chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Research and Design

r .com/ locate /cherd

Optim errespo

Zoran A , Ka INA-Oil Inb Faculty of reb, C

a

Is re in

h onve

in mer

d ion p

data was performed to obtain second order polynomial model and the optimum conditions were determined: hydro-

gen/feed ratio of 6, space velocity of 2 h1 and temperature of 170 C. At optimum conditions conversion of n-hexane

was 70 wt.%. In addition, temperature dependency of product composition was investigated at optimum values of

hydrogen/feed ratio and space velocity. Obtained results show that methylpentanes greatly depend on temperature,

unlike dimethylbutanes, in the studied range from 130 to 170 C. Isomer that was produced in highest quantities

w

w

th

K

1. In

Requiremethe contenpressure, care increasmental conquality, treaof motor fment, as wof hydrocamerizate, iincrease thizate gasolioctane numdemands (W

C5/C6 in-pentane meric form

CorresponE-mail aReceived

0263-8762/$http://dx.doas 2-methylpentane, while 3-methylpentane forms in somewhat smaller amounts. 2,2- and 2,3-dimethylbutanes,

hich contribute the most to the octane number value, are formed in relatively small quantities, amounting to less

an 10 wt.% of the total amount of isomers formed.

2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

eywords: Isomerization; n-Hexane; Zirconium sulfate; Catalyst; Optimization

troduction

nts for the motor fuels quality which are related tot of sulfur, olens and aromatics, as well as vaporonsidering the amount of volatile hydrocarbons,ingly stringent due to increased global environ-cerns. As a solution to achieve the required fueltment processes for increasing the product qualityuels and reducing harmful impacts on environ-ell as on motor vehicles, must be used. The processrbons isomerization, hence the production of iso-s an efcient and economically acceptable way toe octane number of commercial gasoline. Isomer-ne actually represents an ideal product, it has highbers and can meet the strictest environmentaleyda and Khler, 2003; Jones and Pujad, 2006).

somerization is the catalytic process in whichand n-hexane are transformed into their iso-s, which are then added to gasoline pool for the

ding author. Tel.: +385 14597129.ddress: kserti@fkit.hr (K. Sertic-Bionda).

16 December 2011; Received in revised form 11 April 2012; Accepted 14 June 2012

purpose of octane number increase. Isomerization reactions ofn-pentane take place very quickly, while iso-pentane contentin isomerizate gasoline is directly related to the thermody-namic equilibrium of n/iso-pentane (Matsuhashi et al., 1999).By isomerization reactions of n-hexane, 4 forms of iso-hexaneare created: 2-methylpentane (2-MP), 3-methylpentane (3-MP),2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) (Leprince, 2001; Duchet et al., 2001). C5/C6 isomerizationreactions take place in the catalyst bed in the presence ofhydrogen. Catalysts that are used in the isomerization processare active at lower reaction temperatures, where thermody-namic equilibrium favors creation of the parafn with one ormore side chains. Bifunctional catalysts which are used in thisprocess are chlorided aluminum oxide (Pt/Al2O3-CCl4), zeo-lites (Pt/zeolite), and recently, zirconium sulfate on platinum(Pt/SO4-ZrO2) (Anderson et al., 2004; Kimura, 2003; Volkovaet al., 2007).

The available technologies for the isomerization pro-cess can be compared on the basis of performance, the

see front matter 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.i.org/10.1016/j.cherd.2012.06.012journa l h o me pa ge: www.elsev ie

ization of the n-hexane isomnse surface methodology

dzamica, Tamara Adzamica, Marko Muzica

dustry d.d., Lovinciceva bb, 10000 Zagreb, CroatiaChemical Engineering and Technology, Marulicev trg 19, 10000 Zag

b s t r a c t

omerization reactions on commercial zirconium sulfate catalyst a

ydrogen/feed ratio, space velocity and temperature on n-hexane c

cludes values that are applied in the industrial practice of the iso

esign was carried out in order to optimize n-hexane isomerizatization process using

atica Sertic-Biondab,

roatia

vestigated in order to determine inuence of

rsion. Investigated range of inlet parameters

ization process. BoxBehnken experimental

rocess. Statistical analysis of experimental

chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105 101

product oczirconium much large2006). Howson with thalumina ccatalyst is tance to caof acid cenchoice (WaYadav and

This woiments efto determinresponse bgreatest cothe isomervalues thatization pro

2. Ex

2.1. Ma

99.5 wt.% nfor the isomin all expeThe catalysFig. 1 Isomerization apparat

tane number and cost. Process based on thesulfate catalyst has the lowest price, with ar efciency than zeolite catalyst (Ahari et al.,ever, its efciency is slightly lower in compari-e one obtained in the process using chlorinated

atalyst. On the other hand, zirconium sulfatemuch more simple to regenerate, has high resis-talyst poisons, and does not require the additionters, which makes it environmentally acceptablekayama and Matsuhashi, 2005; Deak et al., 2008;Nair, 1999).rk was motivated by the need to conduct exper-ciently by a proper choice of design, in ordere operating conditions according to the optimal

ased on a set of the controllable variables. Thentribution of this paper is an attempt to describeization process at conditions which include the

are applied in the industrial practice of the isomer-cess in the presence of zirconium sulfate catalyst.

perimental

terials

-hexane was used (Carlo Erba, Italy) as a feederization process investigation. Carrier gas used

riments was 99.99% hydrogen (Messer, Croatia).t employed in the isomerization process was the

commerciaZrO2) in th3 mm.

2.2. Isoproduct

The experiusing 10 cmshowed in and activattions. Hydrintroduced61.5 cm3. Thglass balls wreactor conreactor wasit was coolwere separis equippedthe sampliis reached on the presthe GC-201with the Z30 m; internthe ame iwas set to hus.

l zirconium sulfate on platinum catalyst (Pt/SO4-e form of palettes with sizes ranging from 2 to

merization of n-hexane and analysis of the

ments were carried out in a plug-ow reactor3 of catalyst. A schematic of the apparatus is

Fig. 1. Prior to reaction, catalyst sample was drieded according to the manufacturers recommenda-ogen and feed were heated in the preheater and

into the reactor which has the total volume ofe catalyst bed in the reactor is preceded by a bed ofhich are 5 mm in diameter. After the steady state

ditions were established, gaseous mixture from the directed into the high-pressure separator whereed and liquid isomerizate and leftover n-hexaneated from the hydrogen. High-pressure separator

with a liquid level control mechanism enablingng of liquid products when a preset volume levelwhile incurring the least amount of disturbancesure in the system. Samples were analyzed using0 (Shimadzu, Japan) gas chromatograph equippedB-1 (Phenomenex, USA) capillary column (length:al diameter: 0.53 mm; lm thickness: 1.50 m) and

onization detector (FID). The column temperatureold for 1 min at 33 C, then to increase from 33 C

102 chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105

Table 1

Run

: T,

1 130 2 150 3 170 4 130 5 150 6 150 7 130 8 150 9 170

10 150 11 150 12 15013 150 14 170 15 150 16 130 17 170

to 50 C at 30 C min1

The n-h

X(nC6) = x(

2.3. Stamethodolog

Design of Eother thinprocess, imuct qualitythe DOE mthe responThis RSM cparameterstation of recurved surmented uswhich Boxinvestigatioof zirconiuiments by ratio (H2/Hthree levels12 experimcess paramtotal of 17 eby determipolynomiaconversion

3. Re

3.1. Efftemperatur

Three paraexperimenthe aim

sion rang isoratuhownns is of

of lgatey imf theitat

cted.l res

mma

Staalys

terac statBBD experiments and results.

Experimental conditions

A: H2/HC, mol/mol B: LHSV, h1 C

11 3 6 3 1 3 6 2 1 2

11 2 6 4 6 3

11 3 6 3

11 4 6 36 3 6 2 1 4 1 3 6 4

3 C min1, then to increase from 50 C to 90 C atand then to hold at 90 C for 1 min.

exane conversions were calculated as follows:

nC6)in x(nC6)outx(nC6)in

100 (1)

tistical experimental design and analysisy

xperiments (DOE) is a statistical tool which, amonggs, can be used to better explain and model aprove process efciency, as well as improve prod-. Response surface methodology (RSM) is one ofethods that assumes a nonlinear dependence ofse on the input parameters and their interactions.an be applied for a wide range of values of input

on the process response, with a graphical presen-sults which take the form of three-dimensional

faces (Montgomery, 2001). The RSM can be imple-

convercover areninTempebeen sreactiothe losmationinvestivelocitbasis oand limconduimentaare su

3.1.1. The anthe intest ofing different types of experimental designs, ofBehnken Design (BBD) has been chosen for then of the n-hexane isomerization in the presence

m sulfate catalyst. BBD involves performing exper-varying three process parameters (hydrogen/feedC), space velocity (LHSV) and temperature (T)) at

of values. BBD with three parameters consists ofents, plus 5 experiments with the values of pro-eters at center-level. Thus, the design comprises axperiments. The statistical analysis was performednation of the levels of factors, so the second-orderl model could be tted to describe the n-hexane, X(nC6) (Montgomery and Myers, 2009.).

sults and discussion

ects of hydrogen/feed ratio, velocity ande on hydrotreating conversions

meters were used for developing a BoxBehnkental design (available in Expert Design 6.0.6.) withto investigate their effects on the n-hexane

of squares are dened(MSeffect) a2001). Accoeffect has lobe concludlinear effecratio, quadand interachydrogen r

It shoulmaximum 10 indicatethe responour case, t32.2 indicato obtain thThe transfoural logaritthe optimuinterval bedence inteResponse

C X(nC6), wt.% Y: ln[X(nC6)]

4.8 1.5714.5 2.6729.2 3.377.8 2.056.1 1.81

10.3 2.335.8 1.76

15.8 2.7655.3 4.0113.5 2.609.5 2.25

14.1 2.6514.4 2.6770.9 4.265.1 1.632.2 0.79

60.9 4.11

. Temperature values were chosen in such a way toge of the standard conditions used in an industrialmerization process with zirconium sulfate catalyst.res over 170 C were not investigated because it has

that at higher temperatures the share of crackingn the observed system increases, which leads tovaluable isomeric compounds in favor of the for-ight hydrocarbons (Demirci and Garin, 2002). Thed range of values of hydrogen/feed ratio and spacepact on n-hexane conversion was chosen on the

preliminary results, considering the possibilitiesions of the apparatus in which experiments were

Pressure was kept constant at 25 bar. The exper-ults obtained at described conditions by the BBDrized in Table 1.

tistical analysisis of variance (ANOVA) of the main effects andtions for the chosen response, together with theistical signicance are shown in Table 2. The sum

(SS) are used to estimate the F-values (F), which

as the ratio of the respective mean square effectnd the mean square error (MSerror) (Montgomery,rding to the calculated Fisher F-test, signicantw probability value (P-value < 0.05), therefore it can

ed that following effects are statistically signicant:ts of temperature, space velocity and hydrogenratic effects of hydrogen ratio and temperature,tion effects of temperature with space velocity andatio.d be noted that the response was checked for theand minimum ratios. Generally, a ratio greater thans a higher probability that the transformation ofse may improve the model (Stat-Ease, 2008). Inhe ratio of maximum to minimum response wasting that the transformation is required. In ordere appropriate response a BoxCox plot was used.rmed BoxCox plot (Fig. 2) is showing that the nat-hmic value of response is situated in the region ofm value. Used quadratic model has the condencetween 0.03 and 0.3. The current point of con-rval (0.1) lies closely to the model design value of

chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105 103

Table 2

Source ean s

H2/HC 0.8LHSV 0.0T 11.4(H2/HC)2 1.7(LHSV)2 3.2(T)2 0.6(H2/HC) (LH 2.4(H2/HC) (T) 5.0(LHSV) (T) 5.2Model 1.6Total Error 3.3Residual 7.8

Fig

0.12, indication of resp

ln[X(nC6)] =

The coeby multipleindependentheir signicoded facto

Y = 2.67 + B2 +

From Eqcan be seension in the values, canthe systemANOVA results.

Sum of squares (SS) df M

0.82 1 0.063 1

11.49 1 1.71 1 3.2 103 1 0.68 1

SV) 2.4 103 1 5.0 103 1

5.2 103 1 14.86 9 14.74 160.013 40.055 7. 2 BoxCox plot for power transforms.

ting that the appropriate natural log transforma-onse was used.

15.65 + 0.41 H2/HC 0.22 LHSV 0.25 T 0.03 (H2/HC)2 0.03 (LHSV)2

+ 1.01 103 (T)2 + 4.91 103 H2/HC LHSV 3.54 104 H2/HC T + 1.80 103

LHSV T (2)

fcients in Eq. (2) (actual factors) were determined regression analysis. This analysis includes all thet variables and their interactions, regardless ofcance levels. Eq. (2) can be presented with thers in the form of Eq. (3).

0.32 A 0.089 B + 1.20 C 0.64 A2 0.028 0.40 C2 + 0.025 AB 0.035 AC + 0.036 BC

(3)

. (3) and the coefcients values for each effect, it that the greatest impact on the n-hexane conver-

isomerization process, for the investigated range of be attributed to the amount of hydrogen present in, i.e. the molar ratio of hydrogen and feed (code C).

Fig. 3 Calconversion

Next most and space inuence, fA2, B2 and these three

R2 valueing that it in the systesion valuesR2-predicteboth are lais greater tEq. (2) can b

Model Emeaning, sical interprthe reactiohexane conversus obsthat pointsexperimenresenting thused for prepresence oquare (MS) F-value P-value

2 105.14

104 chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105

Fig. 4 Resisomerizatfunction of

Fig. 5 Resisomerizatfunction of

3.1.2. Isosurface metAs mentionform of dihexane conspace velocand tempe

In all of thydrogen r150 C.

In Figs. 4hexane conat the end, around 6. Tcient of Hthe isomerfate catalyintroducedthe surfacegoes spillovhydride ion

With thtion of the et al., 2001

Resrizatn ofponse surface plot for n-hexane conversion inion process with zirconium sulfate catalyst as a

space velocity and hydrogen/feed ratio.

Fig. 6 isomefunctioponse surface plot for n-hexane conversion inion process with zirconium sulfate catalyst as a

temperature and hydrogen/feed ratio.

merization process optimization using responsehodologyed earlier, model Eq. (2) can be presented in the

mensional surfaces, i.e. contour plots for the n-version for various values of hydrogen ratio andity (Fig. 4), temperature and hydrogen ratio (Fig. 5)rature and space velocity (Fig. 6).hese cases third variable was maintained constant,atio at 6, space velocity at 3 h1 and temperature at

and 5 it is shown that with the H2/HC increase n-version increased at the beginning, but decreasedreaching maximum at intermediate level of H2/HChis effect is the result of a negative quadratic coef-2/HC, derived from the reaction mechanism of

ization process in the presence of zirconium sul-st (Lften, 2004; Ono, 2003). Molecular hydrogen

into the system dissociates on the platinum (at of the catalyst) to hydrogen atom which under-er, so catalyst surface is covered with proton and.e increase of this newly formed hydrides, desorp-carbenium intermediates is also increased (Duchet; Manoli et al., 1998). However, when hydrogen is

Fig. 7 Profunction of

present in tmediates otoo quickly

From Figversion, to in the studgreater at looptimal H2ity and tem

Figs. 5 aon n-hexan

Conversperature inat followingIn additionzation expeconditions,did not exc

3.2. Effcomposition

An effect ofstudied (Figsteps of 10and 6, respeby the expponse surface plot for n-hexane conversion inion process with zirconium sulfate catalyst as a

temperature and space velocity.duct composition of isomerization process as a temperature at LHSV = 2 h1 and H2/HC = 6.

he excess amounts, it will compete with the inter-n the active sites and the intermediates will desorb

(Adeeva et al., 1998).s. 4 and 6 it can be concluded that n-hexane con-

a lesser extent, depends on the space velocity valueied area. Still, it is evident that the conversion iswer speeds, i.e. at higher retention times. With the/HC ratio of 6, and the average values of space veloc-perature, n-hexane conversion is around 15 wt.%.nd 6 show positive effect of temperature increasee conversion in the investigated range of values.ion increases almost proportionally with the tem-crease and it reaches maximum value of 70 wt.%

conditions: H2/HC = 6, LHSV = 2 h1 and T = 170 C., to verify obtained optimum conditions, isomeri-riments were conducted three times under these

and standard deviation between obtained resultseeded 0.1%.

ects of process parameters on product

temperature on the product composition was also. 7). Temperature range was from 130 to 170 C inC. LHSV and H2/HC were kept constant at 2 h1

ctively, based on the optimum conditions obtainederimental design. Pressure was 25 bar. Increase of

chemical engineering research and design 9 1 ( 2 0 1 3 ) 100105 105

the temperature generally leads to increases in hydrotreat-ing conversions (Jones and Pujad, 2006). In this work it wasfound that methyl pentanes ratio in the product increaseswith temperature increase, in such a way that from 130 to150 C increase was slow, but from 150 to 170 C this increasewas much faster. In the range from 130 to 150 C, 2-MP and 3-MP are present in almost equal amounts in the product, thanby further temperature increase 2-MP is found in the higheramounts than 3-MP about 15 wt.%. The highest amounts of2-MP and 3-MP were found at 170 C and were 41 wt.% and23 wt.%, respectively.

Amounts of dimethylbutanes in the isomerization prod-uct were much less dependent on temperature change. Theratio increase was evident until 150 C, after which no increasewas observed. The highest amount of 2,3-DMB was found at160 C and was 7 wt.%, while the highest amount of 2,2-DMBwas found

4. Co

The goal oferization psulfate cataof this studhydrogen/fnicant, wiimportant and velocitactions betvelocity wesecond-ordof n-hexan

R2 valulent abilitymaximum LHSV = 2 h

ency of provalues of hof the prodties of 2-MPand 23 wt.%found at 162,2-DMB wadid not cha

Reference

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Anderson, G.C., Rosin, R.R., Stine, M.A., Hunter, M.J., 2004. NewSolutions for Light Parafn Isomerization. UOP LCC.

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al. Toat 150 C and was 3 wt.%.

nclusion

this study was to mathematically describe isom-rocess of n-hexane in the presence of zirconiumlyst, using the BoxBehnken Design (BBD). Resultsy indicate that effects of investigated parameters:eed ratio, space velocity and temperature were sig-thin the experimental range considered. The mostfactor was hydrogen/feed ratio, than temperaturey, while quadratic effect of temperature and inter-ween temperature and hydrogen/feed ratio andre less inuential, but still signicant. An empiricaler model was developed to predict the conversione as a function of three process parameters.e of the model was 0.9963 showing its excel-

to account for the variability of the system. Theconversion of 70 wt.% was achieved at H2/HC = 6,1 and T = 170 C. In addition, temperature depend-duct composition was investigated at optimum

ydrogen/feed ratio and velocity. With the analysisuct it has been proven that the highest quanti-

and 3-MP were found at 170 C and were 41 wt.%, respectively. The highest amount of 2,3-DMB was0 C and was 7 wt.%, while the highest amount ofs found at 150 C and was 3 wt.% and these valuesnge with further increase in temperature.

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Optimization of the n-hexane isomerization process using response surface methodology1 Introduction2 Experimental2.1 Materials2.2 Isomerization of n-hexane and analysis of the product2.3 Statistical experimental design and analysis methodology

3 Results and discussion3.1 Effects of hydrogen/feed ratio, velocity and temperature on hydrotreating conversions3.1.1 Statistical analysis3.1.2 Isomerization process optimization using response surface methodology

3.2 Effects of process parameters on product composition

4 ConclusionReferences