- Email: [email protected]
Simulation and optimization of a membrane system for chromium recovery from tanning wastes
DE!SLINATION EL-SEWER
Desalination 141 (2001) 315-324 www.elsevier.com/locate/desal
Simulation and optimization of a membrane system for chromium recovery from tanning wastes H.F. Shaalan*, M.H. Sorour, S.R. Tewfik Chemical Engineering Department and Pilot Plant Laboratory Engineering Division, National Research Center, Dokki, Cairo, Egypt Tel. +20 (2) 337-0933; Fax +20 (2) 337-0931; email: hayam64@hotmaikom
Received 14 March 2001; accepted 12 April 200 1
Abstract Leather tanning processing involves treatment of skin in chrome baths where the percentage exhaustion of chromium salts is usually in the range of 60-70%. Excess chromium salts are discharged to the sewer system or an effluents treatment facility. Thus, extensive efforts have been exerted on both R&D and application levels to recover chromium for economic and environmental concerns. Several methods have been developed for chromium recovery or reuse including precipitation, adsorption, redox-adsorption, ion-exchange and membrane systems. Recent research indicated that chromium salts recovered by membranes manifested improved characteristics for tanned and retanned skins. This paper is dedicated to the simulation and optimization of dual-membrane systems involving inorganic ultrafiltration and nanofiltration for the recovery of chromium from effluents of exhausted baths. In the first section, parameters governing the separation of protein/fat mix by ultrafiltration are obtained by analysis of published results on separation of proteins by ultrafiltration. The parameters for the concentration of chromium by nanofiltration are also deduced. A simulation model based on the resistance model and involving material balance and energy requirements for the ultrafiltrationnanofiltration scheme has been developed. A cost-objective function involving annual costs and revenues has been formulated. Optimum design parameters have been defined using a Box COMPLEX constrained optimization routine. Results indicated that under the stated optimum conditions the system is cost effective. Keywords: Membrane; Tanning waste; Chromium recovery; Optimization
1. Introduction Membrane schemes have witnessed considerable development in the last decade permitting difficult separation to be achieved. Numerous *Corresponding author. 001 l-9 164/Ol/$- See front matter Q PII:SOOll-9164(01)00414-3
separation alternatives could be developed through combinations of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). The exact system design is governed by several factors including, but not limited to, physico-chemical characteristics ofthe
2001 Elsevier Science B.V. All rights reserved
316
H.F. Shaalan et al. /Desalination
feed stream, target separation efficiency, economics of processing and price of recoverables. UF systems have been widely adopted for the separation of proteins in the food industry. Also the application of complex membrane schemes for the recovery of heavy metals is in commercial practice, especially for the surface finishing industry. The cost savings based on the application of membrane schemes stimulated numerous endeavors for the recovery of chrome from leather tanning effluents as an alternative for the precipitation and chemical treatment recovery approach. Application of dual-membrane systems including MF/NF, UF/RO and NF/RO for treatment of tannery waste and recovery of chrome and/or other chemicals is challenged by the presence of considerable BOD load and proteins, which may cause considerable fouling and subsequent system failure temporarily or permanently. However, the rapid development of inorganic membranes enable numerous mitigating technical problems affecting the processing of tannery waste. The present paper presents critical assessment of previous endeavors, explores a viable separation UF/NF scheme based on inorganic membranes and finally presents the optimal conditions governing the successful application of the proposed membrane scheme in view of a profit/cost objective function.
2. Previous endeavors The separation of proteins and heavy metals using UF and NF membranes has been experimentally investigated by several workers. Results of previous work have been compiled and are discussed below. Several previous studies address protein separation using UF membranes, and it is not surprising that research has found proteins to be a major foulant in UF of food and biological
141 (2001) 315-324
systems since proteins are rejected by the membrane and tend to have high concentrations at the membrane surface. Fouling of UF membranes by cheese whey is a classic and particularly vexing problem in the industry [l]. Nakao et al. [2] studied the effect of operating pressure and protein concentrations on flux decline using a tubular polymeric membrane (PAN with cut-off 20,000-40,000) at 0.62 m/s velocity. Further, Cheryan [3] reported that a-lactalbumin had the greatest flux-depressing effect. They also studied the fouling of UF membranes by applying cottage cheese whey and skimmed milk and soy proteins. On the other hand, because of the high toxicity for human health, heavy metal concentrations in wastewater are limited by strict standards. Membrane separations are used more and more frequently. Several authors studied the separation of heavy metals using UF membranes, being the most economic membrane technique. Solpan and Sahan [4] studied the separation and concentration of metal ions (Cu’+, Ni2+, Fe3’) using EC-PEG4000 alloy UF membranes. Benchscale experiments were performed to assess the effectiveness of RO and NF membranes in removing arsenic (As) from synthetic fresh water and source water. Waypa et al. [5] examined the effects of operating conditions (applied pressure and feed water temperature) and solution chemical composition. Also, Kune et al. [6] studied the treatment of cyanide/heavy metals bearing wastewater by the micelle-enhanced UF technique. Experimental results based on enhanced UF was proposed by Sodaouic et al. [7] in order to separate cadmium and chromate ions from wastewater using zirconium oxide coated on a carbon membrane by adding ionic surfactants. The concentration of both cadmium and hexavalent chromium in the feed are 750, 200 kg/m3 and in the permeate 0.2 and 0.1 kg/m3, respectively. Fabiani et al. [8] used a coupled MF inorganic membrane as a pretreatment of the
H.F. Shaalan et al. /Desalination
waste solution followed by an UF polysulfone membrane to produce a clean chromium solution (permeate) with 28% recovery. Cassano et al. [9] used an integrated membrane scheme (UF followed by NF polymeric membranes) to recover and concentrate the chromium exhaust tanning baths. They found that the quality of the concentrated chromium was improved with respect to the conventional recovery process, based on precipitation and dissolution, as showed by the low ratio of organic lipolytic components: chromium. This process also permits the reuse of the permeate from NF in the pickling phase considering the high content of chlorides in the solution. The flux in the NF membrane is about 86% after 1 h at 16 bar and then remained approximately constant for the following 2 h. NF was tested for the recovery of trivalent chromium in the residual tanning floats [lo]. The economics of low-pressure high-rate membrane process are governed by several factors including the adopted membrane system, flowrate and characteristics ofthe feed, operating conditions, site characteristics and prevailing cost factors. However, cost data and models have been reported by several authors. As an example, Pickering presented a cost model for membrane filtration which may be used to evaluate the sensitivity of capital and operating costs to design and operating parameters in preliminary cost estimates for pressure-driven membrane filtration facilities [l 11.
3. Proposed UF/NF scheme for treatment tannery waste
of
3.1. Technical considerations Performance characteristics for protein and chromium separation by membranes have been predicted according to reported results and by statistical analysis of variables based on published experimental data as outlined below.
317
141 (2001) 315-324
3.1.1. Characteristics of chrome bath effluents Based on the analysis of typical samples from Egyptian tanneries, using standard methods, it has been found that the protein and chrome concentrations in a chrome bath effluent vary around 2 and 3 kg/m3, respectively. This is in agreement with published data which indicate that typical analysis of chrome bath effluents is as shown in Table 1. 3.1.2. Proposed hybrid system Several dual schemes have been initially proposed. These include MF/UF, UF/NF and NF/RO. Preliminary investigations indicated that the UF/NF system exhibits superior performance. This dual scheme is further investigated in this work. The system is depicted in Fig. 1. It is recommended that both the UF and NF membranes be inorganic. This selection is based on the following justifications. Lower adsorption to solutes (protein) in the membrane surface in the UF stage. Withstand high temperature application. Extreme tolerance for chemical attack. High resistance to biological attack. Longer service life. Fouling tendency is lower with tubular membranes.
Table 1 Typical analysis of chromium bath effluent [lo] Parameter
Concentration,
Chrome Sulfate Chloride N-Kijeldal N-NH, Oils and fats COD TSS
3.6 22-23 11-16 1.07-1.11 0.6 0.3 8.6 0.7-2.9 3.74.2
PH
kg/m’
318
I
H.F. Shaalan et al. /Desalination
141 (2001) 315-324
reuse for retanning baths
prefilter
Equalization
I
I
I
UF
Tank
preparation of new pickling baths or waste treatment
Settler Fig. 1. Process flow sheet of the proposed UF/NF system.
3.1.3. Formulated simulation model
(d) Energy consumption
The model adopted in this work for the simulation and optimization of the UF/NF scheme is represented by the following set of relationships for each of the UF and NF stages. (a) Relationship between flux and pressure for UF and NF:
J= APIR,
(1)
(b) Area for NF or UF:
A, or A, = Q,/J
(2)
(c) Material balance equations: For specified recoveries and rejections, the following material balance equations are valid for both the UF and NF stages: l
Q,=Q,+Q, l
Component
Component
(E> in kWh (6)
3.1.4. Performance characteristics based on statistical analysis of previous work The dynamic membrane resistance (RJ) for the UF membrane is calculated from the data of Nakao et al. [2] referred to in Section 2. Fig. 2 depicts the effect of protein concentrations and operating pressures on the dynamic resistance. Using nonlinear regression analysis, the values of RJ for UF polymeric membranes as a function of the operating pressure (AP) and the protein concentration (C,rf> was obtained as presented in Eq. (7).
(7)
(3) material balance for chrome
Qr*Ccti= Qp~Ccp+Q;c,, l
E = (e,- AP/36/r$f
““=*;
Over all material balance
NFf
(4)
material balance for protein
Qf-Cpti= Q;~,,+Q$,,
(5)
where a, = -2.5x10h5, b, = 3.28x10e5, b, = 0.11796 and b, = -0.2778. Inorganic membranes have lower dynamic resistance than the corresponding polymeric ones. A multiplication factor of about 0.57 at a protein concentration of 2 kg/m3 has been deduced from Nakao et al. [2]. For NF membrane, the dynamic membrane resistance (RJ) has been assumed to be 2.4~ IO6
H.F. Shaalan et al. /Desalination
319
141 (2001) 315-324
P (bar)
O.OE+OO
3.OEt05
2.OE+05
4.OEi05
5.OE to5
6.OEt05
7.OE+05
fLOEtO
Dynamic resistance (bar.sec/m)
Fig. 2. Effect of pressure and concentration
Table 2 Performance system
on flux and dynamic resistance for protein (ovalbumin)
[3].
3.2. Economic considerations characteristics
of the proposed
UF/NF
System performance
UF system
NF system
Area, m2 Length, m
0.022 1.2
0.022 1.2
Recovery, % Rejection, % Protein Chrome
90
70
67 25
Dynamic resistance, (Rd), bar.s.m-’
Eq. (7)
95 94 2.4~ lo6
bar.s.m-’ guided by data available on rejection of magnesium [ 121 and adjusted to account for protein deposition on the NF inorganic membrane surface. The characteristics of the UF/NF system as adopted in this work are shown in Table 2.
3.2. I. Cost estimates For cost estimation and optimization purposes, a cost function has been formulated on the basis of the following assumptions. 1. Capital cost (CC): The capital costs for the proposed integrated scheme essentially comprise the following items: l Membrane costs: As previously mentioned, it is proposed that inorganic membranes should be adopted. The UF and NF membrane costs are given by:
C,,=xA,
(8)
Cm=YA,
(9)
l
Pumping and related pressure items The pumping and related items for UF are assumed to vary between 40-60% membrane area and are directly related
costs: or NF of the to the
320
H.F. Shaalan et al. /Desalination
adopted pressures. Guided by published data from Perry [ 131 on the cost of these items, the following formulas have been proposed: C,, = 0.5 * CAu(AP”J4)o.7
(10)
C,,,, = 0.5 * C,,(AP,vJ12)o.7
(11)
.
Other capital cost items: These include pretreatment, site preparation, civil works, chemical treatment, post-treatment, instrumentation and control, and are assumed to be direct functions of membrane costs. Analysis of diverse cost data suggests the use of 1.25 and 1 multipliers as indicated in Eq. (12) within the proposed operating range.
141 (2001) 315-324
(f) Depreciation estimated as 0.067% of pumping and other capital costs assuming an operating lifetime of 15 years for the integrated system. (g) Other annual expenses estimated to be about 5% of the total operating costs expressed by the items (a) to (d). 3. Revenues: Chrome is recovered in the retentate of the NF system. Revenues are estimated according to the current prices of chromium salt, which is about $3.4/kg Cr,O,.
3.2.2. Optimization
Thus the total capital is the summation of membrane costs, pumping costs and the other capital costs for the UF and NF membrane system.
Simulation and economic trends have been studied for specified recoveries and UF/NF pressure combinations. Optimum design parameters and operating conditions that would maximize profits could only be obtained by applying an optimization routine. In this work a Box COMPLEX method [ 151 for constrained optimization was adopted. The formulated objective function represents the annual profit as represented by Eq. (13):
CC=C,,
P, = CR,, - CA,
Other capital costs = 1.25 * C,, + 1.OO* C,, (12)
+ C,, + C,, + C,, + other capital costs
2. Annual costs (CA,): The annual costs essentially comprise the following: (a) Membrane replacement costs assuming that the lifetime for both UF and NF membranes is 8 years and that the prices of UF and NF membranes are $300 and $600, respectively. (b) Maintenance costs expressed as 5% of the pumping and other capital costs [ 141. (c) Labor costs at an average rate of $1200/y/ person and about 12 persons are employed to operate the system for three shifts. (d) Waste treatment costs as estimated to be about $ l/m3 of permeate from NF system. (e) Annual energy cost (CA,) as calculated on the basis of a price of $O.l/kWh.
(13)
The independent parameters include (1) transmembrane pressures for the UF and NF stages, and (2) recoveries of chrome from the UF and NF stages. The CR,, and CA, have been expressed in terms of the independent parameters. Constraints for the independent parameters have been set as follows: 2 I AP,
I 8 bar
8 I AP,,<
16bar
85 I Ret,
< 95%
60 I Ret, I 80%
H.F. Shaalan et al. /Desalination 141 (2001) 315-324 4. Results Based on the developed models and assumptions, simulation and optimization studies have been conducted for the UF/NF system.
4.1. Simulation For a pressure range of 2-8 bars for the UF system and corresponding pressures of 8 to 20 bar for the NF system, main parameters and variables have been estimated. These include flux and hence required membrane areas for both phases, energy requirements and material balance for the chrome and protein phases, respectively. Simulation was undertaken assuming an average daily discharge of chrome bath effluents of 30 m3, 20 operational hours per day and 330 d/y. Results depicted in Fig. 3 indicate that the flux ranges between 55-88 l/m*/h and 1230 l/m*/h for UF and NF phases, respectively. Correspondingly, the membrane areas range between about 24-l 5 and 78-3 1 m* for UF and NF, respectively, for the estimated recoveries and rejections. The chrome concentration as shown in Fig. 4 is about 7.2 kg/m3 in the recovered stream.
321
with a minimum at pressures of 6.5 and 16 bars for the UF and NF phases, respectively. This is equivalent to about $3.8/m3. It is worth mentioning that costs of waste disposal of permeate from NF were included. This item could be excluded if this permeate is reused for pickling. For a current cost of chrome of $3.4 per kg Cr203, the annual revenues for the assumed recoveries and rejections are estimated to be about $65,000 annually .Thus the net profit is about $2.8/m3. To study the effect of increasing the treatment capacity on costs, unit treatment costs have been estimated for the range between 30 and 300 m3/d as presented in Fig. 7. It is apparent that, from the economic point of view, it is preferable to establish centralized treatment units.
.i
80
. ..a..=.
T
60 70
:’
60
a”
z 50 "E 3 40 x 1 30 y
4.2. Economic trends Based on the aforementioned simulation and cost estimates, economic trends including capital costs and annual costs at varying pressures are presented in Figs. 5 and 6. It is noted that the annual costs range between $40,000 and $37,200
0
5
Fig. 3. Pressure vs. area
Qr CUT CPV
9750 14060
Fig. 4. Material balance for the UF/NF system.
ppm ppm
10
15
20
Pressure (bar)
ccrr Cprr
and flux
for a UF/NF system.
CPrp
33 wm
0.405 m3/h 7105 2123
ppm ppm
322
H.F. Shaalan et al. /Desalination
3 v) %
80000
8
60000
141 (2001) 315-324
F 30000 ;
25000
3 20000
g G
40000
UF&NF Pressures (Bar)
i!$:;!$;;g m
d
u)
(0
b
eL,
Fig. 6. Annual costs vs. pressure for the UF/NF system.
UF and NF Pressures (bar) Fig. 5. Capital costs vs. pressure for UF and NF systems.
6
Q=30
-
-A-
Q=SO
-GE- Q=240
--e
Q=lZO
+
However, there are some constraints governing the centralized units involving organizational aspects relevant to collection of chrome bath effluents from several tanneries and redistribution of recovered chrome.
Q=180 Q=300
4
4.3. Optimization
E S iii 8
3
21
z
wocu*ww
SSG;;8Zri m~*col-~ UF&NF
Pressures
(bar)
Fig. 7. Cost vs. pressures and capacity for the UF/NF system.
The Box COMPLEX routine was run using BASIC language on a 486 PC to identify the optimum parameters that would maximize the profit objective function formulated by Eq. (13) and inequality constraints. It has been found that the optimum pressure for NF was at 16 bar, which is the upper boundary condition. A second run was conducted when the upper limit for the pressure of NF stage was raised to 20 bar. The results for the two cases are compiled in Table 3. It is indicated that optimum conditions could be obtained at pressures of 2 and 19.4 bar and recoveries of 95% and 60% for UF and NF, respectively. Under these conditions, the total operating costs could be as low as 3.23$/m3 and
H.F. Shaalan et al. /Desalination
141 (2001) 315-324
323
Table 3 Optimization results Case 1: Upper limit for NF Pressure = 16 bar
Case 2: Upper limit for NF Pressure = 20 bar
UF
NF
UF
NF
Optimum values of parameters: Pressure, bar Recovery, %
2.026 95
16 60
2.01 95
19.44 60
Technical conditions: Area, m2 Flux, llm’/h Q,, m2m
21.2 8.0 1.42
42.8 2.3 0.85
21.0 8.1 1.42
35.2 2.9 0.85
6,340 9,876 5,211 30,009 69,358 34,138
25,650 41,337 5,211 30,009 69,358 34,138
6,312 9,848 6,204 28,803 69,358 34,350 3.23 3.46
21,107 35,902 6,204 28,803 69,358 34,350 3.23 3.46
Cost indicators: Membrane cost, $ Other capital costs, $ Energy costs, $1~ Other annual costs, $ Annual revenues, $ Net profit, $/y Costs/m3, $ _ Net profit, $/m’
the net profit could reach 3.46 $/m3 (about 24% higher than the corresponding value obtained in Section 4.2).
5. Conclusions Based on the literature, it has been possible to define performance characteristics of a proposed UF/NF scheme for chromium recovery from chrome bath tanning effluents using inorganic membranes. By statistical analysis of published data, generalized correlations for dynamic membrane resistance for UF andNF phases could be obtained. A simulation model has been developed, and the relationships between design and operating parameters such as flux, membrane area, energy requirements and pressure have been obtained. Capital and operating cost functions and revenues have been analyzed in terms of varying design parameters. Optimization has been conducted to define optimum conditions,
-
which maximize profits and minimize costs. Optimum conditions have been found to be about 2 and 19.4 bar pressures for UF and NF, respectively, at corresponding recoveries of 95% and 60%. At these conditions, concentration of recovered chrome from the NF stage is about 5.4 kg/m3. It can thus be concluded that the proposed UF/NF system could be cost effective for the recovery of chrome from tanning effluents at the defined conditions.
6. Symbols A, A,
-
NF and UF membrane areas, m*
00, b,, b,, b,
-
CA, CA*
-
Constants Annual energy cost, $/y Other annual cost comprising labor, waste treatment and other annual expenses, $
H.F. Shaalan et al. /Desalination 141 (2001) 315-324
324 CA,, -
CA,
-
CA,
CANY C CT! C PN? C PU
-
CRC,
C TN, CTU
-
Annual costs for UF and NF systems comprising membrane replacement, and maintenance depreciation, $ Total annual cost, $/y
A&
-
PO
-
Q, Qp Qr -
&
-
-
Acknowledgment
Total capital cost for NF and UF system, $
References
Protein concentrations in feed, permeate and reject respectively, kg/m3 Energy consumption, kWh Factor to account for additional energy consumed in membrane back flushing = 1.1 Flux, l/m*/h Solvent flux, m/s Operating pressure and operating pressure for both NF and UF membranes, respectively, bar Annual profit, $/y Feed, permeate and reject flow, m3/h Dynamic resistance including membrane and cake resistance, bar.s.m-’
R%, Ret,
rl
Prices of UF and NF membranes, respectively, $/m* Pump efficiency
Cr203
Capital pumping costs for NF and UF system, respectively, $ Cost of recovered chromium, $/kg
APNF
AP"F
-
This work was undertaken during the scientific cooperation agreement between the Academy of Scientific Research and Technology -National Research Center, Egypt, and CNRResearch Institute on Membranes and Modeling of Chemical Reactors, Italy. The fruitful comments of Prof. Dr. Enrico Drioli and his staff are gratefully acknowledged.
NF and UF membrane costs, $ Total capital cost, $
Chrome concentrations in feed, permeate and reject, respectively, kg/m3
Flux J
XTY
Recoveries respectively
of both NF and UF,
111J. Hayes, J. Dunkerley, L. Mullerand A.Grifftn, Dairy Technology, 29 (1974) 132.
121 S. Nakao, T. Nomura and S. Kimura, World Congress III of Chemical Engineering, Tokyo, 1986, pp. 262265. [31 M. Cheryan, Ultratilatration Handbook, Technomic, Lancaster, PA, 1986, pp 73-l 26. [41 D. Solpan and M. Sahan, J. Appl. Polym. Sci., 55 (1995) 383. PI J. Waypa, M. Elimelech and J. Hering, J. AWWA, 89 (1997) 102. F51 L. Kune, C. Soon and W. Sang, J. Env. Sci., 30 (1995) 467. 171 Z. Sodaouic, C. Azong, G. Charbit and F. Charbit, J. Env. Eng., 124 (1998) 695. PI C. Fabiani, F. Ruscio, M. Spadoni and M. Pizzichini, Desalination, 108 (1996) 183. [91 A. Cassano, E. Drioli, R. Molinari and C. Bertolutti, Desalination, 108 (1996) 193. UOI M. Aloy and B. Vulliermet, J. Sot. Leather Technologies Chemists, 82 (1998) 140. illI K. Pickering and M. Wiesener, J. Env. Eng., 119 (1993) 772. WI M. Porter, Handbook of Industrial Membrane Technology, Noyes, New Jersey, 1991, pp. 307-348. t131 R. Perry, D. Green and J. Maloney, Chemical Engineers Handbook, 7th ed., McGraw-Hill, New York, 1997, pp. 48-57. [14] S. Chellam, C. Serra and M. Wiesner, J. AWWA, 90 (1998) 96. [ 151 M. Box, Computer J., 8 (1965) 42.
Desalination 141 (2001) 315-324 www.elsevier.com/locate/desal
Simulation and optimization of a membrane system for chromium recovery from tanning wastes H.F. Shaalan*, M.H. Sorour, S.R. Tewfik Chemical Engineering Department and Pilot Plant Laboratory Engineering Division, National Research Center, Dokki, Cairo, Egypt Tel. +20 (2) 337-0933; Fax +20 (2) 337-0931; email: hayam64@hotmaikom
Received 14 March 2001; accepted 12 April 200 1
Abstract Leather tanning processing involves treatment of skin in chrome baths where the percentage exhaustion of chromium salts is usually in the range of 60-70%. Excess chromium salts are discharged to the sewer system or an effluents treatment facility. Thus, extensive efforts have been exerted on both R&D and application levels to recover chromium for economic and environmental concerns. Several methods have been developed for chromium recovery or reuse including precipitation, adsorption, redox-adsorption, ion-exchange and membrane systems. Recent research indicated that chromium salts recovered by membranes manifested improved characteristics for tanned and retanned skins. This paper is dedicated to the simulation and optimization of dual-membrane systems involving inorganic ultrafiltration and nanofiltration for the recovery of chromium from effluents of exhausted baths. In the first section, parameters governing the separation of protein/fat mix by ultrafiltration are obtained by analysis of published results on separation of proteins by ultrafiltration. The parameters for the concentration of chromium by nanofiltration are also deduced. A simulation model based on the resistance model and involving material balance and energy requirements for the ultrafiltrationnanofiltration scheme has been developed. A cost-objective function involving annual costs and revenues has been formulated. Optimum design parameters have been defined using a Box COMPLEX constrained optimization routine. Results indicated that under the stated optimum conditions the system is cost effective. Keywords: Membrane; Tanning waste; Chromium recovery; Optimization
1. Introduction Membrane schemes have witnessed considerable development in the last decade permitting difficult separation to be achieved. Numerous *Corresponding author. 001 l-9 164/Ol/$- See front matter Q PII:SOOll-9164(01)00414-3
separation alternatives could be developed through combinations of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). The exact system design is governed by several factors including, but not limited to, physico-chemical characteristics ofthe
2001 Elsevier Science B.V. All rights reserved
316
H.F. Shaalan et al. /Desalination
feed stream, target separation efficiency, economics of processing and price of recoverables. UF systems have been widely adopted for the separation of proteins in the food industry. Also the application of complex membrane schemes for the recovery of heavy metals is in commercial practice, especially for the surface finishing industry. The cost savings based on the application of membrane schemes stimulated numerous endeavors for the recovery of chrome from leather tanning effluents as an alternative for the precipitation and chemical treatment recovery approach. Application of dual-membrane systems including MF/NF, UF/RO and NF/RO for treatment of tannery waste and recovery of chrome and/or other chemicals is challenged by the presence of considerable BOD load and proteins, which may cause considerable fouling and subsequent system failure temporarily or permanently. However, the rapid development of inorganic membranes enable numerous mitigating technical problems affecting the processing of tannery waste. The present paper presents critical assessment of previous endeavors, explores a viable separation UF/NF scheme based on inorganic membranes and finally presents the optimal conditions governing the successful application of the proposed membrane scheme in view of a profit/cost objective function.
2. Previous endeavors The separation of proteins and heavy metals using UF and NF membranes has been experimentally investigated by several workers. Results of previous work have been compiled and are discussed below. Several previous studies address protein separation using UF membranes, and it is not surprising that research has found proteins to be a major foulant in UF of food and biological
141 (2001) 315-324
systems since proteins are rejected by the membrane and tend to have high concentrations at the membrane surface. Fouling of UF membranes by cheese whey is a classic and particularly vexing problem in the industry [l]. Nakao et al. [2] studied the effect of operating pressure and protein concentrations on flux decline using a tubular polymeric membrane (PAN with cut-off 20,000-40,000) at 0.62 m/s velocity. Further, Cheryan [3] reported that a-lactalbumin had the greatest flux-depressing effect. They also studied the fouling of UF membranes by applying cottage cheese whey and skimmed milk and soy proteins. On the other hand, because of the high toxicity for human health, heavy metal concentrations in wastewater are limited by strict standards. Membrane separations are used more and more frequently. Several authors studied the separation of heavy metals using UF membranes, being the most economic membrane technique. Solpan and Sahan [4] studied the separation and concentration of metal ions (Cu’+, Ni2+, Fe3’) using EC-PEG4000 alloy UF membranes. Benchscale experiments were performed to assess the effectiveness of RO and NF membranes in removing arsenic (As) from synthetic fresh water and source water. Waypa et al. [5] examined the effects of operating conditions (applied pressure and feed water temperature) and solution chemical composition. Also, Kune et al. [6] studied the treatment of cyanide/heavy metals bearing wastewater by the micelle-enhanced UF technique. Experimental results based on enhanced UF was proposed by Sodaouic et al. [7] in order to separate cadmium and chromate ions from wastewater using zirconium oxide coated on a carbon membrane by adding ionic surfactants. The concentration of both cadmium and hexavalent chromium in the feed are 750, 200 kg/m3 and in the permeate 0.2 and 0.1 kg/m3, respectively. Fabiani et al. [8] used a coupled MF inorganic membrane as a pretreatment of the
H.F. Shaalan et al. /Desalination
waste solution followed by an UF polysulfone membrane to produce a clean chromium solution (permeate) with 28% recovery. Cassano et al. [9] used an integrated membrane scheme (UF followed by NF polymeric membranes) to recover and concentrate the chromium exhaust tanning baths. They found that the quality of the concentrated chromium was improved with respect to the conventional recovery process, based on precipitation and dissolution, as showed by the low ratio of organic lipolytic components: chromium. This process also permits the reuse of the permeate from NF in the pickling phase considering the high content of chlorides in the solution. The flux in the NF membrane is about 86% after 1 h at 16 bar and then remained approximately constant for the following 2 h. NF was tested for the recovery of trivalent chromium in the residual tanning floats [lo]. The economics of low-pressure high-rate membrane process are governed by several factors including the adopted membrane system, flowrate and characteristics ofthe feed, operating conditions, site characteristics and prevailing cost factors. However, cost data and models have been reported by several authors. As an example, Pickering presented a cost model for membrane filtration which may be used to evaluate the sensitivity of capital and operating costs to design and operating parameters in preliminary cost estimates for pressure-driven membrane filtration facilities [l 11.
3. Proposed UF/NF scheme for treatment tannery waste
of
3.1. Technical considerations Performance characteristics for protein and chromium separation by membranes have been predicted according to reported results and by statistical analysis of variables based on published experimental data as outlined below.
317
141 (2001) 315-324
3.1.1. Characteristics of chrome bath effluents Based on the analysis of typical samples from Egyptian tanneries, using standard methods, it has been found that the protein and chrome concentrations in a chrome bath effluent vary around 2 and 3 kg/m3, respectively. This is in agreement with published data which indicate that typical analysis of chrome bath effluents is as shown in Table 1. 3.1.2. Proposed hybrid system Several dual schemes have been initially proposed. These include MF/UF, UF/NF and NF/RO. Preliminary investigations indicated that the UF/NF system exhibits superior performance. This dual scheme is further investigated in this work. The system is depicted in Fig. 1. It is recommended that both the UF and NF membranes be inorganic. This selection is based on the following justifications. Lower adsorption to solutes (protein) in the membrane surface in the UF stage. Withstand high temperature application. Extreme tolerance for chemical attack. High resistance to biological attack. Longer service life. Fouling tendency is lower with tubular membranes.
Table 1 Typical analysis of chromium bath effluent [lo] Parameter
Concentration,
Chrome Sulfate Chloride N-Kijeldal N-NH, Oils and fats COD TSS
3.6 22-23 11-16 1.07-1.11 0.6 0.3 8.6 0.7-2.9 3.74.2
PH
kg/m’
318
I
H.F. Shaalan et al. /Desalination
141 (2001) 315-324
reuse for retanning baths
prefilter
Equalization
I
I
I
UF
Tank
preparation of new pickling baths or waste treatment
Settler Fig. 1. Process flow sheet of the proposed UF/NF system.
3.1.3. Formulated simulation model
(d) Energy consumption
The model adopted in this work for the simulation and optimization of the UF/NF scheme is represented by the following set of relationships for each of the UF and NF stages. (a) Relationship between flux and pressure for UF and NF:
J= APIR,
(1)
(b) Area for NF or UF:
A, or A, = Q,/J
(2)
(c) Material balance equations: For specified recoveries and rejections, the following material balance equations are valid for both the UF and NF stages: l
Q,=Q,+Q, l
Component
Component
(E> in kWh (6)
3.1.4. Performance characteristics based on statistical analysis of previous work The dynamic membrane resistance (RJ) for the UF membrane is calculated from the data of Nakao et al. [2] referred to in Section 2. Fig. 2 depicts the effect of protein concentrations and operating pressures on the dynamic resistance. Using nonlinear regression analysis, the values of RJ for UF polymeric membranes as a function of the operating pressure (AP) and the protein concentration (C,rf> was obtained as presented in Eq. (7).
(7)
(3) material balance for chrome
Qr*Ccti= Qp~Ccp+Q;c,, l
E = (e,- AP/36/r$f
““=*;
Over all material balance
NFf
(4)
material balance for protein
Qf-Cpti= Q;~,,+Q$,,
(5)
where a, = -2.5x10h5, b, = 3.28x10e5, b, = 0.11796 and b, = -0.2778. Inorganic membranes have lower dynamic resistance than the corresponding polymeric ones. A multiplication factor of about 0.57 at a protein concentration of 2 kg/m3 has been deduced from Nakao et al. [2]. For NF membrane, the dynamic membrane resistance (RJ) has been assumed to be 2.4~ IO6
H.F. Shaalan et al. /Desalination
319
141 (2001) 315-324
P (bar)
O.OE+OO
3.OEt05
2.OE+05
4.OEi05
5.OE to5
6.OEt05
7.OE+05
fLOEtO
Dynamic resistance (bar.sec/m)
Fig. 2. Effect of pressure and concentration
Table 2 Performance system
on flux and dynamic resistance for protein (ovalbumin)
[3].
3.2. Economic considerations characteristics
of the proposed
UF/NF
System performance
UF system
NF system
Area, m2 Length, m
0.022 1.2
0.022 1.2
Recovery, % Rejection, % Protein Chrome
90
70
67 25
Dynamic resistance, (Rd), bar.s.m-’
Eq. (7)
95 94 2.4~ lo6
bar.s.m-’ guided by data available on rejection of magnesium [ 121 and adjusted to account for protein deposition on the NF inorganic membrane surface. The characteristics of the UF/NF system as adopted in this work are shown in Table 2.
3.2. I. Cost estimates For cost estimation and optimization purposes, a cost function has been formulated on the basis of the following assumptions. 1. Capital cost (CC): The capital costs for the proposed integrated scheme essentially comprise the following items: l Membrane costs: As previously mentioned, it is proposed that inorganic membranes should be adopted. The UF and NF membrane costs are given by:
C,,=xA,
(8)
Cm=YA,
(9)
l
Pumping and related pressure items The pumping and related items for UF are assumed to vary between 40-60% membrane area and are directly related
costs: or NF of the to the
320
H.F. Shaalan et al. /Desalination
adopted pressures. Guided by published data from Perry [ 131 on the cost of these items, the following formulas have been proposed: C,, = 0.5 * CAu(AP”J4)o.7
(10)
C,,,, = 0.5 * C,,(AP,vJ12)o.7
(11)
.
Other capital cost items: These include pretreatment, site preparation, civil works, chemical treatment, post-treatment, instrumentation and control, and are assumed to be direct functions of membrane costs. Analysis of diverse cost data suggests the use of 1.25 and 1 multipliers as indicated in Eq. (12) within the proposed operating range.
141 (2001) 315-324
(f) Depreciation estimated as 0.067% of pumping and other capital costs assuming an operating lifetime of 15 years for the integrated system. (g) Other annual expenses estimated to be about 5% of the total operating costs expressed by the items (a) to (d). 3. Revenues: Chrome is recovered in the retentate of the NF system. Revenues are estimated according to the current prices of chromium salt, which is about $3.4/kg Cr,O,.
3.2.2. Optimization
Thus the total capital is the summation of membrane costs, pumping costs and the other capital costs for the UF and NF membrane system.
Simulation and economic trends have been studied for specified recoveries and UF/NF pressure combinations. Optimum design parameters and operating conditions that would maximize profits could only be obtained by applying an optimization routine. In this work a Box COMPLEX method [ 151 for constrained optimization was adopted. The formulated objective function represents the annual profit as represented by Eq. (13):
CC=C,,
P, = CR,, - CA,
Other capital costs = 1.25 * C,, + 1.OO* C,, (12)
+ C,, + C,, + C,, + other capital costs
2. Annual costs (CA,): The annual costs essentially comprise the following: (a) Membrane replacement costs assuming that the lifetime for both UF and NF membranes is 8 years and that the prices of UF and NF membranes are $300 and $600, respectively. (b) Maintenance costs expressed as 5% of the pumping and other capital costs [ 141. (c) Labor costs at an average rate of $1200/y/ person and about 12 persons are employed to operate the system for three shifts. (d) Waste treatment costs as estimated to be about $ l/m3 of permeate from NF system. (e) Annual energy cost (CA,) as calculated on the basis of a price of $O.l/kWh.
(13)
The independent parameters include (1) transmembrane pressures for the UF and NF stages, and (2) recoveries of chrome from the UF and NF stages. The CR,, and CA, have been expressed in terms of the independent parameters. Constraints for the independent parameters have been set as follows: 2 I AP,
I 8 bar
8 I AP,,<
16bar
85 I Ret,
< 95%
60 I Ret, I 80%
H.F. Shaalan et al. /Desalination 141 (2001) 315-324 4. Results Based on the developed models and assumptions, simulation and optimization studies have been conducted for the UF/NF system.
4.1. Simulation For a pressure range of 2-8 bars for the UF system and corresponding pressures of 8 to 20 bar for the NF system, main parameters and variables have been estimated. These include flux and hence required membrane areas for both phases, energy requirements and material balance for the chrome and protein phases, respectively. Simulation was undertaken assuming an average daily discharge of chrome bath effluents of 30 m3, 20 operational hours per day and 330 d/y. Results depicted in Fig. 3 indicate that the flux ranges between 55-88 l/m*/h and 1230 l/m*/h for UF and NF phases, respectively. Correspondingly, the membrane areas range between about 24-l 5 and 78-3 1 m* for UF and NF, respectively, for the estimated recoveries and rejections. The chrome concentration as shown in Fig. 4 is about 7.2 kg/m3 in the recovered stream.
321
with a minimum at pressures of 6.5 and 16 bars for the UF and NF phases, respectively. This is equivalent to about $3.8/m3. It is worth mentioning that costs of waste disposal of permeate from NF were included. This item could be excluded if this permeate is reused for pickling. For a current cost of chrome of $3.4 per kg Cr203, the annual revenues for the assumed recoveries and rejections are estimated to be about $65,000 annually .Thus the net profit is about $2.8/m3. To study the effect of increasing the treatment capacity on costs, unit treatment costs have been estimated for the range between 30 and 300 m3/d as presented in Fig. 7. It is apparent that, from the economic point of view, it is preferable to establish centralized treatment units.
.i
80
. ..a..=.
T
60 70
:’
60
a”
z 50 "E 3 40 x 1 30 y
4.2. Economic trends Based on the aforementioned simulation and cost estimates, economic trends including capital costs and annual costs at varying pressures are presented in Figs. 5 and 6. It is noted that the annual costs range between $40,000 and $37,200
0
5
Fig. 3. Pressure vs. area
Qr CUT CPV
9750 14060
Fig. 4. Material balance for the UF/NF system.
ppm ppm
10
15
20
Pressure (bar)
ccrr Cprr
and flux
for a UF/NF system.
CPrp
33 wm
0.405 m3/h 7105 2123
ppm ppm
322
H.F. Shaalan et al. /Desalination
3 v) %
80000
8
60000
141 (2001) 315-324
F 30000 ;
25000
3 20000
g G
40000
UF&NF Pressures (Bar)
i!$:;!$;;g m
d
u)
(0
b
eL,
Fig. 6. Annual costs vs. pressure for the UF/NF system.
UF and NF Pressures (bar) Fig. 5. Capital costs vs. pressure for UF and NF systems.
6
Q=30
-
-A-
Q=SO
-GE- Q=240
--e
Q=lZO
+
However, there are some constraints governing the centralized units involving organizational aspects relevant to collection of chrome bath effluents from several tanneries and redistribution of recovered chrome.
Q=180 Q=300
4
4.3. Optimization
E S iii 8
3
21
z
wocu*ww
SSG;;8Zri m~*col-~ UF&NF
Pressures
(bar)
Fig. 7. Cost vs. pressures and capacity for the UF/NF system.
The Box COMPLEX routine was run using BASIC language on a 486 PC to identify the optimum parameters that would maximize the profit objective function formulated by Eq. (13) and inequality constraints. It has been found that the optimum pressure for NF was at 16 bar, which is the upper boundary condition. A second run was conducted when the upper limit for the pressure of NF stage was raised to 20 bar. The results for the two cases are compiled in Table 3. It is indicated that optimum conditions could be obtained at pressures of 2 and 19.4 bar and recoveries of 95% and 60% for UF and NF, respectively. Under these conditions, the total operating costs could be as low as 3.23$/m3 and
H.F. Shaalan et al. /Desalination
141 (2001) 315-324
323
Table 3 Optimization results Case 1: Upper limit for NF Pressure = 16 bar
Case 2: Upper limit for NF Pressure = 20 bar
UF
NF
UF
NF
Optimum values of parameters: Pressure, bar Recovery, %
2.026 95
16 60
2.01 95
19.44 60
Technical conditions: Area, m2 Flux, llm’/h Q,, m2m
21.2 8.0 1.42
42.8 2.3 0.85
21.0 8.1 1.42
35.2 2.9 0.85
6,340 9,876 5,211 30,009 69,358 34,138
25,650 41,337 5,211 30,009 69,358 34,138
6,312 9,848 6,204 28,803 69,358 34,350 3.23 3.46
21,107 35,902 6,204 28,803 69,358 34,350 3.23 3.46
Cost indicators: Membrane cost, $ Other capital costs, $ Energy costs, $1~ Other annual costs, $ Annual revenues, $ Net profit, $/y Costs/m3, $ _ Net profit, $/m’
the net profit could reach 3.46 $/m3 (about 24% higher than the corresponding value obtained in Section 4.2).
5. Conclusions Based on the literature, it has been possible to define performance characteristics of a proposed UF/NF scheme for chromium recovery from chrome bath tanning effluents using inorganic membranes. By statistical analysis of published data, generalized correlations for dynamic membrane resistance for UF andNF phases could be obtained. A simulation model has been developed, and the relationships between design and operating parameters such as flux, membrane area, energy requirements and pressure have been obtained. Capital and operating cost functions and revenues have been analyzed in terms of varying design parameters. Optimization has been conducted to define optimum conditions,
-
which maximize profits and minimize costs. Optimum conditions have been found to be about 2 and 19.4 bar pressures for UF and NF, respectively, at corresponding recoveries of 95% and 60%. At these conditions, concentration of recovered chrome from the NF stage is about 5.4 kg/m3. It can thus be concluded that the proposed UF/NF system could be cost effective for the recovery of chrome from tanning effluents at the defined conditions.
6. Symbols A, A,
-
NF and UF membrane areas, m*
00, b,, b,, b,
-
CA, CA*
-
Constants Annual energy cost, $/y Other annual cost comprising labor, waste treatment and other annual expenses, $
H.F. Shaalan et al. /Desalination 141 (2001) 315-324
324 CA,, -
CA,
-
CA,
CANY C CT! C PN? C PU
-
CRC,
C TN, CTU
-
Annual costs for UF and NF systems comprising membrane replacement, and maintenance depreciation, $ Total annual cost, $/y
A&
-
PO
-
Q, Qp Qr -
&
-
-
Acknowledgment
Total capital cost for NF and UF system, $
References
Protein concentrations in feed, permeate and reject respectively, kg/m3 Energy consumption, kWh Factor to account for additional energy consumed in membrane back flushing = 1.1 Flux, l/m*/h Solvent flux, m/s Operating pressure and operating pressure for both NF and UF membranes, respectively, bar Annual profit, $/y Feed, permeate and reject flow, m3/h Dynamic resistance including membrane and cake resistance, bar.s.m-’
R%, Ret,
rl
Prices of UF and NF membranes, respectively, $/m* Pump efficiency
Cr203
Capital pumping costs for NF and UF system, respectively, $ Cost of recovered chromium, $/kg
APNF
AP"F
-
This work was undertaken during the scientific cooperation agreement between the Academy of Scientific Research and Technology -National Research Center, Egypt, and CNRResearch Institute on Membranes and Modeling of Chemical Reactors, Italy. The fruitful comments of Prof. Dr. Enrico Drioli and his staff are gratefully acknowledged.
NF and UF membrane costs, $ Total capital cost, $
Chrome concentrations in feed, permeate and reject, respectively, kg/m3
Flux J
XTY
Recoveries respectively
of both NF and UF,
111J. Hayes, J. Dunkerley, L. Mullerand A.Grifftn, Dairy Technology, 29 (1974) 132.
121 S. Nakao, T. Nomura and S. Kimura, World Congress III of Chemical Engineering, Tokyo, 1986, pp. 262265. [31 M. Cheryan, Ultratilatration Handbook, Technomic, Lancaster, PA, 1986, pp 73-l 26. [41 D. Solpan and M. Sahan, J. Appl. Polym. Sci., 55 (1995) 383. PI J. Waypa, M. Elimelech and J. Hering, J. AWWA, 89 (1997) 102. F51 L. Kune, C. Soon and W. Sang, J. Env. Sci., 30 (1995) 467. 171 Z. Sodaouic, C. Azong, G. Charbit and F. Charbit, J. Env. Eng., 124 (1998) 695. PI C. Fabiani, F. Ruscio, M. Spadoni and M. Pizzichini, Desalination, 108 (1996) 183. [91 A. Cassano, E. Drioli, R. Molinari and C. Bertolutti, Desalination, 108 (1996) 193. UOI M. Aloy and B. Vulliermet, J. Sot. Leather Technologies Chemists, 82 (1998) 140. illI K. Pickering and M. Wiesener, J. Env. Eng., 119 (1993) 772. WI M. Porter, Handbook of Industrial Membrane Technology, Noyes, New Jersey, 1991, pp. 307-348. t131 R. Perry, D. Green and J. Maloney, Chemical Engineers Handbook, 7th ed., McGraw-Hill, New York, 1997, pp. 48-57. [14] S. Chellam, C. Serra and M. Wiesner, J. AWWA, 90 (1998) 96. [ 151 M. Box, Computer J., 8 (1965) 42.