CA1172831A

CA1172831A - Adiabatic gas reactor

Info

Publication number: CA1172831A
Application number: CA000378061A
Authority: CA
Inventors: Hermann-Josef Siegert
Original assignee: Roehm GmbH Darmstadt
Current assignee: Roehm GmbH Darmstadt
Priority date: 1980-05-23
Filing date: 1981-05-21
Publication date: 1984-08-21
Also published as: DE3019730A1; DE3161568D1; JPS5730544A; EP0040668A1; EP0040668B1

Abstract

ABSTRACT OF THE DISCLOSURE

What is disclosed is an adiabatic gas reactor having a housing wherein are present an inlet zone, an outlet zone, and at least one reaction zone between and in communication with said inlet and outlet zones, said reaction zone having therein a catalyst carrier comprising a plurality of parallel, open, linear gas channels defined by walls in a honeycomb arrangement and extending between and in communication with said inlet zone and said outlet zone, said walls having active catalyst material thereon, said reactor further including a gas circulation pump removing spent or reacted gas from said outlet zone and introducing fresh gaseous starting material into said inlet zone.

Description

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.. ADIABATIC GAS RE~CTOR
, The present invention relates to an adiabatic gas reactor in which gaseous reac-tants ~re conducted through a fixed catalyst.
~ore in particular, there is no heat exchanger for supply or removal of reaction heat and no recovery device for the extraction of certain components of the reaction gas within the path of gas circulation. At high circulation rates, i.e. at a high ratio of the gas stream passed through the reaction zone to the gas stream supplied and/or removed, "stirred vessel behavior'1 is a-ttained in ,~ the reactor. This term designates an operating condition in which the differences in concentration and -temperature within the entire reactor are as small as possi~le and in the theoretical limitlng ca~e -~that l~, at an infinitely high circulation rate~- disappear ent-lrely, Because of the mere ~ Eferential change in operating conditions in the reactor, the term "di:Eferential reactor"
is also used. Differential reactors have previously been known only on a laboratory scale and served exclusively to examine the kinetics of gas reactions, In the case of a tried-and-true design of differential reactors, the catalyst material is in granular form as ballasting in a cylindrical reacton zone. The reaction gas exiting from the reaction zone is conducted to a radial fan which conducts the reaction gas through an .' ' ~

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,~, "~ , ", annular area between the cylindrical reaction zone and an external reactor wall back into an inlet chamber located before the reac~ion zone. One portion of the reaction gas can be continuously extracted, while, at another location in the reactor, fresh reaction gas can be continuously supplied.
Upon flowing through the catalyst ballasting, the . reaction gas undergoes a pressure loss, which is, however, - negligibly low in the known laboratory devices of this . 10 type. The input capacity of the gas circulation pump also plays no role in reactors on this scale. However, the use . of differential reactors of heterogeneous gas reactions on a technical scale is not known. Nevertheless, the expert - will recognize that the pressure loss in the catalyst ballasting would be appreciable and would make a gas circulation pump of consiclerable capacity necessary.
Operating conditions approximating stirred~vessel behav:Lor, which behavior pres~pposes the .same presswre in the entire reac-tion zone, would be unattainable on a large technical scale.
. An adiabatic reactor for heterogeneous gas . reactions is known from published German Patent Applica~ion .
23 24 164. Here too, a portion of the reaction gas is recirculated. In order to keep the pressure loss as low as possible when the gas is flowing through the catalyst bed, the ca~alyst is arranged in an annular catalyst zone throueh which the gas flows radially. The clrculatine ees ~ .

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~; is kept in motion by fresh gas blown in through a nozzle.
Because of the limited degree of effectiveness of such a gas circulation device, the proportion by volume of the recycled reaction gas is relatively low~ The operating conditions of the reactor are therefore far from the ideal stirred-vessel behavior.
D. Arntz and F. Fetting (Chemie-Ing.-Technik ~- MS 722/79) (Chemistry Engineering Technology Manuscript 722/79) have described an experimental arrangement in which a reaction gas is conducted through a honeycomb -' catalyst and several heat exchangers in the circulation path. The unit is not operated adiabatically and is intended for study of the reaction kinetics of honeycomb i~ catalysts.
l'he present invention has as its obiect an adiabatic gas reactor showlng a reduced pressure drop for gas flowi.n~ there~hrough, t'hus reduclng the necessary lnp~lt capaclty oE the gas c-lrcuLation pump. This object is accomplis'hed in a reactor whereln the catalyst is present on a carrier described as a monolithic or honeycomb catalys~ [cf. Ullmanns Enzyklopadie der technischen Chemie, ' (Ullmann's Encyclopedia of Technical Chemis~ry)> 4th edition, " Vol. 13, pp. 561-62.~ Honeycomb catalysts are predominantly used for exhaust gas decontamination and combustion, but have not yet been used in technical units for the production of chemical conversion products. Honeycomb catalysts differ from conventional tube-assembly catalysts, which are also ;;' `

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charac-terized by straigh-t channels containi.ng the ca-talyst, in that -the straight channels are open and in tha-t the active ca-talyst material is arranged only in and/or on the wa]ls of the channels in the honeycomb arrangemen-t.
An adiabatic gas reactor according to the present - invention has a housing wherein are present an inlet zone, an outlet zone, and at l.east one reaction zone be-tween and communication with the inlet and outlet zones. The reaction zone has therein a catalyst carrier comprising a plurality of parallel, open, linear gas channels defined by walls in a honeycomb arranc~ement and extending between and in communi-. cation with the inlet zone and the outlet zone, the walls having active catalyst material thereon. The reac-tor further includes a gas circulation pump, which may be located in the housing for removiny spent or reacted gas from the outle-t zone and introducing fresh gaseous s-tarting ma-terial into .~ . the inlet zone.
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.: Gas ~lows through the open channels wi-th low pressure ~.~ loss. The flow resistance depen~s on the :Eine structure and :,. ; 20 preferably :is cl maximum of 0.3 bar, more pa:rticu:l.arly less than 0.1 bar, per meter of honeycomb leng-th, measured a-t a ; gas velocity (empty tube velocity) of 500 m/min at 260 C
for air.
The reactor according to the invention can be operated under conditions which largely approximate the ideally stirred -~ vessel. This makes it possible to have the entire reaction ~''' ~ occur at or near optimum conversion conditions. In contrast, : the gas mixture in a conventional tube reactor conti.nues -to . heat up and thus diverges from the op-timum operating point. ; 30 With increasing gas tempera-ture, increasingly uncontrolled side reactions can occur. Thus, further reaction heat is , released, so tha-t overheating grows exponentially. Islands ' . -- 4 83~

of sharply increased tempera-ture, known as "hot spo-ts" in the technical jargon, arise in the reaction zone, in which not only is a portion of the yield lost, but often the activity of the catalys-t is irreversibly damaged. This danger can be avoided if -the reaction gas is diluted with considerable quantities of inert moderator gas, such as nitrogen, steam, or carbon dioxide. When using the reactor according to -the inven-tion, a moderator gas need not be used, without any danger of general or localized overheating.
A further advantage of the reactor of the invention is the direct utilization of the reaction heat to heat continuously-supplied fresh gas up to reaction temperature. When the cold fresh gas is added into the circulating gas, the temperature of the latter is lowered only differentially. When the gas mixture flows through the reaction zone, it is reheated by the same differential amount. In an analogous manner, the . concentrations of star-ting ma-terials and final produc-ts are changed only by differen-tial amounts by thé ad~itlon Oe Eresh gas and reac-tion in the reac-tion zone.
: 20 Optl~na:L conver~ion cond:it:i.ons, part:i.cul.arly high selec~iv:ity and yie:Lds, are attained, as a rule with ex-tensive approximation of the conditions in -the ideally stirred vessel.
: With a gradual reduction in the ratio of the ci.rcula-ting gas to the fresh gas supply, yields and selectivity are adversely affected only slightly at first, but then drop when a certain limiting circulation ra-te, typical for the reaction, is not surpassed. In order to keep the necessary input capaci.ty of the gas circulation pumps as low as possible, the system is generally operated as closely above the aforementioned limit value for the circu].ation rate as is possible~ In practice, -, ~

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preferab]y, a circulation ratio of 3 - 100 par-tsby volume of circulating cJa.s to one part by volume of fresh yas is used.
The preferred operation range uses a circulation ratio between 5 and 25.

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A better understanding of the present invention and of its many advantages will be had by re~erring to the accompanying drawings, wherein:
Fig. 1 is a view, in axial section, through a cylindrical reactor according to the present invention; and Figs. 2A, 2B 7 and 2C are each plan views o~ different honeycomb catalyst carrier arrangements suitable for use in the reactor of Fig. 1.
More in particular, the reactor of Fig. 1 comprises a cylindrical reactor housing 10, such as of metal, suitably lined with insulating layer 11, such as of ceramic, to insure operation under essentially adiabatic conditions. (When the reactor is used for e~tremely exothermic reactions, such thermal insulation is often not necessary, i.e. there can be some divergence :~rom - strictly adiabatlc conditions.) T~Ls arrangement de~Lnes reaction zone 12 withln the reactor, Ln wh:ich zone is located honeycomb ca~al.yst 13 comprlslng a plurality of ; open linear channels extending longitudinally parallel -to;: 20 the axis of the cylindrical reactor having 10. Inletarea 14, found in the reactor above catalyst 12, receives fresh gas starting material through inlet 15, in communication with a fresh gas supply (not shown).
The fresh gas is mixed in annular zone 16 with recycled gas introduced in the annular zone through one or more inlets 17 and then passes downwardly through the honey-comb catalyst 12 into discharge zone 18 in which is located .~ ' ~6-radial fan 19 which draws reacting gas through the catalyst and discharges reacted gas through outlet 20.~
The radial fan 19 is driven by shaft 21, found in shaft-way 22, and is connected thereby to drive device 23, such as an el.ectric motor, shown schematically in Fig. 1. In many cases, it is expedient to introduce resh gas starting material, or a component thereof, through inlet 24 into shaftway 22 in communication with discharge zone 18 for admixture there with reacted gas.
Depending on the magnitude and type of the reaction to be performed, the reactor according to the invention can be designed in a manner deviating from the embodiment shown in Fig 1. For example, it may be more expedient to have connecting channels running oukside the actual reactor housing in the Eorm o~ separate pipelines, rather than having gas mixing in annular zone 16 andtor discharge zone L8, L~kewise, the gas circuLation pump can be installed at any desirable locat:ion outside the reactor housing in the connection channel. The gas circulation pump can operate according to any known prinicple of gas conveyance; in addition to the radial ; fan already mentioned, axial fans or rotary pumps, for example, could be considered.
A honeycomb catalyst unit comprising many individual channels is often considered as a reaction zone. The reactors according to the invention may con-tain several or a large number of such reaction zones arranged in parallel or in series.

q Honeycomb catalyst carriers are made by extrusion or a rolling process from the usual carrier materials, such as aluminum oxide, silicon dioxide, silicon carbide, mullite, cordierite, spondumene, or zirconium oxide.
Common cross-section forms are shown in Fig. Z. The diameter of a unit is generally between 5 cm and 50 cm.
If the cross-section is rectangular, triangular, or hexagonal, a large number of units of this type can be combined to give a larger cross-sectional area. Each individual unit contains a number of individual channels having diameters between 0.5 mm and 10 mm. The channels ; are separated from one another by thin walls; the usual wall thickness is between 0.1 mm and 3 mm. In the case of extremely -thin walls, 0.1 mm for example, an accessible sur~ace o:E up to 40 cm2 per cm3, with a free flow area o~ up to 80% of the entire cross-sectional area, can be reached. The leng~h o:E the indivldual mits is hardly ll.mited by extrusion during manu~acture.
Lengths of one or more meters are at~ainable; even greater lengths can be created by connecting several wnits in serles, The catalytically active substances, in the case of non-porous carriers, is deposited only on the inner walls of the channels and, in the case of porous carriers, the wall material can also be impregnated with the catalyst.
The methods for impregnation or deposition and for subsequent drying and calcination correspond to those for other usual ~7~

catalyst carriers. It is advantageous to move the honey-combs back and forth along the axis of the flow channels during impregnation or deposition in order to attain an even coating.
S All materials that are effective as catalysts for heterogeneous gas reactions can be used as the catalytically active material. Examples of typical catalysts are metals such as nickel, silver, and the platinum metals, metal oxides such as the oxides of vanadium, i-ron, chromium, manganese, and uranium, and furthermore, heteropolyacids such as phosphotungstic acid or phosphomolybdic acid, and their salts such as their vanadium or iron salts, and also mixed catalysts comprising a number of metallic and non-metallic elements.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An adiabatic gas reactor having a housing wherein are present an inlet zone, an outlet zone, and at least one reaction zone between and in communication with said inlet and outlet zones, said reaction zone having therein a catalyst carrier comprising a plurality of parallel, open, linear gas channels defined by walls in a honeycomb arrangement and extending between and in communication with said inlet zone and said outlet zone, said walls having active catalyst material thereon, said reactor further including a gas circulation pump for removing spent or reacted gas from said outlet zone and introducing fresh gaseous starting material into said inlet zone.

2. An adiabatic gas reactor as in Claim 1 wherein said catalyst carrier has a flow resistance of at most 0.3 bar per meter of length, measured at a gas velocity of 500 meters/minute at 260°C. for air.

3. An adiabatic gas reactor as in Claim 1 or Claim 2, wherein said gas circulation pump is in said housing.

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