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Intensification of fluiddynamic and thermal performance of thermosiphon reboilers
Applied Thermal Engineering 25 (2005) 2615–2629 www.elsevier.com/locate/apthermeng
Intensification of fluiddynamic and thermal performance of thermosiphon reboilers Stephan Scholl *, Fahmi Brahim Institute for Chemical and Thermal Process Engineering, Technical University Braunschweig, Langer Kamp 7, Braunschweig D-38106, Germany Received 1 September 2004; accepted 26 November 2004 Available online 19 February 2005
Abstract The potential benefit of hiTRAN tube inserts for fluiddynamic as well as heat transfer intensification of thermosiphon reboilers was investigated experimentally. Circulating flow expressed as single phase tube inlet velocity is regarded as primary criterion for fluiddynamic behaviour while product side heat transfer coefficient represents heat transfer performance. Pure water and a 29 mol% glycerol/water mixture were used as test fluids. Fluiddynamic as well as thermal performance of the bare tube vs. five inserts with different loop volume fractions were determined in a single tube lab-scale thermosiphon reboiler. It was found that most beneficial use appears for the glycerol/water system at low driving temperature difference between service and product side employing inserts with a loop volume fraction around 3%. In best cases a more than 50% increase of the product side heat transfer coefficient was obtained. 2005 Elsevier Ltd. All rights reserved. Keywords: Thermosiphon reboiler; Heat transfer; Evaporation; Fluid dynamics; Enhancement; Tube inserts
*
Corresponding author. Tel.: +49 531 391 2780; fax: +49 531 391 2792. E-mail address: [email protected] (S. Scholl).
1359-4311/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.11.028
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Nomenclature di do h* lt Re s Toil Tb,p DTov wt,in ap el mp
inner tube diameter (m) outer tube diameter (m) static liquid head (–) tube length (m) Reynolds number, Re = wt,indi/mp (–) tube wall thickness (m) oil inlet temperature (C) product boiling temperature (C) overall temperature difference between service and product side (K) single phase tube inlet velocity (m/s) product side heat transfer coefficient (W/(m2 K)) loop volume fraction (m3/m3) product viscosity (m2/s)
1. Thermosiphon reboilers: applications and limitations Thermosiphon reboilers (TSR) have proven to be a highly effective and efficient equipment in heating as well as cooling circuits for multiple applications [1,8]. When used in their proper range of operation they combine high heat transfer rates with low capital as well as operating costs. While plate type thermosiphon reboilers receive growing attention installations in chemical as well as petrochemical processes are most frequently equipped with plain cylindrical tubes. Typical dimensions range from 1 m to 4 m tube length and 16 mm to 42 mm inner tube diameter, with a length-to-diameter-ratio around 100. Three different designs are found for thermosiphon reboilers, see Fig. 1: External vertical reboilers, integrated short-tube reboilers, frequently termed Robert-type reboilers, and external horizontal thermosiphon reboilers. Although the heat transfer area for all three types may be
Fig. 1. Design options for thermosiphon reboilers.
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designed by tubes or plates, the subsequent considerations will focus on vertical thermosiphon reboilers with tube side evaporation. Nevertheless, it can be expected that most of the arguments and findings are qualitatively valid for plate type TSR as well. The main advantages of TSR are • their low costs with respect to capital expenditure and operation, • high heat transfer rates and • the possible use of a wide range of construction materials. Their main drawbacks arise from their genuine operating principle with • a strong coupling of fluid dynamics and heat transfer, • resulting in a comparably limited operating range with a minimum driving temperature difference required between service and product side, its value depending on complex inter-actions of multiple parameters such as product viscosity, concentration of volatiles in the product flow, total pressure, tube length or static liquid head. The static liquid head h* is defined as the ratio of the liquid level in the column above the lower tube plate divided by the tube length. Above characteristics result in a limited operating range in terms of capacity fluctuations as well as process and design conditions. The ‘‘Green Range’’ for design and operation of TSR may be characterized by • • • • • • • •
non to low-fouling media, low to moderate solids concentrations, liquid viscosity 6 50 mPa s, total pressure P 200 mbar, inner tube diameter di = 16–30 mm, tube length lt = 1–4 m, high concentration of volatiles in the circulating flow, overall temperature difference between service and product side DTov = 20–50 K.
Like most types of reboilers and evaporators thermosiphon reboilers may be operated according to two different process functions, see Fig. 2: As heat transfer unit or as separation unit. When operated as heat transfer unit the reboiler generates the vapour phase for countercurrent twophase vapour–liquid flow required for component separation in a rectification or stripping column. Performance specification for the reboiler is given by a heat duty and optimisation aims to increase the overall heat transfer coefficient and/or reduce the driving temperature difference. The reboiler does not have a designated separation functionality. TSR operated as separation units will be specified through a concentration of volatiles in the concentrate. Typical applications may be the reduction of monomers or oligomers from a polymer or the recycle of organic solvents from a waste stream. Optimisation will aim at reducing the volatiles concentration in the concentrate, thus leading to increased viscosities and vacuum operation in many cases. A reboiler configuration that combines both process functions is the falling film evaporator with divided sump [2].
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 Separation Unit volatiles
Heat Transfer Unit
LC
VR
V feed
LR
bottom product
L
concentrate
Fig. 2. Process functions for thermosiphon reboilers.
Classical applications of TSR are for pure or well defined mixtures and uncritical evaporation behaviour, i.e. no foaming or liquid/liquid phase separation. Typical systems for these applications are refrigerants, low-chain hydrocarbons, water or ammonia. Due to the absence of a circulation pump TSR find beneficial use for products which are sensitive to mechanical stress. For many of these systems, such as acrylic monomers, acrylates, styrene or butadiene, a phase change at a hot wall may be the starting point for fouling built-up. This leads to operations with high values of static liquid head as this suppresses early boiling in the reboiler tube. Enabling and improving TSR use in these situations plus the optimisation for classical applications motivate research on the intensification of thermosiphon reboiler performance in terms of • • • •
increased product side heat transfer, lower minimum driving temperature difference, increased fluid dynamic stability, operation at – lower vacuum 6 100 h Pa, – lower content of volatiles, – higher viscosities, – higher values of static liquid head, • reduced fouling susceptibility.
Due to the static liquid head the circulating fluid is subcooled at the entrance to the lower tube plate of the TSR, see Fig. 4. In the single phase heating zone the fluid is heated to boiling conditions. In the transition region from single to two phase heat transfer subcooled boiling occurs at the hot inner tube wall. In the evaporation zone product temperature drops according to the reduction in static liquid head along the flow path. For wide-boiling mixtures this pressure effect may be overcompensated by the increase in boiling temperature due to the falling volatiles concentration resulting in an increase of boiling temperature along the tube axis. Intensification of the thermal as well as fluid dynamic behaviour of TSR therefore aims to increase heat transfer for these three dominating mechanisms through
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• increased near wall turbulence in single phase flow region, • support of bubble generation and detachment in subcooled boiling, • providing more nucleation points in the boiling region. Different design options are available to achieve the above aims: • macro- and/or microstructuring of the heat transfer surface with finned or microfinned tubes, see e.g. [3], • coating of heat transfer surface with porous layer or polymer film, see e.g. [4], • energetic modification of heat transfer surface through ion implantation or other techniques, • tube inserts such as twisted tapes, hiTRAN inserts [5] or others. The latter design option has the advantage that the performance of an existing heat exchanger with bare tubes may be improved in a retrofit situation without replacing the complete piece. In this study hiTRAN tube inserts were tested with respect to the intensification of fluid dynamics behaviour and tube side heat transfer. Theoretical calculations have shown that depending on the overall heat transfer situation a 50% increase of the product side heat transfer coefficient could be achieved what may result in a 10–20% increase of the overall heat transfer coefficient [6]. 2. Experimental setup Fig. 3 shows the experimental setup used [7]. The single glass or stainless steel reboiler tube with dimensions do · s · lt = 25 · 2 · 800 mm has an external heat transfer area of 0.063 m2. Thermal
Fig. 3. Thermosiphon reboiler test rig.
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Fig. 4. Axial temperature profile in TSR, loop positions of the insert and flow direction.
oil was used for heating and a coriolis type flow meter (PROMASS 80 F DN 25) to determine the circulating flow. Based on pressure drop calculations it can be estimated that the effect of the metering device on the measured flow value can be expected to be in the order of 5–10%. As test fluids pure water was used to simulate the heat transfer situation of a column reboiler at a rectification unit while a 29 mol% glycerol/water-mixture represents evaporation in a separation unit. Static liquid head was varied from 90% to 135% at oil inlet temperatures from 120 to 170 C. Different inserts with loop volume fraction el ranging from 0.6% to 3.5% were used. The loops covered the full tube length except in one case where only the lower half of the tube was equipped, see Fig. 4, left. In all experiments inserts were mounted with their loops leaning against the flow direction, see Fig. 4(right). The experiments were conducted such that at a constant oil inlet temperature the static liquid head h* was reduced stepwise from its maximum level of 135 to 90%. This range was given due to the design of the test rig. At each position of h* the circulating flow was measured as a function of time with a rate of one data set per ten seconds. The feed stream was varied from 2 l/h to 10 l/h and was introduced into the phase separator. All experiments were conducted at ambient pressure.
3. Experiments with glass reboiler tube Fig. 5 depicts experimental findings for the tube inlet velocity as a function of time at different values for the static liquid head. The glass reboiler tube with a coarse surface was employed, 2 l/h feed were introduced and operating pressure was ambient conditions. It may be seen that for these conditions the circulating flow without inserts at first increases slightly with decreasing static liquid head. When reducing h* from 110% to 105% the circulation rate drops significantly and fluctuations appear. This corresponds to a static liquid head falling below the reboiler return nozzle to the phase separator, see Fig. 6. Fluctuations continue for all values of h* 6 105%. On the right hand side similar experiments with an insert of 3% loop volume fraction are depicted. The absolute value of the tube inlet velocity drops to about 0.18 m/s at high values of the static liquid head, gradually decreasing for h* < 110%. No fluctuations appear over the whole range of h*. Similar experiments were performed for 150 C, 140 C and 130 C oil inlet temper-
0.32
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
90 180
time [min]
Fig. 5. Circulating flow in glass thermosiphon reboiler for water at 1 bar; Toil = 160 C, feed 2 l/h. Left: without insert, right: with insert, el = 0.03.
Fig. 6. Average value for single phase tube inlet velocity. Test fluid water at 1 bar, feed 2 l/h, insert with el = 0.03.
ature with similar results. It was found that with decreasing oil temperature fluctuations increased at decreasing circulation rate. Time averaged circulating flows at the different static liquid heads were divided by the inner cross section of the reboiler tube giving the single phase tube inlet velocity. Fig. 6 depicts tube inlet velocities for Toil = 160 C and 130 C, respectively, without and with insert. In all cases circulation rate or tube inlet velocity with insert was significantly lower then without insert. Its value dropped by about 30 to 40% compared to the bare tube. Additionally no improvement could be found in terms of extending the borders of the operating region towards lower driving temperature difference for the pure water system. A water/glycerol-mixture with a mole fraction of 0.29 mol/mol of glycerol was used to simulate TSR performance under separation conditions. The mixture has a viscosity of 14.7 mPa s at 25 C and 1.4 mPa s at 110 C. Fig. 7 shows tube inlet velocity as a function of time for four different
130
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
static liquid head [%]
tube inlet velocity [m/s]
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80 300
time [min]
Fig. 7. Evaporation of glycerol/water-mixture at 1 bar, Toil = 160 C, feed = 2.9 l/h, el = 0.03.
values of the static liquid head. Experiments were conducted with 2.9 l/h feed at ambient pressure and an oil inlet temperature of 160 C. Significant fluctuations appear in the circulating flow being more pronounced without the insert. Except for the data point at h* = 95% amplitudes of tube inlet velocity fluctuations are reduced to about 25% through the insert. When comparing average values for the tube inlet velocities at different heating conditions a different picture than before appears. Fig. 8 shows the average tube inlet velocities wt,in for Toil = 160 C and 140 C, respectively, without and with insert. wt,in is at about the same level without and with insert. These findings suggest that especially at the lower operating range the absolute value of the tube inlet velocity is higher with insert than without. This could be an indication of an extended operating range with respect to driving temperature difference. As the insert enforces radial mixing of the boundary layer with the bulk stream as well as bubble detachment from the heating surface the driving mechanism for circulation is supported. This positive effect overcompensates pressure drop increase due to the insert.
140 °C / without insert 140 °C / with insert 160 °C / without insert 160 °C / with insert
tube inlet velocity [m/s]
0.20
0.15
0.10
0.05
0.00 90
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130
static liquid head [%]
Fig. 8. Tube inlet velocity for TSR with water/glycerol-mixture at 1 bar, 2.9 l/h feed.
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Above experiments were performed using a glass reboiler tube allowing visual inspection of the fluiddynamic behaviour of the TSR. Due to its low thermal conductivity plus the significant heat transfer resistance on the heating oil side no quantitative conclusions with respect to the influence on tube side heat transfer coefficient could be extracted. Additionally heat transfer as well as bubble generation and detachment of a metal surface differ significantly from a glass surface.
4. Experiments with metal reboiler tube 4.1. Fluiddynamic behaviour Similar experiments were carried out with a stainless steel reboiler tube and different inserts. Their loop volume fraction ranged from el = 0.6% to 3.5%. Experiments were conducted for the separation mode, i.e. with the 29 mol% glycerol/water-mixture. Depending on the overall mass balance evaporation temperature at ambient pressure was between 110 C and 114 C. Fig. 9 depicts the circulation flow for a bare tube (el = 0%) as well as for five different inserts at 135%, 125%, 115%, 110%, 105% and 95% static liquid head and 160 C oil inlet temperature. For this situation no significant fluctuations appear and the circulation rate drops with increasing loop volume fraction, see Fig. 9(left). Fig. 9(right) shows tube inlet velocity wt,in as a function of the static liquid head. With increasing loop volume fraction el circulation rate drops down to about 30% of the bare tube for el = 0.035. For this situation no performance improvement of fluiddynamic behaviour is observed. As stated earlier performance intensification of TSR could address each of the three relevant heat transfer mechanisms: single phase heat transfer in the lower inlet region followed by subcooled boiling and finally two phase forced convective flow boiling. To identify the relative influence on these mechanisms experiments were performed with one insert where only the lower half of the tube was equipped with loops. Loop volume density in the lower half was el = 0.015, giving an overall loop density of the full tube of el = 0.0075. In Fig. 9(right) the line for el = 1.5% (1/2)
ε= 0 %
ε = 0.6 %
0.30
0.32 ε = 1.5 %(1/2)
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0.16 ε=3 % ε = 3.5 %
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time [min]
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= h*
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tube inlet velocity [m/s]
tube inlet velocity [m/s]
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ε= 0 ε = 0,6 % ε = 1,5 % ε= 3% ε = 3,5 % ε = 1,5 % (1/2)
0.25 0.20 0.15 0.10 0.05 90
100
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120
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static liquid head [%]
Fig. 9. Tube inlet velocity at different values of static liquid head for Toil = 160 C, 1 bar, glycerol/water-mixture; without and with five different inserts. 2.9 l/h feed, evaporation temperature 110–114 C.
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 ε ε ε ε ε ε
tube inlet velocity [m/s]
0.05 0.04
=0 = 0,6 % = 1,5 % =3% = 3,5 % = 1,5 % (1/2)
0.03 0.02 0.01 0.00 90
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120
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140
static liquid head [%]
Fig. 10. Tube inlet velocity of a thermosiphon reboiler at Toil = 130 C, glycerol/water-mixture; without and with five different inserts. 2.9 l/h feed, evaporation temperature 110–114 C.
represents single phase tube inlet velocity for this semi-insert. Circulation rate is higher than for the full insert with el = 0.015 and lower than for el = 0.006. A more ambiguous picture is obtained for Toil = 130 C. Feed rate was 10 l/h and product evaporation temperature was at 114–116 C which corresponds to a driving temperature difference between service and product side of 15 K. Fig. 10 gives the time averaged tube inlet velocities for the five inserts plus the bare tube as a function of static liquid head. wt,in is almost one order of magnitude smaller than in the previous case indicating the approach to the lower operating range. For all configurations—without and with inserts—significant fluctuations in the same order of magnitude were observed. The bare tube shows the highest circulation rate except for h* < 100%. This is in contrast to the glass tube experiments depicted in Fig. 8, where the insert produced a higher circulation rate at the lower heating temperature over the whole range of h*. This may be attributed to enhanced nucleation and bubble detachment on the metal surface compared to the coarse glass which dominates the effect of the inserts. Only at the lowest value of h* = 95% high loop density inserts produce a higher circulation rate. Also no clear effect of the loop density may be observed: While for high static heads a lower loop density appears to be advantageous this is reversed as the static liquid head decreases. The previous results suggest a strong influence of loop volume fraction on the effectiveness of hiTRAN inserts especially at the operating borders. Therefore the data points for the highest and lowest vales of h* and Toil were re-evaluated. Fig. 11 depicts tube inlet velocities as a function of loop volume fraction el for the two different values of static liquid head h*. Loop volume fraction el = 0% stands for the bare tube. The data points connected by the solid line represent the inserts with loops over the full length while the single spots stand for the semi-looped insert. Toil = 130 C, Fig. 11(left), represents an operating point at the lower end of driving temperature difference, while Toil = 160 C, Fig. 11(right), stands for a situation in the midst of the operating regime. For Toil = 160 C no improvement due to the inserts may be found. The insert with semilooping falls well in the line with the rest of the data points for the fully looped ones if the average loop density of el = 1.5/2% = 0.75% is used. For Toil = 130 C no enhancement is observed at h* = 135% while for the lower end of static liquid head at h* = 95% a slight improvement may be seen for loop volume fractions above 2%. The semi-insert with el = 1.5% (1/2) also shows a sig-
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 0.05
0.30 T-Oil = 130 °C
T-Oil = 160 °C
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
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tube inlet velocity [m/s]
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0.03 0.02 0.01 0.00
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
0.25 0.20 0.15 0.10 0.05 0.00
0
1
2
3
4
0
1
loop volume fraction [%]
2
3
4
loop volume fraction [%]
Fig. 11. Tube inlet velocity for thermosiphon reboiler with bare tube and five different inserts, 29 mol% glycerol/watermixture. Left: Toil = 130 C, right: Toil = 160 C.
nificant improvement over the bare tube for h* = 95%, while for h* = 135% almost zero flow is obtained. Further investigations are needed to evaluate these findings. 4.2. Heat transfer improvement For the metal reboiler tube the effect of the inserts on product side heat transfer coefficient ap was evaluated. These were determined from an overall heat balance and applying calculated values for service side heat transfer coefficient. Integral fouling resistance on product and service side was assumed to be RPS + RSS = 1 · 10 4 m2 K/W. Typical values for oil side heat transfer coefficient were aoil = 600 W/(m2 K). Heat flux density for the experiments was 5–25 kW/m2. Fig. 12 depicts product side heat transfer coefficients at Toil = 160 C as a function of static liquid head without and with inserts. For high values of h* best performance is achieved for the bare tube. With increasing loop volume fraction the product side heat transfer coefficient decreases. The 1800
ε=0 ε = 0,6 % ε = 1,5 % ε=3% ε = 3,5 % ε = 1,5 % (1/2)
1400
2
α P [W/(m K)]
1600
1200 1000 800 600 90
100
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120
130
140
static liquid head [%]
Fig. 12. Product side heat transfer coefficient ap, thermosiphon reboiler, Toil = 160 C, 29 mol% glycerol/water-mixture, without and with five different inserts.
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semi-insert shows a significantly better performance than both alternative inserts with el = 0.6% and el = 1.5%, respectively. This picture is reversed at the lowest values of static liquid head. The highest product side heat transfer coefficient is achieved for el = 0.03 while it decreases with decreasing loop volume fraction. Also an increase to el = 0.035 shows lower performance. In best case tube side heat transfer coefficient increases by about 50% compared to the bare tube resulting in an increase of the overall heat transfer coefficient of 15% for the present conditions. This agrees well with the initial theoretical considerations [6]. For low values of h* no advantage for the semi-insert is seen for product side heat transfer coefficient ap. As ap is even lower than for the insert with the same average loop density of el = 0.015 it may be concluded that two phase heat transfer is dominant. The presence of loops in the two phase boiling region overcompensates the additional pressure drop by additional nucleation points for bubble generation. Fig. 13 gives a condensed picture on the effect of insert design on product side heat transfer coefficient. For Toil = 160 C no benefit seems to be achieved from the use of the insert except for high density inserts at h* = 95%. For Toil = 130 C improvements are obvious. Loop volume fractions of 1.5% 6 el < 3% seem to be beneficial over the bare tube with a potential of P50% improvement of ap. Although no systematic boiling investigations were conducted in terms of loop design optimisation it appears that an optimum design exists around a loop volume fraction of el 3% for the fully equipped insert. The semi-insert indicates an even better performance than the fully equipped ones but this will need more experimental attention. The observed tube inlet velocities for the water–glycerol system corresponded to Reynolds numbers well within the laminar region. Fig. 14 presents integral values of product side heat transfer coefficient as a function of Re for the bare tube and the inserts. The four data points along one line correspond to one experimental run at different values of h*. With increasing loop density Re drops as seen for wt,in. For identical Re numbers product side heat transfer coefficient increases with increasing loop volume fraction. Nevertheless, one should keep in mind that for thermosiphon reboilers circulation flow is a function of fluiddynamic as well as thermal performance of the system and can not be adjusted independently by the operator. 2000 h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
1500
αP [W/(m K)]
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TOil = 130 °C
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TOil = 160 °C
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0
0 0
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loop volume fraction [%]
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Fig. 13. Product side heat transfer coefficient ap for TSR with bare tube and five different inserts, 29 mol% glycerol/ water-mixture. Left: Toil = 130 C, right: Toil = 160 C.
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
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1200
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Re [-]
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Fig. 14. Product side heat transfer coefficient as a function of single phase Reynolds number; water/glycerol-mixture, variation of static liquid head h*. 1.1 1.0 0.9 0.8 0.7
ε=0% ε = 0.6 % ε=3%
0.6 0.5 0.4 0.3 0.2 0.1 0.0 119
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temperature [°C]
Fig. 15. Axial bulk temperatures for the water/glycerol-system at ambient pressure without and with inserts; Toil = 160 C, h* = 110%.
The metal tube is equipped with axial temperature measurements. The effect of the inserts on the axial bulk product temperature is shown in Fig. 15 for Toil = 160 C and h* = 110%. At the given conditions the single phase heating zone covers almost the full tube length for the bare tube. The temperature maximum indicates the axial position at which bulk convective boiling starts. It is obvious that the inserts enhance single phase heat transfer in the subcooled region leading to an earlier start of bulk convective boiling. Due to an elevated static pressure at lower axial tube positions the fluid temperature has to increase even higher to reach boiling conditions. Fig. 16 summarizes these findings concerning single phase heating zone reduction for different overall temperature differences between service and product side. The normalized length of single phase heating zone for a variation of driving temperature difference and tube inserts is depicted for the water/glycerol-system at ambient pressure and h* = 135%. A significant reduction of the single phase length is achieved. This gives rise to an increase in the overall product side heat transfer coefficient. These findings are of special interest in cases with a low content of volatiles in the circulating flow. Early start of bulk flow boiling will increase and stabilize flow circulation and by that broaden the window of operation for TSR.
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 normalized length of heating zone [-]
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1.0 0.9 0.8 0.7 0.6 0.5
ε=0% ε = 0.6 % ε=3%
0.4 0.3 20
25
30
35
40
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50
∆TOV = (TOil - Tb, Product) [K]
Fig. 16. Normalized length of single phase heating zone for a variation of driving temperature difference and tube inserts, water/glycerol-system, ambient pressure, h* = 135%.
5. Conclusions and outlook The effect of hiTRAN tube inserts on the fluiddynamic as well as thermal performance of vertical thermosiphon reboilers was investigated experimentally in a single tube lab-scale reboiler. Pure water was used as test fluid to model a rectification system while a water/glycerol-mixture with a glycerol mole fraction of 0.29 mol/mol stood for a separation system. Five different inserts were tested in reference to a bare tube. The inserts varied in loop volume fractions ranging from el = 0.006 to 0.035. All experiments were conducted at ambient pressure. Encouraging results were obtained for the water/glycerol-system showing an improved circulation at low values of DTov and h*. No benefits were found at high values of DTov and h*. Circulation proved to be more stable with reduced fluctuations. Best cases showed a 50% increase of the overall tube side heat transfer coefficient. For the pure water system no benefits could be identified except slight improvements in circulation at low DTov and h* when inserts with a loop volume fraction of larger than 2% were used. With respect to loop design best results were achieved for a loop volume fraction of el = 3%. Future investigations will aim at a quantification of the observed experimental findings through modeling and simulation. The extension of operation limits for TSR through inserts for the different system, design and process parameters is of key importance. Also alternative insert types, like twisted tapes, and other designs of heat transfer surface modifications, like micro- and macrofinned or coated tubes, will be tested.
Acknowledgement Supply of the inserts by Cal Gavin Process Intensification Engineering, Alcester, Warwickshire/ UK, is gratefully acknowledged.
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References [1] G.F. Hewitt (Ed.), Heat Transfer Design Handbook, Begell House Inc., New York, 2002. [2] U. Eiden, S. Scholl, Use of simulation in rating and design of distillation units, Comp. Chem. Eng. 21 (Suppl.) S199– S204. [3] Wieland-Werke AG, Metallwerke, Graf-Arco-Straße, 89070 Ulm. [4] UOP LLC, 25 East Algonquin Road, Des Plaines, IL 60017-5017, USA. [5] Cal Gavin Ltd., Process Intensification Engineering, Alcester, Warwickshire/GB, Available from. [6] A. Reddy, D.R. Webb, G.T. Polley, Using heat transfer enhancement to improve the performance of thermosiphon reboilers. Presentation at 5th UK National Heat Transfer Conference, London/UK, September 1997. [7] S. Pfau, Experimentelle Untersuchung von Wa¨rmeu¨bergang und Betriebsverhalten von Naturumlaufverdampfern ohne und mit Turbulenzpromotoren (Experimental investigation of heat transfer and operation behaviour of thermosiphon reboilers without and with turbulence promotors, in German). Diploma Thesis TU Braunschweig 2003. [8] S. Arneth, Dimensionierung und Betriebsverhalten von Naturumlaufverdampfern (Sizing and operation behaviour of thermosiphon reboilers, in German). Dissertation TU Mu¨nchen 1999.
Intensification of fluiddynamic and thermal performance of thermosiphon reboilers Stephan Scholl *, Fahmi Brahim Institute for Chemical and Thermal Process Engineering, Technical University Braunschweig, Langer Kamp 7, Braunschweig D-38106, Germany Received 1 September 2004; accepted 26 November 2004 Available online 19 February 2005
Abstract The potential benefit of hiTRAN tube inserts for fluiddynamic as well as heat transfer intensification of thermosiphon reboilers was investigated experimentally. Circulating flow expressed as single phase tube inlet velocity is regarded as primary criterion for fluiddynamic behaviour while product side heat transfer coefficient represents heat transfer performance. Pure water and a 29 mol% glycerol/water mixture were used as test fluids. Fluiddynamic as well as thermal performance of the bare tube vs. five inserts with different loop volume fractions were determined in a single tube lab-scale thermosiphon reboiler. It was found that most beneficial use appears for the glycerol/water system at low driving temperature difference between service and product side employing inserts with a loop volume fraction around 3%. In best cases a more than 50% increase of the product side heat transfer coefficient was obtained. 2005 Elsevier Ltd. All rights reserved. Keywords: Thermosiphon reboiler; Heat transfer; Evaporation; Fluid dynamics; Enhancement; Tube inserts
*
Corresponding author. Tel.: +49 531 391 2780; fax: +49 531 391 2792. E-mail address: [email protected] (S. Scholl).
1359-4311/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.11.028
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Nomenclature di do h* lt Re s Toil Tb,p DTov wt,in ap el mp
inner tube diameter (m) outer tube diameter (m) static liquid head (–) tube length (m) Reynolds number, Re = wt,indi/mp (–) tube wall thickness (m) oil inlet temperature (C) product boiling temperature (C) overall temperature difference between service and product side (K) single phase tube inlet velocity (m/s) product side heat transfer coefficient (W/(m2 K)) loop volume fraction (m3/m3) product viscosity (m2/s)
1. Thermosiphon reboilers: applications and limitations Thermosiphon reboilers (TSR) have proven to be a highly effective and efficient equipment in heating as well as cooling circuits for multiple applications [1,8]. When used in their proper range of operation they combine high heat transfer rates with low capital as well as operating costs. While plate type thermosiphon reboilers receive growing attention installations in chemical as well as petrochemical processes are most frequently equipped with plain cylindrical tubes. Typical dimensions range from 1 m to 4 m tube length and 16 mm to 42 mm inner tube diameter, with a length-to-diameter-ratio around 100. Three different designs are found for thermosiphon reboilers, see Fig. 1: External vertical reboilers, integrated short-tube reboilers, frequently termed Robert-type reboilers, and external horizontal thermosiphon reboilers. Although the heat transfer area for all three types may be
Fig. 1. Design options for thermosiphon reboilers.
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designed by tubes or plates, the subsequent considerations will focus on vertical thermosiphon reboilers with tube side evaporation. Nevertheless, it can be expected that most of the arguments and findings are qualitatively valid for plate type TSR as well. The main advantages of TSR are • their low costs with respect to capital expenditure and operation, • high heat transfer rates and • the possible use of a wide range of construction materials. Their main drawbacks arise from their genuine operating principle with • a strong coupling of fluid dynamics and heat transfer, • resulting in a comparably limited operating range with a minimum driving temperature difference required between service and product side, its value depending on complex inter-actions of multiple parameters such as product viscosity, concentration of volatiles in the product flow, total pressure, tube length or static liquid head. The static liquid head h* is defined as the ratio of the liquid level in the column above the lower tube plate divided by the tube length. Above characteristics result in a limited operating range in terms of capacity fluctuations as well as process and design conditions. The ‘‘Green Range’’ for design and operation of TSR may be characterized by • • • • • • • •
non to low-fouling media, low to moderate solids concentrations, liquid viscosity 6 50 mPa s, total pressure P 200 mbar, inner tube diameter di = 16–30 mm, tube length lt = 1–4 m, high concentration of volatiles in the circulating flow, overall temperature difference between service and product side DTov = 20–50 K.
Like most types of reboilers and evaporators thermosiphon reboilers may be operated according to two different process functions, see Fig. 2: As heat transfer unit or as separation unit. When operated as heat transfer unit the reboiler generates the vapour phase for countercurrent twophase vapour–liquid flow required for component separation in a rectification or stripping column. Performance specification for the reboiler is given by a heat duty and optimisation aims to increase the overall heat transfer coefficient and/or reduce the driving temperature difference. The reboiler does not have a designated separation functionality. TSR operated as separation units will be specified through a concentration of volatiles in the concentrate. Typical applications may be the reduction of monomers or oligomers from a polymer or the recycle of organic solvents from a waste stream. Optimisation will aim at reducing the volatiles concentration in the concentrate, thus leading to increased viscosities and vacuum operation in many cases. A reboiler configuration that combines both process functions is the falling film evaporator with divided sump [2].
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 Separation Unit volatiles
Heat Transfer Unit
LC
VR
V feed
LR
bottom product
L
concentrate
Fig. 2. Process functions for thermosiphon reboilers.
Classical applications of TSR are for pure or well defined mixtures and uncritical evaporation behaviour, i.e. no foaming or liquid/liquid phase separation. Typical systems for these applications are refrigerants, low-chain hydrocarbons, water or ammonia. Due to the absence of a circulation pump TSR find beneficial use for products which are sensitive to mechanical stress. For many of these systems, such as acrylic monomers, acrylates, styrene or butadiene, a phase change at a hot wall may be the starting point for fouling built-up. This leads to operations with high values of static liquid head as this suppresses early boiling in the reboiler tube. Enabling and improving TSR use in these situations plus the optimisation for classical applications motivate research on the intensification of thermosiphon reboiler performance in terms of • • • •
increased product side heat transfer, lower minimum driving temperature difference, increased fluid dynamic stability, operation at – lower vacuum 6 100 h Pa, – lower content of volatiles, – higher viscosities, – higher values of static liquid head, • reduced fouling susceptibility.
Due to the static liquid head the circulating fluid is subcooled at the entrance to the lower tube plate of the TSR, see Fig. 4. In the single phase heating zone the fluid is heated to boiling conditions. In the transition region from single to two phase heat transfer subcooled boiling occurs at the hot inner tube wall. In the evaporation zone product temperature drops according to the reduction in static liquid head along the flow path. For wide-boiling mixtures this pressure effect may be overcompensated by the increase in boiling temperature due to the falling volatiles concentration resulting in an increase of boiling temperature along the tube axis. Intensification of the thermal as well as fluid dynamic behaviour of TSR therefore aims to increase heat transfer for these three dominating mechanisms through
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• increased near wall turbulence in single phase flow region, • support of bubble generation and detachment in subcooled boiling, • providing more nucleation points in the boiling region. Different design options are available to achieve the above aims: • macro- and/or microstructuring of the heat transfer surface with finned or microfinned tubes, see e.g. [3], • coating of heat transfer surface with porous layer or polymer film, see e.g. [4], • energetic modification of heat transfer surface through ion implantation or other techniques, • tube inserts such as twisted tapes, hiTRAN inserts [5] or others. The latter design option has the advantage that the performance of an existing heat exchanger with bare tubes may be improved in a retrofit situation without replacing the complete piece. In this study hiTRAN tube inserts were tested with respect to the intensification of fluid dynamics behaviour and tube side heat transfer. Theoretical calculations have shown that depending on the overall heat transfer situation a 50% increase of the product side heat transfer coefficient could be achieved what may result in a 10–20% increase of the overall heat transfer coefficient [6]. 2. Experimental setup Fig. 3 shows the experimental setup used [7]. The single glass or stainless steel reboiler tube with dimensions do · s · lt = 25 · 2 · 800 mm has an external heat transfer area of 0.063 m2. Thermal
Fig. 3. Thermosiphon reboiler test rig.
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Fig. 4. Axial temperature profile in TSR, loop positions of the insert and flow direction.
oil was used for heating and a coriolis type flow meter (PROMASS 80 F DN 25) to determine the circulating flow. Based on pressure drop calculations it can be estimated that the effect of the metering device on the measured flow value can be expected to be in the order of 5–10%. As test fluids pure water was used to simulate the heat transfer situation of a column reboiler at a rectification unit while a 29 mol% glycerol/water-mixture represents evaporation in a separation unit. Static liquid head was varied from 90% to 135% at oil inlet temperatures from 120 to 170 C. Different inserts with loop volume fraction el ranging from 0.6% to 3.5% were used. The loops covered the full tube length except in one case where only the lower half of the tube was equipped, see Fig. 4, left. In all experiments inserts were mounted with their loops leaning against the flow direction, see Fig. 4(right). The experiments were conducted such that at a constant oil inlet temperature the static liquid head h* was reduced stepwise from its maximum level of 135 to 90%. This range was given due to the design of the test rig. At each position of h* the circulating flow was measured as a function of time with a rate of one data set per ten seconds. The feed stream was varied from 2 l/h to 10 l/h and was introduced into the phase separator. All experiments were conducted at ambient pressure.
3. Experiments with glass reboiler tube Fig. 5 depicts experimental findings for the tube inlet velocity as a function of time at different values for the static liquid head. The glass reboiler tube with a coarse surface was employed, 2 l/h feed were introduced and operating pressure was ambient conditions. It may be seen that for these conditions the circulating flow without inserts at first increases slightly with decreasing static liquid head. When reducing h* from 110% to 105% the circulation rate drops significantly and fluctuations appear. This corresponds to a static liquid head falling below the reboiler return nozzle to the phase separator, see Fig. 6. Fluctuations continue for all values of h* 6 105%. On the right hand side similar experiments with an insert of 3% loop volume fraction are depicted. The absolute value of the tube inlet velocity drops to about 0.18 m/s at high values of the static liquid head, gradually decreasing for h* < 110%. No fluctuations appear over the whole range of h*. Similar experiments were performed for 150 C, 140 C and 130 C oil inlet temper-
0.32
130
0.24
120
0.16
110
0.08
100
0.00 0
50
100
150
time [min]
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250
90 300
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0.40
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0.00 0
30
60
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120
150
static liquid head [%]
140
tube inlet velocity [m/s]
0.40
static liquid head [%]
tube inlet velocity [m/s]
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
90 180
time [min]
Fig. 5. Circulating flow in glass thermosiphon reboiler for water at 1 bar; Toil = 160 C, feed 2 l/h. Left: without insert, right: with insert, el = 0.03.
Fig. 6. Average value for single phase tube inlet velocity. Test fluid water at 1 bar, feed 2 l/h, insert with el = 0.03.
ature with similar results. It was found that with decreasing oil temperature fluctuations increased at decreasing circulation rate. Time averaged circulating flows at the different static liquid heads were divided by the inner cross section of the reboiler tube giving the single phase tube inlet velocity. Fig. 6 depicts tube inlet velocities for Toil = 160 C and 130 C, respectively, without and with insert. In all cases circulation rate or tube inlet velocity with insert was significantly lower then without insert. Its value dropped by about 30 to 40% compared to the bare tube. Additionally no improvement could be found in terms of extending the borders of the operating region towards lower driving temperature difference for the pure water system. A water/glycerol-mixture with a mole fraction of 0.29 mol/mol of glycerol was used to simulate TSR performance under separation conditions. The mixture has a viscosity of 14.7 mPa s at 25 C and 1.4 mPa s at 110 C. Fig. 7 shows tube inlet velocity as a function of time for four different
130
0.32
120
0.24
110
0.16
100
0.08
90
0.00 0
50
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80 300
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0.16
100
0.08
90
0.00 0
50
time [min]
100
150
200
250
static liquid head [%]
0.40
tube inlet velocity [m/s]
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
static liquid head [%]
tube inlet velocity [m/s]
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80 300
time [min]
Fig. 7. Evaporation of glycerol/water-mixture at 1 bar, Toil = 160 C, feed = 2.9 l/h, el = 0.03.
values of the static liquid head. Experiments were conducted with 2.9 l/h feed at ambient pressure and an oil inlet temperature of 160 C. Significant fluctuations appear in the circulating flow being more pronounced without the insert. Except for the data point at h* = 95% amplitudes of tube inlet velocity fluctuations are reduced to about 25% through the insert. When comparing average values for the tube inlet velocities at different heating conditions a different picture than before appears. Fig. 8 shows the average tube inlet velocities wt,in for Toil = 160 C and 140 C, respectively, without and with insert. wt,in is at about the same level without and with insert. These findings suggest that especially at the lower operating range the absolute value of the tube inlet velocity is higher with insert than without. This could be an indication of an extended operating range with respect to driving temperature difference. As the insert enforces radial mixing of the boundary layer with the bulk stream as well as bubble detachment from the heating surface the driving mechanism for circulation is supported. This positive effect overcompensates pressure drop increase due to the insert.
140 °C / without insert 140 °C / with insert 160 °C / without insert 160 °C / with insert
tube inlet velocity [m/s]
0.20
0.15
0.10
0.05
0.00 90
100
110
120
130
static liquid head [%]
Fig. 8. Tube inlet velocity for TSR with water/glycerol-mixture at 1 bar, 2.9 l/h feed.
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
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Above experiments were performed using a glass reboiler tube allowing visual inspection of the fluiddynamic behaviour of the TSR. Due to its low thermal conductivity plus the significant heat transfer resistance on the heating oil side no quantitative conclusions with respect to the influence on tube side heat transfer coefficient could be extracted. Additionally heat transfer as well as bubble generation and detachment of a metal surface differ significantly from a glass surface.
4. Experiments with metal reboiler tube 4.1. Fluiddynamic behaviour Similar experiments were carried out with a stainless steel reboiler tube and different inserts. Their loop volume fraction ranged from el = 0.6% to 3.5%. Experiments were conducted for the separation mode, i.e. with the 29 mol% glycerol/water-mixture. Depending on the overall mass balance evaporation temperature at ambient pressure was between 110 C and 114 C. Fig. 9 depicts the circulation flow for a bare tube (el = 0%) as well as for five different inserts at 135%, 125%, 115%, 110%, 105% and 95% static liquid head and 160 C oil inlet temperature. For this situation no significant fluctuations appear and the circulation rate drops with increasing loop volume fraction, see Fig. 9(left). Fig. 9(right) shows tube inlet velocity wt,in as a function of the static liquid head. With increasing loop volume fraction el circulation rate drops down to about 30% of the bare tube for el = 0.035. For this situation no performance improvement of fluiddynamic behaviour is observed. As stated earlier performance intensification of TSR could address each of the three relevant heat transfer mechanisms: single phase heat transfer in the lower inlet region followed by subcooled boiling and finally two phase forced convective flow boiling. To identify the relative influence on these mechanisms experiments were performed with one insert where only the lower half of the tube was equipped with loops. Loop volume density in the lower half was el = 0.015, giving an overall loop density of the full tube of el = 0.0075. In Fig. 9(right) the line for el = 1.5% (1/2)
ε= 0 %
ε = 0.6 %
0.30
0.32 ε = 1.5 %(1/2)
0.24 ε = 1.5 %
0.16 ε=3 % ε = 3.5 %
0.08 135
0.00 0
125
115 110
50
100
105
95
150
time [min]
200
= h*
250
300
tube inlet velocity [m/s]
tube inlet velocity [m/s]
0.40
ε= 0 ε = 0,6 % ε = 1,5 % ε= 3% ε = 3,5 % ε = 1,5 % (1/2)
0.25 0.20 0.15 0.10 0.05 90
100
110
120
130
140
static liquid head [%]
Fig. 9. Tube inlet velocity at different values of static liquid head for Toil = 160 C, 1 bar, glycerol/water-mixture; without and with five different inserts. 2.9 l/h feed, evaporation temperature 110–114 C.
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S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 ε ε ε ε ε ε
tube inlet velocity [m/s]
0.05 0.04
=0 = 0,6 % = 1,5 % =3% = 3,5 % = 1,5 % (1/2)
0.03 0.02 0.01 0.00 90
100
110
120
130
140
static liquid head [%]
Fig. 10. Tube inlet velocity of a thermosiphon reboiler at Toil = 130 C, glycerol/water-mixture; without and with five different inserts. 2.9 l/h feed, evaporation temperature 110–114 C.
represents single phase tube inlet velocity for this semi-insert. Circulation rate is higher than for the full insert with el = 0.015 and lower than for el = 0.006. A more ambiguous picture is obtained for Toil = 130 C. Feed rate was 10 l/h and product evaporation temperature was at 114–116 C which corresponds to a driving temperature difference between service and product side of 15 K. Fig. 10 gives the time averaged tube inlet velocities for the five inserts plus the bare tube as a function of static liquid head. wt,in is almost one order of magnitude smaller than in the previous case indicating the approach to the lower operating range. For all configurations—without and with inserts—significant fluctuations in the same order of magnitude were observed. The bare tube shows the highest circulation rate except for h* < 100%. This is in contrast to the glass tube experiments depicted in Fig. 8, where the insert produced a higher circulation rate at the lower heating temperature over the whole range of h*. This may be attributed to enhanced nucleation and bubble detachment on the metal surface compared to the coarse glass which dominates the effect of the inserts. Only at the lowest value of h* = 95% high loop density inserts produce a higher circulation rate. Also no clear effect of the loop density may be observed: While for high static heads a lower loop density appears to be advantageous this is reversed as the static liquid head decreases. The previous results suggest a strong influence of loop volume fraction on the effectiveness of hiTRAN inserts especially at the operating borders. Therefore the data points for the highest and lowest vales of h* and Toil were re-evaluated. Fig. 11 depicts tube inlet velocities as a function of loop volume fraction el for the two different values of static liquid head h*. Loop volume fraction el = 0% stands for the bare tube. The data points connected by the solid line represent the inserts with loops over the full length while the single spots stand for the semi-looped insert. Toil = 130 C, Fig. 11(left), represents an operating point at the lower end of driving temperature difference, while Toil = 160 C, Fig. 11(right), stands for a situation in the midst of the operating regime. For Toil = 160 C no improvement due to the inserts may be found. The insert with semilooping falls well in the line with the rest of the data points for the fully looped ones if the average loop density of el = 1.5/2% = 0.75% is used. For Toil = 130 C no enhancement is observed at h* = 135% while for the lower end of static liquid head at h* = 95% a slight improvement may be seen for loop volume fractions above 2%. The semi-insert with el = 1.5% (1/2) also shows a sig-
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 0.05
0.30 T-Oil = 130 °C
T-Oil = 160 °C
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
0.04
tube inlet velocity [m/s]
tube inlet velocity [m/s]
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0.03 0.02 0.01 0.00
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
0.25 0.20 0.15 0.10 0.05 0.00
0
1
2
3
4
0
1
loop volume fraction [%]
2
3
4
loop volume fraction [%]
Fig. 11. Tube inlet velocity for thermosiphon reboiler with bare tube and five different inserts, 29 mol% glycerol/watermixture. Left: Toil = 130 C, right: Toil = 160 C.
nificant improvement over the bare tube for h* = 95%, while for h* = 135% almost zero flow is obtained. Further investigations are needed to evaluate these findings. 4.2. Heat transfer improvement For the metal reboiler tube the effect of the inserts on product side heat transfer coefficient ap was evaluated. These were determined from an overall heat balance and applying calculated values for service side heat transfer coefficient. Integral fouling resistance on product and service side was assumed to be RPS + RSS = 1 · 10 4 m2 K/W. Typical values for oil side heat transfer coefficient were aoil = 600 W/(m2 K). Heat flux density for the experiments was 5–25 kW/m2. Fig. 12 depicts product side heat transfer coefficients at Toil = 160 C as a function of static liquid head without and with inserts. For high values of h* best performance is achieved for the bare tube. With increasing loop volume fraction the product side heat transfer coefficient decreases. The 1800
ε=0 ε = 0,6 % ε = 1,5 % ε=3% ε = 3,5 % ε = 1,5 % (1/2)
1400
2
α P [W/(m K)]
1600
1200 1000 800 600 90
100
110
120
130
140
static liquid head [%]
Fig. 12. Product side heat transfer coefficient ap, thermosiphon reboiler, Toil = 160 C, 29 mol% glycerol/water-mixture, without and with five different inserts.
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semi-insert shows a significantly better performance than both alternative inserts with el = 0.6% and el = 1.5%, respectively. This picture is reversed at the lowest values of static liquid head. The highest product side heat transfer coefficient is achieved for el = 0.03 while it decreases with decreasing loop volume fraction. Also an increase to el = 0.035 shows lower performance. In best case tube side heat transfer coefficient increases by about 50% compared to the bare tube resulting in an increase of the overall heat transfer coefficient of 15% for the present conditions. This agrees well with the initial theoretical considerations [6]. For low values of h* no advantage for the semi-insert is seen for product side heat transfer coefficient ap. As ap is even lower than for the insert with the same average loop density of el = 0.015 it may be concluded that two phase heat transfer is dominant. The presence of loops in the two phase boiling region overcompensates the additional pressure drop by additional nucleation points for bubble generation. Fig. 13 gives a condensed picture on the effect of insert design on product side heat transfer coefficient. For Toil = 160 C no benefit seems to be achieved from the use of the insert except for high density inserts at h* = 95%. For Toil = 130 C improvements are obvious. Loop volume fractions of 1.5% 6 el < 3% seem to be beneficial over the bare tube with a potential of P50% improvement of ap. Although no systematic boiling investigations were conducted in terms of loop design optimisation it appears that an optimum design exists around a loop volume fraction of el 3% for the fully equipped insert. The semi-insert indicates an even better performance than the fully equipped ones but this will need more experimental attention. The observed tube inlet velocities for the water–glycerol system corresponded to Reynolds numbers well within the laminar region. Fig. 14 presents integral values of product side heat transfer coefficient as a function of Re for the bare tube and the inserts. The four data points along one line correspond to one experimental run at different values of h*. With increasing loop density Re drops as seen for wt,in. For identical Re numbers product side heat transfer coefficient increases with increasing loop volume fraction. Nevertheless, one should keep in mind that for thermosiphon reboilers circulation flow is a function of fluiddynamic as well as thermal performance of the system and can not be adjusted independently by the operator. 2000 h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
h* = 95 % h* = 135 % h* = 95 % (1.5/2) h* = 135 % (1.5/2)
1500
αP [W/(m K)]
300
2
TOil = 130 °C
2
α P [W/(m K)]
400
200
100
TOil = 160 °C
1000
500
0
0 0
1
2
3
loop volume fraction [%]
4
0
1
2
3
4
loop volume fraction [%]
Fig. 13. Product side heat transfer coefficient ap for TSR with bare tube and five different inserts, 29 mol% glycerol/ water-mixture. Left: Toil = 130 C, right: Toil = 160 C.
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629
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1200
800
2
αP [W/(m K)]
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600 ε = 0% ε = 0,6 % ε = 1,5 % ε = 3% ε = 3,5 % ε = 1,5 % (1/2)
400 200
0
500
1000
1500
2000
2500
3000
3500
Re [-]
normalized tube length z/ltube [-]
Fig. 14. Product side heat transfer coefficient as a function of single phase Reynolds number; water/glycerol-mixture, variation of static liquid head h*. 1.1 1.0 0.9 0.8 0.7
ε=0% ε = 0.6 % ε=3%
0.6 0.5 0.4 0.3 0.2 0.1 0.0 119
120
121
122
123
124
125
126
temperature [°C]
Fig. 15. Axial bulk temperatures for the water/glycerol-system at ambient pressure without and with inserts; Toil = 160 C, h* = 110%.
The metal tube is equipped with axial temperature measurements. The effect of the inserts on the axial bulk product temperature is shown in Fig. 15 for Toil = 160 C and h* = 110%. At the given conditions the single phase heating zone covers almost the full tube length for the bare tube. The temperature maximum indicates the axial position at which bulk convective boiling starts. It is obvious that the inserts enhance single phase heat transfer in the subcooled region leading to an earlier start of bulk convective boiling. Due to an elevated static pressure at lower axial tube positions the fluid temperature has to increase even higher to reach boiling conditions. Fig. 16 summarizes these findings concerning single phase heating zone reduction for different overall temperature differences between service and product side. The normalized length of single phase heating zone for a variation of driving temperature difference and tube inserts is depicted for the water/glycerol-system at ambient pressure and h* = 135%. A significant reduction of the single phase length is achieved. This gives rise to an increase in the overall product side heat transfer coefficient. These findings are of special interest in cases with a low content of volatiles in the circulating flow. Early start of bulk flow boiling will increase and stabilize flow circulation and by that broaden the window of operation for TSR.
S. Scholl, F. Brahim / Applied Thermal Engineering 25 (2005) 2615–2629 normalized length of heating zone [-]
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1.0 0.9 0.8 0.7 0.6 0.5
ε=0% ε = 0.6 % ε=3%
0.4 0.3 20
25
30
35
40
45
50
∆TOV = (TOil - Tb, Product) [K]
Fig. 16. Normalized length of single phase heating zone for a variation of driving temperature difference and tube inserts, water/glycerol-system, ambient pressure, h* = 135%.
5. Conclusions and outlook The effect of hiTRAN tube inserts on the fluiddynamic as well as thermal performance of vertical thermosiphon reboilers was investigated experimentally in a single tube lab-scale reboiler. Pure water was used as test fluid to model a rectification system while a water/glycerol-mixture with a glycerol mole fraction of 0.29 mol/mol stood for a separation system. Five different inserts were tested in reference to a bare tube. The inserts varied in loop volume fractions ranging from el = 0.006 to 0.035. All experiments were conducted at ambient pressure. Encouraging results were obtained for the water/glycerol-system showing an improved circulation at low values of DTov and h*. No benefits were found at high values of DTov and h*. Circulation proved to be more stable with reduced fluctuations. Best cases showed a 50% increase of the overall tube side heat transfer coefficient. For the pure water system no benefits could be identified except slight improvements in circulation at low DTov and h* when inserts with a loop volume fraction of larger than 2% were used. With respect to loop design best results were achieved for a loop volume fraction of el = 3%. Future investigations will aim at a quantification of the observed experimental findings through modeling and simulation. The extension of operation limits for TSR through inserts for the different system, design and process parameters is of key importance. Also alternative insert types, like twisted tapes, and other designs of heat transfer surface modifications, like micro- and macrofinned or coated tubes, will be tested.
Acknowledgement Supply of the inserts by Cal Gavin Process Intensification Engineering, Alcester, Warwickshire/ UK, is gratefully acknowledged.
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