Acid Mist Cyclone Separation Experiment on the Hydrochloric Acid Regeneration System of a Cold Rolling Steel Plant

In the hydrochloric acid regeneration system of a cold rolling steel plant, the large amounts of hydrochloric acid droplets and iron oxide dust in the flue gas at the absorption column outlet cause serious air pollution. To reduce pollution, a highprecision cyclone separator is utilized to recycle hydrogen chloride acid mist and iron oxide dust simultaneously. The key factors of the high-precision cyclone separator, such as pressure drop, separation efficiency, and fluid density, are investigated to determine the optimal operation parameters under practical working conditions. Results show that the separation efficiency of the cyclone separator increases rapidly initially and then decreases gradually with an increase in pressure drop. Pressure drop and separation efficiency increase slightly with an increase in inlet fluid concentration. When the velocities of the inlet gas are 4.8–10 m s and 5.0–9.2 m s, the separation efficiencies of hydrochloric acid and iron oxide are both higher than 80%. When the velocity of the inlet gas is 6.67 m s, the separation efficiencies of hydrochloric acid and iron oxide reach the highest values of 92% and 89%, respectively.


INTRODUCTION
With the continuously improving requirements of energy conservation and environmental protection, industrial emission standards have become increasingly stringent.Hydrochloric acid regeneration devices in early-founded cold rolling steel plants need to be improved to adapt to the new emission standards of the steel rolling industry.In the existing production process, flue gas that contains hydrochloric acid mist and iron oxide dust is often transmitted to downstream equipment or is discharged into the atmosphere directly after being washed.The purification effect of the column washer for flue gas is not good and results in downstream equipment corrosion and excessive exhaust emission.Therefore, reducing the amount of hydrochloric acid mist and iron oxide dust in flue gas emission is highly significant for emission reduction and improvement of the efficiency of acid regeneration systems.Gravity settler cannot capture the fine particles which lest than 10 µm in flue gas.Although demist nets can capture fine particles, fouling and plugging often occur.Furthermore, these nets are unsuitable for situations involving high liquid content.By contrast, a cyclone separator, which has a simple structure and stable performance, can separate fine particles.In addition, it does not become blocked easily (Movafaghian et al., 2000;Wang et al., 2010).With the continuous development of computer simulation technology, the computational fluid dynamics (CFD) technology has been applied to calculate the gas flow field in a cyclone separator with different entrance structure, the results showed that the effect of changing the inlet width was more significant than changing the inlet height (Elsayed and Lacor, 2011a, b;Avci et al., 2013).By comparing with the conventional models which include the theoretical and statistical models as well as the artificial neural network model, Chen and Shi (2007) established a universal model to calculate cyclone pressure drop.Zhao (2009Zhao ( , 2010Zhao ( , 2012) ) found that the SVM (support vector machine) model provides the higher generalization performance in pressure drop.Li et al. (2015) established a new pressure drop model which indicates this pressure drop model was suitable for the cyclone splitters with pure or droplet-laden gases.Tsai et al. (2004Tsai et al. ( , 2007Tsai et al. ( , 2011) ) remove nanoparticles in low pressure or vacuum conditions by an axial-flow cyclone.Quan et al. (2009Quan et al. ( , 2010) ) invented a new type of gas-liquid stripping equipment-the water jet air cyclone separator (WSA) which greatly improves the efficiency of the stripping of ammonia.Yang et al. (2015) established an innovative operation mode to a new pressure drop model of gas -liquid cyclone.Zhang et al. (2015) calculated the residence time distribution in short-contact cyclone reactors by CFD simulation and experiment.Ma et al. (2013Ma et al. ( , 2014aMa et al. ( , b, 2015) ) applied the micro cyclone to the ultra-high pressure environment of removing hydrocarbon or ammonia in recycle hydrogen, compared with the traditional gas-liquid separation device has a better effect.
Many scholars have investigated the high-precision cyclone separator in many aspects, but no study has been conducted on the high-precision cyclone separator in the hydrochloric acid regeneration system of a cold rolling steel plant.In consideration of the process conditions of a hydrochloric acid regeneration system, we design a high-precision cyclone separator.An industrial sideline test is conducted to measure the effects of several factors (e.g., pressure drop and separation efficiency) on separation performance, determine the optimum operation parameters for the high-precision cyclone separator, and effectively remove hydrogen chloride acid droplets and iron oxide dust in flue gas.

METHODS
The high-precision cyclone separator adopted in the experiment is a rectangular tangential inlet micro cyclone separator, as shown in The industry sideline experimental flow chart is shown in Fig. 2. Waste hydrochloric acid is regenerated and recycled through the reactor (1).Waste acid, with SiO 2 removed, enters the preconcentrator for heating concentration.Concentrated waste acid is expelled from the pump to the reactor for Ruthner spray, and Fe 2 O 3 powder is generated from this reaction.Dust particles with a small diameter enter the low-precision cyclone separator (2) for the separation of HCl and Fe 2 O 3 and then go into the preconcentrator (3).After cooling and dust removal, they enter the absorption column (4).By spraying rinsing water at the top of the absorption column, HCl gas is absorbed and changed to regenerated acid.With the help of the fan (8), the flue gas exhausted by the absorption column enters the high-precision cyclone separator (7).Fine hydrogen chloride acid mist droplets and iron oxide dust particles are then further separated.The flue gas from the overflow port of the high-precision cyclone separator ( 7) is washed in the column washer (5) and then exhausted into the atmosphere.The separating medium at the bottom flow port is discharged with regenerated acid.
The experimental device is shown in the line box of Fig. 2.During the experiment, gas samples in the inlet and separating medium samples at the underflow port of the highprecision cyclone separator were obtained and measured.The contents of Fe 2 O 3 and HCl were measured with a plasma emission spectrometer and an ion chromatographer, respectively.In unit time, the total amount in the inlet is g i , the total collection at the underflow port is g u , and separation efficiency E can be represented as E = g u /g i .

Experimental Materials
The temperature of the flue gas extracted from the experimental site was approximately 78°C.The content of steam, N 2 , CO 2 , O 2 , and HCl in flue gas was 71.44%, 22.83%, 4.66%, 0.9%, and 0.17% respectively.The variation range of water droplets concentration in the flue gas was 179-283 g m -3 , that of Fe 2 O 3 concentration was 460-600 mg m -3 , and that of HCl concentration was 2070-2680 mg m -3 .Fe 2 O 3 in flue gas is reddish brown, insoluble in water, and does not react with water.Hydrogen chloride has a suffocating odor, and its density is higher than that of air.Its aqueous solution is hydrochloric acid, which can corrode downstream devices and pollute air if exhausted into it.The Fe 2 O 3 can react with HCl in the flue gas and separated liquid.The HCl react with Fe 2 O 3 to FeCl 3 , and dissolved in the solution,  A Melvin droplet and particle size analyzer was utilized to determine the diameter of the Fe 2 O 3 particle in hydrochloric acid liquid.The measured average diameter of Fe 2 O 3 dust was 1.796 µm, and the diameter of more than 90% of the particles was larger than 0.502 µm.The Fe 2 O 3 particle sizevolume curve is shown in Fig. 3, and the Fe 2 O 3 SEM image is shown in Fig. 4.

Experimental Results and Discussion
As for the industrial application of the high-precision cyclone separator, we mainly investigated pressure drop and separation efficiency.A small pressure drop can reduce energy consumption, and the system fan need not be modified when installing the cyclone separating device.A fluctuation in flue gas flow occurs in the industrial production.This fluctuation affects the pressure drop and separation efficiency of the high-precision cyclone separator.Based on previous engineering designs and uses, we selected the flue gas velocity of 4.167-11.677m s -1 as the experimental range.
When the inlet droplet concentration was 263 g m -3 , we measured the pressure difference ΔP in the cyclone separator inlet and overflow port.The flue gas velocity in the inlet was changed.The effect of different velocities in the inlet on droplet separation efficiency and pressure drop was tested, and the results are shown in Fig. 5.The ΔP increases with an increase in inlet velocity V in , and the increase rate is rapid.The separation efficiency of the droplets in the flue gas increases initially and then decreases with an increase in gas velocity in the inlet of the cyclone separator.When the gas velocity in the cyclone separator is low, centrifugal force reduces the migration velocity of droplets to the side wall, and the droplets far from the wall are brought out by gas before separation.With an increase in the inlet flow rate, the centrifugal force of droplets increases, and the droplets move fast to the side wall.During this process, the droplets form larger droplets because of collision and coalescence; these larger droplets are easy to separate.Thus, the separation rate increases rapidly.As the gas flow rate continues to increase, the droplet collision and shear breaking effects increase.A large number of small droplets is formed.These droplets are discharged with the internal circulating flow, resulting in low separation efficiency.The increased flow velocity results in a short retention period of droplets in the separation zone, which in turn leads to increased short-circuit flow.The droplet separation efficiency of the cyclone separator is high and can reach up to 95%.The area with separation efficiency higher than 80% is considered a highly efficient separation zone.The variation range of the highly efficient separation zone can reach 4.4-10.3m s -1 .In the actual production process, the frequency control of the system fan changes according to the working condition to adapt to the process.This situation requires the cyclone separator to be flexible to some extent.The system fan flow rate can change fluid concentration C i in flue gas.
Therefore, the effect of inlet fluid concentration on separation performance needs to be investigated.The relationship between pressure drop at the overflow port and fluid concentration at different inlet flow velocities was tested in this study.The results are shown in Fig. 6.Pressure drop increases slightly with an increase in concentration.When the velocity at the inlet is 11.67 m s -1 , the pressure drop increase is the largest (only 3.5%).The increased pressure drop is mainly caused by the increase in internal rotation loss in the cyclone separator because an increase in liquid concentration in flue gas increases the viscosity and density of gas-liquid two-phase flow in the cyclone separator's inlet.Thus, if the fluid concentration in the cyclone separator's inlet is in the range of 179-283 g m -3 , it would have minimal impact on the pressure drop of the cyclone separator.
The most important indicator utilized in acid regeneration by a cyclone separator is the separation efficiency for Fe 2 O 3 and HCl.Five groups of experiments were conducted at different velocities.In each experimental period, the average concentrations of Fe 2 O 3 and HCl in the inlet and outlet and the corresponding separation efficiency were measured.The results are shown in Figs.7 and 8.The concentration of Fe 2 O 3 and HCl in the inlet fluctuates.Comparison of the concentrations in the inlet and outlet indicates that the flue gas purification effect by the cyclone separator is obvious.The velocity ranges in the highly efficient zones of Fe 2 O 3 and HCl are 5.00-9.17m s -1 and 5.00-9.72 m s -1 , respectively.When the velocity is 6.67 m s -1 , the highest separation efficiencies are 89% and 92%.
With 280 working days a year and average cyclone separation efficiency of 85%, each acid separation station can directly separate 32.5 t of ferric oxide powder and 145.8 t of hydrogen chloride.Thus, the workload of the column washer is reduced significantly, and a substantial amount of resources is recycled.
Fig. 9 shows the relationship curve of concentration and separation efficiency at different velocities at the inlet, namely, 5.00, 6.67, and 11.67 m s -1 .With an increase in fluid concentration, separation efficiency increases slightly.When the velocities are 5.00, 6.67, and 8.33 m s -1 , the corresponding increments are 3.6%, 3.0%, and 2.2%, respectively.With an increase in inlet velocity, the increase in separation efficiency becomes small.This result shows that at a high velocity, the influence of droplet breakage on separation efficiency increases.

CONCLUSIONS
When the inlet liquid content of the cyclone separator is 263 g m -3 , the separation efficiency of acid fog droplets in the cyclone separator increases initially and then decreases with an increase in pressure drop.The velocity in the highly efficient zone of the cyclone separator to separate droplets is 5.00-10.3m s -1 , and the corresponding pressure drop at the overflow port is 0.20-0.82KPa.When the velocity is 6.67 m s -1 , droplet separation efficiency reaches the highest value of 95%, and the corresponding pressure drop is 0.32 KPa.At the same time the highest separation efficiency of Fe 2 O 3 and HCl is 89% and 92%, respectively.Within 179-283 g m -3 , the increase in fluid concentration in the cyclone separator inlet causes a slight increase in pressure drop at the overflow port and droplet separation efficiency, with the largest increments of 3.5% and 3.6%, respectively.This result shows that the cyclone separator has wide operation flexibility.With 280 working days a year and average cyclone separation efficiency of 85%, each acid separation station can directly separate 32.5t of ferric oxide powder and 145.8t of hydrogen chloride.

ACKNOWLEDGMENTS
We would like to express our thanks for the sponsorship of the National Key Research and Development Program of China (2016YFC0204500), the Fundamental Research Funds for the Central Universities (22A201514028), the China Postdoctoral Science Foundation (2015M571505).[mg m -3 ] HCl concentration in gas P in [MPa] inlet pressure of cyclone

SYMBOLS USED
Fig. 1.The material of cyclone separator is Fiber Reinforced Plastics (FRP), and the dimension parameters (mm) are as follows: D = 70, a = 50, b = 20, di = 32, dc = 32, s = 100, H 1 = 150, H 2 = 225, H 3 = 150, I n = 150.The separator was optimized based on the Stairmand high-efficiency cyclone.The diameter of the cylindrical section of the cyclone separator was reduced, centrifugation was enhanced, and a parallel guide plate was added to the inlet to make the air flow smooth.