THEORY OF CATALYSTS
In
the chemical industry, Catalysts are used in order to bring some chosen
reaction as close as possible to a selected equilibrium point in the shortest
possible time for a reversible type of reaction. Catalyst accelerates the rates of forward and
backward reaction so as to attain faster the system towards equilibrium and the
Catalyst remains unaltered without taking part in the chemical reaction.
The
rate of a Catalysed reaction is changed by varying the temperature, the
Catalyst, or the concentrations of reactions and products. Although these variables are not independent,
the responses are especially sensitive to the type of Catalyst because each
solid of Catalyst introduces unique rate of reaction. For example, in case of
Ammonia synthesis reaction appropriate Catalysts must have the highest activity
for the Hydrogenation of Nitrogen at as low a temperature as possible. Similarly, in case of reaction of
Carbonmonoxide and Hydrogen, an active Catalyst without selectivity should
produce CH4 and H2O, which are very stable molecules, whereas a selective
Catalyst may produce methanol. It is
evident that the selection of suitable Catalysts depends upon the extent of
knowledge and the correctness of suppositions concerning the nature of the
reaction and of the activity of solids.
THE ACTIVITY OF SOLIDS
The
catalyst, as separate particles or agglomerates of particles, is immersed in a
fluid medium in motion. Reactants and
products diffuse in the gas or liquid phases bathing the boundaries of the
solid, and also in the pore spaces of the agglomerates. These diffusions are rate controlling, the
reactant, the intermediates and the products combine loosely (Physisorption) or
more lightly (Weak and Strong Chemisorption) with the surface of the
Catalyst. Solubilities and rates of
diffusion in solids are small, so the reaction almost invariably takes place on
the solid surface and involves solid substrate interactions, which stretch or dissociate
the bonds of the reactants. The overall
rate is controlled by diffusion, adsorption, desorption, or by an inter action
between surface complexes in a simple reaction, or at some intermediate step in
a complex process.
THE PRINCIPAL CHARACTERISTICS
The efficiency of a Catalyst rests upon
three factors:
- Activity
-
Selectivity
-
Life
The
activity is the extent to which the catalyst influences the rate of change of
the degree of advancement of the (as assessed by the disappearance of reactants)
conversion), per unit weight or per unit volume of Catalyst, under specified
conditions. The activity per unit volume
is of practical importance because process economics can depend critically upon
the cost of packed reactor space. The
bulk density of the Catalyst must always be as small as possible, consistent
with other requirements. The performance
of a catalyst is generally assessed in terms of the rate at which it promotes a
desired (or sometimes an undesired) reaction.
Reaction rate is expressed with well known proportionally termed as
'Catalyst activity'. At equivalent
temperatures, pressures, reaction volumes and mole fractions of reactants,
reaction rate is proportional to Catalyst activity. Where other reactions are
also possible, an assessment is made of Catalyst selectivity. Catalyst selectivity is the ratio between
activity for desired and undesired reactions.
With
most catalyst used for the production of Synthesis gas and Ammonia this is
unimportant because, by careful catalyst selection, high selectivity is
achieved under normal operating conditions.
The life of the Catalyst is the period during which the Catalyst
produces the required product at a space-time yield in excess of or equal to
that designated. The activity of most
Catalysts decreases sharply at first and then declines much more slowly with
time. The selectivity may worsen or
improve. The life of a Catalyst
terminates because of loss of mechanical strength or because of unacceptable
changes in activity and selectivity.
The Catalytic efficacy of a homogeneous solid
phase is influenced by the four factors as follows:
a. The exposed area in contact the fluid termed
as specific area.
b. The intrinsic chemical characteristics of
the surface of the solid. The species
forming the surface may be atoms or ions and their chemical properties must
depend upon their electronic structure and arrangement.
c. The topography of the surface. Because of the dependence of activity upon
geometry and electronic structure, the faces, edges and corners of crystals
must possess different activities.
d. The occurrence of lattice defects. The activities to be associated with defects
such as grain boundaries; dislocations differ from solid to solid and with the
type of reaction Catalysed.
THE SPECIFIC AREA:
The
specific area of the solid phases of the Catalyst must be adjusted to suit the
requirements of the process. Usually the
area is made and maintained large by procedures, which produce either small
particles or porous bodies in more or less stable states. The specific area increases with the particle
diameter and density of the particles.
Higher the specific area, higher is the length of edges and greater
number of corners introduces more regions of different activity. Real Catalysts are composed of particles of a
range of sizes. One detrimental effect
of Catalyst activity is due to phenomena called 'Sintering'. High temperature increases the Sintering of
solid particles. Due to sintering, the
specific area, length of edges and the number of corners is effected.
Sintering
can be restricted to smoothing by dispersing the particles of active phase on
the surface of another inert refractory solid of high area called support of
the Catalyst or by separating them with refractory spacing blocks called
stabilisation surface area can be obscured by debris (dust, rust) or
encapsulated by such products of parasitic reactions as liquid polymers and
solid coke. If thereby the pore size
distribution is changed and the reactions become diffusion limited, then the
selectivity as well as the activity may be impaired.
COMPOSITE CATALYSTS
A
Catalyst is composite when it contains more than one chemical entity. The addition of a second component may be
necessary to support or stabilise the active phase by a second and more
refractory solid or because the reaction is complex, involving a series of
steps, each requiring selective Catalysts.
Almost all industrial Catalysts are composites, if only because the use
of a support decreases manufacturing costs and facilitates handling.
The
rudiments of Catalyst design rest upon the facts that the efficiency of a solid
phase Catalyst is determined by its selectivity, specific activity (activity
per unit area) and specific area and by the effects of specific inhibitors or
promoters e.g. the Catalyst for Hydrogenation of nitrogen to Ammonia, consists
of about 94% Iron oxide with the approximate composition Fe3 O4, the balance
being promoters i.e. mainly oxides of Ca, Al and K. The steam reforming of methane over nickel
containing Catalysts (such as R-67 and
R-67-7H) consists of Nio-16-18 wt%, Magnesium aluminate with free Mg content below
0.5% and SiO2 max. 0.1% in which Ni is the active metal for complex steam
reforming reaction and Magnesium aluminate acts as the stabilising agent plus
it suppresses the formation of carbon.
OPERATION FOR CATALYTIC REACTOR
Conversion
taking place in a reactor filled with selective Catalyst is a function of the
following parameters:
1. Gas composition
2. Space Velocity
3. Pressure
4. Catalyst Temperatures
5. Catalyst Activity
The
first four parameters do, to some extent, depend on the reactor design, but
are, within certain limits, functions of the fifth parameter, the Catalyst
activity. Activity of Catalyst, rather
specific activity expressed to unit area of the Catalyst cannot be directly
measured. However it is a measure of
reaction rate which again depends upon the Catalyst size, voidage for packed
bed and surface area.
SPACE VELOCITY
Space
velocity for catalytic reactor is an indication of the contact time of the
following fluid with the surface of Catalyst.
It is defined as follows:
Space
Velocity = Number of reactor volumes of feed at specified conditions which can
be treated in unit time.
= Volumetric feed rate
Volumetric feed rate
---------------------------
= ---------------------
Reactor volume Catalyst
volume
During
heating of Ammonia converter Catalyst, a high space velocity is maintained to
reduce the reduction time by better uniform heating of the catalyst. However, for normal operating reactor the
lowering of the space velocity reduces the conversion.
CATALYST POISON
The
activity of solid catalyst is sometimes appreciably altered by traces of
foreign substances. Foreign substances,
which tend to inhibit catalytic activity, are known as anti- catalysts or
Catalyst poisons. They are more firmly
and preferentially adsorbed than the reactants at the surface of a solid
catalyst, which is thereby rendered ineffective. The activity of the solid catalyst is reduced
or destroyed by adsorption and by alloy or compound formation, when the processor
gives rise to a less active or inactive surface. The non-metal species like Oxygen, Sulphur,
Arsenic, halogens (Chlorides, Fluorides) and also Carbonmonoxide, Ammonia,
Water, Hydrogen Sulphide, Phosphine etc.and their derivatives can be powerful
poisons when present as accidental impurities or as reactants, depending upon
the facility of the Catalysed reaction.
These poisons may be adsorbed or they may react to form compounds in
surface layers (Oxides, Sulphides, halides etc).If activity recovers on
exclusion of the poison from the reactants, the poisoning is said to be
reversible, otherwise the poisoning is permanent.
Temporary
poisoning is effected by Oxygen and Oxygen-containing compounds such as H2O, CO
and CO2. In case of temporary poisoning,
Catalyst activity is restored when feedstock is restored. Permanent poisoning
is effected by S, As, C1, F,P, Pb and Carbon deposition from the cracking of Hydrocarbons such as compressor
lubrication oil or by the polymerisation of higher Hydrocarbons. The above elements cause permanent damage to
the catalyst. Carbon formation on the
outer surface of the Catalyst may however be termed as 'Semi- permanent'
poisoning since the initial activity can be regained by a regeneration process
such as steam blowing of Ni-reforming catalyst.
CRUSHING STRENGTH OF CATALYST
A
successful commercial heterogenous Catalyst not only must be capable of
Catalysing the desired reactions selectively, it must be also mechanically
robust. It must be suitably shaped, so
that the fluid or gas can flow through a bed made of it without excessive
pressure drop or uneven distribution.
And finally it must retain both its reactivity and its mechanical
properties over a long life, which includes start ups and shutdowns. Normally,
the Catalysts are formed by pelleting (tablets), granulation and extrusion
along with various types of binder agent to give adequate strength to the
finished Catalyst. A high Catalyst
strength is adopted to withstand the following forms of stress:
1. Abrasion (during transit)
2. Impact (when loaded into the Reactor)
3.
Internal stresses (resulting from
phase changes during reduction or when initially brought online).
4. External
stressing (caused by pressure drop,
Catalyst weight and possibly, thermal cycling).
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
Features
|
R-201
|
TK-251
|
5X2.5
|
8.44
|
480
|
2220
|
NiO:2-3%
MoO:10%
|
>5
|
|
R-202A
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
||
R-202B
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
Guard
|
|
F-201
|
Topsoe
R-67-R-7H
prereduced
|
16X11
cylinder with7 holes
|
6.68
|
960
|
NiO:16-18%
SiO2:<0.1%
MgAl2O4
|
3-5 Years
|
Carrier surface area:12-20m2/gm
|
|
R-67-7H
|
16X11 cylinder with 7 holes
|
20.05
|
970
|
NiO:16-18%
SiO2<0.1%
MgAl2O4
|
3-5 Years
|
Ni surface area:3.5 to 5
m2/gm
|
||
R-203
|
RKS-2-7H
|
20X18
|
30.0
|
950
|
2800
|
MoO:9.0%
|
>10
|
|
R-204
|
SK-201
|
6X6
|
70.0
|
1200
|
4400
|
>5
|
Cu promoted
|
|
R-205
|
LSK
LK-821
|
4.5X4.5
4.3X3.2
|
80.3
|
1100
1000
|
4670
|
CuO,ZnO,Cr2O3
CuO,ZnO,Al2O3
|
>5
|
|
R-301
|
PK-5
|
5
|
26.0
|
>10
|
||||
R-501
|
KMR (pre-reduced)
KM
|
1.5-3.0
1.5
|
26.9
68.4
|
2220
2850
|
Prereduced free iron+2wt%O2
94% Fe3O4 balance:CaO,Al2O3, K2O
|
5-10
|
Catalyst promoters:Ca,Al,K
Not pyrophoric at ambient temp.
|
2.2 DESULPHURISATION CATALYST (NICKEL- MOLYBDENUM CATALYST)
The natural gas feed stock supplied to NFCL
contains no H2S, but it is anticipated that future supplies may contain sulphur
compounds which have to be removed in order not to poison the reforming catalysts
and the LT shift catalyst. Natural Gas from battery limit is heated to 385
deg.C in the Feed stock preheater F-203, and is passed through the Hdrogenator.
A bed of Nickel-Molybdenum catalyst is provided to catalyse the hydrogenation of
organic sulphur compounds to hydrogen sulphide. There are two types of organic sulphur
compounds that may be present in the feed stock. One is called 'Normal Sulphur' containing
H2S, COS, CS2 and Mercaptans and the other is called 'Less Reactive Sulphur',
containing Thiophenes, Thioethers etc.
In case of normal sulphur except Mercaptan Hydrogen recycle gas is not
consumed where as for less reactive sulphur, recycle hydrogen is consumed as
per the following hydrogenation reactions:
RSH
+ H2 RH + H2S
(Mercaptans)
R1SR + 2H2
RH + R1H + H2S
(Thioethers)
R1SSR + 3H2 RH + R1H + 2H2S
(Thiophenes)
If sulphur
is present, natural gas is mixed with recycle gas from synthesis gas compressor
first stage discharge with flow of recycle gas around 1306 NM3/hr., in order to
avoid Carbon deposition on the catalyst due to catalytic cracking of higher
hydrocarbons if any. After preheating to
385 deg.C,the gas mixture passes to Hydrogenator Reactor R-201 and reacts to
produce H2S. The above reactions are
exothermic but insignificant (which depends on the type of Sulphur that determines the number of moles
of hydrogen taken up). H2S produced in
R-201 and that already present in Natural Gas is then removed in H2S Absorbers
R-202 A/B, thereby the gas will be free of H2S.
Each absorber contains one bed of Zno
catalyst to absorb the sulphur. The
absorbers are operating in series with the second vessel acting as guard. When the Zno in the first vessel is getting
exhausted, a break through of H2S from the first vessel may be observed. The operation will then continue with the second
vessel in service, while the first vessel is being reloaded with fresh
catalyst. The sulphur content at the
exit of R-202B shall be less than 0.1 ppm on dry volume basis at
all times which is tolerant to reforming catalyst.
The sulphur removal reaction in Zno bed
takes place as follows:
Zno +
H2S
ZnS + H2O Zno + COS ZnS + CO2
Zno reaction with 'S' depend on :
1. Type
of sulphur compounds.
2.
Temperature: Increase in
temperature will generally increase the ability of Zno to remove sulphur.
3.
Capacity : As Zno reacts with sulphur it gets saturated
with sulphur and looses its activity. Normal life of Zno catalyst depends on the
H2S and sulphur concentration in the natural gas.
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
R-201
|
TK-251
|
5X2.5
|
8.44
|
480
|
2220
|
NiO:2-3%
MoO:10%
|
>5
|
R-202A
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
|
R-202B
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
Guard
|
(GENERAL2.3 REFORMING
SECTION)
STEAM REFORMING
CATALYST
Steam
reforming is a vital part of the front end in plants producing Ammonia. Developments in metallurgy have allowed steam
reformers to be operated at higher and higher levels of temperatures, pressures
and heat flux. The reforming process and
the design of the reformer are based on the reaction between methane and higher
hydrocarbons present in the natural gas with steam thereby generating CO, CO2
and Hydrogen. The Hydrogen produced by
the Methane reforming reaction is used to produce Ammonia by combining with
Nitrogen in the ratio of 3 : 1 which is the stoichiometric ratio of hydrogen
and nitrogen to produce ammonia.
The most
important reactions which are taking place in the reformer are
CH4
+ H2O CO
+ 3H2
CO
+ H2O CO2
+ H2
These reactions are taking place in
the presence of a nickel-based
catalyst.
The operating parameters maintained close to the equilibrium, are 769 deg. C
and 30.5 Kg/cm2g and the maximum tube wall skin temperature allowable is 880
deg.C. The methane concentration at the
exit of the reformer is 14.03 %. The
efficient operation of the reformer at the above conditions will give rise to a
methane leakage at the exit of secondary reformer of 0.6%. The higher slippage of methane has been
designed taking advantage of P.G.R. unit incorporated. In addition to the recovery of hydrogen from
the purge gas, there is energy saving due to the lower heat flux required in
the reformer resulting in reduced firing, there by increasing the life of
catalyst and reformer tubes.
PRIMARY REFORMING
Raw synthesis gas is produced by reforming
natural gas to an intermediate level in the primary reformer F-201 using super
heated HS steam in presence of a Nickel based catalyst at 33 Kg/cm2g
pressure. The hot desulphurised natural
gas and recycle gas mixture from R-202B outlet is combined with HS steam (37
Kg/cm2g 370 deg.C) to give a steam to carbon mole ratio of 3.3 : 1.0. A
small quantity of condensate from P-353 A/B containing small
percentage of Methanol and Ammonia is also mixed with steam for subsequent
recovery of H2 and to avoid pollution. The combined steam-natural gas-recycle
gas mixture (Mixed feed) is preheated to 520 deg.C in the process gas
convection coil E-201 located in the waste heat recovery section of the primary
reformer F-201 utilising the heat from the hot flue gases, leaving the reformer
radiant section.
Following preheat, the gases are
distributed through hairpin tubes into vertical reformer tubes filled with Nickel catalyst.
The tubes are placed inside a Furnace, where sensible heat and endothermic
heat of reaction are absorbed in the tubes by radiation from a number of wall
burners to the tubes. The primary
reforming of natural gas is done in a Topsoe design side fired furnace, in
comparison to the top fired furnace, where the maximum heat input is
concentrated in the top part of the furnace.
In the top fired furnace during start-up conditions with low flow,
little or no heat of reaction in the tubes, the maximum temperatures may well
be found at the level of flames. In such
furnaces, higher than desirable temperatures may be present in the top part of
the tubes even when the outlet temperature is not higher than the level
recommended.
In the upper part of the top fired
reformer, where the methane concentration is high and hydrogen concentration is
low, the potential for carbon formation is present. Due to the radial temperature and
concentration gradient in the tube, the risk zone extends somewhat down along
the hot tube wall. If this zone reaches
a temperature level where the rate of the cracking reaction becomes
sufficiently high, carbon formation will take place resulting in a "hot
band". Top fired furnaces are more
prone to this kind of problem. The above disadvantages of using top fired
furnace are eliminated by using the side-fired furnaces. In case of side fired furnace, the reformer
outlet temperature increases gradually from the top towards bottom. The tube skin temperature along the length of
the tube can be better controlled in side fired furnace and more over, the
potential for carbon formation with the age of the catalyst, the possibility of
higher tube skin temperature at the bottom than from the top, is better
controlled using side fired furnace.
The primary reformer furnace
consists of 190 tubes, inserted in two parallel chambers called the radiant
zone. Each chamber has got 95 tubes in a
single row. Each row has been divided
into 5 sections. Each section has got 19
tubes. Reformer tube outlets from both
chambers are connected to hot collectors through pig tails, which are placed
outside the Primary Reformer radiant zone.
Hot collectors are again connected with cold collector. The furnace operates with side firing of fuel
gas on both sides of each row of tubes to develop a process gas temperature of
about 769 deg.C at the catalyst tube outlet.
There are 360 side-fired burners arranged in 6
rows per wall. Each row is having 15
burners. The type of primary reformer
burners is of LP Radial Burner, which is of rugged construction. Natural gas pressure reduced to 3 Kg/cm2g at
Offsite is used as fuel for the LP Radial Burners. The flame shape should be flat against the
heated Muffle Block surrounding the air nozzle.
The excess air shall be 10%. The Reformer is loaded with 20.05 M3 of R-67-7H
Nickel based unreduced catalyst at bottom and 6.68 M3 of R-67R-7H
Nickel based pre- reduced catalyst at top, both cylindrical having
seven holes with OD 16 mm and height 11 mm.
Inside the catalyst tubes, the natural gas steam reforming reaction
takes place.
REFORMING REACTIONS
The following reactions take place
simultaneously, producing a mixture of H2, CO, CO2, CH4 and excess H20 when
hydrocarbons undergo steam reforming over Nickel catalyst :
Cn
Hm +
2n H2O
n CO2 + (2n+m/2) H2 - Heat (1)
CH4
+ 2 H2O
CO2 + 4H2 - Heat (2)
CH4
+ H2O CO
+ 3H2 - Heat (3) CO2
+ H2 CO
+ H20 - Heat (4)
Reactions start at 500 deg. C for the
higher hydrocarbons and 600 deg. C for methane.
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
Features
|
F-201
|
Topsoe
R-67-R-7H
Prereduced
|
16X11
cylinder with 7 holes
|
6.68
|
960
|
NiO:16-18%
SiO2:<0.1%
MgAl2O4
|
3-5 Years
|
Carrier surface area:12-20m2/gm
|
|
R-67-7H
|
16X11 cylinder with 7 holes
|
20.05
|
970
|
NiO:16-18%
SiO2<0.1%
MgAl2O4
|
3-5 Years
|
Ni surface area:3.5 to 5
m2/gm
|
REFORMING
VARIABLES
Reaction (3) requires small amount heat
only, whereas the heat required for 1 and 2 dominate the picture.
TEMPERATURE
The effect of increasing reforming temperature
on the effluent gas composition is to reduce the methane and carbon dioxide
content. On decreasing reforming
temperature, the effects are reversed.
On decreasing temperature, there is a theoretical risk of carbon
formation according to the Boudard reaction when the gas is cooled.
2 CO CO2 +
C (Soot)
Under the chosen conditions, the above
reaction can only take place below 750 deg.C because of the equilibrium
conditions and above 650 deg. C because the soot formation reaction rate is too
slow to have any practical importance below 650 deg.C.
STEAM/CARBON RATIO
The unit operation is most economical at
conditions closely approaching the design steam to carbon mole ratio of 3.3 :
1.0. However, increasing steam to carbon
ratio will shift the above equilibrium reactions to the right with a net effect
of decreased methane and carbon dioxide and increased carbon monoxide and
hydrogen in the reformed gas.
PRESSURE
As the
reforming reaction proceeds with an increase in the number of moles, a higher
pressure will drive the reaction in the undesired direction, resulting in a
higher methane content at the reformer exit, if other factors remain unchanged.
Inspite of the above effect, economy of design has required reforming pressure
to be increased in order to save on synthesis gas compression costs. Also
higher-pressure results in the additional advantage of making the heat of
condensation of gases exit the L.T. Co
Converter available at higher temperature thereby facilitating the recovery of
this heat by B.F.W. preheating.
Hence,
reforming pressure is fixed by an optimal balance between the reaction
equilibrium on one hand and compression power and heat recovery on the other.
Equally important is the pressure drop across the reformer tubes. An increase in pressure drop indicates
possible catalyst fouling or partial blockage of tubes due to some other
reason.
CARBON FORMATION(THE CATALYST DEACTIVATOR)
In
the operation of the primary reformer carbon may be formed partly outside the
catalyst, partly inside the catalyst.
Carbon deposits outside the particle will increase the pressure drop
over the catalyst bed and deposits inside the particles will reduce their
activity and their mechanical strength.
Thermodynamically carbon formation is not possible under the conditions
foreseen, if equilibrium is obtained for each step. If the catalyst, however, is poisoned,
e.g. by sulphur, it will loose its activity and carbon formation is likely to
occur. At very low steam to
carbon ratio, there will be a possibility of carbon formation, which would
result in carbon deposits, especially inside the catalyst particles. If the catalyst is insufficiently reduced, or
if it is partly oxidised during production upsets, without subsequent
reduction, carbon formation may take place.
Carbon deposition will hinder reforming and reduce heat transfer so that
the tube wall temperature will rise in that zone producing 'hot bands' and
subsequently 'hot tubes'. Precautions
should be taken to prevent carbon formation on reforming catalyst for
successful reformer operations.
FLUE GAS SYSTEM
The
reformer furnace is designed to obtain maximum thermal efficiency by recovering
heat from the flue gases leaving the reformer radiant section.The hot flue gas
from top at 980 deg. C passes through downward and horizontal flue gas duct. The desired draft of 375 MMWC is induced by
the flue gas blower, K-201. The flue
gases enter the waste heat recovery section and give up heat successively to
the various coils. At the outlet, the flue gas temperature is reduced to
approx. 170 deg. C as any further reduction in temperature may result in
condensation of sulphur compounds if any present in the flue gas.
COMBUSTION AIR
Combustion
air to the forced draught radiant burners of primary reformer and auxiliary
steam superheater is supplied by Combustion Air Blower, K-202 after preheating
to 293 deg.C in combustion air preheater E-204 by recovering sensible heat of
the flue gases.
SECONDARY REFORMING
The
partially reformed gas exit Primary Reformer contains 14.03 mole percent of CH4
(dry basis). The methane content is
further reduced to 0.6 mole percent (dry basis) at high temperature in the
secondary reforming step. In the
Secondary Reformer, R-203, the heat is supplied by combustion of part of the
gas achieved by mixing air into the gas as
compared to the indirect heat by firing in the Primary Reformer. This combustion provides heat for the rest of
the reforming in R-203. The methane slip
exit Primary reformer is so adjusted that the process air supplying the
reaction heat in the Secondary Reformer will give the Hydrogen/ nitrogen ratio
of 3:1 in the syn. gas. It is desirable to reduce the methane content of the
process gas to a low level in order to keep the level of inert gases low. The methane content exit R-203 is dependent
upon the methane slip at the Primary Reformer outlet at specified
conditions. The high CH4 slip at F-201
outlet gives rise to CH4 slip of 0.6 mole percent (dry) at R-203 outlet at 943
deg.C. Since air quantity is fixed when PGR Unit is running, the H2:N2 ratio in
the make up gas is 2.78 which gives rise to 3:1 at Ammonia Synthesis Converter
inlet by recovering H2 from PGR. The
inert concentration is maintained at 8% in Synthesis loop at Converter inlet.
The
partially reformed gas from primary reformer is directed to the refractory lined
Secondary Reformer R-203 at 769 deg.C and 31 Kg/cm2g. Process air supplied by
Process Air Compressor K-421 at 33 Kg/cm2g and 177 deg.C is preheated to 550
deg. C in E-202A/B coils located in convection section of F-201, passes
vertically downward through the centrally located Air Mixer to the Secondary
Reformer R-203. Instantaneous mixing and
rapid combustion of part of the partially reformed gas takes place with air in
the upper empty space of R-203, resulting in a sharp rise of temperature to
about 1200 deg. C. This combustion provides heat for the rest of the
reforming. From the empty space, the gas
passes down the Nickel catalyst bed
where the reforming reaction is completed with simultaneous cooling of the
gas. The outlet temperature will be about
943 deg. C and the methane concentration will be approx. 0.6 mole percent. In the combustion zone of Secondary Reformer,
the following reaction takes place between process gas and air and O2 gets
completely consumed.
2H2 + 02 2H20 +
Heat (1)
CH4 + 202 CO2 +
2H20 + Heat
(2)
Reaction (1) is
predominant.
In the catalyst bed methane-reforming
reaction takes place as follows :
CH4 + 2 H2O CO2 + 4H2 - Heat
CO2 + H2 CO +
H20 - Heat
(Shift
Conversion)
The catalyst in the secondary reformer
comprises of a layer of alumina balls and alumina guard tiles at the top and 30
M3 Nickel based catalyst in the middle.
A layer of electrofused alumina lumps are also provided at the bottom
over the refractory dome as catalyst support.
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
|
R-203
|
RKS-2-7H
|
20X18
|
30.0
|
950
|
2800
|
MoO:9.0%
|
>10
|
HEAT RECOVERY FROM REFORMED GAS
Reformed gas with unreacted process steam at
943 deg.C from the bottom of Secondary Reformer R-203 passes through the tube
side of special type of Waste Heat Boiler E-208. This Waste Heat Boiler consists of two
compartments, which is castable refractory lined. The total gas is passed through tube side of
the first compartment where as the second compartment has been provided with
internal bypass to control exit temperature.
A temperature controller regulates flow through the bypass to maintain
HT Shift Converter inlet temperature at about 360 deg.C. Sensible heat of the process gas exit R-203
is utilised to generate KS steam in waste heat boiler. Boiler feed water from steam drum is fed to
the shell side of E-208 through number of down comers and steam/water produced
are sent back to the steam drum through number of risers by thermo-syphoning. During start up, the drum pressure is
controlled to maintain R-204 inlet temperature and by bypassing the second
compartment of E-208 through temperature controller.
2.4 CO-CONVERSION CATALYST
Carbon
monoxide present in the reformed gas is converted to CO2 in two shift
converters R-204 and R-205. The
following reaction is taking place in the shift converters R-204 and R-205:
CO + H2O CO2 + H2 + Heat
The
exit gas from R-205 will contain only about 0.22 mole percent CO, thereby
increasing the yield of H2.The above mentioned shift reaction taking place in
the converters R-204 and R-205 will only proceed in contact with a
catalyst. The equilibrium is favoured by
lower temperatures and high steam to gas ratio, while the reaction rate will be
higher at higher temperatures. More
steam to gas ratio may give an apparently lower conversion due to the larger
total volume resulting in a shorter contact time. This means that for each catalyst there
will be an optimum temperature, depending on the activity and the quantity,
which will give optimum conversion.
As the reaction results in a temperature rise, the outlet gas will be at
an unfavourable equilibrium if removal of heat has not taken place before the
conversion is completed. Thus the
conversion is performed in two steps.
The first step takes place in the HT shift converter R-204 where a copper promoted Iron Oxide Catalyst is
installed. The major part of the conversion takes place in R-204, causing
a temperature rise of 64 deg.C. The outlet temperature is about 424 deg.C and
outlet co-concentration is 2.68% which
is fully acceptable for a conventional catalyst being more rugged than the low
temperature catalyst used in the second step of the shift conversion. The low temperature catalyst consists of
specially prepared copper, zinc and aluminium oxides having a much higher
activity, which means that it can be used at the lower temperatures of 200
deg.C at inlet and 218 deg.C outlet.
The inlet temperature is fixed taking into consideration the dew point
of the gas mixture. The catalyst is
less rugged and loses it activity if the temperatures are higher than 250 to
270 deg. C.
Here the CO content is further reduced to
about 0.22 mole % (Dry Basis)
HIGH TEMPERATURE SHIFT CONVERSION CATALYST
The
HT shift converter R-204 contains 70 M3
copper promoted iron oxide catalyst.
The reformed gas enters the HT shift converter at 360 deg. C and
29.8 Kg/cm2g and flows through the
catalyst bed. The outlet temperature is
424 deg. C. At the main start-up of the plant, the catalyst must be activated,
which is mainly a reduction of Fe203 to Fe304.
The reduction will take place at a temperature above 250 deg.C, but the
temperature should not be allowed to exceed 400 deg. C during the reduction in
order not to decrease the activity of the catalyst. When new, the catalyst can operate at a gas
inlet temperature of 350 deg. C.
Afterwards, the optimum inlet temperature will be higher, but as long as
the outlet temperature has not reached 470 deg. C, the activity will only
decrease slowly.
The
cold catalyst can be heated by steam alone, both when oxidised and
reduced. Drops of liquid water on the
hot catalyst may result in disintegration of the catalyst. The catalyst is very sensitive to salts,
which may be introduced with the steam.
The content of chlorine in the gas should be well below 0.1 ppm. The catalyst is not influenced by sulphur in
the quantities present in this plant. The
fresh catalyst contains, however, about 0.3% sulphate which will be given off
as H2S during the first week of operation. Normally the catalyst is not
oxidized by steam alone, but should be
oxidized by adding a small amount of air to the steam before it is accessible
as it is pyrophoric in reduced state.
 |
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
Features
|
R-204
|
SK-201
|
6X6
|
70.0
|
1200
|
4400
|
>5
|
Cu promoted
|
HEAT RECOVERY FROM HT SHIFTED GAS
The final
shift reaction is completed in Low temperature shift converter R-205. The gas leaving HTS is cooled to 200 deg.C
before entering LT shift converter by recovering waste heat successively in
Waste Heat Boiler after Co-converter E-210 and BFW Preheater E-211A/B and Trim
Heater E-209. In E-210, KS steam is
generated while cooling the gas to 340 deg.C.
Part of gas is sent to the Trim Heater E-209 to preheat the methanator
feed inlet gas partly. The gas then
passes through the shell side of E-211A/B and gets cooled to 200 deg.C. There is a bypass of E-211A/B for controlling
LT Shift Converter gas at desired inlet
temperature.
LOW TEMPERATURE SHIFT CONVERTER :
The
LT Shift Converter R-205 contains 80.3
M3 of the catalyst consisting of oxides of copper, zinc and aluminium. As the catalyst is extremely sensitive to
sulphur which may be liberated not only from the preceding HT shift catalyst
but also from secondary reforer refractory material, the LT shift converter is bypassed during initial stage until
the gas is practically sulphur free. The
chlorine may be present in process steam and quench water, due to maloperation
of the water treatment system and process air due to atmospheric air pollution
in very small amounts. Besides chlorides
and sulphur, gaseous Si - compounds are also catalyst poisions. When the catalyst is in a reduced state,
temperatures above 250 deg. C must normally be avoided. A short exposure to 300 deg.C. will have no
adverse effect on the catalyst. Normal
operation should take place at as low a temperature as possible.However, at
temperatures near the dewpoint, the activity will decrease because of capillary condensation of
water inside the catalyst, thus reducing the free area. During operation, the temperature should, therefore,
be kept at least 20 deg. C above the dewpoint of the gas. The reduced catalyst
is pyrophoric and has to be oxidized before opening of the converter.The normal
operating temperature is between 200 deg.C and 218 deg. C. The actual
temperature of the inlet gas to R-205 to be selected is dependent on the
activity of the catalyst.
 |
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
|
R-205
|
LSK
LK-821
|
4.5X4.5
4.3X3.2
|
80.3
|
1100
1000
|
4670
|
CuO,ZnO,Cr2O3
CuO,ZnO,Al2O3
|
>5
|
2.5 CO2 REMOVAL SECTION :
This
unit provides process gas free of CO2 (limit 1000 ppm) for the production of
ammonia and necessary CO2 for Urea
production. In this unit, CO2 in
the process gas is absorbed by the GV solution in the Absorber, C-301 thus
providing process gas with less than 1000 ppm of CO2. Stripping of the absorbed CO2 is done in the
two regenerators and CO2 stripped is supplied to Urea Plant. CO2 removal section know how is by
Giammarco-Vetrocoke of Italy. The Vetrocoke solution consists of K2 CO3,
Vanadium Pentoxide, Glycine and DEA
where V2O5 (Vanadium Pentoxide) is the corrosion inhibitor and glycine/DEA are
the activators. The chemistry involved
in this unit is chemisorption and is explained as follows :
CO2 +
H2O
HCO3- + H+
(1)
K2CO3 +
HCO3- + H+ 2KH CO3 (2)
--------------------------------------------------------------------------------
K2CO3 +
CO2 + H2O 2KH CO3 (3)
The
reaction rate of (3) depends on the reaction rates of (1) and (2). Reaction rate of (1) is slow and the
activator activates this reaction by quickly introducing the gaseous CO2 in the
liquid phase. The activator glycine
reacts with CO2 and forms glycine carbonate according to the reaction.
NH2 CH2 COO- +
CO2
COO-NH CH2 COO- + H+
(4) COO-NH CH2 COO- +
H2O NH2 CH2
COO- +
HCO3- (5)
The sum of (4) and (5) gives (1).
In
solution regeneration, reaction (3) is reversed by application of heat and
pressure reduction and the lean and semilean K2 CO3 solution is recirculated
for further absorption of CO2. The
process gas from V-208 enters the CO2 removal section at 27.5 Kg/cm2g and 165
deg.C and passes through the reboilers and LP Boiler E-302 and then to E-
306A/B (DM Water Heater) getting cooled down to 113.5 deg. C and condensate is
seprated in V-301 before entering the Absorber.
The process gas enters the tube side
of E-301A/B giving its heat energy to the GV solution at the shell side of
E-301A/B. The solution from the bottom
tray of C-302 (Regenerator under pressure) circulates through the reboiler by
thermal siphoning. The CO2 and H2O
vapour along with solution enters C-302 bottom below the bottom tray and serves
as stripping medium. The heat energy
released in E-302 shell is used to produce LS steam which is boosted into C-302
through the ejectors L-301A/B. The
outlet gas temperature of E-302 is 126.5 deg.C.
The gas outlet from E-302 is further cooled in DM water preheaters
E-306A/B. The gas is cooled down to
113.5 deg. C. The resulting condensate
in the process gas is separated in V-301 before entering the CO2 absorber.
In the CO2
Absorber C-301 process gas flows upwards counter current to the solution flow
(the solution is the regenerated GV
solution from C-303). Semi-lean solution
pumps P-302A/B/C takes suction from the take off tray below the packing of
C-303 and pumps the solution to the middle of Absorber as semilean solution at
106 deg.C . Lean solution pumps P-301A/B
takes suction from the bottom of C-303 through the cooler E-303 and pumps the
solution to the top of the absorber as lean solution. E-303 cools the solution from 109 deg.C to 65
deg.C and in turn heats DM water from 40 deg.C to 104 deg.C. The make up condensate to CO2 removal system
is added at the suction of P-301A/B pumps at 59 deg.C to maintain the water
balance in the system.
At the bottom of absorbers C-301
where the bulk of CO2 is absorbed, the high temperature improves the reaction
rate for reaction No. (3) and for reaction No. (5) according to which the CO2
is absorbed by K2 CO3. In the top part
of the absorber, the lower temperature reduces the CO2 vapour pressure in the
solution thereby minimising the CO2 content in the process gas. This is made possible by keeping the reaction
rate (5) sufficiently high even at this lower temperature by the OH
concentrations in the lean solution fed at the top.
Solution regeneration is carried out
at two pressure levels, one at 1.04 Kg/cm2g and other at 0.1 Kg/cm2g for better
utilization of stripping steam compared to the usual technique in which great
part of the stripping steam exits the regenerator top as unused excess. The pressure in regenerator C-302 is
regulated to obtain a temperature increase between the solution inlet and
outlet of the regenerator in order to condense the above mentioned excess
steam. The heat stored in the rich GV
solution exit the regenerator C-302, is recovered as flash steam which has been
experimentally verified to be practically pure steam.
From C-302 top is taken off a rich
solution stream at 106.5 deg.C that feeds Regenerator at low pressure
C-303. In C-303 the flashed steam
regenerates the rich solution stream taken off from C-302 top. The liquid levels at the bottom of C-303 and
at the take off tray are maintained by controlling the flow of lean and
semilean solution from C-302. The lean
solution from the bottom of C-303 at 109 deg.C gets cooled in E-303 and is
pumped by lean solution pumps P-301A/B at 65 deg.C to the top of C-301. From the take off tray of C-303 the solution
goes to the Semilean pumps P-302A/B/C at 106 deg.C to be pumped to middle of
C-301.
The
acid gas stream from the top of the Regenerator C-302 is cooled in the DM water
preheater E-307 from 102 deg.C to 96 deg.C at 1.04 Kg/cm2g pressure. C-302 pressure is maintained by PIC-015. The vapour condensed is removed in V-304 (OH
condensate separator). The acid gas
stream outletting the Regenerator C-303 at 94 deg.C and 0.1 Kg/cm2g is cooled
in the O/H DM water heater E-308 to 91 deg.C and the vapour condensed is
removed in the C-303 1st O/H condensate separator V-305. C-303 pressure is maintained by PIC-001. Again the acid gas is cooled in the
condensers E-304A/B to 40 deg.C by cooling water. The vapour condensed is separated in the
C-303 2nd OH condensate separator V-302.
The CO2 is fed to the Booster compressor K-301 or it can be vented to
atmosphere through PIC-026. K-301 boosts
the pressure of CO2 from 0.1 Kg/cm2g to 0.96 Kg/cm2g at 96 deg.C. The discharge of Booster compressor joins the
stream of CO2 from C-302 at the outlet of V-304 and the mixed stream gets
cooled in the final OH condensers E-305A/B from 102 deg.C to 40 deg.C by
cooling water. The water vapour
condensed is removed in final OH condensate separator V-303 and the CO2 saturated
with water flows to Urea Plant. The
ammonia Plant battery limit conditions for the CO2 sent to Urea Plant are 0.6
Kg/cm2g and 40 deg.C.
The use of compressor K-301 on a
very limited acid gas stream allows to utilise in the most advantageous way,
the two pressure levels regeneration technique, since it allows to keep C-303
pressure at a lower level, thereby increasing the flashing steam of the
solution coming from C-302 with evident energy saving. At the same time it
allows to obtain all CO2 for Urea production at higher pressure.
The condensate separated out at
V-304 and V-303 flows to V-305 and V-302 respectively under pressure where as
condensate from V-305 and V-302 are pumped out by P-304 and P-305 condensate
pumps respectively as make up to CO2 removal section and balance as process
condensate to stripping unit. There are
two numbers lean solution pumps (P-301A/B) one steam turbine driven and the
other motor driven. Out of three
semilean solution pumps (P-302A/B/C), two are steam turbine driven and the
other motor driven.
Two
hydraulic turbines (DPTP-302 A/B) are connected to the turbine driven semilean
solution pumps P-302A/B through auto clutch.
The letdown turbines sends the rich solution from Absorber bottom which
is at a pressure of 27.5 Kg/cm2g to the Regenerator C-302 which is at a
pressure of 1.04 Kg/cm2g. The discharge
side pressure of hydraulic turbine will be about 9 Kg/cm2g. The differential pressure 18.5 Kg/cm2g is
utilised to drive the semilean solution pumps.
This pressure energy approximately amounts to a power of 215 KW in each
hydraulic turbine thus energy on steam driven turbines DSTP-302A/B is conserved
to an extent of 215 KW on each turbine, by clutching Hydraulic turbine to the
Semilean solution pumps.
2.6 METHANATION CATALYST
As CO and CO2 are poisons to the Ammonia
converter catalyst, the unconverted CO and unabsorbed CO2 in the process gas
are reduced to a limit of less than 10 ppm by methanation reaction. In the process, CO and CO2 get converted to
CH4 which is an inert in the synthesis of ammonia. In the Methanator R-301, the reverse of
reforming reaction takes place in presence of Nickel catalyst.
The reactions are as follows :
CO + 3
H2 CH4 + H2O +
Heat
CO2 + 4
H2 CH4 + 2
H2O +
Heat
The
main reason why the reaction is reversed is the lower temperature favouring
formation of methane. Other critical
variables governing the reactions are pressure and steam content. However, within the allowable temperature
range, the equilibrium conditions are so favourable that practically only the
catalyst activity determines the efficiency of the methanation. The higher the temperature, the better the
efficiency, but at the same time it means a shorter life time for the catalyst.
Further more in case of a possible
break through of CO2 and CO to the methanator which would result in a higher
temperature rise, a low inlet temperature is preferred as this limits the
temperature rise. After the methanator
the gas normally contains 10 ppm of CO + CO2. The temperature rise of gas in
methanator will normally be about 21 deg.C. Methanator contains 26 M3 of catalyst
and has approximately the same characteristics as of reformer, being nickel
catalyst on a ceramic base. As the
reactions take place at much lower temperature than those prevailing in the
reformers, the catalyst must be very active at low temperatu- res. The catalyst is sensitive to Arsenic, Sulphur and Chloride
Compounds. The adiabatic temperature
rise per mole % of CO is 74 deg. C and per mole% of CO2 is 60 deg. C. The methanation reaction starts at a
temperature of about 240 deg. C but in order to ensure a sufficiently low
concentration of CO and CO2 in the effluent gas, the operating temperature
would be from 280 deg. C to 350 deg.C, depending on the catalyst activity and
gas composition. The methanator catalyst
should not be exposed to catalyst temperature above 440 deg. C for longer periods
of time as it will damage the vessel R-301.
Washed gas from CO2 removal (outlet
of V-314) goes to E-311 Gas/Gas exchanger at 26.8 Kg/cm2g and 65 deg. C and
gets heated upto the inlet temperature of 310 deg. C by exchange of heat with
hot methanator effluent gas. A part of
the outlet gas from E-311A/B passes through
E-209 and gets heated up with R-204 outlet gas and joins at the inlet of
methanator (resultant temperature is 320 deg. C.. The outlet gas from methanator at 341 deg.C
gets cooled in the Gas/Gas exchanger E-311A/B to 87.3 deg.C and is further
cooled down in E-312A/B (final gas cooler) to 41 deg. C by cooling water and
the condensate formed is
separated in the final gas
separator V-311. The pure synthesis gas enters syn. gas compressor suction at a
pressure of 25.1 Kg/cm2g and 41 deg.
C. Surplus syn. gas is taken out and
sent to Auxiliary boiler to be used as fuel.
In case of synthesis gas compressor trip, or prior to the start-up of
synthesis gas compressor, gas is vented at PIC-074 and PIC-071 and thus front
end pressure is maintained. From the
outlet of V-311, the process gas is also taken as recycle H2 to header to feed
K-204 (recycle gas compressor). This
recycle H2 line is provided with HIC-003, which controls the recycle H2 flow to
header. This valve is connected to the
Methanation trip signal IS-6 which closes HV-003 on trip signal of IS-6.
 |
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
|
R-301
|
PK-5
|
5
|
26.0
|
>10
|
NICKEL CARBONYL GAS
Nickel
carbonyl gas is a poisonous and toxic gas which may be present in R-301. Under certain conditions, CO in the process gas reacts with the
catalyst Ni to form Nickel carbonyl gas.
4 CO
+ Ni Ni (CO)4
The favourable temperature range of the
formation of this gas is between 45 deg.C and 205 deg.C. Hence R-301 catalyst should never be allowed
to cool in the presence of CO containing gas.
Rather it should be purged out with N2 at the time of shut down.While
heating up the catalyst with the process gas containing CO, heating should be
done faster in the range of 45 deg. C to 205 deg. C.
2.8 AMMONIA SYNTHESIS CATALYST
The
Ammonia Synthesis takes place in the Ammonia converter R- 501 as per the
following reaction.
N2 + 3H2
Iron Catalyst 2NH3 + Heat
The
reaction is limited by the equilibrium concentration and only part of the
Hydrogen and Nitrogen can be converted into Ammonia per pass through the
Catalyst bed . The equilibrium
concentration of Ammonia is favoured by high pressure and low temperature. However, reaction rate is very much enhanced
by high temperature operations. There is
a compromise between thermodynamic equilibrium & reaction kinetics. As a result there is an optimum level for the
Catalyst temperatures at which the maximum production is obtained. At higher temperatures the equilibrium
percentage (which is the theoritically highest obtainable concentration of
Ammonia) will be too low while at lower temperature the reaction rate will be
too low. The Synthesis loop is designed
for a miaximum pressure of 155 Kg/cm2g and the normal operating pressure is in
the range of 131-141 Kg/cm2g. The
reaction temperature in the catalyst bed is 360 to 520 deg. C which is close to
the optimum level. The catalyst is a
promoted iron catalyst containing small amounts of non-reducible Oxides. A considerable amount of heat is liberated by
the reaction (about 750 Kcal/Kg of Ammonia produced), and this is utilized for
production of KS steam and for preheating Boiler feed water. Only about 20% of the Hydrogen and Nitrogen
flow contained in the Synthesis gas at converter inlet is converted into
Ammonia per pass, and it is therefore necessary to recycle the unconverted
synthesis gas to the converter. The
Ammonia Converter, R-501 is a Topsoe series 200, Radial Type converter with the
gas flowing through the two catalyst beds in RAdial direction. The advantage of the Radial flow converter is
that the pressure drop is less. The
catalytic activity of small particles is very high and the special advantage of
the radial converter is to allow the use of small catalyst particles without a
prohibitive pressure drop.
The converter contains two catalyst beds
with interbed cooling after 1st bed.
There is also a provision of cold shot injection for better control of
bed temperatures. A total of 96 M3
Catalyst of type Topsoe KMI/KMIR is used.
The first bed has a volume of 28 M3 of KMIR Catalyst and the 2nd bed
contains 68 M3 of KMI Catalyst. The KMIR
Catalyst is the pre-reduced and stabilized catalyst of KMI type. Stabilisation involves skin Oxidation of the
Catalyst where it takes-up an amount of 2% (Wt) of Oxygen. This prereduced catalyst is stable in air
below 100 deg.C. Above 100 deg. C it
will react with air and spontaneously heats up.
The catalyst is activated by reducing Iron-oxide to free Iron. This reduction is carried out with
circulating Synthesis gas. The Catalyst
activity will decrease slowly during normal operation and the lifetime of
Catalyst is 8 to 10 years. This is again
influenced by the actual process conditions. notably the temperature in the
Catalyst bed and the concentrations of Catalyst poisons in the Synthesis gas at
converter inlet. Sulphur compounds and compounds containing
Oxygen such as water (H2O), Carbon Monoxide (CO) and Carbon dioxide (CO2) are
all poisons to the Catalyst and small amounts of the catalyst poisons will
cause a considerable decrease in Catalyst activity. Part of the poisoning effect is only
temporary and catalyst activity will recover somewhat when the gas is clean
again. A certain permanent decrease in
the Catalyst activity will however remain and high concentrations of Oxygen
compounds at converter inlet even for
short duration should therefore be avoided.
PROCESS CONDITIONS
Ammonia Synthesis reaction is affected by the
following parameters :
- Ammonia content in the feed gas
- Inert
gas content in the feed gas
- H2 to N2 ratio in the feed gas
- Reaction temperature
- Circulation Rate
- Operating pressure
- Catalyst activity
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
Features
|
R-501
|
KMR (pre-reduced)
KM
|
1.5-3.0
1.5
|
26.9
68.4
|
2220
2850
|
Prereduced free iron+2wt%O2
94% Fe3O4 balance:CaO,Al2O3, K2O
|
5-10
|
Catalyst promoters:Ca,Al,K
Not pyrophoric at ambient temp.
|
AMMONIA CONTENT IN THE FEED GAS
A
low Ammonia concentration at converter inlet gives a high reaction rate and
thus a high production capacity.The Ammonia concentration at converter inlet is
dependent on the cooling level in the refrigeration chillers and the operating
pressure.4.1% NH3 at converter inlet corresponds to -5 deg.C at a pressure of
132 Kg/cm2g in the Ammonia Separator, V-501.
INERT GASES
The
Makeup gas contains 1.33% (Vol.) of argon and methane. These gases are inerts in the sense that they
pass through the Synthesis converter without undergoing any Chemical
changes. But a high concentration of
inerts reduces the partial pressures of Hydrogen and Nitrogen thereby reducing
the conversion. Therefore a constant
purge of gas from the loop is maintained to keep the inerts level in the
converter inlet at about 8%. The
catalyst activity decreases with the catalyst age. This can be compensated by
either increasing the loop pressure and the circulation rate or by decreasing
the inert level.
HYDROGEN/NITROGEN RATIO
By
the Synthesis reaction, 3 volumes of Hydrogen react with 1 volume of Nitrogen
to form 2 volumes of Ammonia. Therefore
the H2/N2 ratio in the loop and makeup gas must be close to 3:1. A small change in H2/N2 ratio of the make up
gas will result in a much bigger change in the H2/N2 ratio of the circulating
Synthesis gas. The H2/N2 ratio of the
makeup gas should normally be about 2.78
so that after addition of recovered hydrogen from PGR Unit the ratio will be
about 3.0. The Synthesis loop is
designed for operating at the H2/N2 ratio of 3.0, but special conditions may
make it favourable to operate the loop at a slightly different ratio in the
range of 2.5 to 3.5. When the ratio is
decreased to 2.5, the reaction rate will increase slightly (but fall again for
ratios below 2.5), while on the other hand, the circulating Synthesis gas will
be heavier. Therefore the pressure drop
through the loop will increase and the Ammonia separator efficiency may
decrease, leading to increased Ammonia concentration at the converter inlet.The
H2/N2 ratio in the loop should be kept as constant as possible. The ratio is controlled by the H2/N2 ratio in
the makeup gas which will have to be adjusted to get desired ratio in the
circulating gas. After making any change
in the H2/N2 ratio of the makeup gas sufficient time should be allowed for the
system to find its new equilibrium before making further changes.
REACTION TEMPERATURE
The
temperatures in the Catalyst bed are usually in the order of 360 deg.C to 520
deg.C. At the inlet to ech Catalyst bed,
a certain minimum temperature of 360 to 380 deg.C is required to ensure a
sufficient reaction rate. If the
temperature at catalyst inlet is below 360 deg. C, the reaction rate may become
so low that the heat liberated by the reaction becomes too small to maintain the
temperature in the converter, and the reaction will quickly extinguish itself
if proper process adjustments (lowering the gas circulation and / or closing
the cold shot) are not made immediately.
On the other hand it is desirable to keep the catalyst temperatures as
low as possible to prolong the catalyst life.It is therefore recommended that
the catalyst inlet temperature be kept as close as practically possible to the
minimum temperature without extinguishing the reactor. The Synthesis gas entering the converter
at 252 deg. C is heated in the interbed
heat exchanger by the hot gas coming out of the 1st bed. Before entering the 1st bed, the temperature
of this gas is controlled to about 370-380 deg.C by mixing with cold shot. As the gas passes through the catalyst bed,
the temperature increases to a maximum temperature at the outlet from the 1st
catalyst bed, which is normally the highest temperature in the converter,
called the "hot spot". The
temperature of the hot spot may be upto 520 deg.C, but should not exceed 530
deg. C. The gas from the 1st bed is
cooled in the interbed heat exchanger by the main part of the cold inlet gas to
the 1st bed in order to obtain a temperature of approx. 380 deg.C before entering
the 2nd bed. In the 2nd bed the gas
outlet temperature is about 439 deg. C.
CIRCULATION RATE
The
capacity of the synthesis loop with regard to Ammonia production rises with
increasing circulation rate. However,
the Ammonia production per cubic metre of circulation gas which is proportional
to the temperature difference between converter exit and converter inlet, will
decrease.
OPERATING PRESSURE
The
Synthesis loop is designed for a maximum pressure of 155 Kg/cm2g and it is
foreseen that the Synthesis loop can operate at a pressure of 142 Kg/cm2g when
operating at design production rate,
design inert level and design gas composition. The actual operating pressure is not directly
controlled and is dependent on the other process conditions, notably production
rate, inert level, ammonia concentration at converter inlet, H2/N2 ratio and
Catalyst Activity. The production rate increases with rising pressure and for
a given set of process conditions,
the pressure will adjust itself so that the production rate corresponds to the
amount of Makeup gas fed into the loop.
The loop pressure will be increased by increasing the Makeup gas flow to
the loop, by decreasing the circulation rate, increasing the inert level or the concentration of Ammonia at
converter inlet and by changing the H2/N2 ratio away from the optimum. The decreases in Catalyst activity will also
increase the operating pressure.
A TYPICAL REACTOR WITH CATALYST LOADED
LT CATALYST REDUCTION
When the fresh catalyst is loaded
it has to be reduced in order to attain full activity before it is actually
lined up for the shift conversion reaction i.e. water gas shift reaction.The
loaded catalyst vessel is heated to 180oc with nitrogen Circulation
continuously in the loop and then hydrogen gas is introduced into the
circulation
Loop such that the H2 concentration in the loop is around
1.5% slowly the catalyst
Reduction takes place and water is thus formed is
continuously removed in the
Separator the H2 concentration is maintained in the loop and
the inlet and outlet
Temperatures and H2 concentrations are monitored every half
an hour so as to
Keep close control over the reduction. It generally took
36-45 hrs for the reduction
To complete when the inlet and out let H2 gas concentration
equals.
During the
reduction the temperature of the gas and the catalyst bed rises
Owing to the exothermic reaction i.e. oxidation of H2 is
taking place as a result
Water is formed in the loop and it has to be removed in
order to protect the
Catalyst from loss of activity.
When the
reduction is completed then the nitrogen circulation is stopped
And the process gas is introduced into the catalyst bed and
is run for one week
At about 80% of the rated load. After one week the catalyst
gets full activity
And the reactor is ready for full load. For the next one
month the reactor outlet
CO concentration is monitered on daily basis.The bed
temperature is given utmost
Care for the first month and it should never cross 240oc crossing which may
Result in sintering of the catalyst.
A
TYPICAL 45 Hr LT CATALYST
REDUCTION
H2 INLET AND OUTLET H2 CONCENTRATION
DURATION (Hrs)
|
H2 Conc.,(%)
|
|
Inlet
|
Outlet
|
|
I
|
0.58
|
0.04
|
5
|
0.78
|
0.02
|
10
|
0.90
|
0.01
|
15
|
0.94
|
0.06
|
20
|
1.38
|
0.16
|
25
|
1.44
|
0.19
|
30
|
1.55
|
0.38
|
35
|
2.24
|
1.27
|
40
|
4.53
|
3.97
|
45
|
8.53
|
8.37
|
CATALYST
DISPOSAL
The
catalyst which lose its activity has to be disposed in a proper way
As the
catalysts contain metals their oxides and sometimes salts
Which on
surface dumping could lead to leaching of metals in to
The ground water
or surface water causing contamination of the land
And water
bodies. As most of the catalysts are composed of copper
Nickel,
iron, platinum. Zinc, molybdinum, vanadium,and chromium.
Also some
of the organic compounds which acts as promoters
Or activators
such as D.E.A, Glycine etc.,which are potential pollutents
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