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Recovery of sulfur from sour acid gas: Areview of the technology

ARTICLE in ENVIRONMENTAL PROGRESS OCTOBER 2002

Impact Factor: 1.31 DOI: 10.1002/ep.670210312

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Recovery of

Sulfwr

from

Sour

Acid

Gas:A

Review of

the

Technology

John

S

Eow

Department

of

Chemical Engineering, University

of

Leeds, Leeds, LS2 9JT United Kingdom

i e modified Clausprocess is the major technology

f o r

the

recovery of elemental sulfur rom

H S

nd

SO2 A

number of

commercial technologies or the recovery of sulfur rom acid

gases are also highlighted here. A Claus tail-gas clean-u p

treatment

is

essential to give high su lfur recovery efficiency

from sour acid gases. Generally, the ex isting tail-gas clean -up

technologies can be classified into tw o groups: those that

attain 99 overall sulfu r recovery efliciency, an d those that

achieve

99.9

efficiency , includ ing the sulfur recovered in

the C laus uni t. Theseprocesses are the Amocos Cold Bed

Adsolption CBA), the SNPMLurgi Sulfreen, the IF8 the SCOT

the Beavon, a nd the Wellman-Lordprocesses. The SCOT

process is generally the most reliable and flexible technology.

Process comparisons are also summarized in terms

of

the

sul-

f u r recove efficiency, hazards and disadvantages, reliabili-

ty and advantages, plan t ca pacity and ecological impacts.

Several changes and new trends are also highlighted here,

such as the introduction of non-permselectivecatalytic mem -

brane reactorsfor

the

Claus reaction, and the in situ adsop-

tion of water inside the Clam cata lytic reactor. ?be successful

utilization of

S

y converting it to sulfur a nd H 2 attains

the triple objectives of waste min imization, resource utiliza-

tion, and environmentalpollution reduction . Photochemical

and plasmochem ical m ethods are still in the development

stage. Application of electrochemical technology to H2 S

requires fur th er developm ent. Research

for

an optimum

porous catalyst structure is ongoing

o r

obtaining a relation

of

micropores and macropores which would provide effective

conversion of H S nd SO2

INTRODUCTION

Sulfur is often considered one of the four basic raw

materials in the chemical industry. It can

be

produced

from various sources using many different methods,

such as conventional mining methods, or it can be

recovered as a byproduct from sulfur removal and

recovery processes [l l .

However, changes worldwide have affected sulfur

sources and the amounts consumed in the last 30

years

[ l l .

Recovered sulfur production has become

more significant as sour feedstocks are increasingly uti-

lized, and environmental laws on emissions and waste

streams have continued to tighten worldwide [ 2 , 31. For

example, volunta ry sulfur from the Frasch mining

process supplied only 25% in 1995, compared

to

about

53% in 1980. Recovered sulfur increased from 5 of

the total production in 1950, to 67% in 1996

111.

Discov-

ery and development of large sour natural gas fields in

many countries have also been important factors in this

rapid growth. Increased processing of sour crude oil

and tighter pollution control has caused most refineries

to

recover the sulfur content of its crude oil.

Historically, sulfur recovery processes focus on the

removal and conversion of hydrogen sulfide (H2S) and

sulfur dioxide

(S02)

to

elemental sulfur

[4 ,

51, as these

species represent the largest source

of

potential sulfur

emission [61. H2S occurs naturally in many natural gas

wells, and is produced in large quantities in the desul-

furization

of

petroleum stocks [7-91.

It

has been consid-

ered a liability, which only occasionally can be an

asset, depending on the international sulfur price [51.

It

has a high heating value, but its use as a fuel is not

possible because o ne of

its

combustion products

is

S o l , which is not environmentally acceptable. There-

fore, one of the immediate alternative routes

for

the

utilization of H2S is to break it down to

its

constituent

elements of hydrogen and sulfur [lo, 111.

Various processes for the removal

of

SO,

in the

combustion gases have been reviewed

[121.

The majori-

ty of the processes are based on a throw-away process,

in which alkali or alkali earth metal reacts with SO, to

form metal sulfate 113-171. However, this ap proach

results in the disposal of large quantities

of

sulfate

waste materials. Direct catalytic oxidation

of

SO2 to

SOg, and subsequent absorption of

SO3

in water to

produce sulfuric acid, is an alternative method [17, 181.

This approach applies to process or combustion gases

containing moderate

to

high concentrations of SO2.

Copper smelters are the primary example.

Environmental Progress (V01.21, No.3)

October 2002

143


3/21

Currently the modified Claus technology is widely

used, compared to other processes, to produce ele-

mental sulfur from H2S presen t in gases from

oil

refineries, natural gas, coal gasification and o ther

industries [9, 19-23. It was developed by C.F. Claus, in

1883 1241 and was significantly modified in the late

1930s by I.G. Farbenindustrie AG [251. Major improve-

ments were not made in the technology itself o r its

application until such a process was required in the

United States in 1950s. This technology h as n ow

advanced to a stage where overall recovery has

increased from 9042% to the level of 98-99%

of

inlet

sulfur [3,

23,

261.

STUDIES AN D RESEARCH ON CIA US PROCESS MECHANISMS AND TECHNOLOGY

Figure

1

shows a simplified process diagram of a

Claus plant. Acid gas contains HzS, C0 2 and H 20 as

major components, and

N 2

and hydrocarbons as

minor components. Ammonia (NH3) is also present in

sour water stripper gas [27, 281. The Claus furnace and

the waste heat boiler are normally treated as a single

unit [29, 301. Monnery,

et

al.

[311

and Nasato,

et

al.

[301

identified the reaction furnace as on e of the most

important, yet least understood, parts

of

the modified

Claus process. The initial sulfur conversion occurs

there, the SO2 required by downstream catalytic reac-

tors is produced there, and contaminant destruction is

supposed to take place there

[22, 321.

However, many

side reactions also occur, reducing sulfur recovery and

producing unwanted subs tances [201. According to

Nasato, et al. [301, Kaloidas and Papayannakos

[331

and Dowling, et al. [34], the disassociation and re-for-

mation of H2S in the furnace

is

important as it pro-

vides a portion of the sulfur and a majority of the H2

for other reactions and consumes H2S that could

be

used in the Claus reactions. At temperatures below

1,000

C and residence times below 0.5 second, the

H2.S cracking rate is insignificant [321. Below 950 C,

the overall conversion of H2S is low even at a long

residence time. Therefore, the main purpose of the

reaction furnace is to provide optimum temperature

and residence time so that the exiting ratio

of

H2S to

SO2 is 2: 1, maximizing catalytic conversion down-

stream [221.

The waste heat boiler, usually a shell and tube heat

exchanger, cools the furnace exit gases from 1,188 C to

154 C in one or

two

ube passes, generating low-pres-

sure steam [301. This

is

to condense the sulfur products

(mostly Sg and

S6,

and a small amount of S2> [4,

21,

221.

Moreover, at 154 C, the sulfur products are at their low-

est viscosities [35, 361. Hence, the products would easily

flow through the pipes into the sulfur pit. To prevent

the pipes from becoming blocked, a low pressure jack-

eted steam generated in the waste heat boiler is intro-

duced around the pipes. Two reactions are believed to

occur in the waste heat boiler tubes:

The principal reaction of the Claus process are as

follows

[ 4 ,9,

22, 37,

381:

(4)

n

2H,S SO,

2H20 + beat

where n is the average molecular species of the sulfur

vapor product, with

n

=

2

to

8

and possibly more. In

Reaction

(3),

about a third of the H2S is combusted in

the reaction furnace

to

form a stoichiometric amount

of

S 0 2 , which

is

then reacted with the remaining H2S

in Reaction 4

to

yield elemental sulfur and water [23,

391. Reaction 3

is

carried out in the furnace at 1,188 C,

usually under partial oxidation [40, 411.

Reaction 4 is an equilibrium reaction favored at low

temperature in the presence of a catalyst [21, 37, 421. In

order

to

increase conversion, Bonsu and Meisen 1431

proposed using fluidized bed reactors, rather than con-

ventional fixed-bed reactors, so that the last reactor

could be continuously operated below the sulfur dew

point. According to Puchyr,

et

al.

[211

and Bonsu and

Meisen [431,

if

equilibrium conversion could be

achieved in each reactor, the use of fluidized-bed reac-

tors could result in an overall H2S conversion of 99.5Yo.

The most widely used Claus catalyst is non-promot-

ed spherical activated alumina [23, 441. However, Paik

and Chung [17] reported that Co-Mo/AlzOg, which

is

usually used for hydrodesulfurization of a petroleum

feed stock, can convert SO2 with H2S selectively to

elemental sulfur at lower temperature than that com-

monly used . However, the hydrogenation of SO2 to

H2S occurring on metal sulfide sites was found to be

much slower than the Clam reaction on alumina 1181.

The active sites for the SO2 hydrogenation was

believed to be sulfur vacancies in metal sulfide, and

the most effective catalyst had an ability to form and

regenerate sulfur vacancies most easily.

In the Claus process, other sulfur compounds will

form, such as carbon disulfide (CS2) and carbon oxy-

sulfide (COS), and these compounds can of ten con-

tribute from 20 to

50 of

the pollutants in the tail-gas

[44, 451. COS and CS2 are usually hydrolyzed in the

catalytic converter

[21, 381,

as shown below:

CS2

+

2H20 2H2S+ C02

(6)

Studies carried ou t by Laperdrix, et al. [461 also

reveal that the presence of 0 2 traces in the CSZ-H~O

mixture caused a decrease in the activity of alumina

and titania catalysts due

to

sulfate formation. IR stud-

ies show that sulfate species are reduced by H2S at

320 C on titania, in contrast to the sulfate species on

alumina, implying that titania

is

much more effective

than alumina when the CS2 20 feed also contains

H2S and

0 2

races [461.

The temperature of the first catalytic reactor is

maintained at about 350 C to hydrolyze COS an d

CSz,

while that of the subsequent

r e a c t o r s

is just

above the sulfur vapor de w poi nt [421. Tra nsi tio n

metal oxides

can

be used to

modify gamma-alumina

44

October

2002

Environmental Progress

(V01.23, N0.3)


4/21

~~ ~

Recycle fromTail-gas Unit

I

I ~

Preheater Preheater

I I

feed

I

I

I

Steam G-l

t I

furnace

boiler

1

Boilkr feed

water

Figure 1.

Simplified two-stage Claus process flow diagram.

p

eactor

I

Condenser

TO

Tail-gas Unit

Reactor

Condenser

wulphur Pit

to form a catalyst that is effective at temperatures

higher than the dew point of sulfur [47-491. However,

thermodynamics provide a strong incentive to operate

the catalytic converters at low temperature [3, 501 as a

lower temperature should increase the exothermic

reaction efficiency. Under these conditions, the pro-

duced sulfur would be deposited, thus deactivating

the catalyst by fouling [3, 511 and/or decreasing the

specific surface area and pore volume [23, 52-541.

Uncondensed gas, mainly H2S, S02, COS,

CS,,

N2,

unburned hydrocarbon and NH3, are reacted in the

lower temperature catalytic reactors [381. Alvarez, t

al.

[531

and Pineda and Palacios

[541

showed that

H2S

conversions higher than

90%

can

be

achieved using

concentrations in the range of 1-5% with a relatively

slow catalyst deactivation, especially if the operation

conditions and catalyst properties are optimized.

The adverse effect of water on alumina catalyst,

especially at low temperatures, has been recognized

as being responsib le for low activity in th e COS

hydrolysis [55, 561 and a decrease in H2S conversion

[571. Conversion with low water content, such as 5%

water vapor, was found to be 2 to 2.5 times higher

than that obtained with 35 water content, apparently

due to a competition with SO2 and H2S for adsorption

sites. The results by Laperdrix,

t

al.

[261

and Steijns

and Mars [571 also indicate that, in the presence

of

Sn

and H20, H2S and SO2 can be produced. However,

according

to

Ledoux, t

al.

[31, the use of a new type

of support, such as Sic, and a nickel-based active

phase provide an active, extremely selective and sta-

ble catalyst for the oxidation of

H2S

into elemental

sulfur by 0 2 at relatively low temperature. The cata-

lyst exhibited a high and stable H2S conversion even

at a sulfur loading of more than 60 .While in the

feed

without water, a rapid deactivation was

observed. Water assists in the mechanical removal and

transport of the sulfur formed by the particles of the

active phase on the hydrophilic part of the support

(i.e., oxycarbide or oxide

of

Si) to

the hydrophobic

part (i.e., Sic), leaving free access

to

the active parti-

cles even at high sulfur loading [31.

From thermodynamic calculations, Laengrich and

Cameron [21,Ledoux, t al. [31,Anon [191,Opekar and

Goar 1271, Grancher

1581,

and Pearson 1591 recom-

mended three or four catalytic converters operating

under steady state conditions at low temperature .

Thermodynamic calculations indicate the possibility

of

reaching efficiencies > 99 [501. Unfortunately, these

results cannot be obtained with current technology

due to reaction kinetic limitations and, particularly,

because of sulfur deposition in the catalyst pores [37,

52, 54, 601.

A s

a catalyst is being covered by sulfur, a

change in the process kinetics should be expected,

Environmental

Progress (V01.21, No.3)

October 2002 145


5/21


6/21

~~ ~~ ~~~~

Table 1. Comparison

of

the alternative processes for sulfur recovery. Part one

of

two.

Adsorption

Process

-

Overtime,all of the iron

oxide becomes sulphided

and its adsorptive capacity

becomes exhausted.

-

The zinc sulphide formed

cannot

be

oxidised back to

zinc oxide.

The sulphur removed via

this process

is

usually not

recovered. The sulphur

and sorbent both undergo

disposal.

- Limited capacity of

sorbentbed.

-

Limited

to gas

streams of

limited volumetric rate

having low concentration

Safety is most important

of H2S.

because

H S

s extremely

toxic and quickly paralyses

the sense of smell.

-

Molecular sieves

developed to extend the

operating range.

Molecular sieves can be

controlled to target the

removal of certain

components selectively.

-

Molecular sieves can be

regenerated.

- The advancement of

integrated gasification

combined cycle (IGCC)

power plants develops the

fluidisedbed adsorption

bed processes which

are

abletowithstand severe

operating condition.

The sorbent

bed

has a

Limited volumetric rate

limited capacity.

having low concentration

of H2S.

Absorption Process

-

Other components in the

feed gas may react with

and degrade the amine

solution.

Solution must be purged

and fiesh amine added

periodically.

higher solvent circulation

rates and higher

regeneration energy.

MEA

process has shown a

higher tendency towards

corrosion and foaming.

- Safety is most important

as

H2S is extremely toxic.

- MEA

process require

Solvent can be regenerated

for reuse.

- Many absorption processes

also

removed COz and to a

lesser extent COS,

So2

and

mercaptans.

MEA

removes both H2S

and COz nonselectively.

MEA

lowest solvent cost

and lowest hydrocarbonco

absorption relative to other

mine process.

Amine absorption

processes can be applied

when H2S Concentration is

relatively low (e.g.


7/21

Table 1. Comparison of the alternative processes

for

sulfur recovery. Part two

of

two.

Ecological

Impact

costs

The sulphur removed is

generally not recovered,

therefore

it

affects the

environment.

- The disposal of sorbent

material also affects the

environment.

- High operating costs due to

sorbent materials disposal.

2

adsorption towers.

-

Chemical and physical

solvents leakage

can

affect

the surrounding ecology.

health.

- Also affect workers'

- High solvent costs.

-

1

absorber, 1 regenerator,

1

cooler, 1 heat exchanger, 1

reboiler, 1 condenser

of feed gas with high

~ ecological impact with

a tail-gas treatment.

- Suitable to employ

various Claus tail-gas

treatment processes.

- Consists of 1

combustion furnace,

1

waste heat boiler,

1

condenser and a series

of catalytic stages, each

employing 1 reheat, 1

catalyst bed and

1

sulphur condenser.

feed rate containing low concentrations of H2S. The

Claus process has also proven to be very reliable and

mature [691.Only the Claus process, among the three,

can treat large amounts

of feed

gas with high H2S con-

centration, and produce minimum ecological impact

with a tail-gas treatment unit [40

501.

At

the same time

that the capabilitiesof the conversion process have dra

matically improved, innovations and process optimiza-

tion have reduced its capital and operating costs

[ll.

CLAUS TAIL GAS TREATMENT TECHNOLOGIES

In the early Claus sulfur recovery plants, the tail-

gases were usually exhausted

to

atmosphere through a

stack without any treatment. Sometimes the gases were

incinerated after leaving the last converter, and the SO2-

containing tail-gas was passed through a tall stack

[581.

As

the need to reduce SO2 emissions receives greater

emphasis, Claus technology has to be improved to

obtain higher recovery rates.

At

the present time, most

Claus plants are unable to meet existing or proposed air

pollution regulations in developed countries without

additional methods

of

reducing o r eliminating the sulfur

content of the exhaust gas

[5, 661.

Adding a tail-gas

cleanup process should be the last

resort,

as it is expen-

sive in terms

of

investment and energy consumption,

depending on the process selected

[51.

Several processes have been studied for application

as a Claus plant tail-gas cleanup service

[2, 791.

Many

commercial processes are based on low temperature

Claus reactions or on the removal of H S from tail-

gases by absorption and adsorption

16

8 However,

these processes require batch or periodic operation,

and, sometimes, heavy installation costs

[91. A

Claus

tail-gas desulfurization process should preferably be:

(1) easy to operate and flexible; (2) based on familiar

technology and easily adapt to existing Claus units;

(3)

generate no secondary air/water pollution or

waste; and

(4 )

deliver a high degree of desulfurization

over a wide range of operating conditions.

Generally, there are two broad classes of tail-gas

cleanup treatment [21, as illustrated in Figure 2. The for-

mer consists

of

processes which allow the Claus reac-

tions to take place under more favorable conditions.

These processes claim an overall sulfur-recovery effi-

ciency

of

approximately

93 ,

including sulfur recovered

in the Claus main unit. Three processes under this

group are Amoco's Cold Bed Adsorption (CBA), the

SNPMLurgi Sulfreen, and the

IFP

processes

[71].

In the adsorption process, gas from the main Claus

plant last condenser is

fed

to

an adsorption reactor,

operating between

130

C and

150

C, and containing

conventional Claus catalyst. The low temperature

favors equilibrium conversion. Sulfur vapor condenses

on the bed and is removed, shifting the equilibrium

towards higher conversion. Gas from the reactor is

then incinerated. While one reactor

is

on adsorption

cycle, a second reactor is being regenerated. Howev-

er, regeneration for removing sulfur deposit from cata-

lyst surface le ads to a decre ase in sulfur storage

capacity and in initial desulfurization activity

[31.

Hot

gas from the first Claus reactor vaporizes the con-

densed sulfur and reactivates the catalyst. The gas

is

then cooled and the sulfur vapor condenses. Gas is

returned to the Claus cycle just downstream of the

first Claus sulfur condenser.

The Sulfreen process, develope d by the Societe

Nationale des Petroles d'Aquitaine (SNPA) and Lurgi

Gesellschaft GmbH, uses a vapor-phase extended Claw

reaction carried out below the sulfur dew point. The

process operates in the same temperature range as the

CBA method, with the produced sulfur being deposited

on a alumina catalyst bed. In the two-reactor case, one

reactor

is

in service while the other is being regenerated.

The Sulfreen design uses a closed regeneration loop con-

taining a sulfur condenser and a regeneration gas heater,

usually an indirect-fred unit with stainless steel tubes.

The Institute Francais de Petrole (IFP) developed a

treating process used for Claus plant tail-gas cleanup,

148 October 2002

Environmental

Progress

V01.21, No.3)


8/21

Tail-gas Cleanup Treatment

rn

99

sulfur

1 )

Amocos

Cold Bed Adsorption

2)

SNPALurgi Sulfreen Process

(3) IFP Process

(CBA) Process

Figure

2.

Major tail-gas cleanup treatment processes.

recovery efficiency

+

1 )

Shell Claus

Off-Gas

reating

(SCOT) Process

2)

Beavon Process

(3) Wellman-Lord Process

Stack

Reducing

gas steam

orbent Bed A -)+.+

(Sorption)

sulfur

-

*,orbent Bed B*

Regeneration off-gas (Regeneration)

Figure

3.

Simplified MOST process flow diagram

[391.

in which the tail-gas from the Claus unit is contacted

by an IFP solvent, a high boiling point glycol. Both

HZS and S O 2 are thus absorbed. The Claus reaction

then converts these compounds to sulfur. This entire

process occurs above the sulfur melting point.

A s

sul-

fur has low solubility in the

IFP

solvent, liquid sulfur

accumulates at the bottom

of

a packed contacter and

is

withdrawn. Treated gas leaves the top

of

the tower

and is incinerated. Solvent is circulated back

to

the

top of the tower. In normal operation, no fuel or

steam

is

required except the condensate for make-up.

The Mobil Oil SO, Treatment (MOST) process con-

sists

of combusting the Claus tail-gas with air, convert-

ing all sulfur species to SOz/SOg [801. The SOx

is

then

sorbed onto a solid sorbent, and the sulfur is reduc-

tively desorbed as a concentrated stream of mainly

SO2

and

HlS,

which can then be recycled to the Claus

plant for further processing. Catalyst screening for this


(V01.21,

No.3) October 2002 149


9/21

Table 2. Comparison among three-stage Claus, 1 CBA, and 2 CBA processes for Claus plant. Part one of two.

Tail-Gas

Cleanup Unit

Conversion

efficiency

(Overall

recovery

capacity)

Hazards

&

Disadvantages

3-Stage Claus with

indirect reheat

95 %

-

97.8 %

- The kinetics of the Claus

process

are

incompletely

understood.

adversely the equilibrium

of the reaction o f

HIS

and

SOz

over the catalyst.

-

Fog

formation can also

be a problem during

condensation

of

the

sulphur vapour.

and quickly paralyzes the

sense of smell.

- One major problem can

occur with the operation

of

the main burner (on

reaction furnace).

- Water vapour affect

-

HzS

gas extremely toxic

2-Stage Claus (96

recovery)

with 1

CBA

stage in tail gas

cleanup unit

Up to approximately

99

%

- Mechanical and

maintenance problems

associated with the gas

switching valves and the

regeneration gas blower

and heater.

- Requires good control of

stoichiometric

Hz S /S OZ

ratio.

gradually declines

as

sulphur condenses on the

bed.

Catalyst activity

- The process requires

multiple reactors.

2-Stage Claus (96

recovery)

with

2 CBA

stages in tail

gas

cleanup unit

Up to approximately

99.5%

- Mechanical and


associatedwith the

gas-


regeneration gas blower

and heater.

- Requires good control of

stoichiometric

H2S /S 02

ratio.

gradually declines

as

sulphur condenses on the

bed.

multiple reactor.

- Catalyst activity

The process requires

application focuses on examining alumina and mag-

nesium aluminates, with oxidation promoters such as

ceria, vanadia, and platinum, where effective SO2 oxi-

dation promoters are required. The materials with the

highest SOx uptake are a commercial FCC SOx trans-

fer additive, and a vanadia/ceria-promoted, magne-

sium aluminate (V/Ce/Mg2A1205) spinel, with 54

and

46 SO,

uptake, respectively. During most of the

adsorption period, the SO2 level in the effluent from

the sorbent bed is below 1 ppmV [801.

According to Stern, et al. [391, the MOST process,

which can combust sulfur containing species and

selectively capture SO2 produced, offers operational

advantages over other wet scrubbing processes.

A

simplified process flow diagram of the MOST

process is shown in Figure 3. The tail-gas is sent

to

a

burner which oxidizes the remaining H2S to SO2

and SO3. The burner effluent, which contains

1

to

4

0 2 , goes to sorbent Bed A, where adsorption of

the SOx takes place. The tail-gas is then sent to the

stack. Reducing gas flows through Bed B to desorb

the sulfur as a concentrated stream

of

H2S and S 0 2 ,

which is then diverted

to

the Claus unit. At the end

of the cycle, Bed

A

is loaded with sulfur, while Bed

B

had its sulfur removed. At this moment, the valve

positions are changed, causing the regeneration gas

to flow through Bed

A

and the tail-gas

to

flow

through Bed B. The process is described in detail by

Stern, et al. [391.

The second class includes processes capable

of

achieving overall sulfur recoveries in the range of

99.5% to 99.9%. This level corresponds to about 300

ppmV or less total sulfur in the exhaust gas. Three

commercial processes of this type are the Shell Claus

Off-Gas Treating (SCOT), the Beavon, and the Well-

man-Lord processes. The SCOT process consists

of

a

reduction stage, followed by a concentration stage

that provides a H2S-rich stream

to

be recycled

to

the

Claus plant.

A

simplified flow diagram of the process

is shown in Figure 4 . The concentration process is

similar to the amine gas sweetening process common-

ly used in gas processing. In the reduction section, all

sulfur compounds and any

free

sulfur in the Claus tail-

gas are completely converted into H2S with H2 o r a

mixture of Ha and CO over a cobalt/molybdenum on

alumina catalyst at a temperature of about 300 C [44,

811. The tail-gas contains some H2 and CO. The hot

gas is then cooled, and water is condensed in a cool-

ing tower. The cooled gas, which normally contains

up to 3 vol. % H2S and 20 vol. % C 0 2 , is then coun-

tercurrently scrubbed by an alkanolamine solution in

an absorption column

[

51. A conventional stripper can

be used

to

strip the acid gases from the solvent. These

gases are recycled to the Claus plant inlet. The

remaining tail-gas, normally containing 200 to

300

ppmV HzS, is then incinerated. The SCOT process has

been designed for minimum pressure drop

so

that it

can be easily added to an existing Claus unit.

150

October

2002

Environmental Progress (V01.21,

No.3)


10/21

Table

2.

Comparison a mo ng three-stage Claus,

1

CBA, and 2 CBA processes for Claus plant. Part two of two.

Reliability

&

Advantages

Plant capacities

Ecological

impact

costs

- Has proven very reliable

and isconsidered as

mature technology.

- Use of modem , high-

intensity, efficient mixing

main burners can result in

a more stable flame,

especially with leaner

feeds.

destruction fo r

compounds such as

hydrocarbons, NH3,

mercaptons, etc.

- Reduced or nil oxygen

breakthrough.

-

Improved Claus thermal

conversion, and much

wider turndown or turnup

operations.

- Improvements are being

made in better Claus

catalysts and improved

process control.

In

most U.S. states, a

sulphur recovery unit

of

20 Itd or larger will

require some form of tail

gas cleanup.

recovery units of50 ltd or

larger normally require a

tail gas cleanup unit.

Safety is very important

in plants handling and

processing hydrogen

sulphide gas.

Poisoning by H2S.

Nowadays, units without

tail gas treatment cannot

meet the regulations.

-

Much better contaminant

InCanada, Sulphur

Has

the lowest cost

because no tail-gas

treatment unit. However

it is the least efficient and

unable

to

meet the

specifications.

- Low energy

consumption.

-

260F

to 300F for

operation; favours

equilibrium conversion.

- Can reduce

SO2

content

to about 1500 ppmv.

Uses the same

construction and

materials proven in the

Claus plant.

- Requires little plot space

and only minor

modification to an

existing plant.

Example, Amoco

built a

1500

ltd sulphur

plant with CBA near

Requires

2

reactors,

1

condenser and 1 blower

for addition.

Calgary.

Level of COS and CS2 s

not reduced, therefore

affecting the surrounding

air quality.

Problems have occurred

with H2S spikes during

the regeneration

procedure which have

resulted in occasional

environment violations.

A new sulphur recovery

costs 1.5 times more than

a standard 3-stage Claus

unit.

Capital cost to convert is

about that

of

the Claus

plant.

~ ~~

-

260F to 300F for

operation; favours

equilibrium conversion.

- Low energy

consumption.

-

Can reduce

SO2

content

to less than 1000ppmv.

- Uses the same

construction and

materials proven in the

Claus plant.

Produce more but need

more equipment and

energy.

costs.

2

blowers.

More efficient at higher

4 reactors, 2 condensers,

Levels of COS and CS2

are not reduced very

much.

Two new sulphur

recovery units cost 3

times more than a

standard 3-stage Claus

unit.

Capital cost to convert is

about that of the Claus

Dlant.

Environmental

Progress

(V01.21, No.3)

October

2002 5


11/21

Fuel Air

Furnace

l

Uni

Reactor

t Emuent

Fuel

Air

Absorber

Reactor Effluent

Air

SWS

Stripper

Figure

4. Simplified process flow diagram

of

the SCOT process.

Recycle to front end

Claus Unit

~

Regenerator

The Beavon process, developed by Ralph

M .

Par-

sons Company an d Union Oil of California, has been

used in several Claus tail-gas cleanup units

[821.

The

first industrial unit, a t Wintershall AG, Linden, Ger-

many, started in January 1978, and h as performed

very satisfactorily, aimed at

98.5 to 99.5%

overall sul-

fur recovery. In this process , tail-gas from the Claus

unit is first treated by a reducing gas over a cobalt-

molybdenum catalyst to convert all sulfur-containing

species to H2S. The Claus. gas usually contains a sig-

nificant portion of the required reducing agents. Addi-

tional reducing gas is supplied by an auxiliary burner,

which is also used to maintain a temperature between

315

C

and 370

C

[821.

Residual concentrations of COS,

CS2

and CH SH

cooled in a condenser to about 150' C

to

190

C,

and

contacted by a sodium carbonate-bicarbonate solution

at a pH

>

7

to

scrub ou t any SO2 that might have

passed through the catalyst

bed

without being

reduced . The cooled gas then g oes to a Stretford

absorber where it is contacted by a sodium carbonate-

sodium bicarbonate solution containing sodium vana-

date , and a n oxidation catalyst, where the H2S in the

feed

gas is absorbed and oxidized to sulfur. Additional

holding time

for

this

reaction is provided by a reaction

tank. Air is then used to oxidize the vanadium back to

the pentavalent st ate. The recovered sulfur forms a

for the reactor are low. The reduced gases are ti n

froth at the top of the oxidizer which is skimmed off,

filtered, washed and dried, and melted [821. The Beav-

on/Stretford process can reduce sulfur emissions to

several ppm , but

is

less effective than the SCOT

process in removing CS2 or COS, or mitigating any

CO which may pass through the Claus plant [391.

In the Wellman-Lord process, the Claus tail-gas is

incinerated, then cooled

to

about

boo

C and fed to an

absorber, where it is contacted by a sodium sulfide

solution. The solution reacts with SO2

to

form bisul-

fide.

Steam is used to drive

off

the SO2 and much

of

the aqueous solution in the evaporator/crystallizer

(831.

Sodium sulfide crystals precipitate here , forming

a slurry. Gas from the evaporator/crystallizer is cooled

to recover most of the vaporized water, which is used

to

dissolve the crystals. The SO2 gas is recycled

to

the

front end

of

the Claus unit. To complete the regenera-

tion process, the solvent is also treated with sodium

hydroxide, reacting with an y remaining bisulfide to

form sodium sulfide and water. The H2S/S02 ratio

control is not critical, as the Wellman-Lord process

removes sulfur after the tail-gas is incinerated. Tank-

age can be added to allow operation for up to three

days while the regeneration cycle is down.

Table 2 provides a comparison among a three-stage

Claus with indirect reheat unit, a two-stage Claus with

on e CBA unit and a two-stage Claus with tw o

C B A

units. Comparison among th e Sulfreen, the IFP, and

152

October

2002


V01.21, No.3)


12/21

~ ~~

Table 3. Comparison among Sulfreen, IFP, and SCOT processes for Claus plant. Part one of

two

Tail-Gas

Cleanup

Unit

Conversion

efficiency

(Overall

recovery

capacity)

2-Stage Claus (95.5

recovery)wth

Sulfieen process tail

gas cleanup unit

Upt to approximately

99

- Mechanical and


associated with the gas-


regenerationgas blower

and heater.

- Limited by equilibrium

conversion and sulphur

vapour pressure losses.

- Require careful operation

of the parent sulphur

plant

to

achieve

maximum recovery.

- More complicated

than CBA process.

- Not very energy efficient.

operating temperature is

quite low a t 2609: to

300OF.

- Facility

is

compact.

-

Some

H2S

is oxidised by

injection

of

a small

quantity of air, monitored

by

an

analyser, in

order

o

provide an optimal

H2S/S02ratio at the

Sulfieen reactor inlet.

operation makes operator

familiarisationmore

simple.

- The simplicity of

2-Stage Claus (95

recovery)wth IFP

process tail gas

cleanup unit

98.1

t o

99.4

%

-

Air control needed for

correct H2S/S02ratio.

Proper operation of the

Claus plant is required to

m inii ise COS,

CS2

in

the Claus tail gas for

optimum performance.

- Operation of an IFF'

installation is quite

different fiom the parent

Claw unit, presenting a

new

set

of operating

problems.

- Some difficulties present

in the CBA and S u lh e n

processes.

IFP solvent has good

thermal and chemical

stability, and low

volatility reducing

solvent losses.

- Recovered sulphur is

high quality.

No uel or steam is

required other than

condensate for makeup.

Low operating

ternperatwe at 125OC.

Retrofit is not

complicated

as

installation requires little

plot space and does not

recycle any

gas

to the

Claus feed.

Clam plant (94

recovery)wth

SCOT

process tail gas

99.9

+

%

cleanup unit

-

High temperature needed

for catalyst at 575F.

- Not a good selection for

direct treating of the tail

gas from Claus plant that

processes a feed gas with

a high C0 2, low H2S

content.

The concentration

process is similar to the

m i n e gas-sweetening

processes, making SCOT

process easier

to

operate.

- Flexibility and overall

process reliability are

good.

- Catalyst life is good.

-

Presulphiding of the

- Controlled bum-off

catalyst is not critical.

followed by resulphidmg

is said to restore catalyst

to its original activity.

- The reduction step

converts essentially all

sulphur-containiig

compounds o

H2S.

-

Absorption column is

aimed

at

achieving

essentially com plete

removal of H2S while

coabsorbing only a

fraction of the C 02

present.

Can be designed to

operate from 20

of

design fedr ate up to full

rate.

-

Changes in the feed have

only

a

small effect on

Environmental Progress (V01.21,

N0.3)

October

2002

153


13/21

Table 3. Comparison among Sulfreen, IFP, and SCOT processes for Claw plant. Part

two

of two.

-

2 reactors using

conventional Claus

catalyst beds, 1

condenser, 1 regeneration

gas heater.

operation (in 1992) after

Claus units om about 50

to 2200

tpd

of sulphur

and presently under

construction.

40

Sulfreen units in

Proper operation of the

Claus plant to minimise

COS and CS2 in the Claus

tail gas. COS and CS2

can cause air pollution.

amounts to 30% to

45

of the Claus unit cost for

the conventionalversion

and

40

to

55

for the

improved version.

Utility requirements per

ton of sulphur: electricity

300kW, atalyst about 4

lb for conventional

version and 5 Ib for the

improved version.

Operating costs are much

lower than solvent-based

Drocesses.

Sulfreen investment

Capable of reducing the

SO2

content of the

incineratedtail

gas

to as

low

as

1000 ppmV.

-

Require only a contactor,

1 pump,

1

solvent heater

for start up.

Tail

gas

contains some

H2and CO which is toxic

in significant quantity.

Does

not affect CS2 and

cos.

The IFP solvent is

relatively inexpensive,

keeping initial and

operating costs down.

overall sulphur recovery.

-

Reduces the sensitivity

of

the overall sulphur

recovery facility to

variations in the air

supply rate.

- Can be designed for

minimum pressure drop

to make it more suitable

for add-on installation.

Proven and familiar

equipment is used in each

step of this process.

- Produces no secondary

waste streams.

-

From 10 todstream day

(sulphur intake) to 2100

todstream

day

equivalent

Claus capacity.

virtually complete

conversion of elemental

is obtained (i.e. residual

S02contents

= 10

ppm).

- With an excess of

H2,

Sulphur and SO2 into H2S

- Most

widely used.

- 130 units (in 1992) are

committed, with capacity

from 3 to 2100 tpd of

fresh sulphur feed.

The tail gas contains

some

H2

and CO. CO

in

significant quantity can

cause health problems.

Capital costs equal to the

costs of the Claus plant.

- For a new facility with

SCOT esign optimised

for the best possible

fit,

the cost can be

as

low

as

75-85

of the Claus unit.

154 October

2002


(V01.21, No.3)


14/21

Table

4.

Comparison between Beavon and Wellman-Lord processes

for

a Claus plant.

Tail-GS

Cleanup Unit

Conversion

effciency

(Overall

recovery

capacity)

Hazards&

Disadvantages

Reliability&

Advantages

Plant capacities

Ecological

impact

costs

Claus plantwthBeavon process

tail-gas cleanup unit

99.4 Yo

-

At high temperature around 600 to

-

Big space is needed.

-

Absorber has to be clean-out about

Absorber plugging.

Plugging

in

the sulphur fioath lines.

The reducing catalyst life is only about

2

years.

Concerns over vanadium used in the

process have limited its application.

- Residual concentration of COSYCSz

and CH3SHfiom the reactor are low.

-

Some sections of the unit are coated

with plastic to avoid corrosion by

deposited sulphur.

holding time for reaction.

during nonnal operations.

typeof sulphur recovery plant if

adequate plot space is needed.

-

All pressures are near atmospheric.

-

To be able to achieve 100 ppmV or

- The clean tail gas containing less

700F.

every 6 months.

-

A reactiontankcan provide additional

- No ail gas incinerator is required

-

Suitable for add-on installations to any

less total sulphur in tail gas.

10 ppm

HzS

when using the newer

solvents, whichare highly selective

mine type solvents.

-

There are more than 15 Beavon MDEA

plants in the

U.S.and Japan. 2Beavon-

Selectox plants are in theU.S. nd

Germany.

- After cooling in the reactor,HzS,CSZ,

COS

gases

are

treated by the Stretford

process. The exit gas is discharged

with no further processing.

-

Total investment is approximately

-

High operating

costs

for sour gas

equal

o that of the parent Claw plant.

disposal during absorber clean outs

every 6 months, reduction of catalyst

changeouts and any mechanical failure.

Claus plant wth Wellman Lord

process tail-gas cleanup unit

99.9+Yo

-Process chemistry and equipment are

not familiar

to

many plant personnel,

thus compounding operating and

training difficulties.

- HzS/SOz

ratio control

is

not critical

to

- Tankage can also be designed into the

design tail gas treating.

facility

to

allow design operation for up

to 3 days while the regeneration cycle is

down.

This process is well-suited for high COz

streams

as

it does not recycle C02with

the SO2.

To achieve

SOt

emission level at

200

ppmV or less.

-

A bleed stream must

be

treated to take

out sodium sulphate.

High capital cost

because

it requires

exotic metallurgy.

- Capital cost about 130-150% of the

parent Claus unit for a 100 ltd unit.

Environmental

Progress

(V01.21, N0.3)

October

2002

155


15/21

b

the SCOT processes are given in Table 3, while Table

4 illustrates the comparison between the Beavon and

the Wellman-Lord processes for a Claus plant. By con-

sidering the advantages and disadvantages of all the

six methods in Tables 2 to 4 ,

it

is clear that the SCOT

process

is

generally the most suitable for a tail-gas

treatment unit. A Claus plant with a SCOT unit can

achieve conversion of 99.9 or more.

If

the feed gas

to the Claus plant contain a low CO2 concentration

and a high H2S content, then the SCOT process is a

good selection for direct treating of the tail-gas. More-

over, the amine system in the SCOT process is much

easier to operate compared to other processes. The

SCOT process, with good catalysts, is very reliable

and flexible to disturbances.

One of the most important features of the SCOT

process is that it can operate from 20% of design feed

rate to full rate. Therefore, its ability to cope with

changes in the feed conditions minimizes any effect on

overall sulfur recovery. The SCOT process can also be

designed for minimum pressure drop, thus making it

more suitable for add-on installations. I t is also quite

environmentally-friendly since it produces no secondary

waste streams. Using excessive H2, the SCOT process

can achieve a residual SO2 contents of less than 10

ppm. The capital cost can be as low as 75 to 85 of

the main Claus unit if the design is optimized [441.

MODOP and Superclaus processes seem to be very

attractive as they can convert H2S directly to elemen-

tal sulfur by selective catalytic oxidation and d o not

require periodic operation

[91.

Superclaus seems to be

superior to MODOP since the catalyst for the former

can tolerate the presence

of

water. However, Super-

claus uses ten times more

0 2

han the stoichiometric

amount for converting H2S to sulfur, and cannot be

applied to treat H2S higher than 5 . Recently, i t

is

claimed that Fe-Cr/Si02 catalyst can give sulfur yields

of more than 90 at the Superclaus condition [841.

The catalyst is known

to

show

little

decrease in the

sulfur yield, even in the presence of

30

vol. % water

vapor in the feed. Vanadium/silica (V/SiO2) catalyst

shows a decrease in the yield when excess water is

introduced in the feed. The use of a stoichiometric

amount

of

0 2 with V/SiO2 is possible to treat highly

concentrated H2S

I31,

whereas the Superclaus catalyst

is limited to

H2S

concentration

of less

than

5

vol.

Yo.

The following is a simplified guide for selecting a

sulfur-recovery process configuration: (1) t ry a best

.: :ata&st.3:

wat


16/21

O2 + S

+H20

Figure

6

A

non-permselective ceramic catalytic membrane reactor for the Claus reaction

[861.

to

the main Courtaulds plant. The composition

of

the

feed

to

the BOC development facility can be modified

to

represent virtually any commercial installation. The

research program will also develop computational

fluid dynamics models for detailed kinetic studies of

the Claus process.

In another development, Kenneth Klabunde and

Shawn Decker, both of Kansas State University, are

developing a sulfur content removal technique. They

have produced calcium oxide crystals coated with

iron oxide, offering a greater surface area and a coat-

ing that helps increase the reactivity of calcium oxide

with the acid gases. The

7

nm crystals are twice as

efficient at removing SO2 as the current method.

Klabunde also perceives many other possible applica-

tions for his research, including an alternative to incin-

eration of industrial waste, protection of soldiers from

chemical agents, and the removal

of

chlorinated com-

pounds.

RESEARCH ON NEW CONCEPTS FOR REMOVAL

OF

H2S FROMTAIL GASES

Thermal cracking of H2S at temperatures between

1,370 C and 1,650 C is being studied by the Alberta

Sulfur Research Laboratory (ASRL), Calgary. ASRL has

built a semi-works unit and installed it at Petro-Canada's

Wildcat Hills plant (near Cochrane, Alberta). The com-

pany plans to use a special ceramic membrane

to

sepa-

rate the produced H2 from the elemental sulfur. The

laboratory is also working on a further development

stage of the thermal cracker in which the ceramic mate-

rial will also serve as a semipermeable membrane

to

allow the removal of H2 formed in the cracker.

A new configuration

of

catalytic membrane reactor,

introduced by Sloot,

et

al. B61, consists of two cham-

bers separated by a non-permselective ceramic mem-

brane, as shown in Figure 6. The active components

of the catalyst can be easily incorporated within the

membrane. The membrane functions as a physical

barrier between the reactants which are fed to the

opposite sides of the membrane. Figure

6

also shows

the arrangement of the ceramic membrane reactor for

carrying out the Claus reaction 1861.This reactor type

has specific advantages for reactions requiring strict

stoichiometric feed of reactants. Any variation in the

molar fluxes of the reactants will result in a shift of the

reaction z one without affecting the reaction stoi-

chiometry [861. This allows greater flexibility of the

reactor

to

feed rates of H2S and S 0 2 . It is also often

desired that all the products of a reaction be directed

to

one side of the membrane. In this case, the pro-

duced sulfur should be directed to the SO2 side

[871.

This can be achieved by applying an overpressure at

one side of the membrane to generate the combined

effect of convective and diffusive flows [87-891.Further-

more, a homogeneously active membrane can be pro-

duced by using sintered stainless steel as the membrane

for the concept of separated feed of reactants [891.

Veldsink, et

al.

I901 suggest that the membrane

reactor shown in Figure 6 can be used

for

kinetically

fast exothermic heterogeneous reactions. By feeding

the reactants on both sides

of

the membrane, premix-

ing of the reactants is avoided. Therefore, thermal

problems, such as the formation

of

explosive mixtures

and the occurrence of thermal runaways, will not take

place 90, 911. However, accurate controlling of heat

balances

of

the membrane reactor will be a major task

for

any large-scale industrial unit. Therefore, efficient

means to supply or remove heat from any large-scale

membrane reactor will have

to

be developed. Accord-

ing to Adris and Grace [921and Mlezko, et al. 1931, the

combination of membranes and fluidized-bed reactors

are advan tageous because fluidized beds provide

good temperature control.

Much attention is paid

to

the search for an opti-

mum porous catalyst structure, i.e., the relationship

between micropores and macropores, which would

provide effective conversion

of

H2S

and

SO2

during

the e ntire period of adsorption until the reactor

Environmental Progress (V01.21,No.3)

October 2002 157


17/21

switches into the catalyst regeneration mode

[661.

The

larger the volume of micropores, and the smaller their

size, the gre ater the amount of sulfur that can be

extracted by the catalyst, and the smaller the sulfur

losses in the vapor phase. The catalyst efficiency in

tail-gas treatment processes is determined not only by

the relation of micropore and macropore volume, but

also by the number of micropores that the reactant

molecules must go through

to

get from one macrop-

ore

to

the next [661.

Raymont

[LO 111

came up with an alternative route

for the utilization

of

H2S by breaking it down to

its

constituents. The interest in utilization

of

H2S as a

source of H2 and sulfur has intensified in recent years

due to:

(1)

global prospect for hydrogen energy and

waste minimization; (2) the unavoidable production

of H2S from gas plants, refineries and metallurgical

processes; and (3) the cost of a tail-gas clean-up

process for Claus plants that can exceed the value of

the recovered sulfur

if

the environmental regulations

are made more stringent [81.

A

suitable technology for

the production of H2 and sulfur must meet the triple

objectives of waste minimization, resource utilization,

and environmental pollution reduction.

Photochemical 194-971 and plasmochemical [98-1031

technologies are still in the development stages and

are not mature enough to be applied

to

large-scale

chemical processing. Electrochemical technology [104-

1101 is established in certain areas, such as biochemi-

cal and biomedical separation processes, but its appli-

cation to H2S requires further development in the area

of storage and disposal techniques, proper equipment

materials, and knowledge

of

possible side reactions.

In addition, it is unlikely that electrochemical process-

es can be competitive at today's electricity costs. Of

the thermal methods, membrane, thermal diffusion,

and solar technologies have not

yet

developed very

far

[81.

In fact, membrane technology, which appears

very attractive, is essentially a technology for the

future. For chemicals as difficult as H2S, the applica-

tion has to wait until the technology matures

to

less

demanding processes.

As an alternative to the physico-chemical processes,

Basu, et al. [201 demonstrated that the anaerobic, photo-

synthetic bacterium, Chlorobium thiosulfatophilum,

could convert H2S to elemental sulfur in a single

step

at

atmospheric conditions. The autotrophic bacterium uti-

lizes

C 0 2

as carbon source, while energy for cell metab-

olism is provided by incandescent light and H2S oxida-

tion [201. Almost all the H2S could be converted in a res-

idence time of a few minutes. Moreover, high concen-

trations

of

H2S or organics did not seem to affect the

conversion efficiency.

CONCLUSIONS

The modified Claus process is the major technolo-

gy currently used to recover elemental sulfur from

H2S and SO2. Studies and re search o n the Claus

process mechanisms and technology have be en

described.

A

number of current commercial technolo-

gies for the recovery of sulfur from sour acid gas have

also been described and compared. Under modern

environmental regulations in developed countries, a

Claus tail-gas cleanup treatment is essential to achieve

very high sulfur recovery efficiency. Established tail-

gas cleanup processes are Amoco's Cold Bed Adsorp-

tion, the Sulfreen, the IFP, the SCOT, the Beavon, and

the Wellman-Lord processes. The SCOT process

is

the

most reliable and flexibleto disturbances.

Several changes and new trends in the conversion

of

H2S and SO2

to

elemental sulfur have also been highlight-

ed in this review paper. Two examples of the recent

improvement in the Claus tail-gas treatment process are

the introduction of the non-permselective catalytic mem-

brane reactors and in

situ

water separation by zeolite

adsorbent.

The

success in the utilization of H2S by break-

ing it down to elemental sulfur will signlfy the attainment

of the three objectives of waste minimization, resource uti-

lization, and environmental pollution reduction.

Based on the considerations in this review, the

fol-

lowing two processes for the conversion of H2S

and/or SO2 merit further analysis

to

act as the basis

for a prospective commercial technology:

1)

catalytic

thermal decomposition at reduced pressure between

1,000 C

and 1,200

C

in a fixed bed reactor; and

(2)

two-step sulfide processes in the temperature range

of

500

C

to 650 C in fluidized bed reactors as reactor-

regenerator systems. However, there may, in fact, not

be a universal approach for the selection

of

a desulfu-

rization process. The economics

of

a process may be

influenced by diverse factors, making different

processes desirable based on plant size, the source,

temperature and concentration

of

H2S, the local ener-

gy situation and/or the environmental regulations.

Therefore,

it

is important to explore new ways of H2S

and SO2 elimination which will lead to high conver-

sions at minimum cost,

to

increase sulfur recovery.

Photochemical and plasmochemical methods are still

in the development stage, while the electrochemical

technology is established in certain areas, but its appli-

cation to H2S requires further development. Research

for an optimum porous catalyst structure

is

ongoing in

order to obtain a relation

of

micropores and macropores

which would provide effective conversion of H2S and

SO2 during the entire period of adsorption.

ACKNOWLEDGMENTS

The author would like to acknowledge the assis-

tance given by Mohd. Rusydan Abdul Naim and

Ikhsan Masadi, currently with the National Petroleum

Company of Malaysia (Petronas). Appreciation is also

due to Yasser Hussain, Ron Towers and Dr. John

Lamb of University

of

Surrey for their helpful com-

ments and suggestions. Special appreciation is due

to

Professor Mojtaba Ghadiri

for

his various supports.

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Documents

Recovery of Sulfur (1)