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Wetting enhancement of polypropylene plate for falling film tower application
Chemical Engineering and Processing 108 (2016) 1–9
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Wetting enhancement of polypropylene plate for falling film tower application Digvijay Patila , Ritunesh Kumara,* , Fu Xiaob a b
Mechanical Engineering Department, Indian Institute of Technology Indore, MP, 453446, India Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 30 May 2016 Accepted 5 June 2016 Available online 14 June 2016 Keywords: Falling film tower Polypropylene Surface modification Surfactant Wetting factor
A B S T R A C T
Falling film towers are popular for simultaneous heat and mass transfer applications due to modest air pressure drop and low energy demanding liquid distribution. Conventionally, metals are used as gas liquid contacting surface in these towers. Corrosion prone metallic surfaces need expensive anticorrosive coating or frequent replacement, whereas non metallic surfaces such as plastics have poor wetting characteristics. In current study, wetting factor of plain polypropylene and aluminum plates has been measured. Wetting enhancement methods: surfactant addition (sodium lauryl sulfate-SLS) and surface modification have been used on polypropylene plate. Improvement of 50% (SLS 300 ppm) and 80% (Modified Surface C) wetting characteristics of polypropylene have been achieved. Modified Surface C average wetting factor is 41% superior to aluminum surface. A new generalized empirical correlation for wetted area estimation considering influences of solid surface and fluid characteristics has been proposed. Proposed equation shows good agreement with current and past studies (MAPE 8.5%). ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conventionally three types of towers namely packed bed, spray and falling film towers are used in simultaneous heat and mass transfer applications [1]. For transfer of potential (heat and mass) from one fluid to another fluid in these chemical towers, one of the fluids (liquid) is dispersed across continuous phase of another fluid (gas). In the spray towers, liquid dispersion is required in form of fine liquid droplets. In case of the falling film towers, liquid flows in form of thin straight liquid film on solid surface. In case of the packed bed towers, liquid moves in form of distorted liquid film at solid surfaces and as liquid sheet between solid surfaces. Out of above, the packed bed towers are the most versatile and extensively used design. They relish benefit of high efficiency and very large contact area to volume ratio. Major drawbacks of these are high air side pressure drop and high ml requirement [2]. Spray towers are quite popular in air pollution control [3]. Advantages associated with spray towers are simple design, and low initial and maintenance cost. Their major
Abbreviations: Al, aluminum; LiCl, lithium chloride solution; LPM, liter per minute; MAPE, mean average percentage error; PP, polypropylene; ppm, parts per million; SLS, sodium lauryl sulfate; S.S., stainless steel. * Corresponding author. E-mail addresses: [email protected], [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.cep.2016.06.005 0255-2701/ã 2016 Elsevier B.V. All rights reserved.
limitations are: poor efficiency, ineffective/absence of external heat interaction, high speeding liquid droplets (poor residence time of liquid droplets) and large liquid side pressure drop [4]. Falling film towers are better equipped to handle limitations of both packed bed and spray towers. Thin film of liquid attributes towards high heat transfer & mass transfer coefficient. Their air side pressure drop is quite less in comparison to the packed bed towers and they can operate efficiently at low ml/ma ratio. Liquid jet can be easily dispersed in form of a falling film at the expanse of less energy than the spray tower. Distinguished feature of the falling film tower is the instant removal/addition of heat along with mass transfer operation. Total power requirement is even lesser than packed bed and spray towers [5]. Hence, processes that can abide moderate parasitic losses use falling film towers (i.e. cooling tower, absorption refrigeration). Two main limitations of the falling film towers are the incomplete wetting of the solid surfaces and non-uniform distribution of the liquid across solid surfaces [6]. Unlike packed bed towers and spray tower, state-of-the-art of the falling film tower is still not matured. Proficient designs of liquid distributors and highly efficient solid liquid contacting surfaces (high wetting at low flow rates) can help in upgrading the performance of the falling film tower up to the packed bed towers. Many researchers have carried out experiments related to design of liquid distributor with objectives of either to ensure
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Nomenclature
Greek symbols ew Wetting factor a Thermal diffusivity (m2/s) r Density (kg/m3) s Surface tension (N/m) @s Partial differential of surface tension of liquid @T m Viscosity (Pa s) n Kinematic viscosity (m2/s) g Surface energy (J/m2) u End cutting edge angle ( ) Symbols A Area (cm2) Aw Area ratio in Eq. (2) Aw;o B Breadth (cm) d Depth of cut (mm) g Acceleration due to gravity (m/s2) H Height (cm) L Length (cm) ln Viscosity length scale (m) Ka Kapitza number Ma* Modified Marangoni number m Mass flow rate (kg/s) n Number of data points p Distance between two consecutive slots (cm) Re Reynolds number Ref film Reynolds number T Temperature ( C) Tl Average liquid film temperature ( C) t Thickness (cm) W Width of plate (cm) Z Number of slots Sub- and Superscripts a Air ext Extended i Interception point l Liquid o Isothermal condition s Solid surface w Wet
uniform flow of liquid across each solid surface or to ensure stable liquid film across solid surfaces. Luo et al. [7] designed a tangential inlet (liquid flow direction) single vertical tube falling film evaporator. They investigated the effect of an annular slit opening (0.5–2 mm) and liquid spray density on the film uniformity and stability. They recommended the annular slit opening of 1.5–2 mm and spray density between 0.07–0.19 kg/ (m s) for optimum film stability, distributor inlet tube was rotated tangentially by 270 inside distributor header for ensuring uniform film thickness around whole periphery. Qi et al. [8] investigated experimentally the factors affecting the wetted area and film thickness on single channel internally heated vertical regenerator of a liquid desiccant system in the mass flow range of 0.025–0.15 kg/s. They observed that due to increase in slit opening from 1.0 to 1.25 mm decreased the wetted area by around 51%. A rectangular box distributor with parallelly welded lamellas was suggested by Glebov and Settervall [9], liquid was fed in the distributor through three inlet pipes. They reported that
minimum specific flow rate has to be 0.26 kg/(m s) for full wetting of lamella type surface. Distributor and redistribution components in combination were used by Gonda et al. [10] for getting uniform distribution of solution on both sides of a plate. Water flows firstly through the distributor, each side edges of distributor are drilled 13 semi-cylindrical holes of 3 mm diameter. Then, water flows along the internal wall of the header until it reaches the redistributor. They reported removal of dry patch formation due to shrinkage of liquid film with proposed assembly. Wetting or adhesion of the liquid on the solid surface depends on properties of contacting mediums i.e. surface tension of the liquid and surface energy of the solid. Hence, spreading of the liquid on the solid surface can be improved by increasing surface energy of solid and reducing surface tension of liquid [11]. Many experimental studies have been carried out by researchers using surfactant addition or surface modification techniques as wetting enhancing medium. Effect of the surfactant 2-methyl-1-pentanol (500–700 ppm) on absorption chiller (LiBr/H2O) was investigated by Glebov and Settervall [12]. They found 30–35% overall enhancement in cooling capacity of chiller. Cheng et al. [13] experimentally investigated the effect of the surfactants (2-ethyl1-hexanol and 1- octanol) on the heat transfer in falling film absorber. They found improvement up to 100% in heat transfer coefficient with optimum concentration at (40 ppm) for these surfactants and noticed decrease in the heat transfer coefficient with any further increase in the surfactant concentration. Kang et al. [14] observed 34% improvement in the wetted area with triton X 100 surfactant (500 ppm) addition in water. Kim et al. [15] investigated the effect of enhanced surface (micro-scale hatched tube) on horizontal falling film tubular absorber using LiBr/H2O solution. They reported 10% improvement in wetting characteristic of a hatched tube surface than smooth one. Koroglu et al. [16] studied the effect of copper oxide layer deposition on copper tubes in the horizontal tube falling film tower. They reported full wetting in case of the oxidized tubes at low flow rate (Re 86) than the plain tube (Re 114). Hoffman et al. [17] used knurled tubes and 2ethyl-1-hexanol (80 ppm) for the horizontal tube falling film tower. They found 20–40% improvement in mass transfer coefficient for knurled tubes and 60–140% improvement for surfactant addition case. Kim and Ferreria [18] experimentally compared the effects of modified surface geometry (clamping of copper wire mesh on copper surface) and a surfactant 2-ethyl-1hexanol (100 ppm) addition on film pattern of copper plate using LiBr–water absorbent solution. They reported enhancement in mass transfer of 40% and 60–110% through copper wire clamped surface and surfactant addition respectively. Zhang et al. [19] experimentally investigated the role of Marangoni effect on the mass transfer of falling film tower (inclined SS plates). They proposed generalized effective enhancement factor correlation in terms of ethanol concentration, inclination angle of surface and liquid flow condition. Use of metallic gas-liquid contacting surface in the falling film tower is very common due to good wettability of metallic surface. But, corrosion problem is the main limitation of metallic surface and it mandates the replacement of solid surfaces after certain time interval thus adding up significant maintenance cost. Plastic plates can be used as an alternative of metallic surface provided wetting characteristics of plastic surface can be elevated up to metallic surface. They have excellent corrosion resistance property, low cost and light weight. In the present investigation, wetting factor of aluminum (Al) and polypropylene (PP) plates have been experimentally measured. Effect of the sodium lauryl sulfate (SLS) surfactant addition and the surface modification technique have been used for improving the wetting characteristics of PP for vertical falling film tower application.
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2. Experimental set up and procedure In order to ensure accurate measurement of wetting pattern on both sides of the solid surface, single plate falling film tower setup has been fabricated (Fig. 1 (a)). The set up consists of distributor header, vertical tower, solution tanks, and solution supply and return lines. Except of vertical tower, rest of the setup has been from PP sheet (t = 10 mm) and PP pipe (1/200 inch dia.). Welding of PP sheet has been carried out using hot air gun machine (Bosch, GHG 630). Distributor header is a hollow square (L x B) (22.5 22.5) cylinder of H = 18 cm. The topside of the distributor has been kept open for regular cleaning of slit opening around test plate. Pocket in the bottom plate of the distributor header provides tight holding of the test plate and slit opening of 1 mm on both side provides scope of uniform liquid distribution from both faces of the test plate. This pocket has been cut on CNC machine. Vertical tower (B L H) (15 15 60) is hollow cylindrical structure made from acrylic sheet (t = 4 mm) for clear visualization of liquid film. Vertical tower houses the test plate of size (t L H) (1.0 10 62). PP solution tanks of capacity 100 l (Collection tank) and 200 l (supply tank) have been used. Solution heater of 3.5 kW controlled with PID controller (0.5 C of set value) has been installed in the supply tank for analyzing the effect of solution temperature on wetting characteristics. Red color added (for clear visualization of wetted area) water has been used as the working fluid. Solution line consists of supply line, bypass line and return line. Desired flow rate through the supply line can be set with the help of two ball valves (3 & 30 ) and monitored with help of rotameter (1.5–15 LPM), monoblock pump (Kirloskar, 0.5 HP) has been used to circulate the liquid across supply loop. Solution is conveyed back to the supply tank with help of submerged pump (0.25 HP). Camera (Canon, 550 D) and light source (FloLight, MicroBeam128) have been used to capture images of flow pattern. Camera is leveled at the center of vertical test plate to avoid any vertical asymmetry in post processing of captured image. Contact angle of surfactant solutions (60–300 ppm of SLS) on plain test plate of PP has been measured by goniometer (Dataphysics, OCA 15 EC).
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Before each run of test, water is first set at desired value and then thoroughly remixed by closing ball valve 3 for 10 min. Afterwards, required solution flow rate at specific temperature is set precisely by mutual regulation of two ball valves 3 & 30 . The system is then allowed to stabilize for 10 min. Subsequently four images of each side (Side I–facing liquid distributor and Side II– away from liquid distributor) are captured at the time span of 1 min. Four images have been taken in order to avoid any chance of error due to flow pattern fluctuations. Post processing of these images for wetted area estimation has been carried out using ImageJ software. The maximum difference in estimated wetted area is found to be up to 1%. 3. Results and discussion 3.1. Flow patterns and wetting factor of plain PP and Al surfaces Fig. 2(a) and (b) show the flow patterns of falling film on both sides of plain Al and plain PP surfaces with increasing flow rate of solution. In general liquid flow pattern is found to be similar except at low flow rate of 0.038 kg/s, irregular liquid films form at low flow rate on Side I. Also, only at low flow rate wetted area of Side II is found to be larger than Side I. It is found that average wetting factor of Side I (of Al and PP) is 9.0% higher than of Side II. Higher wetting factor of Side I can be attributed to high velocity of liquid approaching slit opening of Side I. In the beginning liquid film contracts in the transverse direction, deformed liquid revolute forms at both side ends due to these contraction, these liquid revolutes were named as liquid rims [20]. The pattern of liquid falling film depends very much on the dynamics and size of these liquid rims. These liquid rims gradually approaches towards each other, rate of approach of liquid rims decreases with increase in flow rate. If these rims intercept each other at a point (apex of rims) then liquid film expands slightly in downward direction. Distance of apex (Hi) from the top and length of liquid film at apex (Li) both increases with increase in flow rates. If they do not intercept at a clear distinct point then liquid film does not expand in a lower half portion. On comparing the flow patterns of PP and Al surface it is
Fig. 1. (a) Schematic diagram of experimental set up (1–supply tank, 2–pump, 3 & 30 –ball valve, 4–rotameter, 5–supply line, 6–distributor header, 7–vertical tower, 8–solid surface, 9–collection tank, 10–submerged pump) (b) design of bottom plate of distributor header.
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Fig. 2. Flow patterns on test plates (a) Plain Al (b) Plain PP (c) Plain PP with SLS 300 ppm on Side I-1,2,3,4 and Side II- 10,20 ,30 ,40 (1, 10 –0.038 kg/s, 2, 20 –0.083 kg/s, 3, 30 –0.139 kg/ s, 4, 40 –0.189 kg/s).
found that liquid film contraction (liquid rims approach rate) is more rapid in case of PP surface. It is also clear from Fig. 2 that contraction length and height ratios for plain PP are lower than plain Al surface. Wetting factor has been calculated using Eq. (1).
ew ¼
Aw As
ð1Þ
Wetting factor for both surfaces has been measured and average wetting factor of each plate (both sides) has been reported in
present study. Fig. 3 compares the wetting factor of PP and Al surface with increasing flow rates. Wetting factor for both surfaces increases with increase in flow rates. Average wetting factors of plain PP and Al surfaces are found to be 0.46 and 0.59 respectively. Especially at low flow rate wetting characteristics of plain PP surface is very poor in comparison to Al test plate. Hence, finding suitable wetting augmentation method for PP is the main concern such that PP surface can be used satisfactorily as replacement of Al surface in falling film towers even for low flow rate applications. It is well evident that wetting characteristics of the surface can be improved by reducing the surface tension of the liquid. Wetting of solid surface can also be improved in case liquid rim formation at edges can be disrupted forcibly to allow liquid spread in transverse direction towards dry patches. The same has been achieved in current study through surface modification technique. These two methods have been explored as possible ways of improving wetting characteristics of PP surface. 3.2. Effect of surfactant on wetting factor of plain PP surface
Fig. 3. Comparison of wetting factor for PP and Al.
Surfactants are compound that are used to lower the surface tension of the solution. They are extremely popular for variety of applications such as detergent, emulsifier, foaming agent and of course as wetting agent. SLS solution of different concentrations (60, 100, 140, 200 and 300 ppm) in water have been used in current work. Fig. 2(c) shows film patterns of 300 ppm surfactant solution on PP surface. Similar patterns are observed for other SLS solution concentration. It is clear that in case of SLS surfactant solution film contraction reduces as compared to plain PP plate. Fig. 4 shows the effect of SLS solution concentration on wetting factor of plain PP with increase in mass flow rate. It is observed that significant
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system in case carried away with process air. Other potential limitations of surfactant additions are biodegradation, settling down, clogging of surfactant at bottom of tank and pump parts. 3.3. Effect of surface modification
Fig. 4. Effect of SLS solution concentration on wetting factor of plain PP test plate.
improvement in wetting factor is offered until SLS solution concentration of 100 ppm. This can be attributed to decrease in solid liquid contact angle with increase in solution concentration. Thus, solution spreads uniformly across entire solid surface. On increasing SLS solution concentration from 100 to 140 ppm, little improvement in wetting factor has been observed. At 200 ppm solution even slight decrease in wetting factor has been found. Wetting factor is again found to have increasing trend in the solution concentration range of 200–300 ppm. To study the reason behind this trend, contact angle on plain PP sheet of different SLS solution concentration have been measured. Fig. 5 shows variation of contact angle and average wetting factor with increased SLS solution concentration. Contact angle variation closely justifies the trend observed for wetting factor variation. Contact angle significantly reduces up to the solution concentration of 100 ppm then it remains almost constant up to the solution concentration of 140 ppm. At 200 ppm slight decrease in contact angle is observed then again it reduces on increasing concentration. Average wetting factor in case of 60, 100, 140, 200, and 300 ppm SLS solution are found to be 0.56, 0.62, 0.64, 0.62 and 0.69 respectively. 100 ppm SLS solution concentration seems to be optimal for air-conditioning applications considering side effects of high surfactant concentration on skin, eyes and respiratory
In order to decide final design of texture required on PP surface for enhancing vertical plate wetting characteristics, four primary surfaces are fabricated; vertically slotted, horizontally slotted, diamond shape and horizontal inclined recessed. These surfaces are shown in Fig. 6. For all these surfaces depth of cut and distance between two slots have been kept 3 mm and 4 mm respectively. First two surfaces are machined with end mill tool 3 mm diameter and remaining two surfaces are prepared by single point cutting tool (u = 60 ). These surfaces are tested to check their liquid retaining capability in transverse direction while keeping surfaces vertically held. Surfaces are dipped completely in the solution (red colour in water). Then, they are taken out of solution one by one and after settling time of 1 min; images have been taken. Fig. 6 shows liquid retention at the end of experiment on machined surfaces. It is clear that horizontal inclined recessed (opposite to flow direction or gravity) surface retains maximum solution. Hence, this texture has been finalized for further study and one PP test plate ((t L H) (1.0 10 62 cm) Modified Surface A—Fig. 7 (a)) has been prepared by machining similar horizontal inclined recessed groves. Same set of experiments have been carried out on Modified Surface A. Fig. 8(a) shows the flow patterns of the Modified Surface A, wetting factor is found to increase with increase in flow rates. Fig. 9 shows the comparison of wetting factor on plain PP and Modified Surface A. The average wetting factor for Surface A is found to be 0.67, which is around 45.7% more than plain PP and 13.6% more than metallic Al surface. Through the flow visualization during the experiment it is found that area remaining between two consecutive inclined recesses remains non-wetted especially at low flow rates. Hence, Modified Surface B—Fig. 7(b) (3 mm depth of cut with no gap between two consecutive slots) has been prepared for improving the wetness level from Modified Surface A. Fig. 8(b) shows the flow patterns of Modified Surface B. It is found that average wetting factor of Modified Surface B is (0.73), around 9.0% more than Modified Surface A. Modified Surface B significantly improves wetting characteristics of PP surface at high flow rate but at low flow rates wetting factor improvement is not encouraging. Further, to find optimum depth of recesses for achieving maximum wetting, Modified Surface C (1.5 mm depth of cut with no gap between two consecutive slots) and Modified Surface D (0.5 mm depth of cut with no gap between two consecutive slots) have been prepared. Average wetting factors for Modified Surface C and Modified Surface D are found to be 0.83 and 0.57 respectively. Modified surface C improves average wetting factor of plain PP by around 80.4%, which is around 40.7% more than plain metallic surface Al. Modified Surface C wetting factor is around 20.3% higher than SLS 300 ppm solution wetting factor. 3.4. Effect of solution temperature on wetting characteristics
Fig. 5. Contact angle and wetting factor variation at different SLS concentration on plain PP test plate.
For many applications such as cooling tower, solar desalination, and regenerators in liquid desiccant systems, solution temperature requirement is in the range of 55–85 C. Experiments have been carried on plain PP, plain Al, plain PP with SLS 300 and Modified Surface C surfaces at elevated solution temperatures (40 and 60 C) for finding the influence of temperature on wetting factor. It is found that wetting factor increases in all above cases with increase in temperature, shown in Fig. 10. This can be credited to the decrease in surface tension of solution at elevated temperature. Surface tension decrease of water is found to be 4.6% at 40 C and
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Fig. 6. Photograph of preliminary surfaces (a) vertical slots, (b) horizontally slots, (c) diamond shape, (d) horizontal inclined recessed.
Fig. 7. Schematic representation slot geometry and pictorial side view of (a) Modified Surface A (b) Modified Surface B (c) Modified Surface C (d) Modified Surface D.
9.3% at 60 C in comparison to initial temperature of 18 C [21]. Wetting factor of plain PP increases by 13.0% and 30.4% at solution temperature of 40 and 60 respectively, whereas wetting factor of
plain Al surfaces increases just by 11.9% and 18.6% for the same temperature range. Hence, effect of solution temperature increase on wetting factor is more significant for plain PP surface in
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Fig. 8. Flow patterns on test plates (a) Modified Surface A (b) Modified Surface B (c) Modified Surface C (d) Modified Surface D on Side I-1,2,3,4 and Side II- 10,20 ,30 ,40 (1, 10 – 0.038 kg/s, 2, 20 –0.083 kg/s, 3, 30 –0.139 kg/s, 4, 40 –0.189 kg/s).
comparison to plain Al surface. Similarly, wetting factor increases by 10.14% and 14.5% for SLS 300 solution, whereas wetting factor increase is found to be 4.8% and 7.2% for Modified Surface C for the same temperature increase. Wetting factor of Modified Surface C is found to be 12.7% higher than SLS 300 solution even at 60 C.
Ma ¼
ma
3.5. Wetting factor correlation ln ¼ Estimation of actual gas-liquid contact area (wetted area) and liquid film thickness are important for the accurate designing and modeling purpose of falling film tower. As per general practice, entire surface area of solid plate/tube is generally assumed to be wetted. But, it is found that wetted area is much smaller than solid surface area at low flow rates and especially for non-metallic surfaces. This may result in to under sizing of the falling film tower. In the open literature many equations are available for the prediction of liquid film thickness around the vertical plates [22–25] and tubes [7,26]. But, only one equation (Eq. (2)) is available in the open literature for the estimation of wetted area [27]. Similarly, heat and mass transfer improvements offered by surface modification methods had been verified by many past researchers [9,15–18] but they did not try to estimate the wetted area improvement provided by these techniques. Aw ¼ 1 1:15 106 Ka0:472 e21:8jMa j Aw;o
@s ðT T Þ l ln n L l @T s
n2
13
g
ð2Þ
where Ka ¼
g m4
rs 3 Fig. 9. Effect of surface modification on wetting factor.
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three non dimensional terms film Reynolds Number of liquid, area enhancement ratio and ratio of surface tension (assuming water at 25 C as the reference fluid). Using all experimental observations of the current study, new wetting factor correlation (Eq. (3)) has been developed using MATLAB version 7.7.0 (R2008b) regression tool. 0:03 As;ext 0:43 10:17ðRef Þ0:24 gs As e¼ ð3Þ 0:55 sl sWater
where film Reynolds number and extended surface area of Modified Surfaces are calculated as: Ref ¼
4ml
mW
As;ext ¼ 2WZ p þ
Fig. 10. Effect of temperature on wetting factor.
The above equation showed poor response for the current experimental observations (MAPE 24.6% for plain Al, MAPE 51.2% for plain PP, MAPE 18.6% for SLS 300, MAPE 45.3% for Modified Surface A, MAPE 22.8% for Modified Surface B, MAPE 21.6% for Modified Surface C and 45.6% for Modified Surface D). The large error in their correlation may be due to negligence of solid surface characteristics. Therefore, it is decided to develop generalized correlation including the solid surface energy, and
! d d u þ 3u Cos 4 Cos 4
Slot geometry parameters p, d and u are shown in Fig. 7. Proposed equation predicts wetting factor of plain and modified surfaces with good accuracy (MAPE 8.9%). MAPE in current study has been calculated by Eq. (4). Validity of the new correlation (Eq. (3)) has also been checked by comparing its prediction against of past experimental studies carried by authors [8,14,27,28]. Table 1 presents the summary of MAPE values of current Eq. (3) and Eq. (2) against of these studies. From the MAPE values it is clear that current proposed correlation is better suited for the estimation of wetted area than equation suggested by Zhang et al. [27]. Fig. 11 shows the comparison of calculated results of wetting factor with experimental observations of current and past studies. Around 88% of data points are found to lie within error band of 15% for the current correlation. MAPE ¼
n ew 1X j n i¼1
ew prediction;i j 100% ew experiment;i
experiment;i
ð4Þ
4. Conclusion In the present work, wetting characteristics of plain Al and plain PP surfaces have been experimentally investigated with the purpose of finding suitable alternative of corrosion prone metallic surface to be used as gas liquid contacting surface in falling film towers. Average wetting factor of plain Al and PP are found to be 0.59 and 0.46 for the flow range of (0.038–0.189 kg/s). Two wetting characteristic improvement methods: surfactant addition and surface modification technique have been used for improving the wetting characteristic of PP surface. SLS surfactant solutions of concentration (60, 100, 140, 200, 300 ppm) have been tested in present study, local maxima of wetting factor is found between 60 ppm and 140 ppm. The average wetting factor of SLS 300 on plain PP is found to be around 50% higher than plain PP with water and around 17% more than plain Al with water. For the surface
Fig. 11. Comparison of experimental and calculated results of wetting factor.
Table 1 Parameters range and MAPE for correlation. Source
Qi et al. [8] Kang et al. [14] Zhang et al. [27] o Quan et al. [28] Current o
n
12 11 10 20 89
Solid-liquid pair
S.S.-LiCl Al-Water, Al-Titron X100 S.S.-Water S.S.-Water P.P.-Water, P.P.- SLS, Al-Water
MAPE (%)
ml
W
m
gs
sl
(kg/s)
(m)
(103 Pa s)
(J/m2)
(N/m)
–
Zhang et al. [27]
Current model
0.02–0.20 0.06 0.03–0.36 0.016–0.033 0.038–0.189
0.65 0.27 0.09 0.10 0.19
0.92 0.9–1.002 0.546 0.8007–0.3565 0.467–1.002
1.1 0.84 1.1 1.1 0.03, 0.84
0.083–0.085 0.031–0.072 0.067 0.059–0.065 0.059–0.072
1 1 1 1 1–1.85
146.4 12.9 20.8 20.4 33.1
13.6 6.0 10.2 10.0 7.6
Isothermal condition, n—number of experimental points.
As;ext As
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modification technique it is found that inclined recess provides maximum liquid holding. Four types of inclined recess surfaces have been prepared for optimizing the depth of cut and structure of surface. Maximum wetting factor is found for Modified Surface C (depth of cut 1.5 mm and no gap between two consecutive inclined recesses). Average wetting factor of Modified Surface C is found to be 0.83, which is around 80.4% more than plain PP surface and around 40.7% more than plain metallic Al surface. Importantly, Modified Surface C provides significant wetting factor improvement in low flow rate range. Hence, it can expedite performance of falling film tower more effectively in comparison to surfactant addition method. Effect of solution temperature increase on wetting factor increase is found to be more pronounced for plain PP surface than plain Al surface. A new generalized correlation has been developed for the estimation of wetting factor. Proposed correlation shows very good agreement (average MAPE 8.5%) for different combination of solid-liquid pair. Acknowledgement The authors acknowledge financial help provided by the Department of Science and Technology (SB/FTP/ETA-0039/2014), Govt. of India for carrying out current work. The funding organization has not played any role in study design, decision to publish or preparation of the manuscript. References [1] V.M. Ramm, Absorption of Gases, 2nd ed., Israel Program for Scientific Translation, Jerusalem, 1968. [2] N. Fumo, D.Y. Goswami, Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration, Sol. Energy 73 (2002) 351–361. [3] K.C. Schifftner, Air Pollution Control Equipment Selection Guide, 2nd ed., CRC Press, Taylor & Francis group, 2014. [4] R. Kumar, P.L. Dhar, S. Jain, Development of new wire mesh packing for improving the performance of zero carry over spray tower, Energy 36 (2) (2011) 1362–1374. [5] R. Kumar, P.L. Dhar, S. Jain, A.K. Asati, Multi absorber stand alone liquid desiccant air-conditioning systems for higher performance, Sol. Energy 83 (2009) 761–772. [6] S. Jain, Studies on desiccant augmented evaporative cooling systems, Ph.D. Thesis, IIT Delhi, India, 2007. [7] C. Luo, W. Ma, Y. Gong, Design of single vertical tube falling-film evaporation basing on experiment, J. Loss Prev. Process Ind. 24 (2011) 695–698. [8] R. Qi, L. Lu, H. Yang, F. Qin, Investigation on wetted area and film thickness for falling film liquid desiccant regeneration system, Appl. Energy 112 (2013) 93–101.
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[9] D. Glebov, F. Settervall, Experimental research of heat exchange and distribution of viscous liquids on vertical lamels, Chem. Comput. Simul. Butlerov Commun. 2 (2002) 65–68. [10] A. Gonda, P. Lancereau, P. Bandelier, L. Luo, Y. Fan, S. Benezech, Water falling film evaporation on a corrugated plate, Int. J. Therm. Sci. 81 (2014) 29–37. [11] R. Goss, Practical guide to adhesive bonding of small engineering plastic and rubber Parts, Smithers Rapra Technol. (2010) 93–99. [12] D. Glebov, F. Settervall, Experimental study of heat transfer additive influence on the absorption chiller performance, Int. J. Refrig. 25 (2002) 538–545. [13] W.L. Cheng, K. Houda, Z.S. Chen, A. Akisawa, P. Hu, T. Kashiwagi, Heat transfer enhancement by additive in vertical falling film absorption of H2O/LiBr, Appl. Therm. Eng. 24 (2004) 281–298. [14] B.H. Kang, K.H. Kim, D.Y. Lee, Fluid flow and heat transfer on a falling liquid film with surfactant from a heated vertical surface, J. Mech. Sci. Technol. 21 (2007) 1807–1812. [15] J.K. Kim, C.W. Park, Y.T. Kang, The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber, Int. J. Refrig. 26 (2003) 575–585. [16] B. Koroglu, K.S. Lee, C. Park, Nano/micro-scale surface modifications using copper oxidation for enhancement of surface wetting and falling-film heat transfer, Int. J. Heat Mass Transf. 62 (2013) 794–804. [17] L. Hoffmann, I. Greiter, A. Wanger, V. Weiss, G. Alefeld, Experimental investigation of heat transfer in a horizontal tube falling film absorber with aqueous solutions of LiBr with and without surfactants, Int. J. Refrig. 19 (1996) 331–341. [18] D.S. Kim, C.A.I. Ferreira, Flow patterns and heat and mass transfer coefficients of low Reynolds number falling film flow on vertical plates: effects of wire screen and an additives, Int. J. Refrig. 32 (2008) 138–149. [19] H. Zhang, X. Yuan, K. Yu, M.A. Kalbassi, B. Liu, Influence of the marangoni effect on mass transfer in inclined falling films, Chem. Eng. Technol. 37 (7) (2014) 1095–1102. [20] F. Zhang, Z. Zhang, J. Geng, Study on shrinkage characteristics of heated falling liquid film, AIChE J. 51 (2005) 2899–2907. [21] N.B. Vargaftik, B.N. Volkov, L.D. Voljak, International tables of the surface tension of water, J. Phys. Chem. Ref. Data. 12 (1983) 817–820. [22] W. Ambrosini, N. Forgione, F. Oriolo, Statistical characteristics of a water film falling down a flat plate at different inclinations and temperatures, Int. J. Multiphase. Flow 28 (2002) 1521–1540. [23] K. Moran, J. Inumaru, M. Kawaji, Instantaneous hydrodynamics of a laminar wavy liquid film, Int. J. Multiphase. Flow 28 (2002) 731–755. [24] F. Zhang, J. Peng, J. Geng, Z.X. Wang, Z.B. Zhang, Thermal imaging study on the surface wave of heated falling liquid films, Exp. Therm. Fluid Sci. 33 (2009) 424–430. [25] Y.Q. Yu, S.J. Wei, Y.H. Yang, X. Cheng, Experimental study of water film falling and spreading on a large vertical plate, Prog. Nucl. Energy 54 (2012) 22–28. [26] W.P. Yan, X.M. Ye, H.T. Li, Flow and heat transfer characteristics of liquid thin film flow, J. North. China. Electr. Power Univ. 32 (1) (2005) 59–65. [27] F. Zhang, X. Zhao, J. Geng, Y.T. Wu, Z. Zhang, A new insight into marangoni effect in non-isothermal falling liquid films, Exp. Therm. Fluid Sci. 31 (2007) 361–365. [28] X. Quan, Y. Geng, P. Yuan, Z. Huang, C. Liu, Experiment and simulation of the shrinkage of falling film upon direct contact with vapor, Chem. Eng. Sci. 135 (2015) 52–60.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Wetting enhancement of polypropylene plate for falling film tower application Digvijay Patila , Ritunesh Kumara,* , Fu Xiaob a b
Mechanical Engineering Department, Indian Institute of Technology Indore, MP, 453446, India Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 30 May 2016 Accepted 5 June 2016 Available online 14 June 2016 Keywords: Falling film tower Polypropylene Surface modification Surfactant Wetting factor
A B S T R A C T
Falling film towers are popular for simultaneous heat and mass transfer applications due to modest air pressure drop and low energy demanding liquid distribution. Conventionally, metals are used as gas liquid contacting surface in these towers. Corrosion prone metallic surfaces need expensive anticorrosive coating or frequent replacement, whereas non metallic surfaces such as plastics have poor wetting characteristics. In current study, wetting factor of plain polypropylene and aluminum plates has been measured. Wetting enhancement methods: surfactant addition (sodium lauryl sulfate-SLS) and surface modification have been used on polypropylene plate. Improvement of 50% (SLS 300 ppm) and 80% (Modified Surface C) wetting characteristics of polypropylene have been achieved. Modified Surface C average wetting factor is 41% superior to aluminum surface. A new generalized empirical correlation for wetted area estimation considering influences of solid surface and fluid characteristics has been proposed. Proposed equation shows good agreement with current and past studies (MAPE 8.5%). ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conventionally three types of towers namely packed bed, spray and falling film towers are used in simultaneous heat and mass transfer applications [1]. For transfer of potential (heat and mass) from one fluid to another fluid in these chemical towers, one of the fluids (liquid) is dispersed across continuous phase of another fluid (gas). In the spray towers, liquid dispersion is required in form of fine liquid droplets. In case of the falling film towers, liquid flows in form of thin straight liquid film on solid surface. In case of the packed bed towers, liquid moves in form of distorted liquid film at solid surfaces and as liquid sheet between solid surfaces. Out of above, the packed bed towers are the most versatile and extensively used design. They relish benefit of high efficiency and very large contact area to volume ratio. Major drawbacks of these are high air side pressure drop and high ml requirement [2]. Spray towers are quite popular in air pollution control [3]. Advantages associated with spray towers are simple design, and low initial and maintenance cost. Their major
Abbreviations: Al, aluminum; LiCl, lithium chloride solution; LPM, liter per minute; MAPE, mean average percentage error; PP, polypropylene; ppm, parts per million; SLS, sodium lauryl sulfate; S.S., stainless steel. * Corresponding author. E-mail addresses: [email protected], [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.cep.2016.06.005 0255-2701/ã 2016 Elsevier B.V. All rights reserved.
limitations are: poor efficiency, ineffective/absence of external heat interaction, high speeding liquid droplets (poor residence time of liquid droplets) and large liquid side pressure drop [4]. Falling film towers are better equipped to handle limitations of both packed bed and spray towers. Thin film of liquid attributes towards high heat transfer & mass transfer coefficient. Their air side pressure drop is quite less in comparison to the packed bed towers and they can operate efficiently at low ml/ma ratio. Liquid jet can be easily dispersed in form of a falling film at the expanse of less energy than the spray tower. Distinguished feature of the falling film tower is the instant removal/addition of heat along with mass transfer operation. Total power requirement is even lesser than packed bed and spray towers [5]. Hence, processes that can abide moderate parasitic losses use falling film towers (i.e. cooling tower, absorption refrigeration). Two main limitations of the falling film towers are the incomplete wetting of the solid surfaces and non-uniform distribution of the liquid across solid surfaces [6]. Unlike packed bed towers and spray tower, state-of-the-art of the falling film tower is still not matured. Proficient designs of liquid distributors and highly efficient solid liquid contacting surfaces (high wetting at low flow rates) can help in upgrading the performance of the falling film tower up to the packed bed towers. Many researchers have carried out experiments related to design of liquid distributor with objectives of either to ensure
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Nomenclature
Greek symbols ew Wetting factor a Thermal diffusivity (m2/s) r Density (kg/m3) s Surface tension (N/m) @s Partial differential of surface tension of liquid @T m Viscosity (Pa s) n Kinematic viscosity (m2/s) g Surface energy (J/m2) u End cutting edge angle ( ) Symbols A Area (cm2) Aw Area ratio in Eq. (2) Aw;o B Breadth (cm) d Depth of cut (mm) g Acceleration due to gravity (m/s2) H Height (cm) L Length (cm) ln Viscosity length scale (m) Ka Kapitza number Ma* Modified Marangoni number m Mass flow rate (kg/s) n Number of data points p Distance between two consecutive slots (cm) Re Reynolds number Ref film Reynolds number T Temperature ( C) Tl Average liquid film temperature ( C) t Thickness (cm) W Width of plate (cm) Z Number of slots Sub- and Superscripts a Air ext Extended i Interception point l Liquid o Isothermal condition s Solid surface w Wet
uniform flow of liquid across each solid surface or to ensure stable liquid film across solid surfaces. Luo et al. [7] designed a tangential inlet (liquid flow direction) single vertical tube falling film evaporator. They investigated the effect of an annular slit opening (0.5–2 mm) and liquid spray density on the film uniformity and stability. They recommended the annular slit opening of 1.5–2 mm and spray density between 0.07–0.19 kg/ (m s) for optimum film stability, distributor inlet tube was rotated tangentially by 270 inside distributor header for ensuring uniform film thickness around whole periphery. Qi et al. [8] investigated experimentally the factors affecting the wetted area and film thickness on single channel internally heated vertical regenerator of a liquid desiccant system in the mass flow range of 0.025–0.15 kg/s. They observed that due to increase in slit opening from 1.0 to 1.25 mm decreased the wetted area by around 51%. A rectangular box distributor with parallelly welded lamellas was suggested by Glebov and Settervall [9], liquid was fed in the distributor through three inlet pipes. They reported that
minimum specific flow rate has to be 0.26 kg/(m s) for full wetting of lamella type surface. Distributor and redistribution components in combination were used by Gonda et al. [10] for getting uniform distribution of solution on both sides of a plate. Water flows firstly through the distributor, each side edges of distributor are drilled 13 semi-cylindrical holes of 3 mm diameter. Then, water flows along the internal wall of the header until it reaches the redistributor. They reported removal of dry patch formation due to shrinkage of liquid film with proposed assembly. Wetting or adhesion of the liquid on the solid surface depends on properties of contacting mediums i.e. surface tension of the liquid and surface energy of the solid. Hence, spreading of the liquid on the solid surface can be improved by increasing surface energy of solid and reducing surface tension of liquid [11]. Many experimental studies have been carried out by researchers using surfactant addition or surface modification techniques as wetting enhancing medium. Effect of the surfactant 2-methyl-1-pentanol (500–700 ppm) on absorption chiller (LiBr/H2O) was investigated by Glebov and Settervall [12]. They found 30–35% overall enhancement in cooling capacity of chiller. Cheng et al. [13] experimentally investigated the effect of the surfactants (2-ethyl1-hexanol and 1- octanol) on the heat transfer in falling film absorber. They found improvement up to 100% in heat transfer coefficient with optimum concentration at (40 ppm) for these surfactants and noticed decrease in the heat transfer coefficient with any further increase in the surfactant concentration. Kang et al. [14] observed 34% improvement in the wetted area with triton X 100 surfactant (500 ppm) addition in water. Kim et al. [15] investigated the effect of enhanced surface (micro-scale hatched tube) on horizontal falling film tubular absorber using LiBr/H2O solution. They reported 10% improvement in wetting characteristic of a hatched tube surface than smooth one. Koroglu et al. [16] studied the effect of copper oxide layer deposition on copper tubes in the horizontal tube falling film tower. They reported full wetting in case of the oxidized tubes at low flow rate (Re 86) than the plain tube (Re 114). Hoffman et al. [17] used knurled tubes and 2ethyl-1-hexanol (80 ppm) for the horizontal tube falling film tower. They found 20–40% improvement in mass transfer coefficient for knurled tubes and 60–140% improvement for surfactant addition case. Kim and Ferreria [18] experimentally compared the effects of modified surface geometry (clamping of copper wire mesh on copper surface) and a surfactant 2-ethyl-1hexanol (100 ppm) addition on film pattern of copper plate using LiBr–water absorbent solution. They reported enhancement in mass transfer of 40% and 60–110% through copper wire clamped surface and surfactant addition respectively. Zhang et al. [19] experimentally investigated the role of Marangoni effect on the mass transfer of falling film tower (inclined SS plates). They proposed generalized effective enhancement factor correlation in terms of ethanol concentration, inclination angle of surface and liquid flow condition. Use of metallic gas-liquid contacting surface in the falling film tower is very common due to good wettability of metallic surface. But, corrosion problem is the main limitation of metallic surface and it mandates the replacement of solid surfaces after certain time interval thus adding up significant maintenance cost. Plastic plates can be used as an alternative of metallic surface provided wetting characteristics of plastic surface can be elevated up to metallic surface. They have excellent corrosion resistance property, low cost and light weight. In the present investigation, wetting factor of aluminum (Al) and polypropylene (PP) plates have been experimentally measured. Effect of the sodium lauryl sulfate (SLS) surfactant addition and the surface modification technique have been used for improving the wetting characteristics of PP for vertical falling film tower application.
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2. Experimental set up and procedure In order to ensure accurate measurement of wetting pattern on both sides of the solid surface, single plate falling film tower setup has been fabricated (Fig. 1 (a)). The set up consists of distributor header, vertical tower, solution tanks, and solution supply and return lines. Except of vertical tower, rest of the setup has been from PP sheet (t = 10 mm) and PP pipe (1/200 inch dia.). Welding of PP sheet has been carried out using hot air gun machine (Bosch, GHG 630). Distributor header is a hollow square (L x B) (22.5 22.5) cylinder of H = 18 cm. The topside of the distributor has been kept open for regular cleaning of slit opening around test plate. Pocket in the bottom plate of the distributor header provides tight holding of the test plate and slit opening of 1 mm on both side provides scope of uniform liquid distribution from both faces of the test plate. This pocket has been cut on CNC machine. Vertical tower (B L H) (15 15 60) is hollow cylindrical structure made from acrylic sheet (t = 4 mm) for clear visualization of liquid film. Vertical tower houses the test plate of size (t L H) (1.0 10 62). PP solution tanks of capacity 100 l (Collection tank) and 200 l (supply tank) have been used. Solution heater of 3.5 kW controlled with PID controller (0.5 C of set value) has been installed in the supply tank for analyzing the effect of solution temperature on wetting characteristics. Red color added (for clear visualization of wetted area) water has been used as the working fluid. Solution line consists of supply line, bypass line and return line. Desired flow rate through the supply line can be set with the help of two ball valves (3 & 30 ) and monitored with help of rotameter (1.5–15 LPM), monoblock pump (Kirloskar, 0.5 HP) has been used to circulate the liquid across supply loop. Solution is conveyed back to the supply tank with help of submerged pump (0.25 HP). Camera (Canon, 550 D) and light source (FloLight, MicroBeam128) have been used to capture images of flow pattern. Camera is leveled at the center of vertical test plate to avoid any vertical asymmetry in post processing of captured image. Contact angle of surfactant solutions (60–300 ppm of SLS) on plain test plate of PP has been measured by goniometer (Dataphysics, OCA 15 EC).
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Before each run of test, water is first set at desired value and then thoroughly remixed by closing ball valve 3 for 10 min. Afterwards, required solution flow rate at specific temperature is set precisely by mutual regulation of two ball valves 3 & 30 . The system is then allowed to stabilize for 10 min. Subsequently four images of each side (Side I–facing liquid distributor and Side II– away from liquid distributor) are captured at the time span of 1 min. Four images have been taken in order to avoid any chance of error due to flow pattern fluctuations. Post processing of these images for wetted area estimation has been carried out using ImageJ software. The maximum difference in estimated wetted area is found to be up to 1%. 3. Results and discussion 3.1. Flow patterns and wetting factor of plain PP and Al surfaces Fig. 2(a) and (b) show the flow patterns of falling film on both sides of plain Al and plain PP surfaces with increasing flow rate of solution. In general liquid flow pattern is found to be similar except at low flow rate of 0.038 kg/s, irregular liquid films form at low flow rate on Side I. Also, only at low flow rate wetted area of Side II is found to be larger than Side I. It is found that average wetting factor of Side I (of Al and PP) is 9.0% higher than of Side II. Higher wetting factor of Side I can be attributed to high velocity of liquid approaching slit opening of Side I. In the beginning liquid film contracts in the transverse direction, deformed liquid revolute forms at both side ends due to these contraction, these liquid revolutes were named as liquid rims [20]. The pattern of liquid falling film depends very much on the dynamics and size of these liquid rims. These liquid rims gradually approaches towards each other, rate of approach of liquid rims decreases with increase in flow rate. If these rims intercept each other at a point (apex of rims) then liquid film expands slightly in downward direction. Distance of apex (Hi) from the top and length of liquid film at apex (Li) both increases with increase in flow rates. If they do not intercept at a clear distinct point then liquid film does not expand in a lower half portion. On comparing the flow patterns of PP and Al surface it is
Fig. 1. (a) Schematic diagram of experimental set up (1–supply tank, 2–pump, 3 & 30 –ball valve, 4–rotameter, 5–supply line, 6–distributor header, 7–vertical tower, 8–solid surface, 9–collection tank, 10–submerged pump) (b) design of bottom plate of distributor header.
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Fig. 2. Flow patterns on test plates (a) Plain Al (b) Plain PP (c) Plain PP with SLS 300 ppm on Side I-1,2,3,4 and Side II- 10,20 ,30 ,40 (1, 10 –0.038 kg/s, 2, 20 –0.083 kg/s, 3, 30 –0.139 kg/ s, 4, 40 –0.189 kg/s).
found that liquid film contraction (liquid rims approach rate) is more rapid in case of PP surface. It is also clear from Fig. 2 that contraction length and height ratios for plain PP are lower than plain Al surface. Wetting factor has been calculated using Eq. (1).
ew ¼
Aw As
ð1Þ
Wetting factor for both surfaces has been measured and average wetting factor of each plate (both sides) has been reported in
present study. Fig. 3 compares the wetting factor of PP and Al surface with increasing flow rates. Wetting factor for both surfaces increases with increase in flow rates. Average wetting factors of plain PP and Al surfaces are found to be 0.46 and 0.59 respectively. Especially at low flow rate wetting characteristics of plain PP surface is very poor in comparison to Al test plate. Hence, finding suitable wetting augmentation method for PP is the main concern such that PP surface can be used satisfactorily as replacement of Al surface in falling film towers even for low flow rate applications. It is well evident that wetting characteristics of the surface can be improved by reducing the surface tension of the liquid. Wetting of solid surface can also be improved in case liquid rim formation at edges can be disrupted forcibly to allow liquid spread in transverse direction towards dry patches. The same has been achieved in current study through surface modification technique. These two methods have been explored as possible ways of improving wetting characteristics of PP surface. 3.2. Effect of surfactant on wetting factor of plain PP surface
Fig. 3. Comparison of wetting factor for PP and Al.
Surfactants are compound that are used to lower the surface tension of the solution. They are extremely popular for variety of applications such as detergent, emulsifier, foaming agent and of course as wetting agent. SLS solution of different concentrations (60, 100, 140, 200 and 300 ppm) in water have been used in current work. Fig. 2(c) shows film patterns of 300 ppm surfactant solution on PP surface. Similar patterns are observed for other SLS solution concentration. It is clear that in case of SLS surfactant solution film contraction reduces as compared to plain PP plate. Fig. 4 shows the effect of SLS solution concentration on wetting factor of plain PP with increase in mass flow rate. It is observed that significant
D. Patil et al. / Chemical Engineering and Processing 108 (2016) 1–9
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system in case carried away with process air. Other potential limitations of surfactant additions are biodegradation, settling down, clogging of surfactant at bottom of tank and pump parts. 3.3. Effect of surface modification
Fig. 4. Effect of SLS solution concentration on wetting factor of plain PP test plate.
improvement in wetting factor is offered until SLS solution concentration of 100 ppm. This can be attributed to decrease in solid liquid contact angle with increase in solution concentration. Thus, solution spreads uniformly across entire solid surface. On increasing SLS solution concentration from 100 to 140 ppm, little improvement in wetting factor has been observed. At 200 ppm solution even slight decrease in wetting factor has been found. Wetting factor is again found to have increasing trend in the solution concentration range of 200–300 ppm. To study the reason behind this trend, contact angle on plain PP sheet of different SLS solution concentration have been measured. Fig. 5 shows variation of contact angle and average wetting factor with increased SLS solution concentration. Contact angle variation closely justifies the trend observed for wetting factor variation. Contact angle significantly reduces up to the solution concentration of 100 ppm then it remains almost constant up to the solution concentration of 140 ppm. At 200 ppm slight decrease in contact angle is observed then again it reduces on increasing concentration. Average wetting factor in case of 60, 100, 140, 200, and 300 ppm SLS solution are found to be 0.56, 0.62, 0.64, 0.62 and 0.69 respectively. 100 ppm SLS solution concentration seems to be optimal for air-conditioning applications considering side effects of high surfactant concentration on skin, eyes and respiratory
In order to decide final design of texture required on PP surface for enhancing vertical plate wetting characteristics, four primary surfaces are fabricated; vertically slotted, horizontally slotted, diamond shape and horizontal inclined recessed. These surfaces are shown in Fig. 6. For all these surfaces depth of cut and distance between two slots have been kept 3 mm and 4 mm respectively. First two surfaces are machined with end mill tool 3 mm diameter and remaining two surfaces are prepared by single point cutting tool (u = 60 ). These surfaces are tested to check their liquid retaining capability in transverse direction while keeping surfaces vertically held. Surfaces are dipped completely in the solution (red colour in water). Then, they are taken out of solution one by one and after settling time of 1 min; images have been taken. Fig. 6 shows liquid retention at the end of experiment on machined surfaces. It is clear that horizontal inclined recessed (opposite to flow direction or gravity) surface retains maximum solution. Hence, this texture has been finalized for further study and one PP test plate ((t L H) (1.0 10 62 cm) Modified Surface A—Fig. 7 (a)) has been prepared by machining similar horizontal inclined recessed groves. Same set of experiments have been carried out on Modified Surface A. Fig. 8(a) shows the flow patterns of the Modified Surface A, wetting factor is found to increase with increase in flow rates. Fig. 9 shows the comparison of wetting factor on plain PP and Modified Surface A. The average wetting factor for Surface A is found to be 0.67, which is around 45.7% more than plain PP and 13.6% more than metallic Al surface. Through the flow visualization during the experiment it is found that area remaining between two consecutive inclined recesses remains non-wetted especially at low flow rates. Hence, Modified Surface B—Fig. 7(b) (3 mm depth of cut with no gap between two consecutive slots) has been prepared for improving the wetness level from Modified Surface A. Fig. 8(b) shows the flow patterns of Modified Surface B. It is found that average wetting factor of Modified Surface B is (0.73), around 9.0% more than Modified Surface A. Modified Surface B significantly improves wetting characteristics of PP surface at high flow rate but at low flow rates wetting factor improvement is not encouraging. Further, to find optimum depth of recesses for achieving maximum wetting, Modified Surface C (1.5 mm depth of cut with no gap between two consecutive slots) and Modified Surface D (0.5 mm depth of cut with no gap between two consecutive slots) have been prepared. Average wetting factors for Modified Surface C and Modified Surface D are found to be 0.83 and 0.57 respectively. Modified surface C improves average wetting factor of plain PP by around 80.4%, which is around 40.7% more than plain metallic surface Al. Modified Surface C wetting factor is around 20.3% higher than SLS 300 ppm solution wetting factor. 3.4. Effect of solution temperature on wetting characteristics
Fig. 5. Contact angle and wetting factor variation at different SLS concentration on plain PP test plate.
For many applications such as cooling tower, solar desalination, and regenerators in liquid desiccant systems, solution temperature requirement is in the range of 55–85 C. Experiments have been carried on plain PP, plain Al, plain PP with SLS 300 and Modified Surface C surfaces at elevated solution temperatures (40 and 60 C) for finding the influence of temperature on wetting factor. It is found that wetting factor increases in all above cases with increase in temperature, shown in Fig. 10. This can be credited to the decrease in surface tension of solution at elevated temperature. Surface tension decrease of water is found to be 4.6% at 40 C and
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Fig. 6. Photograph of preliminary surfaces (a) vertical slots, (b) horizontally slots, (c) diamond shape, (d) horizontal inclined recessed.
Fig. 7. Schematic representation slot geometry and pictorial side view of (a) Modified Surface A (b) Modified Surface B (c) Modified Surface C (d) Modified Surface D.
9.3% at 60 C in comparison to initial temperature of 18 C [21]. Wetting factor of plain PP increases by 13.0% and 30.4% at solution temperature of 40 and 60 respectively, whereas wetting factor of
plain Al surfaces increases just by 11.9% and 18.6% for the same temperature range. Hence, effect of solution temperature increase on wetting factor is more significant for plain PP surface in
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Fig. 8. Flow patterns on test plates (a) Modified Surface A (b) Modified Surface B (c) Modified Surface C (d) Modified Surface D on Side I-1,2,3,4 and Side II- 10,20 ,30 ,40 (1, 10 – 0.038 kg/s, 2, 20 –0.083 kg/s, 3, 30 –0.139 kg/s, 4, 40 –0.189 kg/s).
comparison to plain Al surface. Similarly, wetting factor increases by 10.14% and 14.5% for SLS 300 solution, whereas wetting factor increase is found to be 4.8% and 7.2% for Modified Surface C for the same temperature increase. Wetting factor of Modified Surface C is found to be 12.7% higher than SLS 300 solution even at 60 C.
Ma ¼
ma
3.5. Wetting factor correlation ln ¼ Estimation of actual gas-liquid contact area (wetted area) and liquid film thickness are important for the accurate designing and modeling purpose of falling film tower. As per general practice, entire surface area of solid plate/tube is generally assumed to be wetted. But, it is found that wetted area is much smaller than solid surface area at low flow rates and especially for non-metallic surfaces. This may result in to under sizing of the falling film tower. In the open literature many equations are available for the prediction of liquid film thickness around the vertical plates [22–25] and tubes [7,26]. But, only one equation (Eq. (2)) is available in the open literature for the estimation of wetted area [27]. Similarly, heat and mass transfer improvements offered by surface modification methods had been verified by many past researchers [9,15–18] but they did not try to estimate the wetted area improvement provided by these techniques. Aw ¼ 1 1:15 106 Ka0:472 e21:8jMa j Aw;o
@s ðT T Þ l ln n L l @T s
n2
13
g
ð2Þ
where Ka ¼
g m4
rs 3 Fig. 9. Effect of surface modification on wetting factor.
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three non dimensional terms film Reynolds Number of liquid, area enhancement ratio and ratio of surface tension (assuming water at 25 C as the reference fluid). Using all experimental observations of the current study, new wetting factor correlation (Eq. (3)) has been developed using MATLAB version 7.7.0 (R2008b) regression tool. 0:03 As;ext 0:43 10:17ðRef Þ0:24 gs As e¼ ð3Þ 0:55 sl sWater
where film Reynolds number and extended surface area of Modified Surfaces are calculated as: Ref ¼
4ml
mW
As;ext ¼ 2WZ p þ
Fig. 10. Effect of temperature on wetting factor.
The above equation showed poor response for the current experimental observations (MAPE 24.6% for plain Al, MAPE 51.2% for plain PP, MAPE 18.6% for SLS 300, MAPE 45.3% for Modified Surface A, MAPE 22.8% for Modified Surface B, MAPE 21.6% for Modified Surface C and 45.6% for Modified Surface D). The large error in their correlation may be due to negligence of solid surface characteristics. Therefore, it is decided to develop generalized correlation including the solid surface energy, and
! d d u þ 3u Cos 4 Cos 4
Slot geometry parameters p, d and u are shown in Fig. 7. Proposed equation predicts wetting factor of plain and modified surfaces with good accuracy (MAPE 8.9%). MAPE in current study has been calculated by Eq. (4). Validity of the new correlation (Eq. (3)) has also been checked by comparing its prediction against of past experimental studies carried by authors [8,14,27,28]. Table 1 presents the summary of MAPE values of current Eq. (3) and Eq. (2) against of these studies. From the MAPE values it is clear that current proposed correlation is better suited for the estimation of wetted area than equation suggested by Zhang et al. [27]. Fig. 11 shows the comparison of calculated results of wetting factor with experimental observations of current and past studies. Around 88% of data points are found to lie within error band of 15% for the current correlation. MAPE ¼
n ew 1X j n i¼1
ew prediction;i j 100% ew experiment;i
experiment;i
ð4Þ
4. Conclusion In the present work, wetting characteristics of plain Al and plain PP surfaces have been experimentally investigated with the purpose of finding suitable alternative of corrosion prone metallic surface to be used as gas liquid contacting surface in falling film towers. Average wetting factor of plain Al and PP are found to be 0.59 and 0.46 for the flow range of (0.038–0.189 kg/s). Two wetting characteristic improvement methods: surfactant addition and surface modification technique have been used for improving the wetting characteristic of PP surface. SLS surfactant solutions of concentration (60, 100, 140, 200, 300 ppm) have been tested in present study, local maxima of wetting factor is found between 60 ppm and 140 ppm. The average wetting factor of SLS 300 on plain PP is found to be around 50% higher than plain PP with water and around 17% more than plain Al with water. For the surface
Fig. 11. Comparison of experimental and calculated results of wetting factor.
Table 1 Parameters range and MAPE for correlation. Source
Qi et al. [8] Kang et al. [14] Zhang et al. [27] o Quan et al. [28] Current o
n
12 11 10 20 89
Solid-liquid pair
S.S.-LiCl Al-Water, Al-Titron X100 S.S.-Water S.S.-Water P.P.-Water, P.P.- SLS, Al-Water
MAPE (%)
ml
W
m
gs
sl
(kg/s)
(m)
(103 Pa s)
(J/m2)
(N/m)
–
Zhang et al. [27]
Current model
0.02–0.20 0.06 0.03–0.36 0.016–0.033 0.038–0.189
0.65 0.27 0.09 0.10 0.19
0.92 0.9–1.002 0.546 0.8007–0.3565 0.467–1.002
1.1 0.84 1.1 1.1 0.03, 0.84
0.083–0.085 0.031–0.072 0.067 0.059–0.065 0.059–0.072
1 1 1 1 1–1.85
146.4 12.9 20.8 20.4 33.1
13.6 6.0 10.2 10.0 7.6
Isothermal condition, n—number of experimental points.
As;ext As
D. Patil et al. / Chemical Engineering and Processing 108 (2016) 1–9
modification technique it is found that inclined recess provides maximum liquid holding. Four types of inclined recess surfaces have been prepared for optimizing the depth of cut and structure of surface. Maximum wetting factor is found for Modified Surface C (depth of cut 1.5 mm and no gap between two consecutive inclined recesses). Average wetting factor of Modified Surface C is found to be 0.83, which is around 80.4% more than plain PP surface and around 40.7% more than plain metallic Al surface. Importantly, Modified Surface C provides significant wetting factor improvement in low flow rate range. Hence, it can expedite performance of falling film tower more effectively in comparison to surfactant addition method. Effect of solution temperature increase on wetting factor increase is found to be more pronounced for plain PP surface than plain Al surface. A new generalized correlation has been developed for the estimation of wetting factor. Proposed correlation shows very good agreement (average MAPE 8.5%) for different combination of solid-liquid pair. Acknowledgement The authors acknowledge financial help provided by the Department of Science and Technology (SB/FTP/ETA-0039/2014), Govt. of India for carrying out current work. The funding organization has not played any role in study design, decision to publish or preparation of the manuscript. References [1] V.M. Ramm, Absorption of Gases, 2nd ed., Israel Program for Scientific Translation, Jerusalem, 1968. [2] N. Fumo, D.Y. Goswami, Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration, Sol. Energy 73 (2002) 351–361. [3] K.C. Schifftner, Air Pollution Control Equipment Selection Guide, 2nd ed., CRC Press, Taylor & Francis group, 2014. [4] R. Kumar, P.L. Dhar, S. Jain, Development of new wire mesh packing for improving the performance of zero carry over spray tower, Energy 36 (2) (2011) 1362–1374. [5] R. Kumar, P.L. Dhar, S. Jain, A.K. Asati, Multi absorber stand alone liquid desiccant air-conditioning systems for higher performance, Sol. Energy 83 (2009) 761–772. [6] S. Jain, Studies on desiccant augmented evaporative cooling systems, Ph.D. Thesis, IIT Delhi, India, 2007. [7] C. Luo, W. Ma, Y. Gong, Design of single vertical tube falling-film evaporation basing on experiment, J. Loss Prev. Process Ind. 24 (2011) 695–698. [8] R. Qi, L. Lu, H. Yang, F. Qin, Investigation on wetted area and film thickness for falling film liquid desiccant regeneration system, Appl. Energy 112 (2013) 93–101.
9
[9] D. Glebov, F. Settervall, Experimental research of heat exchange and distribution of viscous liquids on vertical lamels, Chem. Comput. Simul. Butlerov Commun. 2 (2002) 65–68. [10] A. Gonda, P. Lancereau, P. Bandelier, L. Luo, Y. Fan, S. Benezech, Water falling film evaporation on a corrugated plate, Int. J. Therm. Sci. 81 (2014) 29–37. [11] R. Goss, Practical guide to adhesive bonding of small engineering plastic and rubber Parts, Smithers Rapra Technol. (2010) 93–99. [12] D. Glebov, F. Settervall, Experimental study of heat transfer additive influence on the absorption chiller performance, Int. J. Refrig. 25 (2002) 538–545. [13] W.L. Cheng, K. Houda, Z.S. Chen, A. Akisawa, P. Hu, T. Kashiwagi, Heat transfer enhancement by additive in vertical falling film absorption of H2O/LiBr, Appl. Therm. Eng. 24 (2004) 281–298. [14] B.H. Kang, K.H. Kim, D.Y. Lee, Fluid flow and heat transfer on a falling liquid film with surfactant from a heated vertical surface, J. Mech. Sci. Technol. 21 (2007) 1807–1812. [15] J.K. Kim, C.W. Park, Y.T. Kang, The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber, Int. J. Refrig. 26 (2003) 575–585. [16] B. Koroglu, K.S. Lee, C. Park, Nano/micro-scale surface modifications using copper oxidation for enhancement of surface wetting and falling-film heat transfer, Int. J. Heat Mass Transf. 62 (2013) 794–804. [17] L. Hoffmann, I. Greiter, A. Wanger, V. Weiss, G. Alefeld, Experimental investigation of heat transfer in a horizontal tube falling film absorber with aqueous solutions of LiBr with and without surfactants, Int. J. Refrig. 19 (1996) 331–341. [18] D.S. Kim, C.A.I. Ferreira, Flow patterns and heat and mass transfer coefficients of low Reynolds number falling film flow on vertical plates: effects of wire screen and an additives, Int. J. Refrig. 32 (2008) 138–149. [19] H. Zhang, X. Yuan, K. Yu, M.A. Kalbassi, B. Liu, Influence of the marangoni effect on mass transfer in inclined falling films, Chem. Eng. Technol. 37 (7) (2014) 1095–1102. [20] F. Zhang, Z. Zhang, J. Geng, Study on shrinkage characteristics of heated falling liquid film, AIChE J. 51 (2005) 2899–2907. [21] N.B. Vargaftik, B.N. Volkov, L.D. Voljak, International tables of the surface tension of water, J. Phys. Chem. Ref. Data. 12 (1983) 817–820. [22] W. Ambrosini, N. Forgione, F. Oriolo, Statistical characteristics of a water film falling down a flat plate at different inclinations and temperatures, Int. J. Multiphase. Flow 28 (2002) 1521–1540. [23] K. Moran, J. Inumaru, M. Kawaji, Instantaneous hydrodynamics of a laminar wavy liquid film, Int. J. Multiphase. Flow 28 (2002) 731–755. [24] F. Zhang, J. Peng, J. Geng, Z.X. Wang, Z.B. Zhang, Thermal imaging study on the surface wave of heated falling liquid films, Exp. Therm. Fluid Sci. 33 (2009) 424–430. [25] Y.Q. Yu, S.J. Wei, Y.H. Yang, X. Cheng, Experimental study of water film falling and spreading on a large vertical plate, Prog. Nucl. Energy 54 (2012) 22–28. [26] W.P. Yan, X.M. Ye, H.T. Li, Flow and heat transfer characteristics of liquid thin film flow, J. North. China. Electr. Power Univ. 32 (1) (2005) 59–65. [27] F. Zhang, X. Zhao, J. Geng, Y.T. Wu, Z. Zhang, A new insight into marangoni effect in non-isothermal falling liquid films, Exp. Therm. Fluid Sci. 31 (2007) 361–365. [28] X. Quan, Y. Geng, P. Yuan, Z. Huang, C. Liu, Experiment and simulation of the shrinkage of falling film upon direct contact with vapor, Chem. Eng. Sci. 135 (2015) 52–60.