Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
1999-05-21
2001-09-18
Knode, Marian C. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C422S186220, C422S186220
Reexamination Certificate
active
06290920
ABSTRACT:
DESCRIPTION
1. Field of Application
The present invention relates to a mixing assembly for gaseous flows at different temperatures, in particular for heterogeneous exothermic synthesis reactors comprising a plurality of superimposed catalytic beds and in which at least one of the beds is provided with a gas permeable wall for outlet of a flow of hot gas, said assembly including:
a baffle extending below and parallel to said gas permeable wall with which it defines an air space for collection of said hot gas flow;
an annular opening for outlet of said hot gas flow from said air space defined between said baffle and a side wall supporting said at least one bed associated with the reactor;
a perforated distributor for a cooling gas flow supported below said baffle at a predetermined distance from said annular opening.
In the description given below and in the followings claims, the term: hot gas, is understood to mean a partially reacted gas or gas mixture leaving a catalytic bed of a synthesis reactor, in which it has reacted exothermically. By way of example, in the exothermic heterogeneous synthesis reaction of methanol said gas has a temperature generally between 240° C. and 290° C.
On the contrary, the term: cooling gas, is understood to mean a cooling gas having a temperature lower than that of the partially reacted gas leaving the catalytic bed. Usually said gas consists of a predetermined part of reagent gasses and, again in the case of methanol synthesis, has a temperature generally between 60° C. and 200° C.
In the terminology of the field the cooling gas is also indicated by the term ‘quench gas’.
As known, during performance of an exothermic heterogeneous synthesis reaction it is necessary to cool the hot gas flow leaving each catalytic bed so as to bring the temperature back to a value such as to allow reaching the maximum conversion yield in the next bed.
2. Prior Art
For this purpose, the reactors assigned to this type of synthesis are provided with devices or mixing assemblies designed to cool the partially reacted gas mixtures flowing through a single catalytic bed or between a plurality of superimposed catalytic beds.
Thus for example in European patent application EP-A-0 359 952 of this applicant there is described an exothermic heterogeneous methanol synthesis reactor, in which are supported a plurality of superimposed catalytic beds in mutually spaced relationship between which are provided respective mixing assemblies for intermediate cooling of the reaction products by a flow of quench gas.
Said mixing assembly comprises a baffle extending below and parallel to the gas permeable bottom of at least one catalytic bed with which it defines an air space for collection of a hot gas flow leaving said at least one bed.
An annular opening for outlet of said hot gas flow from said air space is defined between said baffle and the side wall of the reactor supporting said at least one bed.
Below said baffle at a predetermined distance from the annular opening is supported a perforated distributor for a cooling gas flow.
Mixing of the hot synthesis gas flow with a cooling gas flow takes place in the peripheral zone of the reactor near the side wall supporting the catalytic beds.
If on one hand this type of assembly allows obtaining a certain degree of mixing of the gaseous flows, on the other hand it is not possible to achieve substantially complete mixing of the hot gases and cold gases before they enter in contact with the next catalytic bed.
In addition, the device in accordance with the prior art is not able of correcting and uniforming any local flow rate and temperature unevenness of the hot gases leaving said annular opening.
This local unevenness in flow rate and temperature are due to the fact that the hot gas flow coming from a bed containing a generally granular catalytic mass is subject to uncontrolled variations in flow rate and, if there are very extensive so-called catalyst “packing” phenomena, in temperature as well.
In other words, the mixing assembly just described does not allow achievement of an optimal degree of hot and cold gaseous flows mixing nor compensation in an independent manner for any local flow rate and temperature unevenness of the hot gas flow.
The inability of known devices to correct these variations results inevitably in a non optimal mixing between the hot gases and the cold gases and, consequently, in the formation of temperature differences in the local flow rates with deviations even over 20° C. in the gaseous flow leaving the mixing assembly.
All this causes reduced conversion yield of the synthesis reactor compared with that theoretically possible.
SUMMARY OF THE INVENTION
The technical problem underlying the present invention is to make available a mixing assembly for gaseous flows at different temperatures, in particular for heterogeneous exothermic synthesis reactors, allowing achievement of a substantially complete and uniform mixing of hot gases and cold gases and capable of independently absorbing any local unevenness in the hot gas flow rate.
The problem is solved by an assembly of the type set forth above and characterized in that the air space for collection of the hot gas flow has a constant thickness in correspondence of the annular opening and in that the ratio of the width of said annular opening to said thickness of said air space is between 0.2 and 1.
In accordance with the present invention it was found that when the thickness of the air space is constant in correspondence of the annular opening and when the ratio of the width of said annular opening to the thickness of said air space is between the above said values any local unevenness of the hot gas flow leaving said air space is absorbed since the hot gas flow is subjected to a pressure drop of predetermined magnitude.
The flow rate of the hot gas flow thus redistributed remains uniform for a period of time inversely proportional to the above said ratio.
This condition is essential for achieving a gaseous flow leaving the mixing assembly with homogeneous temperature, temperature deviations being limited to a few degrees centigrade.
Said pressure drop to which is subjected the hot gas flow depends on said ratio between the width of said annular opening and the thickness of said air space and is preferably between 0.5×10
−3
bar and 6×10
−3
bar, depending on the characteristics of the heterogeneous synthesis process.
In heterogeneous exothermic synthesis reactors of the type considered the thickness of said air space depends on the dimensions of the corresponding catalytic beds and is between a minimum of 60 mm and a maximum of 300 mm, generally between 120 mm and 180 mm.
The distributor is preferably of substantially toroidal shape and comprises at least one row of holes mutually spaced and pitched, and extending along a generatrix thereof.
The diameter of the holes of the row or rows of holes must be such as to ensure that the cooling gas jets have both an entrainment or suction effect on the entire hot gas flow coming from the air space above and an optimal penetration in it. The pitch is further defined in such a manner as to ensure a slight interference between the cooling gas jets.
For this purpose the holes have advantageously a diameter between 5 mm and 15 mm while the pitch is preferably between 20 mm and 60 mm.
The choice of the values within the above defined ranges depends on the physical characteristics of the gases to be mixed, in particular on the ratio between the flow rates of the gaseous flows.
The above said characteristics of pressure drop, entrainment and penetration allow achievement of an optimal mixing degree of the hot gases and cold gases immediately downstream of the distributor.
Depending on the characteristics of the heterogeneous synthesis process, the number of rows and the diameter of the holes, and the selected pitch, it is quite easy for those skilled in the art to determine the pressure of the cooling gas flow in said distributor in order to achieve the desired entrainment effect and penetration desc
Pagani Giorgio
Zardi Umberto
Doroshenk Alexa A.
Knode Marian C.
Methanol Casale S.A.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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