WO1986006299A1

WO1986006299A1 - Lining for a circulating or non circulating fluidized bed

Info

Publication number: WO1986006299A1
Application number: PCT/FR1986/000128
Authority: WO
Inventors: Ardeshir Fakhrian Langroudi
Original assignee: Ardeshir Fakhrian Langroudi
Priority date: 1985-04-19
Filing date: 1986-04-18
Publication date: 1986-11-06
Also published as: EP0218661A1; FR2580520A1

Abstract

A lining enables to increase the solid retention and the gas-solid contact of fluidized beds. Said lining is comprised of one or a plurality of vertical tubes (fig. 1:8, fig. 2:1) open at both ends and perforated on the cylindrical surface. Said tubes enhance the gas exhaust while maintaining a high dispersion of gases in the gas-solid emulsion.

Description

OR NOT CIRCULATING

1

The present invention relates to a lining for improving the performance of gas-solid fluidized bed apparatus. In these devices, where solid particles are maintained in the fluidized state by the passage of a gas stream, there are two inherent difficulties in the fluidization process which prevent optimal conditions from being obtained: on the one hand, the presence gas bubbles reduces the transfer of material between the gas and the solid, on the other hand, large quantities of solid particles are entrained by the gas stream.

Techniques are already known, according to which fillings such as Raschig rings, Berl saddles, Pall rings, wire rings, horizontal or vertical tubes or rods are used to reduce the size of the bubbles. These linings regulari¬ feel the fluidization, which thus becomes relatively homogeneous over the entire section of the enclosure, and improve the transfer of material. However, by suppressing the escape of gas in the form of large bubbles, these techniques have the drawback of causing a large increase in the expansion of the fluidized bed: the retention of solid becomes weak or zero as soon as a speed of gas stream which is close to the terminal free fall velocity of the largest particles. There is thus thus, at high gas speeds, an apparatus which has both the drawback of operating with low retention of the solid and that of operating with high entrainment of these solid particles.

The object of the present invention is to remedy the drawbacks mentioned by proposing a new type of packing which, while promoting gas-solid contact, causes a controlled escape of the gas intended to reduce the expansion of the bed and the particle entrainment.

An installation in which the entrainment of particles is relatively large is generally designated by the name of circulating bed; as soon as the speed of the gas stream exceeds the terminal speed of the smallest particles, a fluidized bed apparatus can be considered as a circulating bed: the inventory of the bed in solid particles is generally kept constant by recirculation of the entrained particles after separation â using cyclones or dust collectors.

REPLACEMENT SHEET The packing element, object of the invention, is a perforated tube, placed vertically in the circulating fluidized reactor. This tube is open at both ends. When placed in a fluidized bed, under certain operating conditions, the linear speed of the gas inside the perforated tube is greater than that of the fluidized bed located outside of this tube, while the solid retention is weaker. There is therefore an escape of gas through the interior of the tube which, to a certain degree, shortens the bed. This exhaust reduces the speed outside the tube which contributes to increase the retention in this area. The role of the perforations of the tube is to promote, at all levels of the enclosure V, interchange between the gas and the particles located in the two zones; the pressure fluctuations due to the heterogeneities of the fluidization cause this interchange, that is to say that the radial dispersions of the gas and of the ide soil in the enclosure are very large owing to the presence of horizontal jets of gas emulsion -soil idea

The packing elements can be arranged inside a fluidized bed reactor as shown in FIG. 1. This device comprises a fluidizing grill 1, a disengagement zone 2, a gas inlet tube. 3 and a gas-solid emulsion outlet tube 4. A heat transfer fluid can be introduced at 5 and withdrawn at 6, after circulation in tubes 7 ensuring heat exchange. The perforated tubes 8, object of the invention are arranged vertically in the reactor. The distance between the perforated tubes is chosen appropriately to improve the performance of the reactor; for each specific application a compromise must be found between the three quantities which characterize the efficiency of the reactor:

- good transfer of gas-solid material in the vicinity of a tube

- high retention of ide soil between the perforated tubes - high radial dispersion of particles.

A high retention of the ide soil signifies that, for the same circulation rate of the solid particles and the same gas flow, the quantity of ide soil in the enclosure is greater than in the absence of the lining. The radial dispersion of the ide soil must be high in certain cases, because it makes it possible to reduce the number of points of injection of the solid into the fluidized bed, and it promotes heat exchanges.

In each particular case, the diameter of the tubes, the number and the diameter of the perforations "and the distance between tubes can be chosen to obtain the desired result.

As is the case with most packings, the radial profiles of velocities and concentrations are relatively uniform over an entire horizontal section of the reactor; the very significant effect of the walls of the reactor often observed with the circulating beds in diluted phase is thus minimized which favors on the one hand the study of the reactor and on the other hand its extrapolation.

A second mode of use of the packing element according to the invention is inside the ascending tubes of a circulating multi-tubular reactor. FIG. 2 shows an example of a multi-builder reactor: a perforated tube 8 is arranged concentrically in each of the ascending tubes £. Descending tubes o can be used to recirculate a large part of the solid towards the fluidization chamber n situated above above the fluidization grid l and which makes it possible, thanks to the reactive feeders, to obtain identical gas and solid flow rates in all the ascending tubes. In this version of the fluidized reactor, which can therefore be both internal and external circulation, the heat transfer fluid is introduced in a set withdrawn in g, that is to say that it circulates outside the tubes.

Other modes of use of these vertical perforated tubes inside circulating or non-circulating fluidized reactors could be cited the examples of Figures 1 and 2 being illustrative but not limiting.

The multitubular reactor being modular, and the reactor of the diagram of figure 1 being not very sensitive to the wall effects, we built two reactors of simulation of reduced diameters (10 cm and 20 cm) to compare their performances in presence and in the absence of perforated tubes. The two 6 m high reactors have either a concentric perforated tube (Figure 3) or three perforated tubes (Figure 4). The inside diameter of the perforated tubes was 4.2 cm and the outside diameter 5 cm; by way of example - nonlimiting - two holes of 2 cm in diameter were drilled on a diameter of the tube this pattern being reproduced every 4 cm after rotation of 90 °.

The two reactors were adapted to the study apparatus of FIG. 5 where they held the role of ascending tube 9 A tub of fluidization fitted with its grid ή. and its air supply 3, as well as an exhaust13 used to control the pressure was used to feed the solid into the riser. The flow of solid is a function of the flow of air injected into the ascending tube via the tube 12 and the pressure of the fluidized tank. The solid is separated from the gas in the settling chamber 2 equipped with a filter 4 and a shutter 15 In the high position this shutter lets the solid circulate towards the downward tube 10 where the solid is in dense fluidized regime. The solid therefore circulates continuously in this device in a closed loop (closed for the solid, open for the gas). Solid retention is measured using a Y radiation absorption hydrometer 16; the flow of solid is measured by interrupting with the shutter the arrival of the solid in the downward tube and by measuring the drop in the level of the dense phase for a determined time.

With the ascending tube 10 cm in diameter of Figure 3, or that of 20 cm in diameter of Figure 4, it was found that the presence of the perforated tubes increased the retention of solid, as the

*> will show the following examples. But it has also been found that the presence of the perforated tubes increases the conversion of a typical catalytic chemical reaction: the decomposition of ozone into oxygen by means of a catalyst consisting of ferric oxide (Fe ^ _) deposited on a support. silica gel. This reaction had already been often cited in the literature as a standard reaction for measuring the efficiency of fluidized bed reactors.

The support is a macro and microporous silica with a specific surface 2 475 m / g; the apparent density of the particles is

3 1.05 g / cm. The average particle diameter is 620 µm. This support is impregnated with an aqueous solution of ferric nitrate and then calcined at 300 ° C. The catalyst thus prepared circulates in a closed loop, at room temperature in the reactor of FIG. 5. The transport gas is air containing 500 ppm of ozone; a gas sample is taken at the outlet of the reactor to measure its ozone content and therefore the conversion, ie the percentage of ozone which has reacted.

Example 1:

The device of FIG. 5 is used with the tube shown in FIG. 3. The air flow, injected at 12 measured under the conditions

10 3 normal temperature and pressure is 30 m / h; the catalyst flow rate is 620 kg / h. The densimëtrelβ indicates a retention of 41%; this retention represents the fraction of the volume occupied by the catalyst. The ozone composition at the outlet of the tube is equal to

30 ppm, i.e. the conversion is equal to 94%.

- * ^• With the same gas flow and the same catalyst flow, in the absence of the perforated tube, the retention is 10% and the conversion 15%. It will be noted that in these two tests the speed in empty barrel was equal to 1.06 m / s and that the mean terminal velocity of free fall was equal to 2.5 m / s.

20 Example 2:

With the same device as in the previous example, but with

3 3 a ggaazzeeuuxx ddee 110,000 mm flow rate // hh eett uunn ddéébbiitt ddee ssoolliiddje of 1400 kg / h, a retention of 18% and a conversion of 30% are measured.

At the same flow rates, but in the absence of the perforated tube, the

25 retention is 4% and conversion 3%.

It will be noted that in these two tests the speed in empty barrel was equal to 3.5 m / s and that the mean terminal speed of free fall was equal to 2.5 m / s.

FIRE. IT REPLACES ^ Eϋiπr Example 3:

The process is carried out under the conditions of Example 1, but eliminating the flow of catalyst to the riser. The speed of the gas in empty barrel (1.06 m / s) being lower than the terminal speed of free fall of the particles (2.5 m / s), the fluidized bed is stable and non-circulating. The retention is 37% and the ozone conversion is 92%.

At the same gas flow rate, in the absence of the perforated tube, the bed is pistonable, the retention is equal to 8% and the conversion of ozone is equal to 12%.

Example 4:

The device of Figure 5 is used with the tube shown

3 in FIG. 4. The air flow rate is equal to 280 m / h and the solid flow rate equal to 4000 kg / h. We measure a retention of 31% and a conversion of 61%.

At the same flow rates, but in the absence of the three perforated tubes, the retention is 6% and the conversion 5%.

It will be noted that in these two tests the speed in empty barrel was equal to 2.47 m / s and that the mean terminal velocity of free fall was equal to 2.5 m / s.

Example 5:

The catalyst used in the previous examples is ground and sieved so as to obtain particles whose average diameter is 65 μm, this diameter being defined by measuring the minimum fluidization speed. The apparatus of Figure 5 is used with the tube of Figure 3.

3 The gas flow rate is 10 m / h and that of the catalyst

1050 kg / h, the solid retention is 12% and the ozone conversion by 2035. The comparative test, in the absence of the perforated tube, had given a retention of 4.2% and a conversion of 3%.

It will be noted that in these two tests, the speed in empty barrel was equal to 3.9 m / s and that the mean terminal velocity of free fall was equal to 0.14 m / s (this value being calculated from the mean diameter defined above).

Example 6:

This example shows that the use of the lining according to the invention makes it possible both to increase the retention of large particles and of fine particles in a hybrid bed, that is to say a non-circulating bed of large particles traversed by a current. fine particles entrained by the gas.

The apparatus of FIG. 5 is used with the tube of FIG. 4. The ascending tube is partially filled with large particles (620 m) while the descending tube is partially filled with fine particles (65) and kept fluidized. In this test, the fluidization tank is eliminated, the circulation of the solid being carried out using a fluidized tubing.

3 We choose a gas flow equal to 120 m / h, that is to say a speed in empty barrel of 1.06 m / s, lower than the free fall speed of large particles (2.5 m / s) but greater than that of small particles (0.14 m / s on average). It can be seen that the large particles do not circulate but that the small particles are entrained upwards; we can thus establish a continuous circulation of small particles. We have chosen a flow rate of 1800 kg / h.

After a certain period of operation, the gas flow rates of transport and fluidization are cut and the riser tube is emptied. It is sieved, weighed and the retention in large particles and the retention in fine particles is calculated.

A retention in large particles equal to 25% and a retention in fine particles equal to 13% was thus measured.

RE ^ PLACE SHEET ώ.- T In the comparative test, carried out at the same flow rates after removal of the three perforated tubes, a retention of large particles equal to 8% and a retention of fine particles equal to 2% was measured.

It is concluded that the improvement in the retention of large particles due to the lining contributes to retaining the fine particles passing through the bed.

It can be seen that the performances obtained thanks to the lining according to the invention are very interesting. The increase in conversion is not only due to the increase in the retention of the catalyst but also to the better gas-solid contact due to the intense mixing caused by the perforations of the tubes. The invention is not limited to the embodiments described, it encompasses many variants and in particular with regard to the geometry and the dimensions of the enclosure, of the tubes and of the perforations. It should be noted, in particular, that superimposed but not joined tubular sleeves constitute a particular case of the perforated tube according to the invention.

SHEET OF F. r.ïP ÂCE..i ^ î

Claims

1) Lining for an enclosure containing a fluidized bed, characterized in that one or more perforated tubes are placed vertically in the enclosure and consequently immersed in the fluidized bed, a relatively dilute gas-solid expansion being formed inside the tubes, while a relatively concentrated emulsion is formed outside the tubes.

2) Packing for enclosure containing a fluidized bed according to claim 1, characterized in that the perforated tubes constituting this packing

10 are open at both ends and the number, the geometry and the dimensions of the perforations can be chosen arbitrarily.

3) Packing according to claim 1 or 2 characterized in that it is used in an enclosure containing a circulating fluidized bed, or a non-circulating fluidized bed or a non-circulating fluidized bed traversed

- * - by a stream of fine particles.

4) Packing according to any one of claims 1 to 3, characterized in that it is used in an enclosure containing a fluidized bed operating at gas speeds between 1/5 of the value of the average terminal fall speed free of solid particles and

20 50 times this value.

5) Packing according to any of claims 1 to 4, characterized in that a single perforated tube is used per enclosure but that several enclosures are used in parallel so as to constitute a multitubular reactor with circulating or non-circulating bed.

6) Lining according to any one of claims 1 to 5, characterized in that the perforated tubes are sufficiently close to each other to allow the formation of a relatively concentrated emulsion inside the tubes, while a relatively emulsion diluted is formed on the outside of the tubes.

REPLACEMENT SHEET M EiVT