Revamp of Ammonia Plants

7/24/2019 Revamp of Ammonia Plants

http://slidepdf.com/reader/full/revamp-of-ammonia-plants 1/5

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)



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



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.

Plantloperations Progress Vol. 1, No. 3)





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

Documents

Revamp of Ammonia Plants