HNO3 during pickling acid regeneration

European Symposium on Computer Aided Process Engineering - 10 S. Pierucci (Editor) 9 2000 Elsevier Science B.V. All rights reserved.

919

Simulation and optimization of the reactive absorption of HF/HNO3 during pickling acid regeneration W. Wukovits a , W. Kamer b, A. Lebl b, M. Harasek a and A. Friedl a a Institute of Chemical Engineering, Fuel and Environmental Technology, Vienna University of Technology, Getreidemarkt 9/159, A-1060 Vienna, Austria

b Andritz AG-Ruthner Surface Technologies, Eibesbrunnergasse 20, A-1120 Vienna, Austria The optimal operation of pickling acid regeneration is very important for its applicability and economical feasibility. This paper describes the simulation and optimization of the process conditions with the commercial process simulation programm ASPENplus 9.3. Beside the calculation and regression of physical properties, an important step was the implementation of reactions occuring during NOx-absorption in ASPENplus. The simulation model was improved by fitting with data from a pilot scale plant. Subsequently a sensitivity analysis was done to find process parameters to be optimized. 1. I N T R O D U C T I O N Surface cleaning is an important step in the production of steel to remove oxide layers built during heat-treatment processes or storage. This cleaning process, called pickling, is usually done by treatment with inorganic acids, mainly hydrochloric acid or sulfuric acid. For cleaning stainless steel a mixture of nitric acid, hydrofluoric acid and water is used. Because of economical reasons and environmental protection, it has become increasingly important to regenerate the used pickling acid. In order to ensure the economical operation of a pickling acid regeneration plant, it is necessary to optimize process conditions to: 9 maximize HF regeneration 9 maximize HNO3 regeneration, with regard to a high degree of NOx-oxidation 9 control the dilution of the regenerated acid through condensation of water, built during the burning of the spent acid (see description of the regeneration process) This paper presents the results of a sensitivity analysis of a pickling acid regeneration process done with the commercial process simulation programm ASPENplus 9.3. 2. R E G E N E R A T I O N PROCESS

Depending on the used pickling acid, regeneration can be performed by rectification, crystallization, extraction, absorption or membrane processes [1]. In case of a mixture of nitric acid and hydrofluoric acid, applicable processes are liquid-liquid extraction, fluoride cristallization, electrodialyses with bipolar ion exchange membranes, retardation (a process using ion exchange resins) [2] and absorption. In this study, the regeneration process involves burning and thermal decomposition of the spent acid / salt mixture, by which the salts of the metals are converted into their

920

Fig. 1. Flowsheet of the pickling acid regeneration plant corresponding oxides (PYROMARS | process). Unfortunately, also decomposition of a part of the nitric acid occurs during this process step. The flue gas passes a venturi scrubber, where it is contacted with the used acid (Fig.l). In this scrubber the used pickling solution is preheated and preconcentrated before entering the burning unit. Simultaneously the flue gas is quenched and metall oxides, which have passed the cyclone are removed. After the venturi scrubber, the gas enters the first of two packed absorption columns, where gaseous nitric and hydrofluoric acid are condensed or absorbed. In these columns, also oxidation and absorption of nitrous oxides occurs. The absorption step is modeled as adiabatic. To remove remaining HF and NOx from flue gas after the absorption step, the gas passes a scrubber and a catalytic DeNOx. 3. SIMULATION MODEL Compared with other absorption processes, the absorption of NOx is very complex. There exist various gaseous species which are in equilibrium and are absorbed simultaneously. In the liquid phase two oxy-acids are built, whereby one of these acids decomposes to NO. NO is desorbed from solution and enters the absorption cycle again. The important reactions in NOx-absorption are shown in Fig. 2. R3

N204

N204 + H20

~> HNO 2 + HNO3

2 NO 2

2 NO 2 + H20

> HNO2 + HNO3

NO + NO 2 + H20

R1

~

2 HNO 2

3 HNO 2 ~_- HNO3 + H20 + 2 NO R4

2 N O + O2/

x~ O + NO2 ~

N203

N203 + H20

> 2 HNO 2 R6

NO

Gas

Interface

Liquid

Fig. 2. Mechanism and reactions of NOx-absorption [3, 4, 5]

921 Models for the calculation of NOx-absorption during nitric acid production are presented for example by Wiegand et al. [6] and Suchak et al. [7]. A general view of the features of different models is given by Pradhan [8]. Because of the large number of species involved in the process, it was decided to use a commercial process simulation program for calculating the absorption step. Within this work, calculation and simulation was done using ASPENplus Version 9.3. The implemented calculation algorithms as well as models and databases for physical property calculation should allow a fast determination of process parameters to be optimized. In doing so, it was accepted to calculate NOx-absorption based on equilibrium calculations, not considering heat- and mass-transfer effects on the absorption step. From the four paths of NOn-absorption shown in Fig. 2, the path via N204 and via N203 was implemented in ASPENplus. Absorption of NO2 and formation of HNO2 in the gas phase were neglected. Calculations showed, that the absorption of N203 is of minor importance. 4. PHYSICAL PROPERTIES

The standard thermodynamic model to handle electrolyte systems in ASPENplus is ElecNRTL. It calculates the activity coefficients for molecular and ionic species using binary and pair parameters [9]. Adjustable parameters are the Born radius of ionic species and the NRTL interaction parameters for molecule-molecule, molecule-electrolyte and electrolyteelectrolyte pairs. The comparison of the calculated vapor-liquid equilibrium with data from literature gives good accordance in the interesting concentration range from 0 to 15 mole% for the system HF/H20 (Fig. 3). The difference using the ElecNRTL-model is even less than using the model ENRTL-HF, which takes into consideration the hexamerization of HF in the vapor phase. Although the deviations in the system HNO3/H20 are bigger, no data regression is necessary. The situation is different concerning mass density and heat capacity. Fig. 4 shows the mass density of the system HNO3/H20. The deviation for a solution of 10 wt% HNO3, calculated via Clark Aqueous Electrolyte Volume, the ASPENplus electrolyte mixture standard model, is considerable and increases with increasing HNO3 content. Density data obtained by the 115

1130 1110

110

.............................................................................................................................................................................. ~.:. I~onc.in wt% HNO3

....

%-C. ~-.

1090

~ 1070 ,x

~'105 P.

"A~i,

'-'1050

~

100

~ . .

~^ ~

~'A ~

"-, ~'~,

'~ 1030 c "o 1010 m 990

~ 95

9 literature (10 wt%) [] 9-,i,-- Clark (10 wt%) - 49.--o- .Clark (30 wt%) ~ ~ f i t t e d data (10 wt%) - - -

E

970

90

950

85

930 0

0,1

0,2 0,3 0,4 m o l e f r a c HF [-]

Fig. 3. Vapor-liquid equilibrium of the system HF/H20 [ 10]

0,5

0,6

. 0

20

.

. 40

. 60

literature (20 wt%) Clark (20 wt%) Costald (10 wt% fitted data (20 wt%)

. 80

100

120

temperature [~

Fig. 4. Density of the system HNO3/H20 [11]

922 Tab. 1. Gas composition [wt%] after column 2 obtained from experiment and simulation Pilot Plant 3,6 0,48 0,34 0,28 74,1 13,0 8,2

H20

HNO3 NO NO2 N2

02 CO2

Simulation 2,7 0,44 0,46 0,00 76,6 12,0 7,8

Costald model show better correspondence. But even these data require data regression. A comparison of fitted data with literature is also given in Fig. 4. Simulation results with the described model showed good accordance to data from a pilot scale pickling acid regeneration plant (Tab. 1). Nevertheless, it was necessary to fit adjustable simulation parameters to process data, to optimize the solubility calculation of nitrous oxides during nitric acid formation, because simulation gives a higher NO content than experiment, while the content of NO2 is zero in the results of simulation. The parameter to be fitted is the solubility of N204. ASPENplus uses Henry's law to calculate gas solubility in liquids [9]" P~ = H , * x i

Henry's constant Hi is obtained by the following relation: B i

lnH, = At +--~ + C~ * lnT + D r *T 1

After fitting the solubility of N 2 0 4 with data obtained from the pilot plant, the ratio of NO:NO2 in the gas stream after the absorption step corresponds well with the ratio observed during the experiment. Fig. 5 and Tab. 1 summarizes kinetic- and solubility-parameters used in further simulations. kl

2

krl - - ~ - ~ =

4,321'

* 5421,8 10_,5 T_~,exP(R,TI

RI"

r~ = - - ~ P o, P No

R2:

K 2 = ~2 PNO2

lnK 2 = -32,6 + - - - ~

R3"

r3 =-k3CN~o4

k~3 = k3 = 12994022,5 * exp(26298'4 ]

R4:

r4 =

R5:

Ks =

_ k~

kmol ] ,) [Pa~*m----3*s I

6866 E11

PN204

4

k 4 CHN02 2 2 He(No) CNo PN:o3

kra k4 = 2027,9' exp(515,49;3 / Ikmmol,sl H,No 4 7 4 0 1 1~] a lnK 5 =-28,1+--T---

PNo * PNo2

r6 6C o

'6: 40 Ill

9'rateconstantsaregivenintheformimplementedinASPENplus."

kr,=kp~.*T"*exP(R--@T I [12]

Fig. 5. Kinetic-parameters used in the simulation model [4, 5]

923 Tab. 2. Henry's constants Hi [atm/mole fraction] used in the simulation model (25~ in water) [ 13, 14]

N2 02

CO2

ASPEN 86530 43980 1610 29200 17E06

Literature 86400 43610 1630 28700 0,71 **)

NO N204 *) *) fittedsolubility **) valuefrom literature describes "bulk solubility"; all species of the gas and its reaction products with water are included 5. RESULT OF THE SENSITIVITY ANALYSIS Finally, the developed simulation model was used to find and estimate optimization possibilities for the described pickling acid regeneration process. A sensitivity analysis was realized to obtain the degree of HNO3 and NOx precipitation as well as the concentration of the regenerated acid as a function of different process parameters. Tab. 3 shows the results of the sensitivity analysis. The varied parameter, the area of variation (usually +/- 30% of the value used in the process) and the effect on precipitaton and concentration are given. An increase in HNO3 and NO• precipitation is given by an increase of column pressure and a decrease of column temperature. The reduction of the inert gas flow also leads to a better precipitation. But the sensitivity analysis also shows that all arrangements cause just a small increase in nitric acid concentration. The reason is the increase of water condensation at lower column temperature. The strongest influence on NO• results from the reduction of the inert gas flow and the increase of column pressure. The inert gas flow in the process is given by the energy demand for the evaporation and decomposition of the used pickling solution. Thus a reduction of the inert gas content is coupled with the optimization or change of the energy supply. An increase of the column pressure is only possible in column 2, because the high acid content of the gas stream entering column 1 would cause corrosion problems in the fan. Tab. 3. Results of the sensitivity analysis Precipitation [%] Parameter Temp. Col 1 Temp. Col 2 Gas-Holdup Col. 2 Input Inertgas Input Oxygen Pressure Col. 2

Unit Variation ~ 30-60 ~ 20-45 m3 10-28 m3/h 1930-4500 m3/h 230-530 mbar 963-1963

HNO3

NOx

78-70 80-86 No effect! 84-65 73-71 71-82

12-7 14-7 8-10 23-5 8-9 9-29

Conc. Reg. Acid [wt%] HNO3 HF 11,6-10,6 7,1-7,2 11,5-10,7 7,1-7,2 10,8 7,2 12,2-9,3 7,0-7,5 11,0-10,6 7,2 10,8-12,3 6,8-7,2

924 6. CONCLUSION Sensitivity analysis with the obtained absorption model shows the complexity of the optimization problem. It was found, that in the discussed process most variations of process parameters which give an large increase in precipitation of nitric acid or in NOx-oxidation result in no or only a small increase in the concentration of the regenerated acid because of water condensation. In further simulations special attention will be given to the rearrangement of apparatus to obtain a process, where it is possible to adjust the acid concentration nearly independently from the degree of acid- and NOx-absorption. The work with the developed simulation model in ASPENplus shows that it is possible to find and estimate optimization possibilities in NO• even in using the equilibrium approach. However, for a detailed simulation and process design mass- and heat-transfer calculations have to be taken into consideration. NOTATION ci

mole concentration [kmole/m3] rate constant rate constant (as implemented in ASPENplus) Ki equilibrium constant pi partial pressure [Pa] Pi partial pressure [atm] R gas constant ri reaction rate [kmole/m3*s] T temperature [K] xi mole fraction [-] Hc Henry's constant (mole concentration basis) [atm*m3/kmole] Hi Henry's constant (mole fraction basis) [atm/mole fraction] Ai, Bi, Ci, Di parameters for calculation of Henry's constant

ki kri

REFERENCES [ 1] Ullmann's Encylopedia of Industrial Chemistry, Vol. A 14, 5th Ed., VCH/Weinheim, 1989 [2] C.J. Brown; Iron Steel Eng., 67(1) (1990) 55-60 [3] D. Thomas, S. Brohez, J. Vanderschuren; Trans. Inst. Chem. Eng. Part B, 74 (1996) 52-58 [4] Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 17, 5th Ed.,VCH/Weinheim 1991 [5] F.T. Shadid, D. Handley; The Chemical Engineering Journal, 43 (1990) 75-88 [6] K.W. Wiegand, E. Scheibler, M. Thiemann; Chem. Eng. Technol. 15(5) (1990) 289-297 [7] N.J. Suchak, K.R. Jethani, J.B. Joshi; AIChE J., 37(3) (1991) 323-339 [8] M.P.Pradhan, N.J. Suchak, P.R.Walse, J.B.Joshi; Chem. Eng. Sci., 52(24) (1997) 4569-4591 [9] ASPENplus Reference Manual Release 9.3, Vol. 2, 1996 [ 10] Dechema Data Series I/1 b Suppl.2, Dechema/Frankfurt a. Main, 1988 [ 11] Landolt-B6mstein - Neue Serie, Bd. 1; Teil B, Springer Verlag, 1977 [ 12] ASPENplus User Guide Release 9.3, Vol. 2, 1996 [13] J.M. Kasper, C.A.Clausen, C.D.Cooper; J. Air&Water Manage. Assoc., 46 (1996) 127-133 [ 14] CRC Handbook of Chemistry and Physics, 75 th Ed., CRC Press/Boca Raton, 1994-1995