REDUCING SULFURIC ACID

PLANT SO2 EMISSIONS WITH THE CANSOLV® SO2

SCRUBBING SYSTEM

Valérie Léveillé & Colin Ryan

Cansolv Technologies, Inc.

Presented at the

33rd

International Phosphate Fertilizer & Sulfuric Acid Technology Conference

June 2009

ABSTRACT

As new legislation drives to lower SO2 emissions from sulfuric acid plants, plant managers face the

challenge of keeping their costs under control. The CANSOLV®

SO2 Scrubbing System is a cost

effective solution for plant operators to meet the new SO2 emissions targets.

The CANSOLV®

SO2 Scrubbing System is an amine-based regenerable process that selectively

absorbs SO2 from a variety of gases, including sulfuric acid plant tail gas. The system produces a

pure, water-saturated SO2 byproduct stream, which can be returned to the front end of the sulfuric

acid plant for conversion to sulfuric acid. SO2 emissions in the treated gas can be controlled to as

low as 0.15 lb SO2/ton acid (10 ppmv SO2). Furthermore, the integration of the CANSOLV®

SO2

Scrubbing System to single absorption sulfuric acid plants offers several operational advantages

such as decoupling the acid plant performance from the SO2 emissions, decreasing SO2 emissions

on demand if future legislation requires it, and eliminating the need for expensive sulfuric acid plant

converter catalyst.

This paper will describe the features and cost advantages of applying the CANSOLV®

SO2

Scrubbing System to treat sulfuric acid plant tail gas.

Introduction

Cansolv Technologies Inc. (CTI) was formed in 1997 to commercialize the Cansolv®

SO2

Scrubbing System. In 2008, CTI was acquired by Shell Global Solutions International B.V. (SGSI)

and the company now operates as a wholly owned subsidiary of SGSI. CTI is an innovative,

technology-centered company with a commitment to provide custom designed economic solutions

to clients' environmental problems.

While the Cansolv process was originally targeted to the utility boiler flue gas desulfurization

market, the process was first commercialized on specialty process applications including sulfur

plant tail gases, acid plant tail gases and smelter off-gases. One of the first Cansolv plants to be

commissioned in 2002 was on an acid plant tail gas unit in the state of California since 2002. The

plant was designed to meet SO2 emissions under 50 ppmv as required by the stringent Southern

California air quality regulations and has met or exceeded this level since start-up.

In the ten years since the first technology license was signed, adoption of the Cansolv SO2

technology has been most intense in the non-ferrous smelting. More recently, there has been

renewed interest for the application of the Cansolv®

SO2 Scrubbing System on acid plant tail gases.

In the past couple of years, the US EPA has commenced the process of entering consent decrees

with sulfuric acid producers, mandating further reduction in SO2 emissions during normal operation

as well as during start-up and upset conditions. Furthermore, some alkali scrubbers installed to meet

emissions on single absorption acid plants are subjected to increasing reagent costs and seasonal or

quality constrained markets for by-products.

At this time, ten commercial Cansolv®

SO2 Scrubbing Systems are in operation, five are scheduled

for startup in 2009 and several more are in the detailed engineering or procurement phase. In 2009,

the first Cansolv SO2 coal combustion gas scrubber will also be commissioned. Several of the non-

ferrous industry clients have elected to convert the recovered SO2 to sulfuric acid. When these

clients have had to install a new acid plant, they have chosen to install a single absorption and use

the Cansolv SO2 Scrubbing System to treat the tail gas.

With the US EPA in consent processes with fertilizer and sulfuric acid producers, several will have

to reduce SO2 emissions without creating more effluent or byproducts. This paper will address the

features and advantages of applying the Cansolv®

SO2 Scrubbing System to treat sulfuric acid plant

tail gas.

Cansolv®

SO2 Scrubbing System Description

The Cansolv®

SO2 Scrubbing System is a patented technology that uses an aqueous amine solution

to achieve high efficiency selective absorption of SO2. The scrubbing by-product is pure, water

saturated SO2 gas recovered by steam stripping using low quality heat. The process is regenerable

meaning the chemical absorbent is not consumed. The high costs of consumable reagents are thus

eliminated and effluents are reduced to a minimum.

The Cansolv®

SO2 Scrubbing System applied to a sulfuric acid plant tail gas operates as follows

(refer to figure 1 for the process flow diagram):

� The dry acid plant tail gas must be cooled and saturated with water prior to contact with the

amine solution in the Cansolv SO2 absorber. The purpose is to minimise evaporation of water

from the amine solution in the SO2 Absorber Tower. A simple in-line quench using rich amine

at liquid to gas ratio of 10 USG/ACF is sufficient to quench the gas. Quenching with rich amine

will occur at a pH of 3.5 or higher so duct metallurgy of 316L SS is sufficient.

� The quenched tail gas is then contacted with the lean amine solution in a counter-current SO2

Absorber Tower containing structured packing where the SO2 is absorbed. The treated gas exits

the SO2 Absorber Tower to atmosphere via a stack with a SO2 content as low as 10 ppmv (0.15

lb SO2/t acid). The SO2 laden rich amine from the Absorber Tower is pumped to the SO2

Stripper Tower.

� The rich amine solution is regenerated by indirect steam stripping and the SO2 gas is recovered

as a pure, water saturated product. Various heat sources may be used in the reboiler, such as low

pressure steam or pressurized hot water. In plants where the heat balance is tight, heat recovery

options internal to the Cansolv®

SO2 Scrubbing System may also be used. Two common heat

recovery options are mechanical vapour recompression and double effect split flow (DESF)

regeneration which can be built in to reduce heat requirements by as much as 55% relative to a

simple flowsheet.

� The lean amine leaves the reboiler and is pumped to the SO2 Absorber Tower via the lean-rich

amine heat exchanger, the lean amine tank and the lean amine cooler.

� A slipstream of the amine is treated for Heat Stable Salts (HSS) removal in the Amine

Purification Unit (APU). The most common HSS is sulfate which is formed when SO3 or acid

mist in the tail gas contacts the Cansolv amine. Because sulfate is a much stronger acid than

sulfite or bisulfite, its absorption cannot be reversed by heating and stripping the amine. The

APU therefore uses ion exchange resins to selectively remove these in the liquid phase

� The overhead vapour of the SO2 Stripper Tower is cooled in a condenser. The SO2 gas is

separated in the Overhead Reflux Accumulator from which the condensed water vapours,

reflux, is pumped back to the SO2 Stripper Tower. The water saturated SO2 gas is recycled to

the front-end of the sulfuric acid plant to be mixed in the combustion air upstream of the drying

tower (in the case of a sulfur burning acid plant) or into the clean SO2 laden gas (in the case of a

metallurgical acid plant). Conversion of the recovered SO2 to sulfuric acid rather than a salt

effluent will result in a marginal increase in sulfuric acid production.

Lean Amine

H2SO4 to storage

SO2

Absorber Tower

SINGLE ABSORPTION ACID PLANT

Treated Tail Gas to atm

< 20 ppmv SO2 [0.30 lb SO2 / t acid]

Acid Plant Tail GasSO2 Content: +5,000 ppmv

Lean Amine

Tank

Condensate

Caustic

APU

SO2 product gas, 99.99% basis, dry

SO2 Stripper Tower

APU Effluent

Rich Amine

Cansolv® SO2 SCRUBBING SYSTEM – Treatment of Sulfuric Acid Plant Tail Gas

Sulfur burner offgas

Lean Amine

H2SO4 to storage

SO2

Absorber Tower

SINGLE ABSORPTION ACID PLANT

Treated Tail Gas to atm

< 20 ppmv SO2 [0.30 lb SO2 / t acid]

Acid Plant Tail GasSO2 Content: +5,000 ppmv

Lean Amine

Tank

Condensate

Caustic

APU

SO2 product gas, 99.99% basis, dry

SO2 Stripper Tower

APU Effluent

Rich Amine

Cansolv® SO2 SCRUBBING SYSTEM – Treatment of Sulfuric Acid Plant Tail Gas

Sulfur burner offgas

Lean Amine

H2SO4 to storage

SO2

Absorber Tower

SINGLE ABSORPTION ACID PLANT

Treated Tail Gas to atm

< 20 ppmv SO2 [0.30 lb SO2 / t acid]

Acid Plant Tail GasSO2 Content: +5,000 ppmv

Lean Amine

Tank

Condensate

Caustic

APU

SO2 product gas, 99.99% basis, dry

SO2 Stripper Tower

APU Effluent

Rich Amine

Cansolv® SO2 SCRUBBING SYSTEM – Treatment of Sulfuric Acid Plant Tail Gas

Sulfur burner offgas

Figure 1: Integrated Cansolv®

SO2 Scrubbing System with conversion of SO2 by-product to

sulfuric acid

Key Benefits of Cansolv in Fertilizer Sulfuric Acid Plants

The experience of the commercial units as well as the advantages highlighted by our clients focus

on three main benefits to applying the Cansolv SO2 Scrubbing System to treat acid plant tail gas:

1. Reduce dependence on by-product market;

2. Uncouple SO2 emissions from the acid plant operation/performance;

3. Achieve SO2 emissions of 20 ppmv (0.3 lb SO2/t acid) or less.

1. Reduce dependence on by-product market

A major value proposition to installing the Cansolv SO2 Scrubbing System is the elimination of a

by-product that must be sold or disposed. The by-product from a Cansolv plant is a pure water-

saturated gaseous SO2 stream. This SO2 is recycled to the existing drying tower at the front end of

the acid plant for conversion to additional sulfuric acid. At emissions of 0.3 lb SO2/ton of acid,

99.98% of the sulfur present is converted to acid. This is a substantial difference compared with an

alkali scrubber where the SO2 from the acid plant tail gas is neutralized in a salt effluent and then

either sold to market or dried and disposed.

One of the early licensees of the Cansolv process in California was fundamentally interested in

eliminating the client’s dependence on alkali by-product markets. The acid plant was a single

absorption unit with an ammonia scrubber. The ammonium sulfate by-product was sold in the

regional fertilizer market. The variable market conditions and the challenges of storing the by-

product during periods of low demand were key drivers in the client’s choice of switching to a

regenerable technology.

The FRP absorber tower and recirculating pumps for the ammonium sulfate process was reused for

the Cansolv process and the regenerator skid was modularized to minimize site installation costs.

The installation of the plant was completed during a scheduled shutdown after which the plant

restarted on the new Cansolv system. For minimal investment, this client was therefore able to

eliminate a by-product from his plant, including the required storage and handling facilities, and

increase the sulfuric acid production.

2. Decouple SO2 emissions from acid plant operation/performance

In a conventional double absorption acid plant, maximizing conversion of SO2 to SO3 is critical to

meet SO2 emissions in the tail gas. In order to continuously achieve the desired emissions levels,

performance catalysts are increasingly used in the latter passes of the converter. Plant shutdown

frequency is determined by the requirement to restore catalyst to its design performance to achieve

emissions and/or capacity.

Using a Cansolv SO2 Scrubbing System to treat the acid plant tail gas changes the philosophy of

operating an acid plant. The SO2 emissions from the Cansolv SO2 Scrubbing System are not

determined by the inlet concentration of SO2 so loss in catalyst performance no longer carries the

requirement to lower throughput to meet emissions. It may also be possible, within limits, to

increase throughput beyond design rates without exceeding emissions.

Acid plant converter catalyst degrades over time and it is not unusual to observe an increase in

single absorption acid plant tail gas emissions from 2,000 ppmv SO2 with fresh catalyst to 3,500

ppmv SO2 or higher at the end of a run. If equipped with a Cansolv SO2 Scrubbing System, as

emissions drift higher, the amine flowrate to the SO2 Absorber Tower is increased accordingly. The

increased amine flow will require a proportional increase in stripping steam, allowing emissions to

remain constant.

Furthermore, SO2 emissions are a function of the quantity of steam consumed. Achieving lower SO2

emissions from the Cansolv SO2 Absorber Tower is as simple as increasing the specific steam

consumption.

Clients in other market sectors have taken advantage of the Cansolv SO2 Scrubbing System to treat

gases with variable SO2 concentrations. In smelting applications, the gas sourced from batch

furnaces can cyclically vary from a few thousand ppms to as high as 12% SO2. As described above,

the amine flowrate is controlled according to the SO2 load in the gas and SO2 emissions to the stack

are maintained constant despite the variation of SO2 content in the gas being treated. The table

below summarizes some of the projects where the Cansolv SO2 Scrubbing System is being applied

to treat variable SO2 concentration in gases.

Application Location Flow

(Nm3/hr)

By-

Product

Feed SO2

Concentration

SO2

Emissions Status

SARP Acid Plant

Tail Gas

North

America 40,000 H2SO4 0.3% - 0.65% 15 ppm Operating (s/u 2002)

Lead Smelter Off-

Gas Asia 33,700 H2SO4 0.1-12.5% 90 ppm Operating (s/u 2005)

Anode Furnace

Offgas Asia 43,000 H2SO4 900 ppm-2% 100 ppm Operating (s/u 2007)

Sinter Machine

(secondary) Asia 350,000 H2SO4 2400 ppm 140 ppm

Construction

(s/u 2009)

Sinter Plant

(primary) Asia

2 x

510,000 H2SO4 800 ppm 40 ppm

Construction

(s/u 2009)

Lead Smelter Off-

Gas + APTG Asia 83,000 H2SO4 0.1% – 8% 140 ppm

Construction

(s/u 2009)

Copper Smelter

Furnace Off-Gas Asia 95,000 Sulfur 15% - 35% 250 ppm Engineering

Nickel Smelter

Offgas

North

America 100,000 H2SO4 0.1% - 3% 50 ppm

Engineering

Lead Sinter

Machine Offgas

Asia-

Pacific 265,000 H2SO4 1-2% 50 ppm Engineering

While acid plant tail gas applications don’t vary on a short term basis, they will drift higher over

time as the catalyst efficiency drops and may double or triple between screenings. If the Cansolv

unit is designed for it, this variability can be easily compensated for, a feature that has been

demonstrated at the California acid plant tail gas scrubber mentioned earlier in this paper. When it

first started up, the tail gas contained 3,100 ppmv of SO2 and the unit easily bettered its design SO2

emissions of 50 ppmv.

Over the past six years, the SO2 concentration in the tail gas has increased as high as 6,500 ppmv.

While this was partially due to the deteriorating catalyst condition, it was also a consequence of

running the unit above design capacity to produce additional sulfuric acid. Despite the increased

inlet SO2 concentration, the SO2 emissions from the Cansolv

SO2 Scrubbing System have

consistently remained at or below the design target emissions of 50 ppmv.

The following figure shows data collected during the annual performance assessments. The data

represents a time average so fluctuations in the operation of both the acid plant and the Cansolv

SO2 Scrubbing System are understated.

Figure 1 shows the acid plant tail gas SO2 concentration (feed to Cansolv SO2 Absorber Tower)

during each of the performance assessments. The variations in SO2 concentration are either due to

change in acid production rate or catalyst performance.

Figure 1: Acid Plant Tail Gas SO2 Concentration (feed to Cansolv)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

0 10 20 30 40 50 60 70

Sampling Numbers

SO

2 (

pp

mv

)

Ref Year 1 Ref Year 2 Ref Year 3

Ref Year 3 Avg: 5,500 ppmv

Ref Year 2: 3,700 ppmv

Ref Year 1: 4,200 ppmv

Figure 2 shows the SO2 emissions from the Cansolv

SO2 Scrubbing System during each of the

performance reviews. The SO2 emissions have been consistently below the design target of 50

ppmv.

Figure 2: SO2 Emissions from Cansolv

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

0 10 20 30 40 50 60 70

Sampling Point

SO

2 (

pp

mv

)

Ref Year 1 Ref Year 2 Ref Year 3

Ref Year 3 Avg: 48 ppmv (due to decreased steam consumption)

Ref Year 2 Avg: 24 ppmv

Ref Year 1 Avg: 16 ppmv

It is important to note that the reference year 3 average of 48 ppmv SO2 emissions was achieved

subsequent to a recommendation by Cansolv to reduce the steam consumption in the unit to the

minimum necessary to achieve permitted emissions. The client had leeway to increase the SO2

emissions and chose to do this to reduce heat input. Figure 3 demonstrates this relation.

Figure 3: SO2 Emissions as a function of Steam Consumption

0

10

20

30

40

50

60

70

80

4000 4500 5000 5500 6000 6500 7000 7500 8000 8500

Steam (lb/hr)

SO

2 (

pp

mv

)

The SO2 emissions are a function of the steam

consumption and not of the feed SO2

concentration.

In an effort to optimise the operation of the Cansolv unit, a recommendation was issued in 2006 to

decrease the steam consumption. The result was higher SO2 emissions (yet still acceptable by the

plant) and significant steam savings.

Figure 4 illustrates the advantage of uncoupling SO2 emissions from the acid plant operation

/performance with a Cansolv

SO2 tail gas scrubber. There is no correlation between the acid plant

tail gas concentration and the SO2 emissions from the Cansolv

SO2 Scrubbing System.

Figure 4: SO2 Emisisons as a function of Feed SO2 Concentration

0

10

20

30

40

50

60

70

80

0 1000 2000 3000 4000 5000 6000 7000

Feed SO2 (ppmv)

SO

2 E

mis

sio

ns

(p

pm

v)

The higher SO2 emissions are due to a

decrease in steam consumption (see fig. 3)

The scatter in datapoints indicate that there is no correlation

between acid plant tail gas SO2 concentration and Cansolv

SO2 emissions.

3. Achieve SO2 emissions below 20 ppmv (0.3 lb SO2/t acid)

The SO2 emissions that can be achieved with the Cansolv SO2 Scrubbing System are a function of

heat input to the Cansolv SO2 Stripper Tower and absorption temperature in the SO2 Absorber

Tower. SO2 absorption performance improves as gas absorption temperature decreases. In most

applications, the Cansolv SO2 Scrubbing System is mostly applied on gases with higher saturation

temperatures (typically in the range of 130°F to 150°F) and designed to meet SO2 emissions under

100 ppmv. Since acid plant tail gases are dry, significant adiabatic cooling is achieved when

quenching the gas. Acid plant tail gases typically saturate between 75°F and 80°F. At these low

temperatures, even at the very minimum specific steam consumption, 75 ppm (0.80 lb SO2/t acid)

emissions are achieved. Adding heat over and above the minimum will allow even lower

emissions. The relationship between heat and emissions is illustrated in figure 4 above. With this

feature in mind, it is possible to operate a Cansolv SO2 Scrubbing System at 75 ppmv until such

time as tighter emissions regulations require marginally more heat input.

In most plants, the largest operating cost of the Cansolv SO2 Scrubbing System is the cost of the

heat used in the reboiler to regenerate the amine. The heat input to the SO2 stripper may be sourced

from several options such as low pressure steam from a back pressure turbine, pressurized hot water

or even high heat capacity thermal fluids. Heat availability is site specific and CTI clients have

typically integrated heat recovery from other process units in their plant to source the heat

requirements of the Cansolv SO2 Scrubbing System.

Conclusion

Interest in the application of the Cansolv SO2 Scrubbing System to acid plant tail gases has surged

in recent years as stricter SO2 emission regulations are mandated. Several projects in various stages

of development are currently underway. The most significant benefit of adopting the technology on

acid plants is to allow emissions to be met regardless of the upstream acid plant operations and the

performance of the catalyst.

Some adopters of the technology have used it to replace existing alkali scrubbing equipment to

maximize yields of acid and avoid involuntarily producing secondary byproducts.