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DISTILLATION

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					The Canadian Crude Quality Technical Association
A non-profit Association registered under the Societies Act of Alberta



                                                         DISTILLATION

DEFINITIONS

An atmospheric or vacuum distillation is one in which the test sample is evaporated and subsequently
condensed to produce cuts in the desired temperature range (1).

A simulated distillation employs gas chromatography (GC) to separate compounds by boiling point to
produce a boiling curve (1).


SIGNIFICANCE

Distillation is the most basic and important separation process in refineries, as crudes, all intermediates
and products go through distillation columns (2). That being said, distillation should play a similar role
in the laboratory, in that it should precede many of the analyses. Reasons for this are as follows:
    • Distillation curves provide an important set of data in its own right.
    • Separation by distillation simplifies interpretation of compositional analyses and is the first step
         in a crude assay.

The relationship between carbon number (or molecular weight), boiling point and chemical composition
is illustrated in Figure 1. It indicates how distillation simplifies compositional analysis.


PRINCIPLES AND TYPES OF DISTILLATION

Distillation basically separates molecules by differences in their vapour pressures, which generally
decrease with increasing molecular weight, carbon number and boiling point (2). Laboratory physical
distillations (will be referred to here as TBP or “true boiling point” distillations, even though technically
speaking only D2892 and D5236 are given this designation) are carried out under atmospheric and
vacuum conditions, and consist of two categories, column and short path. Simulated distillations
(simdist), on the other hand, are carried out by gas chromatography (GC) based on the principle that
hydrocarbons elute from a nonpolar column in order of their boiling points.

Atmospheric and Vacuum Distillation
     The bulk of laboratory distillations are carried out in packed columns as opposed to trays used in
     refineries (3). Column efficiency is measured in terms of its number of theoretical trays or
     plates; the higher the number the higher the efficiency. Laboratory distillation columns typically
     have columns with diameters of 25 to 60 mm, heights of 1 to 1.5 m and efficiencies of 15 to 20
     theoretical plates. The spinning band column is constructed of precision-bore glass tubing, in
     which a closely fitted twisted Teflon or metal band rotates. High mass and heat transfer ensure
     good efficiencies (up to 50 theoretical plates).



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      Figure 1: Relationship between carbon number, boiling point and molecular composition


          From the actual boiling points obtained at reduced pressure, the atmospheric equivalent
          temperatures (AET) may be calculated; this is the temperature at which the material would boil
          under atmosphere if it were stable. It basically extends the boiling range beyond that which
          “cracking” would normally occur. The AET may be estimated from the formula given by ASTM
          (4). Final boiling points typically extend up to 525°C.


Short Path Distillation
       As an alternative to column distillation, a high vacuum, thin film technique called short path may
       be used to obtain deep cuts up to AET of 700°C. This distillation is a nonequilibrium process;
       only two fractions are obtained, the flash distillate and the residue. Boiling points must be
       determined afterward using simdist.

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Simulated Distillation
      Simdist represents savings on both sample size and time compared to actual distillations.
      Furthermore, even though it does not produce cuts for analysis, it does produce a more precise
      boiling curve and can extend to a final boiling temperature of 750°C. Boiling points are
      calibrated with retention time standards (n-paraffins up to C110).

Common Terminology
    Reflux ratio: ratio of reflux (liquid running down the column) to distillate coming over.
    Dynamic holdup: quantity of liquid held up in the column
    Flood point: point at which velocity of upflowing vapour obstructs downcoming reflux and the
    column floods with liquid
    Static holdup: quantity of liquid retained in the column after draining
    Takeoff rate: rate of product takeoff from reflux divider


METHODS

D86 is one of the oldest ASTM methods developed for light petroleum products with final boiling point
<350°C. It is a one-plate atmospheric distillation of limited use for bitumen and heavy oil, and therefore
will not be discussed further.

1. ASTM D1160: Distillation of petroleum products at reduced pressure
     a. Scope: This method covers the determination at reduced pressures as low as 1 mm Hg, of the
        range of boiling points of petroleum products that can be vapourized at a maximum liquid
        temperature of 400°C.
     b. Summary: The sample is distilled at a pressure between 0.13 and 6.7 kPa (1 and 50 mmHg)
        under one theoretical plate conditions. The data provide a boiling point curve relating
        volume percent distilled to AET.
     c. Precision: Repeatability and reproducibility are listed in the method. They were obtained on
        the basis of a 1983 study among nine laboratories with eight samples being run.
     d. Comments: Data may be used in engineering calculations to design distillation equipment, to
        determine compliance with regulatory rules and to determine product suitability for refining
        purposes.

2. ASTM D2892: Distillation of crude petroleum (15-theoretical plate column)
     a. Scope: This method covers the distillation of stabilized (RVP<82.7 kPa) crude petroleum to
        a final cut temperature of 400°C AET. Efficiency is 14-18 theoretical plates operated at a
        reflux ratio of 5:1.
     b. Summary: A weighed sample of 1-30 L is distilled at operating pressures as low as 0.674 to
        0.27 kPa (5 to 2 mm Hg). Mass and density of each cut are obtained; distillation yields by
        volume of all cuts are calculated from mass and density.
     c. Precision:
        Repeatability is under statistical review
        Reproducibility – 1.2 vol% at atmospheric pressure; 1.5 vol% under vacuum
     d. Comments: This method is commonly referred to as the 15/5 or true boiling point (TBP)
        method. Reflux ratio is defined as the ratio of reflux to distillate. The cuts produced can be

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               analyzed as generated (for example crude assays) or combined to produce samples for
               analytical studies, engineering or product quality evaluations.

3. ASTM D5236: Distillation of heavy hydrocarbon mixtures (Vacuum potstill method)
     a. Scope: This method covers the distillation of mixtures with IBP>150°C such as heavy
        crudes, bitumens, residues or synthetic mixtures. The maximum AET obtainable is 565°C.
     b. Summary: A weighed volume of sample is distilled at absolute pressure between 6.6 and
        0.013 kPa (50 and 0.1 mm Hg) at specified rates. From the mass and density of the cuts, a
        distillation curve may be determined.
     c. Precision:
        Repeatability – 4 to 6°C over the 10 to 90 vol% distilled range
        Reproducibility – 4 to 17°C over the 10 to 90 vol% distilled range
     d. Comments: This method is useful for distilling bitumens and heavy oils. Lighter crudes or
        diluted bitumens first require D2892 for distilling off the lighter portion, followed by D5236.

4. UOP 109: Vacuum distillation of topped crude oils and similar petroleum products
     a. Scope: This method describes the determination of distillate cuts and residue from topped
        crudes with IBP>200°C; a maximum AET of 560°C may be achieved.
     b. Summary: Samples of 0.5 to 4 L are distilled under vacuum of 0.2 to 0.3 mm Hg. This
        method is similar to D1160 except for provisions to collect the following cuts: diesel, gas oil,
        lubricating oil, heavy fuel oil and asphalt.
     c. Precision: not given

5. Syncrude Analytical Method 5.2: Spinning band distillation of liquid hydrocarbons and
   bitumen (4)
       a. Scope: Fractions of bitumen and liquid hydrocarbon samples with IBP above room
          temperature at atmospheric pressure may be prepared. The pot temperature cannot exceed
          360°C in order to minimize cracking.
       b. Summary: Samples are distilled under atmospheric and reduced pressure in a still equipped
          with a spinning band column. In practice distillation of bitumen may not be able to be
          carried out beyond AET of 450°C. Up to 50 theoretical plates are possible.
       c. Precision: Data obtained from the Syncrude Manual

                                                   Mean (wt%)            Repeatability
                                                                            (wt%)
              LGO (195-343°C)                            12.5                 0.5
              HGO (343-524°C)                            27.2                 1.5
              Resid (>524°C)                             58.2                 0.8
              Recovery (%)                               99.3                 0.3

          d. Comments: The large number of theoretical plates obtainable make possible closely spaced
             distillation cuts (as close as 20°C apart), suitable for carrying out thermodynamic studies.




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6. Short path distillation (Thin film, Flash still or DISTACT)
      a. Scope: This distillation is used to generate high boiling cuts with endpoints up to 700°C
          AET for further analysis.
      b. Summary: The material is introduced at constant rate onto the hot inner wall of the
          evaporator under high vacuum (at least 10-3 mm Hg). Rotating rollers ensure a thin film as
          feedstock is distilled at fixed temperature and pressure. Vapours condense on a cold surface
          a short distance from the hot wall.
      c. Precision: not available
      d. Comments: Because of the high vacuum, short distance between evaporator and condenser
          surfaces (2-3 cm), and short residence time due to the thin film, a deep distillation without
          decomposition (cracking) is possible. Only two fractions are produced, a distillate and
          residue. It is a non-equilibrium distillation; boiling points cannot be measured, so are
          determined afterward by simdist.

7. ASTM D2887: Boiling range distribution of petroleum fractions by gas chromatography
     a. Scope: This method covers the determination of the boiling range distribution of petroleum
        products having an FBP<538°C and an IBP>55°C. It is obviously not applicable to whole
        bitumen, but is used for its distillation cuts in for example crude assays.
     b. Summary: A nonpolar packed (little used currently) or open tubular (capillary) GC column
        is used to elute the hydrocarbon components in order of increasing boiling point. Boiling
        points are assigned to the time axis from a calibration curve using a mixture of known
        paraffins.
     c. Precision: given in the method based on nine samples analyzed by 19 labs using both packed
        and open tubular columns.
     d. Comments: This method has been compared to D2892, the standard for conventional
        distillation of petroleum products. In all cases, agreement between methods has been good
        using wt% off data. D2887 is often used to determine the “purity” of cuts obtained by other
        distillations, eg. D2892, spinning band.

8. ASTM D5307: Boiling range distribution of crude petroleum by gas chromatography
     a. Scope: This method covers the determination of the boiling range distribution of water-free
        crude petroleum to 538°C and reports material above this temperature as a residue. It is
        applicable to whole crudes that can be solublized in CS2.
     b. Summary: The crude is diluted in CS2 and injected into a GC that separates on the basis of
        increasing boiling point. Boiling points are assigned to the time axis from a calibration curve
        using a mixture of known paraffins. The amount of sample boiling above 538°C is estimated
        by means of a second analysis of the crude to which an internal standard (usually C14 to C17)
        has been added.
     c. Precision:
        Repeatability – 4 to 25°C over the 0 to 90% off range
        Reproducibility – 11 to 45°C over the 0 to 90% off range
     d. Comments: This method has shown a good correlation with D2892 in round robin testing
        (4). The D2892 distillation must be terminated at 400°C AET or even lower to avoid
        cracking.




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9. ASTM D6352: Boiling range distribution of petroleum distillates in the range 174 to 700°C by
   GC
      a. Scope: This method covers the determination of the boiling range of petroleum distillates
         with IBP>174°C and FBP<700°C. It is not applicable to low molecular weight components
         (eg. naphthas, reformates, gasolines), heterogeneous components (eg. alcohols, acids, esters),
         or residues.
     b. Summary: A nonpolar open tubular (capillary) column is used to elute hydrocarbons in order
         of increasing boiling point. Quantitation is accomplished by means of flame ionization
         detector (FID) using n-paraffins to calibrate the retention time axis in terms of boiling point.
         The normalized cumulative corrected sample areas are used to calculate the boiling
         distribution.
     c. Precision:
         Repeatability – 2 to 14°C over the 0.5 to 99.5% off range
         Reproducibility – 6 to 49°C over the 0.5 to 99.5% off range
     d. Comments: This method extends the scope of the boiling range of D2887 to include medium
         and heavy distillate fractions. Since it assumes that the entire sample elutes from the column,
         it should only be used if it is known that the FBP < 700°C. Otherwise D7169, which reports
         material beyond this temperature using an external standard, should be used.

10. ASTM D7169: Boiling point distribution of samples with residues such as crude oils and
    atmospheric and vacuum residues by high temperature gas chromatography
       a. Scope: This method covers the boiling range distribution of crude oils and residues; the
          sample recovery (< 720°C) is determined using an external standard. This method
          essentially extends the applicability of D6352 to samples which do not completely elute from
          the GC column.
       b. Summary: Like D6352 this method uses a capillary column and FID, only in this case a
          reference oil which fully elutes from the column is used to determine the response factor,
          from which the amount of sample eluted (recovery) may be calculated. The boiling point
          distribution to 720°C (C100) may be determined.
       c. Precision: based on four samples of crudes analyzed in duplicate by four labs during 2000 to
          2002
          Repeatability – 1 to 19°C over the 0 to 90% off range (although it should be pointed out (as
          stated below) that the method is unreliable in the IBP to 126°C range.
          Reproducibility – will not be available until 2010
       d. Comments: Typically the boiling range distribution of the response factor reference oil
          standard, as determined using D6352 and given in the current method, is verified to ensure it
          is within the consensus values. Because of incomplete separation in the presence of large
          amounts of CS2 (the solvent), this method results in unreliable boiling distributions in the C4
          to C8 range.




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DISCUSSION

Water in Test Samples
      Water may cause problems during TBP distillations since it may cause bumping; therefore, if
      possible it should be removed by centrifugation. Distillation itself is, however, sometimes used
      as a means to remove water before carrying out other tests on the bitumen as explained in the
      method, “Sample preparation considerations”. For simdist applications, it is only necessary to
      know the water content in order to make a correction, since water does not dissolve in the CS2.

Flooding of the Column
      Flooding during TBP distillations, which results from an excessive rate of heating, is the physical
      movement of liquid up the column into the receiver (1). It can essentially ruin a distillation, and
      drives home the fact that distillations have to be watched at all times.



Cracking
      Cracking is thermal degradation caused by high temperatures in the pot during a TBP distillation,
      and may occur at temperatures as low as 250°C and certainly by 350°C. It is indicated by the
      formation of a fog, a sudden drop in vapour temperature, or the presence of light components in
      the heavy cuts as measured by simdist (1). Cracking may often occur during distillation of
      bitumen, and may make it difficult to obtain cuts greater than 510°C.

Quality of TBP Distillations
       The quality of a TBP is indicated by the degree of overlap between one cut and the next. This is
       usually measured by carrying out a simdist of each of the cuts. The chromatograms shown in
       Figure 2 for a crude point out the high quality of a spinning band distillation, with little of no
       overlap observed between adjacent cuts.




             Figure 2: Simdist chromatograms of whole crude and its spinning band distillation cuts

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Comparison of TBP to Simdist
     Distillation data obtained from an Alberta Committee on Oil Sand Analysis (ACOSA) Round
     Robin in 1983 are reproduced in Figures 3 and 4 for bitumen and heavy oil. This study involved
     16 laboratories and provided the opportunity to compare TBP and simdist (1). The TBP methods
     employed were D2892, D1160, Hivac potstill (evolved into D5236) and spinning band. The
     simdist methods were not specified, but at the time the commonly used one for heavy crudes was
     in the proposed stage, and eventually became D5307. Considering the number of laboratories
     and different methods used, the agreement between the corresponding curves was surprisingly
     good.



                                                             Athabasca Bitumen

                                           600

                                           500

                                           400
                              AET (degC)




                                                                                                       Simdist
                                           300
                                                                                                       TBP
                                           200

                                           100

                                            0
                                                 0   10      20          30   40      50      60
                                                                     % Off



           Figure 3: Comparison between simdist and TBP methods from 1983 ACOSA round robin
                                          on Athabasca bitumen




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                                                          Lloydminster Heavy Oil

                                           600

                                           500

                              AET (degC)   400
                                                                                                       Simdist
                                           300
                                                                                                       TBP
                                           200

                                           100

                                            0
                                                 0   10      20          30   40      50      60
                                                                     % Off



           Figure 4: Comparison between simdist and TBP methods from 1983 ACOSA round robin
                                        on Lloydminster heavy oil

          From the 1983 ACOSA round robin it was possible to determine the following reproducibilities:
                 TBP:       10-48°C over the 0-50% range
                 Simdist:   6-25°C over the 0-50% range

Comparison of Simdist Methods
     The two low temperature simdist methods were developed for analysis of samples which either
     completely elute (distil) from the column by 538°C (C42) or for which a percentage recovery of
     the sample to that temperature can be made. The latter is accomplished by running the sample
     alone and spiked with an internal paraffin standard. In both of these methods the column
     temperature does not exceed 390°C, hence their designation as low temperature methods.

          On the other hand, the two high temperature methods were developed to extend the maximum
          temperature to 720°C (C100). Analogous to the low temperature methods, D6352 applies to
          samples which completely elute (distil) by this temperature, whereas D7169 provides a sample
          recovery to that temperature by employing an external standard, a reference oil which fully
          elutes from the column. From this oil response factors may be calculated from which the sample
          recovery or >720°C residue may be calculated. The assumption is made here that the
          components in this oil give a similar response to that of the sample being analyzed.

          In both of these methods, the column temperature should be capable of sustaining 435°C, hence
          the term high temperature methods. There are a number of columns on the market for which the
          manufacturers make these claims. The high temperatures and high boiling material (eg. residues
          and asphaltenes) to which these columns are subjected during use will, however, cause
          deterioration. Therefore, it is suggested that this deterioration be monitored using a check
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          standard such as a bitumen similar to samples being run. When control limits are exceeded the
          usual practice is to change the column liner; if this does not work the complete column must be
          changed.

Equivalent Distillation
      Boduszynski and Altgelt (4) conceived of a novel method to extend AET into the non-distillable
      residue range (>700°C), in other words beyond that obtainable by short path distillation or
      D7169. Since distillation basically separates on the basis of molecular weight (proportional to
      AET), they developed a separation which does the same, but instead is based on solubility, called
      sequential elution fractionation (SEF). The SEF fractions are obtained by depositing the residue
      on glass beads and eluting with solvents of increasing polarity. The molecular weights of the
      SEF fractions are determined and related to the mid-AET from known equations (4).

          Using this method, examples of distillation curves of the atmospheric residues of three different
          crudes are shown in Figure 5. The lower boiling points were obtained from TBP distillations,
          whereas the higher points were obtained using the equivalent distillation method. Noteworthy is
          the smoothness of the curves between the distillate points and those of the SEF fractions. This
          method pushes the envelope to a FBP of approximately 1200°C.




                     Figure 5: Cumulative AET curves obtained from TBP and SEF data


SUMMARY

Generally, if actual distillation cuts are required for further analysis such as a crude assay, then a TBP is
done rather than a simdist. Common practice for bitumens is to first carry out a simdist (D7169) to get
an overall picture of the distillation range before choosing the type of TBP. For crude assays the cuts
are usually prepared using D2892 and D5236 (or D5236 alone if there is a low amount of volatile
material). This will allow cuts to be obtained up to 538°C AET, and is often considered to correspond to
a refinery vacuum distillation and achieves a maximum of 15 theoretical plates. The D1160 distillation
is only capable of one theoretical plate, and is rarely used for obtaining bitumen cuts.



Office of the Secretary                                                  P.O Box 21059 Terwillagar Postal Outlet
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A spinning band distillation yields a much higher number of theoretical plates (up to 50), and hence a
“cleaner” distillation, with a smaller amount of overlap between adjacent cuts. This distillation,
however, has not yet been recognized by ASTM. If deeper cuts are desired, a short path still may be
used. In this case a distillate with FBP up to 700°C and a residue may be obtained.

On the other hand, a simdist can be determined much more quickly, and can give similar boiling point
data which have been correlated to actual distillations. One does have to be careful, however, in
choosing the correct simdist method according to the expected boiling range of the bitumen. For
bitumen cuts which are known to have a FBP<538°C, D2887 is usually used unless the FBP is close to
or may exceed 538°C. For a cut which is known to have a FBP<700°C ASTM D6352 should be used;
in this case the sample will fully elute from the column. For all conventional and heavy oils including
bitumens, for which the FBP>720°C, one of the methods which employ an internal or external standard
and give a recovery, must be used. The older method, D5307, gives the sample recovery only to 538°C,
whereas the higher temperature method D7169 provides a recovery to 720°C.


REFERENCES

     1. D. Wallace, Ed., A review of analytical methods for bitumens and heavy oils; Alberta Oil Sands
        Technology and Research Authority: Edmonton, 1988; pp 81-85.
     2. K.H. Altgelt and M.M. Boduszynski, Composition and analysis of heavy petroleum fractions;
        Marcel Dekker, Inc.: New York, 1994.
     3. J.G. Speight, The chemistry and technology of petroleum 3rd Ed.; Marcel Dekker, Inc.: New
        York, 1999.
     4. M.M. Boduszynski and K.H. Altgelt, “Composition of heavy petroleum. 4. Significance of the
        extended AEBP scale”, Energy & Fuels, 6, 72-76, 1992.




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