588
Ind. Eng. Chem. Process Des. Dev.
many such processes have been described. The pyrolysis of biomass to give a maximum direct yield of liquid products had received much less attention, and very few studies have been reported, for example, Duncan et al. (1981), Kosstrin (1981), Roy et al. (1982),Scott and Piskorz (1984). Of these, only the work of Kosstrjn and Scott and Piskorz appears to have both used a fluid bed reactor and been directed to maximizing liquid yields. Roy et al. pyrolyzed batch-wise under vacuum, and Duncan et al. (hyflex process) used a transport reactor under pressure. Comparison of the results reported here with those of others, whether using fluid bed reactors or other types, shows that as high or higher organic liquid yields have been achieved in this work. Although there is an extensive literature on the mechanisms of biomass pyrolysis reactions, a fairly detailed knowledge of primary reaction mechanisms together with a knowledge of the heat and mass transfer behavior in a particular reactor is required to determine why liquid yields may be better in one process than another. Registry No. Ni, 7440-02-0; C02, 124-38-9; CO, 630-08-0; CHSCHO, 75-07-0; CH&H20H, 64-17-5; CH&02H, 64-19-7; CH,COCH,, 67-64-1; CH,OH, 67-56-1; HCHO, 50-00-0; CH4,
1985,24. 588-592
74-82-8; acrolein, 107-02-8; furan, 110-00-9; cellulose, 9004-34-6.
Literature Cited Be[gougnou, M. A.; Graham, R. G.; Mok, L. K.; Freei, 8. A.; Overend, R . P. Ultrapyrolysis: The Continuous Fast Pyrolysis of Biomass”, Bio Energy 84,GGteborg, Sweden, June 1984. Duncan, D. A.; Bodle, W. W.; BanerJee, D. P. ”Production of LiquM Fuels from Biomass by the yrflex Process. Energy from Biomass and Wastes, V”; Inst. Gas Tech., 1981;pp 917-938. Funazukuri, T. Ph.D. Thesis, Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., 1983. HabUgOl, M. R.; Howard, J. B.; Longweii, J. P.; Peters, W. A. Ind. Eng. Chem. ProcessDes. D e v . 1982, 21, 457-465. Kosstrin, H. M. “Direct Formation of Pyrolysis Oil from Biomass”; Proc. S p a cialists Workshop on Fast Pyrolysis of Biomass, Copper Mountain, C SERIlCP 622-1096,Solar Energy Research Institute, US. Dept. of Energy, Oct 1981;pp 105-121. Kumar, A.; Mann, R. S. J. Anal. Appl. pvrolysis 1982, 4 , 219-226. Martin, S.B. “Proceedings, Tenth International Symposium on Combustion”; 1985;p 877. Roy, C.; de Caumia, B.; Chwnet. E. “Liquids from Biomass by Vacuum Pyrolysis-Production Aspects”, Proc. Specialists Meeting on Biomass Liquefaction, Saskatoon, Sask. ENFOR Program, Canadian Forestry Service, Envkonment Canada, Feb 1982;pp 57-74. Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1982, 60, 866-674. Scott, D. S.;Piskwz, J. Can. J. Chem. Eng. 1984, 62, 404-412. Shafizadeh, F.; Stevenson, T. T. J. App. Polym. Sci. 1982, 27, 4577-4585. Tyler, R. J. Fuel 1970, 58, 880-686.
Receiued for review Accepted
August August
19, 1983 20, 1984
Effectiveness of Magnetic Water Treatment in Suppressing CaCO, Scale Deposition Davld Haseon” and Dan Bremson Department of Chemical Engineering, Technlon-Israel Institute of Technology, Halfa 32000, Israel
The effecttveness of a commercial magnetic device in suppressing CaC03 scale deposition was investigated in a system consisting of a cast iron pipe through which hard water flowed at ambient temperature. The main variable studied was the supersaturation level of the CaC03-formingions over a range represented by (Ca*+XCO~-)= 20 X lo3 to 65 X lo3 (ppm as CaC03)*. The effect of magnetic exposure on scale suppression was evaluated from measurements of the rate of deposit growth, the extent of the induction period, and the adhesive nature of the incrustation. The accurate rate data showed that magnetic exposure had no effect on deposit growth. Similarly, magnetic exposure exerted no effect on the adhesive nature of the deposits. The less accurate induction period data did not reveal a statistically significant difference either.
Introduction Scale suppression by magnetic treatment consists of passing a potentially scaling water stream through a magnetic field provided by an adequately sized device. Sizing is based primarily on the water flow rate. The intensity of magnetic fields of research and industrial devices that have been used for inducing a scale suppression tendency ranges from 100 to 10000 G and exposure times are of the order of a few seconds, at most. Promoters of magnetic devices claim that this simple operation provides a viable scale control method, even for waters having a marked scaling tendency. It is asserted that magnetic treatment can prevent or markedly reduce the amount of scale precipitated. Moreover, it is said that precipitate morphology is altered. Any deposit accumulating on a flow surface is said to precipitate in the form of an easily washable sludge rather in the form of a trou-
blesome tenacious incrustation. It is also often claimed that magnetic exposure can inhibit corrosion. The intense controversy regarding the effectiveness of magnetic water treatment devices has a long history (Cowan and Weintritt, 1976). Currently, there is a revival of the controversy and renewed interest, stemming apparently from favorable reports published in the Russian literature (Hibben, 1973; Troup and Richardson, 1978; O’Brien, 1979). The present work was initiated as a result of an aggressive promotion drive of magnetic treatment in Israel, by a representative of a US. company. This led Mekorot, the national water supply authority, to sponsor tests of a magnetic device under well-controlled laboratory conditions. The results described in this paper provide unambiguous quantitative data on the effect of magnetic exposure on the rate of CaC03 scale deposition and on the
Ol96-43O5/85/1 124-O588$Ol.50/0 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 589 N~oH
adhesive nature of the deposits, at various supersaturation levels.
Background Laboratory studies reported in the literature have shown that magnetic exposure can sometimes exert an influence on the kinetics of precipitation of sparingly soluble salts and the flocculation properties of colloidal dispersions. The conditions under which such effects occur, are, however, ill defined and difficult to reproduce even under laboratory conditions. In carefully conducted experiments, Duffy (1977) attempted to reproduce several effects reported by previous researchers. He was unable to reproduce observations relating to changes in the spectral curves of water, nor was he able to detect any change in the precipitation rate of CaC03 in magnetically treated water. His experiments did confirm, however, that slight pH changes occurred when colloidal solutions of Fe(OH), were exposed to a magnetic field. Recently, Ellingsen and Kristiansen (1979) conducted similar experiments, in order to verify the existence of an effect of magnetic exposure on bulk precipitation of CaC03. They were able to observe an increased rate of bulk precipitation of CaC03due to magnetic exposure. The effect was systematically larger, the higher the field strength. There is still no substantiated mechanism for guiding experiments and enabling quantification of phenomena of practical interest. The speculative mechanisms suggested in the literature have been summarized by several critical authors (Troup and Richardson, 1978; Duffy, 1977; 0’Brien, 1979; Gruber and Carda, 1981). The proposed mechanisms fall into three categories: (1) structural changes in the ordering of water molecules which change the physical properties of water and thereby alter supersaturation conditions, (2) action of Lorentz forces on the motion of ionic species and colloidal particles which influence nucleation and flocculation phenomena, and (3) effects related to the presence of iron impurities. Duffy (1977) found that magnetic exposure can accelerate the corrosion rate of steel. He showed that iron impurities retard the rate of CaC0, precipitation and hinder the crystallization of calcite. From these results, he hypothesized that CaC03scale suppression by magnetic treatment stems from the Fe(OH), generated by magnetically induced corrosion. Bulk precipitation is only one of the factors involved in scale deposition. From recent progress in understanding of scaling phenomena (Epstein, 1978; Hasson, 1981), the following parameters are considered to be of significance in determing the kinetics of scale deposition and the tenacity of the deposit. (a) Supersaturation Level of Potentially Scaling Ions Present in the Water. The primary cause of scale formation lies in the existence of supersaturation conditions. The supersaturation level determines the extent of the scaling potential. Crystallization of a relatively pure salt creates the strongest scale structure. Mixed salt crystallization and particulate deposition form scales of weaker structures. (b) Flow Velocity. Velocity exercises opposing effects. It promotes deposition by enhancing mass transfer toward the scaling surface, but the increased interfacial shear acts to reduce the probability of adhesion of material reaching the scalewater interface. The first effect predominates in pure salt deposition, while the second predominates in mixed salt deposition. A strongly bonded layer tends to grow linearly with time while a weakly bonded layer tends to reach an asymptotic thickness. The asymptotic thick-
ONCE -THROUGH
FLOW SCHEME
r--ll 60 L I I HIN
60 L I l M l N TAP WATER
FEED
-
VESSEL
CONDITIONER
60 LI I MIN (ZMISEc
,
DIA
IRON W E , 13M LONG
RECYCLE FLOW SCHEME &OH
Figure 1. Experimental system.
ness is lower, the higher the flow velocity. (c) Temperature Level and Heat Transfer Mode. Under typical industrial conditions, the surface integration step is rate controlling and scale formation rate increases exponentially with surface temperature. Phase change heat transfer accelerates scaling, due to the well-known concentration effect associated with bubble formation on heat transfer surfaces. The extent of any effect that might be generated by magnetic exposure should depend on the one hand on parameters related to the scaling phenomenon, as outlined above, and on the other, on parameters related to the magnetic field (intensity, geometry, homogeneity, etc.). Commercial devices differ in their field configuration (Gruber and Carda, 1981), but the significance of design differences is not clear. Criticism of magnetic treatment has often centered on disbelief in the existence of a magnetic effect. The real issues are that, despite the long existence of magnetic devices, their performance remains unpredictable, that no data are available for characterizing the limited range of conditions a t which a scale suppression effect is exerted, and that extensive skilled efforts are required to measure and interpret scaling data, in order to assess the economic advantage of this scale control technique in comparison with other well-proven alternatives. As pointed out by O’Brien (1979), any process affecting deposit formation must either alter solubility limits or influence nucleation and crystallization kinetics. The relative extent of either of these effects in suppressing deposition rates can be of significance only at relatively low scaling tendencies. It thus seems reasonable to assume that the ill-defiied limits, under which magnetic treatment is capable of exerting a detectable scale-suppression tendency, fall in the domain of relatively low supersaturation levels (i.e., the less severe scaling problems). Since the less severe scaling problems are easier to control, the contention that magnetic treatment offers an inherent economic advantage has yet to be justified.
Experimental Section The aim of the experimental program was to provide quantitative data on the effect of magnetic exposure on the rate of CaCO, scale deposition and the adhesive nature of the deposits formed, at different scaling potentials. The tests were conducted at relatively high supersaturation levels, but within the range of conditions specified by the supplier of the magnetic device used in this work. According to the literature supplied with the magnetic device, the unit was capable of treating water with a
590
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
hardness of up to 2250 ppm, expressed as CaCO,. The unit was described as most efficient with water at about 80 "C but losing only 3% of ita efficiency at 0 "C. The tests were conducted with water at ambient temperature (19 to 23 "C) under isothermal flow conditions. The experimental system (Figure 1)consisted of a 1-m3 feed vessel, a feed pump, and a 13 m long horizontal cast iron pipe, 1 in. in diameter. All tests except one were carried out under once-through flow conditions. The flow was held constant at 2 m/s, corresponding to a flow rate of about 15 US. gpm. A magnetic device, rated at 25 US. gpm, was supplied at no cost from the manufacturer. The device was installed in the middle of the test pipe, under the technical supervison of the supplier, who also verified that operating instructions were being properly observed. The device used was of type 111, according to the classification of Gruber and Carda (1981). It is characterized by an alternating field polarity, created by permanent magnets. The experimental pipe was provided with ten equally spaced removable test segments, situated upstream and downstream of the magnetic device. Each test segment was a rectangular curved section, cut from a 1-in.diameter pipe, having a length of 15 cm and a curved width of 2 cm. The test segments were fitted into portholes of similar dimensions on the upper surface of the experimental pipe such that they formed an integral part of ita flow surface. The process of scale formation was monitored by periodic weighing of the deposit formed on test segments. Two tests segments, one upstream and the other downstream of the magnetic device were removed a t periodic time intervals and replaced by fresh segments. These measurements provided growth curves, characterizing the rate of scale formation without and with magnetic exposure. AU testa except one were carried out under once-through flow conditions, which allowed accurate and unambiguous examination of the effect of magnetic exposure on scale formation. Since the deposits were exposed to the same water throughout the experiment, the measurements were free from any uncertainty due to unavoidable experimental fluctuations in water quality. The recycle test had the object of increasing the magnetic exposure time. The flow through the test pipe was kept a t the same rate but 2/3 of the water was recycled. Under these conditions, the test segments upstream of the magnetic device were in contact with water which had received three times the magnetic exposure provided by once-through flow. The downstream segments were in contact with water of four magnetic exposures. The results of the recycle test were compared with a separate control run carried out under the same recycle conditions, but without exposing the water to the magnetic device. The adhesive nature of the deposits was characterized according to the cross cut test procedure (BS 3900,1974). This procedure involves scratching a grid on the deposit and assigning a figure of merit to the degree of flaking occuring on the deposit. Best adhesion results are ranked as zero and worst adhesion results are given the number 5. Hard tap water, ,at room temperature, was used in the tests. Ita composition was: Ca2+ = 200-250 ppm (as CaCOd; Mg2+ = 165-175 ppm (as CaCO,); total alkalinity = 300-340 ppm (as CaC03), pH = 7.2-7.4; T.D.S. = 800 to 900 ppm. The precipitation potential of CaC03 in the various tests was controlled by dosage of NaOH, using a pH control system. The pH levels in the various tests covered the range of 9 to 10, providing carbonate con-
m
3
10 p M I h r
MAGNETIC TREATMENT 10~M/hr
-
I.)
,'..
0 4-
1
/*
oo~ioo
A0
$00
ebo lboo
1;Oo
11oo
TIME ,MINUTES
Figure 2. Deposit growth with and without magnetic treatment at pH = 10.2.
,
2-
ONCE THROUGH cot'= 140-200 ppm COf = 160-260ppm 1 0 -Na,S03 = 2 0 0 p p m
/. RUN 282 pH.96
,,/< 0
/'
/,/
/ o ,
NO TREATMENT 66pMlhr (0)
.c
e,'
MAGNETIC
1 -
centrations from 90 to 400 ppm (as CaCO,). The pH increase caused some bulk precipitation of CaC03, giving particle concentrations ranging from 30 to 100 ppm in the various runs. Sodium sulfite dosage (150-300 ppm) was used in all tests except one in order to achieve defined conditions with respect to the iron impurities that are cbnsidered by some (Duffy, 1977) as highly relevant to the action of magnetic treatment and in order to prevent rust contamination, which has an effect on scale tenacity. From previous work, it w& anticipated that the above conditions would provide linear growth of the deposit, at the termination of the induction period, thus facilitating kinetic analysis of the deposit growth data. Nine tests were carried out (Table I). The duration of the various testa (15 to 35 h) was determined by the time necessary to obtain a scale layer having a thickness of at least 80 pm. The first five experiments were carried out under once-through conditions, with a nominally constant sulfite dosage and a varying pH level. Two recycle experiments (277,278) were carried out at nominally identical water compositions. One run (283) served to test the effect of an iron impurity on the performance of the magnetic device. A solution of FeCl:, was dosed to the water to provide a nominal concentration of 1.2-1.4 ppm of Fe. The test was repeated (284) withobt sulfite dosage. The last run (285) was conducted with soft water, dosed with Ca(OH)> The reason for this test was the claim of the supplier that the magnetic device would perform more effectively with this type of scaling water. Chemical analyses show that the deposits contained over 92% CaCO,. Bulk density of the deposit was evaluated from weight and thickness measurements. Values ranged between 1500 and 2500 kg/m3 and were not sufficiently accurate for comparing relative densities of deposits
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 501 ONCE THRCUGH pH-95
t
0.5
I n .
CO j = 160 - 240 ppm *SOy = 150 -250ppm
NU No TREATMENT 4.6 u M I h r
5 IATMENT
t
TREATMENT 3 1 )AM I h r \
04
(0)
m
6.5 UM I hr
5 0.3 a.
'MAGNETIC
-
TREATMENT
3 2 uMIhr ( 0 )
O0.1' f
la00 TIME
lb
lA00
zoo0
MINUTES
Figure 4. Deposit growth with and without magnetic treatment at
Figure 7. Deposit growth in the presence of iron impurities with
pH = 9.5.
sulfite dosage.
0.3
m
:
7
ONCE THROUGH Ca"= 160 -210 p p m CO;'= 60 -100 p p m M+SOy; 150-200ppm
I
ONCE THROUGH Fe*'. Fe"' D 1 2 -1 4 p p m co"=130-160 p p m CO; 210-260ppm NatSOy = 0 p p m
.
pH.9 6-
5-
m
0 2-
RUN 284 pH = 9 7
4-
4
a
0
17pMIhr
//'
-
5 01-
P;'
/
TREATMENT 17pMlhr
( 0 )
L
1
I
TIME, MINUTES
Figure 8. Deposit growth in the presence of iron impurities without sulfite dosage. I
07
ONCE THROUGH RUN 285 Ca(OH), AOOEO TO SOFT WTER pH= 9 5
I
RUN _ _CO"
0 6- 277 278
Na+O, -
130-170 180-260 200-275 125-160 160-240 200-275
RECYCLE RUNS pH=95
. ,
0 4-
/
0 51
$03LT
o;4
2-
0 1-
0'
TIME
MINUTES
Figure 6. Deposit growth at an increased magnetic exposure time.
formed with and without magnetic exposure.
Results Figures 2 to 9 show the deposit growth in the various
runs. It is seen that deposit accumulation was substantially linear in all experiments. Linear regression of the data of each run was used to evaluate the growth rate and the induction period of deposits formed with and without magnetic exposure. Table I summarizes the values obtained, with estimates of the experimental error based on 95% confidence limits. It is seen that the accuracy in measurement of the growth rate of the deposit is quite satisfactory (around 10%). The induction period data are far less accurate. It is evident from the scaling rate data that the magnetic device exerted no effect at all on the rate of CaC03 de-
100
200
300
400 500 TIME ,MINUTES
600
700
800
9W
Figure 9. Deposit growth with and without magnetic exposure using Ca(OH), for water hardening.
position on the flow surface. The adhesion number measurements also do not provide an indication that the magnetic device had any effect on the tenacity of the scale layer formed. The induction period data are far less accurate. They do not reveal either a statistically significant effect of magnetic treatment on the extent of the induction period. The growth rate measurements obtained reaffirm the profound influence of the supersaturation level on the rate of deposit formation. Figure 10 shows that deposition rate increases systematically with the supersaturation level. The figure also illustrates the considerable influence of impurities on CaC03 scaling rate. Addition of 1.2 to 1.4 ppm of Fe lowers the scaling rate by as much as 40%. Sulfite acta as an inhibitor (Hasson and Karmon, 1981) and in the absence of sulfite, the scaling rate increases by as much as 60%. Magnesium and many other common
592
Ind, Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
Table I. Scaling Rates and Induction Periods with and without Magnetic Treatment run no. 274
flow condition once through
282
once through
275
once through
276
once through
278 277
recycle recycle
283
once through
dosage NaOH + sulfite
magnetic treatment
9.5 9.5
scaling rate, g/(cm2 min) X lo5 3.33 f 0.38 3.33 f 0.40 2.19 f 0.17 1.95 f 0.14 1.53 f 0.22 1.50 f 0.19 0.52 f 0.10 0.56 f 0.11 1.32 f 0.20 1.39 f 0.09
scaling rate lm/h 10 f 1.2 10 f 1.2 6.6 f 0.5 5.9 i 0.4 4.6 f 0.7 4.5 f 0.6 1.6 f 0.3 1.7 f 0.3 4.0 i 0.6 4.2 f 0.3
ind period, min -18 f 50 71 f 49 59 f 59 31 f 54 104 i 86 110 f 77 -40 f 245 118 f 234 100 f 110 154 f 50
9.5
1.03 f 0.09
3.1 f 0.3
23 f 75
pH 10.2
+ sulfite NaOH + sulfite NaOH + sulfite NaOH + sulfite NaOH + sulfite NaOH + sulfite + NaOH
9.6 9.45 9.0
FeCI, 284 285
once through once through
0
+ FeClz Ca(OH), + sulfite NaOH
*
1.06 0.10 2.70 f 0.46 2.58 f 0.47 1.57 f 0.38 1.47 f 0.17
9.7 9.5
NO TREATMENT MAGNETIC TREATMENT
i
i
1
i
/
71
1”;;;r”H=go~ 1 ,
,
60
? O x lo3
1 -
, 0‘
i
i
i
i
10 20 30 40 50 SCALING POTENTIAL, [Ca”1lCO,l
,(ppm os COCO,)’
Figure 10. Effect of the supersaturation level on deposit growth rate with and without magnetic treatment.
ions are known to exert a similar effect. Conclusions The results of this study serve to illustrate several points that should be taken into consideration in the evaluation of magnetic treatment for scale suppression. The most significant parameter in scale formation is the scaling potential as determined by the supersaturation level of dissolved constituents. Impurities and commonly present nonprecipitating ions can have a considerable effect on deposit formation. Hydrodynamic and thermal
3.2 f 0.3 8.1 1.4 7.7 1.4 4.7 f 1.1 4.4 f 0.5
*
105 f 75 61 f 84 73 f 87 -1.5 f 127 -20 f 59
adhesion no. 1 1 1 1
3 3 2 2 2 2
5 4
3 1
parameters are also of major significance. It does not seem plausible to expect magnetic treatment to exert a meaningful scale suppression effect at sufficiently high supersaturation conditions. This has been demonstrated in the present study. There are virtually no data for delineating the range of relatively moderate scaling conditions under which a useful scale suppression effect can be rationally anticipated. Thus, magnetic treatment, despite ita long existence, remains in the status of a technology unbacked by adequate development requirements and unsupported by essential characterizing data. Acknowledgment This study was supported by Mekorot-Water Co. Permission t~ publish the results is gratefully acknowledged. Literature Cited British Standards Insthution, ”Cross Cut Test”, BS 39800Part E6, 1974. Cowan, J. C.; Weintritt, D. J. “Water-Fcfmed Scale Deposits”; Gulf Pubiishlng Co.: Houston, TX, 1976 pp 300-306. Duffy, E. A. Ph.D. Thesis, Ciemson University, Clemson, SC, 1977. Eiiingsen, F. T.; Kristlansen, H. Srtryck ur Vatten 1070, 3 5 / 4 , 309 (in English). Epstein, N. Roc. Int. Heet Transfer Conf., 6th, 1978; 1978, 6, 235. Gruber, E. C.; Carda. D. D. ”Performance Analysis of Permanent Magnetic Type Water Treatment Devices”, Final Report to Water Quality Association; South Dakota School of Mines and Technology, Rapid City, SD, 1981. Hasson, D. “Precipitation Fouling”, in “Fouling of Heat Transfer Equipment”, Somerscaies, E. F. C.; Knudsen, J. G. Ed., Hemisphere Publishing Corp.: Washington, DC, 1981; pp 527-566. Hasson. D.; Karmon, M. U.S. Patent 4264651, 1961. Hibben, S. G. “Magnetic Treatment of Water”; ARPA Order No. 1622-3, U.S. Department of Defense, 1973; 9 pp. O’Brkn. W. P. “On the Use of Magnetic (and Electric and Ultrasonic) Fields for Controlling the Deposition of Scale in Water Systems. A review of Several Papers Translated from Russian"; Civil Engin. Lab., Port Hueneme, CA, 1976; PO No. N62583-79 M-R-770. Troup, D. H.; Richardson, J. A. Chem. Eng. Commun. 1078, 2 , 167.
Received for review August 25, 1983 Revised manuscript received August 30, 1984 Accepted September 17, 1984
