Effect of Fines Content on Fluidity of FCC Catalysts for Stable Operation of Fluid Catalytic Cracking Unit

energies

Article Effect of Fines Content on Fluidity of FCC Catalysts for Stable Operation of Fluid Catalytic Cracking Unit

Sung Won Kim 1,* , Chae Eun Yeo 1 and Do Yeon Lee 2,*

1 School of Chemical and Material Engineering, Korea National University of Transportation, Chungju-si, Chungbuk 27469, Korea; [email protected] 2 Greenhouse Gas Laboratory, Korea Institute of Energy Research, Daejeon 34129, Korea * Correspondence: [email protected] (S.W.K.); [email protected] (D.Y.L.); Tel.: +82-43-841-5228 (S.W.K.); +82-42-860-3138 (D.Y.L.) Received: 20 December 2018; Accepted: 16 January 2019; Published: 18 January 2019

Abstract: Effect of fines content (weight % of particles with diameter less than 45 µm) on bed fluidity was determined to get a base for good fluidization quality in the fluid catalytic cracking (FCC) unit. The fines content in equilibrium FCC catalysts (Ecat) from commercial units were controlled by adding or removing the fines to simulate commercial situation. To get the fluidity values (Umb/Umf) of seven different FCC catalysts (2 Ecats and 5 fresh catalysts) and their mixture, minimum fluidization velocity (Umf) and minimum bubbling velocity (Umb) were measured in a fluidized bed reactor (0.05 m ID). The fluidity decreased with loss of fines content and increased with increments of makeup of fresh catalysts or additive with the controlled fines content. The fluidities of catalysts increase with increases of normalized particle diameter variation by the fines addition. The obtained fluidities have been correlated with the fines contents and the catalyst and gas properties. The proposed correlation could guide to keep good catalyst fluidity in the FCC unit.

Keywords: fluid catalytic cracking; catalysts; fluidization; fluidity; fines content

1. Introduction Fluid catalytic cracking (FCC) processes are used to upgrade heavy hydrocarbon fractions into valuable light fractions, such as propylene, gasoline, and gas oil, etc. [1]. The FCC unit consists mainly of a riser, stripper, and catalyst regenerator. The FCC process utilizes a catalyst circulation system, whereby a hot circulating catalyst contacts the oil feedstock in the riser where the catalytic cracking reaction occurs. The cracked product vapor from the reactor is sent to a main fractionator for product separation. The catalyst after the reaction is stripped of volatiles with steam in the stripper, then is sent to the regenerator. The catalyst is reactivated and heated by burning off the coke with air in the regenerator. The heated catalyst is transported to the riser to meet again with the fresh feedstock. The catalyst circulation loop is completed by re-injecting the regenerated catalyst into the riser [2,3]. It is essential to define fluidization phenomena of catalysts in the FCC unit, since all catalyst beds in the circulation loop are in in fluidized beds at all times. The fluidity or fluidization index is defined in many ways and there have been as yet no unique definition and correlation to be fully accepted [4]. Nevertheless, it has long been known in the industry that fluidity (Umb/Umf) is a useful criterion for judging the hydrodynamic behavior and fluidization properties of the FCC catalysts in fluidized beds [5–7]. To get the fluidity value, many experimental studies have provided correlations between operating parameters and minimum fluidization velocity (Umf), while less work has been directed at minimum bubbling velocity (Umb)[8]. In the experimental measurement of Umf and Umb, the Umb has ambiguity in judgment of the first bubble, unlike Umf. Recently, computational fluid dynamics (CFD) simulation determined Umb point of the bubble can

Energies 2019, 12, 293; doi:10.3390/en12020293 www.mdpi.com/journal/energies Energies 2019, 12, 293 2 of 10 be well defined [9,10]. However, experimental measurements are often recommended in the refinery, because difficulties may arise with fluidization characteristics of the catalysts changing over time [2,8]. Many studies have tried to determine the fines’ on the fluidization quality of the catalyst beds [7]. Direct measurement of the FCC catalyst fluidity could provide refiners a tool to routinely check for the catalyst circulation problems in the FCC unit [11]. The control of catalyst fluidity is often difficult due to changing operation conditions and the limited range of available treatment options. The operational condition, hardware design, and catalyst characteristics causes the emission of catalysts, especially catalyst fines. The loss of fines in catalyst inventory can lead to a reduced catalyst fluidity, catalyst defluidization in FCC circulation loop, or worse, shut-down of the unit. In the case of poor fluidization trouble caused by the fines content loss, it is usually recommended to quickly shift the catalysts size distribution (PSD) to the normal distribution of the equilibrium catalyst in the unit. Refiners purposely add fresh catalyst with high fines content, because the catalyst fines cannot be added independently for fast effect in the fluidity. The catalyst vendor can change the particle size distribution of the fresh catalyst, such as adding fines and re-design of the catalyst to assist in restoring normal PSD of the bed. Finally, the refiners take the action of adding the fresh catalysts or fluidity-enhancing-additives with high fines content articles into the regenerator [11,12]. However, the refiners often face the challenging task of judiciously selecting catalysts and additives and estimating the amount of catalyst required for fast effect in the fluidity. It is very difficult to select a catalyst, either by the catalyst vendor’s claims or through a performance test directly at the plant [13]. Therefore, it is important to apply reliable prediction for the selection and to exploit the selected catalyst capabilities to the full extent based on the prediction. A few correlations [5,6,12] to predict the fluidity have been reported relating to the fluidity prediction, because less correlation work has been directed at Umb compared to Umf [8]. Abrahamsen and Geldart’s [3] correlation for prediction of fluidity has been widely used in the refinery [2]. However, Whitcombe et al. [8] showed that previous correlations on fluidization characteristics did not predict well experimental data of Ecat due to metal contamination on the catalyst surface. It indicates that the fluidity correlation should be improved with experimental data of Ecat only and Ecat-fresh catalyst mixtures for reliable prediction and its application. For the same reason, it is very important to investigate the effect of fresh catalyst and additive addition on the catalyst fluidity in an Ecat fluidized bed, to ensure stable operation even under changing operating conditions of the FCC unit. In this study, the fluidities of commercial fresh and equilibrium FCC catalysts with various contents of fine particles were measured to investigate the effect of fresh catalyst and additive addition for maintaining good fluidization quality of the FCC unit. An improved correlation based on the experimental data has been proposed for practical application in the FCC unit, such as fines makeup of the Ecat bed considering the situation of fines loss. A guidance for control of bed fluidity has been proposed based on the correlation.

2. Materials and Methods Experiments were carried out in a cold model reactor, which is made of a transparent Plexiglas column, as shown in Figure1. It consisted of a main column (0.05 m ID) and an expanded column (0.20 m ID), which is covered with wire mesh screen to retain fluidized particles inside the bed. Primary components are the compressor (NH-5, Hanshin), cylindrical column, wind box, sintered plate distributor (Stainless Steel Filter: pore size 40 µm, Sunwoo sintered filter Co.), flow meter (RK1150, Kojima instrument), and manometer (series 1221, Dwyer). Air was injected into the column through the wind box and the distributor. Pressure taps were mounted flush with the wall of the column to measure pressure drops with gas velocity. Bed materials of 0.45 kg were loaded. To get the fluidity values (Umb/Umf) of the FCC catalysts, the Umf and the Umb were measured in a fluidized bed reactor. Energies 2019, 12, 293 3 of 10 Energies 2019, 12, x FOR PEER REVIEW 3 of 10

FigureFigure 1. 1. SchematicSchematic diagram diagram of of experimental experimental apparatus. apparatus.

TheThe U Umfmf ofof the the catalyst catalyst was was obtained obtained from from a plot a plot of ofpressure pressure drop drop across across the the bed bed as a as function a function of superficialof superficial velocity velocity (Ug). (U Theg). point The point of minimum of minimum fluidization fluidization was taken was takenat the atintercept the intercept of the offixed the bedfixed pressure bed pressure drop, usually drop, usually a linear afunction linear function of Ug, and of the Ug ,pressure and the drop pressure when drop the bed when is thefluidized, bed is essentiallyfluidized, essentiallydetermined determined by the weight by the of weight particles of particlesper unit per area unit of areathe ofbed the [14]. bed The [14]. U Themb was Umb determinedwas determined by the by inspection the inspection of the of theappearance appearance of the of the first first obvious obvious bubble bubble in in the the bed bed after after the the minimumminimum fluidization fluidization state state as as U Ugg isis increased increased [12,14]. [12,14]. SevenSeven types types of of FCC FCC catalysts catalysts (Geldart (Geldart group group A A [5]) [5]) and and their their mixtures mixtures were were used used as as the the bed bed material.material. The The FCC FCC catalysts catalysts have have four four major major componen components:ts: zeolite, zeolite, matrix, matrix, filler, filler, and and binder. binder. They They have have anan internal internal porous porous structure structure with with acid acid sites sites to crac to crackk larger larger molecules molecules to desired to desired size ranges size ranges [2]. The [2]. catalystsThe catalysts include include two types two types of Equilib of Equilibriumrium catalysts catalysts (Ecats), (Ecats), four types four types of fresh of freshcatalysts catalysts (Fcats), (Fcats), and anand FCC an additive. FCC additive. The Ecats The were Ecats obtained were obtained from a fromcommercial a commercial FCC unit FCC in unitKorean in Koreanrefineries, refineries, which useswhich the uses different the different type of type catalysts of catalysts depending depending on main on mainproducts products of the of unit, the unit, such such as gasoline as gasoline or propylene.or propylene. The Fcats The Fcatsand additive and additive in the study in the ar studye commercial are commercial FCC catalysts FCC catalystsprovided providedby different by catalystsdifferent companies. catalysts companies. The Fcat-1 The was Fcat-1 manufactured was manufactured by BASF, by and BASF, the andFcats-2, the Fcats-2,3, and 3,4 andwere 4 manufacturedwere manufactured by Albemarle. by Albemarle. The Theadditi additiveve is a is commercial a commercial product product (Smoothflow (SmoothflowTMTM, ,Albemarle) Albemarle) havinghaving a a high high content content of of fine fine particles particles for for improving improving catalyst catalyst fluidity fluidity in in the the process. process. The The particle particle size size distributionsdistributions (PSDs) of allall catalystcatalysttypes types are are shown shown in in Figure Figure2. All2. All catalysts catalysts show show a broad a broad particle particle size sizedistribution, distribution, mostly mostly in the in the range range between between 10 to10180 to 180µm, μm, while while the the additive additive is inis in the the range range between between 10 10to to 100 100µm. μm. In In the the PSD PSD comparisons comparisons of FCCof FCC catalysts, catalysts, the the Ecats Ecats show show lower lower amounts amounts of particles of particles less lessthan than 50 microns 50 microns than than the the Fcats, Fcats, due due to the to the gradual gradua lossl loss of fine of fine particles particles in the in the catalyst catalyst bed bed through through the thecyclone cyclone system system during during the numerousthe numerous catalyst catalyst circulations circulations in the in unit. the Theunit. Fcats The differ Fcats in differ the amount in the amountof fine particles of fine particles less than less 50 microns than 50 between microns the betw catalysts,een the because catalysts, the because catalyst the vendors catalyst produce vendors the producecatalysts the by catalysts controlling by thecontrolling amount the of the amount fine particles of the fine according particles to according the requirements to the requirements of the refiners. of theThe refiners. physical The properties physical of properties the FCC catalysts of the FCC are showncatalysts in are Table shown1. In thein Table table, 1. d p,45+In therepresents table, dp, the45+ representsSauter mean the diameterSauter mean of catalysts, diameter except of catalysts, for the except fine particles for the fine with particles a diameter with of a diameter less than of 45 lessµm. thanThe scanning45 μm. The electron scanning microscopy electron (SEM:microscopy S-4800, (SEM: Hitachi) S-4800, images Hitachi) of the images catalyst of particles the catalyst are shownparticles in areFigure shown3. As in shown Figure in 3. theAs figure,shown thein the Ecats figure, have the a different Ecats have shape a different compared shape to the compared Fcats and to additives,the Fcats andbecause additives, the Ecat because particles the Ecat are exposed particles to are severe expose conditions,d to severe such conditions, as high su temperaturech as high temperature and attrition andduring attrition the circulation during the in circulation the unit. X-ray in the fluorescence unit. X-ray fluorescence spectroscopy spectroscopy (XRF: Axios-Petro, (XRF: Axios-Petro, PANalytical) PANalytical)was used for thewas metal used contentsfor the metal analysis contents of the catalysts.analysis of The the Ecats catalysts. show The high Ecats metal show contents, high such metal as contents,0.53 wt. %such of Ni, as 1.210.53 wt.wt. %% ofof V, Ni, and 1.21 0.70 wt. wt. % % of of V, Fe, and while 0.70 no wt. Ni % and of VFe, were while detected no Ni inand Fcats, V were and detectedthe amount in Fcats, of Fe and was the at 0.3 amount wt. %. of Fe was at 0.3 wt. %.

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Figure 2. Cumulative particle size distributions of FCC catalysts used in this study. Figure 2. Cumulative particle size distributions of FCC catalysts used in this study. Figure 2. Cumulative particle size distributions of FCC catalysts used in this study.

FigureFigure 3.3. SEMSEM (scanning(scanning electronelectron microscopy)microscopy) imagesimages ofof FCCFCC catalystscatalysts usedused inin thisthis study:study: ((aa)) Ecat-1,Ecat-1, Figure((bb)) Ecat-2,Ecat-2, 3. SEM ((cc)) Fcat-1, Fcat-1,(scanning ((dd)) Fcat-2,electronFcat-2, ((ee )microscopy)) Fcat-3,Fcat-3, ( (ff)) additive. additive. images of FCC catalysts used in this study: (a) Ecat-1, (b) Ecat-2, (c) Fcat-1, (d) Fcat-2, (e) Fcat-3, (f) additive. Table 1. Physical properties of FCC particles in the study. Table 1. Physical properties of FCC particles in the study. Table 1. Physical properties of FCC particles in the study. Average Particle (a) Fine Contents Apparent Density, FCC Catalysts Average Particle(a) dp,45+ [m] Fine Contents Apparent3 Density,(b) Diameter, dp [m] [wt. %] p [kg/m ] FCC Catalysts dp,45+ [m] (a) AverageDiameter, Particle dp [m] (a) Fine[wt. Contents%] Apparentp [kg/m Density,3] (b) FCCEcat-1 Catalysts 65.7dp,45+ 67.5 [m] (a) 6.8 1647 (a)  3 (b) Ecat-1Ecat-2 Diameter,65.7 68.6 dp [m] 67.5 70.1 [wt.6.8 4.9 %] 1647p 1646 [kg/m ] Fcat-1 67.8 71.1 10.4 1402 Ecat-1Ecat-2Fcat-2 65.768.6 65.3 67.570.1 74.0 6.84.9 20.0 16471646 1608 Fcat-3 50.3 63.1 33.9 1580 Ecat-2Fcat-1 68.667.8 70.171.1 4.910.4 16461402 Fcat-4 60.5 67.8 23.0 1720 Fcat-1Fcat-2Additive 67.865.3 47.2 71.174.0 60.5 10.420.0 37.4 14021608 1625 Fcat-2Note:Fcat-3 (a ) based on the65.350.3 average of 3 repeated measurements;74.063.1 (b) based20.033.9 on the average of 516081580 repeated measurements. Fcat-3 50.3 63.1 33.9 1580 InFcat-4 the experiments,60.5 4 sets of catalyst67.8 mixtures were prepared23.0 in order1720 to simulate the refinery situation,Fcat-4Additive as summarized 60.547.2 in Table2. In set67.860.5 1, the fluidity of23.037.4 each catalyst particle17201625 (7 catalysts) was measured.AdditiveNote: ( Ina) based set 2, on the47.2 the content average of of fine 3 repeated particles60.5 measurements; was adjusted37.4 (b by) based addition on the or1625 average removal of 5 of repeated the fines in the Ecat-1Note:measurements. ( anda) based Ecat-2 on bedsthe average to simulate of 3 repeated the plant’s measurements; differential (b fines) based loss on situation, the average such of 5 as repeated the cyclone troublemeasurements. [15]. In set 3, the Fcat-1 and Fcat-2 were added into the Ecat-1 bed without fine particles to simulate the Fcats make-up for restoring the fluidity of the bed from the fine loss situation. In set 4,

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In the experiments, 4 sets of catalyst mixtures were prepared in order to simulate the refinery situation, as summarized in Table 2. In set 1, the fluidity of each catalyst particle (7 catalysts) was measured. In set 2, the content of fine particles was adjusted by addition or removal of the fines in the Ecat-1 and Ecat-2 beds to simulate the plant's differential fines loss situation, such as the cyclone trouble [15]. In set 3, the Fcat-1 and Fcat-2 were added into the Ecat-1 bed without fine particles to Energies 2019, 12, 293 5 of 10 simulate the Fcats make-up for restoring the fluidity of the bed from the fine loss situation. In set 4, the additive was added into the Ecat-1 bed without the fine particles to show the effect of make-up theof additivecatalyst with was addedmuch intohigher the fines Ecat-1 cont bedent without on the fluidity the fine of particles the Ecat to bed. show the effect of make-up of catalyst with much higher fines content on the fluidity of the Ecat bed. Table 2. Summary of experimental sets on the fluidity measurement. Table 2. Summary of experimental sets on the fluidity measurement. SET Method Remark 1 SET Measurement Method with 7 FCC catalysts in Table 1. Remark Effect of physical properties. 2 1 MeasurementMeasurement with with the 7 FCCEcat catalysts bed with in Tablecontrolled1. Effect ofEffect physical of properties.fines contents in Ecats. fines content by Measurement with the Ecat bed with (1) addingcontrolled fines fines to contentEcat-1 bywithout fines. 2 Effect of fines contents in Ecats. (2) adding(1) adding fines fines to toEcat-2 Ecat-1 without without fines. fines. 3 Measurement(2) adding fines with to the Ecat-2 Ecat without bed with fines. controlled Effect of Fcats make-up in Ecat FcatMeasurement content by with the Ecat bed with bed without fines. controlled Fcat content by Effect of Fcats make-up in Ecat bed 3 (1) adding Fcat-1 to Ecat-1 without fines. (1) adding Fcat-1 to Ecat-1 without fines. without fines. (2) adding Fcat-2 to Ecat-1 without fines. (2) adding Fcat-2 to Ecat-1 without fines. 4 Measurement with the Ecat bed with controlled Effect of additive make-up in Measurement with the Ecat bed with 4 additivecontrolled content additive by adding content additive by adding to Ecat-1 Effect ofEcat additive bed. make-up in Ecat bed. withoutadditive fines. to Ecat-1 without fines.

3.3. Results Results

3.1.3.1. Effect Effect of of Fcat Fcat or or Additive Additive Make-up Make-up on on Bed Bed Fluidity Fluidity EffectEffect of of fines fines contents contents on on fluidity fluidity of of FCC FCC catalysts catalysts (Ecats) (Ecats) is is shown shown in in Figure Figure4. 4. The The fluidity fluidity of of thethe Ecats Ecats increases increases with with increase increase of of the the fines fines content. content. It It is is often often speculated speculated that that the the fines fines act act as as a a kind kind ofof lubricant. lubricant. This This lowers lowers the the apparent apparent viscosity viscosity of of the the fluidized fluidized bed bed and and leads leads to to suppression suppression of of the the formingforming of of bubbles, bubbles, with with increased increased overall overall bed bed expansion expansion by by the the more more uniform uniform gas-solid gas-solid distribution. distribution. TheThe amount amount of of gas gas flowingflowing interstitiallyinterstitially is a function function of of the the fines fines content content [7,12]. [7,12 ].Figure Figure 4 shows4 shows that thatincrement increment of 1 of wt. 1 wt. % of % fines of fines increases increases about about 0.03–0.04 0.03–0.04 of fluidity, of fluidity, depending depending on onthe theEcat Ecat type. type. The TheGeldart Geldart Group Group A Aparticles particles with with high high fines fines contents contents retain retain gas gas longer thanthan thosethose withwithlow low fines, fines, indicatingindicating that that a certaina certain fines fines loss loss from from the the bed bed could could make make poor poor fluidization fluidization in some in some regions, regions, such such as standpipeas standpipe or wall or wall region region of the of reactorthe reactor [16]. [16].

3.0

2.8

2.6

2.4

Fluidity[-] 2.2 Ecat-1+fines 2.0 Ecat-2+fines 1.8 0123456789 Fines contents [wt %]

FigureFigure 4. 4.Effect Effect of of ﬁnes fines content content on on catalyst catalyst bed bed ﬂuidity. fluidity.

Effects of the fines content and average particle diameter (dp) on minimum fluidization velocity (Umf) are shown in Figure5. The U mf decreased with increasing content of fine particles in the catalysts with make-up of Fcats or additive into the bed. The Umf is sensitive to parameters such as particle diameter, particle density, and fluid properties [17]. The fine particles introduced between the coarse particles could act as a lubricant in the bed to reduce the bed viscosity with the reduction of the Energies 2019, 12, x FOR PEER REVIEW 6 of 10

Effects of the fines content and average particle diameter (dp) on minimum fluidization velocity (Umf) are shown in Figure 5. The Umf decreased with increasing content of fine particles in the catalysts with make-up of Fcats or additive into the bed. The Umf is sensitive to parameters such as particle Energies 2019, 12, 293 6 of 10 diameter, particle density, and fluid properties [17]. The fine particles introduced between the coarse particles could act as a lubricant in the bed to reduce the bed viscosity with the reduction of the frictionfriction betweenbetween the the coarse coarse particles, particles, thereby thereby leading leading to ato decrease a decrease in the in Uthemf U[5mf]. [5]. Additionally, Additionally, the fine the contentsfine contents can be can related be related to the to voidage the voidage in the bedin the [6]. be Thed [6]. increase The increase of fine contentsof fine contents decreases decreases the average the particlesaverage diameterparticles ofdiameter the mixed of the FCC mixed catalysts. FCC The catalysts. increased The fines increased and decreased fines and dp decreasedaffect the decreasedp affect ofthe the decrease Umf as of in the Figure Umf5 asb. in Figure 5b.

0.004 0.004

0.003 0.003

0.002 [m/s] 0.002 Ecat-1+fines [m/s]

mf Ecat-2+fines mf U

U Ecat-1+Fcat-1 0.001 0.001 Ecat-1+Fcat-2 (a) FCC catalysts (b) Ecat-1+Additive 0.000 0.000 0 10203040 02468 Fines contents [wt %] Fines contents [wt %]

0.004 0.004

0.003 0.003

Ecat-1+fines [m/s] 0.002 [m/s] 0.002 Ecat-2+fines mf mf Ecat-1+Fcat-1 U U 0.001 0.001 Ecat-1+Fcat-2 (c) FCC catalysts (d) Ecat-1+Additive 0.000 0.000 30 40 50 60 70 80 64 66 68 70 72 Average dp [μm] Average dp [μm]

FigureFigure 5.5. Effects of of fine ﬁne contents contents and and average average particle particle diameter diameter on on minimum minimum fluidization ﬂuidization velocity: velocity: (a) (seta) set1, ( 1,b) ( bsets) sets 2–4, 2–4, (c) (setc) set 1, ( 1,d) ( dsets) sets 2–4 2–4 in Table in Table 2. 2.

Effects of the fine contents and average particle diameter (dp) on minimum bubbling velocity Effects of the fine contents and average particle diameter (dp) on minimum bubbling velocity (Umb) are shown in Figure6. The U mb did not show noticeable change with increasing the content of (Umb) are shown in Figure 6. The Umb did not show noticeable change with increasing the content of fine particles and average particle diameter in the catalysts with make-up of Fcats or additive into the fine particles and average particle diameter in the catalysts with make-up of Fcats or additive into bed. The Umb is dependent upon fines content and particle diameter [5]. The increase of fines content the bed. The Umb is dependent upon fines content and particle diameter [5]. The increase of fines in the bed enhances the inter-particle force, influencing the stability of the bubble-free regime [6]. content in the bed enhances the inter-particle force, influencing the stability of the bubble-free regime However, the increase of fines content simultaneously decreases dp, resulting in decrease of Umb. [6]. However, the increase of fines content simultaneously decreases dp, resulting in decrease of Umb. However, it can be seen that the gap between Umf and Umb was obviously increased with the fines However, it can be seen that the gap between Umf and Umb was obviously increased with the fines content from Figures6 and7, indicating expansion of the bubble-free regime after the fluidization of content from Figures 6 and 7, indicating expansion of the bubble-free regime after the fluidization of FCC particles. FCC particles Effects of fines content and normalized variation of particle mean diameter on the catalyst fluidity (Umb/Umf) are shown in Figure7. It is well known that high contents of fines help to maintain good fluidity of the FCC catalyst bed. The increase of fine contents increases the fluidity of the Ecat bed, as in Figure7a, indicating the addition of the catalyst with a high fines content will help to restore fluidity. The effect of fines in the catalysts on the fluidity has been well defined from a lot of studies with varying fines content in given FCC catalysts and the correlation derived from the results [7]. However, the particle size distribution (PSD) and the average particle size change, as well as the change of the fines content when the catalysts have different PSDs, are mixed with the existing catalyst bed. Therefore, it is necessary to consider the relative influence of the fine particles on coarse ones in the overall bed as the fines content varies [7,18], because it is difficult to quantify the PSD change of the bed. Regarding the PSD change, several studies [6,19] introduced a concept of relative spread in the PSD, but the results for its effect were not shown. In this study, the particles in the bed were Energies 2019, 12, x FOR PEER REVIEW 7 of 10

0.010 0.010 (a) 0.008 0.008

0.006 0.006 FCC Catalysts FCC Catalysts [m/s] [m/s] 0.004 Ecat-1+fines 0.004 Ecat-1+fines mb mb Ecat-2+fines Ecat-2+fines U U Ecat-1+Fcat-1 Ecat-1+Fcat-1 0.002 0.002 Ecat-1+Fcat-2 (b) Ecat-1+Fcat-2 Ecat-1+Additive Ecat-1+Additive 0.000 0.000 0 10203040 30 40 50 60 70 80 Fines contents [wt %] Average dp [μm] Energies 2019, 12, 293 7 of 10 Figure 6. Effects of fine contents (a) and average particle diameter (b) on minimum bubbling dividedvelocity. into coarse particles and fine particles based on 45 µm. A concept of variation of mean particle size with fines addition (normalized particle diameter variation) is applied to quantify the effect of Effects of fines content and normalized variation of particle mean diameter on the catalyst mixedEnergies catalyst2019, 12, x bed FOR fluidityPEER REVIEW when the fine particle content in the coarse particle is changed. 7 of 10 fluidity (Umb/Umf) are shown in Figure 7. It is well known that high contents of fines help to maintain good fluidity of the FCC catalyst bed. The increase of fine contents increases the fluidity of the Ecat 0.010 0.010 bed, as in Figure 7a, indicating the addition of the catalyst with a high fines content will help to restore (a) fluidity.0.008 The effect of fines in the catalysts on the fluidity0.008 has been well defined from a lot of studies with varying fines content in given FCC catalysts and the correlation derived from the results [7]. 0.006 0.006 However, the FCCparticle Catalysts size distribution (PSD) and the average particle size change,FCC Catalystsas well as the [m/s] [m/s] 0.004 Ecat-1+fines 0.004 Ecat-1+fines mb

changemb of the fines content when the catalysts have different PSDs, are mixed with the existing Ecat-2+fines Ecat-2+fines U U catalyst bed. Therefore,Ecat-1+Fcat-1 it is necessary to consider the relative influence of the fine particlesEcat-1+Fcat-1 on coarse 0.002 0.002 ones in the overallEcat-1+Fcat-2 bed as the fines content varies [7,18], because(b) it is difficult to Ecat-1+Fcat-2quantify the PSD Ecat-1+Additive Ecat-1+Additive change0.000 of the bed. Regarding the PSD change, several studies0.000 [6,19] introduced a concept of relative spread in the0 PSD, but 10203040 the results for its effect were not shown.30 In this 40 study, 50 the 60particles 70 in the 80 bed Fines contents [wt %] Average dp [μm] were divided into coarse particles and fine particles based on 45 μm. A concept of variation of mean particleFigureFigure size 6.6. Effectswith fines ofof finefine addition contentscontents ((normalized(aa)) andand averageaverage particle particleparticle diameter diameter ( b(variation)b)) on on minimum minimum is applied bubbling bubbling to velocity. quantify the effectvelocity. of mixed catalyst bed fluidity when the fine particle content in the coarse particle is changed. The normalized particle diameter variation is defined as Equation (1). The normalized particle diameter variation is defined as Equation (1). Effects of fines content and normalized variation of particle mean diameter on the catalyst dp,n = (dp,45+ − dp)/ dp,45+ (1) fluidity (Umb/Umf) are shown in Figure 7. It is well known that high contents of fines help to maintain dp,n = (dp,45+ − dp)/ dp,45+ (1) good fluidity of the FCC catalyst bed. The increase of fine contents increases the fluidity of the Ecat The fluidities of catalysts increase with increases of dp,n, due to the increase of PSD and the bed, asThe in Figure fluidities 7a, indicatingof catalysts the increase addition with of the increases catalyst withof dp,n a ,high due fines to the content increase will of help PSD to restoreand the decrease of mean diameter compared initial catalyst bed, as in Figure7b. The increase of fluidity is fluidity.decrease The of effectmean ofdiameter fines in compared the catalysts initial on thecatalyst fluidity bed, has as been in Figure well defined7b. The fromincrease a lot of of fluidity studies is affected not only by the PSD of the added catalyst, but also by the physical properties of the catalyst, withaffected varying not onlyfines by content the PSD in ofgiven the addedFCC catalysts catalyst, and but alsothe correlation by the physical derived properties from the of theresults catalyst, [7]. considering the difference of the increasing slope. However,considering the the particle difference size distributionof the increasing (PSD) slope. and the average particle size change, as well as the change5.0 of the fines content when the catalysts have 5.0different PSDs, are mixed with the existing catalyst bed. Therefore, it is necessary to consider the relative influence of the fine particles on coarse ones in4.0 the overall bed as the fines content varies [7,18],4.0 because it is difficult to quantify the PSD change of the bed. Regarding the PSD change, several studies [6,19] introduced a concept of relative 3.0 3.0 spread in the PSD, but the results for its effect were not shown. In this study, the particles in the bed FCC Catalysts FCC Catalysts were 2.0divided into coarse particlesEcat-1+fines and fine particles based2.0 on 45 μm. A concept of variationEcat-1+fines of mean Ecat-2+fines Ecat-2+fines

particle [-] Fluidity size with fines addition (normalized particle diameter variation) is applied to quantify the Ecat-1+Fcat-1 [-] Fluidity Ecat-1+Fcat-1 1.0 1.0 effect of mixed(a) catalyst bed fluidityEcat-1+Fcat-2 when the fine particle content(b) in the coarse particleEcat-1+Fcat-2 is changed. Ecat-1+Additive Ecat-1+Additive The0.0 normalized particle diameter variation is defined0.0 as Equation (1). 0 10203040 0.00 0.05 0.10 0.15 0.20 0.25 Fines contents [wt %] Normalized dp variation, d [-] dp,n = (dp,45+ − dp)/ dp,45+ p,n (1)

FigureTheFigure fluidities 7. 7.Effects Effects of of of finecatalysts fine contents contents increase (a )(a and) and normalizedwith normalized increases particle particle of diameterd p,ndiameter, due variation to variation the (increaseb) ( onb) on catalyst catalystof PSD fluidity. and the decreasefluidity. of mean diameter compared initial catalyst bed, as in Figure 7b. The increase of fluidity is 3.2. Correlation on Fluidity of FCC Catalysts affected not only by the PSD of the added catalyst, but also by the physical properties of the catalyst, considering3.2.Correlation Correlation the differenceon for Fluidity the fluidity ofof FCCthe is increasing Catalysts useful for slope. monitoring the unit of a given catalyst and interpreting the bed behaviors which may cause problems related to the fluidization [12]. Abrahamsen and 5.0 5.0 Geldart’s [5] correlation has been widely used for the fluidity. However, their correlation is not enough for exact4.0 prediction of catalysts make-up rate in urgent4.0 decisions after fines loss issues, such as catalyst circulation problems, because the correlation is for a wide range of Geldart A particles [11,20]. In this study,3.0 a guiding correlation has been proposed for3.0 fines makeup to the E-cat bed. The obtained FCC Catalysts FCC Catalysts fluidities2.0 for FCC catalysts with differentEcat-1+fines fine fractions2.0 (F45) have been correlated withEcat-1+fines dimensionless numbers based on the results of thisEcat-2+fines study: Ecat-2+fines Fluidity [-] Fluidity Ecat-1+Fcat-1 [-] Fluidity Ecat-1+Fcat-1 1.0 1.0 (a) Ecat-1+Fcat-2 Ecat-1+Fcat-2 −0.25 0.76(b) 1.37 Umb/UEcat-1+Additivemf = 2.09 Ar (exp F45) (dp,n+1) Ecat-1+Additive (2) 0.0 0.0 0 10203040 0.00 0.05 0.10 0.15 0.20 0.25 Fines contents [wt %] Normalized dp variation, dp,n [-] Figure 7. Effects of fine contents (a) and normalized particle diameter variation (b) on catalyst fluidity.

3.2. Correlation on Fluidity of FCC Catalysts

Energies 2019, 12, x FOR PEER REVIEW 8 of 10

Correlation for the fluidity is useful for monitoring the unit of a given catalyst and interpreting the bed behaviors which may cause problems related to the fluidization [12]. Abrahamsen and Geldart’s [5] correlation has been widely used for the fluidity. However, their correlation is not enough for exact prediction of catalysts make-up rate in urgent decisions after fines loss issues, such as catalyst circulation problems, because the correlation is for a wide range of Geldart A particles [11,20]. In this study, a guiding correlation has been proposed for fines makeup to the E-cat bed. The obtained fluidities for FCC catalysts with different fine fractions (F45) have been correlated with dimensionless numbers based on the results of this study: Energies 2019, 12, 293 8 of 10

Umb/Umf = 2.09 Ar−0.25 (exp F45)0.76(dp,n+1)1.37 (2) with a correlation coefficient number of 0.93 and a standard error of 0.183. The range of variables in with a correlation coefficient number of 0.93 and a standard error of 0.183. The range of variables in Equation (2) covers 5.8 ≤ Ar ≤ 19.3, 0 ≤ F45 ≤ 0.37, and 0 ≤ dp,n ≤ 0.22. EquationThe fluidity (2) covers values 5.8 measured≤ Ar ≤ 19.3, in 0 the≤ F45 present ≤ 0.37, and study 0 ≤are dp,n compared≤ 0.22. with the values calculated by EquationThe fluidity (2) as shownvalues measured in Figure8 in. Asthe can present be seen, stud they are calculated compared fluidity with the values values predict calculated well by theEquation experimental (2) as datashown within in Figure± 18 8. %. As Thecan be experimental seen, the calculated results are fluidity compared values with predict published well the correlationsexperimental [5,6 ,data12] by within the absolute ± 18 %. The average experimental error, E as results [21,22 ]:are compared with published correlations [5,6,12] by the absolute average error, E as [21,22]: − E(%)E(%) = (100/N) = (100/N)Σ | Σ (prediction | (prediction experimental)/experimental− experimental)/experimental (3) (3) wherewhere N isN datais data number. number. As As can can be be seen seen in in Table Table3, the3, the Equation Equation (2) (2) shows shows relatively relatively good good prediction, prediction, withwith an an E valueE value of of 5.3%, 5.3%, compared compared to to previous previous studies studies [5, [5,6,12].6,12]. The The correlation correlation of of the the previous previous studies studies underestimatedunderestimated the the experimental experimental values. values. The The E valueE value results results are are well well matched matched with with the the experimental experimental reportreport by by Whitcombe Whitcombe et al.et al. [8 ],[8], where where they they concluded conclude thed the fluidity fluidity and and fluidization fluidization characteristics characteristics are are affectedaffected by by the the different different surface surface characteristics characteristics and and shape shape of theof the Ecat Ecat compared compared to theto the Fcats Fcats [8, 22[8,22].].

6 FCC Catalysts

5 Ecat-1 +fines

3 Predicted Fluidity [-] 2 23456 Measured Fluidity [-]

FigureFigure 8. Comparison8. Comparison of theof the ﬂuidity fluidity obtained obtained by by the th proposede proposed correlation correlation and and the the experimental experimental data. data.Table 3. Mean percentage deviation between predicted and experimental ﬂuidity values.

Table 3. AuthorsMean percentage deviation between pred Correlationsicted and experimental fluidity E [%]values. 2300ρ 0.126exp(0.716F ) Abrahamsen and Geldart [5] Umb = g 45 . 22.3 Um f 0.934 0.934 0.8 Authors Correlations(ρp−ρg ) g dp E [%] 0.19 0.37 U 330.ρg µg exp(0.716F45) 𝑈 2300𝜌mb = 𝑒𝑥𝑝(0.716𝐹). Xie and Geldart [6] Um f 0.934 0.934 0.8 37 Abrahamsen and = (ρp−ρg ) g dp . . 𝑈 1.13 .−0.0384 0.74 22.3 U 0.5231𝜌(d−𝜌p/Dc ) (Dc𝑔/hs) 𝑑(ρp/ρg ) Geldart [5] mb = h i . 1 Singh and Roy [12] Um f 0.934 0.934 1.8 0.066 0.87 674.2 (ρp−ρg ) g dp /(1100ρg µg ) . . 𝑈Umb 330𝜌 −0.25𝜇 𝑒𝑥𝑝0.76(0.716𝐹 ) 1.37 This study = 2.09Ar (exp F45) dp,n + 1 5.3 Um f = . . Xie and1 Geldart [6] 𝑈 . 39.7 The correlation is based on Umb correlation𝜌 from −𝜌 experiments 𝑔 on𝑑 Geldart B particles.

3.3. Guidance for Control of Bed Fluidity

Many industrial fluidized bed reactors operate on a catalyst bed, with a high fines content of 10 to 50 wt % for improved fluidized bed performance, and thus higher yields [7]. It is noticed that the fines loss usually occurs during the change of operating condition in the FCC unit, including start-up, charging-up of feed, and shut-down. Comparative test data on the fluidity may show quickly if a fines loss problem develops [11]. The refiners take the action of adding fresh catalysts or fluidity enhancing additives with a large number of fine particles into the reactor to restore the fluidity of the catalyst in the unit. However, there is an associated cost in maintaining or restoring a high level of fines. A better guidance for effective catalyst make-up for keeping or restoring the bed fluidity is required, because Energies 2019, 12, 293 9 of 10 the catalyst make-up costs may be higher. The proposed Equation (2) is believed to provide a PSD management guide for making the fluidization quality good in the unit by providing the desired PSD information of fresh catalysts for the make-up [18]. On the basis of the available findings and results, the following suggestion would likely be useful in quickly restoring the bed fluidity in various fines or catalyst loss occasions. First, the monitoring of the fines content and fluidity obtained from the direct measurement [2] or the CFD simulation [9,10] in normal operating conditions is required to define the unit’s own fluidity characteristics, due to each FCC unit’s operational limits, such as cyclone configuration and efficiency [23]. Second, check the fines and the fluidity in case of temporary abnormal operating conditions, such as catalyst losses. Finally, determine the make-up rate of the fresh catalysts to reach an optimum fluidity level from the proposed correlation before catalyst fluidity gets worse.

4. Conclusions The effect of catalyst fine contents on the FCC catalyst bed fluidity was determined. The fluidity decreases with loss of fine content and increases with increment of makeup of fresh catalysts or additive with the controlled fines content. The fluidities of catalysts increase with increases of normalized particle diameter variation by the fines addition. A guiding correlation on the FCC catalyst fluidity for the makeup of fines to E-cat bed has been proposed based on the fine contents, Archimedes number, and normalized particle diameter variation. The proposed correlation could guide to keep good catalyst fluidity in the FCC unit.

Author Contributions: S.W.K. and D.Y.L. conceived and designed the research. C.E.Y. analyzed the testing data. S.W.K. wrote the paper. S.W.K. and D.Y.L. supervised. Funding: This work was funded by the National Research Council of Science and Technology (NST) grant by the Korea government (MSIT) (No. CRC-14-1-KRICT). Acknowledgments: This work was supported by the National Research Council of Science and Technology (NST) grant by the Korea government (MSIT) (No. CRC-14-1-KRICT). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Kim, S.W.; Kim, S.D. Void Properties in Dense Bed of Cold-Flow Fluid Catalytic Cracking Regenerator. Processes 2018, 6, 80. [CrossRef] 2. Sadeghbeigi, R. Fluid Catalytic Cracking Handbook, 3rd ed.; Butterworth-Heinemann: Oxford, UK, 2012; pp. 1–42, ISBN 978-0-12-386965-4. 3. Song, J.; Sun, G.; Chao, Z.; Wei, Y.; Shi, M. Gas flow behavior and residence time distribution in a FCC disengage vessel with different coupling configurations between two-stage separators. Powder Technol. 2010, 201, 258–265. [CrossRef] 4. Mandal, D.; Sharma, V.K.; Pant, H.J.; Sathiyamoorthy, D.; Vinjamur, M. Quality of Fluidization in Gas-solid Unary and Packed Fluidized Beds: An Experimental Study using Gamma Ray Transmission Technique. Powder Technol. 2012, 226, 91–98. [CrossRef] 5. Abrahamsen, A.R.; Geldart, D. Behaviour of Gas-Fluidized Beds of Fine Powders Part I. Homogeneous Expansion. Powder Technol. 1980, 26, 35–46. [CrossRef] 6. Xie, H.-Y.; Geldart, D. Fluidization of FCC Powders in the Bubble-free Regime: Effect of Types of Gases and Temperature. Powder Technol. 1995, 82, 269–277. [CrossRef] 7. Lettieri, P.; Macrì, D. Effect of Process Conditions on Fluidization. KONA Powder Part. J. 2016, 33, 86–108. [CrossRef] 8. Whitcombe, J.M.; Agranovski, I.E.; Braddock, R.D. Impact of Metal Ridging on the Fluidization Characteristics of FCC Catalyst. Chem. Eng. Technol. 2002, 25, 981–987. [CrossRef] 9. Wang, J.; vanderHoef, M.A.; Kuipers, J.A.M. CFD study of the minimum bubbling velocity of Geldart A particles in gas-fluidized beds. Chem. Eng. Sci. 2010, 65, 3772–3785. [CrossRef] Energies 2019, 12, 293 10 of 10

10. Sande, P.C.; Ray, S. Fine Mesh Computational Fluid Dynamics Study on Gas-Fluidization of Geldart A Particles: Homogeneous to Bubbling Bed. Ind. Eng. Chem. Res. 2016, 55, 2623–2633. [CrossRef] 11. Brown, G.W. Direct measurement of FCC fluidizability helps spot problems. Oil Gas J. 1990, 15, 46–49. 12. Singh, R.K.; Roy, G.K. Prediction of Minimum Bubbling Velocity, Fluidization Index and Range of Particulate Fluidization for Gas–solid Fluidization in Cylindrical and Non-cylindrical beds. Powder Technol. 2005, 159, 168–172. [CrossRef] 13. Chiranjeevi, T.; Ravichander, N.; Gokak, D.T.; Ravikumar, V.; Choudary, N.V. The Selection of Fluid Catalytic Cracking Catalysts and Additives: The Significance of Attrition Studies. Petrol. Sci. Technol. 2014, 32, 470–478. [CrossRef] 14. Davies, C.E.; Krouse, D.; Carroll, A. A New Approach to the Identification of Transitions in Fluidized Beds. Powder Technol. 2010, 199, 107–110. [CrossRef] 15. Whitcombe, J.M.; Agranovski, I.E.; Braddock, R.D. Categorisation of Particlulate Emissions from a Fluidised Catalytic Cracking Unit. Part. Part. Syst. Charact. 2004, 21, 463–472. [CrossRef] 16. Knowlton, T.M.; Karri, S.B.R.; Issangya, A. Scale-up of Fluidized-bed Hydrodynamics. Powder Technol. 2005, 150, 72–77. [CrossRef] 17. Coltters, R.; Rivas, A.L. Minimum fluidization Velocity Correlations in Particle Systems. Powder Technol. 2004, 147, 34–48. [CrossRef] 18. Zhong, Y.; Gao, J.; Wang, Z.; Guo, Z. Influence of Particle Size Distribution on Agglomeration/defluidization of Iron Powders at Elevated Temperature. ISIJ Int. 2017, 57, 649–655. [CrossRef] 19. Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinman: Oxford, UK, 1991. 20. Kim, S.W.; Kim, G.R.; Shin, J.W.; Yoo, I.S.; Kang, H.S.; Park, S.H. Fluidization Technology for Stable Startup of Commercial FCC Unit; Fluidization XIII; Kim, S.D., Kang, Y., Lee, J.K., Seo, Y.C., Eds.; ECI: Gyeong-ju, Korea, 2010; pp. 773–780. 21. Kim, S.W.; Kim, S.D. Heat transfer characteristics in a pressurized fluidized bed of fine particles with immersed horizontal tube bundle. Int. J. Heat Mass Transfer 2013, 64, 269–277. [CrossRef] 22. Kim, S.W. Measurement of Carbon Nanotube Agglomerates Size and Shape in Dilute Phase of a Fluidized Bed. Korean Chem. Eng. Res. 2017, 55, 646–651. 23. Kim, S.W. Pyrolysis Conditions of Biomass in Fluidized Beds for Production of Bio-oil Compatible with Petroleum Refinery. J. Anal. Appl. Pyrolysis. 2016, 117, 220–227. [CrossRef]

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