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Unfortunately, there is no ‘silver bullet’ to
optimize overall performance of a sulphuric
acid plant. Plant equipment and operation
must be viewed holistically to effectively
integrate improvements to sustain efficient
operation and profitability. This paper
illustrates how tower and plant performance
can be improved by an integrated approach
that addresses both tower gas/acid distribution
and mist elimination.
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Proper dry tower design, maintenance, and
operation impact overall plant performance and
economics. Running the dry tower outside
design conditions can lead to mist carryover
into the blower and excessive acid carryover in
the plant, resulting in corrosion, crusting of
catalyst beds, and excessive acid in areas
where it should not be present.
When designing a tower, it is therefore
important to consider the following aspects of
tower design:
Acid distribution
– Uniform and accurate acid
distribution
– Limiting mist generation
– Wetting the walls
Gas distribution
– Gas velocity
– Optimum inlet gas nozzle and
packing support design
Liquid/gas contact
– Packing performance
– Gas and acid conditions
The greater the open area in the tower,
the less the pressure drop
An improvement of the drying tower
(DT) acid distribution will reduce sub-
micrometre acid mist formation. Figure 1
shows the effect of water vapour slipping past
the DT as measured by the exit dry tower
water dew point. This water vapour eventually
combines with SO3in the process gas,
resulting in sulphuric acid vapour entering the
inter-pass absorption tower (IPAT). Normal
dew point temperature is less than -40°C;
however, well-performing dry towers have
much lower dew points. This can be a low as -
50 to -60°C. The higher the dry tower dew
point, the higher the inlet acid vapour content
entering the IPAT and the higher the potential
for acid mist formation once the gas enters the
IPAT and cools below the sulphuric acid dew
point.
The amount of sub-micrometre acid mist
formed in the IPAT versus the amount of acid
vapour condensing on wetted packing surfaces
depends on the amount of acid vapour
entering the IPAT, the concentration of
nucleating agents in the gas (e.g. very fine
dust particles from air or from ash resulting
from sulphur burning) and fine particles from
upstream metallurgical process, along with
how the IPAT is designed and operated.
Sulphuric acid plant optimization and
troubleshooting
by J. Hanekom
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In an ideal world in which sulphuric acid plant operators could have
anything they wish for, many would request an acid plant that never
experiences corrosion, never has gas leaks, and never needs catalyst
screening. And yet that is not the reality. All sulphuric acid plant
operators experience acid mist carryover from sulphuric acid towers at
some stage. If this is not detected and corrected quickly, extensive damage
can be caused to downstream equipment. Not all mist carryover issues are
related to poorly performing mist eliminators. This paper presents a review
of the entire tower as a unit and discusses how process conditions,
packing, acid distribution, mechanical factors, or mist eliminators affect
the performance of the tower, as well as early detection and
troubleshooting tools and methods.
51+/%'
acid mist carryover, process conditions, packing, acid distribution,
mechanical factors, mist eliminators.
*Sulphur EMEA, MECS, South Africa.
© 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.
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VOLUME 117
http://dx.doi.org/10.17159/2411-9717/2017/v117n11a7
Sulphuric acid plant optimization and troubleshooting
Improved mist elimination in the DT also helps reduce the
acid vapour level at the inlet to the IPAT, since any acid mist
reaching the downstream sulphur burner will decompose to
H2O and SO3and ultimately result in more acid vapour at the
inlet to the IPAT.
On metallurgical acid plants the DT are often blamed for
poorly performing upstream wet electrostatic precipitators
(WESPs) often found in these plants. Sub-micrometre mist
from WESP will pass through the drying tower and
accumulate in the blower. The WESP sight glasses should
always be optically clear.
When designing an acid tower, it is imperative to keep the
following in mind:
1. Good absorption of SO3(absorbing towers) or H2O
(drying tower)
= Uniform distribution
2. Low mist generation (saturation or overloading of
mist eliminators)
= No droplet carryover
3. Safe and reliable operation / maintenance
= Easy access, no sulphate build-up, no plugging
4. Acid quality => low Fe content
= Steel with low corrosion rates (Figure 2a).
Uniform acid distribution is achieved by even irrigation of
the packing both in the centre and near the walls, and of an
equal flow per pour point (Figure 2b).
Droplet carryover can be avoided if the acid is discharged
slowly, just under the packing without decreasing the tower
open cross-section.
Flat packing support grids, such as the ZeCor®packing
support grids, ensure equal packing thickness and equal gas
distribution, and require no maintenance. The open area is
more than 80% and the packing (random Intalox saddles) is
installed directly on top of the grid. No grid blocks or cross-
partition rings are required. ZeCor®packing support grids are
easy to install and are supplied in easy-to-handle sections.
Dew point is defined as the temperature at which water in a
gas condenses. The dew point for sulphuric acid plants is in
fact the point at which the first droplet of sulphuric acid
condenses. However, the quantity is so small that it is
invisible to the naked eye. An effective way for sulphuric acid
plant operators to obtain accurate readings of the dew point
is to use a vacuum pump to take measurements at the outlet
of the dry tower if it is a suction tower. Dew-point
measurement is an early detection method for any potential
issues.
The H2O volume is 30 times that of H2SO4, making the
water dew point visible first. For example:
Vapour pressure over 96% H2SO4at 65°C (typical DT
operating point) mmHg
- H2O0.02
- H2SO40.0007
- SO30.000002
The dew point matters because it affects drying tower
efficiency, a crucial factor in plant optimization and one that
has an impact on the life of a plant. As mentioned above, any
moisture in the process gas will increase the dew point
temperature before the IPAT. This will also increase the risk
of acid condensation in economizers and in the converter
during long hot shutdowns, increasing corrosion and damage
to catalyst structure.
Tower performance is also affected by the choice of mist
eliminator. Many different mist eliminator options are
available in the market. They all intend to capture
submicrometre liquid and soluble mists. However, they don’t
all function in the same way. Three mechanisms are used to
capture mist –impaction, interception, and diffusion. MECS
products use all three of these mechanisms – the impaction
mechanism with the help of mesh pads, the interception
mechanism with a combination of mesh pads and glass fibre,
or the diffusion mechanism with the help of fibre beds to
collect mist effectively.
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Fibre bed collection intercepts particles ranging in size
from <1 m to larger particles >3 m, trapping them in the
fibre bed as shown in Figure 3.
The focus of this paper will be on diffusion bed mist
eliminators, which are predominantly used in IPAT and final
absorption towers (FAT) in modern acid plants. Some plant
still uses impaction/interception-type mist eliminators in the
FAT
Diffusion bed mist eliminators are produced by wrapping
glass fibre (in rope form) around an inner cage of metal wire
or by compressing bulk glass fibre between two metal cages.
The commonly used diffusion bed mist eliminators are the
ones where glass fibre rope is wrapped around an inner cage
– ES-type mist eliminators.
Two methods are used for placing fibre roving onto a mist
eliminator – angle wrap and parallel wrap. The positioning of
the fibre has a significant impact on the efficiency of the mist
eliminator, as demonstrated with the help of thermographic
imaging (Figures 4 and 5). Angle-wrapped mist eliminators
tend to allow less heat to escape than parallel-wrapped fibre
beds.
Velometry testing of angle- and parallel-wrapped fibre
bed mist eliminators also shows a difference in performance
(Table I, Figures 6 and 7).
Whatever the type of mist eliminator chosen, it is
normally easy to install, but not to remove. Maintenance
must be carried out on site. It is therefore vital, at
installation, that enough space is planned for maintenance
and parts replacement.
Although mist eliminators are relatively simple devices, it
is important that they are installed properly. Many factors
will affect the efficiency of a mist eliminator:
Orientation of the mist eliminator – hanging or
standing
Type of gaskets used
Sulphuric acid plant optimization and troubleshooting
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VOLUME 117
Table I
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Standard packed (angle-wrapped) ES 30 102 1
Hand-packed (parallel-wrapped) ES 30 685 4
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Is it installed on a pedestal or directly on the tube
sheet?
Are the correct flange bolts used and are they torqued
properly?
Are seal pots or seal pipe used?
Are seal pots filled up with acid before start-up?
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Preventing high acid mist carryover requires an
understanding of its possible root causes. This involves a
good knowledge of the plant design, an insight into the plant
maintenance history (e.g. have there been tower distribution
problems in the past?), a familiarity with the functioning of
mist eliminators, plans for dealing with expected mist, and
awareness of what constitutes a problem symptom.
These may include an abnormal drop in pressure, re-
entrainment, ‘drip acid’, stack opacity e.g. EPA method 9,
and measurements that show high exit loading. Downstream
systems such as high stack opacity, high EPA 8 emissions, or
a charred stick often have the same root cause (SO3/H2SO4
vapour) or can be due to very specific causes:
Although it is possible to run a plant with partially blinded
candles, i.e. a blocked mist eliminator, there is a substantial
risk of damaging the cages. In the case of sulphur
sublimation, the best approach is to shut down the plant and
wash or replace the mist eliminator elements. Hanging
elements should never be operated with a dP more than 680
mmWC (27 inWC).
Fouled packing results in sulphate accumulation in mist
eliminators and an increase in operating pressure drop.
Sulphuric acid plant operators can deploy a range of
testing and troubleshooting methods aimed at addressing a
variety of root causes for problems. They can vary from
plotting element pressure drop divided by the actual gas flow
versus time, to acid vapour and acid mist stick tests, drip acid
volumes and concentration, dry tower stick tests, the
installation of viewports and, of course, physical inspections,
though these require shutdowns and will lead to a loss of
production unless the inspections are scheduled to coincide
with planned turnarounds.
Several ways to identify the reason for acid mist
carryover from acid towers are listed below. The most
common fault-finding method is a stick test on the outlet of
the tower (Figure 8). The condition of the stick after 30
seconds, 1 minute or 3 minutes gives a sound qualitative
assessment of acid mist carryover.
Pressure drop and flow rate history—plant records and
while operating
Stick test—while operating
Drip acid measurements—while operating
Physical inspection—during plant shutdown
View ports—requires planning and shutdown to install
Smoke test—primarily for checking leaks at gaskets,
liquid seals, and tube sheet
Tyndall beam—while operating – requires some
finesse/practice
Water dew-point—while operating
Acid dew-point—while operating – not commonly used
(expensive equipment)
MECS method 104—requires planning and hot
shutdown to install sampling nozzles and/or full-bore
4-inch gate or ball valves—last resort if other
troubleshooting methods do not identify root causes.
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Improving the performance of an acid plant is not a simple
matter. Equipment cannot be placed in a haphazard,
piecemeal fashion. Rather, the entire acid plant must be
viewed holistically and improvements need to be made in a
way that consider the interplay of all the various unit
operations. Acid mist emissions cannot be reduced by simply
optimizing tower operation, nor by simply enhancing mist
capture. Indeed, the operational performance, and ultimately,
the profitability, of many sulphuric acid plants will be
improved only by incorporating and maintaining efficient
tower acid distribution and superior mist elimination. This
holistic approach is likely to reduce maintenance costs, avoid
loss of production, and reduce stack acid mist emissions.
Sulphuric acid plant optimization and troubleshooting
1034 VOLUME 117
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