RU2057745C1 - Method for preparing methanol - Google Patents

Abstract

FIELD: chemical industry. SUBSTANCE: methane-containing gas is used as reagent 1, oxygen or air is used as reagent 2. Reaction is carried out at 325-500 C and pressure 30-100 atm, concentration of oxygen is 2-8 vol %. Duration of contact between said gases is 0.2-10 s, it should be not more then duration of diffusion of reacting particles to reactor's wall. Mentioned above gases are supplied into reactor separately. Reagent 1 is preliminary heated to 380-450 C and it is supplied under pressure 10-100 atm, reagent 2 is supplied under pressure 10-100 atm and 40-50 C. Initially only portion only portion of reagent 2 (preferable not more 1.5 vol %), residual portion of said reagent being fed into reaction mass without preliminary cooling of the stream. Duration of mixing of said gases into reactor is not more than 0.1 of reaction time. EFFECT: improves efficiency of method. 4 cl, 2 dwg

Description

 The invention relates to organic chemistry, and in particular to methods for producing methanol by direct oxidation of natural gas (methane), and can be used in the chemical and gas industries.

 The reserves of natural gas are large, it is an environmentally friendly fuel, but the direct use of gaseous energy in many cases, in particular in transport, is inconvenient. The problem is solved by converting natural gas into a more versatile liquid fuel, in particular methanol, which is also the initial product of many chemical industries.

A number of methods are known for converting methane to methanol. The steam conversion of methane to synthesis gas (a mixture of CO and H ₂ ) with its subsequent catalytic conversion to methanol has wide industrial applications, but this process has a number of significant drawbacks: the complexity of the equipment, the very high energy consumption for producing synthesis gas, the multi-stage process, high requirements for gas purity, unprofitability of small and medium-sized enterprises (with a capacity of less than 2000 tons / day) (1).

 Currently, the most interesting is the direct, bypassing the stage of synthesis gas production, gas-phase oxidation of methane to methanol at high pressures. Despite numerous research works, there is still no sufficiently technologically advanced method suitable for commercial production for carrying out the process of direct oxidation of methane to methanol at high pressures (2).

 Patent-protected methods for producing methanol by direct oxidation of methane at elevated temperatures and pressures (without the use of catalysts) are known (3-5), but all currently known methods are not very suitable for industrial development, as they are inferior in efficiency to the traditional method through synthesis gas.

Closest to the proposed method and the best known method for the direct conversion of natural gas to methanol by its oxidation with an oxygen-containing gas (oxygen or air), including thorough mixing of the reacting gases (to achieve the most complete uniformity) and carrying out the process in an inert reactor with the inner walls of quartz or Teflon, reducing the negative impact on the inner surface of the reactor process at a temperature of 300-500 ^C and a pressure of 10-100 atm, conc tion of oxygen in the mixture of the reacting gases 2-20% and a contact time of reactant gases 2-1000, followed by separation of the desired product by cooling the reaction mixture, and condensing methanol (6).

 The known method (6) is carried out in a reactor with a diameter of 0.5-2.5 cm and a length of 40-45 cm.

9,5•10^-3 нм³/c
Поскольку при 90% селективности и конверсии 10% выход метанола составит около 120 кг/1000 нм³, часовая производительность установки по метанолу не превысит
[Me]

4 кг/ч
Следует отметить также, что реализация высоких показателей известного способа (6) в сильной степени зависит от выполнения требований высокой степени однородности перемешивания реагирующих газов и инертности внутренней поверхности реактора. Известный способ сопровождается также возникновением экспериментально наблюдаемых неустойчивых, в частности, колебательных режимов (7), что недопустимо при промышленной эксплуатации.The known method (6) differs from other known processes, for example (3-5), in a high methanol yield: up to 80-90% for reacted methane, methane conversion in the best experiments reaches 15-18%, but like other known methods, there are also few It is suitable for industrial development because of the need for a very long residence time of gases in the reactor (2-1000 s) and the associated low linear velocity of the gas flow, which limits the productivity of the process. Thus, for reactor diameter D of 25 mm, the length L of 45 cm, the reaction time t 10, a pressure P of 100 bar and a reactor temperature of T 700 K (400 ° ^C) gas volume v under normal conditions (T _o and P _o), passing through the reactor is
V

9.5 • 10 ^-3 nm ³ / s
Since at 90% selectivity and 10% conversion, the methanol yield will be about 120 kg / 1000 nm ³ , the hourly capacity of the installation for methanol will not exceed
[Me]

4 kg / h
It should also be noted that the implementation of the high performance of the known method (6) strongly depends on meeting the requirements of a high degree of uniformity of mixing of the reacting gases and the inertia of the inner surface of the reactor. The known method is also accompanied by the appearance of experimentally observed unstable, in particular, oscillatory modes (7), which is unacceptable during industrial operation.

 The objective of the invention is to develop a simple, technologically advanced and stable method for the direct oxidation of methane to methanol, which would be effective enough for industrial use instead of the traditional one and suitable for use directly in the conditions of gas production and gas transmission enterprises.

This object is achieved by a process for producing methanol by oxidation of a methane-containing gas, including natural gas, oxygen or air at a temperature of 325-500 ^C and a pressure of 30-100 atm, and the oxygen concentration of 2-8 vol. followed by separation of the target product by cooling and condensation, in which the contact time of the reacting gases (reaction time) is maintained within 0.2-1.0 s, moreover, the reaction time should not exceed the diffusion time of the reaction products to the inner surface of the reactor, the supply of matane-containing and oxygen-containing gases in the reactor is carried out separately, wherein the methane-gas is preheated to 300-500 ^{° C} and fed to the reactor under pressure of 10-100 atm and oxygen or air is fed into the reactor under pressure of 10-100 atm and at the temperature erature of 40-50 ^{° C,} wherein the first portion of the oxygen supplied only required to start the reaction, preferably no more than about 1.5. and the main part of oxygen is fed into an already developing and stabilized reaction without preliminary cooling of the gas stream and the mixing time of each supplied portion of oxygen or air with the main stream in the reactor does not exceed 0.1 of the length of time that this portion of oxygen is consumed.

The process is carried out at 325-500 ^{° C,} preferably at 380-450 ^C, a pressure of 30-100 atm, preferably 50-100 atm, an oxygen feed 2-8. preferably at 2.5-4.5 vol. and unreacted methane-containing gas is recycled.

 The proposed method was developed on the basis of a detailed experimental study of the mechanism of the direct oxidation of natural gas to methanol using methods of mathematical modeling. Studies have shown that the oxidation reaction proceeds according to a chain mechanism with degenerate branching (69 elementary stages are included in the mathematical model). Three main stages of the process were identified.

 The initial stage of the process is a branched chain reaction in which a rapid increase in the concentration of active radicals occurs. At this stage, an important role is played by the reactions of nucleation, branching and death of free radicals.

 The second stage is a stationary branched chain reaction, characterized by the equality of the rates of branching and quadratic recombination of active radicals. At the second stage of the process, the production of intermediate products (peroxides and aldehydes) occurs, leading to the last rapid heating. The first and most of the second stage correspond to the experimentally observed period of the induction of the process. At the end of the second stage, as a result of the reaction of accumulated intermediate products, a sharp auto-acceleration of the process occurs, accompanied by a rise in temperature. In this case, the main divergence of the oxidizing agent (oxygen) occurs.

 At the third and final stage of the process, after the consumption of one of the oxygen reagents, radical recombination and a relatively slow thermal non-oxidative conversion of unstable reaction products occur.

 The principal result of the conducted research is the establishment of the fact that a decrease in the oxygen concentration not only does not increase the reaction time, but, on the contrary, shortens it by reducing the induction period, which makes up most of the duration of the process. Therefore, a decrease in the ratio of reaction time to the diffusion rate of reagents can be achieved by reducing the oxygen concentration at the initial stage of the process. Another important result is the establishment of the fact that the likelihood of unstable vibrational and cold-flame processes increases with increasing oxygen concentration in the initial stages of the reaction, especially at concentrations exceeding 4-5%. Therefore, the supply of the minimum amount of oxygen necessary to start the reaction at the initial stage is not only reduces the duration of the process, but also dramatically increases its stability. In addition, as is well known from the literature (2), a decrease in the oxygen concentration decreases the selectivity of the formation of products of deep oxidation of methane, primarily carbon oxides, and, accordingly, increases the yield of the target products of methanol and formaldehyde.

 The obtained data on the reaction mechanism made it possible to propose a method fundamentally different from the known one (6).

 In the known method (6), the quality of gas mixing and the inertia of the inner walls of the reactor play an important role, since the interaction of free radical particles with materials commonly used for gas phase reactors (stainless steels and other metal alloys) leads to their rapid death. Since in the known method (6) the gas flow is very slow, the required degree of mixing is achieved by using an additional mixing chamber in front of the reactor filled with Teflon chips, and surface inertness is achieved by using inserts of “chemically inert” materials, for example glass or Teflon, and eliminating the possibility of contact of reacting gases "with chemically active" materials, such as metals, which creates significant difficulties in the implementation of the process.

 In addition, the use of an additional mixing chamber, in which there are partially mixed, but still unreacted gases, sharply increases the explosiveness of the process, especially when the oxygen concentration is above 5 vol.

 In the proposed method, the negative influence of the inner surface of the reactor on the process is also minimized, but in a completely different way, namely, the process is carried out at a contact time of the reacting gases not exceeding the diffusion time of free radical particles to the walls of the reactor, while free radicals have time to react gas phase until reaching the walls. The necessary rapid mixing of gases in this case is carried out in a turbulent mode by using multi-section mixing blocks, the design of which is shown in figure 1.

The process is carried out in a two-section stainless steel reactor (see Fig. 1). The diameter of the sections is d 20 cm, the length of each section, which has its own BS gas mixing unit, is 400 cm. For the diameters of the oxidizing nozzles d ₁ 0.5 cm and the natural gas nozzles d ₂ 1.5 cm (see Fig. 1) the mixing channel length I is 18 cm, which is necessary for complete mixing of the reagents, and the number of nozzles in each mixing unit is N 49. The gas flow rate in the nozzle is V 56 m / s and in the reactor 11.7 m / s. Plant productivity ≈ 1200 kg of methanol / h (10 thousand tons of methanol / year).

The turbulent diffusion coefficient D is calculated by the formula:
D εUL
where ε = 0.01 is the degree of turbulence; v linear velocity of the gas stream; L 0.15d _{2 the} turbulence scale for our conditions, given by the diameter of the mixing channel d ₂ .

1,37 с
т. е. t_дифф > τ_р= 0,2-1 с, при этом узел смешения указанной конструкции, помимо основной функции выполняет роль устройства, уменьшающего масштаб турбулентности (увеличивающего время турбулентной диффузии) в основной части реактора.Since the turbulence scale L and the degree of turbulence ε specified by the mixing nozzles are practically preserved when gases pass through the main part of the reactor, the same values can be used to calculate the diffusion time in the reactor t $\genfrac{}{}{0ex}{}{\text{R}}{\text{diff}}$
t $\genfrac{}{}{0ex}{}{\text{p}}{\text{diff}}$

1.37 s
i.e., t _diff > τ _p = 0.2-1 s, while the mixing unit of this design, in addition to the main function, plays the role of a device that reduces the scale of turbulence (increasing the time of turbulent diffusion) in the main part of the reactor.

 In the first stage of the reactor serves natural gas and part of the oxidizing agent so that the oxygen concentration in the reactor does not exceed 1.5 vol. In the second similar section serves the rest of the oxidizing agent. At the same time, this design provides a low scale of turbulence in the second section of the reactor. If it is necessary to obtain a higher degree of conversion of natural gas, a larger number of such sections can be used, however, the overall selectivity of the conversion of natural gas to target products will decrease. To more reliably ensure a low turbulence scale, additional baffles with holes of 1-2 cm in diameter and 15-20 cm in length can be installed in the reactor sections. The gas leaving the reactor after separation of liquid products contains only small amounts of carbon oxides, hydrogen, and in the case of air oxidation nitrogen and can again be fed to the inlet of the reactor (recycled).

 The process is as follows.

Natural gas from the pipeline under a pressure of 10-75 atm. either directly for cleaning and heating and then to the reactor, or first to a gas compressor, where it is compressed to the required pressure (maximum 100 atm). Natural gas before entering the reactor polyacetal was purified on a filter of the residual moisture and gas condensate and first heated in an indirect heat exchanger "gas-gas" to 350 ^{° C,} then the heater (due to heat of combustion of the fuel gas) to 380-450 ° ^C. Oxygen or air is compressed by a compressor to a pressure of 90-100 atm, and sent to the reactor (temperature of the oxygen stream 40-50 ^about C). The reactor at a temperature of ^about 325-500 C (preferably ^about 380-450 C), a pressure of 30-100 atm, and the oxygen supplying about 2-8. (preferably 2.5-4.5%), the process of oxidation of methane-containing (natural) gas to a methanol product proceeds.

From the reactor the reaction mixture at a temperature ^of 390-500 C enters the tube side of the regenerative heat exchanger where it gives off heat the cold natural gas from the pipeline and is cooled to 120-140 ° ^C. Then, the reaction mixture enters the air cooler-condenser where the liquid reaction products of ( methanol with impurities of ethanol, formaldehyde, acetic acid, etc.) condense. The resulting gas-liquid reaction mixture enters the acid neutralization apparatus, then to the separator, in which the methanol product is separated from unreacted methane-containing gas, which is sent either to the main or to re-oxidation to the reactor.

PRI me R 1 implementation of the process: Pressure, MPa 10 Temperature, ^о С 410 Natural gas consumption, nm ³ / h 58573 Oxygen consumption, nm ³ / h: section 1 889 section 2 889
The degree of conversion of oxygen, 95 Reaction time, s 0.3
Yield, kg / 1000m ^{3 of} passed gas: crude methanol product 40 methanol 20
Productivity in methanol, kg / h 1170
The average composition of the crude methanol product, wt. methanol 49.8 ethanol 2.0 propanol 0.30 acetone 0.50 formaldehyde 8.0 formic acid 0.50 water 38.2 other 0.70
PRI me R s 2-4.

 Thus, a method is quite simple in technology and equipment for producing methanol from natural gas with a productivity of 1200 kg / h, suitable for use in the chemical industry, as well as directly at gas producing and gas transmission enterprises.

Claims (4)

1. METHOD OF PRODUCING METHANOL by oxidation of a methane-containing gas, including natural gas, with oxygen or air at 325 500 ^o С, 30 100 atm and an oxygen concentration of 2 8 vol. followed by separation of the target product by cooling and condensation, characterized in that the contact time of the reacting gases is maintained within 0.2 10.0 s, and the reaction time should not exceed the diffusion time of the reacting particles to the walls of the reactor, the supply of methane and oxygen-containing gases to the reactor carried out separately, while the methane-containing gas is preheated to 380 450 ^o C and fed into the reactor under a pressure of 10 100 atm, and oxygen-containing gas is supplied under a pressure of 10 100 atm and at 40 50 ^o C, and at first only part sulphide, preferably not more than 1.5 vol. and the rest is fed into an already developed reaction without preliminary cooling of the flow and the time of mixing of gases in the reactor is not more than 0.1 reaction time. 2. The method according to p. 1, characterized in that the process is carried out at 370 - 470 ^o C, preferably at 380 450 ^o C.  3. The method according to claim 1, characterized in that the process is carried out at an oxygen concentration of 2.5 to 4.5 vol.  4. The method according to claim 1, characterized in that the gas leaving the reactor after separation of the liquid products is again fed to the inlet of the reactor (recycle).

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