RU2608092C2 - Ammonia synthesis reactor with split-flow and tubular nozzle - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to production of synthetic ammonia by catalytic reaction of gaseous crude stream, containing nitrogen and hydrogen. Ammonia synthesis reactor comprises vertical cylindrical housing, mechanically isolated reaction zones with catalyst, located one above another, gas ducts to bypass reaction zones of gases, related to other reaction zones, and heat exchange tubes in catalyst layer for cooling of reaction zones.

EFFECT: invention provides high concentration of ammonia at outlet of reaction zone, as well as low hydraulic resistance of catalyst layers.

4 cl, 5 dwg

Description

FIELD OF THE INVENTION

The present invention relates to the production of synthetic ammonia by a catalytic method by reacting a gaseous feed stream containing nitrogen and hydrogen.

State of the art

The prior art there are many different designs of reactors for the synthesis of ammonia from nitrogen and hydrogen in the gas phase in a fixed catalyst bed.

The design of the reactor is complicated by the fact that the ammonia synthesis reaction is exothermic and proceeds at high temperatures and pressures. Because the reactor has a high cost.

In addition, the equilibrium in the reaction of the synthesis of ammonia from nitrogen and hydrogen is strongly shifted to the left. For example, in large-scale industries with a synthesis pressure of 323 atm and an average contact temperature of 500 ° C, the ammonia concentration at the reactor outlet is about 18%. This entails high energy and mechanical costs for compression of fresh synthesis gas for its circulation.

The most important parameters to reduce the cost of a reactor are to reduce its volume and diameter (relative to productivity), and to reduce energy-mechanical losses, increase the concentration of ammonia at the outlet of the reactor and decrease the gas-dynamic resistance of the reactor.

In the 70s of the last century, axial type design was widespread on large-capacity ammonia production units, in which the reaction mixture moves through the catalyst layers along the axis of the reactor. The reactors consisted of 3-4 catalyst beds (shelves) in front of each of which a gas supply through a cold bypass was arranged to reduce the temperature of the reaction mixture.

The main volume of gas (65-80%) was passed sequentially through all layers of the catalyst (Synthesis of ammonia, edited by KTN Kuznetsova LD, M. Chemistry 1982). The reaction proceeded with a stepwise decrease in the temperature of the reaction mixture and a sharp decrease in the concentration of ammonia in front of each catalyst bed (see Fig. 1).

The disadvantage of this design was the high hydraulic resistance of the catalyst layers and a decrease in the concentration of ammonia at the outlet of the column due to the constant dilution of the reaction mixture with a gas with a low ammonia content through “cold bypasses”.

Recently, the design of a radial-axial type reactor has become the most widespread (RU 2070827, 12/27/1996; US 4372920, 02/08/1983). The reaction gas successively passes through two catalyst zones: radial (transverse to the axis of the reactor) and axial (along the axis of the reactor). Using a radial design can significantly reduce hydraulic resistance.

The reaction gas is cooled between the catalyst beds using an integrated intermediate heat exchanger. This avoids dilution of the reaction gas, reduces the temperature in the reaction zone and, as a result, increases the concentration of ammonia at the outlet of the reactor (Fig. 2).

The most significant drawbacks of the radial type design are: uneven distribution of the gas flow in the layer, the effect of catalyst shrinkage, the possibility of fluidization of the catalyst in the upper layer, the difficulty of creating an optimal temperature regime (temperature field), the complexity of loading and unloading the catalyst.

It is for these reasons that a combined type of reactor is used - radial-axial, partially reducing the influence of the disadvantages of a purely axial and purely radial design, but, in general, retaining the listed disadvantages of both designs.

In addition, the design of ammonia synthesis reactor with a tubular packing is known from the level of technical development (it was used in ammonia production units with a capacity of 100-300 t / day). In such reactors, the reaction gas was passed axially through the catalyst beds; the catalyst was penetrated by heat exchange tubes of a cold bypass or the main gas stream entering the reactor. Thus, the heat of synthesis reaction and its removal through heat exchange tubes simultaneously passed in the catalyst bed. One of the options for this design is described in RU 2241667, 12/10/2004.

The advantage of the design with a tubular nozzle is that the temperature regime approaches the optimal and high process stability. In addition, this design was relatively simple and reliable.

The disadvantage of this design is the high hydraulic resistance, and therefore it has not found application on large-capacity ammonia production units.

In addition, methods are known for reducing the hydraulic resistance of contact devices with a limited cross-sectional area (RU 2469765, 12/20/2012; US 7081230 B2, 07/25/2006). The apparatus is arranged in such a way that the reaction stream is divided into two (or more) parts, each of which is directed to its own isolated part of the apparatus, where the reactions proceed; moreover, these reaction zones are divided by the height of the apparatus. Then the threads are merged again. Thus, the cross section of the reaction zone is artificially doubled (or tripled, etc., by the number of flows), the flow rate of the reaction mixture is reduced, and the hydraulic resistance decreases with the initial diameter of the apparatus.

Closest to the claimed technical solution is the design described in US 7081230. The synthesis gas in front of the first shelf is divided into two streams. Intermediate catalyst shelves are also divided into two parts, located one above the other and mechanically isolated. Thus, each flow is passed through its shelves, the hydraulic resistance is reduced. The gas is cooled in the built-in heat exchanger after the reaction gas exits the shelf.

The reaction in this design corresponds to the diagram of FIG. 2.

Thus, the prototype contains the previously known disadvantages: suboptimal temperature conditions (the heat exchanger is between the catalyst layers), the complexity of loading and unloading the catalyst.

Disclosure of invention

The objective of the invention is an ammonia synthesis reactor in which the reaction proceeds at the optimum temperature (so that the ammonia concentration at the outlet of the apparatus is as high as possible), and at the same time, such a reactor has a reduced hydraulic resistance with a similar diameter.

This is achieved according to the invention due to the fact that:

a) the reaction gas is continuously cooled during the reaction using heat transfer tubes located in the catalyst bed;

b) the reaction gas stream in the reactor is divided into two or more parts, and each stream flows through its individual catalyst bed (s).

A block diagram of such a reactor is shown in FIG. 4, where the reactor (101), catalyst beds (102), flow of reaction gas (103) and coolant (104) are conventionally displayed.

The technical result, in comparison with the prototype, is a higher concentration of ammonia at the outlet of the reaction zone, achieved due to the continuous cooling of the reaction gas during the reaction.

Compared with all other known structures, the technical result is a reduction in the hydraulic resistance of the reactor.

Description of drawings

FIG. 1 is a diagram of the reaction in an axial type reactor in coordinates of temperature / ammonia concentration.

FIG. 2 is a flow diagram of a reaction in a radial-axial type reactor and a prototype reactor in temperature / ammonia concentration coordinates.

FIG. 3 is a diagram of the reaction in the invented reactor in coordinates of temperature / concentration of ammonia.

FIG. 4 is a structural diagram of an invented reactor.

FIG. 5 is a longitudinal embodiment of a synthesis reactor device.

The implementation of the invention

Consider an example implementation of a reactor (see Fig. 5) based on the principles disclosed in the invention.

The main flow of circulating gas (about 40 ... 50%) is introduced into the reactor from below (201) and rises up the annular gap between the body and the inner lining (202).

Next, the gas is passed through the annulus of the heat exchanger built into the reactor head (203) and is divided into two equal flows, each of which is directed to its part of the apparatus - the second flow freely passes through the gas duct (204) through the upper half of the apparatus to the lower.

Next, each of the streams is mixed with cold bypass gases (205) coming through heat-exchange catalyst tubes (206, 207), and with the temperature of the onset of the reaction (390 ... 420 ° C) they are fed to the upper catalyst layer (208, 209).

The reaction gas begins to react violently with heat, heats up to the temperature of the maximum reaction rate (480 ... 500 ° C) and, moving further through the catalyst bed, is continuously cooled to temperatures close to the temperature of the onset of the reaction (410 ... 430 ° C), at which the equilibrium concentration of ammonia is maximum (37 ... 33% vol. for synthesis pressure of 24 MPa and inert gas content of 9%).

I note that the equilibrium concentration of ammonia in reactors with a heat exchanger between the layers, for example, as in the prototype reactor, is 20% vol. (for the gas temperature at the outlet of the catalyst bed 460 ° C), the measured concentration of ammonia at the outlet of the reactor is 18.5 ... 19% vol.

Considering that a decrease in the gas temperature at the outlet of the catalyst zone from 460 ° C to 420 ° C will reduce the reaction rate by only 3%, and the residence time of the gas in the reaction zone will almost double, it can be expected that the concentration of ammonia at the exit of the catalyst zone of the invented reactor will be close to equilibrium and will be 34 ... 31% vol.

Further, the reacted flow from the lower part of the apparatus along the central gas duct (210) rises upwards - miscible along the path with the reacted gas of the upper half of the apparatus. The total flow from the central gas duct is passed through the pipe space of a heat exchanger integrated in the reactor head (203), the main gas stream entering the reactor is cooled, and (214) is removed from the reactor.

The supply of cold bypass gas (205) to the heat exchange tubes (206, 207) is carried out in such a way as to create the possibility of controlling the temperature field under changing reaction conditions. For example, in FIG. 4, one part of the cold bypass gas cools the catalyst layer over the entire height, and the second part cools the upper, most loaded layer.

To save the cross section of the reactor in the example shown, pitot tubes are proposed as heat exchange catalyst tubes. In addition, as shown by the practice of using tubular reactors, such a design (the so-called nozzle with double straight-through tubes) provides the most favorable temperature conditions (Synthesis of ammonia, edited by KTN Kuznetsova L.D., M .: Chemistry, 1982).

Leaving the cooling tubes, cold bypass gas is mixed with the mainstream gas and sent to the upper catalyst bed. Subsequently, the mixing of the reaction gas with the cold bypass gas is excluded and, thus, the reaction gas does not dilute, which allows to obtain the maximum concentration of ammonia at the outlet of the reactor.

Loading and unloading of the catalyst can be carried out through the upper (211) and lower hatches (212) without the need to remove the reactor cover and dismantling the nozzle. In FIG. 5 shows a method for loading and unloading pipes, in which the separation of the catalyst of the two reaction zones is carried out using a longitudinal partition (213, 214), passing inside the pipe and reaching the common hatch.

For emergency unloading of the catalyst, it is possible to dismantle the heat exchange pipes through the dismantled reactor lid.

The reactor shown in the example can be obtained by reconstructing the existing reactors of large-capacity ammonia production.

Claims (8)

1. An ammonia synthesis reactor containing at least: vertical cylindrical body, two or more mechanically isolated reaction zones arranged one above the other with a catalyst for working with two or more descending parallel flows, gas ducts for free bypass of the reaction zones with gases belonging to other reaction zones, heat transfer tubes in the catalyst bed for cooling the reaction zones. 2. The reactor of claim 1, wherein in each reaction zone two or more catalyst beds may be present. 3. The reactor according to claim 2, in which at least one catalyst bed is provided with heat sink tubes. 4. The reactor according to claim 1, in which the catalyst is located inside the tubes, and the coolant is fed into the annulus.

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