Available via license: CC BY 4.0
Content may be subject to copyright.
3*+-)&'+,)*
Most non-ferrous smelter operations are
coupled with a metallurgical sulphuric acid
plant to treat SO2-containing off-gases before
discharge to the atmosphere. With the
increasing use of oxygen enrichment to
increase production in existing smelters, and
due to advances in smelting technology, more
and more smelting operations are producing
off-gases with SO2concentrations well above
30 vol%. The concentrated gases may be
mixed with lower concentration off-gases from
secondary processing or other emission
sources before entering the off-gas cleaning
and acid plant.
As smelter off-gases are generally deficient
in oxygen, it is necessary to add oxygen,
typically using ambient air, to the gas prior to
the drying tower in the acid plant. The
resulting SO2concentration in the process gas
after this O2:SO2ratio adjustment is typically
between 15 and 25 vol% SO2at the acid plant
converter. However, the conventional
sulphuric acid plant (double contact – double
absorption) is limited to no more than 13 vol%
SO2at the converter inlet in order to keep the
gas temperature leaving the first catalytic stage
below the thermal stability limit (approx.
630°C) of the vanadium-based catalyst.
Air addition in excess of the amount
required for adjusting the oxygen content
increases the gas volume processed through
the acid plant and consequently the equipment
size, and capital and operating costs. Higher
gas throughput also increases the heat loss to
the acid circuit, thereby reducing energy
recovery from the gas contact section and
increasing cooling water demand. With newly
built and future smelter operations designed to
produce high-concentration SO2gas at
increasing throughputs, ever more air dilution
is required to keep within the limits of
conventional acid plants. In some cases, gas
volumes exceed the design limit of single-train
acid plants (currently around 5000 t/d), which
forces designers to resort to multiple train
contact plants, thus further increasing plant
footprint and CAPEX/OPEX.
To counteract these shortcomings of the
conventional acid plant, Chemetics Inc., with
more than 50 years of experience in sulphuric
acid technology, now offers two solutions: The
Chemetics High Strength (CHS™) process and
the Chemetics Pseudo-Isothermal process
utilizing the CORE™ reactor technology.
%$0!0+,'/1,($1#+-0*(+$1%#
-)'0//
The CHS™ process is designed for two typical
situations:
Locations where the gas received from
the smelter is high in SO2but deficient
in oxygen, such that dilution air or
oxygen is required to achieve the
required O2:SO2ratio for the conversion
Locations with multiple SO2off-gas
sources (such as a flash furnace coupled
with Peirce-Smith converters) that have
different SO2concentrations but are not
necessarily deficient in oxygen when
Economical abatement of high-strength
SO2off-gas from a smelter
by R. Dijkstra, B. Senyard, U. Shah, and H. Lee
#*) /,/
With increasing use of oxygen enrichment and advances in smelting
technology, SO2concentrations in smelter off-gases are increasing, which
necessitates larger acid plant equipment and increases in capital and
operating. To counteract the shortcomings in conventional acid plants,
Chemetics provides two unique solutions: the Chemetics High Strength
(CHS™) process and the Chemetics Pseudo-Isothermal process utilizing the
CORE™ reactor technology. In this paper we present a general outline of
these two solutions and how they can be implemented in new or existing
acid plants.
40)-/
smelter off-gas, high-strength SO2, pseudo-isothermal process.
*Chemetics In. Canada.
© The Southern African Institute of Mining and
Metallurgy, 2017. ISSN 2225-6253. This paper
was first presented at the 6th Sulphuric Acid 2017
Conference’, 9–12 May 2017, Southern Sun Cape
Sun, Cape Town.
1003
VOLUME 117
http://dx.doi.org/10.17159/2411-9717/2017/v117n11a2
Economical abatement of high-strength SO2off-gas from a smelter
mixed together, and where the client is considering
installing separate gas-cleaning systems for these gas
sources for operational (reliability) or process (e.g.
different gas cleaning requirements) reasons.
The CHS™ process capitalizes on the difference in the SO2
concentrations of the feed streams, resulting in the ability to
process gases containing up to approximately 18% SO2.
The CHS™ design (Figure 1) processes the two gas streams
(high and low SO2concentrations) by reconfiguring the
contact section. A separate drying tower (with a common acid
system) and blower is used for each stream. After drying, the
weak gas (which also includes all required dilution air to
maintain the correct O2:SO2ratio) is mixed with part of the
strong gas to provide a gas containing approximately 13%
SO2at the inlet of the first catalyst bed. After part of the SO2
is converted to SO3, the now SO3-rich gas from bed 1 is
combined with the remaining strong SO2gas and processed
in a further four catalyst beds. The overall arrangement is a
4+1 DCDA configuration. Energy recovery from the hot gas
leaving beds 3, 4, and 5 enables the production of high-
pressure steam.
Taking full advantage of Chemetics’ experience and
expertise in acid plant equipment design, the converter used
for the CHS™ process is a single stainless-steel five-bed
converter with two internal heat exchangers (see Figure 2).
The advantages of the Chemetics stainless steel converter
design are well known and include all-welded construction,
rapid heat-up time, improved reliability, excellent gas
distribution, and less external hot gas ducting.
%#121')*0*+,)*."1 ".*+151'./01/+&1
For comparison between the CHS™ and a conventional acid
plant, the off-gas sources from a recent study are considered
(Table I).
In a conventional acid plant, these sources would be
blended and delivered to the drying tower as a single feed
stream. Additional dilution air, required to control the first-
pass converter bed temperature, results in about 27%
increase in the gas throughput. The CHS™ design receives
these streams separately and requires no further dilution air.
Gas flow through any of the CHS™ converter passes is no
more than the total feed gas flow rate. This flow reduction,
effectively 25% lower in the study case, translates to capex
and opex savings in addition to approximately 20%
improvement in energy recovery.
Compared with competitor technology, which recycles
hot SO3-rich gas from bed 3 to suppress the temperature rise
in the first pass, the CHSTM design offers many benefits
(Table II). These include a significantly lower gas flow rate
1004 VOLUME 117
Table I
%./01/+&1(./100/
#+-)*( 0. %)!,*0
#(./ #(./ #(./
SO2concentration 25 8 16.5
O2/SO2ratio 0.5 2 0.86
% of total feed 50 50 100
,(&-01%#1(0*0-,'1 -)'0//1")1,.(-.!
,(&-01%$0!0+,'/1,0 .//1')*0-+0-1,+$1+)1,*+0-*."1$0.+
0'$.*(0-/
through the converter beds and the heat exchangers
upstream of the intermediate absorption tower, higher
equilibrium conversion prior to intermediate absorption,
reduced power consumption, and fewer reliability concerns
associated with a hot SO3gas recycle fan.
/0&),/)+$0-!."1')*0-+0-151%$0!0+,'/1%)-0
In August 2016 Chemetics acquired all patents and know-
how for the BAYQIK®converter technology from Bayer AG.
This converter technology, now marketed under the CORE™
name, is a proven pseudo-isothermal reactor system capable
of converting high-strength SO2gas without diluting the gas
with air or recycled process gas. The first commercial
installation in Germany (see Figure 3) has been operating
continuously for more than 8 years, processing gas with up
to 21 vol% SO2. A second plant was commissioned in January
2017 and is processing gas up to 15 vol% SO2. The
technology is most valuable in treating a single strong gas
source, but can also be an economical pre-converter for a
plant with multiple off-gas sources
The Chemetics CORE™ converter is the only commercially
available isothermal converter system for SO2oxidation.
Continuous removal of the reaction heat using air or molten
salt allows the process temperature to be controlled within
the operating limit of the catalyst.
In addition to the ability to convert high-concentration
SO2gas, the pseudo-isothermal process also operates farther
from the equilibrium curve than the conventional multi-pass
adiabatic process, as shown in Figure 4. This translates into
lower overall catalyst loading and significantly higher
conversion in a single pass.
The pseudo-isothermal process is carried out in a
patented tubular converter (see Figure 5). The SO2process
gas flows through the tubes, which are filled with a carefully
selected mixture of vanadium-based catalyst. A cooling
medium (air or molten salt, depending on the reactor size) is
introduced on the shell side to remove the reaction heat. Heat
transfer is optimized in the design of the reactor. Energy
recovered from the circulating cooling medium can be used
for preheating the process gas and for generating high-
pressure steam. CORE™ reactors designs for capacities up to
100 000 Nm3/h (equivalent to approx. 2000 t/d acid
production) are currently available, with higher capacity
designs under development.
There are several approaches to using the Chemetics
CORE™ technology in handling high-strength SO2gas.
In an in-line configuration, the CORE™ reactor can simply
replace the primary contact plant, which typically includes
beds 1 through 3 and intercooling gas exchangers in an
adiabatic design. As a result, the in-line Chemetics CORE™
design reduces not only the gas flow through the plant but
also the number of major equipment items and the overall
plant pressure drop.
This significant reduction in plant size is demonstrated in
the following comparison between the various technologies
using a baseline case of 25 vol% SO2off-gas with adequate
oxygen content (O2/SO2ratio ≥ 0.8). In a conventional
design, air addition required to reduce the SO2concentration
to 13 vol% results in near-doubling of the gas flow through
the contact plant. The CHS™ design can reduce the dilution
air requirement by adjusting about half of the feed gas to
13 vol% prior to bed 1. The resulting gas flow through the
acid plant is reduced to 75% of that in the conventional
design, but is still elevated at 150% of the feed gas flow. If
the hot SO3recycle approach is used, the gas flow rate to bed
1 will be reduced to about 140% of feed gas flow (or
approximately 70% of the gas flow for a conventional
Economical abatement of high-strength SO2off-gas from a smelter
1005
VOLUME 117
Table III
%./01/+&1-0/&"+/
.-.!0+0-1 %#1 %)*0*+,)*."
Gas throughput
Bed 1
Vol% SO213% 13%
Gas flow (% of total feed) 80 127
Bed 2
Vol% SO29.2% 4.5%
Gas flow (% of total feed) 96 122
Energy efficiency
Blower power 88 100
Energy recovery 120 100
,(&-01%1,*/+."".+,)*1 -)'0//,*(1(./1& 1+)11)"1#.*
) 0-.+,*(1/,*'01
,(&-01#')*0-/,)*1'&-01)1')*0*+,)*."1.,..+,'1 -)'0//1
/0&),/)+$0-!."1 -)'0//
design). However, in the absence of dilution air, the number
of adiabatic passes must increase to achieve the same SO2
emission allowance. This results in a contact plant with up to
six passes and three absorption towers. Finally with the
Chemetics in-line design using a CORE™ reactor coupled
with secondary contact, the gas flow to bed 1 is not only the
lowest but also the number of reaction stages is minimized
(see Figure 6).
Since SO2conversion in excess of 90% can be achieved
using a single Chemetics CORE™ reactor, the downstream
secondary contact section can be customized based on the
client’s specific needs. This second SO2abatement process
can be (i) a conventional single or dual adiabatic design, (ii)
another single-pass CORE™ reactor, or (iii) a regenerative
SO2tail-gas treatment unit. For instance, if a client desires
the smallest plant footprint and the flexibility of equipment
modularization, a Chemetics CORE™ reactor coupled with a
regenerative tail-gas unit would be the preferred solution, at
the expense of steam consumption in the tail-gas unit. This
combination is especially suitable for smaller capacities or
locations where a regenerative scrubbing system is already
required to capture the SO2gases prior to conversion to acid.
Another application of the technology is to treat only a
portion of the strong SO2feed gas, with the SO3-rich gas
leaving the isothermal pre-converter directly mixed with rest
of the strong feed gas and processed through a standard
adiabatic contact plant. This configuration may be attractive
for large-capacity new plants, where the benefits of directly
treating a strong feed gas are fully realized with a smaller
CORE™ reactor and cooling system. This same concept also
highlights the value of the reactor technology for brownfield
plant expansion, where a planned upgrade in the smelter
operation would increase the SO2gas concentration going to
an existing acid plant. The conventional design would require
diluting the strong gas with air, resulting in a gas flow rate
beyond the capacity of the existing acid plant. In many cases,
a new double-contact acid plant or a costly debottlenecking of
the existing contact plant would be required. In such
applications, an add-on Chemetics CORE™ module is a more
economical solution. The CORE™ module converts and
removes the extra SO2, resulting in a gas to the existing acid
plant that is the same volume and concentration as before the
Economical abatement of high-strength SO2off-gas from a smelter
1006 VOLUME 117
,(&-010'$*)")(,0/1)-1+-0.+,*(1.1/,*("01$,($/+-0*(+$1#/)&-'0
,(&-01#+.*.")*01%$0!0+,'/1%1//+0!
smelter expansion. This solution thus offers a compact
design, smaller footprint, and improved energy efficiency at
lower capital and operating cost.
An example of the pre-converter line-up is shown in
Figure 7. Optional add-ons such as a dedicated SO2booster
fan and intermediate absorption tower are offered, depending
on the project requirement. Acid produced in the intermediate
absorption tower is of high quality as any remaining
impurities in the process gas have already been washed out
in the drying tower. In some cases this premium quality acid
can be sold at significantly higher prices, improving
profitability.
The Chemetics CORE™ converter operation can be
adjusted by controlling the temperature of the cooling
medium (controlling conversion) or by adjusting the gas flow
through the reactor. If the SO2concentration from the smelter
is low, the unit can be taken off-line into ’hot standby’ and
can stay in this mode for any length of time while
maintaining optimum catalyst temperature for immediate
restart. From hot-standby mode the plant can be switched to
on-line by simply restarting the gas flow. This operational
flexibility maintains a steady gas concentration to the
downstream acid plant despite variability in feed gas, and
thereby improves the acid plant reliability.
The versatility of the Chemetics CORE™ technology, with
its ability to handle high-strength SO2gas and fluctuating
process conditions, makes it a powerful solution for
metallurgical SO2off-gas abatement which also allows for
increased steam production. When used in smelter
expansions to accept higher concentration gas, the CORE™
technology is by far the most cost-effective solution.
#&!!.-1.*1%)*'"&/,)*/
Chemetics offers several solutions for treating high-strength
SO2off-gas without requiring excess dilution air or recycling
of hot process gas. While CHS™ is best suited for
applications with multiple large SO2gas streams, process
designs integrating the Chemetics CORE™ reactor can directly
treat concentrated gas streams as high as 50 vol% SO2. Both
approaches use proven equipment and catalyst and simple
controls. Their advantages over the conventional acid plant
design are summarized in Table III. Chemetics’ success in the
sulphuric acid industry has been built on focusing on the
client’s needs. With the evolution of increasingly high-
concentration SO2off-gases, Chemetics is able to offer
customized solutions for any conditions by selecting the
appropriate process. The optimized solution is reached by
closely working together with our clients to ensure that their
needs are fully realized.
Economical abatement of high-strength SO2off-gas from a smelter
VOLUME 117 1007
,(&-01.! "01)1+$01 -0')*0-+0-1",*0&
Table III
#&!!.-1)1..*+.(0/1)1%#1.*1%
%$0!0+,'/ %$0!0+,'/
,($1#+-0*(+$ %
%#
Capital improvement
Reduced plant size
- No air dilution beyond O2:
SO2ratio adjustment,
- No process gas recycle
Reduced footprint
Ease of integration
Reduced reaction stages
Operating Improvement
Lower utility consumption
(power, cooling water)
Improved energy recovery
and steam production
Improved acid plant availability
Increased catalyst life
(reduced thermal stress)
Higher purity product acid
