Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ ~3L2~L8327 ca~e 196-6
AMMONIA S YNT~ S I S C O~ Rl~R
This inventlon relates to converters for the exothermic, catalytic
synthesis of ammonia from hydrogen and nitrogen. More particularly, the
invent~on relates to a hot wall converter for ammonia ~ynthesis carried out
over a highly active, lnw temperature synthesis catalyst.
5 j Conventionally, ammonia synthesis i8 carried out over predominantly
iron catalyst at temperatures within ehe range from 340C to 530C and
¦ pressures withln the range from 70 kg/cm2 to 280 kg/cm2 in a large,
complex, high pressure converter having from two to six catalyst beds
~ arranged for series and/or parallel flow and provision for inter or intra
, bed cooling of partially converted synthesis gas by internal, indirect heat
exchangers or by the introduction of cool quenching gas. The catalyst beds
have been arranged varlously for axial, radial, or transverse flow of gas.
In general, the high temperature, high pressure syntheqis conditions have
j required that the converters be built with a double Rhell in order that
I cooling gas may be clrculated in the ~hell snnulus to cool the outer,
j pressure shell
Converters of the type described are not suitable for synthesis cats-
lyst that i3 very actlve at low temperatures in the r~nge between 315C
I and 435C and employed in modern synthesis loops operating at pres~ures
I in the range between 50 kg/cm2 and 150 kg/cm2 owing to thelr ~ize, com-
plexity, and cost.
According to the invention, the ammonla synthesis converter comprises
a horizontal reactor having a single, continuous, cylindrical shell with
¦ heads flxed eo each end of the shell and a plur~lity of pressure compart-
ments therein. Each of the pressure compartment~ ha~ a sin~le, adlabatlc,catalyst bed of Rlab 8hape extending across the pressure compartment at
~ 7
~ubstantially lts mid~ection, a gas inlet in the upper portion of the pres-
sure compartment Eor lntroducing synthesls ga~ to an upper plenum above the
catalyst bed, and a gas outlet in the lower portion of the pressure compart-
ment for withdrawing ga3 from a lower plenum below the cataly~t bed. The
converter additionally comprises indirect heat exchange means located exter-
nally of the reactor and pipe means lnterconnecting the pressure compart-
ment~ and ~ndirect heat exchange means for serial flow of gas through the
catalyst beds and interbed cooling of gas ln the indirect heat exchange
means.
ll
Figure 1 lllustrate~ an embodiment of the lnvention where~n the syn-
I thesis converter has three pre~sure compartments and intermediate indirect
i heat exchangers.
.
¦~ Figure 2 is a transver~e cross-section of a pressure compartment taken
j at Section A-A of Figure 1.
i
I The reactor of the converter i8 a single vessel typically between 1
I and 5 meters diameter and between 10 and 50 meters length depending on the
¦ size and type of ammonia plant in which it i5 u~ed but, in any event, will
~ have a length to dlameter ratio between lO and 20. Transverse, circular
i bulkheads are spaced wlthin the reactor to divide it into at least first,
¦ second, and third pressure compartments. The bulkheads abut and are ~ealed
to the inner surface of the cylindrical ~hell at their re~pective peripher-
ies to prevent leaka~e between the pressure compartments.
The catalyst bed within each of the pressure compar~ments is supported
by a foraminou6, flat, horizontal ~uppor~ extending across the pressure
compartment and contsin~ ~ynthesi~ cataly3t optionally underlald by a layer
of coar~e, inert particulates. Since the reactor is e~pecially adapted for
use of very active synthesls cataly6t, the catalyst bed 1B unconventionally
thin, typically between 30 and 150 centimeters in depth, and ha8 a ~urface
area to depth ratio between 15 and 75 m2/m, A~ prevlously recited, the
3~
catalyst bed ls of slab shape nnd i8 disposed for vertically downward flow
; of Yynthesis gas through the bed. The bed extends horizontally across the
entire midsection of the pressure compartment and i~ bounded laterally ln a
transver6e direction by opposlng inner surfaceg o~ the cylindrical shell
¦ and, in the directlon of the reactor a~ls, by a vessel head and transverse
¦ bulkhead in the instance of end compartments and by two transverse bulk-
¦ heads ln the instance of an intermedlate pre~sure compartment. The ratlo
of aggregate volume for all the catalyst beds to the volume of the reactor
I wlll preferably be between 0.2 and 0.6.
10 I The gas inlet in the upper portion of each pressure compartment ls
i disposed in the upper portion of the cyllndrlcal shell and i~ preferably
i centered over the catalyst bed. The gas outlet for each pressure compart-
ment is similarly disposed in the lower portion of the cylindrical shell.
~ In a preferred embodiment of the invention, the reactor has only three
15 , pressure compartments and an overall length to diameter ratio between 12
I and 16. The catalyst bed of the first compartment will have a surface area
¦~ to depth rstio between 30 and 55 m2/m, the catalyst bed of the second
¦ compartment a surface area to depth ratio between 25 and 45 m2/m, and the
¦, catalyst bed of the third compartment a surface area ~o depth ratlo between
20 ! 15 and 30 m2/m. We recognize that incoming synthe~is ga~ having a low
¦, content of ammonia reacts relatively fast over the very active catalyst in
the first pressure compartment ~nd, thereafter, the reaction rate progres-
sively decreases as ammonia content of ehe gas increases during lts subse-
quent passage through the second, and finally, the third pressure compar~-
ments. Accordingly, catalyst volume in the third pressure compartment wlll
be greater than in the second compartment and catalyst ~olume ln the second
pressure compartment wlll be greater thnn that in the flr~t compartment.
To accommodate progressively lncreasing catalyst volume, we may hold the
I catalyst bed depth substantially constant and u~e progressively longer
pre~ur co=psrt=ent~ ao cbot~ in Figure 1. Sioce, however, tbe rescror i~
~2~832~ ~
long and narrow, the la~t pres~ure compartment may, ln some circum~tances,
¦ have poor gas dl6tributlon. In the latter lnstance, we prefer to use pres-
¦ sure compar~ments of ~ubstantlally equal length with cstalgst bed~ having
, subs~antially the same surface area and, accordingly, p20gressively
increase the catalyst bed depth.
Since the cat~lyst bed~ are adiabatic but the synthesis reaction is
exothermic, the converter has at least first indirect heat e~change means
~i for cooling gas between the first and second pressure co~partments and
second indirect heat exchange means for cooling gas between the second and
~ third pressure compartments. Whlle cool quenching gas may be used to aug-
i ment heat removal by the indirect heat e~change mean~, we prefer not to useI lt. Preferably, the first and second indirect heat exchange means are
¦ embodied in a unitary heat exchanger having first and second hot sides for
~ synthesis gas but only A single cold side for the cooling medlum. The
~ cooling medium is preferably boiler feed water AO that steam may bP gene-
rated in the heat exchange means or an a~ociated steam drum and lntegrated
$nto the ammonia plant steam system.
i
Figure l illu~trates an embodiment of the invention wherein the reac-
, tor of the synthesis converter haa ~hree adJoining pressure compartments.
~ The converter also has an e~ternal, unitary heat exchanger shown above the
~I reactor for clarity of illu6tration but which would actually be located
near grade level ad~acent the reactor.
I
Reactor l is comprised of a single, continuous, horizontal, cylindri-
cal ~hell 2, spherical heads 3, and tranaverse bulkhsads 4, 5 which define
a flrst pressure compartment 6, a second pressure compartment 7, snd a
third pressure compart~ent 8. The reactor is 27 meters long and 2 meters
in diameter. Each pressure compartment hAs a gas inlet 9 disposed in the
¦ upper portion of cylindrical ~hell 2 and a gas outlet 16 di~posed in the
lower portion of the ~hell as well as a narrow, slab-~haped, catalyst bed
ll.
~ 2~
Unitary heat e~changer 17 18 of the shell and tube type wherein ~hell
side 18 i~ the cold side fitted wlth boiler feed water lnlet l9 and hot
ll water outlet 20. The hot water outlet i~ connected to A steam drum (not
¦ shown). In contrast, the hot side 18 divided lnto fir8t and second heat
¦ exchange ~ones by first U-tube bundle 21 and ~econd U-tube bundle 22. The
flrst U-tube bundle 21 is in fluid communication with hot ~ide inlet 23 and
hot side outlet 24 and the second U-tube bundle 22 is in fluid communica-
tion with separate hot side inlet 25 and hot slde outlet 26. The re~pec-
¦l tive hot side lnlets and outle~s are dlaposed ln the e~changer heads whichlO ¦ have psrtitions 27 for flow separatlon.
I .
i In operation, preheated syntheais gas having a low concentration of
j ammonia is introduced via line 28 to the upper plenum of first pressure
compartment 6, flows downwardly through the catalyst bed therein where a
portion of the synthe~ls gas is adlabatically converted to ammonia, and is
I discharged from the lower plenum of first pressure compartment 6 via lts
, gas outlet and line 29.
The hot, partially converted gas is then introduced to U-tube bundlè
1 21 in the first heat exchange zone through hot ~ide gas inlet 23, cooled,
i and directed v~a hot side outlet 24 and llne 30 to the upper plenum of
I second pressure compar~ment 7. In like manner, additional conversion takes
place ln pressure compartment 7, the additlonally converted gas flows via
line 31 to U-tube bundle 22 in the ~econd heat e~change zone, and the
resulting cooled gas i8 then directed vla llne 32 to third pressure com-
partment 8 for flnal conver~ion to the target ammonin concentrntion and
then withdrawal through line 33 to an ammonla recovery system.
Figure 2, as prevlously noted, i~ a transverse cross-~ectlon of a pres-
sure compartment of the reactor taken at Section A-A of Figure l. Top ga~
¦ inlet 9 and distributor lO ere tisposed In the upper part of cyllndrical
shell 2. The ga~ dlstributor comprlses a ~pnced serles of annular plates
! ~Z9~3327
ll
to provide even flow of gas through the upper plenum deflned by the upper,
inner surface of the cylindrical ~hell and the top surface af catalyst bed
1` 11.
I
Catalyst bed 11, having a depth d, is ~upported by a wlre screen, in
turn, ~upported by perforated, horizontal ~upport plate 12, ln turn,
supported by lower strip 13 which 18 fi~ed to the cylindrical shell. The
. catalyst bed is compri~ed of a layer of highly active ammonia synthesis
' catalyst underlaid by coarse inert material and extends horlzontally across
! the pressure compartment at its vertical midpolnt to the oppo3ing lnner
10 1 surfaces of the shell. A perforated, flat gss dlstributor plate 14
i supported by upper strip 15 i6 removably mounted above the catalyst bed to
¦I provide uniform distribution of synthesis gas from the upper plenum into
I the catalyst bed. Partially converted gas leaving the bottom surface of
¦ the cataly~t bed passes through support plate 12 and lnto a lower plenum
1. dPf~ned by the lower, inner surface of the cylindrical Rhell and bottom
~urface of ~ e ca ~lv~t bed and le~ve; th~ ve~sel thrDugh g ~ outlet 16.
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