WO1993003065A1

WO1993003065A1 - Loop reactor for the oxidation of olefins

Info

Publication number: WO1993003065A1
Application number: PCT/US1992/006082
Authority: WO
Inventors: Michael C. Fullington
Original assignee: Olin Corporation
Priority date: 1991-08-07
Filing date: 1992-07-23
Publication date: 1993-02-18
Also published as: CN1069975A; AU2399992A

Abstract

A loop reactor for the oxidation of hydrocarbons, such as propylene, has at least one reaction tube (18) immersed in a thermal transfer medium (42). The thermal transfer medium (42) is preferably a pressurized liquid (51) such as water (51). The pressure appplied to the liquid (51) is varied so that the liquid (51) boils at the temperature desired for the oxidation reaction.

Description

LOOP REACTOR FOR THE OXIDATION OF OLEFINS

This invention relates to a reactor for the direct oxidation of olefins. In particular, an isothermal loop reaction is employed for the direct oxidation of propylene to propylene oxide. Alkylene oxides (vicinal epoxy alkanes), and particularly propylene oxide, are widely used chemicals. The alkylene oxides have been polymerized with a wide variety of monomers to yield polymers which are useful in coating compositions and in the manufacture of molded articles such as urethane foams. They are also reactive with alcohols to yield monoalkyl ethers which have utility as solvents in many commercial processes and which are useful as components for synthetic turbojet lubricants. Many methods to produce propylene oxide are known throughout the art. One method, referred to as the chlorohydrin process, involves the reaction of chlorine and water to form hypochlorouε acid which is then reacted with propylene, forming propylene chlorohydrin. The propylene chlorohydrin is then dehalogenated to yield propylene oxide.

U.S. Patent Nos. 4,785,123 and 4,943,643 to Pennington, assigned to a common assignee, disclose the vapor phase oxidation of olefins by bubbling the gases through a molten nitrate salt catalyst. The salts are a mixture of potassium and sodium salts containing 20-80 weight percent sodium nitrate. Besides a catalyst, the molten salts serve as an isothermal medium for any co-catalyst and absorb large quantities of heat generated during the exothermic oxidation reaction.

Non-catalytic oxidation reactions have also been disclosed. U.S. Patent No. 5,117,011 to Pennington et al., disclose that propylene is oxidized under isothermal, non-catalytic conditions with a propylene partial pressure of from about 80 to about 300ρsia and a reaction temperature of from about 200°C to about 350°C. The oxidation of . propylene is highly exothermic. During gas phase oxidation, heat must be continuously removed from the reaction vessel to maintain a constant reaction temperature. If not adequately removed, the reaction temperature can rise rapidly, leading to the formation of undesirable byproducts by coking or combustion. The heat rise may be sufficient to cause over pressurization damaging the reactor.

The removal of heat from a gas phase reaction is particularly difficult because the efficiency of the transference of heat from a gas phase to a solid, such as the reactor walls is poor. U.S. Patent No. 2,530,509 by Cook, discloses reacting propylene with oxygen in a plug flow reactor. The reactor has a large surface area relative to the volume occupied by the reacting gases. The reaction tube may be packed with a solid such as porcelain to increase surface area.

U.S. Patent No. 3,132,156 to Lemon et al discloses a reaction vessel for the oxidation of propylene. Reaction gases are circulated within the reaction vessel, either by mechanical stirring or directed gaseous flow to achieve a substantially isothermal condition.

The circulation of reaction gases within the reactor will achieve an isothermal condition. However, transfer of heat from the reaction vessel is limited so while uniform, a rise in temperature is typical. Further, the reactor is manufactured from a material chemically resistant to reaction gases such as stainless steel. Chemically resistant materials are generally characterized by poor thermal capacity.

Better heat transfer may be achieved with a loop reactor than with a batch reactor or a continuous stirred tank reactor as disclosed in an article by Geddes entitled "The Loop Reactor Process". While loop reactors have been widely used for the conversion of liquid olefins to solid particles as disclosed in U.S. Patent No. 3,324,093 to Alleman and for the dimerzation of ethylene in the presence of a catalyst as disclosed in U.S. Patent No. 4,242,531 to Carter, a loop reactor has not been used for the gaseous . phase direct oxidation of an olefin such as propylene.

Accordingly, as a first object of the invention, a gas phase loop reactor is provided. As a second objective of the invention, a process for the use of the gas phase loop reactor for the direct oxidation of propylene is provided. One feature of the reactor is that gaseous oxidation occurs in at least one reaction tube immersed in a thermal transfer medium capable of maintaining an isothermal reaction. By regulating the pressure of the thermal transfer medium, the temperature of the reaction is controlled. It is an advantage of the reactor of the invention that the heat transfer coefficient is on the order of about (70 BTU/hour-sq.ft. •°F) 73.78 x 10³ joules which is sufficient to remove (1220 BTU/hour) 3 1285.88 x 10 joules of reaction generated heat. Yet another advantage of the invention is that the temperature of the reaction may be further controlled by regulation of both gas velocity and oxygen concentration. As yet another advantage of the invention, only a portion of the offgas is removed from the loop. The remainder of the gas is combined with fresh feed gas and recirculated.

In accordance with the invention, there is provided a reactor for the gas phase oxidation of a hydrocarbon. The hydrocarbon is selected from the group consisting of alkylenes, alkanes, mixtures and derivatives thereof. The reactor has at least one reaction tube with first and second opposing open ends. The reactor further has a means for isothermally regulating the temperature of each reaction tube. An inlet delivers a mixture containing the hydrocarbon and oxygen to one end of each reaction tube. An outlet proximate the opposite end of each reaction tube removes a portion of the mixture. A recirculating means transfers the remainder of the gaseous mixture past the inlet to the first end of each reaction tube.

The above stated objects, features and advantages, as well as others, will become more apparent from the specification and drawings which follow.

Figure 1 shows in schematic a gas phase loop reactor in accordance with the invention.

Figure 2 shows in cross-sectional representation a bundle of reaction tubes in a thermal transfer medium. Figure 1 illustrates in schematic a loop reactor 10 for the oxidation of olefins. A desired mixture of gases is introduced to the loop reactor 10 by means of inlet 12. The reactor is suitable for the oxidation of hydrocarbon gases broadly defined as alkylenes, alkanes and derivatives thereof, generally having from 3 to 22 carbon atoms. When the hydrocarbon is a straight chain molecule, it is preferred that the molecule not have more than 5 carbon atoms. When a cyclic compound is used, it is preferred that there be not have more than 12 carbon atoms per molecule. Illustrative reactants include propane, propylene, isobutane, butane, cyclohexene and mixtures thereof. A preferred reactant within this group is propylene or a mixture of propylene and propane, based on commercial availability.

Oxygen is provided either as pure gas or in a mixture with other gases. One such mixture is air which is preferred based on ready availability. Pure oxygen will be preferred in a commercial setting to minimize contamination from trace constituents.

In the oxidation of propylene, in addition to the hydrocarbon and oxygen, a diluent is preferably present. When propylene is provided at high partial pressure and in high concentration, thermal cracking may break apart the carbon chains. The diluent reduces the concentration of propylene to eliminate or minimize thermal cracking. Among the suitable diluents are inert gases such as nitrogen and argon or mixtures of oxidation byproduct gases such as acetaldehyde, methane and carbon dioxide. Diluents having high thermal capacity and high thermal conductivity are preferred to assist in the circulation of heat. A most preferred diluent is carbon dioxide generated as a byproduct of the oxidation reaction.^' Within the reactor, it is preferred that the gas feed stock comprise from about 30 to about 85 volume percent (vol.%) propylene, from about 1 to about 20 vol.% oxygen and the balance CO„ or other diluent. If air is the oxygen source, the ratio of oxygen to nitrogen in the air is considered in determining the feed stock ratio. More preferably, the propylene concentration is from about 40 to about 75 vol.%, the oxygen from about 2 to about 17 vol.% and the balance CO,,. Most preferably, the concentration of oxygen is from about 5 to about 15 vol.%.

Oxygen initiates the exothermic reaction. It is, therefore, preferred to charge the reactor with a mixture of propylene and carbon dioxide. For example, a mixture of propylene and carbon dioxide in a 50/50 molar ratio may be used to charge the loop reactor. Once the reactor is fully charged and the gas at a suitable velocity and pressure, the desired amount of oxygen is added to the feed mixture. The feed gases flow through inlet 12 and in flow pipe 14 to U-tube 16. Figure 1 provides a series of arrows to indicate the direction of gas flow within the reactor 10. From U-tube 16, the reaction gases flow into at least one reaction tube 18. Each reaction tube 18 has a first open end 20 for receiving the reaction gases and a second opposing open end 22 for delivering the reacted gases back to U-tube 16.

The reaction tubes 18 are formed from any material which is chemically inert to the reaction gases and reaction products having good thermal transfer characteristics. One preferred material is a stainless steel such as 304 stainless steel. The reaction tubes may be any diameter. It is preferable to minimize the diameter of the tubes to maximize the surface area of the interior walls of the tubes relative to the volume of reaction gas. The tube thickness should be sufficient to avoid rupture when pressurized. The reactor pressure is in the range of

3 (400 psig) (pounds per square inch gauge) 2758 x 10 pascal (Pa) and the tube thickness should be sufficient to provide a rupture safety margin at that pressure.

In stainless steel, a wall thickness in excess of about

(.100 inches) 2.54 mm and preferably from about (.110 inches) 2.80 mm to about (.150 inches) 3.81 mm. Preferably, the outside diameter of the stainless steel reaction tubes is from about (.5 inches) 12.7 mm to about (1 inch) 25.4 mm.

The number of tubes is limited only by- the thermal conduction of the thermal transfer medium and the size of the reactor. While at least one tube is required, up to several thousand tubes may be present for high flow and high product yield. The reaction tubes 18 are bundled in any desired arrangement as illustrated in cross-section in Figure 2. The plurality of reaction tubes generally form two adjoining bundles of tubes. A first bundle of tubes 24 transmits the ingaε and a second bundle of tubes 26 transmits the outgas. Spacing 28 between the individual tubes facilitates the flow of thermal transfer medium.

With reference back to Figure 1, the reacted gases exit the reaction tubes 18 at the second open end 22 flowing into U-tube 16. The U-tube 16 has a geometric design suitable for good mixing of the gases to prevent localized accumulations of oxygen which could form hot spots within the reactor leading to the generation of undesired byproducts. The outgas exits U-tube 16 through outflow tube 30. Outlet 32 removes a portion of the offgas. A pressure sensitive valve 34 permits from about 5 to about 50 vol.% and preferably, from about 5 to 15 vol.% of the offgas to exit the reactor 10 The product gas is then collected by any suitable collection means known in the art. The pressure sensitive valve 34 maintains pressurization within the reactor while allowing the collection of offgas.

The remainder of the offgas proceeds to a recirculating means 36. The recirculating means 36 may^¬ be any means capable of transferring gas at a relatively high velocity. The velocity is generally on the order of about (50 fps) 15.24 m/sec. to about (100 feet per second) (fps) 30.48 m/sec, and preferably on the order of about (60 fps) 18.29 m/sec. to about (80 fps) 24.39 m/sec. in each tube 18. The internal gas velocity is critical to promote the necessary heat transfer coefficient to remove reaction heat. It is therefore necessary that the recirculating means provide a high flow rate with good flow control. One suitable means is a mechanical gas blower. Other circulating means such as a gas-powered venturi are also suitable.

The recycled portion of the offgas flows through back tube 38 past inlet 12. New reaction gases in approximately the same volume fraction removed at outlet 32 are added to the recycled portion by means of the inlet 12. To promote good mixing, internal baffles 40 may be provided within inflow pipe 14 to provide static mixing. The mixture of recycled gas and new reactant products enter U-tube 16 for transfer back into reaction tube 18 and the continuous loop process is repeated. The oxidation of propylene is highly exothermic. While air or forced draft air may be used as a thermal transfer means to cool reaction tubes 18, the higher thermal capacity of a liquid is preferred. The reaction tubes 18 are preferably immersed in a liquid thermal transfer medium 42. As illustrated in the cross-sectional view in Figure 2, the thermal transfer medium 42 preferably surrounds substantially all surfaces of the inflow bundle 24 and outflow bundle 26. The reactor may be manufactured with the individual tubes of the inflow bundle 24 and outflow bundle 26 bonded together such as by welding, it is possible that the thermal transfer medium will not be able to completely surround each individual tube. However, to maximize thermal conduction from the tubes, the thermal transfer medium 42 preferably surrounds as much of the tubes as possible.

With reference back to Figure 1, the thermal transfer medium is provided under pressure through liquid inlet 44. During operation of the reactor, the thermal transfer medium is preferably heated to boiling and converted to a vapor. The boiling heat transfer fluid takes the heat of reaction from the loop at a constant temperature and at high heat transfer conditions. The low density vapor rises in the reactor tube to outlet ports 46 from which it flows into containment drum 48. A portion of the vapor phase thermal transfer medium escapes from the containment drum 48 through outlet port 50 with the remainder condensing back to liquid 51. A back check valve 52 maintains a desired pressure within the thermal transfer medium.

Adjusting the pressure to activate the back check valve 52 varies the pressure applied to the thermal transfer medium 42. By varying the pressure, the boiling point of the thermal transfer medium may be adjusted to exactly correspond to that desired for the reaction. The oxidation of propylene to propylene 5 oxide requires a minimum temperature of about 200°C. A supplemental heating means, such as electric strip heaters, may be immersed in the thermal -transfer medium to achieve that temperature prior to introduction of the reaction gases or during the gas charging step. ₀ The pressure of the back check valve is simultaneously adjusted so the thermal transfer medium boils at that temperature. Once the exothermic oxidation is underway, the external heaters may be disengaged and the pressure adjusted as necessary. _$ A most preferred thermal transfer medium is water. Water is convenient, inexpensive and chemically compatible with the stainless steel reaction tubes 18. Other fluids such as kerosene or any commercially available heat transfer fluid may be used. The key o aspect is that regulation of the boiling temperature of the fluid gives close control and consistency of temperature. However, a non-boiling fluid could also be applied.

For example, while at atmospheric pressure, water 5 boils at 100°C. Raising the pressure of the water to

355ρsig raises the boiling temperature to 225°C which is a preferred temperature for the isothermal direct oxidation of propylene.

The working of the gas phase loop reactor of the 0 invention will become more apparent from the example which follows. The example is intended to be exemplary and in no means to limit the scope of the invention. Theoretical Example

A stainless steel tube with an outside diameter of (.75 inches) 19.05 mm and an inside diameter of

(.620 inches) 15.75 mm and a length of (-76 inches) 1930.4 mm is formed into a continuous loop having an inlet and outlet for admitting fresh feed and exhausting product gas, respectively. The loop is connected to a blower to circulate the gases at a desired rate. The reactor loop, except for the blower, is immersed in a pressure vessel containing water.

The loop reactor is brought to temperature

(225°C) by heating the water to boiling at (355 psig)

3 2447.73 x 10 (Pa) using immersed electrical heating strips. If a different reaction temperature is required, the pressure of the water may be suitably adjusted to change the boiling temperature. In U.S.

Patent 5,117,011 it is disclosed that the reaction temperature may be in the range of from about 200°C to about 350°C. The higher temperatures give improved propylene selectivity. Propylene selectivity is that molar percent of reacted propylene which forms propylene oxide. It is believed that with the reactor of the present invention, propylene selectivity in excess of about 50% may be achieved. When the loop reactor is up to temperature, a charging gas of propylene and carbon dioxide in a 50/50 mole ratio is added and the reaction tubes 18 are

3 pressurized to about (400 psig) 2758 x 10 (Pa)

The partial pressure of propylene should be in the range of (100 psig) 689.5 x 10 (Pa) to about (300

3 psig) 2068.5 x 10 (Pa) . The partial pressure of propylene is calculated by multiplying the total pressure (absolute) and^' the volume percent of propylene as determined by analytical means.

The blower is then energized to begin circulating the reactor contents through the tube at a desired velocity, typically in the range of from about (50 fps) 15.24 m/sec. to about (100 feet per second) 30.48 m/sec. In the hypothetical Example, at a velocity of about (67 feet per second) 20.42 m/sec, the average residence time of the reaction gas in the loop is about 20-40 seconds generating a heat of reaction of about (1220 BTU/hour) 1285.88 x 10³ (J) .

To begin the reaction, oxygen gas is added to the feed mixture in a desired concentration. The desired concentration of oxygen will be from about 1 to about 20 vol.%. The electrical heating strips are de-energized and the reaction temperature controlled by adjusting the pressure of the boiling water, and thereby, the temperature of the water. The reactor temperature is controlled to give about 80% conversion of the oxygen and 15% conversion of the propylene. Under these conditions, the ratio of circulating gas to fresh feed is about 20-1. The internal circulation rate of (67 feet per second) 20.42 m/sec. is also sufficiently high to give a considerable degree of back-mixing, at both the internal baffles 40 and the U-tube 16 which is important to the high yields of the reaction.

The reaction gas exiting the loop theoretically contains by volume, 42.5% propylene, 41.8% carbon dioxide, 4.5% propylene oxide, 3.2% water, 2.3% carbon monoxide, 2.1% oxygen, 1.4% methanol, 1.2% acetaldehyde, 0.5% propionaldehyde and 0.5% allyl alcohol.

While the specification and above Example illustrate the oxidation of propylene at specific temperature and pressure, the described reactor may be used with any gas phase reaction where it is necessary to remove a large amount of reaction heat and maintain isothermal reaction conditions. It is to be understood that the above-described embodiments of the invention are illustrative only and that modifications throughout may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited as defined by the appended claims.

Claims

XN THE CLAIMS

1. A reactor for the gas phase oxidation of a hydrocarbon selected from the group consisting of alkylenes, alkanes and derivatives thereof, characterized by: at least one reaction tube (18) having first (20) and second (22) opposing open ends; a means (42) for isothermally regulating the temperature within each of said reaction tube (18) ; an inlet for delivering a mixture containing said hydrocarbon and oxygen to said first open end (20) of each said reaction tube (18); an outlet (32) for removing a portion of said mixture and a means (36) for recycling the remainder of said mixture to said reaction tube (18) via said first open end (20) .

2. The reactor of claim 1 characterized in that said hydrocarbon is propylene, propane, derivatives or mixtures thereof.

3. The reactor of claim 2 characterized in that said hydrocarbon is propylene.

4. The reactor of claim 1 characterized in that said reaction tubes (18) are formed from a material chemically inert to the reaction gases and products and is rupture resistant at pressures in excess of about (400 psig) 2758 x 10³ (Pa).

5. The reactor of claim 4 characterized in that said reaction tubes (18) are formed from stainless steel having a wall thickness in excess of about 2.54 mm (.100 inches) 2.54 mm.

6. The reactor of claim 5 characterized in that said reaction tubes (18) are formed from 304 stainless steel having a wall thickness in the range of from about (.110 inches) 2.80 mm to about (.150 inches) 3.81 mm.

10 7. The reactor of claim 5 characterized in that said reaction tubes (18) have an outside diameter of from about (.5 inches) 12.7 mm to about (1.0 inches) 25.4 mm.

8. The reactor of claim 4 characterized in that

-_L the portion of said mixture removed at said outlet (32) constitutes from about 5 to about 50 vol.% of the offgas .

9. The reactor of claim 8 characterized in that the portion of said mixture removed at said outlet (32)

20 constitutes from about 5 to about 15 vol.% of the offgas.

10. The reactor of claim 4 characterized in that said recirculating means is a mechanical blower (36).

25 11. The reactor of claim 10 characterized in that said recirculating means (36) propels said mixture at a velocity of from about (50 fps) 15.24 m/sec. to about (100 feet per second) 30.48 m/sec.

12. The reactor of claim 11 characterized in that said recirculating means (36) propels said mixture at a velocity of from about (60 fps) 18.29 m/sec. to about (80 feet per second) 24.39 m/sec.

13. The reactor of claim 11 further characterized in that internal baffles (40) to combine the remainder of said mixture with fresh ingas.

14. The reactor of claim 4 characterized in that said means (42) for isothermally regulating the temperature of said reaction tube (18) is surrounding each said reaction tube (18) with a thermal transfer medium (42) .

15. The reactor of claim 14 characterized in that said thermal transfer medium (42) is a liquid (51) .

16. The reactor of claim 15 characterized in that said thermal transfer medium (42) is water (51) .

17. The reactor of claim 15 characterized in that it has a means (52) to pressurize said liquid (51) .

18. The reactor of claim 17 characterized in that said liquid (51) is pressurized to have a boiling temperature in the range of from about 200°C to about 350°C.

19. The reactor of claim 18 characterized in that said liquid (51) is pressurized to have a boiling temperature - in the range of from about 200°C to about 250°C.

20. A process for the direct oxidation of propylene, characterized by:

(a) providing a mixture of propylene and oxygen to at least one reaction tube (18) of a loop reactor; (b) isothermally maintaining said mixture at a desired temperature;

(c) removing a portion of reacted offgas; and

(d) recirculating the remainder of said offgas with fresh ingas.

21. The process of claim 20 characterized ^'in that said reaction tube (18) is immersed in a pressurized liquid (51) .

22. The process of claim 20 characterized in that prior to step (a) said reaction tube (18) is preheated to a temperature in excess of about 200°C.

23. The process of claim 21 characterized in that said desired reaction temperature is maintained by adjusting the pressure of said pressurized liquid to boil at said desired temperature.

24. The process of claim 23 characterized in that said desired temperature is from about 200°C to about 350°C.