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ESTIMATION OF VISBROKEN AND SELECTIVE

CATALYTIC STEAM CRACKED PRODUCT

STABILITY USING IATROSCAN TLC-FID.

Ingrid Higuerey, Miguel Orea, Pedro Pereira

PDVSA-INTEVEP. P.O. Box 76343. Caracas 1070. Venezuela.

INTRODUCTIONCrude oil vacuum residues are characterized by a high

concentration of high-molecular-weight compounds such asasphaltenes. These compounds impart undesirable properties to theresidue (i.e. low viscosity) and affect thermal and catalyticconversion processes1. In many cases, the efficiency of conversionis related to the formation of insoluble carbon-rich deposits2-3. Invisbreaking (VB) and selective catalytic steam cracking (SCSC)

processes, severity of the operation is limited by heater fouling andproduct instability, which is dependent on the asphaltene content ofthe feed4.

An experimental measuring of the stability of the visbroken andselective catalytic steam cracked residues is given by the Shell P-value. This parameter is a measure of the flocculation potential ofasphaltenes and their trends to form solid deposits. P-Value isdirectly proportional to the stability. Its determination involves thevisual detection of the flocculated asphaltenes by using amicroscope. In consequence, results depend on the operator abilityand can vary from an analyst to another. Additionally, the

procedure is time consuming.One approach to determine the instability of heavy oil residues is

through the colloidal instability index (CII), which is based on thechemical composition of the residue. The CII is defined as ratio ofthe sum of asphaltenes and saturates to the sum of the peptizingresins and aromatics5-6:

CII = (Asphaltenes + Saturates) / ( Resins + Aromatics)

Calculations of this parameter involve a hydrocarbon group typeanalysis. Saturate, Aromatic, Resin and Asphaltene (SARA)quantification can be achieved either by High Performance LiquidChromatography (HPLC) or by Thin Layer Chromatography-Flame Ionization Detection (TLC-FID). The former requires theuse of liquid chromatography after asphaltene precipitation andquantification by gravimetry7-8; besides, it is time consuming andneeds considerable volumes of solvents.

TLC is becoming increasingly popular in the crude oil industry.This technique is recognized as an efficient, fast and cost-effectiveway to obtain quantitative information about crude oil composition9. For separation and quantification, correct use of Iatroscan TLC-FID offers good precision and accuracy, in addition to rapidanalysis and low solvent consumption.Cebolla et. al 7, Karlsen et. al 10-11 and Sol et. al 12, are some of the

researchers that have used this technique for the analysis of SARAconstituents in crude oils and solvent extracts.

In this work, we evaluate TLC-FID as an alternative tool toestimate the stability of visbroken and selective steam crackedresidues by CII calculations. Results were compared to theconventional P-Value.

EXPERIMENTAL

The samples studied in this work came from test carried out at a pilot plant of 1 barrel/day capacity, using a short Venezuelanresidue operated both visbreaking and selective catalytic steamcracking. The reactions were stopped when the bottom (350C+)

product reached a critical P-Value of 1.2. P-Value was determinedusing hexadecane titration, as indicated in reference 13.

SARA TLC-FID Analysis. An Iatroscan TH-10 instrument(Iatron Labs Inc.), equipped with a flame ionization detector (FID)and interfaced with a computerized acquisition system ( DIONEXAI 450 Optimize) was employed. This instrument was calibratedfor quantitative SARA analysis. Pure hydrogen (190 mL/ min,supplied by a hydrogen generator) and pure air (2.0 l/min, supplied

by an air cylinder) were used for the detector. Two sets of 10 silicarods, type Chromarod-SIII (pore diameter 60 , particle size 5m)were used: One for the saturate and aromatic quantification and theother one for asphaltene quantification. Samples were prepared bydissolution in a mixture of toluene: chloroform 1:1 vol:vol toobtain a final concentration of 25 mg/mL. Before sampleapplication, Chromarods were passed twice through the FID toremove contaminants, and to obtain constant activity of the silicalayer. Each rod was spotted by 1.0 l of the sample solution,applied dropwise by using a 1.00 l syringe. For saturate andaromatic determinations, Chromarods were developed in n-heptanefor 40 min; air dried (3 min) and developed again in toluene for 20min. For asphaltene determinations, the second set of Chromarodswas developed in n-heptane/ isopropanol 95:5 vol. % for 20 min.Each set of rods was dried for 3 min and then, pyrolized at a scanspeed of 0,32 cm/s.

RESULTS

Selective catalytic steam cracking (SCSC) is a process based onchemicals reactions carried out in presence of steam andultradispersed catalysts14. The technology based on SCSC is calledAquaconversion. Table 1 shows the conversion achieved by thefeedstock (350C+) during VB and SCSC operations. P-values ofthe feedstock and its products are shown as well. When in VB amaximum conversion established by a critical P-Value of 1.15 isreached, the stability of the 350C+ SCSC product (given by a P-Value of 1.45) allows to enhance the conversion by increasing theseverity of the process. Even, under this severe condition, a newSCSC product is obtained with higher stability than VB product.

Table 1. Conversion and P-values of feed and products14

Feed VB SCSC-1 SCSC-2(*)Conversion (wt.%)

To 165C-Of 500C+

------

2.928.2

5.228.5

7.536.3

P-Value 2.7 1.15 1.45 1.20

(*) SCSC-2 refers to severe conditions in SCSC process (5C over the fixedtemperature)

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0,50

1,00

1,50

2,00

2,50

3,00

P-Value

0,40

0,48

0,56

0,64

0,72

0,80

Feed VB SCSC-1 SCSC-2

CII(adimentional

0

5

10

15

20

25

30

35

40

Feed VB SCSC-1 SCSC-2

Wt.%

Saturates Aromatics Resins Asphaltenes

Figure 1. SARA distribution of the studied 350C+ residue and the350C+ VB and SCSC products.

It is known that during VB, asphaltene formation increases withconversion. This is a consequence of thermal cracking reactionsthat follow a free radical mechanism, causing asphaltenecondensation and formation of heavier products and coke15. On the

contrary, the mechanism proposed for SCSC suggests thesaturation of these free radicals by the incorporation of hydrogenfree radicals formed by the catalytic dissociation of water. Thus,asphaltene formation is avoided as well as the early appearance ofcondensed products potentially unstable

14.

Figure 1 shows TLC-FID SARA distributions of the feedstockstudied and its VB and SCSC 500C+ products. It is interesting tonote that the addition of steam and catalysts to VB produces aresidue with higher resin content and less quantity of asphaltenes.This observation indicates that, while asphaltenes are normally

produced in VB, they are converted in SCSC. This particularbehavior of SCSC favors the quality of the final product.

Figure 2. Comparison of CII values calculated from TLC-FIDresults with the P-Value of the VB and SCSC products of thestudied 350C+ residue.

On the other hand, an identical behavior was found when the P-Value was compared to the Theoretical Distribution of SolubilityParameter of these pure asphaltenes obtained by n-heptane

precipitation4. These findings confirm that asphaltene nature playsan important role in the stability of the residues.

Some authors have pointed out that the nature of asphaltenesseem to play an important role in the stability of residues regardlesstheir content16; however, others factors such as the content of

asphaltenes, the presence of resins and the quality of the oil mediamust be taken into account17. CONCLUSION

Stability of VB and SCSC products was evaluated by calculationof CII, using TLC-FID SARA distribution. Results obtained by thisfast and accurate method are in total agreement with theconventional P-Value. Additionally, the CII showed the same trendfound in the theoretical studies of the solubility parameter.

Considering the crude oil residues as colloidal systems, resinsand asphaltenes comprise the dispersed phase while saturates andaromatics form the continuous phase19. The interaction among both

phases and their influence on the stability of the system is relatedto the peptizing power of the resins, the solvent effect of thearomatics, the precipitant properties of saturates and theflocculation tendency of asphaltenes.

CII is proposed as a potential alternative parameter to obtain arapid evaluation of the bottom product stability during VB andSCSC processes. However, a higher number of samples are beingconsidered to confirm the results shown in this work.

Lower values of CII indicate higher asphaltene stability due tothe dispersant effect of aromatic and resins18. Figure 2 shows theCII values calculated from TLC-FID data and the P-Value. As can

be observed, CII exhibits an inverse trend to the one showed by P-Value, giving surprisingly a mirror image. As can be seen in Figure2, higher CIIs correspond to lower P-Values. It is amazing the

excellent correlation found between a chromatographic methodsuch TLC-FID and the titration method (P-Value). This interestingcorrelation offers a potential tool to evaluate product stability in avery fast and accurate way.

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