Direct heat and mass transfer in structured packings

Chem~

ELSEVIER

Engineering and Processing Chemical Engineering and Processing 35 (1996) 479-485

Direct heat and mass transfer in structured packings 1 L. Spiegel, P. Bomio , R. Hunkeler Sulzer Chemtech Ltd.• Winterthur. S witzerland

Received 11 March 1996; accepted 29 April 1996

Abstract

Direct heat transfer is an important method in the exchangeof heat betweentwo countercurrent process streams within a column. The processcan be simulated using either the theoretical stage or the rate based concept. With both concepts. a reliable heat transfer coefficient is needed. Additionally, the rate of the heat transfer coefficient is influenced by the simultaneous mass transfer. A number of application-dependent methods to estimate the heat transfer coefficient have been developed. mainly for random packings. It is the purpose of this paper to extend this work to structured packings. A number of experiments with air/water have been performed in a column of300 mm inner diameter with Mellapak 250.Y.250.X and 12S,X at ambient conditions. A second group of measurements were done using an oil/air system where only sensible heat was transferred. Based on these experimental results a method was developed to predict the heat transfer coefficient for structured packings. The method is applied to examples of industrial importance, like a gas quench, a gas saturator and a pump-around zone in an atmospheric tower. Keywords: Direct heat transfer; Mass transfer; Process streams; Heat transfer coefficient

1. Introduction Direct heat transfer is an important method in the exchange of heat between two countercurrent process streams within a column. Well known examples are, among others, pump-around zones in refinery columns and ethylene quenchers. The heat transfer takes place between the gas and the liquid phase that are in close contact. Because no additional heat exchanger equipment is needed, investment costs are saved. The process can be simulated using either the theoretical stage or the rate based concept. With both concepts, a reliable heat transfer coefficient is needed. Additionally, the rate of the heat transfer coefficient is influenced by the simultaneous mass transfer. A number of application dependent methods to estimate the heat transfer coefficient have been developed, mainly for random packings [I]. The performance of the structured packing Mellapak under distillation and absorption conditions is well known . However, a similar knowledge of packing per-

formance under heat transfer conditions was lacking. Therefore, a test program was started to gain a better understanding of packing behaviour under such conditions. . For most experiments the air/water system was used . This system has well known properties, a large enthalpy of vaporization and a Lewis number of I, which greatly simplifies the evaluation of the test results. The presence of the transfer of both latent and sensible heat is characteristic. In order to investigate the purely sensible heat transfer (conduction), the system air/dibutylphthalate (DBP) was selected. DBP has a reasonably low viscosity and a very low vapour pressure in the temperature range investigated. The data can be easily evaluated with the methods used for tubular heat exchangers, yielding overall heat transfer coefficients. Based on the experimental results a method was developed to predict the heat transfer coefficient for structured packings. 2. Pilot plant

I Prepared for presentation at the AIChE Spring National Meeting, March 19-23, 1995 in Houston, Texas (Distillation Column Design and Operation-II).

0255-2701/95/$15.00 © 1996 PI] S0255 -2701(96)04162-1

Elsevier Science S.A. All rights reserved

The pilot plant consisted of a well-insulated pyrex glass column of 400 mm internal diameter, as well as

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L. Spiegel et al. / Chemical Engineering and Processing 35 (1996) 479-485

The pressure was 96 kPa (atmospheric conditions). For a test series, the liquid load was kept constant (between 1 and 25 m" m -2 h -I), the gas load F-factor Fv varied between 0.8 and 2.2 Pao. s. The first group of experiments was done with the air/water system with exchange of both latent and sensible heat, and the second group with the air/DBP system where only sensible heat was transferred.

Exhaust

Llquld

3. Air/water tests

Back up

water

, Hl ••,. H•• ter

z

I

Fig. I. Sketch of the pilot plant.

liquid tanks and pumps, gas ventilators and auxillary equipment. A sketch is given in Fig. 1. The plant was operated in a once-through mode for air (open loop), whereas the liquid was recirculated. Provision was made for air and liquid heating and/or cooling. The gas flow rate was measured with an U-tube manometer and propeller, and the liquid flow rate with an inertial meter. For humidity measurements, on-line sensors were available. Special attention was devoted to the measurement of the temperature in the column. Twenty high precision Pt100 thermoresistors were used, together with an on-line hybrid recorder for data aquisition. Painstaking calibration of the Ptl00 sensors allowed reproducible temperature measurements to a real accuracy of 0.1 K. The following packings were tested: Mellapak 250.X and 250.Y The geometric packing data are listed in Table 1. The packing height corresponded to three layers rotated to each other by 90°. More information on the packings is given in [9]. Table 1 Geometric packing data

Type Mellapak 250.x Mellapak 250.Y

Height (m) 250 250

30 45

The measured temperatures on the top and bottom of the packing in both phases, together with the flow rates and humidities, were used to calculate the operating line in the enthalpy-temperature diagram. The equilibrium line for air/water was calculated based on published data [2]. The method described by [3] was used to evaluate the experimental data. The number of overall transfer units NTU oo was calculated based on the enthalpy difference on the gas side assuming no mass transfer resistance on the liquid side: t op dho NTU oo = bottom ho - hi where bo is the enthalpy of the bulk gas phase and hi is the enthalpy of the gas phase at the interface. The number of overall transfer units per meter NTUMoo was calculated:

0.675 0.63

NTUM oo = NTUoo/Z with Z being the packing height. In Figs. 2 and 3, the NTUM oo is plotted against the gas load F-factor F v for Mellapak 250.X and 250.Y for different liquid loads. The NTUM oo depend on the gas and the liquid load. For a system that is gas side controlled, a dependence on the gas load is expected. The influence of the liquid load may be attributed to the effective interfacial area, which depends on the liquid load for the air/water system. This system does not wet a metallic packing surface well at low liquid loads. The effect has been described by de Brito et at. [4] and others. McNulty et at. [5] published measurements of Flexipac 2Y with the same system which agree with our measurements of Mellapak 250.Y quite well (see Fig. 4). His data demonstrate a strong dependence on liquid load also. The influence of the liquid load on the effective interfacial area is found by a regression analysis to obey the following power law:

where al,efT is the effective interfacial area and superficial velocity of the liquid phase.

VL

is the

481

L. Spiegel et al. / Chemical Engineering and Processing 35 (1996) 479-485

4. Air/DBP tests

MELLAPAK 250.x airlwater heat and mass transfer 10

Fig. 5 shows the overall heat transfer coefficient U of Mellapak 250X for the system air/DBP in function of the gas load F v . Because of the low volatility of DBP, there is only sensible heat transferred. The temperature of the DBP at the inlet was around 50°C. The overall heat transfer coefficient was calculated using the theory of tubular heat exchangers:

I liquid load

9

m3/m 2 h

•

8

5.9 12.5 25.1

A

7

I

•

6

....

•

e0

5

~

:::>

IZ

where Q is the heat flow, A is the heat transfer area of the packing, and ATLM is the mean logarithmic temperature difference. Analysis of the data yields the following dependence of the overall heat transfer coefficient

-

E

A

4

•

A A

3

••

•

A

..

A

b

A

•

A•

2

•

°

0.8

1.0

1.2

1.4

1.6

Gas load FV1 mls

A

••

1.8 2.0 3 (kg/m )0.5

A

•

-

which is completely in accordance with the air/water tests. There seems to be no dependence on liquid load. This is typical for systems wetting the packing surface well. MELLAPAK 250.Y air/water heat and mass transfer

2.2 10

Fig. 2. Heat and mass transfer of Mellapak 2S0X measured with the air/water system.

liquid load m3/m2h 5.9 A 8.5 12.5 <> 25.1

9

•

8

This has to be compared to the random packings where a much stronger dependence on V L is found. This may be due to the fact that aI.efT of random packings depends much more on the liquid load than structured packings. Because the Lewis number of the air/water system is close to one, the overall heat transfer coefficient U is calculated from NTUM oo as follows:

U= NTUMooPGvG al.errCP,G

where VG is the superficial velocity of the gas phase, PG is the gas density, and cP,G is the specific heat capacity of the gas. Combining these results together we find the following correlation for U of Mellapak 250.X: UOCV~8

The exponent 0.8 of the gas load F-factor is typical for gas side controlled systems.

7

.... I

•

6

E 0 5

A

:::>

•

0 ~

IZ

4

•

•

<>

•~

A

A

•

3

<>

..

<()

•

<>

~

-

."

2

°0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Gas load FVt mls {kg/m 3 )0.5 Fig. 3. Heat and mass transfer of Mellapak 2S0.Y measured with the air/water system.

L. Spiegel et al.; Chemical Engineering and Processing 35 (1996) 479-485

482

FLEXIPAC2Y airlwater heat and mass transfer 10

m3/m2h

6.1. Atmospheric crude oil tower

~"'-12.2

8

The first example is an atmospheric crude oil tower in the USA. It has been described by Roza et al. [7] and McNulty et al. [8]. Only the bottom pump-around bed was equipped with structured packing: 0.91 m Flexipac

r---e-24.4 ~.-48.8

7 6

3Y.

E 8 5 0

:E ::>

...

We analyzed industrial heat transfer data of two different applications.

liquid load

9

.-I

6. Industrial heat transfer data

.1"--....

-I---

4

Z

3

............

I---

... '---

---------.-

r-.. I - -

I--

2

--

...

°0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

The published overall volumetric heat transfer coefficient of 17.9 kW m - 3 K -1 (see Table 2) is based on the external heat duty of 8.4 MW that includes latent and sensible heat transport [7]. An analysis analogous to the procedure given above based on the internal sensible heat transfer only shows that the overall volumetric heat transfer coefficient is 19.2 kW m - 3 K - 1. If we consider the complete heat transfer including the latent heat, we find a value of 62.7 kW m - 3 K - 1. This is in agreement with the analysis of McNulty et al. [8] who found a heat transfer coefficient of 61.8 kW m - 3 K - 1 simulating the tower with RATEFRAC (ASPENTECH).

Gas load Fv• m/s (kg/m 3)0.5

MELLAPAK 250.X air/DBP heat transfer

Fig. 4. Heat and mass transfer of Flexipac 2Y measured with the air/water system [5].

70

Additional tests with cold DBP showed that U does not depend on the direction of the heat flux.

Liquid load

65

m 3/m2h 3

60

• •

5. Correlation of the data

555

.,

In Fig. 6, the overall heat transfer coefficient U is plotted against the gas side Reynolds ( - Reo) number:

::i 50

-

•

Reo = PoVo dtJ(p.o cos(y»

~ 45

Q' C'l

...

~

c
U = 0.0925Re~8 This theoretical U is also shown in Fig. 6 (labelled "model") for comparison.

. •

...8 40 ~

•

II)

...gbo

•

:c

~

25

.;

•

•

~.

•

t

•

•

•A

..

~

-

c 35

~

..

25

...


where dh is the hydraulic diameter of packing, Jlo is the dynamic viscosity of the gas phase, and y is the corrugation angle of packing. The cos(y)-term in the Reynolds number is used to account for the effective velocity of the gas within the packing. Then one must not distinguish between packings of different corrugation angles (X- or V-types). The experimental data of the two tests fit together well. A regression analysis yields the following correlation:

...

6 10 19

•

20 o~

1~

1~

1A

1~

1~

2~

2~

2A

Gas load Fv• m/s (kg/m 3)O.5 Fig. 5. Direct heat transfer of Mellapak 250.x measured with the system air/DPB.

483

L. Spiegel et al. / Chemical Engineering and Processing 35 (/996) 479-485

correlation for heat transfer for forced convection (see Bird et al. [6]):

MELLAPAK 250.X direct heat transfer 100

Nuo/PrM3 a: Re~8

»,

90

•

Q' N

airlDBP airlwater model

•

80

f-----

E 70

~

-

~

60

c(1)

•

'0 50

-. ~ -~

IE

8... 40

•• ':L • ••

~ II)

c

g

eu

'(1)

J:

30 /

20 10

o

-:

/

V

~

1/ o

500

1000

1500

2000

2500

where Nuo is the Nusselt number (= 4U/(aIko U is the overall heat transfer coefficient, ko is the thermal conductivity, and Pro is the Prandtl number (= f.loCp,o/ ko)· The abscissa in the Nusselt diagram are NUo/PrM 3, and the ordinate is Reo. The following procedure was used to obtain the data points for the Nusselt diagram. In the first step, the column was simulated using PRO/2 (SimSci). From the profiles, the mean logarithmic temperature difference tiTL M and the amount of sensible heat Qs (difference of total heat minus latent heat) was calculated. Then U, Nu, Pr and Re were calculated. The physical properties were taken from the simulation. The correction factor according to Ackermann [10] for high mass fluxes was considered also. The data are compiled in the Table 2. In Fig. 7, the Nusselt correlation is tested by this industrial heat transfer data. We see that the data agree fairly well in the Nusselt diagram. The deviation between correlation and data is between 10 and 25%.

3000

Reynolds number ReG Fig. 6. Comparison of experimental and model data of sensible heat transfer of Mellapak 250.x.

The second example is the Kerosin pump-around zone of an atmospheric tower in Japan. The diameter was 3.05 m. A mixed bed consisting of Mellapak l25.x and 250.x was installed. Three different operating cases were measured. The external heat duty was 6.15, 6.93 and 7.39 MW. 6.2. Water quench tower in an Ethylene plant

The first example has already been described in [7]. The tower is located in Japan and has two sections. The top section has a diameter of 3.5 m filled with a mixed bed of Mellapak 250.x and l25.Y. The bottom section has a diameter of 4.5 m filled with Mellapak l25.Y. The external heat duty is 14.5 MW in the top PA and 26.7 MW in the bottom section. The second example is a German tower operated at a top pressure of 12.4 kPa. Only the top section was analysed. The diameter was 4.3 m filled with Mellapak l25.x. Three different operating cases were analyzed, according to a heat duty of 10.2, 14.7 and 25.6 MW. The physical properties of the fluids in the industrial applications described above are very different. To make possible a comparison of the heat transfer data, we used the Nusselt diagram which is based on the

7. Designing a direct heat transfer section The column was simulated with a rigorous column design software. With this information, the amount of latent heat transferred (mass transferred times heat of vaporization) for the considered direct heat transfer section was calculated. This was subtracted from the total heat flow to get the sensible heat transferred. From the temperatures of both phases at the ends of the bed, the mean logarithmtic temperature difference was be calculated. U was taken from Fig. 7. The necessary area for the heat transfer was then calculated: A = Qs/(UtiTL M ) It was assumed that U depends on the gas velocity

(and on the liquid velocity for non wetting systems). For a column of given diameter d, the necessary packing height was then calculated. Z = A/(rrd 2/4aI )

For operational reasons, the packed height installed should comprise at least four packing layers. 8. Conclusion

A Nusselt type correlation of direct heat transfer based on the sensible heat flow was developed. The correlation was tested with measurements using the air/water system and the air/DBP system. The correla-

484

L. Spiegel et al. / Chemical Engineering and Processing 35 (1996) 479-485

Table 2 Data points for the Nusselt diagram Application

U (W (m2 K)-I)

Reo

Pro

(Nu/Pr l / 3)o

Atmospheric tower USA, bottom PA Atmospheric tower Japan, case I Atmospheric tower Japan, case 2 Atmospheric tower Japan, case 3 Water quench Japan, bottom PA Water quench Japan, top PA Water quench Germany, case 1 Water quench Germany, case 2 Water quench Germany, case 3

167 7I.S 78.0 75.1 64.5 87.9 29.7 47.3 100.6

23969 6479 7381 6817 11194 4739 6371 11054 13755

I.10 I.10 I.10 I.10 0.75 0.75 0.75 0.75 0.75

21I.S5 76.16 82.31 79.25 85.70 58.47 42.53 67.73 142.66

cp

Directheat transfercorrelation

,

,

220

atm. tower 1 atm. tower 2 Ethylene quench 1 Ethylene quench2 - - model V

• '"

180 160

l:2

.....oS' 140

w

120

/

~ :J

...

Z

~ 100 E :J c:

~

/

./

80

CI>

II) II)

:J

Z

60

-II

I

40 20

o

•

• •

200

I

d dh

/'

/

V

NTUMoo Nu Pr

/

V

/

Qs

Re !1TL M

•

(K)

U

'"

v

w

Z

/

o

5000

Fy h !1h LO k NTUoo

10000

specific heat capacity (J (kg K)-I) column diameter (m) hydraulic diameter of packing ( = 4/alo m) gas F-factor ( = Va' plfl, Pao. S) specific enthalpy (J kg") latent heat of vaporization (J kg-I) thermal conductivity (W (m K)-I) number of overall transfer units based on gas phase number of overall transfer units based on gas phase per meter (m- I) Nusselt number ( = Udh/k) Prandtl number (= pCp/k) sensible heat flow (W) Reynolds number (= vpdh/(p cos(y» mean logarithmic temperature difference

15000

20000

25000

overall heat transfer coefficient based on gas phase (W (m2 K)-I) superficial phase velocity (m S-I) packing height (m)

Greek letters y corrugation angle of packing (0) p dynamic viscosity (N s m -2) p gas density (kg m")

Reynolds number ReG Fig. 7. Test of the heat transfer model by operating data of industrial applications.

tion is confirmed by heat transfer data from various industrial applications.

9. Nomenclature al

geometric interfacial area of packing (m" m ")

al.cIT

effective interfacial area of packing (m' m ")

Subscripts

G I L

gas phase interface liquid phase

References [I] R.F. Strigle Jr., Random Packings and Packed Towers, Gulf, Houston, 1987. [2] VDI-Wiirmeatlas, 5th edn, VOl-Verlag, 1988. [3] H.S. Mickley, Design of forced draft air conditioning equipment, Chern. Eng. Prog., 45(12) (1945) 739-745. [4] M.H. de Brito, U. von Stoekar, A.M. Bangerter, P. Bomio and M. Laso, Effective mass-transfer area in a pilot plant column equipped with structured packings and with ceramic rings, Ind. Eng. Chern. Res., 33(3) (1994) 647-656.

L. Spiegel et al. / Chemical Engineering and Processing 35 (1996) 479-485

[5] K.J. McNulty and C.H. Hsieh, Hydraulic performance and efficiency of Koch F1exipak structured packings, AIChE Annual Meeting, Los Angeles, November, 1982. [6] Bird, Stewart and Lightfoot, Transport Phenomena, Wiley, New York, 1960. [7] M. Roza, R. Hunkeler, O.J. Berven and S. Ide, MeUapak in refineries and the petrochemical industry, I. Chem. E. Symp. Ser., 104 (1987) BI65-BI78. [8] K.J. McNulty and S.G. Chatterjee, Simulation of atmospheric

485

crude towers including packed bed pump-around zones using a rate-based simulator, I. Chem. E. Symp, Ser., 128 (1992) A329-A344. [9] Sulzer Brothers, Winterthur, Switzerland, Separation Columns for Distillation and Absorption, Publication No. 22.13.06, 1991. [10] G. Ackermann, Wiinneiibergang und molekulare StofTiibertragung im gleichen Feld bei grossen Temperatur- und PartialdruckdifTerenzen, VDI-Forschungsheft 382, VOl-Verlag, 1937.