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Ind. Eng. Chem. Res. 2008, 47, 7490–7495
Effect of Reactor Length on the Disinfection of Fluids in Taylor-Couette Photoreactor Aklilu T. G. Giorges* and John A. Pierson Food Processing Technology DiVision, Georgia Tech Research Institute, Atlanta, Georgia 30332
Larry J. Forney School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
When designing a Taylor-Couette flow ultraviolet (UV) disinfection system, the proper reactor size is obtained by balancing the diameter and height of the reactor. The disinfection level, which is based on reactor length (short and long reactors) and outside UV sources, is investigated to study the implications of reactor geometries. The experiment was performed by introducing three different sets of photon energy into the reactor from circumferential sources with different lengths. Three sets of equivalent photon energy are delivered to the system by limiting the photon exposed region and the number of sources. In addition, water-soluble caramel is used to establish the photon attenuation. The result indicated that there is no significant difference between long and short reactors with an equivalent UV-exposed region and similar flow rates. Indeed, the result shows that disinfection is strongly affected by the dosage delivered to the system. A short and wider photoreactor may yield similar disinfection levels as a tall and slender device, as long as the dosage delivered and the flow fields are similar. Thus, the reactor height and width can be sized depending on the application and other space limitations. A method is also presented to predict the mean intensity of the photoreactor with outside photon sources. Introduction Photoreactors are used in many engineering applications. Geometrical parameters (e.g., physical size) play a major role when selecting and then estimating the photoreactor performance. A common practice is to use average values when designing photoreactors, because of the lack of detailed information on the photon and reactor flow fields. Thus, the photoreactor dosage may be predicted from the applied photon energy and the residence time. However, estimating or determining the dosage distribution is not straightforward. Even by simplifying the photoreaction as first-order with ideal flow and perfect mixing, the reactor dosage distribution has a tendency to be nonuniform, because of the attenuation effect of the medium. For example, there is a degree of nonuniformity associated with the concentration distribution within the reactor, because of the change in concentration during photoreaction, which can obscure the photon. Moreover, the reactor shape, size, location of sources, operation and maintenance, and reactor wall properties change, because of wall deposits, which limits photoreactor performance.1,2 Based on the source locations, reactors with sources placed inside (annular reactor) and outside are the two most common cylindrical photoreactor models. In both cases, a uniform radiation is assumed to be delivered to the nearest reactor surface.3 For reactors with a concentric light source, symmetry can be used to reduce the intensity field dimensionality. Using the reactor symmetry and neglecting the upper and lower surface photon losses will simplify radiation intensity fields that yield a uniform photon energy radiated on the reactor wall. Yet, delivering uniform intensity is not a simple task for a cylindrical reactor with outside sources. Several closely placed sources must be applied around the reactor to deliver a uniform intensity. * Corresponding author. Tel.: 404-407-8837. Fax: 404-894-8051. E-mail address:
[email protected].
Even with closely placed sources, the degree of uniformity is affected by source proximity both to each other and to the reactor, which ultimately affects the reactor wall photon distribution. However, cylindrical reactors with outside sources have significant advantages in applications, because these designs utilize the largest surface area of the reactor and provide easier source accessibility for maintenance. Previously, the Taylor-Couette photoreactor is shown to provide advantages in the treatment of photon-absorbing fluids.4,5 When a fluid is placed between concentric cylinders with the inner cylinder rotating while the outer cylinder is at rest, unstable flow stratification results. When the rotation rate of the inner cylinder exceeds a critical value, the flow becomes well-ordered with laminar counter-rotating vortices6 (Taylor vortices). In a cylindrical photoreactor, the vortices provide an effective mechanism of mass transfer between the photons exposed and shaded regions in the photon-absorbing fluid as the fluid flows through the reactor. Therefore, the degree of uniformity is affected by both geometric and operational parameters. In applications, the photon energy from the source passes through different mediums before it reaches its target within the reactor. In a typical photoreactor, the radiation passes through air and quartz before it reaches the fluid inside the reactor. Thus, the intensity near the source and the reactor wall is estimated using the conservation of energy for a non-photonabsorbing medium. After the reactor surface photon distribution is estimated, the photon energy balance is used to model and estimate the mean intensity through the reactor wall and eventually the photon energy delivered to the system. In this paper, we will discuss the intensity fields for a cylindrical reactor with outside sources. In the first two sections, the intensity fields from single and multiple sources are outlined. The effect of reactor length then is investigated for disinfection levels, using two types of arrangements of ultraviolet (UV)
10.1021/ie800250r CCC: $40.75 2008 American Chemical Society Published on Web 08/29/2008
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factor of the medium. For example, Figure 1 shows a cylindrical photoreactor with the photon sources located outside the reactor. It also illustrates the location of the sources and the reactor wall. The system consists of the photon sources, the region between the reactor and the sources, the reactor wall, and the inside of the reactor. Essentially, three mediums with different photocharacteristics are involved. The first region is a nonphoton absorbing medium that is comprised of air between the sources and the reactor. The second medium is the reactor wall that has preferably a small absorption (glass or quartz), followed by the third medium, which is the photon-absorbing fluid. To determine the photon energy that is delivered to the reactor, the photon intensity at the reactor surface must be obtained. The intensity field of the photon energy, based on the energy balance and source photon energy conservation surrounding the reactor, can be estimated using the source output distribution.8-12 Depending on the reactor surface distance from the source, the intensity varies as
()
Figure 1. Schematic of the concentric ultraviolet (UV) reactor with outside photon sources.
I ) Io
ro -µ(r-ro) e r
(2)
where r g ro Here, r is the distance from the source center and µ is the medium attenuation. If the fluid is a non-photon-absorbing medium, the intensity varies only with the distance from the source and the aforementioned equation is reduced to
()
ro (3) r The distance r between the reactor wall and the source surface is not uniform, because of the geometric variations resulting from the curvature of the reactor wall and the distance to the source. For a single source, the intensity profile at the reactor surface varies from its strongest at the reactor wall near the source to the weakest at the outermost distance from the source surface. The intensity at the reactor wall is reasonably estimated, because the intensity at and around the source can be estimated based on the source output and the distance between the source and the reactor wall. Furthermore, the estimated surface values can be used to estimate the total photons delivered to the reactor by integrating the local predicted values: I ) Io
Figure 2. Measured and predicted UV intensity profiles, relative to distance from a single source.
sources to represent long and short reactors, such that an equal amount of dosage is delivered to the system. The first arrangement uses six symmetrically placed lamps with partially blocked lengths, while the second uses the entire lamp length. By adjusting the height of the six sources, an equivalent dosage for one, two, and three UV sources is established. The results can then be used to outline the significance of the reactor length. Model Prediction and Evaluation. The radiation from the source (cylindrical lamp surface), including the bottom of the lamp, can be reasonably calculated from the lamp power output:7,8 I0 )
E 2πro [( L + ro ⁄ 2)]
(1)
where E is the source power, ro the radius of the lamp, L the length of the lamp, and I0 the irradiance at the lamp surface. If the lamp length is significantly larger than the radius of the lamp, or if the distance from the source is significantly smaller than the source length, then the photon energy lost through the bottom may be neglected and the intensity at the surfaces can be estimated as I0 ) E/(2πroL). In conventional photoreactors, the photon energy from the sources passes through different mediums. Depending on the location and the characteristics of the medium, the intensity field can be reasonably estimated from the inverse law and attenuation
Es )
∫I
ow
ds
(4)
where ds is the path length of the reactor outside, Iow the outer reactor wall intensity, and Es the energy delivered to the photoreactor (per unit length). Single Source. For a long cylindrical source, the photon intensity can be estimated from lamp photon energy output and area. If reflection and refraction are neglected for a non-photonabsorbing medium when the source is in close proximity of the reactor wall, eq 1 can be used to predict the surrounding intensity fields. To verify the intensity fields, the intensity was measured at several distances from the source (5.4-cm-long UV lamp) and at the middle (2.7 cm from the end), off middle (1.35 cm), and end of the lamp. The lamp is rated to deliver 4.4 mW/ cm2 at a distance of 1.91 cm away from the lamp, and the total lamp output is calculated to be 270 mW. The intensity around the source can be predicted using the measured data, because of symmetry. The theoretical intensity profile is predicted using the source rating and eq 3. Figure 2 demonstrates the change in intensity relative to the distance from the source. The measured values at the middle and off middle
7492 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 1. Measured Intensity between Two Photon Sources Relative to the Distance from the Two Sources intensity distance distance intensity from superimposed measured from from from source 2 intensity intensity source 1 source 2 source 1 2 2 (mW/cm2) (mW/cm2) (cm) (cm) (mW/cm ) (mW/cm ) 0.778 1.098 1.7338
Figure 3. Reactor outside surface within a 2D intensity profile from a single source.
do not indicate significant differences, confirming that intensity is almost uniform along the lamp, except at the edge, where the intensity does not follow the line source model. This was expected, because the edge effect increases with the distance from the source. Although the measured values are slightly higher, the intensity measured along the middle and off middle follow a similar trend. In addition, the intensity profile based on the source rating and the average measured intensity is shown in the plot. The measured profile with the distance from the source almost follows the intensity predicted using the line source assumption. In fact, the experimental and predicted data from the lamp rating show similar values and trends. As expected, this indicates that the intensity away from the source varies with the inverse of the distance, and the line source assumption, in this case, can be used to estimate the source proximity intensity fields. Figure 3 shows the two-dimensional (2D) intensity field of a single source: I(r, θ) ) Io(ro, θ)
1 r
3.195 2.451 1.7338
10.77 7.63 4.83
2.62 3.42 4.83
13.40 11.05 9.67
14.70 9.28 7.94
Thus, the measured average intensity (14.7 mW/cm2) was estimated using six values (in the range of 14.0-16.0 mW/ cm2). The measured intensity at a particular location was compared with the intensity predicted based on the distance from each source. This indicates that, by superimposing the intensity field from each source, it is possible to predict the intensity field from multiple sources. The intensity difference shows a parabolic profile between the two sources, as with the distance away from the sources. However, the contribution from a nearby source showed a significant change in local values predicted from the single source only. Indeed, using the source rating and the proximity superimposed intensity field, the reactor intensity distribution may be predicted and the local intensity distribution may be used to design effective photoreactors. Moreover, the present measurements, although not designed to provide accurate values of the mean intensity, confirm the trend of the superimposed intensity profile. Figure 4 illustrates the intensity field for the reactor located at the middle of six symmetrically located photon sources. The
(5)
where r g ro and 0 < θ < 2π The superimposed reactor wall on the intensity field clearly illustrates the intensity change at the reactor wall. Indeed, it is clear that the intensity at the reactor wall can be reasonably predicted using the 2D intensity profile from the source within the view factor limitation, using the upper and lower tangents. Multiple Sources. To study the multiple source intensity field, two UV light sources were suspended 60° apart with a radius of curvature of 3.10 cm, similar to the configuration shown in Figure 1. However, the lamps were suspended horizontally, and the intensity field was mapped under, along, and between the sources, in addition to away from the sources. To minimize the measuring error, and for simplicity, the data presented here were measured near to the sources. The average measured values and the predicted intensity from source ratings are compared to the intensity predicted from each superimposed source in Table 1. For example, the measured value along one of the sources was measured and compared with the measured intensity from an identical location in the second source. As expected, the measured value indicated similarity due to the geometry.
Figure 4. Reactor outside surface intensity: (a) superimposed reactor surface within a 2D intensity profile and (b) reactor wall predicted intensity value and variation.
Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7493
contours indicate the superimposed intensity field. The medium was assumed to be a non-photon-absorbing medium; therefore, the contours show the contribution from all sources. This model can be further modified to include the contribution from nearby sources as necessary. Figure 4b shows the intensity predicted around the outer surface of the reactor. The intensity variation between the sources and its profile was similar to that previously measured from multiple sources. However, the superimposed values at the reactor outer wall (see Figure 4b) are different from the measured values (see Table 1), because the locations of the measured and predicted values were different. Mean Intensity. In conventional reactors, the reaction is dependent on the reactor volume, the reacting material available for reaction, and the flow and mixing rates. However, in photoreactors, the photon intensity and distribution are also major factors. The reactor concentration and photon distribution have a tendency to be nonuniform, because of the location of the reaction and the attenuation effect of the fluid. Furthermore, the concentration distribution within the reactor and the change in concentration during the photoreaction process compound the complexity of the photoreactor. Moreover, reactor shape, size, lamp location, operation and maintenance, and the change in reactor wall properties due to surface deposition are also limiting factors in photoreactor applications. The mean intensity can be estimated from the reactor shape and simplified reaction conditions. For a cylindrical photoreactor with external radial sources, the intensity profile can be predicted using eq 2.2,11 The mean intensity is defined as I)
1 d
∫
d
0
I ds
(6)
where s is the characteristic length and I is the intensity. The mean intensity is related to the incidental power entering the reactor per unit volume (P/V). The general expression for the mean intensity of the photochemical reactor based on the power delivered to the reactor (P) and the characteristic length (d) is12 I)
(P ⁄ V) (1 - e-µd) µ
(7)
where µ is the attenuation factor. It is common practice to use average values when designing photoreactors, because of the lack of detailed information on the local photon, photoreaction, and reactor flow fields. For a highphoton-attenuating medium within a cylindrical reactor with outside sources delivering uniform radiation, photons penetrate a fraction of the reactor depth and most of the reaction occurs within the reactor-photon-infiltrated region (the shaded region in Figure 5). Thus, these local mean intensities can be predicted by integrating the intensity profile from the inner reactor wall through the photon penetration depth. Particularly, for a disinfection process where a threshold dosage is required to kill the organism of interest, this local region with the fluid transport mechanism determines the effectiveness of the reactor. The region of effective photoreaction (disinfection) can be estimated by integrating the intensity from the inner reactor wall (Iiw) to a 90% drop in intensity. When µ ) 2.3A, where A is absorbance (A ) 1/λ, and λ is the penetration depth), the last part of the term on the right-hand side in eq 7 is reduced to 1 - exp[-µ(ris - ri)] ) 0.9. Therefore, the local mean intensity becomes I)
1.8Iiwris µ(2ris - λ)λ
(8)
where ris is the reactor inner wall radius and ri is the inside reactor photon penetration depth radius. This local mean
Figure 5. Effective photoreaction (disinfection) region, based on absorbance.
intensity with the fluid mechanics may be used to address the local disinfection process and design and/or modify an effective photoreactor. Reactor Length Effect Experiment. The reactor length effect and lamp configuration, and their implication on the reactor design, were investigated in this study. The experiment was designed to study the disinfection rate of E. coli when equal amounts of UV energy were applied from the total and partial lamp height to a Taylor-Couette flow reactor. Figure 1 shows a schematic representation of the bench-scale reactor used in this experiment. A Taylor vortex column consisting of a Teflon rotor with an outside diameter of 3.45 cm contained within a Vycor stator with an inside diameter of 4.59 cm and an outside diameter of 5.14 cm. The reactor height was 7.3 cm, as measured from the inlet to the outlet, although the six low-pressure mercury UVC lamps 0.66 cm in diameter have effective lengths of 5.4 cm (Pen-Ray Lamps). The six UV sources were positioned symmetrically every 60° around the reactor. The transmission ratio of UV radiation (Iiw/Iow ) e-µtros/ (ros - t)) was measured to be 50% (0.5) through the 2.75-mmthick reactor wall, where ros is the outer reactor wall radius and t is the reactor wall thickness. Three equivalent lamp outputs (one, two, and three lamps), as well as six lamps, were tested for UV disinfection to study the effect of reactor length (height). Three equivalent lamp outputs for short and long reactor lengths were investigated. The short reactor lamp output provided equivalents for a total of one, two, and three lamps by varying the partial lengths of all six lamps (i.e., 1/6, 2/6, and 3/6). In addition, disinfection from the entire length of all six lamps was studied. For simplicity, no reflector was placed around the UV sources, so that only the direct incoming UV effect was studied. Commercially available caramel coloring was used to set the UV absorbing medium characteristics. An absorbance of 1 mm-1 (a penetration depth of 1 mm) was established using 2.152 g of caramel per liter of deiodized water. The flow rate and Taylor number (Ta) for all tests were fixed to 20 mL/min and 300, respectively. The Taylor number is defined as follows: Ta )
ωrrd ν
d rr
(9)
where ω is the rotational speed, d the gap width, ν the kinematic viscosity, and rr the rotor radius. The UV sources were marked and numbered to simplify the UV source changing process, namely, the lamp heights and the
7494 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008
Vexp (12) Q The concentration of a local surviving organism in an ideal plugflow reactor is given as13 τexp )
dC ) -kI(r) dV C0
(13)
where k is the inactivation rate constant. Integrating I(r) and dV from the reactor inner wall to UV penetration depth λ ) 1 mm yields eqs 10 and 11 for the local mean UV intensity and UV exposed volume, respectively. Furthermore, using the photon-exposed region retention time, eq 13 becomes
Figure 6. UV disinfection rate for one, two, and three equivalent sources ((0) dataset for circumferential sources and (4) dataset for vertical sources).
number of UV sources. In the first three tests (partially exposed or shorter reactors), the six lamps were moved downward 9, 18, and 27 mm (lamp lengths of 1/6, 2/6, and 1/2) from the top of the reactor. Next, three cases of fully exposed (long reactors) were tested by placing one, two, and three fully exposed sources to the transparent region of the reactor. In the case of two and three fully exposed sources, the sources were placed symmetrically. In the last test, all lamps were placed in the transparent region. Sampling occurred 9 min after the UV source placement and operation. The E. coli ATCC15597 and ATCC25253 samples were obtained from the American type Culture Collection (Manassas, VA). Culturing procedures were described in greater detail elsewhere.13,14 For caramel-based solutions, dilution blanks contained 0.1% trypic soy broth (Acumedia 7164A, Neogen Corporation, Lansing, MI). Then, the samples were diluted and plated and the colony forming units were enumerated. Figure 6 illustrates inactivation of E. coli versus equivalent UV source output. The single source with the entire length and the equivalent partial exposed lamps length for six sources show a comparable disinfection rate. The same pattern is also observed with two and three UV source equivalents. The inactivation rate for the similar flow conditions (Ta and flow rate) and intensity is almost identical. As expected, the comparison among one, two, three, and six sources shows a significant difference in disinfection. The disinfection difference with similar intensity delivered was not significant. The reactor performance is not significantly affected by the lamp shape (short or long sources). It only significantly varies with the increase in dosage. Thus, the reactor effectiveness is significantly affected by the dosage, which is a direct result of the flow dynamics and the fluid attenuation not size (long or short reactor). The total number of photons delivered to the reactor from eq 4 is determined by integrating the local intensity at the reactor wall. Using eq 8, the local mean intensity (Ij) for a reactor radius of 22.95 mm and a penetration depth of 1 mm yields I ) 0.40Iiw
(10)
Assuming plug-flow conditions, the photon-exposed volume (Vexp) is calculated using the penetration depth (λ), the stator inner radius (ris), and the source length (L): Vexp ) π(2risλ - λ2)L
(11)
The maximum exposed time (τexp) is defined as the time spent in the photon-exposed region and is estimated as
[
( )
]
Iiw C ) exp(-kIτexp) ) exp -k × 0.4π λ(2ris - λ)L (14) C0 Q For example, for a single source with the current experimental arrangements, ∼60° (1/3π) of the reactor side is directly exposed to the source. However, ∼1/6 of the source energy is directed toward the reactor. Based on the reactor size and penetration depth, the effective volume is 1.27 cm3, the flow rate is 0.33 cm3/s, and the wall intensity is 2.6 mW/cm2. The mean intensity was 1.0 mW/cm2, with a mean dosage of 3.8 mW s/cm2. The dosage delivered to the system is the product of the photon intensity in the reactor and the time the fluid spent in the photonexposed region. To simplify this complex dynamic process and gain some understanding of the local region effect, the dosage delivered to the system is predicted from the product of the local mean photon intensity (eq 10) and the photon-exposed time (eq 12). Although the mean dosage value of the reactor is similar, the local mean intensity-based data yield the region where most of the photoreaction (disinfection) is occurring. This region is very important in disinfection, especially where there is a threshold dosage required for killing the harmful organisms. Thus, the local mean intensity with the flow parameters can be used to estimate whether the threshold dosage can be exceeded to provide adequate disinfection. It also provides valuable information for designing an effective photoreactor. Conclusion This study shows that superimposing the intensity distribution of each source allows a reasonable prediction of the intensity profile of multiple sources. It also provides an insight on the local intensity field. The model can be used to estimate the intensity field of the reactor using the source rating, and it also serves as a reasonable model for predicting the photoreactor intensity profile with smaller gaps and absorption. Indeed, the disinfection of the ultraviolet (UV) reactor is strongly affected by the dosage delivered to the system. The geometric aspect of the reactor, either long and thin or short and wide, did not show a significant difference for similar flow conditions as long as the dosage delivered to the system was equivalent. Thus, the reactor may be designed to fit a particular space and still provide appropriate disinfection. Acknowledgment The authors acknowledge the support of the Southern Co. and the State of Georgia Agricultural Technology Research Program (ATRP). Nomenclature A ) absorbance (base 10) C ) organisms concentration
Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7495 C0 ) initial organisms concentration d ) gap width E ) source power I ) intensity Iiw ) inner reactor wall intensity I0 ) intensity at the lamp surface Iow ) outer reactor wall intensity L ) length of the lamp Q ) flow rate r ) distance from the source center ri ) reactor inside radius ris ) reactor inner wall radius ro ) outer radius of the lamp ros ) outer reactor wall radius rr ) rotor radius s ) path length of the reactor outside t ) reactor wall thickness Ta ) Taylor number; Ta ) ωrr d/[ν(d/rr)1/2] Vexp ) photon exposed region λ ) penetration depth µ ) absorbance (base e) ν ) kinematic viscosity τexp ) exposure time ω ) rotational speed
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ReceiVed for reView February 12, 2008 ReVised manuscript receiVed July 3, 2008 Accepted July 15, 2008 IE800250R
