INTRODUCTION
New standards for hydrochloric acid emissions are expected to be
proposed by the EPA soon, and these will affect the design of scrubbers for pickle line
fume exhaust systems. This paper discusses hydrochloric acid fume scrubbing, with
particular reference to the impact of new regulations on the type and design of equipment
required.
At this time, the new standards for hydrochloric acid scrubber
efficiency are not known, but we can be sure that they will be more rigorous than present
requirements. At present, most fume scrubbers are designed for efficiencies of 90-95% -
new requirements are expected to be in the 98-99% range. What does this mean for the
pickle line operator?
WHY SCRUBBERS ARE NEEDED
The liquid hydrochloric acid used in pickling is actually a solution of
hydrogen chloride gas in water. Raw acid is generally purchased as ‘20°Baumé’,
which means it has a specific gravity of 1.16 and contains about 32% hydrogen chloride
(HCl) and 68% water. For pickling purposes, this acid is usually diluted to about 18-22%
HCl.
The HCl solution used in pickling emits HCl vapors into the space above
the liquid surface. The tendency of the HCl to escape is measured as a vapor pressure -
the higher this pressure, the more HCl escapes. Figure 1 shows the vapor pressure curves
for HCl at various temperatures as a function of liquid concentration - the vapor pressure
increases as temperatures and concentrations increase. These curves are for the vapor
pressure over solutions in pure water - the vapor pressures over pickling acid solutions
are significantly higher, due to the accumulation of iron chloride in the acid, which
increases the solution HCl vapor pressure.
Figure 1 shows why pickle acids need fume control. The threshold limit
value (TLV) for HCl is 5 ppmv, which is equivalent to a vapor pressure of about 0.004
mmHg. This pressure is in equilibrium with a 5% solution at 120°F, 2% at 140°F and less
than 1% at 176°F - all concentrations well below what is used in pickling, whether in
open or closed tanks. This means that the air above the surface of the acid during
pickling is well above the allowable limits, and some form of containment or control is
needed.
Besides operator hygiene, control is needed because HCl is extremely
corrosive to the building fabric and equipment. Referring again to Figure 1, the 80°F
curve shows that there is negligible vapor pressure up to about 5% - this means that HCl
is very soluble in cold water, and even low gas concentrations can generate high liquid
acid concentrations. Thus, HCl in the air can migrate through the interior fabric of the
building, and dissolve in condensation on cold surfaces, producing a strong and corrosive
acid in hidden or inaccessible areas.
Figure 1 - Vapor pressure of hydrogen chloride over aqueous
solutions (ref 1,2)
The ideal fume control would be to pickle in a sealed vessel, isolated
from the air - however, the need for openings to insert, guide, access and remove the
steel from the acid generally precludes this, so usually the fumes are controlled by
creating a draft above the acid surface. This causes air to be drawn in through the
openings, thereby preventing escape of acid fumes. However, as the air flows across the
acid surface, it sweeps away the HCl vapors, which are then carried out of the tank in the
exhaust air. The concentration of HCl in the exhaust gas is a complex function of tank
open surface area, air velocity, temperature and acid composition, and can vary from 50
ppmv in well-controlled open tank systems with lateral exhaust, to as much as 2000 ppmv in
continuous pickle lines with tight lids and 6000 ppmv in tower picklers. These
concentrations are far too high for discharge to the atmosphere, even without considering
the effect of such emissions on the building roof and nearby structures.
SCRUBBER DESIGN
There are as many types of scrubber as there are inventors - but they
all make use of the physical chemistry shown in Figure 1. By contacting the HCl bearing
gas with cold water, the fumes are dissolved, and removed from the air. This is
illustrated in figures 2 and 3. In figure 2, at point ‘A’, which represents
conditions in the pickle tank, the HCl vapor pressure over 10% acid at 180°F is 0.86
mmHg, whereas the air entering the tank has no HCl in it (if the fume control system is
working properly). Thus, the tendency is for the HCl to evaporate into the air. In figure
3, at point ‘B’, which represents conditions in the scrubber, the air from the
pickle tank, now
Figure 2 - Conditions in the pickle tank
Figure 3 - Conditions in the fume scrubber
containing perhaps 500 ppmv of HCl (equal to 0.4 mmHg pressure), is
contacted with water that has a negligible HCl vapor pressure, and the tendency is for the
HCl to dissolve in the water.
When HCl dissolves in water, a substantial amount of heat is generated
- in a scrubber, this heat is absorbed by evaporation of water into the air. The
evaporation of water will also cool the incoming air to near the adiabatic saturation
temperature, so that the air leaving a wet scrubber will have almost 100% humidity.
Although there are numerous scrubber designs, three types dominate the
market - packed, plate and crossflow.
The packed scrubber is by far the most widely used type. Figure 4 shows
the construction of a packed scrubber. The air enters the bottom of the scrubber, and
flows upwards through a bed of packing. Scrubbing water is sprayed on top of the packing
bed, and flows downwards by gravity - the purpose of the packing is to promote turbulence,
and good mixing of the liquid and the gas, so that the HCl will be absorbed.
Figure 4 shows the components of a packed scrubber , which are
the shell, the packing support, the packing, the liquid distribution system and the
demister. The shell is usually plastic - PVC, polypropylene or FRP - and is most
economically constructed as a cylinder. At the bottom is a grid which supports the
packing, and which needs to have a large open area to allow passage of the air and gas in
opposite directions, without creating excessive pressure drop.
Figure 4 - Components of a packed fume scrubber
The packing consists of plastic shapes, typically 2" to 4" in
size. There are numerous proprietary packing designs - some common ones are: rings, which
are short hollow cylinders; saddle shaped packing; donuts of small dia. rod wound in a
spiral; and intersecting plates, usually with a spherical external profile. Each packing
claims superiority, backed by lab and pilot testing - in fact, they all have about the
same efficiency under industrial conditions.
Good liquid distribution on the top of the packing is essential - if
more liquid is added in one area of the packing, it runs down that area, while the gas,
taking the path of least resistance travels up areas where there is less liquid, and
contacting is poor.
Finally, the droplets entrained in the air stream must be removed, in
order to stop them being discharged from the stack as acid rain. This is done by passage
through a device which changes flow direction, and provides a surface for the removed
droplets to grow, in order to prevent re-entrainment. The two main types of entrainment
eliminator are: chevron, consisting of parallel S-shaped blades; and mesh, which is a bed
of knitted polypropylene fibers. They are both equally efficient, but the chevron is
preferred, because it lasts longer, has large openings, a lower pressure drop, and less
tendency to block with dust, crystals or deteriorated fibers.
The advantages of packed scrubbers are simplicity and cheapness (mainly
due to traditional use of cheap materials and flimsy construction methods), plus the
ability to accommodate variations in air flow. Disadvantages are the tendency of the
packing to plug with dirt and the need for a large water flow over the packing (about 100
gal water/10,000 cu.ft. gas). Usually, to avoid high water consumption, and generation of
large amounts of very weak acid effluent, this water flow is obtained by pumping water
from the bottom of the scrubber back to the top of the packing. This requires a pump to be
maintained, and also puts acidic water, instead of clean water, in contact with the clean
air.
Figure 5 - Components of a plate fume scrubber
The components of a plate scrubber are shown in figure 5. The
shell and demister are the same as for a packed scrubber, but the gas/liquid contact is
brought about by a number of perforated plates. The plates are plastic plates, perforated
with numerous small holes, 1/8" to 3/8" in diameter. As the air passes through
these holes at high velocity, it prevents the water from falling through the holes, and
generates a highly turbulent water/air mixture on the plate. The water depth on the plate
is controlled by weirs - as more water is added to the top plate of the scrubber, it
displaces water over the weir, and this flows by gravity through a downcomer pipe to the
plate below, eventually discharging from the base of the scrubber.
Plate scrubbers use relatively low amounts of water (1-2 gal
water/10,000 cu.ft. gas) on a once-through basis, and require minimal maintenance. The low
volume of relatively concentrated effluent makes for easier recovery. However, these
scrubbers are not suitable for systems in which the air flow varies widely.
The features of cross-flow scrubbers are shown in figure 6.
These are packed scrubbers in which the gas flow is horizontal instead of vertical - the
water still flows downwards by gravity. In these scrubbers, the shell is frequently
rectangular in cross section, and the packing is held between two grids. The water from
the sump is pumped back to the top of the packing.
These scrubbers have the same advantages and disadvantages as packed
scrubbers, but are less efficient. Their main advantage is a low profile when headroom is
limited, and the ability to locate the fan on ground level with minimum ducting.
Figure 6 - Components of a cross-flow fume scrubber
SCRUBBER PERFORMANCE AND EFFICIENCY
This discussion relates to scrubbers which are operating with effluent
acid concentrations of 5% or less, so that the HCl vapor pressure of the liquid can be
considered negligible. Tower picklers, where recovered acid up to 15% can be generated,
required more complex calculations.
The efficiency of removal of HCl in a packed scrubber depends on
the air and liquid flow mass velocities, and the size of packing. Generally, the volume of
packing needed for a given efficiency is approximately constant, so the same results can
be obtained by a short, large diameter, scrubber, or a taller, small diameter scrubber.
Large diameter requires a lot more water volume to irrigate the packing - small diameter
gives high pressure drop, and the risk of flooding (when the water is held up in the
packing due to the high gas velocity). A superficial air velocity of 5-8 fps is usually
found to be the best compromise.
In packed scrubbers, the height of the packed bed is given by the
equation
Z = Hog x Nog
where Hog is the height of a transfer unit (which is
an experimentally determined property of the packing used), based on gas concentrations,
and Nog is the number of transfer units, based on gas concentrations. The
number of transfer units required, at low HCl concentrations, is:
Nog = ln [1/(1 - Eo)]
where Eo is the desired overall scrubbing efficiency (as a
fraction).
The number of transfer units required for various overall efficiencies
is given in Table 1.
TABLE 1:
NUMBER OF TRANSFER UNITS REQUIRED
FOR VARIOUS SCRUBBING EFFICIENCIES
Thus, present day plate
scrubbers usually have 2 or 3 plates - to meet future efficiency requirements, 4 or 5
plates will be needed. For each additional plate, the height of the scrubber increases by
24-30", and the pressure drop by 1" to 1.5" of water.
SCRUBBER OPERATIONS
The scrubber is only one component of a fume exhaust system, although
there is a tendency to look upon it as being the only thing that matters. However, the
function of the scrubber is to remove HCl from the air - collection of the fumes is
determined by hood and duct design, and the amount of draft is determined by the fan
performance.
Some areas where the scrubber design is important are:
- variable loads
- entrainment
- stack droplet emissions
The HCl concentration in the scrubber inlet gas can vary substantially,
particularly in batch pickling operations, in which high HCl loads can occur as the steel
is being removed from the tank - this is especially the case in modern systems that have
crane-mounted hoods to control these fumes. Essentially, a fume scrubber is a
constant-efficiency device, so, if the inlet concentration goes up 10 times, the outlet
concentration also goes up 10 times. Usually, these high concentrations only occur for
short periods, so that the average emissions remain low. However, such excursions increase
the acidity of any droplets in the gas after the scrubber, and make entrainment control
even more important.
In order to prevent acid rain from the stack, water droplets must be
eliminated from the air stream. In the scrubber, this is done by having an efficient
entrainment eliminator. However, all wet scrubbers discharge air saturated with water
vapor, so that contact with cold equipment or stack walls can cause condensation after the
scrubber - this condensation absorbs the residual HCl content of the gas, and becomes
highly acidic. If the stack velocity is too high, or if the stack wall is very cold, or
the scrubber exhaust air temperature high, the condensate may be discharged as droplets
from the stack. In order to prevent this, an additional entrainment eliminator may be
necessary at the top of the stack. This presents problems in compliance testing, because
the rules require testing in the stack, at a point which does not represent the actual
stack emission!
Another factor that needs consideration is what to do with the scrubber
effluent. The scrubber only removes the HCl from the air, generating a dilute acid
effluent stream in the process. This effluent requires treatment or re-use. With plate
scrubbers, which use little water, it is possible to generate effluent acid up to 2-3%
strength, which can often be returned to the pickle tanks to make up for evaporation -
this is not generally possible with packed scrubbers, that generate effluent in the 0.1 to
0.5% concentration range. Recycle of the scrubber effluent makes economic sense - a
continuous strip pickle line can discharge acid costing as much as $25,000/yr, and then
spend as much again in neutralizing chemicals.
CONTROLLING COSTS
Costs for scrubbers are always of concern, and will become even more
important as more efficient scrubbers are required. The initial cost of the scrubber can
be minimized by designing the fume exhaust system to handle the smallest volume of air
that will give satisfactory control of fumes. It is tempting to oversize exhaust systems,
just to be safe, and to avoid having to pay too much attention to maintenance. However,
oversizing results in higher capital costs, more fan hp, more effluent generation, and
higher acid losses from the pickle tanks.
The ideal fume control system is a closed cover, with no exhaust, as in
‘fumeless picklers’ used for wire strands, but this is not technically practical
for batch pickling or high speed strip lines. Techniques for minimizing exhaust rates
include:
- tight, well-maintained covers
- locate exhaust ducts near openings in hoods and tanks
- minimize open area with local seals or closures
- use of double covers
- regular maintenance of fan, hoods and ducting.
- careful, well-balanced duct design
CONCLUSION
Future emission regulations are going to require larger, more efficient
HCl fume scrubbers. The new limits are easily achievable using scrubbers that are 30-50%
larger than those presently in use. The cost of upgrading fume scrubbers can be reduced by
improving fume control practices to minimize exhaust rates.
REFERENCES
1. Perry, R.H., and Green, D.W., ‘Perry’s Chemical
Engineers’ Handbook’, 6th ed., McGraw-Hill, New York, 1984, p 3-64
2. Fritz, J.J., and Fuget, C.R., ‘Vapor pressure of aqueous
hydrogen chloride solutions, 0 to 50°C’, Ind.Eng Chem., Chem. and Eng. Data, Vol1,
#1, p 10, 1956
