CASE STUDY OF A SUCCESSFUL HCl MIST CONTROL SYSTEM

Shreerang Thergaonkar (Design Consultant, ‘Integrated Treatment Systems’, 78, New Ramdaspeth, Behind Shree Ram Bhawan, Nagpur-10). R.V. Dalvi (Director Technical, ‘FACOR Steels’, Ltd, Plot No 46 A&B, MIDC, Hingna, Nagpur-26).

V.M. Motghare (Air Pollution Abatement Engineer, ‘Maharashtra Pollution Control Board’, Sion, Mumbai-22).

Sanjay Joshi (Assistant General Manager, Projects, ‘FACOR Steels’, Ltd, Plot No 46 A&B, MIDC, Hingna, Nagpur-26).

INTRODUCTION• Success of any air pollution control system depends on individual

success of each of the three constituent sub-systems i.e. system for capturing the emitted air pollutants, system for conveying the pollutants up to the control system and the control system itself.

• In this paper, a case study of a successful HCl mist control system for pickling process at ‘FACOR Steels’ Ltd., MIDC, Nagpur, is presented.

• Objectives:• To explicate the necessity of control.• To establish guidelines for effective capture and scrubbing

system.• To report some findings which indicate need for further

investigation.

NECESSITY OF CONTROL• (PEL) for HCl is 5 ppm, which is equivalent to a vapor

pressure of about 0.004 mmHg. This pressure is in equilibrium with a 5% solution at 120°F (48.9°C), 2% at 140°F (60°C) and less than 1% at 176°F (80°C).

• These acid concentrations are significantly below those used for pickling.

• Vapor pressures of HCl over pickling acid solutions are significantly higher; due to formation of iron chloride (Iron Chloride binds water molecules and raises the vapor pressure of HCl). These concentrations are far too high for uncontrolled discharge to the atmosphere. Hence, control is necessary.

CAPTURE SYSTEM:CHOICE AND DESIGN (4-9)

• Amongst the options for local exhaust systems, pure lateral suction systems turned out to be unfeasible (this being generally the case with open tanks with the least dimension 0.6m).

• Because of the requirement for access for machinery and operators at the top, an overhead canopy was not considered.

• The only option was to provide the push-pull exhaust system wherein a push-flow is generated by a jet of air blown over the free liquid surface towards an exhaust hood (placed along one side of the tank which functions to remove the contaminants contained within the push jet) which pulls and removes the fluid from the jet containing the contaminants volatilized from the liquid.

• The control is primarily provided by the push-jet which forms a wall jet along the free liquid surface. Efflux from the push jet curves towards the free liquid surface and gets attached. Hence the name: wall jet.

• Distribution of velocity field in a wall jet can be obtained from that of an equivalent free jet by cutting a section by a plane below the line of symmetry.

• If this field is probed for the magnitude of horizontal velocity vectors in the direction normal to the plane then (1) the local maximum, um, is near the plane and (2) because of cutting a section, the spectrum of decayed velocities from um is larger in the up direction than in the direction towards the plane.

• There exists a unique normal distance, Z, in the up direction at which the velocity magnitude reduces to um/2.

• um reduces, with increase in horizontal distance from the discharge nozzle due to air entrapment and is minimum at x = L (where L = length of the push flow measured up to the exhaust hood), and can be represented16 as:

• um = 3.86 × [(j)/(ρ)] 0.5 × x-0.509

• j = push nozzle initial momentum factor (nozzle flow per unit length × nozzle exit velocity).

• x = horizontal distance from the push nozzle, ρ = fluid velocity at x.

• Contaminants are released from the free liquid surface with some velocity. Therefore, there exits a unique capture velocity, Vcap, which must be induced so as to move the contaminant towards an exhaust system. Hence, the push system must be designed so that:

• um ≥ Vcap : equivalently the push design must ensure:• [(j)/(ρ)] ≥ [(Vcap)/(3.86)]10.2 × L10.2

• There are unbalanced upward components of buoyant and other forces . These are not accounted for in the inequality.

• These impart an upward velocity, vg, to the contaminant. • um reduces along the outward normal. • Hence, at some point, the push-jet induced velocity, vcrit, will be

insufficient to overpower upward movement of the contaminant. • Two options therefore emerge: (1) at this point ensure that the

concentration of the contaminant is below its TLV or (2) design the overall geometry and the push-velocity field so that vcrit ≥ vg. In the present case, the push system and pickling tank geometry were designed so that the practically observed “null point”17 was below the upper bounding plane at which um is located.

• Finally, the exhaust flow is to be attended to. It increases the overall movement of air (to a large extent in close proximity of the exhaust hood). This is beneficial as regards the stated functional objective of the system. The most important consideration is to ensure that the exhaust hood must remove all fluid contained within the jet induced flow. Volumetric exhaust flow rate to ensure this is given by:

• qo = 0.443 × L0.491 × [(j)/(ρ)]0.5

SCRUBBER SELECTION AND DESIGN(10-23)

• Composition of gas film above the free acid surface• Typical Pickling Bath Composition• HCl : 20% = 5.43 mol (%) ; H2O: 80% = 94.57 mol (%) ;

Solution Temperature = 50 °C.• HCl Concentration In The Gas Film Over Free Acid Surface In

The Pickling Tank (Freshly Prepared Acid) : • Partial Pressure: pHCl = 2.21 mmHg = 0.301 mol (%) =

3007 ppm = 4489 mg/Nm3

• The diffusion flux, j , of HCl (g) can be estimated by assuming “Dilute Condition Steady State Diffusion across a stagnant film” This means that there is no convective contribution to the flux.

• Theoretical estimation of gaseous diffusion coefficient, D, is that developed by Fuller, Schettler, and Giddings (1966).18

• D = [(4.3 × 10-7 × T3/2) × (M1/2)] / [P × Ώ]• Where, D = Coefficient of diffusion of HCl (g) in air (m2/s):, T =

Temperature, K:, P = absolute pressure, atm:, M = square root of the sum of reciprocals of molar masses of air and HCl and Ώ = square of the sum of cube roots of the molar volumes of air and HCl (= 29.9 and 28.3 cm3/atom, respectively).

• In this equation, when the values: T= 303 K, M = 0.25, P = 0.967 atm and Ώ = 38.44 are plugged in, we get the desired diffusion coefficient as: D = 1.52 × 10-5 m2/s = 0.06 m2/hr (approximately). This essentially means that in one hour HCl (g) has diffused up to the distance of about 0.5 m above the surface of the pickling solution.

• In this equation, when the values: T= 303 K, M = 0.25, P = 0.967 atm and Ώ = 38.44 are plugged in, we get the desired diffusion coefficient as: D = 1.52 × 10-5 m2/s = 0.06 m2/hr (approximately). This essentially means that in one hour HCl (g) has diffused up to the distance of about 0.5 m above the surface of the pickling solution.

• With, D in hand, j (per m2 of tank area) can now be estimated as: j = (D/L) × (concentration difference)

• where L = diffusion length measured normal to the free acid surface in the pickling tank, (which in the present case is 1m) and the HCl gas phase concentration difference between the saturated gas film immediately above the free pickling solution surface and at the end of the diffusion length, L. This concentration difference was measured as 4489 mg/Nm3 assuming that the concentration of HCl (g) at the end of the diffusion length = 0. Hence,

• j = [(0.06 m2/hr)/(1m)] × (4489 mg/Nm3) = 269 mg/m2/hr (approximately).

• As pickling progresses, the concentration of FeCl2, in the pickling bath increases causing increasing concentrations of HCl in the gas phase. This results in increased flux of HCl (g) into the work-zone. Values of partial pressure of HCl (pHCl) over the FeCl2 – HCl – H2O system, in the temperature range 45°C - 70°C, fit the following equation

• log (pHCl) = (-6.466)+( 0.0175 T) + 0.2028 × HCl (soln) + 0.07313 × FeCl2 (soln) .

• Where, (pHCl) = partial pressure of HCl above pickling solution in grams HCl per cubic feet of inert gas, T = Temperature °F, HCl (soln) = concentration of HCl in pickling liquid in weight percentage and FeCl2 (soln) = concentration of FeCl2 in pickling liquid in weight percentage.

• Computed film concentrations were found to vary in the range: 140 mg/Nm3 – 8700 mg/Nm3.

• Overall transfer process: HCl (g) = H+(aq) + Cl-(aq)• The equilibrium relation for this reaction is: • keq = {[H+] × [Cl-]}/{pHCl}• Where, pHCl is the partial pressure of HCl. • Both H+ and Cl- are entirely due to dissolution of HCl which

may be considered to be dissociated entirely. Therefore, pHCl = (keq)-1

× [Acid]2

• Hence, the solubility of HCl in water is non-linear in the partial pressure of the gaseous species

• Hence, Henry’s Law cannot be applied for determination of mass transfer equilibrium level for absorption.

• The process of absorption of HCl (g) in water is a two step process which can be written as an ordered sequence of the following reactions:

• HCl (g) = HCl (aq); and HCl (aq) = H+ (aq) + Cl-(aq)•  The corresponding equilibrium expressions are:• kHCl = [HCl (aq)] / pHCl ; ka = {[ H+] × [Cl-]}/ {HCl (aq)}•  Therefore the overall equilibrium expression is:• keq = {[ H+] × [Cl-]}/ pHCl

• The requirement for design is, therefore, the ratio keq/ ka, not kHCl. Henry’s law constant (kHCl) for HCl, has been reported in the range: 3 -3.5 bar in the temperature range: 290 -330K and log (ka) has been reported as 0.71 and 0.802 at 298K and 333K respectively.

• Absorption of HCl (g) in water is exothermic releasing about 2100 kJ/kmol (HCl (g) absorbed).

• This heat release raises the temperature of the scrubbing liquid thereby reducing solubility.

• The rise in temperature is approximately proportional to the change in concentration in the liquid phase.

• Therefore, large concentration changes occurring during absorption causes correspondingly large temperature changes.

• Hence, it is necessary to make some provision for removing this heat.

• Equilibrium diagram for HCl-H2O system at 1 atm reveals that the operating line is higher than the equilibrium line. This favors absorption.

• Its “practical” linearity signifies that weight of water evaporated equals the weight of HCl absorbed.

• Therefore there is a balance between heat gained due to absorption and heat lost due to evaporation and transfer of sensible heat.

• This means that significantly concentrated acid can be produced by absorption of HCl gas in an un-cooled tower, without loss of HCl from the top of the tower.

• This hints at the possibility of using scrubbing liquor as make-up at the pickling tanks. This is a better option than generating copious amounts of dilute liquor requiring disposal.

• “True gases” reduce the combined partial pressures of HCl (g) and H2O (g) causing reduction in the equilibrium temperature, but the equilibrium composition of the liquid phase is changed only negligibly.

• The combined effects of reduction in equilibrium temperature and evaporative cooling is the reason behind progressive concentration of HCl in the scrubbing liquid without exerting HCl load in the stack.

• A packed column was selected because: • It is more economical and effective for larger flows (hence, particularly

useful in situations involving large open sources) and for large fractional removals.

• It operates at a fraction of the pressure drop of plate columns. • It is particularly effective and economical when high volatility

compounds are to be removed because they offer very large transfer areas and the ability to operate counter currently.

• Better turn down and operational flexibility than any other type, particularly in case of situations where there is a considerable variation in the load to be scrubbed.

• The anticipated problem that voluminous dilute spent scrubbing liquor will be generated (because of comparatively large L/G ratios requirement) posing disposal problems was solved by way of study of the equilibrium diagram for HCl-H2O system.

• Volume of packing needed for a given efficiency is approximately constant.

• hence identical results can be obtained by a short, large diameter scrubber, or a tall small diameter scrubber.

• Large diameter requires comparatively larger water volumes to wet the packing –

• small column diameter causes higher pressure drops, and may also cause flooding (when the down-flow of re-circulating water across the packing is prevented due to the high gas velocity).

• A superficial air velocity in the range 1.0-2.0 (m/s) , the Liquid and Gas fluxes in the range 9000 – 10,000 (kg/hr/m2) and pressure drop across the packing in the range 0.5 -1.2 (inches WC per foot of packing height) was found to be the best compromise.

• Initial “frame-work” design, for the purpose of arriving at “first estimates,” was done by estimating the packing height, Hp: Hp = (factor of safety: 1.25 -1.5) × Hog × Nog

• where Hog is the height of a transfer unit (which is an experimentally determined property of the packing used and the data is supplied by the packing manufacturer ) and Nog is the number of transfer units required.

• This was followed by estimating the number of transfer units, Nog, required, in case where the solute reacts rapidly with the solvent (as in the present case):

• Nog = ln [8700/18] = ln [484] = 6.2 (approximately) for peak load. Therefore, Hog × Nog = 1.25 × 6.2 = 7.75 feet = 2.363 m (approximately). Therefore, the packing height of 2.75 m was selected for 1.5” PP Pall rings.

• As part of the design exercise, series of Volumetric Absorption Coefficients ,ky (s/m2), were computed in three steps as follows:– Calculate the mass flow rates, G, of HCl (kg/s) = [(y'-y") × (Va)]/

(3600), where y' & y" are the HCl vapor concentrations at inlet and outlet respectively (y" being fixed at 18 mg/Nm3), and Va is the volumetric flow rate of ventilation air (fixed as per ACGIH Criteria) carrying HCl (g) to the scrubber.

– Compute the “driving force”, Δ P, for absorption: = [(p' - p")]/ [ln (p'/ p")], where, p' & p" are partial pressures (in Pa) of HCl (g) at scrubber inlet and outlet respectively (p" being fixed corresponding to 18 mg/Nm3).

– Finally, a series of volumetric HCl Absorption Coefficients were obtained using: ky (s/m2) = [G] / [(Δ P) × (V)] where, V = Packing Volume (m3). The packing volume being fixed on the basis of the design aspects outlined above.

• HCl concentration in the gas film varies substantially. • Essentially, a fume scrubber is a constant-efficiency device; therefore, if the

inlet concentration increases 5 times, the outlet concentration will necessarily increase 5 times.

• These high concentrations occur only for very short periods, so that the average emissions remain low.

• Such variations increase the acidity of droplets which remain in the gas after the scrubber which may cause detrimental effects on equipment and environment.

• Hence, their formation and carry-over must be prevented. • Droplets, with diameters below 1 μm, are formed due to condensation, those

having diameters in the range 1 μm – 50 μm are formed due to the gas-liquid contacting operation and those having diameters in the range 50 μm -500 μm are formed due to atomization at the nozzles.

• In the present case, use of low-pressure-drop nozzles for wetting the packing elements, low stack velocity and a packed-bed type entrainment eliminator consisting of 1” PP Pall rings (with packed height = 200mm) has shown good results.

NEW FINDING• Computed Values of ky, had the same order of magnitude as the

coefficient of diffusion, D, of HCl (g) in the gas phase (D = 1.521 × 10-5 m2/s at 303 K).

• This indicates the possibility of scientific investigation to assess whether this observation is a pure coincidence or there is a plausible reason on the basis of theoretical considerations and/or by carrying out properly designed and controlled experiments in case of other gases which are highly soluble in and/or reacting with water.

• Alternatively, whether the dimensionless product ky × D, may be treated as design parameter (for equipment for absorption of soluble gases in water) needs further theoretical and experimental work.

FUNCTIONAL ASPECTS• In the present case:• The measured HCl (g) concentration, in the stack after the

scrubber, does not exceed 9 mg/Nm3 as against of the control limit of 35 mg/Nm3 prescribed by the Maharashtra Pollution Control Board (MPCB).

• Observed shop-floor HCl concentration: ≤ 2ppm. This is less than the TLV of 2 ppm = 3 mg/Nm3 (approximately for HCl).

• Author (ST) wishes to thank and acknowledge the support and help extended by Mr. Karan Bagaria, Director Shilpa Re-Rollers, Pvt. Ltd (SRPL). The studies carried out on the state-of-the-art pickling facility at Transmission Line Tower Galvanizing Plant (of M/s SRPL at MIDC, Butibori) formed a firm practical foundation regarding the concepts involved.

• All authors take this opportunity to thank Dr. Ajay Deshpande, I/c Zonal Officer, MPCB, Mumbai for supporting and encouraging the essential spirit of scientific enquiry without which very existence is questionable.

• Authors (ST, RVD and SJ) wish to thank Mr. Lalit Shah of M/s Polyplast Chemiplants India Pvt. Ltd, Mumbai who not only fabricated the scrubber and the ducting but also shared his immense experience and made many visits at site for fine tuning of the system.

• There are two kinds of science, normal science and revolutionary science.

• In any particular field, normal science is conducted in accordance with a set of rules, concepts and procedures, called a paradigm which is accepted by all scientists working in that field. Normal science is similar to puzzle-solving: interesting and even beautiful solutions are obtained but the rules never change.

• In this normal scientific activity, however, unexpected discoveries sometimes are made that are inconsistent with the prevailing paradigm.

• Among the scientists, a tense situation then ensues, which increases in intensity until a scientific revolution is achieved.

• This is marked by a paradigm shift, and a new paradigm emerges under which normal scientific activity can be resumed.

• THE AUTHOR (ST) DEDICATES THIS PAPER TO THE LOVING MEMORY OF HIS MOTHER MRS SHUBHALAXMI THERGAONKAR WHO WAS (AND STILL IS) “THE” CONSTANT SOURCE OF INSPIRATION. HER LIFE AND EVEN THE VERY PROCESS WHICH RESULTED IN HER TRANSCENDENCE ON MARCH 03,2011, “DEMONSTRATED” , Q.E.D., THAT “THE MOMENT ONE COMMITS ONESELF, THEN PROVIDENCE MOVES IN TOO. ALL SORTS OF THINGS OCCUR TO HELP ONE THAT WOULD NEVER OTHERWISE HAVE OCCURRED. A WHOLE STREAM OF EVENTS ISSUES FROM THE DECISION, RAISING IN ONES FAVOUR ALL MANNER OF UNFORSEEN INCIDENTS, MEETINGS AND ASSISTENCE, WHICH NO MAN COULD HAVE DREAMT WOULD COME HIS WAY”.