Field of the Invention

The invention relates to production equipment for gas-liquid processes and can be used in chemical, petrochemical and other industries.

Background Art

For most gas-liquid processes, one or more of starting reagents is in the gas phase, and for a reaction to take place, the reagents are to be converted into the liquid phase or to the boundary of the two phases, which requires high mass transfer rates. Furthermore, processes carried out in liquid and gas phases are often accompanied by a high release or absorption of heat, which requires an efficient heat exchange between the mixture and heat transfer agent.

Also of high importance is the uniformity of conditions for running the heat and mass transfer throughout the apparatus, since local increase or reduction in the reaction mixture temperature or the concentration of reagents can decrease the selectivity and conversion of the process, and the reaction rate. Uniformity of reaction conditions can be attained by using equipment with uniform distribution of tubes by volume, i.e. with distribution of tubes, which equalizes the velocities of liquid phase flow throughout the apparatus and eliminates stagnant zones.

A conventional shell-and-tube apparatus for carrying out exothermic gas-liquid processes (reactions) is disclosed in US 5,864,698 (publ. 08.12.1998). The apparatus comprises a hollow draft tube disposed therein. The tube accommodates an impeller means to cause recirculation of liquid downward through the tube into a bottom mixing chamber. Liquid flow is introduced into the apparatus through a feed line, and gas is introduced above the liquid level via the line. The apparatus exhibits improved heat transfer, high productivity and selectivity owing to the use of forced circulation created by the impeller means. However, fixation of the fast-rotating structure with a single an- chor point (only in upward position) is technically problematic, since while rotating the impeller can misalign from the axis of rotation under tangential forces. Therefore, spe- cial seals will be required for operation of the impeller shaft at high pressures. Further- more, a high velocity of liquid is required to create the descending gas-liquid flow through the central tube, which will cause additional energy losses in rotation of the impeller. The length of the apparatus will be severely limited even with a significant rotational speed of the impeller.

RU2040940 (publ. 09.08.1995) discloses an apparatus for carrying out gas-liquid chemical and heat and mass transfer processes with a high thermal effect, which can reduce the residence time dispersion (i.e. deviation of residence time of actual flows from a rated value) of a liquid reagent in the apparatus through the provision of its multi- pass through the working area. The apparatus comprises a bundle of bubble tubes (through which the gas-liquid mixture flows from the bottom upwards) and circulation tubes (through which the liquid returns (circulates) to the lower part of the apparatus), the tubes being secured in tube sheets and accommodated in a cylindrical vertical hous- ing; an upper chamber with vertical plates and a lower chamber with a gas distribution device. The housing comprises heat transfer agent feed and withdrawal nozzles, the lower chamber has a gas feed nozzle and a drainage nozzle. The apparatus is character- ized by the provision of a vertical partition in the lower chamber and a gas distribution device in the form of a horizontal partition with holes along axes of the bubble tubes. Furthermore, the partitions are offset relative to plates in the upper chamber to create a multi-pass channel for liquid flow from the feed to withdrawal nozzle. The above ele- ments reduce the residence time dispersion of individual portions of liquid. By varying the number and arrangement of plates in the upper chamber and partitions in the lower chamber, it is possible to create apparatuses with a desired number of passes through the tube space, which can vary from 2 to 6-10. However, such apparatus requires a high gas flow rate to prevent entry of a large amount of liquid into the lower gas chamber. But even operating with a high gas flow rate, the entry of liquid into the lower chamber cannot be avoided completely, which may lead to loss of reagents. Consequently, the apparatus disclosed in RU2040940 includes inactive zones occupied by gas. Further- more, due to possible blockage of the holes, it is undesirable to use this apparatus in processes that may be accompanied by precipitation of solid precipitate, high molecular and/or highly viscous compounds, including resins and polymers, and in processes ac- companied by crystallization of one of the reaction mixture components.

In conventional apparatuses described below, uniform gas distribution over the apparatus cross section (i.e. gas concentration is the same in any point of the tube in the horizontal section of the apparatus) is attained owing to the fact that walls of the tubes disposed under the lower tube sheet have holes for transition of gas entering the tubes from the gas blanket formed under the tube sheet. However, use of the apparatuses with holes in tube walls is problematic in the processes accompanied by formation of crys- tallized and precipitated reaction products or catalyst due to blockage of the holes and disturbance of hydrodynamic parameters of the apparatus. The probability of formation of explosive gas mixtures and liquid vapors in the gas space below the lower tube sheet also limits the applicability of the apparatuses.

A gas lift apparatus disclosed in SU1212550 (publ. 23.02.1986) comprises a ver- tical cylindrical housing accommodating an upper and lower tube sheets to secure a ver- tical bundle of circulation and bubble tubes. Upper ends of bubble tubes are disposed higher than ends of circulation tubes. To improve the productivity by increasing the re- action zone of the apparatus, the phase contact surface and creating a stable circulation, the apparatus further comprises a supplementary tube sheet mounted above the upper tube sheet so that a gas chamber is formed between them. Ends of bubble tubes are disposed in the gas phase, while ends of circulation tubes are disposed in the liquid phase, and holes are provided in circulation tube sections disposed in the gas chamber. The apparatus comprises a separating chamber with a drop eliminator and feed and with- drawal nozzles for phases and heat transfer agent. The disclosed apparatus structure does not exclude entry of liquid from circulation tubes into the upper gas chamber, which can impair its operation. In addition, the flow of gas, coming from the upper gas chamber, in circulation tubes can be hindered when moving to the lower part of the apparatus under the effect of the liquid phase.

An apparatus disclosed in SU 129643 (publ. 01.01.1960) is designed as a vertical housing with a central circulation tube. Each tube of the apparatus has an elongated end extending through a lower tube sheet with holes. Lower section of the circulation tube is below the tube cuts. When gas is fed through a branch tube to the apparatus, liquid filling the cavity under the tube sheet is pressed downward, and a gas blanket is formed under the tube sheet; gas bubbles through holes of all tubes and thereby is uniformly distributed over the apparatus section. While raising through the tubes, gas bubbles en- train liquid and create its intensive circulation, which improves heat transfer. However, the document does not disclose that the apparatus provides a stable heat exchange (i.e. unchanged temperature gradient in the apparatus in time) throughout the apparatus. Since the gas-liquid flow in bubble tubes of this apparatus, which are directly adjacent to the central circulation tube, will have a higher velocity than in the other tubes, differ- ent heat and mass transfer conditions will be observed in different parts of the apparatus, and as a result, reduced reaction selectivity. Processes occurring in the apparatus will not be stable even with increased number of circulation tubes, since both circulation and bubble tubes extend beyond the tube sheet, thereby causing accumulation of gas phase in the lower part of the apparatus.

Another conventional shell-and-tube gas lift apparatus, disclosed in SU199087 (publ. 01.01.1967), comprises: an upper chamber and a lower chamber with jackets, tube sheets and circulation tubes passing through them, and bubble tubes, whose ends in the upper chamber are disposed at different levels, a liquid phase feed nozzle and a gas phase feed nozzle. In course of feeding gas, a gas layer forms under the tube sheet, from which gas enters bubble tubes through holes. Owing to the difference in densities of the liquid phase in the circulation tubes and the gas-liquid mixture in the bubble tubes, intensive phase circulation occurs: the gas-liquid mixture moves upwards through the bubble tubes and the liquid phase moves downwards through the circulation tubes. Owing to the arrangement of tube ends at different levels, in the upper chamber the liquid phase is stratified depending on the specific gravity of the components. However, SU199087 is silent on the flow circulation stability. The effect of the amount of tube extensions above the upper tube sheet of the apparatus on liquid circulation is not supported by specific examples in the description of this patent. Furthermore, explosive gas mixtures can form in this space.

Therefore, no apparatus is known at present in the art, which would enable car- rying out gas-liquid processes accompanied by high thermal effects, formation of crys- talline precipitates, high-molecular and/or highly viscous compounds, or accompanied by formation of explosive gases, with high efficiency, in particular, with high selectivity and high yields of desired product, where the process is a chemical reaction.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a bubble shell-and-tube gas-lift apparatus for carrying out gas-liquid processes, exhibiting stable and uniform heat and mass transfer throughout the apparatus, and steady performance.

A technical effect of the invention is the provision of a bubble shell-and-tube gas-lift apparatus that reduces the residence time dispersion of liquid in the reaction zone, improves the hydrodynamic efficiency, and also increases the selectivity of pro- cesses carried out in the apparatus (including chemical reactions).

A further technical effect of the invention is the provision of a bubble shell-and- tube gas-lift apparatus enabling processes with significant thermal effects, as well as processes accompanied by formation of solid precipitate, high-molecular and/or highly viscous compounds, including resins and polymers, and also processes accompanied by crystallization of one of the reaction mixture components.

A further technical effect of the invention is the reduced probability of formation of explosive mixtures of gas and liquid vapors owing to the reduced gas space volume in the apparatus, which does not react with the liquid phase.

Furthermore, the present apparatus is free of inactive zones occupied by gas.

Furthermore, the productivity of the apparatus is enhanced owing to increased volumetric efficiency of the apparatus and increased reaction zone.

In the context of the present invention, stability of heat and mass transfer in the apparatus shall be construed as the constancy of characteristics (composition, tempera- ture, flow rate, etc.) at each flow point over time.

Steady performance of the apparatus shall be construed as the mode of operation, the characteristics of which return to the initial state after a disturbance has been elimi- nated.

Moreover, residence time dispersion of liquid in the reaction zone shall be con- strued as the deviation of residence time of actual flows from a rated value, and hydro- dynamic efficiency shall be construed as approaching characteristics of the actual appa- ratus to a plug-flow apparatus.

In the context of the present invention, the term "substantially" means a deviation within a permissible error range for a particular value determined by a person skilled in the art.

The object and technical effects are attained by using an apparatus, comprising one or more vertical shell-and-tube units formed as a housing with reagent feed devices and reaction product withdrawal devices, heat transfer agent feed and withdrawal de- vices; two tube groups secured by tube sheets in upper and lower parts of the housing, one tube group extending beyond the lower tube sheet, and the second tube group having tube ends substantially flush with the lower tube sheet, the tubes of the first tube group being distributed substantially uniformly over the tube sheet.

The inventors have unexpectedly found that the inventive structure and arrangement of the first ahd second tube groups maintain a constant flow direction in the tubes, which provides stable and uniform heat and mass transfer throughout the apparatus and steady performance of the apparatus, thereby ensuring the claimed technical effects.

A detailed description of various aspects and embodiments of the present inven- tion will follow below.

Tubes of the first tube group extend 10-150 mm, preferably 50-100 mm, beyond a lower tube sheet. If the extension length of the tubes of the first tube group relative to the lower tube sheet is less than 10 mm, the probability of gas slippage through the circulation circuit increases, which disrupts the hydrodynamics and, as a consequence, heat and mass transfer in the apparatus. Extension of the tubes of the first tube group by more than 150 mm relative to the lower tube sheet is not advisable, since this can increase geometrical dimensions of the apparatus, and, consequently, its metal content without improving the apparatus efficiency. Extending parts of the tubes of the first group can be either of the same or different length. The same length of extending parts of the tubes of the first tube group is not a prerequisite for operation of the apparatus. The determining condition is the length of more than 10 mm, sufficient to prevent gas from entering them.

Diameters of the tubes of the first and second tube groups can be either the same or different, however, to increase the gas/liquid contact zone, it is preferable to use the tubes of the second tube group of a larger diameter than the tubes of the first tube group.

The mandatory requirement for operation of the apparatus is the uniform distribution of the tubes of the first tube group throughout the apparatus, i.e. the distribution of the tubes, which equalizes velocities of liquid phase throughout the apparatus and eliminates stagnant zones.

The ratio of the number of the tubes of the first and second tube group is from 1 :1.25 to 1 :5. Preferably, at least one of the tubes of the first tube group is adjacent to each of the tubes of the second tube group in the horizontal section of the unit. More preferably, each of the tubes of the first tube group is surrounded on perimeter by the tubes of the second tube group in the horizontal section of the unit. This arrangement of the tubes is attained at a ratio of about 1 :2. In one embodiment, the tubes of the first tube group are the circulation tubes, and the tubes of the second tube group are bubble tubes.

The overall dimensions of the device, the number of shell-and-tube units, the number of tubes in the unit, and the total number of tubes in the apparatus are selected based on the requirements of the specific application of the device.

Reagent feed devices include a liquid phase feed device and a gas phase feed device, and reaction product withdrawal devices include a liquid phase withdrawal de- vice and a gas phase withdrawal device.

The apparatus can be employed as a reactor for carrying out various liquid-phase reactions, for example, hydrocarbon oxidation, olefin oligomerization, synthesis of car- boxylic acids, ethylene chlorination, hydroformylation, and as an apparatus for micro- biological processes, etc.

A method for carrying out chemical reactions in a bubble shell-and-tube appa- ratus according to the invention comprises:

- feeding a liquid phase to fill the entire free volume in the tube part of the ap- paratus;

- feeding a gas phase to the lower part of the apparatus to cause gas to rise to the lower tube sheet and enter the tubes of the second tube group so that the tubes of the second tube group work as bubble tubes;

- providing movement of liquid under the effect of gas upwards through the bub- ble tubes;

- upon reaching the upper part of the apparatus, the gas-liquid mixture separates;

- withdrawing the gas phase from the apparatus;

- withdrawing a smaller portion of the liquid phase, while a larger portion of the liquid phase starts moving under gravity downwards through the tubes of the first tube group that act as circulation tubes.

Other features and advantages of the present invention will be more apparent in accordance with preferred embodiments of the present invention, which are described in more detail with reference to the accompanying drawings. The embodiments are pro- vided to describe the present invention only as an example and, therefore, should not be construed as limiting the technical scope of the present invention.

Brief Description of the Drawings Fig. 1 illustrates an apparatus for carrying out exothermic gas-liquid reactions according to US 5,846,498.

Fig. 2 illustrates an apparatus for carrying out gas-liquid chemical arid heat and mass exchange processes according to RU2040940.

Fig. 3 illustrates a gas lift apparatus according to SU1212550.

Fig. 4 illustrates an apparatus according to SU 129643.

Fig. 5 illustrates an apparatus according to SU 199087.

Fig. 6 illustrates schematically the structure of an apparatus according to the present invention.

Fig. 7 illustrates schematically the arrangement of tubes in th^ apparatus accord- ing to Comparative Example 2.

Fig. 8 illustrates schematically the arrangement of tubes in the apparatus accord- ing to Comparative Example 3.

Fig. 9 illustrates schematically the arrangement of tubes in the apparatus accord- ing to Example 4.

Fig. 10 illustrates schematically the arrangement of tubes in the apparatus ac- cording to Example 5.

Fig. 11 illustrates a unit of the apparatus according to the invention.

Fig. 12 illustrates a glass apparatus with tubes of the same length according to Comparative Example 6.

Embodiment of the Invention

An apparatus according to the invention, shown schematically in Fig. 6, corn- prises a single vertical shell-and-tube unit 1 with circulation tubes 3 and bubble tubes 4 secured in a tube sheet 2. Lower part of the apparatus comprises a liquid feed device 5 and a gas feed device 6. Upper part of the apparatus comprises a gas phase withdrawal device 7 and a liquid phase withdrawal device 8. Intertubular space of the apparatus comprises heat transfer agent circulation nozzles 9 and 10. Feed and withdrawal device refers hereinafter to any conventional means for feeding and withdrawal of flow, for example, a nozzle, an injector, etc.

Process is carried out in the inventive apparatus in the following manner:

1. Liquid phase is fed through the liquid feed device 5 to completely fill the entire free volume in the tube part of the apparatus; 2. Gas phase is then fed through the gas feed device 6 to the lower part of the apparatus;

3. Gas rises up the tube sheet 2 and then enters the bubble tubes 3;

4. Liquid starts moving upwards through the bubble tubes under the effect of gas;

5. Having reached the upper part of the apparatus, the gas-liquid mixture is sep- arated;

6. Gas phase is withdrawn from the apparatus through the gas withdrawal device

7;

7. Smaller portion of liquid phase is withdrawn through the liquid withdrawal device 8, and larger portion of the liquid phase starts moving downwards under gravity through the circulation tubes 3.

Uniform distribution of circulation tubes ensures equal velocity of liquid through all bubble tubes and, as a consequence, equal conditions for heat and mass transfer pro- cesses running throughout the apparatus. To ensure heat transfer in the intertubular space of the apparatus, a heat transfer agent is circulated through the nozzles 9 and 10.

Examples

Comparative Example 1. Use of an apparatus comprising tubes of the same length.

Tests were carried out in a steel apparatus comprising a vertical shell-and-tube unit 80 mm in diameter having 19 tubes 725 mm long and 13x1.4 mm ip diameter, se- cured in a tube sheet. Lower part of the apparatus comprises a liquid and gas feed nozzle. Upper part of the apparatus comprises gas and liquid phase withdrawal nozzles. Inter- tubular space of the apparatus comprises heat transfer agent circulation nozzles (see Fig. 2).

Tests were carried out at atmospheric pressure. Cyclohexane was used as the liquid phase, and nitrogen was used as the gas phase. The number of tubes in the bub- bling mode was determined visually every minute by the emerging bubbles of gas. Then the results were averaged over the time interval of 30 minutes, and a conclusion was made as to the mode and activity of each tube.

Without extension of tubes, a chaotic alternation of circulation and bubble tubes was observed. Also, in operation, the circulation tube could become bubbling and vice versa. Moreover, the number of bubble tubes was different at different times. All this indicates the instability of gas-liquid mixture flow regime over time. This leads to local jumps in the concentration of dissolved gas and temperature, which were also observed visually. It should be noted that this apparatus comprises about 60% of tubes, through which gas does not pass, which means that no movement of liquid occurs in the part of tubes and stagnant zones are formed.

Comparative Example 2. Use of an apparatus comprising a single extended tube.

Tests were carried out in the apparatus described in Example 1, with the differ ence that the tube disposed in the apparatus center extends 50 mm below the tube sheet (see Fig. 7).

The extension of one central tube strictly defines that this tube is the circulation tube. However, the throughput capacity of the tube is insufficient to provide a stable circulation of liquid throughout the apparatus unit. Furthermore, part of bubble tubes go into the regime of chaotic circulation of liquid, i.e. they work alternately either as circu- lating or bubble tubes, which leads, as in Example 1, to local jumps in the concentration of dissolved gas and temperature.

Comparative Example 3. Use of an apparatus comprising three extended tubes.

Tests were carried out in the apparatus described in Example 1, with the differ- ence that three tubes, disposed next by one around a central tube, extend 50 mm down- wards beyond the tube sheet (see Fig. 8).

The increased number of circulation tubes to three and their distribution over the apparatus significantly stabilized the flow. All bubble tubes worked as bubble tubes only. However, the throughput rate of gas, and hence liquid, in the bubble tubes was significantly different, which led to destabilization of heat and mass transfer conditions in the tube space of the apparatus.

Example 4. Use of an apparatus comprising three circulation tubes disposed near a central tube, with three tubes of outer row plugged.

Tests were carried out in the apparatus described in Example 1, with the difference that three tubes, disposed next by one around the central tube, extend 50 mm be yond the tube sheet, and three tubes of outer row are plugged (see Fig. 9).

This distribution of circulation tubes ensures a steady performance of the unit with an average circulation velocity of liquid throughout the entire apparatus (i.e. aver age liquid circulation velocity is substantially steady over time throughout the apparatus) This results in uniform heat and mass transfer conditions throughout the apparatus.

Example 5. Use of an apparatus comprising three circulation tubes disposed near a central tube and three circulation tubes of outer row.

Tests were carried out in the apparatus described in Example 1, with the differ- ence that three tubes, arranged next by one around the central tube, extend 50 mm be- yond a tube sheet (see Fig. 9).

The increased number of circulation tubes to six leads to a significant increase in the liquid circulation rate. Furthermore, the flow structure is steady, that is, the compo- sition, local velocities and physical characteristics of the medium at each point of the flow in the working zone of the apparatus stay substantially constant over time. This results in uniform heat and mass transfer conditions throughout the apparatus. Compared to Example 4, liquid phase velocities increase, thereby increasing the efficiency of heat removal from the apparatus surface.

Comparative Example 6. Tests on mass transfer in a glass apparatus comprising tubes of the same length.

Tests were carried out in a glass apparatus with a volume of 2 liters, comprising a vertical unit having two metal tube sheets 100 mm in diameter with 19 glass tubes, 800 mm long and 10x1.5 mm in diameter, secured in the tube sheets. All the tubes were extending beyond the tube sheet. Lower part of the apparatus comprises liquid and gas feed nozzles. Upper part of the apparatus comprises gas and liquid phase withdrawal nozzles (see Fig. 2).

Tests were carried out at atmospheric pressure. Aqueous solution of NaOH was used as the liquid phase, and carbon dioxide was used as the gas phase. Liquid velocity was set by a pump, liquid concentration at the outlet was measured with a pH meter. Gas was fed from a cylinder through a flow meter.

Neutralization reaction proceeds with a high rate, respectively, the process lim- iting factor is the transition of carbon dioxide to liquid. By means of the example of reaction of neutralization of carbonic acid with a strong alkali, it is possible to estimate the effectiveness of mass transfer between gas and liquid phases in the apparatus.

Tests were carried out in the following manner: the apparatus was completely filled with liquid through a pump. Then a required constant flow rate (200 ml/min) of liquid phase was set, and gas injection was started (500 ml/min). pH values were de- tected every 2 minutes. Establishment of steady-state operation was fixed on the time where no pH value variatiori occurred at the apparatus outlet within 10 minutes.

With the above parameters, the time to steady-state operation was 40 minutes, with stationary pH value = 10.2. Furthermore, chaotic movement of two-phase flow was observed as in Example 1.

Example 7. Tests on mass transfer in a glass apparatus comprising three circula- tion tubes disposed near a central tube and three circulation tubes of outer row.

Tests were carried out in a glass apparatus with a volume of 2 liters, comprising a vertical unit having two metal tube sheets 100 mm in diameter and 19 glass tubes, 800 mm long and 10x1.5 mm in diameter, secured in the tube sheets. Part of the tubes were extending beyond the lower tube sheet as in Example 5. Lower part of the apparatus comprised a liquid and gas feed nozzle. Upper part of the apparatus comprised gas and liquid phase withdrawal nozzles (see Fig. 9).

Tests were carried out at atmospheric pressure. Aqueous solution of NaOH was used as the liquid phase, and carbon dioxide was used as the gas phase. Liquid velocity was set by a pump, the liquid concentration at outlet was measured with a pH meter. Gas was fed from a cylinder through a flow meter.

Tests were carried out in the following manner: the apparatus was completely filled with liquid through a pump. Required constant flow rate (200 ml/min) of the liquid phase was then set, and gas injection (500 ml/min) was started. pH values were detected every 2 minutes. Establishment of the steady-state condition was fixed on the time where no variation in pH value occurred at the apparatus outlet within 10 minutes.

With the above parameters, the time to steady-state operation was 30 minutes, with stationary pH value = 9.4.

As can be seen from Examples 6 and 7, mass transfer processes are more efficient in the inventive apparatus because the stationary pH value is lower. The decrease in pH indicates that the mass-exchange processes (reactions) proceed in the gas-liquid system faster and more fully. In addition, the time to reach equilibrium is reduced by 25%, which indicates that there are no stagnant zones that can cause noticeable jumps in the liquid phase concentration, and that the apparatus comprises a uniform velocity field, enabling effective mass transfer in the liquid phase. Comparative Example 8. Use of an apparatus comprising tubes of the same length as a reactor for ethylene trimerization.

Tests were carried out in a steel reactor comprising a vertical shell-and-tube unit 80 mm in diameter, having 19 tubes 725 mm in length and 13x1.4 mm in diameter, secured in a tube sheet. Lower part of the reactor comprises a liquid and gas feed nozzle. Upper part of the reactor comprises gas and liquid phase withdrawal nozzles. Intertubu- lar space of the reactor comprises heat transfer agent circulation nozzles (see Fig. 2).

Ethylene trimerization reaction was carried out under a pressure of 14 bar. Cy- clohexane with the addition of a homogeneous catalytic complex was used as the liquid phase, and ethylene was used as the gas phase. Concentration of the reaction product, hexene- 1, and by-products was measured by periodic sampling at the reactor outlet. Gas chromatography was used as analytical control method.

Concentration of hexene- 1 at the reactor outlet varied within the range of 6-7% by weight with a selectivity of 96-97%.

Example 9. Use of an apparatus comprising three circulation tubes disposed near a central tube and three circulation tubes of outer row as a reactor for ethylene trimeri- zation.

Tests were carried out in the apparatus described in Example 8, with the differ- ence that three tubes disposed next by one around the central tube extended 50 mm be- yond the tube sheet (see Example 5).

Concentration of hexene- 1 at the reactor outlet varied, as in Example 8, within the range of 6-7% by weight with a selectivity of 96-97%.

The difference between tests in Examples 8 and 9 is almost imperceptible due to the relatively small reactor tube capacity, which was about 60% of the total reaction volume in the tests.