FR2467015A1 - GAS MIXER APPARATUS - Google Patents

Abstract

The invention relates to a gas mixing apparatus. This mixer has the structure of a shell and tube heat exchanger; a first gas is fed through tubes of relatively small diameters 1 and a second gas is fed into a casing 2 housing a bundle of tubes 1, the second gas entering the tubes 1 through small holes 6 provided in their wall, so that the homogeneous mixture of these gases takes place quickly and without risk of explosion in the tubes. The pressure drop across the device is relatively low. Application to the mixture of organic compounds and oxygenated gases with a view to a reaction in the gas phase. (CF DRAWING IN BOPI)

Description

24 6 7015

  The invention essentially relates to apparatus for mixing two gases quickly and in complete safety. In gas phase reactions, it is often necessary to mix two gases. An example of such reactions is a gas phase oxidation process, which comprises a step of mixing a gas comprising an organic compound and a gas comprising molecular oxygen (including molecular oxygen only). These reactions include, for example, processes for the preparation of ethylene oxide from ethylene, maleic anhydride from benzene or C4 hydrocarbons, anhydride

  phthalic acid from xylene, propylene oxide, acrylonitrile

  or acrylonitrile from propylene, acrylic acid from acrolein, butadiene from C4 hydrocarbons, styrene from ethylbenzene, formaldehyde from methanol, methacrolein from from isobutylene and methacrylic acid from

methacrolein.

  When a gas containing an organic compound to be oxidized and a gas comprising molecular oxygen is mixed in such reaction processes, it is necessary to take fully into account the flammability range of the mixture of organic compound and molecular oxygen. . From the point of view of safety, it is essential to adjust the mixing ratio of the gases so that the composition of the gas mixture to be brought to the oxidation reaction region out of the flammability game. However, it is inevitable that a composition within the flammability range is locally and temporarily formed while a gas comprising a compound is mixed.

  organic and a gas comprising molecular oxygen.

  Also, it is important to be able to mix these two gases rapidly in the mixing step, so as to reduce the volume of the gaseous mixture whose composition

  local is within the flammability range.

  In conventional devices, we aim to reach as quickly as possible a completely mixed state

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  By dividing several times one of the gases and bringing the divided gas in dispersed form to the other gas, as described in the published Japanese Patent Application Nos. 4362/1971 and 16381/1972. In the case where the supply is not carried out in the dispersed state, a practiced method consists of bringing one of the gases to the other gas through a single nozzle and to ensure that the gas injected by the nozzle reaches a deflector placed in the lower course of

  the nozzle 1 so as to disperse the injected gas.

  In either case, these two gases mix together

  in a tubular space. The ratio between the length (L) of the tube and its inside diameter (D), therefore L / D, should be between 5 and 10, approximately, or more, if one

  wants to get a completely homogeneous gas mixture.

  In industrial-scale equipment, very large quantities of gas (several thousand or tens of thousands of cubic meters per hour) are treated and the pressure drop in the pipe system often causes an economic disadvantage. Also, the internal diameter of the pipe system is generally 30 to 50 cm and the distance required to obtain a completely homogeneous gas mixture is about 1.5 to 0 m, starting from the point where the two gases meet. It is thus formed locally, over this distance of 1.5 to 5.0 m, a composition of gas lying in the range

  of ignitability, which is very dangerous.

  One of the objects of the invention is to solve the above problems. The invention proposes to achieve this goal by providing a gas mixing apparatus of the type of shell and tube heat exchangers, in which the characteristics of the small tubes are taken advantage of.

which concerns the flow of gas.

  The invention provides an apparatus comprising: (a) a beam formed of multiple tubes each having at least one small hole in its wall, (b) an envelope housing the bundle of tubes, (c) spaced apart spacing elements. outside of the tubes inside the envelope insulating the interior space of the tubes and the space outside the tubes in the envelope, (d) gas supply means which bring a first and a second gas to be mixed, respectively inside each of the tubes and at a spacing located inside the envelope and formed by the elements (c), this apparatus being characterized in that the first gas passes through inside the tubes of the bundle (a) and that the second gas is brought to the spacing formed inside the envelope by the elements (ó) and enters the tubes through the small holes for

  mix with the first gas passing through the tubes.

  The invention solves the above problems by means of the gas mixing apparatus having a structure of the type of shell and tube heat exchangers, wherein a first gas supplied as a "tube fluid" and a second gas supplied as "shell fluid" mix efficiently inside the tubes, the second gas arriving in each of the tubes through a small hole in the wall of each tube, so that the mixing takes place inside multiple

  tubes having a relatively small diameter.

  Multiple tubes (often several tens or hundreds), which give a safe mixing region having a sufficient volume, are simply combined into a bundle in the form of a structure of the type of shell and tube heat exchangers. It has also been found that the loss of pressure at the crossing of

the device is relatively small.

  The attached drawings represent a form of

  of the invention. In these drawings: FIG. 1 is a longitudinal section in a plane passing through the geometric axis of a concrete example of a gas mixing apparatus according to the invention and illustrates the principle thereof; Figure 2 is a section of the apparatus in the plane indicated by the line II-II in Figure 1; Figures 3a, 3b and 3o are sections through planes

2467015.

  4 is a cross-section in a plane perpendicular to the axis of a concrete example of an envelope comprising tubes and partitions; FIG. 5 is a longitudinal section through a plane passing through the geometric axis of a nozzle type mixer, illustrating the principle thereof. FIG. 6 is a graph drawn from FIGS.

Table 1 results.

  1. Theoretical Analysis It is generally known that, when a gas is passed through a tube, the degree of mixing is governed by the L / D ratio (tube length / inside diameter of the tube) # if the flow passing through the tube is in the turbulent flow range, that is, if the Reynolds number represented by (tube inside diameter) x (gas flow) x (gas density) / (gas viscosity) ) is

equal to or greater than 10,000.

  Therefore, by mixing the gases in a smaller diameter tube, the same degree of mixing is obtained with a shorter tube length than in a larger diameter tube. In other words, the mixture in a smaller diameter tube is carried out more rapidly than in a larger diameter tube # so that the time or the region where a composition is locally formed is

  in the flammability range can be reduced.

  It is also known that the flammability range of a system comprising an organic compound and molecular oxygen in a smaller diameter tube is narrower than in a larger diameter tube. This seems due to the increase in the effective area of the walls

  of the tube, which serve to prevent a combustion reaction.

  Therefore, the degree of existence of the mixture having a dangerous local composition decreases, together with the range of flammability, when mixing an organic compound and molecular oxygen in a smaller diameter tube, so that safety is increased compared to mixing done in a larger tube

246701 5.

  -5- diameter. 2. Gas mixing apparatus structures The gas mixing apparatus according to the invention has a structure similar to that of a jacket and tube heat exchanger. However, there is a fundamental difference between these two types of structure, namely that a small hole is provided in each of the small tubes that constitute the beam and that the fluid (gas) contained in the envelope flows into each of the tubes. through

the little hole.

  As seen in Figures t and 2, multiple tubes 1 are supported within an envelope 2 by tubular plates 2, 2a. A spacing 4 is thus formed outside the casing 3 and is essentially isolated from the internal spacing of the tubes by the tubular plates 2, 2a. The envelope 3 usually presents

extension regions 3a and 3b.

  In the example shown, the extension regions 3a 3b are closed by cheeks 5, 5%. However, it is possible to give the casing 3 an outer diameter equal to that of the gas piping, so that the extension regions 3a, 3b are part of the piping

gas.

  According to the invention, a small hole 6 is provided in each of the tubes 1. Although the drawing indicates a small hole in each tube 1, several holes may be provided in each tube in the direction of its circumference or axis. The size and number of the holes can be suitably determined in order to obtain a homogeneous gas mixture in as short a time as possible. The maximum number of holes exists for example in a tube formed of a lattice. The small holes are usually one to many. In order to achieve homogeneous mixing in a tube of limited length, the small holes must be provided in the tubes as far upstream as

  possible relative to a gas passing through the tubes.

  The inside diameter, the length of the tubes, their number, etc., in the bundle are preferably chosen in such a way that the gas passing through the tubes is in a state of turbulent flow, that is to say say that the number

  Reynolds is equal to or greater than 10,000.

  The inside diameter of each tube constituting

  kill the beam, or the equivalent diameter defined by 4 x (section area of the divided space) / (circumference of the section of the space in contact with the gas) is preferably 5 cm at most and, more preferably, from 1 to 3 cm. On the other hand, the length of the tubes is preferably chosen so that the L / D ratio is equal to or greater than 3. In general, the length of the tubes is chosen so that L / D is from 10 to about 15. Here, the length of the tube is defined as the distance between a position where a small hole is provided in the most downstream portion of the flow in a tube and the end of the tube (end of valle). When several holes are provided in the tube,

  the length starts from the hole located furthest downstream.

  The number of tubes constituting the beam is

  ordinarily of the order of 50 to 1000.

  3. Mixing of gases In the apparatus represented by FIGS. 1 and 2, the first gas to be mixed is fed to the apparatus through an inlet 7, it passes through the envelope in the direction of the arrows,

  then passes through the tubes and is evacuated through an outlet 8.

  The second gas to be mixed is fed through an inlet 9 at the spacing 4 located inside the casing and insulated from the internal spacings of the tubes by the tubular plates 2, 2a and the wall of the tubes, it flows to the tubes through the small holes 6, it then mixes with the first gas

  passing through the tubes and is evacuated through exit 8.

  The first and second gases may be either a single component or a mixture of gases. For example, when the first gas is a gas comprising molecular oxygen, it may be generally air (i.e. a mixture of molecular oxygen and nitrogen) or a mixture air and a diluent gas such as water vapor or carbon dioxide. When the second gas is a gas comprising an organic compound, it may be a mixture

containing a diluent gas.

  In the invention, the mixing apparatus is generally described with respect to two gases which are the first and second gases, since both of these gases can form a gas.

  mixture composition within the range of

  mabilité. Also, the apparatus according to the invention can also be used to mix with the first and second gases a third gas that does not cause any risk of explosion. For example, it is within the scope of the invention to further bring by a 7% input a third gas such as a diluent gas to effect the mixing of these three gases. Since the apparatus of the invention has the effect of mixing rapidly, there may be cases where the first and second gases do not form a mixture within the inflation range. Of course, cases of this kind

  fall within the scope of the invention.

  When the gases to be blended comprise a molecular oxygen-containing gas and a gas containing an organic compound (some examples being mentioned above), it can be conveniently determined from the ratio of gas quantities to flammability ranges and other factors, what is the gas that must be passed through the tubes and what is the gas

to bring to the envelope.

  4. Concrete Examples of the Apparatus A concrete example of the gas mixing apparatus of the invention comprises empty tubes or unfilled tubes, which constitute the bundle shown in FIGS.

and 2.

  Another concrete example of the apparatus comprises packed tubes which constitute the beam. The solid packings to be packed into the tubes may be in the form of spheres, saddles, rings, sieves, bars

  massive or hollow or in any other form.

  In this connection, it is understood that the term "solid packing" refers not only to ordinary packings, but

  deflectors provided on the walls inside the tubes.

  The use of filled tubes ensures a faster mixing of the gases and also increases the effective area of the walls of the tubes, which serves to prevent an explosive combustion reaction. Another concrete example of the apparatus includes

  tubes, each of which is divided into several longitudinal cavities

  tudinal walls in its axial direction, and the small hole is provided in the wall of each tube,

  so that each of the cavities communicates with the space

  cement 4 (see Figure 1) through the holes. Figures 3a, 3b and 3o show in section three examples of such tubes, in which the tubes are divided into two to four cavities by the partitions 10, 10a, 10b. This division of the tubes by partition backs is effective, when it is not possible to give the tubes an inside diameter sufficiently

  small or when you want to get the effect of smaller tubes.

  In addition to these variations on the bundle of tubes, it is also possible to vary the spacing 4 of the envelope

(see Figures 1 and 2).

  Figure 4 shows in section a concrete example of such a device. In this example, the inner spacing 4

  -of the envelope is divided into multiple longitudinal cavities

  dinales (four cavities, 4a, 4b, 4o and 4d in this case) by partitions in its axial direction (11 and 11a in this case) and gas inlets 9 are provided for each of the cavities (9a, 9b 9o and 9d in the present case). This division of the cavity 4 of the envelope makes it possible to effectively prevent the path

  gas and disperse the gas evenly.

  Of course, some of the examples above can

  to be put into practice in combination.

-5. Experiences

EXAMPLE

  A gas mixing experiment is conducted using a gas mixing apparatus having a shell and tube heat exchanger type structure, comprising 37 small diameter tubes. Each tube has an inner diameter of 10 mm and a length of 400 mm and

246701 5

  -9 has a small hole 2 mm in diameter located 50 mm from the upstream end of the tube. The inside diameter of

the envelope is 146 mm.

  Air is used as the first and second gas. A small amount (3% by volume) of a tracer gas is incorporated in the second gas to evaluate the degree of mixing.

gases.

  The flow rate of the first gas through the small tubes is 35 m / s while the speed of the second gas passing through the small hole is 75 m / s. The temperature of these gases is 350C. The gas mixture for all small tubes is measured at 370 mm from the upstream end of the

tube (320 mm from the small hole).

  There are no differences in the concentration distribution and the average concentration of the tracer gas in the small tubes, depending on their position, between the tube placed in the center of the envelope and the tubes adjacent to the

wall of the envelope.

  The mixed gas samples are sampled at five sampling points comprising the center of each tube and the tracer gas concentrations are determined. The result is that the difference in concentration in each

  measuring point is within a margin of + 2% relative-

  at the average concentration (0.24% by volume) in all small tubes. An example of the results is shown in Table 1 and Figure 6 (curve 14). We dose the

  tracer gas by gas chromatography.

  In Table 1 and Figure 6, R denotes the inner radius of the small tubes and r the distance between the sampling position in the small tube and the center.

of it.

  The "average concentration" of the tracer, of which it is

  here question, is reported to the area.

COMPARATIVE EXAMPLE

  To make a nozzle type gas mixing apparatus, a 68 mm inner diameter gas supply nozzle for the second gas is mounted through a steel tube having an inside diameter of 305 mm, such as

- 10 -

shown in Figure 5.

  Air is used as the first gas and is brought to a flow rate of 35 m / s, from left to right in Figure 5. The second gas, which was prepared by incorporating into the air 3% by volume of CH4 as tracer gas, is injected by the feed nozzle, the speed at the end of the nozzle being 75 m / s, in the opposite direction to the first gas. The temperature of the first and second gases is 35 C.

  the same way as in the example, samples are taken

  lons of mixed gas at a point at 320 -m downstream

  the tip of the delivery nozzle and the concentrations

  tracer gas. The results are shown in Table 1 and Figure 6 (Curve 15) where R is the inside radius of the steel tube. The average tracer gas concentration is 0.29% by volume. When the L / D ratio is about 1, even at a mixing distance of 320 m which is the same as in the example, it does not occur

almost no mixing of gases.

TABLE 1

  Experimental results I Repositioning position Measuring rate / R Tracer concentration, C, (% by volume) Average tracer concentration, C, (% by volume (C-E) (%)

_ 0.8 0.241 0.4

0.4 0.245 2.1

Example 0.0 0.244 0.24 1.7

-0.4 0.237 -1.2

-0.8 0.239 -0.4

0.95 0.070 -75.9

0.84 0.104 -64.1

0.70 0.194 -33.1

  Example 0.55 0.318 9.7 comati 0.0 0.705 0.29 143.1 Ratified

-0.55 0.469 61.7

-0.70 0.328 13.1

-0.84 0.159 -45.2

-0.95 0.094 -67.6

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2467015 -

- 12 -

  cavities (4a, 4_, 4_, 4g) by a winding (11, 11_) located in its axial direction and that the supply means (9a, 9b, 9e, 9d) of gas (d) serving to bring the second gas at the spacing of the envelope are provided for each of the cavities.