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Contents lists available at ScienceDirect

Education for Chemical Engineers

j ournal homepage: www.elsevier .com/ locate /ece

Gas solute movementin packed columnsA remote

control experiment

Hugo Silvaa, Sandra Sa, Lcia Brandoa, J.M. Loureirob, Joaquim Gabriel c,Adlio Mendesa,

a LEPAE-Departamento de Engenharia Qumica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias,

4200-465 Porto, Portugalb LSRE-Departamento de Engenharia Qumica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias,

4200-465 Porto, Portugalc IDMEC Plo FEUP, Departamento de Engenharia Mecnica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias,

4200-465 Porto, Portugal

a b s t r a c t

A novel teaching remotecontrolled experiment is describedconcerning thesolutemovement through an adsorption

column. A packed-bed column filled with 5A zeolite was inserted in a thermostatic oven and connected to a TCD

detector. The complete set-up can be remotely controlled and visualized alive using an internet connected camera

to improve the sense of reality. The experiment purpose was to assist and motivate students regarding a topic that

usually they demonstrate difficulty to assimilate, which is the prediction of concentration fronts behavior by using

the solutemovement theory(SMT). Theset-upis versatileto studybreakthroughcurves andfeedpulses.Two loopsof

2 cm3 and5cm3 allowthe injectionof O2or N2adsorbatespecies thathave different isothermstypeandconsequentlyconcentration fronts history. Interaction between shock and diffuse waves is addressed for the narrow pulse case

of2 cm3. Also, students are able to obtain the nitrogen and oxygen isotherms at different temperatures for the 5A

zeolite (chromatographicmethod) andcompare those isothermswith those obtained by thevolumetric method; the

latter is used as reference method.

2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Gas adsorption; Solutemovement theory; Remote control experiment

1. Introduction

Educational laboratories are an important part of theundergraduate chemical engineering study plan. With the

development of computers and the worldwide internet com-

munications network, the laboratory pedagogical approach

has been changing. Since the late 1990s, a new concept

is emerging in engineering education institutions: remotely

controlled laboratories (Selmer et al., 2007). Remote labs

are systems based on real equipment, which allow stu-

dents to perform experimental work through an internet

connection (Coito and Palma, 2008). Although remotely con-

trolled laboratories are in a maturation process, there are

some advantages pointed in the literature when compared

Corresponding author. Tel.: +351 22 508 1695; fax: +351 22 508 1449.E-mail address: mendes@fe.up.pt (A. Mendes).Received23 July 2012;Receivedin revisedform25 June 2013;Accepted30 June2013

to traditional hands-on laboratories. Engineering education

institutions face budget restrictions to create and maintain

traditional hands-on laboratories, due to an increasing needof space,staff,availabilityand safety (Ogotet al.,2003;Wiesner

andLan,2004;Abu-ElHumos etal., 2005;Azad,2007; Jing etal.,

2007; Rafael et al., 2007; Selmer et al., 2007; Murrayet al., 2008;

Wiseman et al., 2008; Gravier et al., 2009). On the other hand,

remotelycontrolled experiments areavailable fromanywhere

at anytime which facilitates the scheduling process. In addi-

tion, students safety is guaranteed, minimum supervision

is involved and more users can perform the experiment in

comparison with hands-on. Finally, it opens the possibility of

sharing experiments among different institutions, increasing

the number and variety available (Bourne et al., 2005; Azad,

1749-7728/$ see front matter 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ece.2013.06.002

http://www.sciencedirect.com/science/journal/17497728http://www.elsevier.com/locate/ecemailto:mendes@fe.up.pthttp://localhost/var/www/apps/conversion/tmp/scratch_9/dx.doi.org/10.1016/j.ece.2013.06.002http://localhost/var/www/apps/conversion/tmp/scratch_9/dx.doi.org/10.1016/j.ece.2013.06.002mailto:mendes@fe.up.pthttp://www.elsevier.com/locate/ecehttp://www.sciencedirect.com/science/journal/17497728

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Fig. 1 Experimental setup for the gas adsorption experiment on a porous adsorbent.

Gas feed system two gases can be supplied to the GC,

nitrogen and oxygen, following two feeding modes, step per-

turbationand conventional injection (pulse perturbation). For

the conventional injection, an automatic injection valve (A,

VICI E36) is used. Three-way electric valve (B), Fig. 1, is used

to select the gas and three-way electric valve (C) is used to

select theinjection volume loop (either 2 cm3 or5cm3).A third

three way electric valve (D) allows the selection of the exit

position (from2cm3 to5cm3 loops) of theautomatic injection

valve to the packed bed column. The carrier gas, helium, is

supplied to the GC by a mass flow controller (E, Bronkhorst,

100cm3min1). For step perturbations, electric on/off vales F,G and H are used. The mass flow controller is then used to

control the flow rate. At the adsorption columntop,a pressure

sensor (I) reads the feeding pressure.

Powersupply andacquisition board a standard 12VDCpower

supply is used to power the mass flow controller, pressure

sensor and electric valves. The valves are controlled using

two in-house built relay boards (with 4 relays each). The relay

boards, mass flow controller, pressure sensor and GC oven

temperature controller (West 8100 PID) are connected to an

acquisition board (Advantech, PLCD-8710).

Web camera an internet surveillance IP camera (PIV-6732,

IP PZT network camera) was connected to a switch (Asus

GX-100SB) together with the computer and the switch wasconnected to an internet socket. Computer software allows

using thecamera that is capable of zoom anddirectional con-

trol, giving a sharp viewof the experimental set-up.

Controlling software a comprehensive controlling program

was developed in LabVIEW7.1 (National Instruments) Fig. 2.

It allows setting (a) the carrier gas flow rate, (b) the solute

species, (c) the pulse injection volume (selectable 2 cm3 or

5cm3), (d) the perturbation type, (e) the adsorption temper-

ature (temperature of the oven) and (f) to trigger the injection

valve. This interface plots on-line the TCD response (related

to the exiting stream concentration), shows the status of all

setting variables and displays the pressure at the columns

top. The variables carrier gas flow rate, TCD response, pres-sure entrance and retention time can be saved in a text file.

The LabVIEW software has a built-in web server facility to

allow easy internet access, thus providing the option of pub-

lishing the graphical user interface (GUI). This GUI is available

at http://elabs.fe.up.pt and students can access and control

the experiment by using a standard web browser, preferably

Internet Explorer6. Fig. 3 shows a schematic representationof

the installation set-up.

The experimentalset-upwas developedmostlyby a gradu-

atestudent, during hismaster thesis, andtook advantage of a

very old GC obtained at a low price. The whole set-up is quite

inexpensive and it was a great opportunity of learning some

different and relevant competences.

3. Solute movementtheory

This section presents a brief description of the underlying

theory to help the student in understanding and analysing

the experiment. Reference books (Wankat, 1990; Seader and

Henley, 1998) could be used to help in a deeper understanding

of the solutemovement theory (SMT).

When a pulse gas enters the packed-bed column, it either

moves at the interstitial velocity defined by the carrier gas

flow rate, or is immobilized inside the adsorbent (adsorbedor

in the fluid stagnant phase). Accordingly, the average veloc-

ity depends on the time that the solute is retained inside the

adsorbent. This is quite effortless to understand and explain.

However, complexity is present when it is necessary to inter-

pret the formationof different concentration-frontwaves and

how they interact. This is when the solute movement theory

is helpful since, despite being a mathematical model based

on simple physical phenomena, it can be used to analyze and

understand rather complex processes.

The SMTestablishesthat an infinitesimalelement of a sin-

gle gas solute, with partial pressurepA, will migrate through

the packed-bed column at a velocity us, which inversely

depends on the slope of the adsorption isotherm (dqA/dpA),

us(pA) =

z

t

pA

=ui

1+ ((1 )/)pKd + ((1 )/)T(dqA/dpA)

(1)

where ui is the interstitial velocity of the carrier gas in the

column, pA is the solute partial pressure in the fluid phase,

qA is the solute concentration in the solid phase, is the

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Fig. 2 Experiment user interface developed in LabVIEW 7.1, with a nitrogen chromatogram response.

Fig. 3 Schematic representation of the experiment.

solid apparent density, is the gas constant, T is the abso-lute temperature, z is the column axial position, t is time,

is the packing (interparticle) porosity, p is the intraparticle

porosity (within the particle) and Kd is the fraction of intra-

particle volume the species can penetrate; Kd = 1 for small

solute molecules (present case). As it can be seen from Eq.

(1), the solute movement is retarded since the solute is in

the adsorbed phase, ([(1 )/]T(dqA/dpA)) or in gas phase

inside the porosity of the adsorbent ([(1 )/]pKd).

The DeVault (SMT) equation can be derived from a solute

mass balance to the absorbent column:

uipA

z

+pA

t

+1

pKdpA

t

+1

TqA

t

= 0 (2)

considering themathematical relations (3) and (4):

dpAdt

pA

= 0 =

pAz

t

z

t

pA

+

pAt

z

(3)

zt

pA

= (pA/t)z(pA/z)t

(4)

sincepA =f(z, t), one obtains Eq. (1).

The solute mass balance is based on the following main

assumptions (Wankat, 1990):

(i) Instantaneous equilibrium between the gas and solid phase;

(ii) Plug flow of the gas phase;

(iii) No pressure drop along the column;

(iv) Isothermal operation;

(v) Ideal gas behavior.

Among these assumptions, the least likely to be valid

concerns the local equilibrium. This assumption states that

q relates to p through the adsorption isotherm, neglect-

ing intraparticle mass transport limitations. The model also

assumes plug flowwithnoaxial dispersion,whichisnotlikely

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q1

q2

p1 p2

Fig. 4 Two-segment discretized favorable adsorption

isotherm.

to happen. However, for favorable isotherms (with negative

secondderivative Fig. 4) and when equilibrium is the domi-

nant phenomenon, normally the SMT approximates well the

adsorption experimental data.

3.1. SMTapplied to Langmuir type isotherms

Eq. (3) shows that in order to correctly predict the velocity

of the solute inside the column, the adsorption equilibrium

isotherm for the solute in the adsorbent should be known.

Following, the formation of shock waves and concentration

frontswhen the solute adsorbs in the adsorbent phase with aLangmuir type isotherm (Fig. 4) will be considered. This type

of isothermis usuallycalled favorable isotherm(withnegative

second derivative).

The adsorption solute movement of species with a favor-

able isotherm can bemore easily conceptualized discretizing

the isotherm into two segments Fig. 4.

For a feed concentration p1 the solute velocity would be

u1; on the other hand, for an adsorption column initially in

equilibriumwith a feed streamof concentrationp1, when thefeed concentration is changed top2, the solute velocitywould

be u2and u2 >u1, since theadsorption columnretains (delays)

more the soluteup to concentrationp1. When feeding a clean

column with a stream of concentration p2, the solute front

wouldbeas sketchedin Fig. 5. Thehigher concentrations (that

could travel at higher velocities, according to Eq. (3)) cannot

move faster than the lower concentrations like when a car at

ahigherspeed cannotovercomea lower speedcar infrontof it.

The following link shows thedescribedanalogy, http://chelta.

fe.up.pt/chelta/student/animations/road.swf. Because of this

limitation in the velocity of the higherconcentrations, a mass

balance is established at the solute front and a shock wave is

formed thataverages thesolute velocityforall concentrations,as it is presented in Eq. (5):

ushock =ui

1+ ((1 )/)pKd + ((1 )/)T(qA/pA) (5)

For the desorption of a column initially saturated in equi-

libriumwithp2, a dispersivewave front is formed; as is shown

in Fig. 6, the lower concentrationmoves more slowly.

The solutemovement is usually explainedbased on solute

movement diagrams, graphical expression of the mathe-

matical method of characteristics used in the solution of

(hyperbolic) partial differential equations Fig. 7. In Fig. 7, it is

represented the shock wave velocity slope ushock, and the gen-eration of the spreading zone, for the response to a feed pulse

p2

p1

z

Shock

Wave

Fig. 5 Conceptual adsorption solute front movementfor the isotherm depicted in Fig. 4; a shock wave front is formed.

p2

p1

z

Fig. 6 Conceptual desorption solute front movementfor the isotherm depicted in Fig. 4; a dispersivewave is formed.

http://chelta.fe.up.pt/chelta/student/animations/road.swfhttp://chelta.fe.up.pt/chelta/student/animations/road.swfhttp://chelta.fe.up.pt/chelta/student/animations/road.swfhttp://chelta.fe.up.pt/chelta/student/animations/road.swf

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education for chemicalengineers 8 ( 2 0 1 3 ) e94e104 e99

ushock(pH)

L

Z

pout

pin

Time

pH

Time

Time

0

0

0

pH

Fig. 7 Feed pulse diagrams according to the solute

movement theory: (a) inlet concentration; (b) shock and

diffuse waves formation and; (c) outlet concentration.

of a solutewith favorable isotherm. In this case, the shockand

diffuse waves for the highest concentrations exit the column

at the same time, without intersection during their migra-

tion. Fig. 8 sketches thepulse shape as it progresses along the

adsorption column.

For favorable isotherms the following relation between

velocity and concentration is of great importance: us(Chigh) >

ushock> us(Clow). Therefore, for a feed pulse perturbation theleading shock wave is intersected by the high concentration

solute movement if the adsorption column is long enough, as

it can be deduced fromFigs. 7 and8. Consequently, the solute

elutesat lower concentrationswhen compared to theoriginal

feedpulse.Regardingtheconcentrationcurves, students often

have difficulty to mentally visualize that the profile shown

in Fig. 8 (t=5), where concentration is represented as a func-

tion of the axial position inside the column, is symmetrical to

the representation of the concentration-fronts as a function

of time (histories), shown in Fig. 7.

For the case of linear isotherms, the solutewaves travel all

at the samevelocity and the waves formedareusually named

simple. The student should refer to known reference bookson SMT to a better understanding of these concepts (Wankat,

1990; Seader andHenley, 1998).

3.2. Adsorption isotherms determined by the SMT

Theadsorption isothermof thesolute species in theadsorbent

packed in the column can be obtained if the concentration

history to a feed pulse is obtained at the exit of the column.

Fig. 8 Sketch of the solute pulse progress along an adsorption column.

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Fig. 9 Chromatograph response to a solute feed pulse

with favorable isotherm.

Theadsorption isothermcanbe obtained from the desorp-

tion response to a feed perturbation (diffuse wave). This

happens because the shape of the diffuse wave is related to

solutevelocityas a functionof thesoluteconcentration(Fig. 9):

uiu(pA)

=trt0

(6)

where t0is thecarriergas retentiontimeand tr(pA)isthesolute

retention time, which is a function of solute partial pressure.

Replacing Eq. (1) in (6), it becomes:

trt0

= 1+1

Tf(pA)+

1

p (7)

and expressing the isotherm derivative it becomes:

dqAdpA

=1

(1 )T

trt0

[(1 )p + ]

(8)

Integrating nowEq. (8) one obtains:

qA =1

(1 )T

pA00

trt0

[(1 )p + ]dpA (9)

Eq. (9) can be solved based on the TCD response obtained

for the solute concentration as a function of the retentiontime, pA =f((tr/t0) [(1 )p + ]). However, the TCD used

is old and has a large dead volume, acting as a stirring

tank at the end of the adsorption column that originates the

peak to exhibit a shape similar to the case of a favorable

isotherm (Wankat, 1990). This is a common source of devi-

ations between experimental andmodel results.

Thepartialpressurevalues ontheordinate axis of the chro-

matogram, h, do not represent the solute partial pressure, but

the h values of ordinate axes are directly proportional to the

solute partial pressure, according to Eq. (10):

pA = k h (10)

where h is thearbitraryscale chosen for theordinateaxis (e.g.

the electrical signal from the GC in mV) and k the propor-

tionality factor. This way, the solute partial pressure can be

obtained since the product between the total pressure (pA0 )

Time / s

0 50 100 150 200 250 300

NormalizedAmplitude

0.0

0.2

0.4

0.6

0.8

1.0

N2

-5 cm3

Fig. 10 Response to a nitrogen feed pulse, at 60C and

20cm3Nmin1 helium flow rate for the injection loops of

5 cm3.

and the injection time tinjis equal to the chromatogram total

area, according to:

pA0 tinj =

0

pAdt = k

0

h dt (11)

where:

tinj =Vinj

U (12)

Vinj is the loop injected volume and U is the carrier gas flowrate.

The proportionality factor is obtained from:

k =pA0 tinj

0 hdt

(13)

After obtaining thepartial pressurepA value, the next step

is the integration of Eq. (9) to obtain the adsorption isotherm,

qA =f(pA).

4. Observations and discussion

4.1. Determination ofadsorption isotherms

In this experimental section, students can determine the

adsorption isotherms of nitrogen and oxygen using the chro-

matographic method. Fig. 10 shows the chromatographic

experimental response to a nitrogen feed pulseof 5 cm3 in the

packed bed column, at 60C and 20cm3Nmin1 helium flow

rate. Students candetermine the nitrogen adsorption equilib-

rium isotherm for these conditions, as described in thesolute

movement theory section.

Fig. 11 reveals that nitrogen follows a Langmuir type

isotherm on the 5A zeolite; also it compares the adsorption

isotherm obtained from the volumetric method (reference

method) with the chromatographic response for a 5cm3

feed pulse of nitrogen at 60 C and for a carrier flow rate

of 20cm3Nmin1. Students can conclude that the nitrogen

equilibrium isotherms determined by both methods are very

similar. Other main conclusion that can be taken in this

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Pressure / bar

0.0 0.1 0.2 0.3 0.4 0.5 0.6

q/m

ol.kg

-1

0.00

0.05

0.10

0.15

0.20Chromatographic method

Volumetric method

Fig. 11 Comparison of the adsorption equilibrium

isotherms for nitrogen in the 5A zeolite obtained by the

volumetric method and from applying the SMT for a

nitrogen feed pulse, at 60 C.

procedure is that temperaturestrongly affects theequilibrium

and consequently the determined isotherms.

In order to apply the SMT, Eq. (1), in the following sections,

students can use the adsorption equilibrium isotherms for

oxygenandnitrogeninthe5Azeolite obtainedby theso-called

volumetricmethod at 40 C and 60 C. These adsorption equi-

librium isotherms are illustrated in Fig. 12. Nitrogen shows a

typical favorableshape isotherm Langmuirtype (represented

by Eq. (15)) as observed above, while oxygen displays a linear

behavior (represented by Eq. (14)).

qO2 = HPO2 (14)

qN2 =QKLPN21+ KLPN2

(15)

Students should use the adsorption isotherms parame-

ters represented in Eq. (14) or (15) obtained by the volumetric

method (Table 1) when they areapplying theSMTmathemat-

icalmodel.

Table 1 Nitrogen and oxygen equilibriumparametersisotherms on the 5A zeolite at 40C and 60 C.

T(C) N2 O2

Q(molkg1) KL(bar1) H (molkg1 bar1)

40 2.42 0.117 0.094

60 2.27 0.077 0.069

Time / s

0 50 100 150 200 250 300

NormalizedAmplitude

0.0

0.2

0.4

0.6

0.8

1.0

Step at 60 CStep at 40 C

Fig. 13 Response to a nitrogen feed step at 40C and 60 C

column temperature and heliumflow rate of 20cm3Nmin1.

4.2. Determination ofconcentration fronts by the SMT

With the equilibrium isotherms, it is now possible for stu-

dents to use the SMT and compare the model predictions

with the results obtained experimentally. With this set of

experiments, students canevaluate which variables affect thevelocityof a concentration-front.Theexperimental procedure

allows to analyze the influenceof thefollowing variables: feed

perturbations, injection volumes (for feed pulse), adsorption

temperature (T) and interstitial velocity (ui).

4.2.1. Feed step

Fig. 13 shows the normalized experimental response for a

nitrogen positive feed step into the packed bed column, with

pressure / bar

0.0 0.5 1.0 1.5 2.0

q/mol.kg-1

0.0

0.1

0.2

0.3

0.4

0.5

O2at 40 C

O2at 60 C

pressure / bar

0.0 0.5 1.0 1.5 2.0

q/mol.kg

-1

0.0

0.1

0.2

0.3

0.4

0.5

N2at 40 C

N2at 60 C

Fig. 12 Adsorption equilibrium isotherms on zeolite 5A of (a) oxygen and (b) nitrogen at 40C and 60 C, determined by the

volumetric method.

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a feedflowrate of 20cm3Nmin1 (average column pressure of

1.17bar) at 40C and 60 C. According to the SMT, this pertur-

bation leads exclusively to the formation of a shock wave; it

is expected that nitrogen exits the column at tshock = L/ushock,

where L is the length of the adsorption column and Eq. (5)

is used to obtain the velocity of the shock wave. Consider-

ing the column temperature at 60C, the exit time of the

shock wave for the data presented in Fig. 13 is ca. 163s

and the SMT predicts an exit time of 156s . The SMT was

expected to approximate much more closely the experimen-

tal data since the dominant phenomenon during nitrogen

migration through thepacked-bed column should be adsorp-

tion. Actually, for favorable isothermsandwhenequilibriumis

the dominant phenomenon the SMT normally approximates

well the adsorption experimental data (Wankat, 1990). As the

temperature increases the adsorption isothermbecomes less

favorable and the concentration front becomes more disper-

sive (Wankat, 1990) (see Fig. 13). Finally, it was assumed that

the column was operating at an average value between the

outlet and inlet pressures (for this particular case, an average

column pressure of 1.17barwas used in the calculations).

4.2.1.1. Effect oftemperature. Inorderto illustratethe temper-

ature influence in theadsorption process, Fig. 13 presents the

experimental results fornitrogen feed stepsperformedat two

different temperatures, 40 C and 60 C, and for a carrier gas

and nitrogen feed flow rate of 20cm3Nmin1.

Nitrogen retention time is higher at 40 C, since the

adsorptioncapacityof theadsorbentdecreases forhighertem-

peratures. This happens because adsorption is an exothermic

process and the equilibrium constants follow the Vant Hoff

equation. In this way, students can verify that adsorption

equilibrium is strongly affected by temperature. Additionally,

the dispersion phenomenon can be compared for both step

results. Thefront shapeof a breakthroughcurve is theresultof

dispersion and compression actions. Thedispersion ismostly

related to the axial dispersion and to the intraparticle mass

transport while thecompressionresults fromthenonlinearity

of the adsorption isotherm as well as the adsorption capac-

ity. Both dispersion andcompression forcesdecreasewith the

temperature but the compression forces decrease normally

more. Fig. 13 indicates that in the present case the concen-

tration front at 60C shows indeed a higher dispersion.

4.2.2. Feedpulse

Students can realize with this set of experiments that com-

plexity of the SMT is increased for a feed pulse mainly when

the shock wave, originated due to the positive concentration

step of the pulse, interacts with the diffuse wave, originated

due to the negative concentration step of the pulse Fig. 8.

4.2.2.1. Influence of solute species and concentration.

Figs. 14 and 15 show the response of the chromatographic

system at different feed pulses. Two volumes of nitrogen

and oxygen were injected: 5cm3 and 2cm3. The carrier gas

velocity and temperature were 20cm3Nmin1 and 60 C,

respectively.

Students can take several conclusions from these exper-

imental observations. The injection of different volumes

of nitrogen (different inlet concentrations) into the column

results in different retention times (Fig. 14). According to the

SMT predictions, the retention time of a shock wave for both

volumes injectedin thenitrogen feed pulseis 156s (see above).

Fig. 14 shows that the 5 cm3 pulse retention timecorresponds

Time / s

0 50 100 150 200 250 300

NormalizedAmplitude

0.0

0.2

0.4

0.6

0.8

1.0

N2pulse of 2 cm3

N2pulse of 5 cm3

Fig. 14 Response to nitrogen feed pulse, at 60C and

20cm3Nmin1 helium flow rate and for two injection loops:

5 cm3 and 2cm3.

Time / s

0 50 100 150 200 250 300

NormalizedAmplitude

0.0

0.2

0.4

0.6

0.8

1.0

O2pulse of 2 cm3

O2pulse of 5 cm3

Fig. 15 Response to oxygen feed pulse, at 60C and

20cm3 Nmin1 helium flow rate and for two injection

loops: 5 cm3 and 2cm3.

to this value. However, the retention time for the 2cm3 feed

pulse is higher, ca. 170 s, suggesting that the shock wavewas

intercepted by the higher concentrations of the diffuse wave.

To confirm this hypothesis, the expected intersection time

(t1) of the concentration-waves was determined. The shock

wave and thediffuse wave at thehighest concentration inter-

sect atz = z1when, ushockt1 = us(t1 tF)where tF = Vinj/U isthe

injection pulse time and us is the diffuse wave velocity at the

highest concentration. Students should obtain usby using Eq.

(3) where dqA/dpAshould be evaluated at the average column

pressure (in this case ca. 1.17bar, see above). The computed

time for the nitrogen 5cm3 injection volume is 234 s, which

corresponds to the column axial position (z1 = ushock t1) of

172cm. Since the packed-bed column is only 115cm long, the

shock and diffuse waves for the 5cm3 nitrogen pulse never

intercept. In the 2 cm3 nitrogen feed pulse, where the pulse

is narrower, the interception time is at t=94 s, which corre-

sponds tothecolumnaxialpositionof 69cm. Therefore, in this

case the shock wave is intersected by the higher concentra-

tions of the nitrogen diffuse wave before exiting the column.

Taking in to account this interception, the SMT predicts a

retention time of 164s for the 2cm3 feed pulse, comparing

quitewell with the experimental result.

Concerning the oxygen feed pulse, experimental observa-

tions show that the retention time is similar for both injected

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education for chemicalengineers 8 ( 2 0 1 3 ) e94e104 e103

Time / s

0 20 40 60 80 100 120 140 160 180

NormalizedAmplitude

0.0

0.2

0.4

0.6

0.8

1.0

Fig. 16 Response to a feed pulse of 5cm3 of nitrogen at

60 C and 40cm3Nmin1 helium flow rate.

volumes (Fig. 15). Students can realize this as a consequence

of the oxygen linear type isotherm. In this case, all the oxygenconcentrations travel through the column at same velocity,

forming a simple concentration front wave. The experimen-

tal retention time is around 90s, while SMT prediction is 83s.

However, the asymmetric shape of the peaks might indicate

a favorable isotherm, forming a shock front and a dispersive

wave on the backward. This is not the case and the shape

is partially related to the dead volume of the TCD, ca. 3 cm3,

which behaves like a perfectlymixed tank.

Figs. 13 and 14 shownormalized responses to respectively

step and pulse nitrogen perturbations. The adsorption dis-

persion to the step perturbation is larger than to the pulse

perturbation but it should be noted that the maximum pres-

sure for the step perturbation is ca. 1bar and for the pulseperturbation is just ca. 0.35bar.

4.2.2.2. Influence of the carrier flow rate. The influence of the

carrier gas flow rate was observed by performing a 5cm3

nitrogen feed pulse at 40cm3Nmin1 and 60C. The obtained

chromatogramis presented in Fig. 16. Students canshow that

theSMT predictsa retention time of 91s, considering an aver-

age column pressure of 1.33bar (higher flow rate than before)

and compare with the experimental retention time. Fig. 16

shows that the retention time is approximately 81s. This 10s

difference between the SMT prediction and the experimental

result canbe related to thehigher pressure drop in thepacked

bed column. In the literature, it is reported that prematurebreakthrough curves can be obtained due to velocity devia-

tions, which result from high pressure drop effects (Zwiebel,

1969). Finally, by comparing these experimental results with

the ones for the carrier flow rate at 20cm3Nmin1 (Fig. 14),

students can conclude that higher carrier flow rates simply

decrease the exit time of the concentration front.

5. Students opinion

Thisexperiment is available to students thatenroll Separation

Processes II subject, third year of the Chemical Engineering

program at Faculty of Engineering of Porto University (FEUP),

when the traveling concentration wave topic is taught. For

performing an experimental run, students must book a time

window. The booking process is currently available by e-mail

but soon will also be possible to book directly from the web-

site of the experiment. 23 students that contacted with this

Table 2 Responses of the students to the inquire.

# Question Fraction of positiveanswers (%)

1 Since remote controlled experiments are

available at anytime, in your opinion would

they be suitable for learning consolidationthrough repetition?

82.6

2 Would you preferto perform the experiments

during the class time or at anytime using the

remote control facility?

73.9

3 Duringtutorialclassesdo youprefer touse

experimental data acquired using a remote

controlled experiment or to use assumed

data?

91.3

4 Doyou consider that thelivecamera

improves the really sensewhen using the

remote controlled experiment?

100

5 Are youfamiliar withshock waves and

dispersive front?

82.6

experiment were inquired and questions and answers orga-

nized in Table 2.

Students generally prefer to use this remotely controlled

experiment to generate the data needed for solving train-

ing problems than to use assumed data. Though they think

this experiment can be used to consolidate their knowledge

because,theyprefer toperformitduringthelabclasses,proba-

blydue tosomeindependent learning insecurity. Thepresence

of the live camera is unanimously viewed as very relevant to

give a sense of reality to the experiment. Finally, the students

state that the shock wave and dispersive fronts are familiar

concepts, showinga relevantassimilation of critical concepts.

6. Conclusions

A remotely controlled educational experiment was developed

to supportstudents acknowledgmentofa quite complextopic

in chemical engineering: gasadsorption throughapacked-bed

column. From anywhere, at any time, students can comple-

ment the theoretical knowledge gathered about this topic,

withreal experimentsperformedonline(http://elabs.fe.up.pt).

The solute movement theory, as oversimplified model, is

the ideal tool for students first contact with gas adsorp-

tionphenomena.Whileperforming theexperiments,students

should critically discuss the results with the model predic-

tions. This way, it is expected that students consolidate their

knowledge on adsorption and on solute movement in an

adsorption columnandunderstand themodel limitations and

all the influence factors inherent to this kind of separation

processes. Regarding students opinion, it was asked if they

preferred to use the remote controlled experiment with a

virtual interface or a real set-up during class time. A sig-

nificant percentage (73.9%) stated a preference for making

experiments in the laboratory class, using real equipment.

Theexperiment targets both laboratory classes as well as lec-

tures/tutorials classes.The greatestnovelty is,however,its use

to assist teaching lecture/tutorial classes and namely Sepa-

ration Processes courses. This experiment can be used, for

example, to obtain the real solution for a problem ques-

tioned to a group of students in a tutorial class. Comparing

their result with real result, students can learn more and

deeper since they are also involving more their emotion.

Finally, students seem to be motivated with this learning

http://elabs.fe.up.pt/http://elabs.fe.up.pt/

8/12/2019 e94-e104

11/11

e104 education for chemicalengineers 8 ( 2 0 1 3 ) e94e104

strategy, since they expressed their preference of using real

experimental data instead of assumed data.

Acknowledgments

The authors would like to acknowledge the Chemical Engi-

neering Department of the Faculty of Engineering at the

University of Porto and paint company CIN SA for supportingthis work. The authors are also grateful to Eng. LuisMatos for

collaborating assembling the experimental set-up.

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