Page 93 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 73
99 C. In many cases kinematic viscosity at 40 C (122 F) or
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line between values of input parameters, a reading can be
60 C (140 F) is reported. One may estimate the kinematic than two input parameters are involved. By drawing a straight
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viscosities at 38 and 99 C through Eqs. (2.128) and (2.129) made where the straight line intersects with the line (or curve)
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and then use the ASTM chart (or Eq. 2.128) to obtain the of the desired property. The best example and widely used
value of viscosity at 40 or 60 C. If the interpolated value is far nomogram is the one developed by Winn in 1957 [25]. This
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from laboratory data then the estimation method cannot be nomogram, which is also included in the API-TDB [2], re-
trusted and other methods should be considered. lates molecular weight (M), CH weight ratio, aniline point,
Another consistency test can be made through estimation and Watson K to boiling point and specific gravity (or API
of the molecular weight by Eq. (2.52) using estimated vis- gravity) on a single chart and is shown in Fig. 2.14.
cosities by Eqs. (2.128) and (2.129). If value of M calculated Application of this figure is mainly for petroleum fractions --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
through Eq. (2.52) is near the value of M estimated from T b and the mean average boiling point defined in Chapter 3
and SG by Eqs. (2.51) or (2.50), then all estimated values can is used as the boiling point, T b . If any two parameters are
be trusted. Such consistency tests can be extended to all other available, all other characterization parameters can be deter-
physical properties. The following example demonstrates the mined. However, on the figure, the best two input parameters
test method. are T b and SG that are on the opposite side of the figure.
Obviously use of only M and T b as input parameters is not
Example 2.11—The viscosity of a pure multiring hydrocar- suitable since they are near each other on the figure and an
bon from an aromatic group (naphthecene type compound) accurate reading for other parameters would not be possible.
with formula C 26 H 40 has been reported in the API RP-42 Similarly CH and SG are not suitable as the only two input
[18]. Data available are M = 352.6, SG = 0.9845, and ν 99(210) = parameters. Previously the computerized form of the Winn
13.09 cSt. Estimate the kinematic viscosity of this hydrocar- nomogram for molecular weight was given by Eq. (2.95).
bon at 38 and 99 C (100 and 210 F) by Eqs. (2.128) and Use of the nomogram is not common at the present time
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(2.129). How can you assess the validity of your estimated especially with availability of personal computers (PCs) and
kinematic viscosity at 38 C? simulators, but still some process engineers prefer to use a
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nomogram to have a quick estimate of a property or to check
Solution—To estimate the viscosity through the Abbott cor- their calculations from analytical correlations and computer
relations, K W and API gravity are needed. However, T b is not programs.
available and should be estimated from M and SG. Since If the boiling point is not available, methods discussed in
M > 300, we use Eq. (2.57) in terms of M and SG to esti- Section 2.4.2 may be used to estimate the boiling point be-
mate T b as follows: T b = 720.7 K. Using Eqs. (2.13) and (2.4), fore using the figure. Equation (2.50) for molecular weight
K W and API are calculated as K W = 11.08 and API = 12.23. may be combined with Eq. (2.13) to obtain a relation for the
Using Eqs. (2.128) and (2.129) the viscosities are calculated estimation of K W from M and SG [51].
as ν 38(100) = 299.76 cSt. ν 99(210) = 11.35 cSt. At 99 C the esti- 0.15178 −0.84573
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mated value can be directly evaluated against the experimen- (2.133) K W = 4.5579M SG
tal data: %D = [(11.08 − 13.09)/13.09)] × 100 =−15.4%. To This equation gives an approximate value for K W and should
evaluate accuracy of estimated viscosity at 38 C a consistency be used with care for hydrocarbons heavier than C 30 . A more
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test is required. Since the actual value of molecular weight, accurate correlation for estimation of K W can be obtained if
M, is given, one can estimate M through Eq. (2.52) using the boiling point is calculated from Eq. (2.56) or (2.57) and
estimated values of ν 38(100) , ν 99(210) , and SG as the input pa- used in Eq. (2.13) to calculate K W .
rameter. The estimated M is 333.8 which in comparison with
actual value of 352.6 gives %AD of 11.36%. This error is ac- Example 2.12—Basic properties of n-tridecylcyclohexane
ceptable considering that Eq. (2.52) has been developed based (C 19 H 38 ) are given in Table 2.1. Use M and SG as available
on data of petroleum fractions and the fact that input param- input parameters to calculate
eters are estimated rather than actual values. Therefore, we a. K W from Eq. (2.133).
can conclude that the consistency test has been successful b. K W from most accurate method.
and the value of 299.8 cSt as viscosity of this hydrocarbon at c. K W from Winn Nomogram.
38 C is acceptable. The error on estimated viscosity at 99 C d. CH weight ratio from M and SG.
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is 15.4%, which is within the range of errors reported for the e. CH weight ratio from Winn method.
method. It should be realized that the equations for prediction
of kinematic viscosity and estimation of molecular weight by f. %D for each method in comparison with the actual values.
Eq. (2.52) were originally recommended for petroleum frac-
tions rather than pure compounds. Solution—From Table 2.1, M = 266.5, T b = 614.7 K,
and SG = 0.8277. HC atomic ratio = 38/19 = 2.0. Using
Eq. (2.122), CH weight ratio = 11.9147/2.0 = 5.957. From
definition of K W , i.e., Eq. (2.13), the actual value of K W is
2.8 THE WINN NOMOGRAM
calculated as K W = (1.8 × 614.7) 1/3 /0.8277 = 12.496.
Development of estimation techniques through graphical a. From Eq. (2.133), K W = 4.5579 × [(266.5) 0.15178 ] ×
methods was quite common in the 1930s through the 1950s [(0.8277) −0.84573 ] = 12.485. %D =−0.09%.
when computational tools were not available. Nomogram or b. The compound is from the n-alkylcyclohexane family and
nomograph usually refers to a graphical correlation between the most accurate way of predicting its boiling point is
different input parameters and desired property when more through Eq. (2.42) with constants given in Table 2.6 which

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