P.1 

OAR Box 1784 

Prepped by Ollie Stewart 

Document Number: ^ / 

79) ll-l-< 

Docket Number: 

A-84-44 

Printed 10/31/2008 8:05:41 AM Sheet 79 of 143 
p-i. 

MINIMIZING GAS TREATING COSTS 

WITH LO-CAT® FOR H2S REMOVAL 

by 

L. C. Hardison 

President 

ARI Technologies, Inc. 

600 North First Bank Drive 

Palatine, Illinois 60067 

For Presentation at the 

Panhandle Plains Regional Meeting 

of the 

Gas Processors Association 

Amarillo, Texas 

October 11, 1984 
P.3 

Abstract 

The LO-CAT® Hydrogen Sulfide Oxidation Process uses dilute aqueous 

solutions of chelated iron to scrub H2S from gas streams and convert 

it directly to elemental sulfur. The main features of the LO-CAT 

process which set it apart from other direct conversion processes 

include: 

- Extremely high catalyst activity 

- Complete non-toxicity of the catalyst solution 

- Great flexibility of application 

Over the past two years, a number of applications in the natural gas 

processing industry have demonstrated the reliabilit y and economy of 

t h e LO-CAT process . This pape r describe s th e most recen t 

application s t o natura l gas processing, and reviews the technical 

and economic incentive s fo r ga s processors t o utiliz e thi s 

technology. 

- 1 -
P.4 

MINIMIZING GAS TREATING COSTS 

WITH LO-CAT® FOR H?S REMOVAL 

Introduction 

The LO-CAT® Hydrogen Sulfide Oxidation Process was developed in the 

early 1970's and first applied commercially in 1978. The initial 

applciations were in oil refining and miscellaneous chemical 

processing plants, and there were no installations of the LO-CAT 

process in natural gas treating until early 1983 when the first unit 

started up at the Seven Sisters Gas Field near Freer, Texas. Since 

then, there has been a second installation at Seven Sisters. 

Additional units have been furnished to Tejas Gas Corp. for their 

Indian Rock, Texas Plant, to Alberta Natural Gas Company for the 

Corchran Gas Treating Plant, to West Bay Explorations and JEM 

Petroleum for Michigan installations, and to Geokinetics, Vernal, 

Utah (in situ shale retorting). With the exception of the West Bay 

unit, which has not been installed, all of these are functioning 

well and verifying the representations made by ARI for catalyst 

consumption, efficiency, etc. In almost every case, the units which 

have been placed in service are "firsts" with respect to the 

mechanical arrangement or the process flow. This is no longer the 

case and we are now able to approach most jobs with a background of 

well demonstrated performance and well defined economics. 

In addition, units are under design and/or construction for ARCO 

Petroleum Company's Sable Plant at Plains, Texas (for CO2 EOR), 

Grace Petroleum, Leonard, Michigan, Coal-Tec, Gillette, Wyoming 

(coal gasification), and Texaco's San Ardo, California fire flood 

project. The most significant addition to this list is the Oil and 

Gas Commission of India (ONGC) who have selected LO-CAT for the 

Hazira Plant to treat acid gas derived from the Bombay Offshore 

Project. This installation will treat over 1 MMMSCFD of gas in 

seven LO-CAT units with 12.5 metric ton/day of sulfur production 

capacity each. Plans call for immediate installation of the first 

three trains, with two additional trains in six months and the 

remaining two within two years. 

This paper describes the technical basis of the process, some 

recent design and operating experience, and outlines the economic 

factors involved in the selection of a LO-CAT unit. 

-2-
P.5 

Technical Basis 

A simplified flow scheme for a conventional LO-CAT® Hydrogen Sulfide 

Oxidation Process is shown in Figure 1. The gas to be treated is 

contacted with an aqueous scrubbing solution in an absorber and the 

H2S is absorbed and converted immediately to elemental sulfur. 

No other chemical reactions take place in the absorber. If CO2 is 

present, it may be absorbed to a limited extent, but the oxidation 

reaction is strictly limited to hydrogen sulfide. 

The scrubbing solution is regenerated by contact with atmospheric 

air in an oxidizer vessel. 

The sulfur forms as small solid particles suspended in the aqueous 

catalyst solution. They circulate around until the size has grown 

large enough that they can be separated by settling. This is 

usually done in the bottom of the oxidizer vessel, although any 

vessel designed with low velocity and a suitable cone-bottom can be 

used for separation of the sulfur. For plants which treat only a 

small amount of H2S - 0 to 0.5 LT/D - there is very little more to 

the process than is shown on the left-hand side of Figure 1. 

Settled sulfur slurry has about 15% sulfur by weight and 85% of the 

dilute catalyst solution in it. This material is ordinarily treated 

as a waste product because of the very small quantity of sulfur 

available, and because the value of the catalyst solution is too low 

to warrant the installation of separation equipment to recover it 

and return it to the process. 

A larger plant, in the 0.5 to 5 LT/D range of sulfur production 

rate, can pay off dewatering of the sulfur by means of a centrifuge 

or filter in order to recover the catalyst solution which would 

otherwise be discarded. Larger plants with capacities of 2 tons per 

day upward can often justify the added expense of a sulfur melter, 

molten sulfur storage facility and truckloading station. In these 

units, no catalyst solution is discarded, and the sulfur can 

ordinarily be sold for use in sulfuric acid manufacture or for other 

industrial uses. 

The unit described in Figure 1 would be quite suitable for a low 

pressure gas treating application at pressures up to about 50 psig. 

There is no problem meeting the usual quarter grain H2S 

specification, and ordinarily no CO2 or mercaptan sulfur compounds 

are removed from the gas stream. 

* 

High pressure units, or units with very large gas volumes, require 

modified adsorbe r designs in order to satisf y the performance 

requirements with a reasonable first cost. Venturi scrubbers, 

packed scrubbers, stati c mixers and other conventional contacting 

devices are al l suitable , and are all being used for this service. 

However, grea t car e must be exercise d in th e design of the 

contacting devices to make sure solid sulfur particles do not cause 

plugging problems. We have units in operation with a wide variety 

of contacting devices. 

- 3 -
P.6 

The oxidizer vessels are generally liquid-filled tanks into which 

oxidation air is introduced through air spreaders or spargers at the 

bottom of the vessel. None of the oxidizers in service have 

mechanical mixing devices or packing in the oxidizer vessel except 

for those which have air in the process gas stream. 

The presence of carbon dioxide in the gas stream being treated 

requires buffering of the solution with an alkali salt such as 

sodium hydroxide, soda ash, KOH, or ammonia. The higher CO2 partial 

pressure, the more buffering is required, and the greater will be 

the change in pH of the operating solution as the liquid circulates 

between the absorber with a high CO2 partial pressure in the 

oxidizer with a greatly reduced CO2 partial pressure. In chelated 

iron solutions using EDTA or Type A chelates alone, this pH 

variation often leads to precipitation of the iron from the 

solution, and a nearly complete loss of catalyst activity. 

This does not happen in the LO-CAT® process because of the unique, 

patented dual chelate system which stabilizes the iron and prevents 

precipitation at all pH levels. 

This very desirable characteristic of the solution permits LO-CAT 

operations on pure CO2 streams, and on CO2 at elevated pressures up 

to and including a supercritical operation. Very high CO2 pressures 

require special buffering techniques, but do not present any 

fundamental process problems for LO-CAT. 

In cases where the gas volume rate of flow is low, liquid-full 

absorbers have proven to be both very efficient and free of 

mechanical problems. In situations where the gas flow is low and 

can also be treated as a waste gas - that is discarded into the 

atmosphere after cleanup - a very simple autocirculation design can 

be applied. The patented autocirculation system is a natural for 

treatment of amine offgases and Figure 2 is a process flow diagram 

for an autocirculation LO-CAT unit. This is the design which is 

currently in operation at the Tejas Gas Indian Rock Plant near 

Longview, Texas. 

The autocirculation design incorporates a number of advantages in 

addition to those listed for the LO-CAT process. These include: 

- Extreme simplicity. There are no solution circulating 

pumps, level controls, etc. 

- Low operating cost. The catalyst concentration can be 

held to low levels (less than 1 lb of catalyst concentrate for 50 to 100 lbs of water), which minimizes 

catalyst losses with the sulfur product; low circulating pump horsepower. Electrical loads are limited 

to slightly more than the air blower horsepower 

requirement. 

-4-
P.7 

- Very good efficiency. We will guarantee these units 

at 99.95% efficiency for any ordinary amine effluent 

treating operation. 

- Very high reliability. 

Operating Experience With Autocirculation Units 

The first full-sized autocirculation process plant started up at the 

Tejas Indian Rock Plant at Gilmer, Texas on May 28, 1984. Figure 3 

is a photograph of this plant. 

This plant was constructed by Tejas to treat and transport 50 MMSCFD 

of gas under contract to Exxon. Tejas contacted several major 

engineering/contracting companies for proposals for the treating 

plant and also contacted ARI Technologies directly. The initial 

proposal was for a LO-CAT® unit to treat 20 MMSCFD at line pressure 

as indicated in Figure 1, with an alternative proposal to treat only 

the acid gas produced by a conventional DEA scrubbing system. 

Because of the high CO2 concentration of the gas, DEA scrubbing 

would be required downstream of the LO-CAT unit for the high 

pressure case. 

Both capital and operating cost appear to be lower for the case in 

which the LO-CAT unit would be treated for atmospheric pressure and 

this approach was selected. The size of the plant was upgraded from 

20 million cubic feet a day to 50 million cubic feet a day, and 

contracts for the equipment were let in 1984. ARI was awarded a 

contract for the LO-CAT plant on a skid-mounted basis, including the 

interconnecting piping betwen skids, and the absorber/oxidizer 

vessel as a stand-alone unit. The amine plant was furnished by 

EHR-Emron Engineering, Inc. and BS&B Engineers, Inc. was awarded the 

contract for installation of all of the equipment. 

A deadline date of June 1, 1984 was established for commercial gas 

processing with substantial penalties for failure to complete the 

project on time. 

Operation began on schedule with cut in of gas to the LO-CAT unit on 

May 28, 1984. 

Initial operation of the plant was uneventful. The startup consists 

of simply turning on the air blowers, chemical injection pumps and 

centrifuge and slurry handling pumps. The initial gas processing 

rate was in the 14 MMSCFD to 28 MMSCFD range with variable H2S 

loadings. Under reduced load conditions, the flows and catalyst 

concentrations in the LO-CAT unit are ordinarily left at design 

rates and 100% turndown is expected during a routine operation. The 

flow variations during early operation showed no adverse effect on 

the plant operation, and no H2S residual was found in the gas stream 

at all. During the first month of operation, chemical additive 

-5-
P.8 

rate s were set a t the design levels, but sulfur production averaged 

less than one-half the normal design rate . Ordinarily, when a unit 

is operated a t below peak capacitie s i t is possible to reduce the 

concentratio n of active chemicals in the solution, and the chemical 

consumption is further reduced automatically by any decrease in the 

amount of wet sulfur discarded from the system. This combination of 

circumstance s led t o an accumulation of activ e catalyst in the 

solution, and concentrations rose from the 500 ppm of active iron on 

which the design was based to over 650 ppm during June and July. 

There is no adverse effec t of such chemical overabundance in the 

system, except tha t the cost of the chemicals being supplied is, of 

course, higher when the addition rate s are se t higher than they need 

be. 

During the initia l operating period, the plant was attended around 

t h e clock . Operators were instructe d t o sample th e catalyst 

solutio n and record the pH and redox potentia l readings once per 

shift . Alkali levels above 7.5 and redox potentia l readings above 

-200 ensur e tha t th e H2S scrubbing efficiency will be adequate and 

tha t th e concentratio n of catalyst i n the system is sufficiently 

high t o handle the maximum H2S load on the unit. Chemical analyses 

for chelat e composition and th e concentratio n ar e not ordinarily 

recommended in the field , and once-monthly samples of solution are 

returned to ARI's laboratorie s for check analyses. 

Afte r 30 days of operation , th e pH redox potentials indicate d 

satisfactor y operatio n and the laboratory analysis suggested tha t 

the catalyst additio n rat e could be reduced substantially without 

interferin g with the plan t operation . Subsequent reduction s in 

chemical additio n rat e brought the chemical consumption per lb of 

sulfu r int o lin e with the design conditions while additions to the 

plant are a t considerably below design addition rates. 

From th e standpoint of performance and process chemistry, the Tejas 

Plan t has performed extremely well. Two minor mechanical problems 

have occurred and been corrected. The first of these involved the 

loss of a bearing on the single sulfur dewatering centrifuge. This 

apparentl y resulte d from initia l misalignment of the centrifuge 

shaft , and afte r correctio n ther e should be no reoccurrences. 

During repair of th e centrifuge , th e acid gas was bypassed around 

t h e LO-CAT® uni t t o the flare . The LO-CAT uni t could function 

normally without the centrifuge a t the expense of a higher chemical 

consumption because of the catalyst solution discarded with the wet 

slurry , and als o with the production of a much large r volume of 

wast e material . Had productio n of a limite d amount of sulfu r 

dioxid e been critica l from an air pollution control standpoint, the 

plan t coul d have been kep t i n servic e throug h th e period of 

centrifuge outage. Alternatively, a plant of this sort could employ 

dua l centrifuge s (al l othe r rotatin g equipment in th e plan t is 

spared), or a t this siz e or slightl y larger, a continuous melter 

could be employed. 

- 6 -
P.9 

The second mechanical problem occurred during September, 1984 as the 

result of a boilover of the amine still. This discharged 

approximately 1000 gallons of MDEA solution into the LO-CAT® plant, 

probably a substantial quantity of liquid hydrocarbons. 

The result of this upset was continued operation of the LO-CAT unit 

without diminished efficiency, but some foaming and carryover of 

LO-CAT solution from the unit with the oxidizer air. Ordinarily, we 

do not expect to see any negative effect on the process when the 

solution is contaminated with amines. The same is true for glycol, 

alcohols and other completely water miscible solvents. 

Hydrocarbons, on the other hand, do have a negative effect on the 

mechanical separation of the sulfur. Liquid hydrocarbons wet the 

sulfur particles preferentially and cause them to agglomerate into 

sponge-like masses that entrap air bubbles. This, again, would 

ordinarily be counteracted by the addition of a larger-than-normal 

dosage of wetting agent supplied to ensure wetting of the sulfur 

particles under normal conditions. This incident did not result in 

any downtime or discharge of any H2S or sulfur dioxide from the 

plant. However, it did result in solution carryover from the 

oxidizer for a period of two or three days while wetting agents were 

being added at a higher than normal rate. 

Overall, operation of the LO-CAT unit has been trouble free and 

performance has been nearly perfect with respect to eliminating 

emissions of SO2 and H2S from the atmosphere. 

Chemical consumption for the unit at design conditions consists of 

additions of each of the following materials: 

- ARI-310 Catalyst Concentrate 

- ARI-310M Chelate-rich Makeup Solution 

- ARI-400 Biostat 

- Wetting Agent 

- KOH 

The addition rates for these materials are lower than the design 

rate, because the sulfur production rate is lower and the chemical 

consumptions tend to be proportional to the sulfur discard rate. 

Gal/hr 

0.38 

1.04 

0.015 

0.0135 

3.873 

$/Day 

86.18 

160.74 

4.57 

2.19 

163.59 

417.27 

-7-
P.10 

The sulfur product was represented by ARI to be of good chemical 

quality, but of little value because of the relatively low 

production rate involved. At 1.5 LT/D, the value of the recovered 

sulfur is not sufficient to pay for the capital cost of the melting 

storage and truckloading facilities required for product sales. The 

sulfur product emitted by the Indian Rock Plant appears to average 

50 to 65% sulfur by weight, with impurities consisting of the 

catalyst solution ingredients. A chemical assay of this material is 

shown in Table 1. In order to evaluate the sulfur which could be 

made by continuous melting, samples of the Indian Rock Plant product 

were melted in a laboratory autoclave. The molten product 

solidified to a near bright yellow color, and was remarkably free of 

impurities. 

Experience at Alberta Natural Gas 

The Corchran natural gas liquids plant operated by Alberta Natural 

Gas Company at Corchran, Alberta, Canada was constructed in 1981-2 

with a design capacity of over one billion cubic feet per day of 

natural gas. The plant consists primarily of an expander type 

refrigeration unit followed by a cryogenic gas separation system 

which produces several hundred thousand barrels per day of natural 

gas liquids. 

The gas entering the plant is of pipeline quality, with less than 

1/4 grain H2S per 100 SCF. Carbon dioxide concentration is also 

quite low and averages on the order of 1.3%. The Corchran plant was 

designed as a "sweet gas" plant throughout. However, provision was 

made for the separation of CO2 from the ethane to liquid product by 

means of a liquid-liquid amine extraction system. 

When this plant was put on stream in 1982, it became apparent that 

along with the CO2 condensed from the gas by refrigeration, 

sufficient hydrogen sulfide was removed to render the C02~rich acid 

gas from the amine extraction unit odorless, and capable of creating 

a local nuisance. 

The Alberta Natural Gas personnel became aware of the LO-CAT® 

process, and were encouraged by the successful operations on high 

CO2 streams at the Seven Sisters Gas Field. In October, 1983, 

Alberta Natural Gas ordered a LO-CAT H2S scrubbing system from ARI. 

The plant was designed by ARI and fabricated in Canada. Startup was 

in August, 1984, and had many features in common with the operations 

at Indian Rock. 

-8-
P.11 

The H2S loadin g t o th e plan t ha s t o dat e bee n considerabl y below th e 

desig n rate . This doe s no t caus e an y operatin g problems, and ha s 

resulte d i n substantiall y complet e remova l o f th e hydroge n sulfid e 

rathe r tha n th e 99% remova l efficienc y fo r whic h th e plan t wa s 

designed . 

The flo w scheme i s significantl y differen t tha n tha t utilize d a t 

India n Rock , becaus e o f th e potentia l futur e us e o f th e CO2 fo r 

beverag e purposes . I n orde r t o avoi d contaminatio n o f th e ai r 

strea m o f th e CO2 'strea m wit h oxygen , separat e absorber/oxidize r 

vessel s wer e installed . The basi c flo w scheme appear s nearl y 

identica l wit h tha t utilize d earlie r wit h th e Seve n Sister s 

operation , bu t th e contacto r vesse l i s markedl y different . Whereas 

t h e Seve n Sister s unit s utiliz e low pressur e dro p packe d absorbe r 

vessels , th e Corchra n ha s a larg e liquid-ful l vesse l instead . The 

liquid-ful l absorbe r i s much large r i n diameter, more costl y tha n 

t h e packe d column , an d ha s a highe r pressur e drop . However, thi s 

typ e o f contacto r ha s almos t n o potentia l fo r pluggin g wit h 

elementa l sulfur , and i s inherentl y a ver y hig h efficienc y device . 

T h e economi c consideration s involve d th e us e o f a single , mor e 

expensiv e liquid-fille d absorbe r o r alternativel y two packe d 

columns, on e of which would serv e a s a spar e shoul d sulfu r pluggin g 

of th e operatin g vesse l occur. 

The Corchra n plan t produce s a relativel y trivia l amount of sulfu r a s 

compare d wit h India n Rock, bu t accomplishe s a ver y importan t ai r 

pollutio n function . Becaus e o f th e .low sulfu r productio n rate , 

chemica l consumptio n i s extremel y smal l and ha s bee n difficul t t o 

ascertain . 

The desig n rate s of each of th e chemica l constituent s are : 

ARI-310 

ARI-310M 

ARI-400 

Wettin g Agent 

KOH 

Gal/hr 

0.195 

0.096 

0.014 

0.012 

0.97 

$/day 

44.22 

14.83 

4.26 

1.94 

1.70 

62.69 

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P.12 

Economic Considerations 

The two plants described previously represent typical autocirculation and conventional LO-CAT® units. The costs for both 

types of units are strongly dependent upon the sulfur production 

rate, and relatively less dependent upon the gas flow rate, so long 

as the gas composition is within the "normal range of compositions" 

for each type of unit. 

Figure 4 is a plot of estimated capital costs for the skid-mounted 

equipment package for small (less than 5 LT/D) LO-CAT units of both 

types. These costs are relatively accurate (+10%) and represent 

currently quoted plants fabricated by ARI, less installation cost. 

Installation ordinarily includes the installation of foundations, 

the vessels and equipment skids in place and securing them, 

installing inter-skid piping and electrical connections, bringing 

utilities such as electric power, instrument air, and the gas piping 

to the unit, and checkout of equipment for proper mechanical 

operation. 

Most of the LO-CAT units in operation are in geographic areas with 

moderate climates, and no provision is made for freeze protection. 

These units do ordinarily have a small trim heater installed in 

these circulating solution streams or in the absorber/oxidizer 

vessel to provide for keeping the solution temperature above the 

temperature of the incoming gas in order to prevent condensation of 

hydrocarbons or water in the unit. In case of brief excursions 

below 32°F, these units should continue to function properly because 

of the heat liberation by the oxidation of hydrogen sulfide, and by 

use of the trim heaters to maintain the solution temperature above 

freezing. However, in colder climates either provisions for heat 

tracing or enclosure of portions of the unit in a heated building is 

ordinarily required. The Tejas unit was installed without 

provisions for freeze protection, and is basicly an outdoor unit. 

Some heat tracing will be installed for protection of the smaller 

lines, and the use of ethylene glycol as an antifreeze is 

contemplated if the weather becomes unusually cold. Inclusions of 

several percent ethylene glycol in the circulating solution do not 

have a deleterious effect on the operation of the process and can 

depress the freezing point of the solution substantially. 

The Alberta Natural Gas unit is fully enclosed in a heated building. 

The unit under construction for ARCO Oil and Gas Company in Plaines, 

Texas will be partially enclosed in a heated building, with the 

absorption columns extending beyond the roof line. 

The costs for equipment packages indicated in Figure 4 are based on 

"normal" ranges of concentration of H2S. Outside these ranges some 

additional capital and/or operating costs may be required. 

-10-
"Normal" for an autocirculation unit on an amine effluent stream 

covers the range from about 10 vol% H2S upwards to 100% H2S. Lower 

concentrations can be handled without difficulty, and do not involve 

any increase in the operating cost. However, below 10 vol% H2S gas 

streams require absorber volumes and add significantly to the first 

cost. Units of this basic design have been built for operations as 

low as 1 vol% H2S. 

"Normal" concentrations for conventional units usually involve H2S 

concentrations below 2 vol%. As in the case for autocirculation 

units, this range can be extended considerably above the 2% level, 

but usually requires a capital cost somewhat higher than indicated. 

The principle difference between conventional and autocirculation 

units from a functional standpoint lies in the greater ease of 

separating the process gas from oxidizer air in a conventional 

design. In some cases, conventional design is used even though 

there is no requirement for recovery of the sweetened gas. This is 

usually because the gas volume is very large and the concentration 

of H2S is so low that an inordinately expensive absorber would be 

required for the autocirculation design. 

Also, the autocirculation approach is sometimes used even though it 

is necessary to separate the process gas from the oxidizer air. 

This is done by the inclusion of a "cap" over the upper end of the 

absorber which places a positive liquid seal between the sweetened 

gas and the spent oxidizer air. 

Operating costs for LO-CAT® units can be divided into utility costs 

and chemical costs. The utility costs consists of electric power to 

drive the solution circulating pumps (for the conventional design), 

the oxidizer air blowers, sulfur slurry pumps and clarified solution 

return pumps and the centrifuge if one is included, or steam for the 

sulfur melter on larger units. Usually, there is a small trim 

heater included in the circulating solution line or within the 

oxidizer vessel to maintain the solution temperature if insufficient 

heat is being generated by the reaction of hydrogen sulfide and 

oxygen to keep the solution temperature at least as warm as the 

incoming gas temperature. The requirement here is quite nominal, 

and on units of one half ton per day or greater capacity, there is 

usually no requirement for external heat. Operating utility 

requirements are ordinarily fixed at the design rate coinciding with 

maximum sulfur handling capacity, and remain fixed regardless of 

actual load of the unit. 

Chemical costs consists of catalyst and makeup solution, alkaline 

buffering material for pH control and trace additives to control wet 

ability of the sulfur particles and to prevent biological attack. 

These chemicals are all used at rates that are proportional to the 

actual sulfur production rate. That is, a plant designed for 5 ton 

-11-
P.M 

per day sulfur production capacity but actually running a t 0.5 ton 

p e r day wil l use only one tent h of th e chemicals specifie d for 

operation a t design conditions. For prolonged operation a t very low 

loads, even greate r savings can be realized by taking advantage of 

the highe r circulatio n rat e buil t int o th e process to reduce the 

catalyst strengt h t o a level adequate for the lower loadings. The 

discar d of catalyst solutio n with the sulfur product decreases in 

proportio n t o th e decreas e i n sulfu r production . If th e 

concentratio n of catalyst in th e solutio n is reduced below the 

desig n concentration , even greate r savings ar e realized . For 

example, the 5 ton uni t operatin g a t 0.5 ton per day and designed 

for 500 ppm iro n concentratio n in the solutio n would discard one 

tent h of th e norma l amount of th e solution , and th e iro n 

concentratio n could be allowed to decrease to only 50 ppm, reducing 

the discard rat e to 1/100 of the design rate . 

For autocirculatio n units, the air blower comprises most of the 

utilit y costs. In small units th e air blower is sized to provide 

adequat e coolin g as wel l a s oxidation , and lower capacitie s 

typically require 30 horsepower per ton per day of sulfur production 

capacity . For larg e units i t is uneconomical t o provide cooling 

with the air blower, and the brake horsepower per ton of sulfur per 

day is reduced to 15 and other means of cooling are provided. 

The only othe r significan t utilit y cost for LO-CAT® units is the 

circulatin g pump for conventional designs. Optimum design involves 

an absorbe r which operate s a t about 20 psi g and an atmospheric 

pressur e oxidize r vessel . In this case there are three levels of 

power requirement based on th e method of disposa l of the sulfur. 

For units wit h melters, th e catalys t iro n concentratio n is 

ordinaril y maintained a t 1500 ppm, and the horsepower per ton per 

day of sulfu r for the circulating pump is approximately 25. Units 

with centrifuge s instea d of melters ordinarily operate with about 

1000 ppm iro n concentration and a brake horsepower per ton per day 

of 37.5 . Small units where th e sulfu r is discarde d without 

dewatering, generally operate a t 500 ppm and use 75 horsepower per 

ton of sulfur per day. 

Of course , on autocirculatio n units ther e is no circulating pump 

horsepower a t all . 

The costs consist principall y of catalyst concentrat e and makeup 

solutio n usage and KOH consumption. Units operating a t atmospheric 

pressure , th e catalyst consumption usuall y is of greate r concern 

than buffering agent losses. 

-12-
ARI-310 catalyst is not consumed in the process and need be added 

only to replace solution which is lost by spillage or withdrawn with 

the sulfur or in a separate blowdown stream. Catalyst costs for 

autocirculation units with catalyst concentration of 200 ppm for the 

three different methods of sulfur disposal, given in dollars per ton 

of sulfur, are tabulated below: 

Discard of 15 wt% slurry $141.20/per ton 

For 50% cake discard from 

centrifuge $ 21.20/per ton 

No discard with sulfur; thiosulfate 

withdrawn based on 2% thiosulfate 

production $ 1.23/per ton 

In the case of units with melters, there is no way for soluble 

sulfur compounds such as thiosulfate or sulfate to leave the LO-CAT® 

unit, and the laboratory operations have suggested that the 

thiosulfate concentration will in time build up to sufficiently high 

levels to require a purge stream to prevent condensation of sulfate 

or thiosulfate salts. For this reason, a calculated loss of 

catalyst with a purge stream based on maintaining no more than 30% 

total soluble sulfur compound salts in solution has been included. 

Actual operating results over 3-1/2 years with two large units with 

melters have indicated that the thiosulfate has not built up to a 

sufficiently high concentration to require purging of any solution. 

It is reasonable to expect that this catalyst loss may not be 

incurred in future units, but there is not sufficient evidence by 

way of a thiosulfate decomposition mechanism to be sure that there 

will not be some catalyst loss via this route. 

ARI-310M is a chelate-rich additive which is put in the unit to 

replace the chelates which degrade with time. 

Both laboratory and field test results indicate that this rate for 

ARI-310 is approximately .07 lbs per lb of sulfur produced. The 

cost of this material varies with quantity, and may be roughly 

characterized as given below: 

Small units 

(0-1 LT/D) $ 94.00/per ton 

Medium units 

(1-5 LT/D) $ 78.50/per ton 

Large units 

(greater than 5 LT/D) § 62.75/per ton 

-13-
It is apparent that many factors enter into the operating cost of 

individual units. However, one may generalize costs per ton of 

sulfur as a function of plant size as follows: 

P.16 

Typical size LT/D 

Small 

Plants 

0.5 

Medium 

Plants 

Large 

Plants 

10 

Typical cost $/annual ton sulfur 

production: 

Autocirculation Plant 

(equipment only) 

Conventional Plant 

(equipment only) 

Operating costs for 

Autocirculation Unit 

Catalyst costs $/Ton 

Utility cost 

Total operating cost, $/Ton 

Conventional Unit 

Catalyst costs $/Ton 

Utility cost $/Ton 

Total operating cost, $/Ton 

$1,000.00 $ 700.00 $ 500.00 

2,000.00 1,000.00 680.00 

235.20 

1.50 

236.70 

450.00 

5.25 

455.25 

99.70 

1.50 

100.70 

184.50 

3.35 

187.85 

64.00 

0.75 

64.75 

81.20 

2.00 

83.20 

A great deal of care must be used in applying these figures to units 

that are unusual, or which involve operation at high pressures or 

with gaseous constituents other than light hydrocarbons, CO2, and 

H2S. 

-14-
Summary 

The LO-CAT® process offers a simple and cost-effective method for 

removal of hydrogen sulfide from either natural gas streams or from 

amine acid gas. Several units have come on stream during the past 

year and have generally worked well and within the representations 

of chemical consumption and operating utilities represented. 

Plant costs per ton of sulfur vary considerably with plant size, and 

in the larger sizes are competitive with Claus plants, although the 

efficiency of the LO-CAT process is much higher and turndown 

capability substantially unlimited. For smaller plants, the capital 

costs are not competitive with the Claus process, but LO-CAT works 

well, whereas very small Claus units do not. 

Chemical costs for small LO-CAT units are high in terms of the cost 

per ton of sulfur produced, but low in total cost, and usually 

insignificant in the evaluation of alternative processes. 

Large plants require much less chemical cost per ton of sulfur, but 

the costs are large in the aggregate and likely to be a significant 

factor in the process evaluation. For large plants with sulfur 

melting equipment, the minimum chemical cost is on the order of $50 

per long ton of sulfur produced under the most favorable conditions. 

The LO-CAT process is the only viable alternative for treating some 

small gas streams which cannot be adequately handled by the Claus 

process, yet are too large for economical application of iron 

sponge. However, plants of substantial capacity are being licensed 

where the outstanding advantages of the LO-CAT unit outweigh the 

chemical cost disadvantage. The most recent of these, a 37.5 LT/D 

system for the Oil and Natural Gas Commission of India's Gujarat 

offshore plant is an example. 

-15-
P.18 

TABLE 1 

CHEMICAL ASSAY OF 

TEJAS SULFUR PRODUCT 

SULFUR. WT% 57.56 

IRON.WT% 0.0431 

PH 7.28 

MELTED SULFUR APPEARANCE SEMI BRIGHT YELLOW 

MELTED SULFUR FE ASSAY 45 PPM WT. 

REACTIVITY 

REACTIVITY SULFIDE <0.25 PPM WT. 

REACTIVITY, CYANIDE <.025 PPM WT. 
P.19 

TABLE 2 

RCRA HAZARDOUS WASTE 

DETERMINATION 

E.P. TOXICITY 

ARSENIC 

BARIUM 

CADMIUM 

CHROMIUM, TOT. 

LEAD 

MERCURY 

SELENIUM 

SILVER 

DETERMINED 

VALUE PPM WT. 

0.003 

<0.01 

<0.001 

0.006 

<0.01 

<0.001 

<0.01 

0.004 

MAXIMUM PERh 

PPM WT. 

5.0 

100.0 

1.0 

5.0 

5.0 

0.2 

1.0 

5.0 
>>SuiJ :-)U vy*! 

o 

SOUR GAS 

SCRUBBER 

VENTURI PACKED 

PRECQNTAGTOR ABSORBER 

OXIDIZER AIR 

COMPRESSOR 

CENTRIFUGE 

I 

GAS* 1 

IN 

*!«>»* 

J 

SWEET GAS OUT 

11 

£** 

TO ATMOS. 

11 

T —0-

SOLUTION 

COOLER 

xt n 

-y— v v v >/ | 

. • - • 

• * • • • o 

* 1 

*. .. -. 

i_j / 

AIR SPARGER 

J Q^TVWN ] 

FIGURE 1 B 

e^ 

NOTE: 

CHEMICAL 

FLOW SCHEME 

FOR CONVENTIONAL 

LO-CAT® UNIT 

ADDITION EQUIPMEN T NOT SHOWN 

SOLUTION 

CIRCULATION 

PUMP 

S3 

CLARIFIED SOLUTION 

RETURN TANK 

SULFUR CAKE 

RECEPTACLE 

SILENCER 

i 

* ^ T 

CLARIFIED SOLUTION 

RETUR N PUM P 

an TECHMXOGES NC 

ANAEROBI C 

LO'CAT^IHYDROGEN SULFIDE 

OXIDATION PROCESS 

A 
SOUR GAS 

SCRUBBER 

t \ 

SOUR 

GAS K 

^vxxx 

NOTE : 

AUTOCIRCULATION AIR 

ABSORBER / OXIDIZER BLOWER 

CENTRIFUGE 

FEED TANK 

CENTRIFUGE 

VENT 

TO 

ATMOS 1 

^ 

1 

•"* •"*

 k ' •-»'«•• ' -•• • ' * * 

J V 

? HI— * 

SULFUR CAKE 

RECEPTACLE 

CLARIFIED 

SOLUTION 

RETURN 

TANK 

FIGURE 2 

J -v 

rr-FWNr 

SLURRY 

PUMP 

Ldi 

FLOW SCHEME FOR 

TAIL GA S 

AUTOCIRCULATION 

LO-CATTM 

< . 

SLURRY 

PUM P 

CHEMICAL ADDITION EQUIPMEN T NOT SHOWN . 

f-\ 

NOTE : 

CENTRIFUGE 

NOT REQUIRED 

CLARIFIED SOLUTION 

RETURN PUMP 

an TECHNOLOGIES NC 

PROCESS FLOW SKETCH FOR 

AUTOCIRCULATION 

LO-CATi»HYDROGEN / SULFIDE 

OXIDATION PROCESS 

—"W.ASJ"' r—== 
P.22 

*»"OI«-' | mi.-'fmm' — *...-•- *?•.'-*" ^1.1 . "r '' 
CO 

cr 

< 

. j 

- i 

o 

a 

H 

CO 

o 

o 

FIGURE 4 

PLOT OF EQUIPMENT COST 

VS SULFUR CAPACITY" ' 

10,000,000 

1,000,000 

100,000 

1 10 100 1000 

SULFUR CAPACITY, LB/HR 

(1) SULFUR CONCENTRATION, FREEZE PROTECTION NOT INCLUDED.