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Microbeam analysis of hydrogen in coked catalyst pellets
Nuclear Instruments and Methods 191 (1981) 379-382 North-Holland Pubhshmg Company
379
MICROBEAM ANALYSIS OF HYDROGEN IN COKED CATALYST PELLETS C J SOFIELD, L B BRIDWELL * and C J WRIGHT
UKAEA, Harwell, UK
The hydrogen content of catalyst pellets has recently been shown to be important in predicting the maximum temperature rise of the catalyst during regeneratmn in industrial plant This is because carbonaceous coke deposits (CH0 4 to CH2 o) accumulate m the pellet during use, whmh then have to be removed by exothermlc oxldatmn It is Important, m this well estabhshed industrial process, to hmlt the maxxmum temperature that the catalyst reaches to level s sufficiently low to prevent permanent deactivation by smterlng In vaew of the importance of the H content as a functmn of the distance from the surface of the catalyst pellets m predmting maximum temperature rises during de-cokmg, we have adopted the technique of elastm recoil analysis for use with the Harwell mlcrobeam to obtam measurements of the hydrogen concentration at varying positions along a pellet diameter The method of analysis Is described and Illustrated wath typical data.
1 Introduction When catalysts are used in various hydrocarbon conversion reactions such as cracking and reforming, the catalyst becomes poisoned. The poisoning takes the form of carbonaceous deposits called "coke" laid down within the catalyst pellets Regeneration of these "coked" pellets by burning off the deposits in hot steam or air/inert gas mixtures is a well established industrial process [1] However during this regeneration procedure the exothermlc de-coking reactions may heat the pellets to temperatures sufficiently high to slnter the catalyst material and permanently de-activate it The de-coklng process has received considerable theoretical attention and mathematical modelling of the temperature rise resulting from regeneration has been carried out [2] The distribution of the "coke" within a catalyst support pellet and its composition are important input parameters for models, but It is only recently that the hydrogen component of the CHx "coke" has also been considered as a variable [21. The analysis of the carbon distribution across sectioned catalyst support pellets has recently been investigated [3] by nuclear reaction analysis using the I2C(d, p ) l a c reaction The spatial resolution required
* Permanent address Murray State University, Murray, Kentucky, USA 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland
to carry out the analysis at many well resolved positions across a sectioned pellet of ~1 mm radius was provided by focusing the 1.3 MeV deuteron beam used in this reaction with the Harwell mlcrobeam system [4] Measurements of the H content of such catalyst support pellets with a similar spatial resolution to these carbon analyses would also be very useful.
2. Experimental method
2.1 Specimens The catalyst support pellets studied were cyhndrlcal or spherical In shape with diameters in the range 0 5 mm to 4 ram. They were fabricated from porous, high surface area A1203 with active catalyst dispersed over the surface The porosity of these materials is generally large, about 20% of the volume being open pore space The analysis technique which is described below, requires the specimens to be sectioned and the exposed surface to be presented to the probing beam at about 10° This was achieved by lightly clamping the samples in a vice-like jig which had two jaws chamfered at 10 °. The specimen was then sliced with a scalpel to remove the protruding portion and expose a fresh surface Microscopic examination showed the slicing technique to provide a smooth flat surface apart from features snch as cracks or holes already present in the catalyst VIII NUCLEAR REACTION ANALYSIS
380
C J Sofield et al / Mtcrobeam analysts of hydrogen
The specimens stall clamped in their jig were mounted in a vacuum chamber and coupled to a steplng motor drive which could move them in the horizontal plane at r]ght angles to the probing beam A quartz viewing screen was also attached to the jig and a microscope mounted on the chamber to view the specimens. The microscope was eqmpped with a graduated scale wh]ch could be related to the beam poSltlon o n the quartz viewer Subsequent observations of features on the specimens could then be related to the beam position 2 2 Hydrogen analysts The Harwell m]crobeam lenses provide a beam of light ions which can readily be focused to a 20/am by 20 /am spot This spot size provides more than adequate spatial resolution for the examination ot catalyst support pellets The mlcrobeam lenses are installed on the Harwell IBIS accelerator, which can operate with ease between 1 and 3 MeV, and is eqmpped with an rf source The only hydrogen analysis method suitable for this facihty is the elamc recod techmque [5,6]. We used a beam of 2 5 MeV 3He ions to recoil H atoms out of the target into a detector. The choice of incident ion is governed by kinematic considerations and He or heavier ions may also be used The glancing incidence geometry used to examine the specimens is shown schematically in fig 1. The sample surface was placed at 10 ° to the
beam axis and the detector at 20 ° This geometry results m the beana intersecting about 100 /lm of specimen along the beam direction even though the initial spot size is 2 0 / l m by 20/~m With this arrangement simple kinematic relations [7] show that 2 5 MeV 3He Ions scatter off the A1 of the pellet with an energy of no more than 2 47 MeV at 20 °. Hydrogen Ions recoil only in a forward &rectlon and those from the pellet surface have an energy of ~1 66 MeV at 10 ° An A1 fod about 7 #m thick placed in front of the detector was sufficient to stop the 3He ions scattered off the substrate whde allowing the recoil H ions from the surface through with a reduced energy of about 1 2 MeV. Hydrogen atoms up to a perpendicular depth of about 0.7 /~m can be detected after passing through the A1 fod Thus the spectrum of recoil H atoms contams Information about the H concentration from the surface to about 0 7 #m depth The data from differing positions on the sample was obtained by electrostatlcally scanning the beam across the sample and dividing the sweep into 32 equal length segments from each of which a H recoil spectrum of about 128 channels was recorded This two parameter mode of data acqulsmon using the mIcrobeam apparatus has been described in more detail by Cookson [4] Early in our investigation we found the signal we measured to depend non-linearly on the beam current and beam density, probably due to beam heating of the specimens This p r o b l ' m was
Microscope// bo
o
~
~-k
~
~ 10°
Catalyst support
3He mmrobeom
pellet
Fig 1 Schemat]c of arrangement of specimen, beam and detector for elastic recoil analysis ot hydrogen
C J Sofield et al
/ Mtcrobeam analysts
reduced to manageable proportions by reducing the beam current to about 10 nA and by the scanning procedure outlined above There are indications that this sensitivity to beam heating at high currents arises tram the presence of low nlolecular weight oily reaction products in the coke These may be outgassed by beam heating or "cracked" by the beam thus giving rise to a change in the H concentration Most of the "coke" is expected to be stable at the temperatures of the reactions which were in progress during ItS formation In future experiments it may be necessary for some samples to be cooled for quantitative analysis To demonstrate the consistency between this method of analysis and more established techniques, an initial experiment on a T1Hv sample on a stainless steel backing was performed The data are shown in fig 2 This sample had previously been examined by 7L1 nuclear reaction H profiling [8]. A scratch was put on the specnnen to generate a distinct feature that could be readily identified in a scan The data contained In a single H recoil spectrum was analysed by integrating the counts in the lowest eight channels
300 ~
~
o f hydrogen
of the broad peak This was done initially to exclude the posslbllity of including signal due to adsorbed H20 on the surfaces of the catalyst specimens, however subsequent examination showed this to be unnecessary The quantity of hydrogen (at %) had already been determined as a function of depth for the T1Hx specimen [8]. An estimate of the relation between counts of recoil H for the porous A1203 catalyst specimens and the hydrogen concentration was made by reference to the observed count and the known concentration in the TIHx spemmen, taking account of the different stopping powers in the two materials However as the catalyst specimens are porous this procedure may be imprecise due to a lack of knowledge of the appropriate stopping power Specimens of catalyst were also analysed after the recoil analysis by thermal vacuunr desorptlon and mass spectrometry to determine their total hydrogen concentratmns These two approaches gave results within 15% of each other Further work on this problem IS being undertaken Hole
Sc/rolc h
i ~ 'm
Scanned
250
Ar eo
/
Scan area
~ Catalyst support pellet
/~C7~_-_/~___TDHx//I
(Sect)oned)
H Analysis
5OO 400
/'% x .
0
381
~-10 \ \
20 30 40 "Channel 1Scratch b ~ \ C'-~ o~1000 k~
%
N°
300 /
\/
;"~"×-~x-.x
\
*
Hole ~ x\
5
~ / 1
m
xx × x
u
200
3:
lOO § soo, I
1
g
i
lO Posthon {mm)
F
E
0
.4 . 6. 8. 10 . 12~ 14 - J -16 ~ 18 20 + Relahve posihon(unlts 03ram)
22-
t lg 2 A recoil spectrum at H tram a TIHx coated stainless steel sample (as shown in sketch) and a graph of H concentration versus position on this sample
o
1 lg 3 Sketdl of catalyst support pellet sample sectioned for analysis and the variation o~ the H concentration across the portion of the specHnen indicated in the sketch. Note the effect of the hole m the specimen on the H concentration versus position graph VIII NUCLFAR RFACTION ANALYSIS
3 82
C J Softeld et al / Mlcrobeam analysis of hydrogen not Intersecting the hole, IS shown in fig 4. Again a sketch of the pellet is Included The quantity o f H is seen to peak at about 5 at % and remain roughly constant across the pellet Typical values for the carbon concentration [3] of these pellets range from about 2 at % to 10 at %. It would therefore seem likely that the coke composition in this support pellet was about CH2 to CHo s depending on the carbon concentration these values are not dissimilar to the values used in model calculations [2]
s Scan QFeQ C a t a l y s t s u p p o r t pellet ( Sect ion ed )
H Analysis 500 × ×x
&O0
×
5
x
4. Conclusion
×
× x
0
x
x
x
X
x
o~
300 x
×
E o
B
200
I
o o ]Z
100
0
o 20
I 10
0
Poslhon
(mm)
Fig 4 Sketch of specimen of fig 3 with a scan across a different positron as indicated and the resultant H concentration as a function of position across the specimen
The elastic recoil technique for hydrogen analysis using 2 5 MeV aHe ions has proved sufficiently sensitive for the investigation of catalyst support pellets The depth at which hydrogen could be assessed ( ~ 0 7 #m) was adequate for the examples investigated This technique was easily realized using a 3He mlcrobeam which thus provided the spatial resolution required to examine the spatial distribution of hydrogen across the diameter o f sectioned "coked up" catalyst support pellets. Examination of the specimen topography was essential to avoid misinterpretation of data on changes in H concentration as a function o f position
3. Results and discussion
References
The value of this H analysis technlqe is illustrated by reference to data taken from a single spherical catalyst support pellet The data taken from a scan across the diameter o f the sectioned specimen, which is also shown as a sketch, are shown in fig. 3. There is clear evidence of a large decrease in the H concentration at the right hand side of the scan. Visual ln-SltU examination of the specimen with a microscope showed that this was an artifact of the specimen topography a hole at the appropriate position of the scan masked the recoil H atoms from the detector Clearly this IS a special problem of the application of the elastic recoil technique but one which can be avoided with adequate visual examination Data taken from the same pellet, but along a chord
[1] R V Shankland, Adv Cat VI (1964) 27l [2] P A Ramachandran, M H Rashld and R Hughes, Chem Eng. Soc 30 (1975) 1391 [3] C.J Wright, J W McMtllan and J A. Cookson, J CS Chem Comrn (1979)968 {4] J A Cookson, Nucl Instr and Math 165 (1979) 477 [5] B Terrault, J G Martell, R G St Jacques and J L'Ecuyer, J Vac Scl Techn 141 (1977)492 [6] B L. Doyle and P S Peercy, Appl Phys Lett 34 (1979) 811 [7] A Mlchalowlcz, Kinematics of nuclear reactions (lhffe, London, 1964) [8] C J Sofield, J.l. Singleton, E S Hotston and G M McCracken, J Nud Mat 76/77 (1978) 348, G M. McCracken, D H J Goodall, L B Bndwell, G. Dearnaley, J H. Shea, C J Sofield and J Turner, IAFA-CN-37/N-52(A) (1978)
379
MICROBEAM ANALYSIS OF HYDROGEN IN COKED CATALYST PELLETS C J SOFIELD, L B BRIDWELL * and C J WRIGHT
UKAEA, Harwell, UK
The hydrogen content of catalyst pellets has recently been shown to be important in predicting the maximum temperature rise of the catalyst during regeneratmn in industrial plant This is because carbonaceous coke deposits (CH0 4 to CH2 o) accumulate m the pellet during use, whmh then have to be removed by exothermlc oxldatmn It is Important, m this well estabhshed industrial process, to hmlt the maxxmum temperature that the catalyst reaches to level s sufficiently low to prevent permanent deactivation by smterlng In vaew of the importance of the H content as a functmn of the distance from the surface of the catalyst pellets m predmting maximum temperature rises during de-cokmg, we have adopted the technique of elastm recoil analysis for use with the Harwell mlcrobeam to obtam measurements of the hydrogen concentration at varying positions along a pellet diameter The method of analysis Is described and Illustrated wath typical data.
1 Introduction When catalysts are used in various hydrocarbon conversion reactions such as cracking and reforming, the catalyst becomes poisoned. The poisoning takes the form of carbonaceous deposits called "coke" laid down within the catalyst pellets Regeneration of these "coked" pellets by burning off the deposits in hot steam or air/inert gas mixtures is a well established industrial process [1] However during this regeneration procedure the exothermlc de-coking reactions may heat the pellets to temperatures sufficiently high to slnter the catalyst material and permanently de-activate it The de-coklng process has received considerable theoretical attention and mathematical modelling of the temperature rise resulting from regeneration has been carried out [2] The distribution of the "coke" within a catalyst support pellet and its composition are important input parameters for models, but It is only recently that the hydrogen component of the CHx "coke" has also been considered as a variable [21. The analysis of the carbon distribution across sectioned catalyst support pellets has recently been investigated [3] by nuclear reaction analysis using the I2C(d, p ) l a c reaction The spatial resolution required
* Permanent address Murray State University, Murray, Kentucky, USA 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland
to carry out the analysis at many well resolved positions across a sectioned pellet of ~1 mm radius was provided by focusing the 1.3 MeV deuteron beam used in this reaction with the Harwell mlcrobeam system [4] Measurements of the H content of such catalyst support pellets with a similar spatial resolution to these carbon analyses would also be very useful.
2. Experimental method
2.1 Specimens The catalyst support pellets studied were cyhndrlcal or spherical In shape with diameters in the range 0 5 mm to 4 ram. They were fabricated from porous, high surface area A1203 with active catalyst dispersed over the surface The porosity of these materials is generally large, about 20% of the volume being open pore space The analysis technique which is described below, requires the specimens to be sectioned and the exposed surface to be presented to the probing beam at about 10° This was achieved by lightly clamping the samples in a vice-like jig which had two jaws chamfered at 10 °. The specimen was then sliced with a scalpel to remove the protruding portion and expose a fresh surface Microscopic examination showed the slicing technique to provide a smooth flat surface apart from features snch as cracks or holes already present in the catalyst VIII NUCLEAR REACTION ANALYSIS
380
C J Sofield et al / Mtcrobeam analysts of hydrogen
The specimens stall clamped in their jig were mounted in a vacuum chamber and coupled to a steplng motor drive which could move them in the horizontal plane at r]ght angles to the probing beam A quartz viewing screen was also attached to the jig and a microscope mounted on the chamber to view the specimens. The microscope was eqmpped with a graduated scale wh]ch could be related to the beam poSltlon o n the quartz viewer Subsequent observations of features on the specimens could then be related to the beam position 2 2 Hydrogen analysts The Harwell m]crobeam lenses provide a beam of light ions which can readily be focused to a 20/am by 20 /am spot This spot size provides more than adequate spatial resolution for the examination ot catalyst support pellets The mlcrobeam lenses are installed on the Harwell IBIS accelerator, which can operate with ease between 1 and 3 MeV, and is eqmpped with an rf source The only hydrogen analysis method suitable for this facihty is the elamc recod techmque [5,6]. We used a beam of 2 5 MeV 3He ions to recoil H atoms out of the target into a detector. The choice of incident ion is governed by kinematic considerations and He or heavier ions may also be used The glancing incidence geometry used to examine the specimens is shown schematically in fig 1. The sample surface was placed at 10 ° to the
beam axis and the detector at 20 ° This geometry results m the beana intersecting about 100 /lm of specimen along the beam direction even though the initial spot size is 2 0 / l m by 20/~m With this arrangement simple kinematic relations [7] show that 2 5 MeV 3He Ions scatter off the A1 of the pellet with an energy of no more than 2 47 MeV at 20 °. Hydrogen Ions recoil only in a forward &rectlon and those from the pellet surface have an energy of ~1 66 MeV at 10 ° An A1 fod about 7 #m thick placed in front of the detector was sufficient to stop the 3He ions scattered off the substrate whde allowing the recoil H ions from the surface through with a reduced energy of about 1 2 MeV. Hydrogen atoms up to a perpendicular depth of about 0.7 /~m can be detected after passing through the A1 fod Thus the spectrum of recoil H atoms contams Information about the H concentration from the surface to about 0 7 #m depth The data from differing positions on the sample was obtained by electrostatlcally scanning the beam across the sample and dividing the sweep into 32 equal length segments from each of which a H recoil spectrum of about 128 channels was recorded This two parameter mode of data acqulsmon using the mIcrobeam apparatus has been described in more detail by Cookson [4] Early in our investigation we found the signal we measured to depend non-linearly on the beam current and beam density, probably due to beam heating of the specimens This p r o b l ' m was
Microscope// bo
o
~
~-k
~
~ 10°
Catalyst support
3He mmrobeom
pellet
Fig 1 Schemat]c of arrangement of specimen, beam and detector for elastic recoil analysis ot hydrogen
C J Sofield et al
/ Mtcrobeam analysts
reduced to manageable proportions by reducing the beam current to about 10 nA and by the scanning procedure outlined above There are indications that this sensitivity to beam heating at high currents arises tram the presence of low nlolecular weight oily reaction products in the coke These may be outgassed by beam heating or "cracked" by the beam thus giving rise to a change in the H concentration Most of the "coke" is expected to be stable at the temperatures of the reactions which were in progress during ItS formation In future experiments it may be necessary for some samples to be cooled for quantitative analysis To demonstrate the consistency between this method of analysis and more established techniques, an initial experiment on a T1Hv sample on a stainless steel backing was performed The data are shown in fig 2 This sample had previously been examined by 7L1 nuclear reaction H profiling [8]. A scratch was put on the specnnen to generate a distinct feature that could be readily identified in a scan The data contained In a single H recoil spectrum was analysed by integrating the counts in the lowest eight channels
300 ~
~
o f hydrogen
of the broad peak This was done initially to exclude the posslbllity of including signal due to adsorbed H20 on the surfaces of the catalyst specimens, however subsequent examination showed this to be unnecessary The quantity of hydrogen (at %) had already been determined as a function of depth for the T1Hx specimen [8]. An estimate of the relation between counts of recoil H for the porous A1203 catalyst specimens and the hydrogen concentration was made by reference to the observed count and the known concentration in the TIHx spemmen, taking account of the different stopping powers in the two materials However as the catalyst specimens are porous this procedure may be imprecise due to a lack of knowledge of the appropriate stopping power Specimens of catalyst were also analysed after the recoil analysis by thermal vacuunr desorptlon and mass spectrometry to determine their total hydrogen concentratmns These two approaches gave results within 15% of each other Further work on this problem IS being undertaken Hole
Sc/rolc h
i ~ 'm
Scanned
250
Ar eo
/
Scan area
~ Catalyst support pellet
/~C7~_-_/~___TDHx//I
(Sect)oned)
H Analysis
5OO 400
/'% x .
0
381
~-10 \ \
20 30 40 "Channel 1Scratch b ~ \ C'-~ o~1000 k~
%
N°
300 /
\/
;"~"×-~x-.x
\
*
Hole ~ x\
5
~ / 1
m
xx × x
u
200
3:
lOO § soo, I
1
g
i
lO Posthon {mm)
F
E
0
.4 . 6. 8. 10 . 12~ 14 - J -16 ~ 18 20 + Relahve posihon(unlts 03ram)
22-
t lg 2 A recoil spectrum at H tram a TIHx coated stainless steel sample (as shown in sketch) and a graph of H concentration versus position on this sample
o
1 lg 3 Sketdl of catalyst support pellet sample sectioned for analysis and the variation o~ the H concentration across the portion of the specHnen indicated in the sketch. Note the effect of the hole m the specimen on the H concentration versus position graph VIII NUCLFAR RFACTION ANALYSIS
3 82
C J Softeld et al / Mlcrobeam analysis of hydrogen not Intersecting the hole, IS shown in fig 4. Again a sketch of the pellet is Included The quantity o f H is seen to peak at about 5 at % and remain roughly constant across the pellet Typical values for the carbon concentration [3] of these pellets range from about 2 at % to 10 at %. It would therefore seem likely that the coke composition in this support pellet was about CH2 to CHo s depending on the carbon concentration these values are not dissimilar to the values used in model calculations [2]
s Scan QFeQ C a t a l y s t s u p p o r t pellet ( Sect ion ed )
H Analysis 500 × ×x
&O0
×
5
x
4. Conclusion
×
× x
0
x
x
x
X
x
o~
300 x
×
E o
B
200
I
o o ]Z
100
0
o 20
I 10
0
Poslhon
(mm)
Fig 4 Sketch of specimen of fig 3 with a scan across a different positron as indicated and the resultant H concentration as a function of position across the specimen
The elastic recoil technique for hydrogen analysis using 2 5 MeV aHe ions has proved sufficiently sensitive for the investigation of catalyst support pellets The depth at which hydrogen could be assessed ( ~ 0 7 #m) was adequate for the examples investigated This technique was easily realized using a 3He mlcrobeam which thus provided the spatial resolution required to examine the spatial distribution of hydrogen across the diameter o f sectioned "coked up" catalyst support pellets. Examination of the specimen topography was essential to avoid misinterpretation of data on changes in H concentration as a function o f position
3. Results and discussion
References
The value of this H analysis technlqe is illustrated by reference to data taken from a single spherical catalyst support pellet The data taken from a scan across the diameter o f the sectioned specimen, which is also shown as a sketch, are shown in fig. 3. There is clear evidence of a large decrease in the H concentration at the right hand side of the scan. Visual ln-SltU examination of the specimen with a microscope showed that this was an artifact of the specimen topography a hole at the appropriate position of the scan masked the recoil H atoms from the detector Clearly this IS a special problem of the application of the elastic recoil technique but one which can be avoided with adequate visual examination Data taken from the same pellet, but along a chord
[1] R V Shankland, Adv Cat VI (1964) 27l [2] P A Ramachandran, M H Rashld and R Hughes, Chem Eng. Soc 30 (1975) 1391 [3] C.J Wright, J W McMtllan and J A. Cookson, J CS Chem Comrn (1979)968 {4] J A Cookson, Nucl Instr and Math 165 (1979) 477 [5] B Terrault, J G Martell, R G St Jacques and J L'Ecuyer, J Vac Scl Techn 141 (1977)492 [6] B L. Doyle and P S Peercy, Appl Phys Lett 34 (1979) 811 [7] A Mlchalowlcz, Kinematics of nuclear reactions (lhffe, London, 1964) [8] C J Sofield, J.l. Singleton, E S Hotston and G M McCracken, J Nud Mat 76/77 (1978) 348, G M. McCracken, D H J Goodall, L B Bndwell, G. Dearnaley, J H. Shea, C J Sofield and J Turner, IAFA-CN-37/N-52(A) (1978)