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to make gases heated to the liquid phase practically superfluous. Such super drying can be achieved even when the feed gas has a high temperature, because the dew points when drying with molecular sieves are not dependent on the inlet temperature and because these are unique absorbents maintain a high absorption capacity even at high working temperatures. Processing of proportionately warm input product for the production of a completely dry gas represents an exceptional property of Molecular sieves. Their effectiveness is also unaffected by the degree of saturation of the feedstock.

Rapid drying of large quantities of fluid is particularly required in petroleum refineries where large quantities are used daily of fluid hydrocarbons are to be processed. Becomes a feed product with such a reduced water content is used, the yield obtained therefrom is in spite of the cost of drying with chemical drying agents more economical than with water-containing feedstock. Although the continuous drying of hydrocarbonaceous Fluids an application of the method according to the invention in large scope, the invention is also for numerous, essentially non-reactive with the respective desiccant liquid or gaseous fluid flows can be used. Water-containing streams such as atmospheric nitrogen, carbon monoxide,

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Carbon dioxide, halogenated hydrocarbons, chlorobenzene and other streams do not react with the corresponding ones inorganic drying agents and are therefore used as starting products for drying processes with the aid of molecular sieves applicable. The molecular sieves are inert to most of the fluids used and even with a water content physically stable at normal bed depths.

The drying properties of molecular sieves reach up to higher temperatures than known adsorbents. That Usual absorption capacity is 16.5% at 95 ° C and 4% even at 230 ° C. The amount of water absorbed has up to the breakthrough point, at which the vapor pressure rises abruptly, hardly any influence on the drying properties of the molecular sieves. Leave it achieve dew points below -75 C even for gases with a temperature of 100 C. Molecular sieves dry gases with high Surface velocities even with low relative humidity of the feed gases. The speed is for Zeolites usually 10 to 50 m / min. For drying, molecular sieves with a smaller pore size of about 3 8 are often used for reduction used by coadsorption of other substances. Such synthetic crystalline alkali metal aluminum silicates des Faujasite types are described, for example, in U.S. Patents 2,882,243 and 2,882,244.

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In preparation for the next adsorption step, the exhausted bed must be regenerated to remove the adsorbate, Usually the main stream is used to maintain a continuous process during this regeneration passed a second adsorption tower or column. at known processes, regeneration was carried out in various ways depending on the technical and economic Conditions carried out. The known regeneration processes were based on the same principle, namely that Process conditions surrounding adsorbents became those corresponding to a very small equilibrium capacity Conditions changed. The greater the difference between the equilibrium absorption capacities of adsorption and regeneration in general, the faster and more complete the regeneration.

In typical cyclic arrangements, the adsorbate is removed from the molecular sieves by heating and purging with a carrier gas expelled. This regenerates the adsorbent and prepares it for the next adsorption cycle. A sufficient amount of heat must be used during regeneration to heat the adsorbent, adsorbate and tower are available and there must also be heat to evaporate the liquid and the heat of wetting to be transported away from the surface of the molecular sieve.

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In order to increase the amount of heat supplied, gas temperatures above the boiling point of the adsorbate are often chosen. In connection with the regeneration temperatures, the The temperature of the molecular sieve bed is a critical factor. Bed temperatures of 200 to 300 ° C. are usually chosen.

After regeneration, the molecular sieve temperature is raised to about 15 to 20 above the treatment temperature in a cooling period of the stream cooled. This is conveniently done using the same gas flow as for heating, but without Heat supply. The thermal process includes heating to a temperature at which the adsorption capacity is at a low value is reduced so that the adsorbate emerges from the molecular sieve surface and in a weak purge gas flow is easily removable. This can be done at operating pressure or at negative pressure.

The pressure change regeneration method is similarly dependent on the reduction in adsorption capacity by a Pressure decrease at an essentially constant temperature.

According to another method, the adsorbate is without temperature or Change in pressure is removed by passing a liquid or gaseous fluid without absorbable molecules through it, in the the adsorbate is soluble or miscible.

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Changing the temperature or pressure by passing through a fluid with a high concentration of absorbables Molecules can also affect desorption. Because of this high concentration, the adsorbable molecules displace the previously adsorbed substance. For liquids, the resulting mixture is then distilled separated into an extremely pure end product and the reused regeneration fluid.

US Pat. No. 3,474,023 describes the regeneration of a wet molecular sieve bed by means of electrolysis by applying a weak direct voltage to the formation of H ₃ O ₂ . The ÜS-PS.3 359 also describes the use of high-frequency electrical energy for dielectric heating of the particulate bed. ·

The heat of adsorption and the heat of evaporation of water are known to be about the same. The well-known regeneration processes use far too much of it to restore the adsorbate. For the widespread use of molecular sieve adsorption, especially when drying air, hydrocarbon-containing feedstocks and industrial gases there is a strong need for a rapid and economical molecular sieve regeneration process.

This task was accomplished with an improved gas dryer a gas line with an inlet opening for the to be dried

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Gas and an outlet opening for the dried gas as well as with a bed of a zeolitic adsorbent lying in the gas line between the inlet opening and the outlet opening. According to the invention, an electric current for example passed through the adsorbent bed by means of two electrodes. The applied current frequency is like this small that sufficient to evaporate the adsorbed water Heating of the bed is excluded and that the current density is not sufficient to hydrolyze the adsorbed water.

In one embodiment of the invention, the electrical. current used to detect changes in the conductivity of the bed as an indication of an increase in moisture in the bed, so that when a predetermined moisture content is reached, a warning signal is given indicating the point in time for an interruption of the adsorption cycle and for the initiation of a regeneration cycle. The invention provides facilities for measuring the electrical current flow in the bed, devices for generating a signal as a function of the adsorbed water content of the bed as well as devices controllable by the signal for interrupting the gas flow through the bed at a given sub-saturation water content.

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In another embodiment of the invention, the electric current is used to regenerate the bed. It was found It is surprising that zeolitic particles can be reactivated by means of electrode desorption to remove sorbed gas. According to the electrode sorption process, molecular sieve particles loaded with sorbed gas are trapped between electrodes, an electrode sorption voltage is applied to the electrodes and the desorbed gas from the molecular sieve separated. The molecular sieve consists essentially of synthetic crystalline alkali metal aluminum silicate in the form of zeolites with particle sizes of at least about 1 / µm. Preferably the adsorption bed contains zeolitic ones Particles in powder form with a mean particle size of about 3 to 5 μm or bound particles of about 100 to 500 μm. Also in accordance with the invention is means for maintaining a substantially constant regeneration flow

2 provided by the bed from about 0.01 to 100 µA / cm. the The device according to the invention is used in particular to regenerate moist zeolites.

The regeneration current is, for example, a direct current or an alternating current with a frequency of up to 10 Hz. The electrical Field for regeneration should be at least 0.2 kV / cm.

Preferably, the two embodiments of the invention Method, namely the attraction of a current

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to detect adsorption of water and to indicate a required regeneration cycle and use of electrical power for regeneration itself can be combined in a single embodiment according to FIG. 1.

In the following the invention is explained in more detail with reference to the figures; show it:

Fig. 1 is a partially sectioned side view of the invention Electrode desorption device;

2 shows an electrical circuit of a controller for the electrode sorption method according to the invention;

Figures 3 and 4 are schematic drawings of other embodiments of the invention;

5 shows a diagram of the adsorbed water and the specific resistance as a function of the flow time;

Fig. 6 is a graph of the electrical resistivity and moisture content of a molecular sieve bed;

7 shows an embodiment for determining changes in the moisture content;

8 shows another embodiment for determining changes in the moisture content.

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Molecular sieves consist essentially of crystalline, hydrated metal aluminum silicates with a number of unusual properties. The most important molecular sieves are produced synthetically, but their structure is the same essentially the naturally occurring minerals known as zeolites. Although the crystal structure of some Molecular sieves is extremely different, with two types, A and X, having the greatest importance, their importance lies as commercial adsorbents in the connecting cavities of the same size separated by smaller openings or pores of the same size. During the formation, this crystalline structure is filled with water, which, however, comes off when the temperature is low the cavities can be driven out without changing the crystalline structure. This leaves the cavities with all of them Surface and pore volume available for adsorption of water or other substances. The evacuation and refilling the cavities can be repeated indefinitely under favorable conditions.

With the help of molecular sieves, a very precise process control is possible possible, since the pores of the crystalline structure are essentially the same size, which is the case for other adsorbents does not apply. Because of this large surface area and pore volume, molecular sieves are used to separate molecules feasible, the uniform pore dimensions serving to differentiate between molecular size and arrangement.

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Molecular sieves are crystalline metal aluminum silicates with three-dimensional lattice structure made of silicon dioxide and aluminum oxide tetrahedron. This extremely uniform crystalline grid structure is the reason for the excellent drying properties together with a high absorption capacity even at elevated temperatures. Some molecular sieves are also able to to indicate relative humidity by changing color, which can be used to determine the reactivation point can.

The crystalline metal aluminum silicates have a three-dimensional one nested lattice structure of silicon dioxide and aluminum oxide tetrahedra. The tetrahedra are by four oxygen atoms surrounding a silicon or an aluminum atom are formed. Each oxygen atom carries two negatives and each Silicon atom carries four positive charges. This forms a tetrahedron that can be evenly connected in four directions. the Trivalence of aluminum causes a negative charge for the aluminum oxide tetrahedron, which is an additional cation for neutralization needed. Thus, sodium, potassium, calcium or other cations are present in a closed lattice. These charge-neutralizing cations are the exchangeable ions of the zeolite.

In the crystal form, up to half of the quadrivalent silicon atoms can be replaced by trivalent aluminum atoms. It

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are zeolites with different silicon to aluminum ion ratios as well as with different crystal structures with different cations.

In the most common zeolite A, the tetrahedra are to a truncated octahedron with a silica or alumina tetrahedra grouped at each point. This structure is called a sodalite cage.

The assembly of sodalite cages into simple cubic shapes results in a grid with 11.5 A cavities, which are open on all six sides. Eight oxygen ions surround each of these openings. One or more exchangeable cations also partially close off this face. In the sodium form, the oxygen ions form Openings to the interior of the zeolite with a diameter of 4.2 A. This crystalline structure is as follows Formula can be represented:

₁₂ [(AlO ₂ J ₁₂ (SiO ₂ ) 12] XH ₃ O

The water of hydration that fills the cavities during crystallization is loosely bound and can by little Heating to be removed. The free spaces previously occupied by the water are due to the adsorption of a multitude of gases or liquids refillable. The number of absorbable water molecules, the value X, can be up to 27.

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The sodium ions belonging to the aluminum tetrahedra tend to block the openings or, vice versa, help to prevent the passage of molecules that are somewhat too large due to their electrical charge. Such a molecular sieve with sodium atoms is known commercially as a 4A sieve and it has openings of approximately the same size and approximately 4 S in diameter.

Because of their base exchange properties, zeolites are with other metals replacing part of the sodium atoms can be produced. Of the synthetic zeolites, industrial uses especially two as extreme proved useful. By replacing a large proportion of the sodium ions with potassium ions, a 3A molecular sieve is made with Openings of about 3 A are formed. Instead, calcium ions are used used in place of the sodium ions, then an SA sieve with openings of about 5 8 in diameter is formed. That The crystal lattice of a zeolite X is formed into a tetrahedral arrangement by the arrangement of the elementary sodalite cells Diamond structure formed with bridges over the six-part oxygen ring. These rings form openings of 9 to 10 S Diameter to the inside of the crystal lattice. As with the Α-type, the entire electrical charge is through positively charged Neutralizes cations. The chemical formula of the unit cell of a sodium type X molecular sieve is as follows:

(SiO ₂ ) _{10 6} ] XH ₂ O

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As with the Α type, the water of hydration is by slight heating removable and the cavities formed can be filled with other liquids or gases. The value of X can be up to 276.

A primary required property of any adsorbent is a large surface area per unit volume. In addition, the The surface is chemically inert and accessible for the adsorbate or adsorbates be. Theoretically, the rate of adsorption of the molecules, together with other factors, depends on their contact with the surface and the diffusion rate after contact. One or the other of these factors is different controllable according to the circumstances. One way to increase mass transfer is to reduce the size of the adsorbent particles.

Although synthetic crystalline zeolites are relatively are small, for example 0.1 to 10 µm, these small particles can be bonded or agglomerated into larger shapes. Typical commercially available spherical particles have a mean bound particle size of 100 to 500 µm, which is a Mesh grid of 4x12 corresponds. Other forms of molecular sieve such as pellets with a diameter of 1 to 3 mm, Raschig rings or saddles can be used for continuous sorption processes. Preferred molecular sieves are zeolites

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- I ο -

Types A, L, X and Y or mixtures thereof with average particle sizes from about 1 to 10, u in powder form or from 100 to 500, u for compound forms.

Fig. 1 shows a drying apparatus 10 in partially sectioned ' Opinion. A vertical cylindrical tower 12 forms a drying chamber. The fluid to be dried is through an inlet conduit 14 and a T-piece 15 passed into the chamber 12. At the The lower end of the tower 12 lies in a circular ring 19 held grid 18. The grid 18 is made of metal or another suitable material of sufficient strength to accommodate an absorption bed 20, for example in Form of a zeolitic molecular sieve. A concentric metal electrode 24 is immersed in the tower 12 in the particles 20. Die Electrode 24 is held in a specific position by a bushing 26 that is electrically insulated with a T-piece 17. It is connected to a power source 40 by means of an electrical cable 41. The power source 40 is via a line grounded. The tower 12 can consist of an electrically conductive material, for example steel, in order to provide a direct current path to the particulate bed 20. The tower is grounded via a line 44. A fluid line 28 with a valve 29 serves as an outlet for the tower 12. The line 14 preferably includes a control valve for interrupting inlet flux flow during regeneration. The chamber

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is connected to an outlet opening 16 via the T-piece 17, which is either connected for product delivery or during the regeneration phase to remove the sorbate vapor leads to the atmosphere.

The electrodeorption regeneration process can be performed with dry air flowing through the bed during regeneration Purge gas can be carried out, or a vacuum can be maintained with the aid of valves and pressure seals.

Fig. 3 shows a schematic representation of an embodiment of the invention with parallel fixed electrode plates 12 and 24 spaced apart in contact with the particulate bed. The fluid to be dried is from below up through the bed across the direction of the electrical current between the electrons 12 and 24, respectively.

FIG. 4 shows another embodiment of the invention, wherein a lower grid electrode 12 and an upper grid electrode 24 are arranged in a drying apparatus 10 with an absorbent bed. The fluid flows through from bottom to top the bed parallel to the direction of current between the electrodes 12 and 24. The dry cell arrangement is very diverse Process requirements adaptable. A high gas throughput can be achieved in numerous processes. A condensation of Water vapor or desorbed liquid can cause a liquid

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water flow through the particulate bed to an outlet port according to FIG. 1 require. In other methods, the electrodesorbed component is only in the vapor phase removed.

The tower 12 preferably consists of an electrically insulating one Fabric such as polyvinyl chloride, nylon, phenolic, acrylic or ABS resin, glass, steel lined with glass, or wrapped Fiberglass / synthetic resin. In the case of a cage electrode, the housing is preferably made of metal or lined with metal .

The electrodes consist, for example, of a sintered metal powder, of steel wool, drilled carbon or of other electrically conductive materials with openings. A powdery one Molecular sieve is held in place by porous metal mesh wool electrodes.

The physical state of the bed need not be the same during the drying phase of a fluid as during the To be electrode desorption phase. The packing density is relatively high within certain working limits changeable. During the regeneration phase, the zeolite particles should have an electrical current path from a first Electrode to a second electrode from particle to particle

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set up. A free space of less than 50% by volume is usually sufficient for this. Loosely packed, permeable to fluids Molecular sieve beds have a free volume macroporosity of about 30 to 40% by volume. It is believed that the electric current flow on the particle surface due to the free mobility of the alkali metal ion takes place in the sorbed water phase.

A comparison of the regeneration process according to the invention with a known regeneration process, namely reactivation using heat, illustrates the effectiveness of the energy expended in the process according to the invention. For example, the heat of adsorption and desorption of water vapor on a NaA molecular sieve is around 1000 cal / g H ₂ O at 20 C. Theoretically, the heat of desorption of water in the liquid phase is calculated from the heat of desorption minus the heat of evaporation, i.e. 1000 - 540 = 460 cal /G. According to the method according to the invention, in which water is mostly desorbed from the molecular sieve in the liquid phase, the minimum value for the heat of regeneration theoretically reaches 460 cal / g. According to the invention, however, it has been shown that the energy required to desbrify molecular water at a reasonable rate from an NaA zeolite is approximately one to two times the heat of desorption of steam, i.e. 1000 to 2000 cal / g H ₃ O.

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In contrast, the amount of heat required for desorption of water vapor from an NaA zeolite according to the known one Heating process such as heat of desorption plus heat of evaporation plus the amount of heat required to heat the adsorbent bed and the desorbed water vapor is required to an increased regeneration temperature at which the desorption done at an advantageous rate.

The following example illustrates that according to the invention Process achieved energy saving. Let it be:

Heat content of H ₂ O steam 0.5 cal / g / ° G

Heat content of H ₂ O, liquid 1 cal / g / ° C

Heat of Evaporation of H ₃ O 540 cal / g

Heat of adsorption from H ₃ O to NaA 1000 cal / c Mass density of NaA 0.69 g / cm '

specific amount of heat of NaA 0.25 cal / g / ° C

Saturation of adsorbed on NaA

H ₂ O 0.20 g / g

Regeneration temperature 23O ° C

A conventional adsorption column has a diameter of 183 cm and a height of 305 cm with a volume of 8.03 × 10 cm, which contains 5.53 × 10 g NaA. It is to be dried in the air stream to a dew point of +10 ⁰ C or 3000 ppm H "0th To achieve this degree of dryness, the bed

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subjected to an adsorption cycle until reaching 5% of its saturation capacity. Therefore, the H ₀ corresponds to the amount of 0

in the

in bed:

(1) 0.05 χ 0.2 χ 5.53 χ 10 ⁶ = 0.6 χ 10 ⁵ g

Is the amount of heat lost through radiation and convection during a typical 2 to 4 hour regeneration cycle neglected, then the amount of heat required to heat the NaA in the bed from 230 C to 20 results in:

(2) 0.25 c (230-20) χ 5.53 χ 10 ⁶ = 2.9 χ 10 ⁸ cal

The heat of desorption of water vapor is calculated as follows;

(3) 1000 χ 0.6 χ 10 ⁵ = 0.6 χ 10 ⁸ cal

The one to increase the steam temperature to the regeneration temperature required amount of heat is:

(4) 0.5 χ 0.6 χ 10 ⁵ χ (230-20) = 6.3 χ 10 ⁶ cal

The total amount of heat required in known regeneration processes is the sum of equations (2), (3) and (4) and is approximately (5) 3.6 χ 10 cal.

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In contrast, according to the method according to the invention the amount of heat required to regenerate the same bed is 1 to 2 times the heat of adsorption of liquid water. Based on this assumption, the calculation according to the invention Procedure required amount of heat to regenerate the bed:

(6) 2 χ 1000 χ 0.6 χ 10 ⁵ = 1.2 χ 10 ⁸ cal

A comparison of the products from equations (5) and (6) shows that which can be achieved by the method according to the invention Energy saving of:

χ 100 = 33.3 3.6 χ 10

This means that according to the method according to the invention only about 1/3 of the regeneration of a known manner thermal reactivation of regenerated molecular sieve drying bed required amount of heat is required.

It is clearly evident that the inventive method under the ^N viewpoint of the expended energy is several times more effective than the known thermal methods. It is even more important, however, that the time required for regeneration according to the invention is several times shorter than with

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known thermal processes. According to the known method, about three hours are required for sufficient regeneration Bed of the type described at 23O ° C required. According to the invention, only three minutes are sufficient Regeneration of the same bed. This allows the use of much shorter beds because the bed length is not are more determined by long reactivation times.

Although the causes of the electrode desorption of wet zeolites have not yet been fully explored, one is View that the water is first desorbed as a liquid and from a thin electrolyte layer when it is heated Applying an electric field is evaporated. After removing water through desorption and evaporation increases the mass conductivity.

At a given moisture content measured by the dew point of the gas phase, the conductivity of the bed the power consumption or other physical properties, the regeneration step is ended and the drying cycle continued.

These data were obtained using a # 18 stainless porous mesh Disc electrodes in a cylindrical acrylic plastic housing achieved with a 28 cm long bed 3.2 cm in diameter. The particles were 4 to 5 S Davison Grade 714

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Type X zeolites (8x12 mesh). The air flow through the packed bed was 53 l / min. (STP) with an inlet water content of 2500 ppm and an outlet water content of about 2 ppm. The sieve adsorbs about 6.8% H ₃ O per hour at ambient conditions. A DC voltage of +18.5 V was applied to the electrodes, with the anode in the downstream stream. The steep increase in resistivity towards the end of the cycle indicates that the bed has reached moisture saturation and that regeneration is required.

By applying a constant electric field of about 4000 V / cm to the bed, the bed was regenerated in about 30 to 60 seconds.

6 shows the electrical conductivity as a function of the moisture content for two different arrangements. the Curve A shows the dependence for at the ends of the bed ^ - placed electrodes according to FIG. 4 versus coaxial rod electrodes, curve B, according to FIG. 1.

In the following, the invention will be described using the exemplary embodiment in which the electric current is used for determination changes in conductivity are passed through the adsorption bed as a signal for an increase in the moisture content, and thereby indicating a required regeneration or a beginning of regeneration.

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While the measuring electrodes can be the regeneration electrodes at the same time, according to FIG. 7 it can also be for each function a separate pair of electrodes can be provided. The electrodes 12A and 24A are connected to a high voltage alternating source 40A, which can be excited in accordance with a signal derived from the direct current between the grid measuring electrodes 12B and 24B is. The measuring current flows parallel to the fluid flow, while the regeneration flow is directed transversely to the fluid flow. It will pointed out that these functions are also interchangeable with one another.

The same electrical power source can be used for moisture measurement and regeneration. As usually for reactivation a moisture-laden bed can perform at full power over a period of half a minute to a minute or more is needed, a short voltage pulse can be used to measure moisture without significantly heating the bed be used. In one embodiment, for example, a DC voltage of 20 kV was used during the drying cycle Measurement of the leakage current applied to the molecular sieve bed.

Fig. 8 shows a common electrode arrangement in one Regeneration dryer with an electrically insulated tower and an enclosed molecular sieve bed. The ones in the distance mutually arranged electrodes 12 and 24 are with a

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Voltage source 40 connected. An ammeter connected in series measures the electric current and provides an indication of the moisture content in the packed particles between the Corresponding signal to electrodes. A controller shown in block form can be controlled and operated by this signal a valve to interrupt the flow during regeneration. Depending on the control, there is one for Regeneration effective full regeneration voltage from the voltage source can be applied to the electrodes of the bed. While During this phase, a purge gas flow or a vacuum is connected to the tower 12. The ammeter signal indicates that the moisture has been removed to a desired value, then the controller starts the drying cycle and switches the power source again in a measuring state.

The activation or regeneration cycle is through a previously set time switch, by measuring the dew point in the purge gas flow, by measuring the electrical Conductivity of the bed, controllable or limited by energy absorption during regeneration or in some other way.

The current used for moisture measurement can be a continuous wave or a current pulse train. the The frequency and length of the pulses as well as the voltage can be used to connect the moisture measuring circuit to the others

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Functions of the control must be coordinated. The control of the regeneration circuits and the fluid flow circuits can by a central control unit or an on-line process computer take place. The functions are selected depending on the process conditions.

It should be noted that certain parts of the dryer can perform different functions, for example The electrodes used to measure the moisture in the bed can also be used as connections for a high voltage for regeneration serve the bed. This requires a known switching of the voltage source.

The current measuring the moisture provides a measure of the moisture content corresponding signal, or an ammeter converts this current to a corresponding to the moisture content electrical, mechanical or fluid signal. The signal is used to trigger a regeneration cycle for the periodic Expelling water from the molecular sieve bed with a specified value compared.

The direct application of low-frequency electrical energy from 0 to 1000 Hz to expel the adsorbate is the Regeneration in a small fraction of the time required to load the molecular sieves during the sorption cycle

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feasible. A well-known typical gas dryer needs more than two hours to reach the moisture capacity or the breakthrough point, the same amount of water is approximately 1 minute or less can be expelled according to the invention, which means a regeneration time of less than 1% of the sorption time.

6 shows the electrical conductivity as a function of the moisture content for two different cell arrangements. The data for curve A were obtained with stainless disc electrodes made from a No. 18 permeable grid in a cylindrical acrylic housing with a 18 cm long bed 3.2 cm in diameter. The particles were 4 to 5 Ά Davison Grade 714 Type X zeolites (8x12 mesh). The air flow through the packed bed was 53 liters per minute (under normal conditions), with an inlet water content of 2500 ppm and an outlet water content of about 2 ppm. The molecular sieve adsorbs about 6.8% H ₉ O per hour at ambient conditions. A DC voltage of +18.5 V was applied to the electrodes, with the anode in the downstream stream. The current flowing between the electrodes was used for electrical measurements. The increase in conductivity indicates that the bed has reached moisture concentration and that regeneration is required.

Curve B shows the dependency for coaxially arranged rod electrodes according to the embodiment according to FIG. 1.

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Claims (11)

-.28.- '7B57803 Claims Gas dryer with an inlet for the gas to be dried, with an outlet for the dried gas and with one intermediate chamber with an adsorbent bed consisting of a molecular sieve, characterized in that that on the adsorbent bed electrodes for generating an electrical current flow in the adsorbent bed are applied, the frequency of the electric current is so low that heating of the Adsorbent bed is excluded until the adsorbed water has evaporated, and the current density is so small that there is no hydrolysis of the absorbed water. 2. Apparatus according to claim 1, characterized in that an electric field of at least to the electrodes 0.2 kV / cm can be laid at a frequency of 0 to 10 Hz and that a current density of 0.01 to 100 µA / cm can be maintained is. 3. \ Device according to claim 2, characterized for example through a saturated bed reactor as shown in Fig. 2 to increase the tension in driving off the adsorbed Water from the adsorbent bed during the regeneration 6098 2 9/0740 rierens to maintain a substantially constant current density for the desorption. 4. Device according to one of claims 1 to 3, characterized through a line (27) and a valve (28) for releasing the electrical current between the Electrodes desorbed liquid water. 5. Apparatus according to claim 1 or 2, characterized by a device for recording the between the electrodes electrical current flowing through the absorbent bed, and through means for generating it a signal indicating a certain water content of the adsorbed water as a function of a increased electrical conductivity of the adsorbent bed. 6. The device according to claim 4 , characterized by a device controllable by the signal for interrupting the gas flow through the adsorbent bed, and by a device for generating a regeneration flow between the electrodes when the gas flow is interrupted. 7. A method for regenerating an adsorbent bed consisting adsorbed into a molecular sieve 609829/0740 R 5? 8 Π 3 Water vapor by passing an electric current through the adsorbent bed, characterized in that that an electric field with a frequency of less than 10 Hz to avoid dielectric heating of the bed and a current density of 0.01 to 10OyU A / cm to avoid electrolysis of the adsorbed Water and a field strength of at least 0.2 kV / cm can be established, with the adsorbed Water is desorbed as liquid molecular water. 8. The method according to claim 7, characterized in that the desorbed water is drained or as a liquid is evaporated and expelled with a purge gas. 9. Method for drying a gas with the aid of a bed of adsorbent in the form of a molecular sieve through selective adsorption of the water vapor from the gas up to a certain water content in the bed and then interrupting the gas flow and regenerating the bed by expelling the adsorbed water, characterized in that while flowing through the adsorbent bed from the gas to be cleaned an electric current passed through the adsorbent bed and as Measure for the electrical 609829/0740 _ ₃₁ _ 75R7803 trical conductivity of the adsorbent bed recorded is that when a predetermined value of the conductivity is reached, a signal is generated and that the Gas flow is interrupted as a function of the signal and a regeneration cycle is started. 10. The method according to claim 8, characterized in that those used to measure conductivity increases Electrodes also serve to generate a regeneration current of essentially constant current density. 11. Device for moisture measurement and moisture desorption for carrying out the method according to claim 10, with one containing a zeolitic adsorbent Tower, characterized by electrodes (12, 24) applied to the adsorbent, by a device for measuring the current flow between the electrodes and a device for generation that can be controlled by them a regeneration stream to drive moisture out of the adsorbent bed. Λ Device according to claim 11, characterized by a with the control of the electrical voltage source for the electrodes connected reactor with saturated bed, whereby the regeneration flow during drying of the adsorption 60 9829/0740 7557803 middle bed and its resistance increase essentially is kept constant. ue: hu: kö 609829/07 ^ 0 Blank page