revamp of ammonia plants
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From our investigation of HK40 material, it was
confirmed that the macrostructure of the metal is the
most significant factor affecting the creep-rupture
properties. Therefore, the macrostructure
of
the speci-
men should b e inspec ted and recorded for discussion of
the creep-rupture properties of heat-resisting alloy
castings.
Our destruc tive investigation of catalyst tubes u sed for
long periods of time reveals that the maximum wall
thickness of the catalyst tubes for steam reformers
should be limited to reduce creep damage due to ther-
mal stresses.
Controlling the macrostructure of individual cast tubes
and of weld metals can contribute to prolonging the
catalyst tube life, even if the material and the wall
thickness are kept the same.
In order to prevent too great an increase in the wall
thickness, modified HK40 material, such as IN519,
Hika, or BST alloys, should be used for the catalyst
tubes of steam reformers.
LITERATURE CITED
1.
2.
3.
4.
5
6.
7.
8.
National Research Institute for Metals,” “NRIM Creep Data
Sheet, No. 16” (1974).
National Research Institute
for
Metals, “NRIM Creep Data
Sheet , No. 16A” (1980).
Kawai, T., K. Takemura, T. Shibasaki, and T. Mohri, “Effect of
Macrostructure on Catalyst Tube Damage and Cr eep Rupture
Properties of HK40”, AIChE, Ammonia Plnnt Safety,
22,
119
(1979).
Ohta, S., “Report of the 123 Committee on Heat-Resisting
Metals and Alloys, Japan Society for the Promotion of Sci-
ence,”
Vol .
18,
383 (1977).
Kawai,
T.,
K. Takemura, T. Shibasaki, and T. Mohri, private
report, “Catalyst Tube No. 3” Topsae-Chiyoda Steam Re-
former Symposium, July (1977).
INCO Europe Limited, Inco Databooks, “IN-519 Cast
Chromium-Nickel-Niobium Heat-Resi sting Steel” (1976).
Sasaki, R., H. Hataya, and Y Fukui, Institute
ofMechanicaZ
Engineer
Vol. 13, 169.1 (1973).
Zaghloul, M.
B. E.
Doctoral Thesis, Tokyo Inst. ofTech. (1976).
Takao Kawai, is assistant principal e ngineer at the
combustion engineering department of Chiyoda
Chemical Engineering Construction Co., Ltd.,
and is responsible for the development of a furnace
for high temperature services. He holds a B. S . and
M . E .
degree from the Tokyo Institute of Technol.
O Y .
Katsuaki Takemura, is assistaut principal eng ineer
at material and welding technology department,
and earne d his B.E. and M.E. degree in metallur-
gical e ngineer ing at the Tohoku University, Sen-
dai. Current responsibilities include material
evaluation and development of welding technol-
ogy.
Toshikazu Shimbasaki is the material and welding
technology department engineer. His work in-
volves materials engineering at elevated tempera-
ture services, welding technology and failure
analysis. He holds a B.E . and
M.E.
degree in met-
allurgical engineering from the Tohoku Univc:
sity, Sendai.
Takaaki
Mohri, is
heater en gineer at the combus-
tion engineering department, and is engaged in
the development of a furnace for high tempera ture
services. He holds a B.S .M.E.degree from the col-
lege of Science and Technology, Nihon Univer-
sity.
Revamp of Ammonia Plants
A
reduction of the steam/carbon ratio below the traditional l e v e l can result in a
considerably higher energy efficiency.
Anders Nie lsen, John
B.
Hansen, Jens Houken, and Erik
A .
Gam, Haldor Topsoe
NS
Lyngby, Denmark.
The rapid escalation of the costs of hydrocarbons during
the last decade has brought about increasing interest in re-
vamping ammonia plants for energy savings. Today’s costs
of natural gas and projected prices may justify investments
in the range of several millions of U.S. dollars.
In connection with the production of synthesis gas in
plants based on steam reforming, an appreciable improve-
ment of the efficiency can be achieved by dec reasing the
overall energy loss du e to the large amounts of excess pro-
cess steam usually consumed.
The present paper deal s with various aspects oft he mod-
ification of existing ammonia plants to operate at a steam/-
carbon ratio appreciably below the usual level of 3.7-4.0.
ISSN 0278-4513-82-6329-0186-$2.00.
h e American Insti tute
of
Chemical Engineers,
1982.
REFERENCE PLA NT
In order to describe the changes of the operating condi-
tions brought about by a substantial decrease oft he steaml-
carbon ratio, and in order to determine the potential en-
ergy savings, a typical ammonia plant designed by Haldor
Topsoe
J S
at the end of the sixties has been taken as a
starting point. I t may be expedien t to mention the process
sequence.
Desulfurization of the hydrocarbon f eed stock is carried
out using hot zinc oxide at about 400°C (750°F). If necessa-
ry-due to the pre sence of refractory sulfur compounds-a
hydrogenation step is inserted upstream of the zinc-oxide
vessels. Reforming of the hydrocarbons at about 34 bar
(480 psig) takes place i n two steps, primary reforming in a
186 July, 1982 Plant/Operotions
Progress
Vol.
1, No. 3)
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tubular furnace of the side-wall fired type, and secondary
autothermal reforming.
CO-conversion of the reformed gas is performed in two
stages, the high-temperature stage with a conventional
chromium oxide-promoted
magnetite-Fe304-catalyst,
and the low-temperature stage using a copper-based cata-
lyst. CO, is removed in an activated hot potassium-car-
bonate wash. The heat for regeneration of the solvent is
obtained from condensation of th e excess steam in the pro-
cess gas. The last st ep of the synthesis-gas preparation
train is methanation for removal of residual CO and COz n
the gas.
Ammonia synthesis is based on the Classical TO PS 0E
Radial Flow Ammonia Synthesis Converter. The pressure
is
typically 265 bar (3830 psig).
The reference plant operates with a steamkarbon ratio
equal to
3.75.
The specific energy consumption is about
8.3
Gcal/MT N H 3 LHV) [30 MMBtu/ST].
OPERATION AT REDUCED STEAMICARBON RA TIO
General Comments
Like most other ammonia plants, the refe rence plant op-
erates with a very large excess of process steam, considera-
bly larger than required by the reforming and shift reac-
tions. Although th e excess steam is utilized as a heat source
for the regeneration of the solvent in the C0,-removal sec-
tion, this operation causes an energy
loss
by using med-
ium-pressure steam as a heat source where low-pressure
steam would be sufficient. This is illustrated in Figure
1
If the flow of excess steam through the reformers and the
CO-conversion is reduced and the corresponding steam
quantity is transferred to the hot potassium-carbonate
C0,-wash via a back-pressure turbine, an appreciable
amount of mechanical energy can be gained.
From Figure
1
it is also obvious that a fur ther improve-
ment
of
the steam balance can be obtained if the steam
consumption of the C0 ,-wash can be decreased . The maxi-
mum steam saving is obtained by introducing a physical
C0,-wash , which does not require steam for regeneration
of the solvent.
Besides the steam savings just mentioned, operation
with a low steamkarbon ratio yields the following addi-
tional benefits, viz.;
reduced pressure dro p in the front-end;
. educed absorption of heat in the reforming furnace and
reduced costs of recovering process condensate.
in the flue-gas channel; and
NAT.GAS PROCESS STEAM
I
REFORMING
TO METHANATION
Figure 1 Efficiency gain due
to low
rteomkorbon rotio.
PIontfOperations Progress Vol. 1, No. 3)
In add ition to an energy saving,
a
lower front-end pressure
drop may contr ibute to a debottle-necking of the plant.
REFORMING AT LOW STEAMICARBON RATIOS
Operating the primary reformer at a decreased steam/-
carbon ratio, e.g . 2.5, may cause two major concerns, viz.:
the risk of carbon deposition;
the effect on the tube-wall temperature.
Carbon Formation
Minor amounts of carbon deposit ion cause “hot bands”
or hot spots, usually 2-3 m from the to
of
the reformer
the reformer tubes. More sev ere cases may necessitate ca-
pacity reduction or even plant shut-down. Therefore, du e
to limitations in the activity of avai lable catalysts, it has un-
til recently been considered necessary to operate the pri-
mary reformer with an ap reciable amount ofexcess steam
as a safeguard against car on deposition. Today, however,
a second generation of reforming catalysts with a consid-
erably higher activity and a better resistance to poison-
ing-especially by sulfur-has bee n made available to in-
dustry. With these catalysts it
is
possible to operate at con-
siderably more severe conditions without carbon
deposition.
Th e higher reaction rate due to the more active catalyst
flattens the temperature profile at the top of the reformer
tubes; the conditions come much closer to equilibrium.
The lower reaction temperatures again cause a decrease
of
the tube-wall temperature. This effect, together with the
increased conversion at the top of the reformer, eliminates
the tendency toward carbon formation. Successful com-
mercial operation, with the improved catalysts installed in
several reforming furnaces, has amply demonstrated that
the problem of carbon formation can be solved and that a
decrease of the tube-wall temperature is really obta ined.
tubes. Such occurrences decrease the ePective lifet ime of
TUBE-WALL TEMPERATURE
Basically, a decrease of the steamkarbon ratio will cause
an increase of the skin temperatures of the reformer tubes.
However, the new reforming catalysts give an appreciable
improvement of the heat transfer, which , to a great extent,
can compensate for the temperature increase due to the
decrease of the steamkarbon ratio. The mechanism has al-
ready been described above.
The more efficient heat transfer can be illustrated by
means ofTable
1.
Case
0
is the reference case (SIC
=
3.75).
For this case, the maximum tube-wall temperature has
bee n determined to be 900°C (1650°F). Th e calculations
assume a side wall-fired Topsee reforming furnace, and a
“conventional” catalyst such as, for instance, the Topsee
RKS-catalyst.
For Case la-with a steam carbon ratio equa l to 2.5-the
basis is the same. The decrease of the steamkarbon ratio
causes the necessary maximum tube-wall temperature to
increase by 30°C (54°F).
In Case
ib
the steam/carbon ratio is again 2.5. However,
one of the new highly active catalyst charges has been se-
lected. Furthermore, advantage has bee n taken of the re-
duced mass flow by using a catalyst with a smaller particle
size. The smaller particle size gives an addit ional improve-
ment of the catalytic effect. Compared to Case la , a 20°C
(36°F)
decrease of the maximum tube-wall temperatur e is
obtained.
In Case
2
the steamicarbon ratio was b een selected as
3.0. Fur ther bases are identical to those for Case Ib. In this
case the maximum tube-wall temperature becomes even
lower than that in the reference case.
Tahle
1
refers to a side-wal l fired reforming furnace. In
top-fired furnaces ev en larger decr eases of the tube skin
July, 1982 187
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TABLE1. REFORMINGECTION PERATINGONDITIONS
Temp.
inleuexit
Abs.
heat
prim. ref. Gcal per
C MT
N H ,
Case 0:
SIC = 3.75
TOPSBE RKS-catalvst 5201796 1.31
19/9/15 mm rings
Case
l a : SIC
=
2.5
TOPSQIERKS-catalyst 5201832
19/9/15
mm rings
Case 1b: SI =
2.5
TOPSQIERKGIRKS 35165% 5201828
13/6/13mm rings
TOPSQIE
RKG/RKS 35-65% 5201808
13/6/13
mm rings
Case
2: SIC = 3.0
.25
.25
.26
temperatures have been experienced after installation of
the new catalysts.
Reformer
E x i t
Temperatures
As
indicated in Table
1,
he exit temperatures ofboth the
primary and the secondar reformer will, of course , in-
mum permissible temperatures must be considered from
case to case.
crease when the steamtcar
{
n ratio is reduced. The maxi-
CO-CONVERSION A T LOW STEAMIDRY GAS RATIO
The operating conditions in the reference plant are
shown in Figure 2. Th e corresponding conditions for oper-
ation with a steamlcarbon ratio equal to 2.5 appear i n Fig-
ure
3. As
can be seen, for equilibrium reasons it is neces-
sary to decrease the temperature level in both reactors.
The new operating conditions require new catalysts for
both reactors. These new catalysts are designated LK-811
and LK-821. LK-811 is used in the 1st reactor and LK-821
in the 2nd reactor. Both catalysts are iron-free copper-
based catalysts.
The properties of the n ew catalysts and the reasons for
selecting the operating conditions shown in Figure
3
will
be explained in what follows.
In connection with CO-conversion at low steam/dry gas
ratios two major problems must
be
taken into considera-
tions, v iz . :
the possibility of by-product formation, particularly hy-
a high demand for catalyst activity.
By-product Formation
Hydrocarbons Application of a traditional high-temper -
ature shift catalyst at the reduced steam/dry gas ratio
drocarbons and methanol, an d
H20/DRY:0.44
0 23
Molo/&/oCb
Figure 2. CO-conversion classical system. SIC
= 3.75.
1 8 8 July, 1982
Max. tube- Cat. press .
wall temp. drop
C Kglcmz
900 1.96
930
1.14
909 1.89
888 2.37
Temp. exit Methane
sec.
ref. leakage
C
Mol%
975 0.3
1028 0.3
1028 0.3
1004 0.0
would cause a substantial formation of by-product hydro-
carbons. The traditional high-temperature shift catalyst in
its active state consists of
magnetite-Fe,O,-promoted
with Cr,03. Under certain conditions magnetite can be
converted into carb ide, e.g., by the following reaction
5
Fe,O,
32
CO
3
Fe,C,
+
26 CO,
Iron carbides are catalysts for the Fischer -Tropsch reac-
tion, which forms hydrocarbons from carbon oxides and
hydrogen. Therefore, conversion of the high-tempera-
ture shift catalyst into iron carbide will result in the forma-
tion of hydrocarbons in the shift reactor. From the reac-
tion above, it appears that th e carbide is stable at a high
CO/CO,-ratio, or, when taking the shift reaction into ac-
count, at a low steam/dry gas ratio.
Thermodynamic studies show that carbide formation
will take place a t the steam/dry gas ratios in question. Fur-
thermore , experiments in Topsoe pilot plants have demon-
strated that hydrocarbons are indeed formed on a tradi-
tional high-temperature shift catalyst wh en it is operated
under these conditions, and the observed occurrence of
hydrocarbons is in good accordance with predictions from
thermodynamic studies.
Methanol:
It is well known that all copper-containing
low-temperature shift catalysts are methanol synthesis cat-
alysts as well. Therefore, due to the low steam content
there is a possibility that significant amounts of methanol
could
be
formed in the 2nd shift reactor. The maximum
possible amount-if both the.shift and methanol synthes is
reactions are at equilibrium- appear in Figure 4. The CO-
equilibrium percentage is also shown on this figure. As
can be seen, in order to obtain an acceptab le decrease of
the CO-leakage, the exit temperature of the 2nd reactor
must be lower than 220°C (428°F). Under these conditions
H 2 0 DRY=0.39
1 4 . 8
MolQ/LcO
Figure
3.
CO-conversion new process. SIC
= 2.50.
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Anders Nielsen
is Manager
of
the Research and
Development Division of Haldor Topsole
NS
Denmark. He has been with the Topsole Organ-
ization since
1943,
except
for
a visit to Columbia
University 1949-50.He is a recipient of the Julius
Thomsen medal. He is on the editorial board of
Journulof Cutulysis
and a member of
the
Danish
Academy ofTechnica1 Sciences, AIChE and ACS.
John Bsgild Hansen graduated from the Techni-
cal University of Denmark with a M.S c . degree in
chemical engineering in 1975.Since graduation he
has been employed with Haldor Topsde NS. First
in the Catalyst Sales & Technical Service Divi-
.sion, later in the Research and Development Di-
\vision.
Jens Houken is assistant manager, catalyst sales
and technical service. He holds a
M.Sc.
in chem-
ical engineering from the Technical University of
Denmark and has worked in the Tops0e Organiza-
tion since 1965.
Erik Andreas Gam graduated
from
the Technical
University of Denmark. Degree in chemical engi-
neering in 1965.Since 1965he has been employed
with the Haldor Topsde NS. He is responsible
for rocess development in the field
of
ammonia
proiuction.
Calculated Process Risks and Hazards
Management
Hazards management at
Du
Pont
is
based on a discriminating use
of
quantita-
tive
risk
analysis.
E.
Neil Helmers and Leon
C.
Schaller, E. I. Du Pont de Nemours
& Co.,
Inc., Wilmington, Del.
19898
CAL CULA TED PROCESS RISKS AND HA ZAR DS MAN AG EMEN T
In the last five years, the use of hazards analysis meth-
ods that quantify chemical industry process risks has in-
creased dramatically. This is certainly true at Du Pont.
Five years ago, consultants in Du Pont’s Engineeri ng De-
partment inc luded o ne specialist in process hazards analy-
sis. Today.we have five specialists who devote full time to
this activity. They spend most of their time consulting on
fault tree analysis (FTA). During this same five years,
nearly
1,000
Du Pont engineers have had some training in
fault tree analysis, and many
of
them have applied this
knowledge to Du Pont processes.
This new approach to process hazards analysis has con-
fronted Du Pont line managers with quantified risk esti-
mates and a new decision problem-namely, what is an ac-
ceptable level of risk under various circumstances? Our
policy has long been that no injury is acceptable and our
goal continues to be zero injuries; but, at th e same time, we
all know there’s no such thin
as an activity with zero risk.
lines on the use oPprocess risk calculations in hazards
management. The objective was not to promote indiscrim-
inate use of fault tree analysis for all process hazards bu t
rather to help line management make better decisions
when confronted with quantifi ed risk assessments. Actu-
ally, our experience in applying the concepts in these
guidelines started a year or more before they were form-
Our corporate res onse ear
i
in 1981was to issue guide-
ISSN 0278-4513-82-6296-0190-$2.00,OThe American Institute
of
Chemical Engineers
1982.
ally issued. Our objective in this paper is to describe the
guidelines and our experience in applying them
so
far.
To begin we want to emphasize two things: first-the
guidelines are tools to aid line management in decision
making. This is vital because line managers bear the pri-
mary responsibility for safety; and second-quantitative
risk assessment is only one of many tools for process haz-
ards management . Th e oint is not to become
so
enamored
ent precision that we slight equally or more important
tools such as management commitment, organization, in-
spections and tests, operating procedures, training, design
and maintenance controls, serious-incident investigation,
information exchange, and auditing.
A
favorable risk as-
sessment should never be an excuse for complacency or a
substitute for other essential approaches to process
haz-
ards management.
of analysis because of t
e
beauty of its logic and its appar-
FAU LT TREE ANAL YSIS
To get on with the description of Du Pont’s guidelines,
we shall review the criteria we use to characterize process
risk. Ou r principal me thod for quantifying risk is fault tree
analysis (FTA) which gives the most probab le time inter-
val between serious incidents. So it’s natural that our at-
tention first focused on this measure of risk, as shown in
Figure
1
Our guidelines use th e terminology “Interval
Between Incidents,” or IBI, and expresses this interval in
years. Many early FTA users considered
an
I B I
of
10,000
years as the dividing line between “unacceptable” and
“acceptab le” risk for serious incidents such as explosions.
Plant/Operations Progress
Vol. 1,
No.
3
90 July, 1982