DE4437078A1 - Process for the removal of hydrocarbons contained in the air by adsorption - Google Patents

Abstract

The invention concerns a process for the adsorption of volatile organic compounds, e.g. hydrocarbons, contained in air entering an adsorption reactor by at least one entrance E and leaving by at least one exit S, characterised in that an adsorbent is inserted in the form of discrete units: cones, tetrahedra or cylinders, in order to considerably reduce the pressure drop in the adsorption reactor without adversely affecting the expected performance expressed in terms of the volume of adsorbent used in the operation. <IMAGE>

Description

The invention relates to a method for eliminating the in hydrocarbons contained in the air by adsorption, and especially in the atmosphere of certain factories from de they are deducted to ensure hygiene and safety precautions writings, which are prescribed by the legislation speak. The invention also relates to the organi volatile compounds other than carbon and what S and N can contain atoms as structure.

The fact, selectively, at low temperature, the organi air connections on an adsorbent adequate Adsorbing mass is well known and has been around for decades practices just like the fact that the process of De sorption by rinsing the adsorbent mass through a Desorption gas stream, for example air, is carried out to a temperature of more than 100 to 200 ° C above the Ar is brought to the adsorption temperature. The most common The most recommended adsorbing masses are the active ones today high surface area coal, the silicalite and the oleophilic Zeolites caused by strong dealumination of the zeolites an increased SiO₂ / Al₂O₃ ratio were obtained, including the zeolites of the ZSM type, the mordenite and in particular the Faujasite; these siliconized compounds (composes siliciques) have the advantage that they can be regenerated in warm air are without experiencing any damage that can be coal must fear. A final adsorbent that at This type of operation can be used from one amorphous silica-alumina are manufactured, the was suitably dealuminated. The performance of these silici umoxide-aluminum oxides are slightly below those of dealuminated zeolites, what adsorption rate and adsorbed amounts of hydrocarbons per gram of solid saturation concerns, the dealumination process is depending but simpler and the cost of the solids obtained are less high.  

The present invention relates to the adsorb described above tion agent and especially the siliconized compounds un original conditions that enable the best benefit to draw from their physico-chemical properties. This The stake is made to get the most favorable result from the Adsorbent activity (expressed as adsorbent speed), its adsorption capacity, the performance as elimination material of the COV (volatile organic compound dungen), and the pressure drops in the middle of the ad to minimize sorption bed. The pressure drop to be overcome between entry and exit of the unit in both Adsorption period as in the desorption period mean tat the most important economic items among the different operating costs.

The prior art addresses adsorption processes based on Adsorbent used in the form of fixed axial beds, fluidized beds or even moveable beds are used (Handbook of separation proccess technolo gy - 1987), which describes the different types of adsorption describe reactors. No one puts them under these reactors Realization contemplated according to the invention.

Based on activated carbon adsorbents: silicon oxides, dealuminated silicon oxides-aluminum oxides, sili caliten, dealuminated molecular sieves capable are to adsorb the hydrocarbons and generally the COV in the presence of water relates to the aim of the invention an original use of the adsorbent materials that allow the pressure loss through the adsorption bed to reduce through and a good restraint of Koh Hydrogen oils (even the COV), which can be eliminated from the solid are. Ensure and thus the Druckver  to reduce losses in the adsorption reactor without the expected Damage benefits that are expressed in terms of the process of adsorption volume used.

According to a first variant of the process, the adsorbent used in the form of granules, the mean equivalent diameter of which is between 0.5 and 5 mm, is encapsulated in the interior of the volumes of woven or knitted fabrics made of metallic siliconized wires or of temperature-resistant composite materials . The mesh is smaller than the diameter of the particles and the equivalent diameter of the elementary volume that affects the particles is adjusted to the dimensions of the reactor. The observed pressure losses are then closely related to the dimensions of the elementary volume and in the adsorption reactor each of the elementary volumes behaves like an adsorption minireactor, which in terms of performance obeys the general laws in the field of chemistry. Under these conditions, the shape of the bed may be one or the other of the following geometries: axial bed, radial bed, insertion into a cylindrical solar cellular bed. The geometries of these three types of beds are shown in Figures 1A, 1B, 1C (where E denotes at least one entrance and S denotes at least one exit). The adsorbing elementary volumes can have different geometric shapes (drawn on the side of FIG. 1B): cylinders, spheres, cubes or, preferably, tetrahedra, as described in European patent application No. 494550-A. In all cases it is observed that the diameter of the adsorption bed is about 50 times larger than the equivalent diameter of the elementary volume.

The reduction in pressure loss caused by a bed free grains of the same thickness is caused, lies above  half a factor of 10. It becomes possible to increase bed thicknesses use that is two to five times larger than the one is allowed by the pressure drop when you get a bed out free grains used. In addition, these require bet from grains pressed tightly in elementary volumes, that Gas flow takes random paths at comparable pressures losses and comparable space velocity, too essential much better cleaning performance than that which leads to the Ad Obtain sorbents of the type monolith with parallel channels were that with adsorbent phase of the monolith type are deleted.

In a second variant of the process, the grains are free grains either in the form of full extrudates or in the form of hollow extrudates which are introduced into a radial mini-bed, as shown in FIG. 2; these elementary reactors have a length between 100 and 200 cm, a thickness between 10 and 30 cm and are arranged inside a collecting volume for the treated gas like pipes of a heat exchanger (FIGS . 2A and 2B). Each adsorption volume is in the form of a radial mini-reactor, which is fed from the center, the treated gases being obtained outside in the calender, where the mini-reactors are stored. This type of design has the advantage that there is little pressure loss and that it is composed of identical metallic fabric elements that can be filled before being inserted into the calender. The capacity of each element is between 50 and 200 l for an element length of 100 cm; the bed thickness is linked to the equivalent diameter of the adsorbent grains and the pressure loss permitted in the unit. Two reactors are arranged in parallel, one in the adsorption phase, the other in the desorption phase ( Fig. 2D). Each reactor can contain, for example, 4 to 36 mini beds and for the large outputs it is possible to place two mini beds against each other in such a way that the installation capacity is doubled. Let us specify that this catalytic bed type of small thickness is particularly suitable for the use of oleophilic dealuminated molecular sieves (mordenites, ZSM, faujasite), for which the characteristic curve QA · f (PA) (where QA is the amount of A, which has been adsorbed on the solid and PA its partial pressure) leads to its stage (platform) (saturation of the solid) for low values of PA, where A is the hydrocarbon to be adsorbed. In this case, a well-designed adsorption front is formed, which allows the use of a small bed thickness.

This type of design can also allow parallel-epiped-shaped unit elements to be used in accordance with the diagram of FIG. 2C.

Whether it is cylindrical or parallel-epipedför unitary elements, the walls of the element for Entry and exit of the gas are made of rigid stainless steel mesh (or reinforced with reinforcing bars tes fabric) whose mesh width is smaller than the smallest Ab measurement of the adsorption particles remains.

In a third variant, the adsorbent is through Coating on a knitted support, either made of aluminum wire or fiberglass to form a mat the one in which the gases will take any path.

However, these elements offer a low pressure drop find applications in the treatment of air where the through rates considerably and where the available pressures are low.  

On the other hand, the volume adsorption capacity, due to the small mass of the adsorbent deposited by coating (in the order of 100 kg per m³) requires the use of such a mass for a continuous regenerative process (see FIG. 1c).

For the adsorption of volatile organic ordinary additives compositions (COV) that are poorly or not polymerizable adsorption works under normal conditions of temperature and pressure, d. H. at a temperature between 0 and 120 ° C and preferably between 20 and 60 ° C when printing the Rejection of the gases to be eliminated. Generally are the COV in the air available at a gauge pressure between 0.1 and 1 MPa and generally between 0.1 and 0.2 MPa. The applied space velocity depends on the considered adsorp agent. For the activated carbons, it is between 1000 and 5000 m³ / h of gas to be treated per m³ of adsorbent and for the dealuminated zeolites between 5,000 and 50,000 m³ / h / m³. The space velocity, especially for the active Carbon depends on the degree of cleaning that is being considered.

Once the adsorbent is used among the Be conditions of adsorption and purification saturated, so the Air (or gas) to be cleaned is prevented and replaced by a desorption stream, the properties of which are used depend on the adsorbent. Is the adsorbent a silica-alumina or a dealuminated zeo lith, the desorbent is formed by a current of air between 140 and 300 ° C and preferably between 160 and 250 ° C is heated, which one on the adsorption bed a space velocity between 100 and 500 m³ / h conducts. Of the Desorbent flow is through to ordinary temperature cooled through an exchanger and the organic vapors  partially condensed before coming in from the gaseous phase be separated in a separator. After the separation, the gaseous phase can be recycled into the adsorption bed, if you have a minimum residue of volatile organic ver wants to have ties. The desorption process is at one Pressure adjacent and slightly higher than atmospheres printing done.

If the adsorbent used is activated carbon, the Desorption realized under the aforementioned conditions, depending on used to avoid degradation of the adsorbent an inert gas (nitrogen or combustion gas) near the stoechiometry as a desorption stream.

The process scheme under consideration can be divided into two Represent variants according to the type of reactor used: mobile or fixed reactor.

If the reactor is movable, of the type shown in FIG. 1C, the adsorption bed is moved from the adsorption zone to that of the desorption. The reactor, which is divided into identical reactor quarters (for example 6 to 10 volumes), rotates periodically about its axis from the value of a circular sector. If the reactor comprises n sectors (for example 6 to 10 identical sectors), there are n-3 sectors in the adsorption phase and the gas to be treated flows through them from top to bottom, as in FIG. 1C, for example, the other three sectors are located in the desorption phase, which includes the following three phases: heating, desorption, cooling. In this case, collector zones are well divided and allow the gaseous streams to be recovered by preventing them from mixing with the gases to be treated. The heating takes place under a desorption gas stream which is heated between 130 and 250 ° C. The regenerated solid is cooled under a cold air stream (or cold desorption gas).

The solid reactor systems can comprise two identical adsorption reactors, regardless of their number and the geometry of the adsorbent beds. One of the reactors is in the adsorption phase, while in others the consequence is: heating, desorption, cooling. The principle of operation remains the same, whether it is Fig. 1A, 1B, 2A, 2C.

Examples example 1

A faujasite with an atomic ratio Si / Al equal to 100, in Form of extrudates with an average diameter of 1.8 mm for an average length of 4 mm it comes out in conical volumes stainless steel mesh used, the mesh length 1.2 mm for a wire thickness of 0.3 mm. The cone has one long edge of 30 mm and a base diameter of also 30 mm. These cones, in an amount of 85 per liter, are in used a cylindrical reactor where they have an adsorp Formation bed of 1.12 m height for 0.07 m² base area. The Bed is perforated at its base by a stainless Steel mesh held, the mesh at 5 mm for a wire thickness of 1 mm. The fluids flow through the adsorption bed from bottom to top.

The adsorption tests are carried out with air, the with Butyl acetate and xylenes in an amount of 200 (parts per Million) in air at 30 ° C at about 70% relative humidity (HR) is loaded. The application space velocity is included 10,000, expressed in volumes of polluted air per adsorb volume and per hour, what linear speeds at corresponds to an empty cylinder of 3.1 m per second. The watch  The pressure loss is 240 mm WS (24 hPa, about 24 mbar). The outflow is analyzed at the outlet and the attempt to Desorption interrupted when the hydrocarbon content containing compounds at a total of 5% of the treated Gases is. Under the experimental test conditions it is found that the zeolite has 8.4% of its weight in Koh Hydrogen desorbed. This corresponds to 2 kg of coal water during an operation of 2.6 hours before Piercing the bed.

For desorption operation, warm air of 180 ° C is left at Space velocity of 300 m³ per m³ bed and hour circulie ren. The temperature of the bed gradually rises from 30 ° C 175 ° C with a progressive desorption of the adsor hydrocarbons and a chromatographic Do. concentration of the gases. The butyl acetate is desorbed faster than the xylene, but after a 1 h 30 of warm air circulation, the product is completely desorbed. The at -15 ° C condensed products correspond to a weight of about 2 kg.

The adsorbent is then circulated at 30 ° C non-polluted air at a space velocity of Chilled in 2000. The measured pressure drop during the adsorp periods is less than 10 mm water column (= 1 hPa = 1 mbar).

Example 2

The conditions of this test are identical to those of test 1, except for the reduced room speed to 5700 h ^-1 . The pressure loss measured across the adsorption bed is then 137 mm WS (= 13.7 hPa = 13.7 mbar). The maximum total amount of adsorbed hydrocarbons rises to 9.8% of the weight of the zeolite used, which corresponds to 2.5 kg of weight before the bed is pierced. Drilling occurs after 4.5 hours of adsorption.

The desorption is realized by circulating a gas from almost stoechiometric combustion of the methane previously cooled to 200 ° C. at a space velocity equal to 150 h ^-1 . Under the conditions used, the process can be regarded as terminated after 1 hour of circulation of the desorption gas.

Example 3

The experiment is carried out using the same zeolite as Example 1 and carried out under the same working conditions, one leads however in tetrahedral volumes with an edge length of 3 cm instead of in conical volumes. The observed pressure drop is slightly lower (190 mm water pressure or 19 hPa = 19 mbar) and the observed performance is slightly lower, that namely the piercing of the bed after only 2.3 h adsorp tion occurs.

Example 4

A trial was carried out under identical working conditions as carried out those of Example 2, d. H. with a space speed 5000, in which, however, as an adsorbent Silica-alumina, which was dealuminated to 98.7% ren used. This silica-alumina is in Shape of balls, the diameter of which is between 2 and 4 mm wearing. The results obtained show an unexpected, ever but not negligible adsorption of the two coals substances, but also a very strong adsorption of water.  

The piercing takes place after 1.2 h instead of 4.5 h as on that dealuminated zeolite observed. At the moment of through drilling has the adsorbent 1% hydrocarbon Solvent and 35% water captured. Such an adsorp can preferably be used for simultaneous adsorption of organic volatile compounds and water vapor be used.

Example 5

A reactor as shown in FIG. 2B is used as the adsorption reactor, which can be designed for large capacities, as indicated in FIG. 2C, in a parallel epipedic or cylindrical space. The walls of the bed are formed by a stiff fabric made of stainless steel with a mesh size of 1.4 mm and consist of wires with a diameter equal to 1 mm, which gives the fabric good rigidity. In addition, each element comprises a rigid truss structure that avoids any deformation of the adsorption volume under the influence of the speeds of the gases or temperature increases during the desorption process. The thickness of the adsorption layer is 9 cm; the central collector has a diameter of 10 cm, the height of the element is 1 m, hence a bed that contains 55 liters of adsorbent and is penetrated radially by the gas streams to be treated. The solid adsorbent is dealuminated zeolite identical to that considered in example 1, but is used in a state without introduction into elementary volumes, regardless of its geometric shape.

At 30 ° C, for a space velocity = 10,000 h ^-1 with a gas identical to that of Example 1, a pressure loss = 61 mm WS (6 hPa) is observed. Drilling through the bed occurs when the adsorbed hydrocarbon content is 2.1% after 3 hours of running or operation.

Desorption carried out with a hot air flow of 220 ° C expires in 35 minutes (35 mm) and gives the treated ad sorbent its original properties again.

A bed of the same volume (55 l) and the same thickness (9 cm), however, parallel-epipedic results in a pressure loss of 50 mm WS and services that are slightly above presses in amount of adsorbed hydrocarbon before passage drill (10.3%).

Example 6

The gas of Example 1 is treated, however, by using activated carbon as the adsorbent, which is introduced into conical micro-volumes which are identical to those of Example 1. The bed of conical volumes has the same dimensions as that of Example 1. The working conditions are the same in terms of temperature and pressure, but the room speed is reduced to 4000 h ^-1 . The carbon used is in the form of particles which have the following properties: average diameter: 1.2 mm; specific surface area 937 m² / g; Filling thickness not sieved: 0.22. Under these conditions, the bed is pierced after 23 hours of operation and hydrocarbons of the coal are retained at 35% based on their own weight.

The desorption is carried out by passing a combustion gas from natural gas over the bed, which cools to 250 ° C and is very low in oxygen, with a space velocity of 200 h ^-1 . The hydrocarbons are condensed after cooling; the gaseous fraction of the outlet separator is recycled for desorption after mixing with the gas from the combustion in which it participates, cooling down to 250 ° C. The desorption process can be considered complete after 3 hours of treatment.

A second adsorption process identical to the first enables es, trapping hydrocarbons at 34% by weight on the weight of the coal used.

A third, then a fourth adsorption process, which De sorption processes under the conditions described above go, it allows 34.5 and 35.3 wt .-% retention of ad to achieve sorbed hydrocarbons.

Example 7

A mat is used either as the adsorbent Aluminum wire or fiberglass, knitted, then coated with a zeolite powder of the same type as that of the previous one examples according to a relevant state of the art, such as he already used for the realization of catalytic pots becomes.

The mat thus obtained is in the form of a cylinder from 80 mm in diameter and 200 mm in length, the mesh of which is 2 to 3 mm and where the diameter of the wire is about 8/10 mm wearing. This adsorption mass contains 92 g of faujasite with a Atomic ratio Si / Al <100 as in Example 1.

The adsorption buffer is placed in a cylindrical reactor 80 mm diameter set by at ambient temperature  Air with a throughput of 30 m³ / h, d. H. at the speed speed of 1.7 m / s loaded with 300 ppm butyl acetate d. H. approximately 1.2 g / m³ of hydrocarbon can circulate.

The concentration is observed at the end by chromatography occurs every 3 minutes.

The hydrocarbon concentration is less than 25 ppm for the first four measurements and suddenly rises to 300 ppm for the following, puncturing the adsorption front is displayed after 12 to 15 minutes of operation.

The measured pressure loss at such an "adsorption bed" is below 2 hPa (20 mbar) or at least 100 mm water per meter length of bed at the speed of 1.7 m / s.

The global VVH is 30,000 h ^-1 and the volume adsorption capacity is of the order of 9 kg butyl acetate per m³ bed.

The regeneration is carried out as in the previous ones Examples with warm air at 150 ° and counterflow.

Claims (6)

1. A process for the adsorption of hydrocarbons which are contained in an adsorption reactor via at least one inlet E and at least one outlet S exiting air, characterized in that an adsorbent is chosen, in particular from the granules, full extrudates, hollow extrudates is introduced in unit volumes which are flowed radially through the gases to be treated and which are arranged in a calender of the exchanger type, these unit volumes being selected from: cones, tetrahedra, cylinders which enable the pressure losses to be observed in a respectable manner to decrease in the adsorption reactor without damaging the expected performance, expressed in terms of the adsorption volume used in the process. 2. The method of claim 1, wherein the desorp tion originating gas is cooled before it is in a separator is introduced where the organic desorbed verbin condenses before the one originating from this separation Fraction to the top of the adsorption is recycled. 3. The method according to any one of claims 1 to 2, wherein the adsorbent used a dealuminated zeolite from Type is faujasite, mordenite or a silicate. 4. The method according to any one of claims 1 to 3, in which the adsorbent used a dealuminated silicon is oxide-alumina, which is particularly recommended when  it is a question of being treated simultaneously in the Gases to extract water and hydrocarbons ren. 5. The method according to any one of claims 1 to 4, in which the Adsorbent is an activated carbon, its regeneration by circulating an inert gas (nitrogen or combustion gas, which is depleted in oxygen, is ensured, the gaseous fraction of the separator after condensation as a diluent and cooling fluid for the desorption gas is used. 6. The method according to any one of claims 1 and 3 to 5, in which the desorption gas is not cooled, condensed and recycled based, but overall catalytically or partly according to Küh treatment and condensation and possibly recycling as desorp diluent is burned when the used Solid is an activated carbon.