GB2061744A

GB2061744A - Gas-mixing device

Info

Publication number: GB2061744A
Application number: GB8032474A
Authority: GB
Original assignee: JGC Corp; Mitsubishi Petrochemical Co Ltd
Current assignee: JGC Corp; Mitsubishi Petrochemical Co Ltd
Priority date: 1979-10-09
Filing date: 1980-10-08
Publication date: 1981-05-20
Also published as: JPS5811247B2; CS683380A2; DD153521A5; DE3037817A1; YU41219B; FR2467015B1; YU248680A; DE3037817C2; CS241033B2; GB2061744B; FR2467015A1; JPS5653733A

Abstract

A gas-mixing device having a shell-and-tube heat-exchanger type structure, in which a first gas is supplied through relatively small-diameter tubes 2, and a second gas is supplied in a shell 3 accommodating a bundle of the tubes the second gas flowing into the tubes through small apertures 6 provided through the tube walls, whereby homogeneous mixing of these gases is carried out rapidly and safely without a risk of explosion in the tubes. Pressure loss through the device is relatively small. <IMAGE>

Description

SPECIFICATION Gas-mixing device This invention fundamentally relates to devices for mixing two gases rapidly and safely.

The need for mixing two gases has often been felt in gas phase reactions. One example of such reac tions is a gas oxidation process which comprises a step of mixing a gas comprising an organic com pound and a gas comprising molecular oxygen (including molecular oxygen alone). Such reaction processes include, for example, processes for the preparation of ethylene oxide from ethylene, maleic anhydridefrom benzene orC4 hydrocarbons, phthalic anhydride from xylene, propylene oxide, acrolein or acrylonitrile from propylene, acrylic acid from acrolein, butadiene from C4 hydrocarbons, styrene from ethyl benzene, formaldehyde from methanol, methacrolein from isobutylene, and methacrylic acid from methacrolein.

Upon mixing a gas containing an organic com pound to be oxidized and a gas comprising molecu lar oxygen in such reaction processes, it is necessary to give full consideration to the range of inflamma bilityformed by the organic compound-molecular oxygen mixture. It is essential from the viewpoint of safety to control the mixing ratio of the gases so that the composition of the gas mixture to be supplied to the oxidation reaction region is outside of the range of inflammability. However, the formation locally and temporaily of a composition within the range of inflammability in the course of mixing a gas com prising an organic compound and a gas comprising molecular oxygen is unavoidable.

Thus, it is of importance how these two gases can be mixed rapidly in the mixing step thereby to reduce the volume of the gas mixture whose local composition is within the range of inflammability.

In the conventional devices, the art is directed toward attaining a fully mixed state as rapidly as possible by multiply dividing one gas and supplying the divided gas dispersively to the other gas as dis closed in Japanese Laid-open Patent Publication Nos. 4362/1971 and 16381/1972. in the case where dispersive supply is not carried out, one method which is being carried out isto supply one gas through a single nozzle to the other gas and to cause the injected gas from the nozzle to strike against a baffle set in the lower course of the nozzle so as to disperse the injected gas.

In either of these cases, these two gases are mixed in a tubular space. The ratio of the tube length (L) to the inner diameter of the tube (D), i.e. (UD), is required to be about 5 to about 10 or more, to obtain a thoroughly homogeneous gas mixture.

In industrial-scale devices, very large quantities of gases (several thousands to several ten thousands m3/hr) are treated, and pressure losses in the piping system often cause economic disadvantage. Thus, the inside diameter of the piping system is generally 30 to 50 cm, and the distance required for obtaining a thoroughly homogeneous gas mixture will be about 1.5 to about 5.0 meters from the points where two gases merge. Therefore, a gas composition within the range of inflammability will be formed locally in the course of this distance of 1.5 to 5.0 meters, which is very dangerous.

We have now found that the above-described problems can be successfully met by providing a gas-mixing device of a shell-and-tube heatexchanger type structure in which the characteristics of small tubes with respect to gas flow are utilized.

The gas-mixing device in accordance with the present invention comprises: (A) a bundle of a plurality of tubes, at least one small aperture being provided through the wall of each tube; (B) a shell accommodating the bundle of tubes; (C) members forming a space outside the tubes within the shell by isolating space within the tubes and the space outside of the tubes in the shell; and (D) gas-supply means which supply respectively a first gas and a second gas to be mixed together to the inside of each tube and to a space within the shell which is formed by the members (C), wherein the first gas is passed through the insides of the tubes of the bundle (A), and the second gas is supplied to the space within the shell formed by the members (C), the second gas being caused to flow into the tubes through the small apertures so as to be mixed with the first gas passing through the tubes.

The present invention succeeds in solving the above-described problems by means of a gasmixing device having a shell-and-tube heatexchanger type structure, in which a first gas supplied as the "tube-side fluid" and a second gas supplied as the "shell-side fluid" are effectively mixed within the tubes, the second gas flowing into each of the tubes through a small aperture provided through the wall of each tube, whereby the mixing is carried out within a plurality of tubes having a relatively small diameter.

A plurality of the tubes (often several tens to several hundreds of tubes), which give a safe mixing region having a sufficient volume, are simply combined as a bundle in the form of a shell-and-tube heat-exchangertype structure. We have also found that pressure losses through the device are relatively small.

In the drawings: FIG. 1 is a longitudinal section taken along a plane passing through the centerline of a specific example of the gas-mixing device of the present invention and indicates the principle thereof; FIG. 2 is a cross-section of the device taken along the plane indicated by line ll-ll in FIG. 1; FIGS. 3a, 3b, and 3c are cross-sections taken along planes perpendicular to the axes of specific examples of tubes; FIG. 4 is a cross-section taken along a plane perpendicularto the axis of a specific example of a shell having tubes and partition walls; FIG. 5 is a longitudinal section taken along a plane passing through the centreline of a nozzle-type mixer and indicates the principle thereof; and FIG. 6 is a graph showing the results set forth in Table 1 (below).

1. Theoretical analysis It is generally known that, when a gas is passed through a tube, the degree of mixing is governed by the value of UD (tube length/inner diameter of tube) if the rate of flow through the tube is within the range of turbulent flow, that is, if the Reynolds number represented by (inner diameter of tube) x (flow rate of gas) x (density of gas)/(viscosity of gas) is 10,000 or more.

From this fact, the gas mixing in a tube having a smaller diameter results in the same degree of mixing, with a shorter tube length, as the mixing in a tube having a larger diameter. That is, the mixing in a tube of smaller diameter is effected more rapidly than the mixing in a larger-diametertube, whereby the time or region in which a composition within the above-described range of inflammability is locally formed can be shortened or reduced.

It is also known that the range of inflammability of an organic compound-molecular oxygen system in a small-diametertube is narrower than the range in a large-diameter tube. This is considered to be due to the increase in the effective areas of the tube walls which serve to prevent a combustion reaction.

In view of this fact, the extent of local occurrence of a mixture of dangerous composition will decrease as the range of inflammability is diminished when an organic compound and molecular oxygen are mixed in a small-diametertube, so that the safety in mixing is enhanced in comparison with mixing in a largediameter tube.

2. Structures of gas-mixing device The gas-mixing device in accordance with the present invention has a structure similar to that of a shell-and-tube heat exchanger. There is a fundamental difference between these two kinds of structures, however, in that a small aperture is formed in each of the small tubes which constitute the bundle of tubes and the fluid (gas) in the shell flows into each of the tubes through the small aperture.

Referring to FIGS. 1 and 2, a plurality of tubes 1 are supported within a shell 3 by tube plates or sheets 2, 2a. A space 4 outside of the tubes 1 is thus formed which is essentially isolated in the shell 3 by the tube sheets 2, 2a from spaces within the tubes. The shell 3 ordinarily has extended regions 3a and 3b.

In the example shown in the drawings, the extended regions 3a, 3b are closed by end plates 5, 5a. It is possible, however, to make the outer diameter of the shell 3 equal to that of the gas piping so that the extended regions 3a, 3b constitute parts of the gas piping.

In accordance with the present invention, a small aperture 6 is provided in each of the tubes 1.

Although a small aperture is shown in each tube 1 in the drawing, a plurality of apertures can be provided in each tube in the direction of the circumference or the axis thereof. The sizes or areas of the apertures and the numbers thereof can be suitably determined from the viewpoint of obtaining a homogeneous gas mixture in as short a period of time as possible. The maximum number of apertures may be found in, for example, a tube composed of a screen material. The number of small apertures is usually 1 to several. In orderto realize homogeneous mixing in a tube of limited length, the small aperture be provided in the tubes as far upstream as possible with respect to a gas passing through the tube.

The inner diameter, tube length, number and the like of each tube which is a part of the tube bundle are preferably selected so that the gas passing through the tubes is in a state of turbulent flow, that is, its Reynolds number is 10,000 or more. The inner diameter of each of the tubes constituting the tube bundle or the equivalent diameter defined by 4 x (a cross-sectional area of the divided room)/(circumferential length of cross-section of the room in contact with gas) is generally 5 cm or smaller, and preferably in the range of 1 to 3 cm.

On the other hand, the tube length is preferably selected so that the ratio UD (tube length/inner diameter of the tube) is 3 or more. In general, the tube length is selected so that UD is in the range of from 10 to 15. By tube length is meantthe distance between a position wherean aperture at the most downstream part of the flow is provided in a tube and the end of the tube (the downstream end). When a plurality of apertures are provided in the tube, the length starts from the most downstream aperture.

The number of the tubes constituting the tube bundle is ordinarily in the range of 50 to 1,000.

3. Mixingofgases In the device shown in FIGS. 1 and 2, the first gas to be mixed is supplied through an inlet 7 into the device, passed through the shell in the direction of the arrows, and then through the tubes, and discharged from an outlet 8.

The second gas to be mixed is supplied through an inlet 9 into the space 4 within the shell which is isolated from the spaces within the tubes by the tube sheets 2, 2a and the tube walls, flows into the tubes from the apertures 6, and is then mixed with the first gas passing through the tubes to be discharged from the outlet 8.

Eitherthe first gas or the second gas can be composed of a single component or can instead be a mixture of gases. For example, when the first gas is a gas comprising molecular oxygen, it can be generally air (i.e., a mixture of molecular oxygen and nitrogen gases) our a mixture of air and a diluting gas such as steam or carbon dioxide. When the second gas is a gas comprising an organic compound, the gas can be a mixture containing a diluting gas.

In the present invention, the mixing device is generally described with respect to two gases, namely, the first and second gases, since a mixture composition within the range of inflammability may be formed from these two gases. Thus, the device of the present invention can also be used for mixing a third gas having no explosive problem with the first and second gases. For example, it is within the scope of the present invention that a third gas such as a diluting gas can be further supplied through an inlet 7a to effect the mixing of these three gases. Since the device of the present invention has the effect of performing the mixing rapidly, there may be cases where the first and second gases do not form a mixture within the range of inflammability. Of course, such cases are within the scope of the present invention.

When the gases to be mixed comprise a gas com prising molecular oxygen and a gas comprising an organic compound (some of the examples being aforementioned), it can be suitably determined, from a consideration of the ratio of the gas quantities, the ranges of inflammability concerned, and other factors, which gas is to be passed through the tubes and which gas is to be supplied to the shell.

4. Specific examples of the device A specific example of the gas-mixing device of the present invention comprises empty tubes or nonpacked tubes which constitute the tube bundle as shown in FIGS. 1 and 2.

Another specific example of the device comprises packed tubes which constitute the tube bundle. The solid packingswhich are to be packed in the tubes can be in the form of spheres, saddles, rings, screens, solid (non-hollow) or hollow bars, or other optional shapes. In this connection, the term "solid packing" should be understood to include, in addition to ordinary packings, baffle plates provided on the walls within the tubes. The use of packed tubes will result in a more rapid mixing of the gases and also increase the effective areas of the tube walls which serve to prevent an explosive combustion reaction.

A further specific example of the device comprises tubes wherein each tube is divided into a plurality of longitudinal chambers by partition walls extending in its axial direction, and a small aperture is provided in each tube wall so that each of the chambers communicates with the space 4 (cf. FIG. 1) through the apertures. FIGS. 3a, Sb, and 3c show cross-sections of three examples of such tubes, in which the tubes are divided into 2 to 4 chambers by the partition walls 10, 10a, and 10b, respectively. Such divisions of the tubes by partition walls is effective when the inner diameter of tubes cannot be made sufficiently small or a small tube effect is further required.

In addition to, or apart from, such variations of the bundle of tubes, the shell space 4 (cf. FIGS. 1 and 2) can also be varied.

FIG. 4 showing such a specific example of the device, is a cross-section. In this example, the space 4 within the shell is divided into a plurality of longitudinal chambers (4 chambers:) 4a, 4b, 4c and 4d in this case) by partition walls extending in its axial direction (11 and 11a in this case), and gas inlets 9 are provided for each of the chambers (9a, 9b, 9c and 9d in this case). This division of the chamber 4 within the shell is effective in preventing gas channeling and in dispersing the gas uniformly.

Some of the above-described examples can, of course, be practised in combination.

5. Experiments Example A gas-mixing experiment was carried out using a gas-mixing device of a shell-and-tube heatexchanger type structure having 37 small-diameter tubes. Each tube had a 1 0-mm inner diameter and 400-mm length and had one small aperture 2 mm in diameter at a position 50 mm from the upstream end of the tube. The inner diameter of the shell was 146 mm.

Air was used as the first and second gases. A small quantity (3% by volume) of a tracer gas (CH4) was incorporated into the second gas to evaluate the degree of mixing of the gases.

The flow velocity of the first gas through the small tube was 35 m/sec., while the velocity of the second gas passing through the small aperture was 75m/sec. The temperature of these gases was 35"C.

The gas mixing was measured for all small tubes at a position 370 mm from the upstream end of the tube (320 mm from the small aperture).

There were no differences in the distribution of concentration and the mean concentration of the tracer gas in the small tubes due to their positions, between the tube placed at the center of the shell and the tubes in the vicinity of the shell wall.

The samples of mixed gases were taken out at 5 sampling points including the center of each tube, and the concentrations of the tracer gas were determined. As a result, the relative difference in concentration at each measured point with within t 2% relative to the mean concentration (0.24% by volume) in all of the small tubes. An example of the results is shown in Table 1 and FIG. 6 (curve 14). The tracer gas was analysed by gas chromatography.

In Table 1 and FIG..6, R denotes the inner radius of the small tubes and r stands for the distance of the sampling position within the small tube from the center of the small tube.

The term "mean concentration" used herein of the tracer means an area mean concentration.

Comparative Example A gas-mixing device of a nozzle type was made by installing a gas-supplying nozzle of 68-mm inner diameter for the second gas through a steel pipe of 305-mm inner diameter as shown in FIG. 5.

Air was used as the first gas and supplied at a flow velocity of 35 m/sec. in the direction from the left to the right of FIG. 5. The second gas which had been prepared by incorporating 3% by volume of CH4 tracer gas into air was injected from the supplying nozzle at a nozzle-tip velocity of 75 m/sec. in a direction opposite to that of the first gas. The temperature of the first and second gases was 35"C. In the same way as in the Example, the samples of mixed gases were taken at a position 320 mm downstream from the tip of the supplying nozzle, and the concentrations of the tracer gas were determined. The results are shown in Table 1 and FIG. 6 (curve 15), wherein R denotes the inner radius of the steel pipe. The mean concentration of the tracer gas was 0.29% by volume.When the ratio UD is about 1, even at a mixing distance of 320 mm which is the same as in the Example, there was almost no mixing of the gases.

Tablet Results of Experiments

Measuring Concentration Mean concen- (C-C) positions of tracer (C) tration of tracer X 100 (C) r/R (% by volume) (% by volume) (%) 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 Comparative 0.55 0.318 9.7 Example 0.0 0.705 0.29 143.1 -0.55 0.469 61.7 -0.70 0.328 13.1 -0.84 0.159 -45.2 -0.95 0.094 -67.6

Claims

1. A gas-mixing device which comprises: (A) a bundle of a plurality of tubes, at least one small aperture being provided through the wall of each tube; (B) a shell accommodating the bundle of tubes; (C) members forming a space outside of the tubes within the shell by isolating space within the tubes and the space outside of the tubes in the shell; and (D) gas-supplying means which supply respectively a first gas and a second gas to be mixed together, to each inside of the tubes and to a space within the shell which is formed by the members (C), wherein: the first gas is passed through the insides of the tubes of the bundle (A), the second gas is supplied to the space within the shell formed by the members (C),the second gas being caused to flow into the tubes through the small apertures to be mixed with the first gas passing through the tubes.

2. A gas-mixing device as set forth in Claim 1, in which the tubes constituting the bundle (A) are empty tubes or packed tubes packed with a solid packing.

3. A gas-mixing device as set forth in Claim 1 or 2, in which each tube constituting the bundle (A) is divided, by a partition wall extending in its axial direction into a plurality of longitudinal chambers, and a small aperture is provided through the tube wall for each of the chambers.

4. A gas-mixing device as set forth in any of Claims 1 to 3, in which the inner device diameter of the tube, or the equivalent diameter of the chambers divided by the partition wall in the tube, is 5 cm or smaller.

5. A gas-mixing device as set forth in any of Claims 1 to 4, in which the space in the shell formed by the members (C) is divided into a plurality of chambers by a partition wall extending in its axial direction, and the gas-supplying means (D) to supply the second gas to the space in the shell is provided for each of the chambers.

6. A gas-mixing device as set forth in claim 1 substantially as herein described with reference to the accompanying drawings and/or any one of the specific examples.