- Email: [email protected]
Performance of a new packing with low pressure drop
79
The Chemical Engineering JoumaL. 45 (1990) 79-81
Short Communication
Performance of a new packing with low pressure drop H. O . Ozbelge& and T. G. Somer Departnaen2 of Chemical Enigineering, Middle East Technical University, Ankara (Trkeg)
(Received July 19, 1989 ; in final form December 6, 1989)
1 . Introduction A new kind of packing is proposed for use in vacuum distillation . It consists of flat circular metal discs with holes at the centre through which metal rods are passed, as seen in Fig . 1 . The rods are installed vertically in a square pattern in a column, so that the discs overlap each other in criss-cross fashion .
With this configuration each layer of circular discs is perfectly horizontal and the liquid flows through a uniform bed ; thus a good liquid distribution is expected . As the liquid descends the packing, its direction changes from vertical on the rods to horizontal on the discs to achieve good mixing, and it does not stay in pockets at any point in the column . The vapour passes through the discs in a zigzag path ; so a good mixing and a high contact time are provided for mass transfer . At high liquid rates the liquid film may start to flow down from the periphery of the circular discs in the form of cylinders ; the rising vapour in the criss-cross movement intercepts this film . The pressure drop occurs mainly because of the changes in the cross-sectional flow area for the gas. However, the porosity is quite high so that the drag and the pressure drop are low .
2 . Experimental
The discs used for making the packing for the preliminary studies are 17 mm in diameter and 1 mm in thickness, and the rods are 4 mm in diameter. For the preliminary experiments the rods and the discs were made of brass ; the packing can be made out of plastic material also to decrease the weight . The packing prepared had a voidage of 0.85 and a dry surface area of 191 .5 mz m_a and was
FIg . 1 . Pleces of disc packing cut from the assembly used
in the experiments . *Present address : Chemical Engineering Department, King Saud University, P .O . Box 800, Riyadh 11421, Saud] Arabia .
0200-9487/90/$3 .50
contained in a steel column of 0 .096 m diameter and 0 .875 m height. The performance of the packing was evaluated using the following experimental data obtained at atmospheric pressure : (i) the dry and irrigated packing pressure drops ; (ii) flooding rates ; (iii) distillation of a binary mixture . The first two groups of experiments were performed using air and water to obtain pressure drop vs . F factor data . In flooding rate experiments, extreme care was exercised to determine the exact F factor corresponding to the flooding point. Distillation experiments were performed with the benzene-toluene system at total refiux. The range of benzene mole fraction In these experiments was 0.85-0 .0235,
C Elsevier Sequoia/Printcd in The Netherlands
s0 100
90
System : Air-Water
70
Symbol L(m3lm 2 hr)
• 0 50 - a 60 -
X
40 -
0
and the calculations were based on the equilibrium data of Rollet et at . [ 11 for the benzene-toluene system . Necessary experimental measures were taken to ensure adiabatic operation, to determine the quality and the rate of the steam used, and to minimize the error in the pressure drop measurements due to condensation and spills in the manometer taps.
Dry Packing 16 .4 26.5 30 .1 34 .0
30 25 20 -
3. Results 15 -
F-Factor , U V +/ry ,m112 sec kg
12
Fig. 2 . Pressure drop data obtained in the dry and Irrigated packing experiments .
Figure 2 shows the results of some of the dry and irrigated packing experiments as well as the flooding tests . At low liquid rates, flooding is possible only with very high vapour -2 rates . In the water rate range 11-35 m$ m h - I and for all F factor values, the pressure drop values obtained were lower than those reported 121 for Intallox saddles, Pall rings, Berl saddles and Rasching rings with sizes of £ in and smaller . In distillation the specific pressure drop (i.e. the pressure drop per theoretical plate) values measured were compared with those reported for conventional contact equipment operating at the same tray loading factor (i.e. Uv(p,1prp,)'"2) ; the performance of the new packing is much better than 25 mm ceramic Pall rings, a,perforated plate with 10 mm hole size, a Spraypak with a 2 mm hole, a .Kittel tray with a 5 mm hole, a floating-cap tray with a 40 mm hole, and a bubble cap
40 1Q 25 mm Pall Rings (B-T )
35
22 35mm Poll Rings (E-5) 30 - () 51 mm Lewa Packing (1-T ) 0 I?mn Disc Pocking(B-T) 25 20 15
f + Iso-Octane E : Ethyl Benzene S : Styrene T : Toluene B : Benzene
10 5 0 0
0 .5
1 .0
1.5
F-Factor, U v I'
2 .0 112
sec"
2 .5
30
3.5
kg 112
Fig. 3 . Comparison of specific pressure drop data with recently commercialized packings.
81
tray with a 46 mm riser diameter and with caps 76 mm in diameter, 70 mm in height and 22 slots . In Figure 3 we present the comparison of the performance of the new disc packing with those of the pacldngs [3, 4] commercially used in vacuum distillation . Although the size of the new packing is smaller than those of the commercial packings that it is compared with, its specific pressure drop data are either better than or similar to those of the commercial packings . There is a change in the slope of the experimental curve for the disc packing at an F factor of around 1 .5 . This change in the performance suggests that near this experimental condition in the packing the momentum of the vapour becomes high enough to deflect the liquid towards the walls of the column . The liquid can no longer descend as described in Section 1 ; the contact area is decreased and the mass transfer rate declines .
4 . Conclusion The experiments performed at atmospheric pressure in a pilot-size column prove that the new packing is superior to almost all conventional packings and gives a better performance than or at least the same performance as the recently commercialized packings . At vapour rates for an F factor of up to 1 .5 the packing can function as expected ; however, improvements arc needed for higher F factor range .
References 1 E . Rollet (ed), 1959, Physicochemical Constants of Binary Systems VI (New York : Wiley-Interscience Publishers) . 2 Packed Towers Engineering Data, U .S . Stoneware Co., Akmn, OH, 1964. 3 R. Billet, Chem. Eng. Prog., 63 (9) (1967) 53 . 4 M . Leva, Chem. Eng. Pmg ., 76 (9) (1980) 73 .
The Chemical Engineering JoumaL. 45 (1990) 79-81
Short Communication
Performance of a new packing with low pressure drop H. O . Ozbelge& and T. G. Somer Departnaen2 of Chemical Enigineering, Middle East Technical University, Ankara (Trkeg)
(Received July 19, 1989 ; in final form December 6, 1989)
1 . Introduction A new kind of packing is proposed for use in vacuum distillation . It consists of flat circular metal discs with holes at the centre through which metal rods are passed, as seen in Fig . 1 . The rods are installed vertically in a square pattern in a column, so that the discs overlap each other in criss-cross fashion .
With this configuration each layer of circular discs is perfectly horizontal and the liquid flows through a uniform bed ; thus a good liquid distribution is expected . As the liquid descends the packing, its direction changes from vertical on the rods to horizontal on the discs to achieve good mixing, and it does not stay in pockets at any point in the column . The vapour passes through the discs in a zigzag path ; so a good mixing and a high contact time are provided for mass transfer . At high liquid rates the liquid film may start to flow down from the periphery of the circular discs in the form of cylinders ; the rising vapour in the criss-cross movement intercepts this film . The pressure drop occurs mainly because of the changes in the cross-sectional flow area for the gas. However, the porosity is quite high so that the drag and the pressure drop are low .
2 . Experimental
The discs used for making the packing for the preliminary studies are 17 mm in diameter and 1 mm in thickness, and the rods are 4 mm in diameter. For the preliminary experiments the rods and the discs were made of brass ; the packing can be made out of plastic material also to decrease the weight . The packing prepared had a voidage of 0.85 and a dry surface area of 191 .5 mz m_a and was
FIg . 1 . Pleces of disc packing cut from the assembly used
in the experiments . *Present address : Chemical Engineering Department, King Saud University, P .O . Box 800, Riyadh 11421, Saud] Arabia .
0200-9487/90/$3 .50
contained in a steel column of 0 .096 m diameter and 0 .875 m height. The performance of the packing was evaluated using the following experimental data obtained at atmospheric pressure : (i) the dry and irrigated packing pressure drops ; (ii) flooding rates ; (iii) distillation of a binary mixture . The first two groups of experiments were performed using air and water to obtain pressure drop vs . F factor data . In flooding rate experiments, extreme care was exercised to determine the exact F factor corresponding to the flooding point. Distillation experiments were performed with the benzene-toluene system at total refiux. The range of benzene mole fraction In these experiments was 0.85-0 .0235,
C Elsevier Sequoia/Printcd in The Netherlands
s0 100
90
System : Air-Water
70
Symbol L(m3lm 2 hr)
• 0 50 - a 60 -
X
40 -
0
and the calculations were based on the equilibrium data of Rollet et at . [ 11 for the benzene-toluene system . Necessary experimental measures were taken to ensure adiabatic operation, to determine the quality and the rate of the steam used, and to minimize the error in the pressure drop measurements due to condensation and spills in the manometer taps.
Dry Packing 16 .4 26.5 30 .1 34 .0
30 25 20 -
3. Results 15 -
F-Factor , U V +/ry ,m112 sec kg
12
Fig. 2 . Pressure drop data obtained in the dry and Irrigated packing experiments .
Figure 2 shows the results of some of the dry and irrigated packing experiments as well as the flooding tests . At low liquid rates, flooding is possible only with very high vapour -2 rates . In the water rate range 11-35 m$ m h - I and for all F factor values, the pressure drop values obtained were lower than those reported 121 for Intallox saddles, Pall rings, Berl saddles and Rasching rings with sizes of £ in and smaller . In distillation the specific pressure drop (i.e. the pressure drop per theoretical plate) values measured were compared with those reported for conventional contact equipment operating at the same tray loading factor (i.e. Uv(p,1prp,)'"2) ; the performance of the new packing is much better than 25 mm ceramic Pall rings, a,perforated plate with 10 mm hole size, a Spraypak with a 2 mm hole, a .Kittel tray with a 5 mm hole, a floating-cap tray with a 40 mm hole, and a bubble cap
40 1Q 25 mm Pall Rings (B-T )
35
22 35mm Poll Rings (E-5) 30 - () 51 mm Lewa Packing (1-T ) 0 I?mn Disc Pocking(B-T) 25 20 15
f + Iso-Octane E : Ethyl Benzene S : Styrene T : Toluene B : Benzene
10 5 0 0
0 .5
1 .0
1.5
F-Factor, U v I'
2 .0 112
sec"
2 .5
30
3.5
kg 112
Fig. 3 . Comparison of specific pressure drop data with recently commercialized packings.
81
tray with a 46 mm riser diameter and with caps 76 mm in diameter, 70 mm in height and 22 slots . In Figure 3 we present the comparison of the performance of the new disc packing with those of the pacldngs [3, 4] commercially used in vacuum distillation . Although the size of the new packing is smaller than those of the commercial packings that it is compared with, its specific pressure drop data are either better than or similar to those of the commercial packings . There is a change in the slope of the experimental curve for the disc packing at an F factor of around 1 .5 . This change in the performance suggests that near this experimental condition in the packing the momentum of the vapour becomes high enough to deflect the liquid towards the walls of the column . The liquid can no longer descend as described in Section 1 ; the contact area is decreased and the mass transfer rate declines .
4 . Conclusion The experiments performed at atmospheric pressure in a pilot-size column prove that the new packing is superior to almost all conventional packings and gives a better performance than or at least the same performance as the recently commercialized packings . At vapour rates for an F factor of up to 1 .5 the packing can function as expected ; however, improvements arc needed for higher F factor range .
References 1 E . Rollet (ed), 1959, Physicochemical Constants of Binary Systems VI (New York : Wiley-Interscience Publishers) . 2 Packed Towers Engineering Data, U .S . Stoneware Co., Akmn, OH, 1964. 3 R. Billet, Chem. Eng. Prog., 63 (9) (1967) 53 . 4 M . Leva, Chem. Eng. Pmg ., 76 (9) (1980) 73 .