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OAR Box 1784 

Prepped by Ollie Stewart 

Document Number: /*%[ 

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Docket Number: 

A--84-44 

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Treating hydrogen sulfide: 

An alternative to Claus 

This process uses a catalytic reagent to selectively oxidize 

the hydrogen sulfide present in gas streams into elemental 

sulfur. It is both economic and environmentally sound. 

L C. Hardison, ARI Technologies, Inc. 

F

or decades, the amine-Claus 

combination has been industry's 

workhorse for removing hydrogen sulfide from gas streams. 

But, strict air-quality regulations promulgated in the late '70s spurred the 

development of a new set of processes 

characterized by higher performance 

and H2S-removal efficiencies. Unfortunately, many of these promising newer 

routes have also been plagued by high 

reagent costs or by persistent operating problems. 

In fact, the first two generations of 

LO-CAT, a process based on a liquidphase-oxidation technique originated 

by Humphreys & Glasgow (London), 

also had their share of teething problems. However, with the introduction 

of a new catalytic reagent—ARI 

310 — in 1982, the third-generation process is now proving itself to be an 

economical, reliable and safe alternative to the Claus process. Indeed, in the 

two years since ARI 310's introduction, 

more than 10 LO-CAT units have been 

put onstream. 

Furthermore, a patented autocirculation scrubber (Fig. 1) commercialized in 

1984 has made the LO-CAT process 

even more attractive. In this innovation, two previously separate operations, absorption and oxidation, are 

now conducted in the same vessel. Capital costs have thus been reduced substantially— the unit now costs less 

than even a simple Claus plant. Further, in contrast to the Claus system, it 

provides more complete air-pollution 

control without having to resort to tailgas cleanup or incineration. In May, the 

first LO-CAT autocirculation system, 

capable of producing 2 tons/d of sulfur, 

was started up to treat the H2S-laden 

gases extracted from 50 million ft3/d of 

natural gas at Tejas Gas Corp.'s Indian 

Rock plant near Gilmer, Texas. 

SOME PROS — Why should users elect to 

use LO-CAT instead of one of the older 

processes? Here are some reasons: 

• The ARI 310 reagent —a dilute 

solution of organically-chelated iron in 

water — is highly active, yet nontoxic. 

It operates by oxidizing H2S ions absorbed into the solution directly to elemental sulfur, thus turning a potential 

pollution problem into a salable prodI 

-'•= 3 

y combining the absorber and oxidizer into a single unit, capital costs are reduced 

Reprinted from CHEMICAL ENGINEERING, Jan. 21, 1985. Copyright © 1985 by McGraw-Hill Inc. 

1221 Avenue of the Americas, New York, N.Y. 10020 
P.3 

f 

uct. And since nothing is discharged to 

the atmosphere, or flared, there is no 

need for any air-pollution controls. 

• The reagent is completely selective 

for H2S. The C02 content of the gas 

does not affect efficacy, an important 

feature in some applications, such as 

enhanced oil recovery or beverage processing, where the concentration of 

C02 in the gas stream is high. If required, some C02 may also be removed 

by strong buffering of the solution. 

• Because of the high activity of the 

LO-CAT catalyst, the plants can even 

operate on air streams or with oxygen 

added to the absorber without being 

affected by byproduct reactions that 

reduce the amount of sulfur recovered. 

• Turndown, with respect to both gas 

flow and H2S content, is 100%. Thus, 

the treated gas will still meet specifications, even if its flow is low. 

• No heat is required to regenerate 

reagent; heat is needed only for freeze 

protection, or for keeping solution temperature above the gas dewpoint. 

HOW IT WORKS —The LO-CAT process 

has two basic designs: aerobic, which 

absorbs hydrogen sulfide from airladen streams, and anaerobic, used 

when the gas stream has little or no 

oxygen present. 

The scrubbing medium used in the 

LO-CAT process is an iron/water solution, buffered with soda ash, potassium 

hydroxide or any other common alkali 

to a pH of about 8. After the H2S is 

contacted with the reagent in an absorber, it reacts with the dissolved iron 

to form elemental sulfur accordingly: 

H2S + 2Fe+ + -

2H+ + S + 2Fe + + 

The iron, now in a reduced ferrous 

form, is not consumed; instead, it is 

circulated to a second vessel, an oxidizer, where it is regenerated by bubbling 

air through the solution: 

y202 + H20 + 2Fe+ + -

2(OH)" + 2Fe + + + 

Although iron isn't consumed, there 

are some chemical replacement costs. A 

small portion is lost as the chelating 

agent degrades. Further, in small 

units, it often is not economical to filter 

and recover the reagent completely 

from the precipitated sulfur: Normally, 

the sulfur is separated by gravity, 

centrifuging or melting. 

The above two reactions, generally 

common to most liquid-phase-oxidation 

processes, are very specifically geared 

to convert H2S into sulfur. However, 

large amounts of a byproduct, thiosulfate, can be formed when H2S ions and 

oxygen come into contact. In practice, 

this happens when sulfide ions "leak" 

out of the absorber and get into the 

oxidizer — because of either inadequate residence time or low reagent 

activity. The high activity of the ARI 

310 reagent, however, tends to suppress such byproduct formation by 

rapidly converting H2S to sulfur. 

Thiosulfate can also result when oxyaammmWmBmsam^mmmKammsm 

500 

400 -

300 

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-Illlllll l Illlllll l 1 111111I I lllllllll l 

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Plant capacity, tons/d (with 30% H2S in feed gas) 

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gen dissolved in the oxidizer is not used 

up and is carried back to the absorber. 

However, in the LO-CAT process, the 

reagent keeps the concentration of the 

oxygen in the re-oxidized solution at 

only about 1 ppm. 

APPLICATIONS — Aerobic uses of LOCAT include treating of process effluents, air discharged to plant ventilation systems, and sewage-plant lift 

stations. Among anaerobic applications: treating refinery fuelgas, digester gas from sewage plants, and acid 

gases from amine treaters; disposing of 

stripper gas from sour waters; and 

processing syngas from coal gasifiers. 

The most common applications for 

LO-CAT, however, are in gas processing and conditioning, Natural-gas 

sweetening, for example, requires H2S 

concentrations to be reduced to a level 

of 4 ppm or less. The usual way of 

doing this is with a simple amine treater, or with a more complex system if 

there is a high ratio of C02 to H2S. 

In low-pressure (150 psig or thereabouts) sweetening operations, the LOCAT system handles both gas-sweetening and air-pollution-control operations; 

and although it costs more than an 

amine treater and flare combined, it 

offers some pollution-control benefits. 

At higher pressures (around 1,000 

psig), and particularly with low H2S 

loadings (200 ppm), the simpler autocirculation unit can be employed, making 

the LO-CAT cost-effective compared 

with alternative systems. 

Besides sweetening natural gas, LOCAT can be used to scrub regenerator 

gases — from amine treaters — before 

venting them. And high-C02-content regenerator gases also can be processed 

for use in enhanced-oil-recovery operations or in beverages. 

Fig. 2 details the capital and operating costs of autocirculation LO-CAT 

units, both with and without centrifuges for separating the sulfur. In 

cases where the amine treater's offgas 

is primarily carbon dioxide and water 

with very little hydrocarbons, incineration with support fuel, instead of flaring, is required. Here, replacing the 

incinerator with a LO-CAT system can 

result in a payout of less than one year. 

Jay Chowdhury, Editor 

The author 

L. C. Hardison is president of ARI Technologies, Inc., 

developer and licensor of the LO-CAT H2S-oxidation 

process. A cofounder of ARI in 1970, he was previously 

employed by UOP, Inc. He holds over 50 patents, 

several of which are related to LO-CAT technology.