CHEMICAL ENGINEERING RESEARCH AND DESIGN 99 ( 2 0 I 5 ) 228-235

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Chemical Engineering Research and Design IChemEj o u rn a l hom epage: w w w . e ls e v ie r . c o m / l o c a te /c h e r d

Influences of the experimental setup configuration on mass transfer measurements in absorption systems

Verena Wolfa’*, Markus Lehnera, Karin Hoffmannb3 Montanuniuersitaet Leoben, Leoben, Austria b RVT Process Equipment GmbH, Steinuiiesen, Germany

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A R T I C L E I N F O A B S T R A C T

Article history: The standardisation of mass transfer measurements for absorption systems is a key factorReceived 20 October 2014 for the deduction of accurate mass transfer models for random and structured packing.Received in revised form 12 March Several papers (Hoffmann et al. (2007). Trans IChemE, A, 85 (Al),40;Kunzeetal. (2012). Chem.2015 Ingen. Techn., 84, 1931; Rejl et al. (2009). Chem. Eng. Res. Des., 87, 695) already deal withAccepted 3 April 2015 this problem and recommendations are given for appropriate test systems, the executionAvailable online 13 April 2015 of the experiments and particularly also for the experimental setup to be used. However,_______ _______________________ systematic investigations of the influences of the experimental setup configuration on theKeywords: results of mass transfer measurements in absorption systems are published rarely.Absorption Mass transfer measurements with the system ammonia-air/water have been performedMass transfer measurement at a pilot plant consisting of a DN 600 saturation column and a DN 450 measuring col-Standardisation umn, both made from polypropylene, and equipped with different random and structuredPacking packing. Although the used experimental setup of the pilot plant follows strictly the rec-Experimental setup ommendations published in Hoffmann et al., experimental results may differ significantly

depending on, for example, the locations of the gas and liquid sampling in the column, the raw gas concentrations of ammonia or the pre-treatment of packing.

The paper presents the results of the test series considering different effects on the derived mass transfer performance of the packing and addresses also problems as well as solutions concerning the sampling in a two-phase regime. The aim is to provide a valuable contri­bution to the efforts for the standardisation of mass transfer measurements in absorption systems.© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction

Mass transfer m easurem ents at pilot plants are the basis for the dimensioning of industrial columns as well as for the m ass transfer models developed during the last years. For this reason the quality of m ass transfer m easurem ents is very im portant which have to be conducted as conscientiously as

possible. A lot of influences on m ass transfer m easurem ents exist which need to be considered. Many of them are already m entioned in Hoffman et al. (2007), Rejl e t al. (2009) as well as in Kunze et al. (2012). Nevertheless, there are further impor­tan t influences to be reflected and in this paper the effects of the chosen raw gas concentration, sampling position, and the pre-treatm ent of plastic packing are considered.

* Corresponding author. Tel.: +43 38424025010.E-mail address: verena.wolf@unileoben.ac.at (V. Wolf).

http://dx.doi.Org/10.1016/j.cherd.2015.04.0050263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 0 I 5 ) 2 2 8 -2 3 5 229

Symbols

A cross sectional area of the column [m2]c concentration [mol/m3]DN diameter nominalF F-factor [Pa0 5]FTIR Fourier transformed infraredH height of packed bed [m]H20 waterHe(TG) Henry constant dependent on gas temperature

[bar]HTUov height of a transfer unit (overall) [m]feNH3 (tg; Henry constant dependent on gas tempera­

ture [mol/(l3 atm)]MM molar mass [kg/mol]NH3 ammoniaNTU number of transfer units [-]V pressure [Pa]T temperature [°C], [K]UL liquid load [m3/(m2 h)]V volume flow [m3/h]X liquid mole fraction [mol/molges]X liquid mole load [mol/moljnert]y gas mole fraction [mol/molges]Y gas mole load [mol/mol;nert;]p density [kg/m3]

IndicesG,g gas phasei component iin inlet streamU liquid phaseout outlet streamOV overallS saturation

2. Methods

2.1. Experimental setup

The experimental plant at the Montanuniversitaet Leoben consists of a DN 600 saturation column and a DN 450 mea­surement column made of polypropylene which can be seen in Figs. 1 and 2.

The air is sucked from the environment and flows through the saturation column where it is saturated with water. The relative humidity of the sucked air and of the air leaving the saturation column are measured. After passing a tube type gas distributor, positioned 1 m below the packing, the gas streams through the measurement column. Afterwards, it enters a radial blower (frequency converted gear, max. 5000 m3/h, max. 5000 Pa pressure increase) and is then finally released to the environment. The gas flow is measured by a flow grid (airflow) downstream of the blower. Furthermore, the temperature of the gas is measured at every in- and outlet of both columns (Endress & Hauser, PT 100 type Omnigrad M TR 13; WIKA, type 55) which have their own liquid circulation. The pack­ing is irrigated by a trough type liquid distributor which is situated 0.15 m above the packing (128 dripping points per m2, free cross sectional area of 41%, three different oper­ating ranges are available: 50-30 m3/(m2 h), 10-60 m3/(m2h), 30-120 m3/(m2h)). While the saturation column possesses a

Fig. 1 - Experimental plant “packed column” at Montanuniversitaet Leoben.

closed liquid circulation, the liquid in the measurement col­umn is only used for one single passage. It is collected below the gas entrance of the column in a pan type collector and conducted to a bin outside of the column. The used radial pumps are able to handle a volume flow of up to 50 m3/h. The liquid flow is measured by a magnetic inductive flow meter (Krohne, type optiflux 4000) and the temperature of the inlet stream at the measurement column is gauged by a resistance thermometer (Voltcraft, type K).

The mass transfer characteristic has been determined by using the absorption model system ammonia (NH3)-air/water. The mass transfer resistance for this system is located in both phases although it is predominant in the gas phase (Hoffmann et al., 2007). This mass transfer system has been chosen because it is established at many experimental plants and therefore measurement data for validation exists.

The gaseous ammonia is injected in the gas duct between the saturation and the measuring column and has to pass a static mixer to ensure an optimal mixing. A constant ammonia dosage is ensured by the use of a pressure regulator, a valve as well as a float-type flow meter.

The concentrations of ammonia in front of and behind the packing are measured for each operating point (constant F-factor and liquid load) by an FTIR spectrometer (Gasmet Technologies Oy, DX4000) in real time at a frequency of 1Hz. Furthermore, the aqueous vapour concentration of the gas stream is also measured every second by the FTIR. This mea­surement is a key factor in monitoring that no water droplets are sucked in at the gaseous sampling location so that the test results do not get falsified as ammonia is readily soluble in water.

The concentration of ammonia in the liquid phase is ana­lysed by photometric determination after it was preserved by the use of hydrochloric acid as receiver. This method has been selected as it is not sensitive to the C02-CaCC>3-equilibrium present in the piped water used due to a lack of huge amounts of deionised water for the necessary measurements (Gucher, 2013).

At the beginning of a test run, the experimental plant is only operated with water and air until steady-state conditions for the chosen operation point are reached. The mass transfer

230 CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 0 I 5 ) 2 2 8 -2 3 5

measurements are operated at these adjusted liquid and gas loads. Then, the metering of ammonia is started where the amount of ammonia is adjusted in a way that the measured outlet concentration above of the packing reaches approxi­mately 100 ppm. Steady-state conditions are assumed when a constant NH3-concentration in the gas outlet stream for a time period of 1-2 min is achieved. Now, the gas measuring position inside the column is switched to the gas sampling location below the packing and again the measurement is operated until a constant value for 1-2 min is realised. After­wards, the liquid samples above and below the packing are taken simultaneously as described in Section 2.1.1.

The concentration of approximately 100 ppm in the gas outlet stream has been chosen to ensure a good measuring result of the FTIR. Furthermore, a bed height of 0.75 m has been chosen to ensure that at least 5% of the inlet gas concentration can be measured in the outlet gas stream as recommended by Rejl et al. (2009).

2.1.1. SamplingConcerning the sampling of the liquid and the gaseous phase, following has to be considered:

• The sample needs to be representative, which means that there should not be any disturbing components present and the sample volume should be high enough.

• On the other side, it is very important to avoid a signifi­cant disturbance of the gas and the liquid phase and thus maintaining the fluid dynamic conditions unchanged.

• Furthermore, the sampling positions should be located in such a way that only the mass transfer capacity of the pack­ing itself is measured, which means the sampling should take place straight above (Fig. 3) and below the packing.

Therefore, various sampling devices have been developed which are described below.

2.1.1.1. Gas sampling. For the analysis of the gas phase an amount of 1.41/min is sucked from the column (optional from above or below the packing) through a heated pipe (180 °C) to the pre-heated FTIR spectrometer, where the concentrations

of water, ammonia, and carbon dioxide are analysed. Based on the fact that ammonia is readily soluble in water and as there is interference between water and ammonia concerning their infrared spectra, it is essential that only gas is sucked in and no water droplets. As the sampling position is located straight above and below the packing, in particular in the two- phase regime, a special sampling device is necessary, which was developed especially for this purpose at the Chair of Pro­cess Engineering. The measurement pipe which leads directly to the FTIR spectrometer is situated in a double-tube, where a small, but long (over the whole cross section of the column) slot is milled in both pipes, the inner one at the top and the outer one at the bottom side as shown in Fig. 4.

2.1.1.2. Liquid sampling. The liquid should also be taken straight above and below the packing. For this purpose a sam­pling device has been developed which is illustrated in Fig. 5.

Fig. 3 - Gas and liquid sampling device directly above the packing.

CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 O I 5 ) 2 2 8 -2 3 5 231

Fig. 4 - View of the gas measuring probe (Photograph and schematic view).

It is designed in such a way that the liquid is collected in the bulk phase of the column. This prevents the sampling of liq­uid close to the column walls which interaction with the gas is limited and which is therefore not representative (Griinewald et al., 2011).

During sampling the total volume of the liquid sample col­lector is emptied three times before the sample is taken to ensure a fresh and representative sample. Furthermore, the horizontal position of the three possible liquid collectors along the height of the column is chosen in a way that they do not interact with each other. As shown in Fig. 6, they collect the liquid at the same diameter inside the column but they are shifted by angles of 45 and 90 degrees. The exact positions of the liquid sampling devices along the column height can be seen in Fig. 7.

Fig. 6 - Arrangement of the liquid sampling devices.

ps is calculated with the Antoine equation (Eq. (3)) with the constants given in Gmehling and Kolbe (1992).

Pair, humid [k&'m3] __________ Pair [Pa]_______287.06 x (273.15 + Tajr [-C])

X ^1 - 0.378 XPs [Pa] \

Pair [Pa]) (2)

P s [Pa] = 10(1019621-(1730.63/233.426+Tair[°C])) (3)

3. Calculations 3.2. Irrigation density

3.1. Gas load factor F

For the description of the gas flow in a mass transfer columns the F-factor is used, where the gas flow is based on the cross sectional area of the column and multiplied with the square root of the gas density to get comparable results.

The liquid flow is also applied to the cross-sectional area of the empty column as shown in the following equation:

u l

V l [ m 3/ h ]

A [m2] (4)

F Pa°'5] = X \ / « ;x [kg/m3] (1)

The therefore required gas density is calculated based on the ideal gas equation by Eq. (2) with the assumption that the air is completely saturated with water after passing the saturation column. The required saturated vapour pressure

3.3. Height of a transfer unit-HTU

The experimental results of the mass transfer measurements are used for the calculation of the required packing height of industrial columns. For this purpose the kinetic mass trans­fer model of Chilton and Colburn as shown in Eq. (5) is used where the number of transfer units (NTU) is multiplied with the height of one transfer unit (HTU).

H [m] = HTU x NTU = HTU®V x NTU9V = HTU|,V x NTU^ (5)

For dimensioning a new column the number of transfer units can be calculated from the specified concentrations in the in- and outgoing streams. Conceptually, this value describes how often the potential of the driving force has to be exhausted to reach the desired outgoing concentrations based on the ingoing concentrations. In Eqs. (6a) and (6b) as well as in the further described equations the mass transfer

232 CHEMICAL ENGINEERING RESEARCH AND DESIGN 99 ( 2 0 I 5 ) 2 2 8 -2 3 5

calculation for the mass transition for the gas side g as well as for the liquid side 1 are outlined.

NTUr

xin

-/<Yout

dYY-Y*

ntu |,v =Xout

(6a)

(6b)

With the assumption of a linear equilibrium as well as balance line, which is fulfilled concerning the used ammonia/air-water test system under the used operating con­ditions, as well as if the calculation is done with molar loads instead of molar fractions, the calculation of NTU can be done with a logarithmic average (Eqs. (7a) and (7b)) (Lehner et al., 2011)nti4 = Yin -Yout x In [' AYin \ (7a)

A Yin — AYout AYout /

NTU[,V = Xout — Xin x In (/ AX0ut \ (7b)AX0ut ~ AXin AXin /

AY describes the difference between the existing molar load in the gas phase and the theoretically to this value corresponding equilibrium load in the liquid phase, besides AX defines the difference between the equilibrium gas load and the existing molar load in the liquid phase. The equilibrium liquid, respec­tively, gas load can be calculated by the use of Henry’s law for molar loads, as shown in the following equations:

(He (Tg) /p) x (Xj/1 + Xj)1 _ (He (Tg) /p) x (X;/l + Xj)

(8a)

AXj = X* - X; (Yj/1 + Yj) x (p/He (Tg))1 _ (Yj/1 + Yj) x (p/He (Tg)) 1

(8b)

The herein used temperature dependent Henry-constant is calculated by the help of Eq. (9) based on the Van’t Hoff equation. The therefore required values, in case ammonia is absorbed by aqueous solutions, are published by Sander (1999) which are established on the relationship given by Edwards et al. (1978).

feNH3 (Tg) [mol/13 atm] = 61 x exp ^4200 x ^Tg [K] 298.15 ))(9)

He (Tg) [bar]

^Solution [kg/m3]MM solution [kg/mol] X feNH3 [mol/13 atm] x 1000 [l3/m3]

(10)

Eq. (10) is used to translate the calculated Henry-constant in the pressure form. The herein used density of the ammonia-water solution which is dependent on the concen­tration of ammonia is shown in Eq. (11) and has been found by fitting the values given in Kiister et al. (1972).

/’solution [k g to ta l^ to ta l] = 4-3 x 10 8 x c£,H3

- 0.0074 x cNh3 + 997.99 (11)

The molar mass of the ammonia-water solution can after­wards be calculated with the density of the solution by using Eq. (12) and the water concentration by Eq. (13).

MMSo]uti0n [kgtotal/moltotal] —^Solution [kg t0ta i /m t30taI]

^Solution [m o lto ta l/m [otal]

___________ /’Solution [kgtotal/rc^otal]___________

ch2o [molH2o/mt3otal] + cNh3 [molNH3/m3otal](12)

CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 0 I 5 ) 2 2 8 -2 3 5 233

ch2o [molHjo/m,3total^Solution [kgtotal/m t0tal] /’partial,NH3 [^S N H s^ to ta l]

MMh2o [kgH20/molH2o]

/^Solution fkgtotal^m !;otal] — ^^3 j molNH3/m 30 j x MMnh3 [kgNH3/m olNH3

MMh2o [kgH20/molH2o]

(13)

It is important to use the molar loads instead of the molar fractions for the calculations as otherwise the mass flow of the inlet and outlet stream would change too much. The cal­culation of the loads out of the fractions is shown in Eq. (14) exemplary for the gas side (Draxler and Siebenhofer, 2014).

Yj [molj/moljnert] = y, [molj/moltotaI] 1 - y ; [mol;/moltotal] (14)

The height of transfer unit is necessary for the calculation of a column height. It can be calculated as shown in Eqs. (15a) and (15b) as the used packing height is known and the number of transfer units can be calculated with the measured in- and outgoing concentrations during the experiment as shown in Eqs. (7a) and (7b).

HTU3V [m] = H(15a)

HTUoV

HTUoV [m] = H(15b)

HTUL

For the calculation of the NTU value the measured concen­trations have to be transformed into molar loads which are calculated with Eq. (14) for the gas phase and Eqs. (15)—(17) for the liquid phase.

Yj [molj/molinert = molNH3/molair] = ^ [PPm l * 10—L J 1 - y; [ppm] x 10-b

(16)

Furthermore, the presented results concerning the location of the sampling as well as the raw gas concentration were repeated to show the reproducibility of the measurements.

4.1. Location of sampling

Hoffmann et al. (2007) recommend that the sampling position for the gas and the liquid phase should be situated directly below the packing to eliminate the mass transfer which takes place below the packing but they also suggest locating the sampling in the inlet and outlet tubes to get a representative sample. Rejl et al. (2009) suggest to situate the sampling loca­tion inside the packing to eliminate end effects. Therefore, the influence of the sampling location has been examined. All examined sampling positions are marked with numbers (gas sampling) or letters (liquid sampling) in Fig. 7. In fact only the sampling positions below the packing were varied. Therefore, the liquid inlet (a) and the gas outlet concentration (1) were measured straight above the packing. The gas inlet concentration was measured straight below the packing (2) and alternatively in the inlet tube (4). The liquid outlet con­centration was measured directly below (b) as well as 60 cm below (c) the packing and in the outlet tube (d). The results of these measurements are depicted in Fig. 8 together with the mass balance calculations, where the HTUov values are plotted against the F-factor. The mass balance is calculated by comparing the total NH3 amount of the inlet gas and liquid stream with the one of the outgoing streams (Eq. (18)) and the determination of the deviation of thus received values.

X, [molj/moijne,, - molBH!/mol„!0] = ai* ["“W C - l * MM- ° [ W ™ 1 h,o]Ppartial,H20 [kgH2CI/ m 30tal]

Ppartial,H20 [^•Stotal^m total] = ^solution [^§total^m total] — ^partial,NH3 [kgNH3/ m tttal]

— Psolution [kgtotal/m?otal] cnh3 [molNH3/m t3otal] X MMNh3 [kgNH3/molNH3]

4. Results

(17)

(18)

Although the used experimental setup of the pilot plant fol­lows strictly the recommendations published by Hoffmann et al. (2007), further influences on the experimental results have been observed, which are described below. All experi­ments were carried out with random plastic (polypropylene) packing with a nominal size of 50 mm, a void fraction of 94%, and a specific surface area of 90m2/m 3. Due to the fact that the used air is sucked in from the environment it is important to mention that the ambient conditions (relative humidity, temperature, and ambient pressure) were recorded and considered for interpretation of the results. All presented experimental results are normalised based on the highest value shown in each diagram due to a non-disclosure agree­ment. The uncertainty analysis has been performed based on the propagation of uncertainty of GaufS (DIN 1319,1983). The measured and the calculated variables as well as their uncer­tainties can be obtained from Table 1.

cNH3,L,in X Vl + CNH3,G,in x Vg = CNH3,L,out X Vl + CNH3,G,out X Vq

(19)

Comparing the measurements [g:l—2, l:a-b] and [g:l—2, 1: a-c] it can be recognised that the HTUov values differ only by 2% but the mass balance has an deviation of about 60%. Fur­ther it can be seen that the HTUov values of the measurements [g:l-2, l:a-b] and [g:l-2, l:a-d] show a deviation of 10% and the mass balance already of about 150-300%.

Of course the huge difference concerning the mass bal­ance can be explained due to the not closed mass balance area. Therefore, the measurement [g:l—4, l:a-d] took this into consideration which is also visible in the best matching mass balance (-3-10%) in Fig. 8. However, these sampling positions lead to 65% lower HTUov values than [g:l-2, l:a-b] due to the

234 CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 0 I 5 ) 2 2 8 -2 3 5

1 Table 1 - Results from the uncertainty analysis.Calculated variable Measured variable Max. uncertainty of calculated variable

F-factor Gas flow (±3 Pa + 0.5% of m easuring range) Column diam eter (±1 mm)Atmospheric pressure (±100 Pa)Gas tem perature (±1°C)

±0.2 Pa0-5 or 7%

Irrigation density Liquid flow (±0.2% of m easured value) Column diam eter (±1 mm)

±0.4m 3/(m2 h) or 0.3%

HTUov Packing height (±1 cm)Gas concentration (±4% of m easured value) Liquid concentration (±5% of m easured value)

±0.025 m or 17%

HTU°0V = f(F)

Fig. 8 - Influence of sampling location and mass balance therefore.

interaction of gas and the liquid droplets in the column section below the packing. Therefore, it is obvious, that the sampling position has a huge influence on the determined HTUov val­ues, whereas the liquid concentrations do not have such a decisive effect on the HTUov values. Furthermore, a closed mass balance is not an indicator for the right sampling posi­tions.

4.2. Raw gas concentrations

The amount of ammonia dispensed into the air should be high enough to limit any measurement mistakes on the one hand, but on the other hand ammonia gas should also not be wasted. Therefore, it was investigated, if the chosen raw gas concen­tration has an influence on the measured mass transfer per­formance. Due to the fact that the mass transfer resistance in the used system ammonia/air-water is located in both phases, the gas as well as the liquid side HTUqv values have been

calculated and are shown in Fig. 9. The gas side HTUov values are lower compared to the liquid side ones. As shown in Fig. 9 an almost doubled raw gas concentration leads to 10% lower HTUov values for the gas phase as well as for the liquid phase. This indicates that the influence of the chosen gas concen­tration cannot be neglected. In order to achieve comparable measurements, additional examinations would be necessary to give an indication what gas concentration should be used.

4.3. Pre-treatment of plastic packing

In general, it is known that plastic packing improve their mass transfer performance during operation compared to their vir­gin state at the start-up. However, there are no publications about how long or in which way plastic packing should be pre-treated before they are examined in mass transfer mea­surements. Results concerning the fluid dynamic behaviour of this issue are published by Wolf and Lehner (2013). Woicke

HTU30V = f(F )D epend an ce o f G as C o n cen tra tio n

-lo w concentration

low concentration repeated

-h igh concentration

high concentration repeated

System: NH3-alr/water H=0.75m uL=30m3/(m2h)

o “ 0,100 — > ’

2,5 3,0F-Factor F [Pa0 5]

. 1,4

i 1,3

! 1,2

L l , 1

! ^ 1>°n °

i P 0’9i 1 0,8

I « 1 0,6

0,5

HTU'ov = f(F ) D epen d an c e o f G as C o n cen tra tio n

• • • lo w concentration

-O low concentration repeated

- • -h ig h concentration

- • high concentration repeated

S ystem : N H 3-a lr/w ate tH =0.75muL=30m 3/(m 2h)

IPa05] [ppml [ppm]low cone.

2.3 115 5123.4 112 4103.9 121 424low cone. rep.

2.3 111 4783.4 109 405

a s * 3.9 122 430C.

2.3 215 10903.4 230 1004

» 3.9 206 833high cone. rep.

2.3 185 9443.4 202 9023.9 198 814

0 2 5 3 0 3 5 4, 0

y« y.

F-Factor F [Pa0’5]

Fig. 9 - Influence of gas concentration.

CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 9 ( 2 0 I 5 ) 228-235 235

HTU% « f(F)

Fig. 10 - Influence of the pre-treatment of plastic packing.

et al. (2012) have already reported the occurring effect concern­ing the mass transfer performance. Nevertheless, the results in Fig. 10 should point out how long it takes to establish stable conditions. The HTUov values at the 4th and 5th irri­gation are about 40% lower at an F-factor of 1.2Pa0-5, 20% at F-factors 2.3 Pa0-5 and 3.2 Pa0-5 and 11% lower at an F-factor of 3.9 Pa0 5 than the ones found at the 1st irrigation. The therefore deduced recommendation is to perform first mass transfer measurements under the same conditions with a brand-new plastic packing as long as it takes to achieve stable conditions. Only then valid measurements are obtained.

Summing up the alternating irrigation times for the presented results it required all together 2h of ammonia-air/water and 2h of simple air/water irrigation to achieve the presented stable conditions after the fourth irrigation. The packing was left wet between the measure­ments of the single irrigation runs, which also can have an influence on the surface of the packing. It needs to be mentioned that the presented results were measured over a complete duration of five days.

5. Conclusion and outlook

It has been shown that the influences of the chosen raw gas concentration, the sampling location as well as the pre­treatment of the plastic packing have a significant influence on mass transfer measurements and should therefore not be neglected. This work is intended to contribute to the efforts to standardise mass transfer measurements although for an accurate understanding of the shown influences further sys­tematic investigations would be necessary, particularly with other test systems as recommended by Hoffmann et al. (2007) and Rejl et al. (2009). Nevertheless, the following recommen­dations can be pointed out:

• Location of sampling: The sampling positions where the concentrations are measured have to be chosen carefully and it is necessary to define whether they should be situ­ated in the pipes to/from the measurement column, directly above or below of the packing or/and inside of the packing. Due to the presented results and to avoid effects of the plant configuration the positions directly above, below, and inside of the packing are recommended. Furthermore, if the sam­pling is done inside of the column, it is essential to get an integral sample over the whole cross section of the column diameter as the mass transfer in the bulk and the wall zone differs.

• Raw gas concentration: It is not useful to leave the deci­sion open about which gas concentration shall be used. The choice of the used raw gas concentration is dependent on the measuring equipment as well as on the packing height as the clean gas concentration has to be in the measuring range. The authors suggest to define special gas concentra­tions for the different available test systems to ensure better comparability. The definition of these concentrations would need further investigations.

• Pre-treatment of packing: A detailed procedure for the pre­treatment of plastic as well as metal packing has to be defined to guarantee that mass transfer measurements in different experimental plants lead to the same results. To develop such procedures further investigations are neces­sary in particular addressing the surface properties and how they are changeable.

Acknowledgement

Financial support from RVT Process Equipment GmbH is grate­fully acknowledged.

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