FIELD OF THE INVENTION
[0001] The present invention generally relates to ammonia synthesis processes, wherein hydrogen
and nitrogen are catalytically reacted at elevated temperature and pressure to produce
ammonia.
BACKGROUND ART
[0002] The synthesis of ammonia by a catalyzed reaction of hydrogen with nitrogen at elevated
pressure and temperature has long been used for the commercial production of ammonia
because of the simplicity of the reaction, which is highly selective in that no by-products
are formed. Despite its simplicity and the absence of deleterious reaction by-products,
the ammonia synthesis reaction is intrinsically limited by thermodynamic equilibrium
and kinetic constraints and is favored by operation at high pressure and low temperature.
The effects of varying process conditions on the ammonia synthesis reaction have been
intensively studied by numerous investigators, as summarized for example in "An Investigation
on Promoted Iron Catalyst for the Synthesis of Ammonia" by Anders Nielsen, The Haldor
Topsoe Research Laboratory, 3rd Edition, and in "Fertilizer Science and Technology",
Volume 2, Part III, "Ammonia".
[0003] In commercial synthesis systems, various ammonia synthesis catalysts are utilized
in fixed beds through which the hydrogen and nitrogen reactants are flowed. Contaminants
or inert gases in the hydrogen and nitrogen containing feed gas, if present in excessive
amounts, may adversely affect catalyst activity or dilute the reaction mass sufficiently
to lower the kinetics and yield of product ammonia. Where the unreacted hydrogen and
nitrogen are recycled after ammonia is recovered from the product effluent gas, such
contaminants and inerts tend to build up in the system or synthesis loop. As high
recycle rates are typically employed in such systems it is necessary to purge feed
contaminants from the system in order to limit their concentration. For example, if
the mol percent of inerts in the feed is increased from 0 to 10 percent, with the
reaction being carried out at 838°F and 200 atmospheres pressure, the mol percent
yield of ammonia is decreased from 27.8 to 22.7 percent.
[0004] Another factor is the pressure at which the reaction is carried out; increases in
pressure while maintaining reaction temperature constant increase the equilibrium
yield of ammonia. For example, at a temperature of 838°F, increasing reaction pressure
from 100 to 500 atmospheres increases the percent yield of ammonia from 13.6 to 40.3
percent. On the other hand, if pressure is maintained at an uniform value while temperature
is increased, the equilibrium yield of ammonia is reduced. For example, at a pressure
of 200 atmospheres, increasing the reaction temperature from 694°F to 968°F decreases
the yield of ammonia from 38.7 to 13.2 percent. The decrease of equilibrium yield
of ammonia with increasing temperature is exacerbated by the fact that the ammonia
synthesis reaction is highly exothermic. Even under advantageous pressure and temperature
conditions, comparatively low conversions of ammonia result. As a result many different
commercial synthesis reaction system designs have been proposed and implemented.
[0005] One approach to removal of the exothermic heat from the ammonia synthesis reaction
system is the use of interstage cooling between multiple fixed beds of catalyst in
series, either by injecting cold feed gas between beds or by installing heat exchangers
between beds of indirect cooling, or both. But even with the employment of such interstage
cooling techniques, the exothermic reaction produces temperature gradients in the
fixed catalyst beds of 50-200°F or more.
[0006] An optimized process design for a commercial fixed bed ammonia synthesis system is
disclosed in "Why Horizontal NH₃ Converters?" by Quartulli and Wagner in HYDROCARBON
PROCESSING, December 1978, page 117, wherein the effluent from sequential fixed beds
in the system is subjected to quenching with cool feed gas to lower the temperature
in the beds. Even with such interstage cooling, fixed catalyst bed temperatures at
the bed inlets ranged from 750 to 824°F, while the outlet temperatures were from 880
to 920°F. Such wide temperature variations have a significant effect on ammonia yield
and system efficiency, as noted in the aforementioned Nielsen text, in Figures 8 and
9 thereof.
[0007] It is apparent that such temperature gradients preclude carrying out the reaction
at an optimum temperature at all points in the fixed bed.
[0008] Another problem with fixed bed designs is the necessity to carefully pack the particulate
catalyst within the fixed beds in order to minimize channeling of the reactant gases
through the beds. Gas flow channeling occurs when a stream of gas forms a channel
through the bed, through which channel the flow rate of the gas increases due to the
reduced pressure drop across the channel as compared to the pressure drop across the
rest of the bed. Channeling causes a portion of the reactant gases to flow at a high
rate through the channel instead of flowing at a slower rate in evenly dispersed contact
with the catalyst bed. This results in less contact with the catalyst and consequently
lower rates of reaction. In an attempt to overcome such channeling tendencies, it
is conventional practice when filling the fixed beds with catalyst to send an operator
into the beds to tamp them with a mechanical vibrating tamper in an attempt to attain
uniform, firm packing and thereby minimize channeling. This not only requires labor
expenditure but due to the construction of the fixed beds it is usually not possible
to uniformly tamp the catalyst at all locations and gas channeling through the beds
remains a problem in the art of fixed bed ammonia synthesis reactors.
[0009] Although various materials have been employed as catalysts in ammonia synthesis,
iron-based materials have come into widespread usage due to their relatively low cost
and high conversion efficiency. such catalyst are generally activated, by which is
meant that they are maintained in a reducing atmosphere, e.g., hydrogen, for sufficient
time to reduce their iron oxide content to yield a catalyst substantially free of
iron oxides. Due to the generally low equilibrium yields of ammonia in ammonia synthesis,
catalyst activity is a highly important factor in the economics of the system design
and operation. It is known that the activity of the typical commercial iron-based
catalyst is increased by decreasing the particle size of the catalyst. Such increase
in activity is in part attributable to the reducing conditions present in the activating
process, during which reduction of the catalyst particles proceeds in a radially inwardly
direction, from the surface of the particle toward the center thereof. When, as is
typical, hydrogen is used as the reducing atmosphere, water vapor is formed by combination
of hydrogen with the oxygen normally bound to the iron. This water vapor migrates
to to the surface of the catalyst particles and significantly damages the outer surfaces
thereof, so that catalyst activity is significantly diminished. Further, the size
of the particulate catalyst material is a factor in such lowering of activity. Specifically,
the larger the particle of catalyst, the longer it takes to complete the reduction
of the particle and the more damage is inflicted on the reduced catalyst portion of
the particle due to the water vapor released in the reduction. The magnitude of this
effect is readily ascertainable by surface area measurements for varying sized catalyst
particles; such measurements show that surface area of the catalyst is higher when
smaller particles are reduced.
[0010] It therefore would appear that significant process improvement can be achieved by
reduction of catalyst particle size. However, due to pressure drop considerations,
this benefit has only partially been realized to date. Specifically, fixed bed pressure
drop is an important consideration in the overall ammonia plant design, since this
pressure drop determines the power requirements for recycling unconverted synthesis
gas therein, with higher pressure drop being associated with increased horsepower
requirements for the recycling operation. In early fixed bed ammonia synthesis designs,
a catalyst average particle diameter of from about 6 to 10 mm was employed, although
some designs utilized catalyst with an average particle size of about 14 to 20 mm.
Over the years, ammonia synthesis plants have used smaller sized catalyst to take
advantage of the correspondingly increased catalytic activity. Nonetheless, the literature
does not report any instances of commercial ammonia synthesis plants using catalyst
having a particle size below about 1.5 to 3 mm diameter.
[0011] Attempts to overcome the pressure drop effect associated with small-diameter ammonia
synthesis catalyst particles have included modifications of the synthesis reactor
to accomodate radial or horizontal flow of the feed gas through shallow fixed beds,
but even in such systems catalysts less then about 1.5 to 3 mm in diameter have not
been employed. As a result, the high activity benefits of catalyst with smaller diameter
have not been realized in the ammonia synthesis systems developed to date.
[0012] Although no commercial ammonia synthesis plants have used catalyst having a particle
diameter below about 1.5 to 3 mm, the above mentioned Nielsen article shows in Figure
26 thereof that reducing the catalyst particle size from a range of 1.2 to 2.5 mm
to a range of 0.5 to 1.2 mm, effects an 80 percent increase in the kinetic relative
activity of the catalyst, at the reported conditions including a temperature of 842°F,
a pressure of 317 atmospheres, and a space hourly velocity of 16,000 hr⁻¹.
[0013] U.K. Patent 1,148,513, published April 16, 1969, discloses a method of synthesizing
ammonia using a fluidized bed of catalyst particles which are magnetic in nature.
A bed of the magnetic particles, advantageously of a particle size of 0.1 to 0.2 or
0.2 to 0.3 mm, are fluidized by the synthesis gas containing nitrogen and hydrogen
and reacted at an elevated temperature and pressure. A magnetic field, preferably
of from 500 to 4000 oersteds is applied to the reaction zone by means of a permanet
magnet or preferably an electromagnet placed around the outside of the reaction vessel
made of nonmagnetic steel or within the reaction vessel when it is made of a magnetically
conductive steel. The applied magnetic field is stated to help in lifting the fluidized
catalyst particles, and to retain the particles in the reaction zone and not allow
them to be removed by the flowing gas. The patent states that under the conditions
of the invention, the catalyst becomes much more active for a given particle size
compared to its activity "upon operation in a stationary layer". Optimum operating
temperatures are stated to be from 50 to 125°K lower than those for the same catalyst
used "in a stationary layer" under the same conditions. At page 1, lines 18-31 the
patentee states that preferably the reaction is carried out at a pressure substantially
greater than atmospheric pressure, and both examples summarized in the Table at page
2 show a pressure of 300 atmospheres.
[0014] U.S. Patent 3,304,249 discloses carrying out chemical reactions in fluidized beds,
including the application of an electrical charge to the particulate matter comprising
the fluidized bed. The patent discloses that the turbulent particles of the fluidized
bed provide excellent contact between the particles and the fluidizing gas and, if
there is a temperature difference, excellent heat flow between them. It further discloses
that a fluidized bed of fluidizable particulate solids having a moderate suface electroconductivity
can be stabilized by applying a high voltage to the fluidizing medium in an amount
sufficient to ionize at least part of the fluidizing medium and create an electric
field throughout the bed. The applied voltage should be sufficient to effect a corona
discharge in at least a portion of the bed. A stabilized fluidized bed is said to
mean an expanded fluidized bed in which turbulent motion of the particles and slugging
effects produced by the growth of gas bubbles can be completely suppressed.
[0015] U.S. Patent 2,818,418 concerns improved catalysts for the catalytic hydrogenation
of carbon monoxide in a synthesis which includes the formation of hydrocarbons and
alcohols. The patent states that the process is most suitably carried out with fixed-bed
catalysts but "is also applicable to a synthesis with a pulverulent catalyst (fluidized
synthesis)". Reaction conditions disclosed in the patent include temperatures of about
150 to 350°C and pressures of one to two hundred atmospheres. The patent also states
that "use of the catalysts according to the invention for a carbon monoxide hydrogenation
in the liquid phase and, above all, in the oily phase is possible in both the pulverulent
and granular state". However, all the examples of this patent concern fixed bed processes.
[0016] U.S. Patent 4,397,964 discloses an unsupported particulate catalyst stated to be
especially useful for methanation reactions. The patent states that in a preferred
embodyment, "the particulate unsupported catalyst is used in a catalytic process in
a fluidized bed reactor". It further states that the disclosed catlyst particles are
resistant to attrition in the fluidized bed and are readily reactivated. Example 3
illustrates the use of the catalyst of the invention in a fluidized bed.
[0017] U.S. Patent 2,623,058 discloses catalytic conversion of carbon monoxide with hydrogen
employing fluidized catalysts.
[0018] U.S. Patent 2,779,777 concerns the reaction of hydrogen and carbon monoxyde in a
fluidized catalyst bed. The drawings show a fluidized catalyst bed in a reactor cooled
by a cooling coil.
[0019] U.S. Patent 4,197,418 discloses the use of fluidized catalyst techniques in Fischer-Tropsch
synthesis.
[0020] U.S. Patent 3,402,999 describes the utilization of a fluidized bed reactor in a process
for decomposing ammonium chloride to ammonia.
SUMMARY OF THE INVENTION
[0021] In accordance with the present invention there is provided a process for producing
ammonia, the process comprising the following steps:
[0022] A synthesis gas mixture comprising hydrogen and nitrogen is contacted with a mass
of solid, particulate ammonia synthesis catalyst, e.g., an activated iron catalyst,
under conditions to fluidized the mass of catalyst. The synthesis gas is passed through
a reaction zone in contact with the fluidized catalyst mass, the reaction zone being
maintained at an elevated temperature and at pressure of from about 20 to 100 atmospheres
to therein react hydrogen with nitrogen to form ammonia and thereby provide a reacted
synthesis gas stream. The reacted synthesis gas is flowed from the reaction zone to
an ammonia recovery zone wherein ammonia produced in the reaction zone is recovered
to provide an ammonia product and an ammonia-depleted gas.
[0023] In other aspects of the invention the pressure may be maintained within a narrower
pressure range, say at a pressure of from about 25 to 90 atzmosperes, or at a pressure
of from about 30 to 70 atmospheres.
[0024] Another aspect of the invention provides for removing entrained particulate catalyst
from the reacted synthesis gas leaving the reaction zone by a catalyst recovery step
consisting essentially of one or both of mechanical recovery and electrostatic recovery.
[0025] Still another aspect of the invention provides for maintaining the catalyst mass
in a fluidized state by a fluidization step consisting essentially of controlling
the volumetric hourly flow rate of gas through the fluidized mass, and the pressure
within the reaction zone.
[0026] Other aspects of the invention provide for recycling the ammonia depleted gas to
the reaction zone and concurrently introducing fresh synthesis gas to the reaction
zone.
[0027] Further aspects of the invention are disclosed in the following detailed description
and schematic diagram, not to scale, of a simplified ammonia synthesis flow chart.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODYMENTS
[0028] In accordance with the present invention, the ammonia synthesis process is carried
out within a pressure range, which is low compared to usual ammonia synthesis pressures,
and in a fluidized bed wherein the solid catalyst is fluidized by a synthesis gas
comprising the hydrogen and nitrogen reactants for the synthesis. Fluidized bed operation
of the synthesis system at such low pressure is a substantial advance in the ammonia
synthesis art, relative to fixed bed processes, because low pressure operation offers
significant savings in capital investment and operating costs for compression of the
synthesis gas, and fluidized bed operation is free of the substantial temperatur gradients
characteristic of fixed bed operation.
[0029] While the art has recognized that considerable savings with respect to compression
of the synthesis gas could be effectuated by reducing the pressure at which the ammonia
synthesis reaction is carried out, the reduction in equilibrium content of ammonia
at such reduced pressures established a practical lower limit of about 100 atmospheres
for the ammonia synthesis reaction. While ammonia synthesis at pressures as low as
500 psig (34 atmospheres) using fixed bed catalysts have been considered, they have
not been put into use because of the requirement for extremely large quantities of
catalyst in the fixed beds when operating at such low pressures. The attraction of
operating at very low pressures, such as 450 to 500 psig, is that this is often the
pressure at which hydrogen gas from hydrocarbon reforming operations is available.
If the ammonia synthesis could feasibly carried out at such low levels, at least the
need to pressurize the hydrogen portion of the synthesis gas is eliminated.
[0030] Although the prior art exemplified by the above mentioned U.K. Patent 1,148,513 discloses
the utilization of a fluidized bed of catalyst in ammonia synthesis, the difficulty
of fluidized bed operation even at the lower end of the pressure range deemed essential
for economical ammonia synthesis constrains the utilization of extraordinary measures
for the operation. Thus, the U.K. Patent discloses the imposition of a magnetic field
of 500 to 4,000 oersted to help maintain the bed in a fluidized condition and to prevent
excessive loss of catalyst particles entrained within the gas at the pressures utilized
(300 atmospheres is exemplified). Among the difficulties encountered in fluidized
bed operations at such high pressures is increased difficulty in separating entrained
particles from the high pressure gases due to the relatively high density of the gas
as compared to gas densities at significantly lower pressures. Loss of the catalyst
from the reaction zone of course represents a problem both in contamination of product
and replacement of catalyst in the reaction zone.
[0031] It has been discovered that the pressure drop/catalyst size constraint is overcome
in the fluidized bed of the present invention, wherein the only pressure drop requirement
is the balancing of the weight of catalyst particles. Accordingly, the process of
the present invention is able to utilize catalyst particles of sizes which typically
are smaller by a factor of ten or more times than catalyst particles utilized in conventional,
fixed bed ammonia plants. Fluidization of the catalyst is according to the present
invention preferably attained solely by the flow of sythesis gas through the bed or
mass of catalyst, i.e., essentially by controlling the volumetric hourly flow rate
of gas through the catalyst , and the pressure within the reaction zone.
[0032] Fluidized catalyst bed operation in ammonia synthesis in accordance with the present
invention provides advantages as compared to fixed bed catalyst operations which are
significant enough to permit lowering the pressure at which the reaction is carried
out. Accordingly the inventive process for producing ammonia includes maintaining
the reaction zone at a pressure of from about 20 to 100 atmospheres to therein react
hydrogen with nitrogen at an elevated temperature to form ammonia and provide a reacted
synthesis gas stream. Such pressure levels permit fluidized bed operation without
the utilization of extraordinary measures such as the application of magnetic or electrical
fields to the high temperature reaction zone.
[0033] The fluidized bed is capable of isothermal operation, either intrinsically or as
augmented for temperature control by heat exchanging a coolant medium with the fluidized
bed, such as indirect heat exchange by flow of coolant liquid through heat exchange
coils disposed in the bed. In the latter case, the coolant liquid removes the exothermic
heat of reaction from the fluidized bed, so that the bed may be closely controlled
at a substantially isothermal temperature throughout its volume. The coolant liquid
may be flowed through cooling coils disposed in a closed path passing through the
fluidized bed for vaporization of the coolant liquid and consequent removal of the
reaction heat from the fluidized bed.
[0034] The inventive ammonia production process may be operated with the catalyst in the
form of a fine particulate material to obtain optimum catalytic activity. As mentioned,
fixed beds have not been able to utilize catalyst having a diameter below about 1.5
mm, since the pressure drop becomes prohibitively high, and the energy requirements
for flowing the gases through the bed increase uneconomically. The process of the
present invention may, as an example, be operated with catalyst having an average
particle diameter of about 0.06 mm, whereby the catalyst activity and conversion efficiency
of the ammonia synthesis reaction are substantially improved over what has heretofore
been attainable in commercial practice. However, catalyst of any desired particle
size are useable in the fluidized bed of the present invention as long as they can
be fluidized under the operating conditions employed.
[0035] Further, the fluidization of the catalyst particles according to the invention permits
operation at optimum temperature condition to be readily achieved, in a manner which
favors uniformity of temperature in the reaction zone provided by the fluidized bed
of catalyst, enabling removal of the exothermic heat of reaction by flow of the fluidized
gas through the bed and, if necessary or desirable, indirect heat exchange therein.
[0036] As a result of the foregoing advantages, the process of the invention permits attainment
of a satisfactory conversion of hydrogen and nitrogen to ammonia, notwithstanding
the use of reaction pressures lower than those conventionally employed, with the result,
that the recycle gas compressor horsepower and catalyst inventory may be reduced,
thereby decreasing equipment and operating costs for the plant and production.
[0037] Another advantage of the present invention is that catalyst may be added to and/or
withdrawn from the fluidized bed on a continuous or periodic basis without necessity
of a shutdown of the plant. Conventional large scale ammonia plants utilize catalyst
which, unless accidentally contaminated, remain in service for years, gradually losing
activity. The present invention enables maintaining the activity of the catalyst
in the fluidized bed at a predtermined high level simply by replacement with fresh
catalyst, as desired or necessary, of a portion of the existing inventory of catalyst
in use during normal operation and without interruption.
[0038] Referring now to the sole figure of the drawing, a fluidized bed reactor vessel 1
is provided with an inlet conduit 2 which, after passing through heat exchanger 9
and being heated therein, is then passed to vessel 1 for introduction therein of heated
reactant gas comprising the the hydrogen and nitrogen reactants for the ammonia synthesis.
A feed gas distributor 3 is provided near the bottom inlet portion of the vessel 1
and may be in the form of a plate or pipe grid to facilitata distribution of the feed
gas across substantially the entire cross section of the vessel 1.
[0039] At the top of vessel 1 is a multiple stage cyclone recovery means 4, which functions
to recover catalyst particles from the product gas flowing into the upper outlet
portion of vessel 1. Outlet conduit 5 communicates the upper end of vessel 1 to heat
exchanger 9 which in turn is connected to product gas conduit 10, in turn connected
to ammonia recovery zone 16. Ammonia gas conduit 17 leads from recovery zone 16 as
does recycle gas line 8. A feed line 7 and recycle gas line 8 are connected to inlet
conduit 22 which leads into head exchanger 9. A purge gas line 21 leads from recycle
gas line 8.
[0040] Vessel 1 contains a fluidized bed of catalyst 6 which may be a conventional activated
iron catalyst, substantially free of iron oxides as described above under "Description
of Related Art". A suitable catalyst may be a solid particulate catalyst comprising
particles having a particle size (diameter) of less than about 1.5 mm, preferably
less than about 0.25 mm, and a particle size distribution such that at least about
90 weight percent of the particles have a diameter not greater than about 3 mm, preferably
not greater than about 0.5 mm. A suitable range of particle sizes useful in the practive
of the present invention is from about 0.005 to about 1.5 mm, preferably 0.01 to about
1.2 mm, more preferably from about 0.02 to about 1.0 mm, and most preferably from
about 0.04 to about 0.8 mm. For example, a suitable catalyst may be an activated
iron-based catalyst having an average particle diameter on the order of about 0.06
mm.
[0041] Extending into the fluidized bed 6 of catalyst particles, the top of bed 6 being
indicated within vessel 1 by dotted line 6a, is a cyclone dip leg 12 of the multiple
stage cyclone recovery means 4. An electrostatic precipitation zone 20 is provided
and outlet conduit 5 is connected thereto and thence to heat exchanger 9. Within the
fluidized bed of catalyst 6 there is also disposed cooling coils 13 joined in flow
communication with a source of coolant liquid (not shown), each cooling coil defining
a closed path through the fluidized bed. At the lower portion of the ves sel 1 is
a catalyst discharge line 15, which may be used in the known manner to intermittently
withdraw from the vessel 1 a quantity of catalyst which is replaced by makeup catalyst
introduced through line 14.
[0042] In operation of the system, feed gas comprising hydrogen and nitrogen, preferably
in a stoichiometric 3:1 volumetric ratio of hydrogen to nitrogen, is introduced to
the system via feed line 7, and joined with ammonia-depleted unreacted recycle gas
in gas line 8 to form a combined feed stream in conduit 19. The combined stream is
passed through heat exchanger 9 in indirect heat exchange relationship with the reaction
product gas from outlet conduit 5, the latter subsequently being discharged from heat
exchanger 9 in product gas conduit 10 to the ammonia recovery zone 16.
[0043] As a result of the indirect heat exchange in heat exchanger 9, the feed gas stream
from conduit 19 is heated to elevated temperature, while the product gas from outlet
conduit 5, initially at high temperature due to the exothermic character of the synthesis
reaction, is cooled to recover a portion of its heat content and passes via product
gas conduit 10 at significantly reduced temperature to ammonia recovery zone 16.
[0044] The heated feed gas stream passes through inlet conduit 2 and enters vessel 1 at
the lower portion thereof where it is distributed across the vessel cross-section
by means of feed gas distributor 3. The feed gas is introduced at sufficient volumetric
flow rate and superficial velocity to fluidize the bed of solid catalyst 6 within
the vessel and thereby form a fluidized bed 6 of catalyst. In the fluidized bed 6,
the hydrogen and nitrogen reactants in the feed gas are contacted with the solid catalyst
particles in the fluidized bed 6 to react and form ammonia. The reaction product gas,
comprising ammonia and unreacted hydrogen and nitrogen (and any inerts introduced
by the feed) is discharged into the disengagement space 11 of vessel 1 where entrained
catalyst solids in the reaction product gas stream are removed by passage of the reaction
product gas stream through the multiple stage cyclone recovery means 4. Collected
entrainment solids are returned to the fluidized bed 6 via dip leg 12, and solids-depleted
product gas is discharged from vessel 1 via outlet conduit 5. Catalyst particles
which may escape cyclone recovery means 4 are recovered in electrostatic precipitator
20 and the recovered catalyst may be returned to reactor 1 via line 22 to make up
catalyst line 14. The nature of the entrained solids separation means is not critical
and any suitable mechanical or electrostatic precipitation apparatus, of which multiple
stage cyclones are one example, could be employed. For example, a series of porous
metal filters could be positioned in the disengagement space of the vessel, in a
known fashion. The present invention does not require the use of extraordinary means,
such as the imposition of magnetic fields or electrical fields on the reaction zone
catalyst, either to recover the catalyst or keep it in the reaction zone, or to maintain
the bed in a fluidized condition. Thus, the removal of entrained particulate catalyst
from the reaction product gas stream leaving the reaction zone is effectuated in a
catalyst recovery step consisting essentially of one or both the mechanical recovery
(e.g., cyclone separation, filters) and electrostatic recovery ( applying an electrostatic
charge to the catalyst particles).
[0045] The product gas stream discharged from the vessel in outlet conduit 5 is passed through
heat exchanger 9 into product gas conduit 10 as previously described and into the
ammonia recovery zone 16 which may utilize any suitable system for recovery of ammonia
from the reaction product gas as is well known in the art. For example, ammonia recovery
zone 16 may employ adsorption with adsorbent materials such as molecular sieves, liquid
scrubbing of the reaction pro duct gas with a scrubbing liquid in which the ammonia
is highly soluble, followed by separation of the solubilized ammonia from the scrubbing
liquid, or any other suitable process or combination of processes whereby ammonia
is separated from the unreacted components of the gas. The recovered ammonia product
is discharged from recovery zone 16 by ammonia gas conduit 17 and passed to suitable
storage or other processes (not shown).
[0046] The unreacted hydrogen and nitrogen-containing gas produced by ammonia recovery in
the ammonia recovery zone 16 is recycled in gas line 8 for joining with the synthesis
feed gas from feed line 7. A portion of the recycled gas is vented from gas line 8
via purge gas line 18 to discharge a purge gas from the system to thereby control
the build-up of inerts in the synthesis loop, as is well known to those skilled in
the art.
[0047] During the operation of the process, if desired, a portion of the catalyst inventory
may be replaced by fresh catalyst to maintain or enhance the level of catalytic activity.
To do so, fresh catalyst is added to vessel 1 by catalyst line 14 and spent catalyst
may be withdrawn from the vessel by catalyst discharge line 15. Although not specifically
shown, the specific mechanism of solids introduction and removal from the vessel
in respective lines 14 and 15 may entail the use of any suitable means, e.g., lock-hopper
means, introduction and withdrawal wells, etc.
[0048] Within vessel 1, the ammonia synthesis reaction may be carried out at varying conditions.
In general, suitable process conditions include temperatures on the order of from
about 600 to about 1100°F, pressures on the order of from about 20 to about 100 atmospheres,
and superficial gas velocities of the fluidizing synthesis gas of from about 0.5
to about 5.0 ft/sec.
[0049] As indicated, the low pressure, fluidized bed operation of the invention facilitates
uniformity in temperature throughout the reaction mass of the fluidized bed, and preferably
the fluidized bed is maintained under substantially isothermal conditions. While such
preferred isothermal operation may be carried out without heat exchange between the
fluidized bed 6 and an external coolant medium, it may be desirable in some cases
to carry out indirect heat transfer from the fluidized bed to a coolant liquid, e.g.,
water, circulated in one or more cooling coils submerged in the bed. It is generally
desirable to recover some of the sensible heat of the reacted synthesis gas by indirect
heat exchange with the feed gas stream passed to the reaction vessel.
[0050] The catalyst bed fluidizing conditions may vary widely, depending on the specific
synthesis system, so that the fluidized bed operation may comprise dense phase fluidization,
dilute phase fluidization, or any other fluidization flow regime which is suitable
in the specific system application being utilized.
[0051] In comparison to the conventional high pressure, fixed bed processes of the prior
art, the low pressure, fluidized bed process of the present invention affords numerous
advantages. Areas of improvement include potential elimination of the synthesis gas
compressor because the lower operating pressures may be able to use synthesis gas
at available pressure, reduction of refrigeration compressor horsepower (due to the
substantially isothermal operation of which the fluidized bed process is capable),
lowered operating pressures in the reforming and feed gas preparation portions of
the ammonia synthesis plant, and a simpler, stabler and more economic synthesis reactor.
[0052] The specific process design of ammonia plants adapted to carry out the process of
the present invention may vary widely insofar as the features and process variables
involved, such process variables including temperature, pressure, space velocity,
flow rates, composition of the synthesis feed gas, and amount of inerts therein and
the method employed to remove inerts from the synthesis loop, catalyst activity, catalyst
particle size, the method of separating ammonia from the effluent gas from the reactor
vessel, and the method of removal of the exothermic heat of the synthesis reaction.
Specifically, the kinetics fo the synthesis reaction depends on space velocity of
the gas flow through the reaction system, catalyst activity, temperature and the degree
to which ammonia conversion approaches the thermodynamic equilibrium value for the
process conditions. The thermodynamic equilibrium ammonia content in turn depends
upon temperature, pressure and the amount of the inerts in the reaction product gas.
It will be appreciated by those skilled in the art that the numerous process variables
involved may be varied within wide limits to achieve optimal system performance in
a given application of the process of the invention.
[0053] In the practice of the present invention, advantage can be taken of the high activity
of fine (small diameter) catalysts, since the catalyst fineness does not, as in the
case of fixed bed reactors, affect the pressure drop through the fluidized bed, which
depends only on the weight of the fluidized catalyst in the bed and the cross-sectional
area of the bed. The type of catalyst employed in the process may vary widely, with
iron-based catalysts being preferred, and preferably being of small diameter particles,
most preferably with the previously described average particle size ranges and maximum
preferred values. A preferred activated iron-based catalyst may have a solid density
(measured without voids) of 6.39 gm/cc; a true particle density (with voids) of 3.73
gm/cc; a pore volume of 0.11 cc/gm; and a particle size distribution as follows: 0-20
microns, 5 weight percent; 20-40 microns, 20 weight percent; 40-80 microns, 40 weight
percent; 80 microns and above, 35 weight percent. (The term "weight percent" as used
herein and in the claims with reference to particle size distribution, means percent
by weight of the sample which is made up of particles of the indicated size range.)
The foregoing distribution yields an average particle size for the catalyst of about
60 microns, which is generally less than one tenth of the smallest catalyst size utilized
in conventional fixed bed ammonia synthesis plants. Conventionally, catalysts are
ground such that at least about 90 weight percent of the catalyst comprises particles
which are of a diameter not more than about twice the average diameter. Usually, at
least about 95 weight percent are coarser than about one-third of the average particle
size. Those skilled in the art will appreciate that the particle size range results
from the size reduction technique and equipment employed in a given case. The ability
to employ in a fluidized bed catalyst particles much finer than those employable in
a fixed bed provides a substantial increase in catalyst activity, the benefit of
which has not heretofore been realized due to pressure drop constraints associated
with fixed bed units.
[0054] The features and advantages of one embodiment of the present invention are illustrated
with respect to the following non-limiting Example, wherein all parts and percentages
are by weight, unless otherwise expressly stated.
Example
[0055] To illustrate the advantages of the present invention, simulation models for a conventional
fixed bed ammonia synthesis process of the prior art (Process A) and for an illustrative
fluidized bed systems representative of the present invention (Process B) were developed.
Process A utilizes three fixed beds of catalyst in a horizontal reactor of the type
described in the above-mentioned article by Quartilli and Wagner in
Hydrocarbon Processing. Process B utilizes a fluidized catalyst bed in accordance with the practice of
this invention. Calculations are based on a conventional iron based catalyst, obtained
by fusing magnetite (Fe₃O₄) particles with alkali promoters and reducing the catalyst
at elevated temperature, e.g., 600 to 1000°F (316 to 538°C) in hydrogen at a pressure
of less than 30 psig. For comparison purposes of this Example, ammonia production
rate, reaction pressure and gas space velocity through the respective catalyst beds
are taken as the same in both cases A and B. Catalyst activity, reaction temperature,
recycle gas rate and catalyst inventory are seen to differ in the respective cases
A and B; the values for these operating variables are likewise set forth in the Table.
[0056] As shown by the tabulated data, Process B, which is representative of the present
invention, shows considerable advantages over Process A, a typical prior art process
utilizing a horizontal, fixed bed reactor. The temperatures within the fixed bed
of Process A vary from 750°F (399°C) at the reactor inlet to 920°F (493°C) at the
reactor outlet, a temperature gradient of 170°F (94.4°C). The synthesis temperature
in Process B is 800°F (427°C) and is substantially constant throughout the reactor
vessel, reflecting the substantially isothermal operation attainable by the process
of the present invention. Temperature variations within the ammonia synthesis reactor
have an adverse effect on ammonia yield as described, for example, in the text "An
Investigation On Promoted Iron Catalysts For The Synthesis Of Ammonia" by Anders
Nielsen of the Haldor Topsoe Research Laboratory. The lower pressures of Cases 1B
and 2B of Process B as compared to Cases 1A and 2A of Process A provided considerable
savings in reduced utilities cost and compressor and pressure vessel fixed costs,
while providing the same quantity of ammonia product.
[0057] As is also well known in the art, a lower conversion rate, with the consequent higher
recycle rate, increases the buildup of inerts in the feed. Such build-up decreases
the thermodynamic equilibrium ammonia content as described above under the heading
"Description Of The Related Art", in which the equilibrium ammonia content at a stated
temperature and pressure is reduced from 27.8 mol percent to 22.7 mol percent by
the inclusion of 10 mol percent inerts in the mixture.
[0058] It will also be observed that the catalyst inventory required in Process B is considerably
less, about 30% less, than that required in the fixed bed catalyst system of Process
A. Because of the much smaller particle size of the catalyst utilizeable in the fluidized
bed system of Process B, and because of the isothermal operating conditions, the catalytic
activity of the catalyst of Process B is conservatively calculated at more than about
three times greater than that of the compositionally identical catalyst of Process
A. For example, in the above-mentioned Nielsen article, if the activity of the catalyst
of the Example having a particle size in the range of 0.3 to 0.8 mm is taken as 1.0,
its use in the particle size range and under the conditions of Process A would show
a relative catalytic activity of 0.6 whereas its use in the particle size range and
under the conditions of Process B would show a relative catalytic activity of at
least 2. (Relative catalytic activity is the ratio of space velocity of the reactant
gases through the catalyst at a given conversion of reactants. For example, at identical
conditions of temperature and pressure, if a catalyst whose a relative activity is
taken as 1 gives 12% conversion of the reactants at 15,000 space velocity, a test
catalyst which gives 12% conversion at 30,000 space velocity has a relative catalytic
activity of 2.)
[0059] The foregoing improvements of the process systems illustrative of the present invention
as compared to the con ventional fixed bed system are, as shown, substantial and
are primarily attributable to operating the reaction (a) at an optimum temperature
level which is substantially uniform throughout the fluidized bed, and (b) with a
more active catalyst for a given composition, due to the much finer catalyst particle
used in the fluidized bed process of the present invention as compared to fixed bed
processes.
[0060] Although the foregoing calculated comparison evidences specific improvements with
regard to certain process variables, it is not essential to achievement of benefits
accorded by the present invention to operate in the specific manner of the example.
For example, in lieu of reducing the catalyst inventory relative to a comparable fixed
bed system, it may be desirable in some instances to maintain a higher the catalyst
inventory and thereby achieve a greater reduction in recycle gas rate. As another
alternative, rather than taking reductions in recycle gas rate and/or catalyst inventory,
a reduction in operating pressure below that required for fixed bed operation may
be elected. Obviously, a combination of two or more such improvements may be selected.
[0061] Although the invention has been shown and described with respect to preferred illustrative
embodiments, it will be apparent that numerous variations and other features are possible
within the broad scope thereof, and accordingly, all apparent variations, modifications
and embodiments are to be regarded as being within the spirit and scope of the present
invention.