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A
Rapid p p r o ~ i l D a t e Meth od For
EstilD ation
of Hydrocarhon Dew
Points
Pipeline Natural
Gases
By
ZIZ
and
W.
G O V I E R ~
13th
Annual
Technical Meeting
Calgary May 1962
Th e
of Clean
ABSTRACT
A
theoretical correlation
of
th e
effect
of pressure
on
the
hydrocar
bon dew
point
temperature
of c lean
pipeline gases is presented.
The
correlation
was developed from cal-
culated dew
point
data on thi rt een
ga s
analyses
from
ten
gas fields
in
Alberta. The
dew points
were cal-
culated by
conventional
trial and
error method an d NGAA equilib·
rium
ratio
charts.
The
data are
correlated
by means
of
a wetness
parameter
based on th e
amounts
of
propane and heavier components
in
the gas and equilibrium ratios
fo r
those components
at
arbitrarily
selected
conditions.
The correlat ion allows th e quick
est imation of
th e
hydrocarbon
dew
points
of c lean
pipeline
gases with.
ou t
th e
usual
trial an d
error
pro-
cedure.
For
more accura te results
th e
dew points calculated
f rom thi s
correlation
may be used as the
first
e st imate in the
conventional
trial
and error calculations. The
calcu-
lated dew
points
ar e usually within
±4°F
of
th e results from
conven-
tional
calculations
over t he norma l
range of pipeline operating
pres·
sure
and, fo r pipeline natural gases
f ree of
contaminants,
such
as glycol
or
compressor
oil, are
within about
th e
same
limits when compared
with measu red dew points.
The correlation
does
no t apply to
th e
prediction
of dew point temp
era tures for pipeline gases contain
ing even small quantities
of
alcohol,
glycol or
compressor
oil.
Assistant Professor
of Petroleum
Engineering
University of
Alber-
ta Edmonton.
Professor of Chemical Engineer-
ing Dean
of Faculty
of Engin-
eering
University
of Alberta
Edmonton.
Technology Spring 1962 Calgary
INTRODUCTION
IPELINE natural gases,
which
ar e
composed
mainly of
a mix-
ture
of
paraffin hydrocarbon
gases,
nitrogen, carbon dioxide an d water
vapor,
normally
exhibit
two
kinds
of
dew points. These ar e hydro
car bon dew points
an d water dew
points.
The
dew
point
is
th e temperature,
at a certain
pressure,
at
which
th e
first drop of liquid is formed
or
is
just about
to form .
In
the
ca se o f
water dew points, the
liquid
formed
is water an d
in
th e case of hydro
carbon dew points the
liquid formed
is
a
mixture
of
th e
hydrocarbons
contained in
th e
gas.
If a pipeline
gas,
as a result
of
processing, con·
ta ins contaminants such as glycol,
a thi rd type of dew
points
may
also
be observed. The liquid formed in
this case will
be
a mixture
of
hydro
carbons
an d
the contaminant.
Be-
cause
of
the higher boiling point o f
th e
contaminant, th e
presence of
thi s component
in
the gas
would
raise
the dew point
temperature.
Small amoun ts
of n it rogen,
carbon
dioxide and
other
non-hydrocarbon
constituents
may
also be
present in
th e dew formed, depending upon
th e concent ra tion of these compon-
ents.
In the
case of clean
natural
gases
(free of con taminant s) the hydro
carbon
dew
point may
be
determ
ined
e ith er b y
direct
measurement
or
by calculation
from the known
ga s
analysis.
The
dew points of
contaminated gases may
only be
determined by
actual measurement.
(If
the amoun t a nd nature of the
contaminants were accurately
k
ow
n
theoret ical calcu la tions
should be
possible.
CALCULATION OF HYDROCARBON
DEW POINTS FROM GAS
ANALYSIS
The
dew point is th e
condition
at
which th e first
droplet
of l iquid
(o r
dew) is formed, an d is computed
as th e temperature, T, an d pressure,
P, at which the
sum
of th e ~
terms equals
unity.
= x = 1.0
at dew
point. 1
where:
y
mole fraction of
th e
hydro
carbon
in
th e gas
phase
on a
water free basis.
x mole
fraction
of the
hydro
carbon
in
th e droplet
o f l iquid
formed on a wate r f ree
basis.
K:
equilibrium
ratio
for the hy-
drocarbon at
pressure P an d
at temperature
T.
The procedure fo r
calculation
of th e
dew point temperature knowing th e
pressure
and th e
composition
of th e
ga s
is
as
follows:
1
Assume
a
dew
point tempera
ture.
(2 )
Determine th e K
values
for
each component at the pres
sure
of
interest and assumed
temperature (K values may
be obtained from the NGAA
correIation).
3 Calculate
the composition
in
th e
liquid
phase
(x) fo r each
component (assuming that
the o ri gin al composition o f
the
gas
is the same as th e
composition
of th e gas
after
the first droplet of
liquid
is
formed).
9
62-01-02
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4)
Check
th e sum of a ll th e
mole
fractions in th e
liquid phase
(x). This sum
must
be
equal
to 1.00
if
the temperature as-
sumed
is correct_
5)
the sum of th e l iquid mole
fractions is no t
equal to
1.00,
repeat
calculations 2)
through
(4),
assuming a dif-
ferent dew point temperature.
In
order
to
perform
th e above
calculations,
an analysis
of
th e gas
an d
th e K-value
fo r
each
compon
ent in the gas as a
function
of
p ress ure an d tempera tu re mus t
be
ava il ab le . The se
ar e
now discussed
separately.
ANALYSIS OF GAS
to
composi tion would be most ac-
curate.
Several correlations are available
fo r
th e
determination
of K
values.
All of these cor re la tions
ar e based
on th e assumption th at th e compo-
nents
of a natural
ga s
behave sim
i la rly with respect to
their
physical
p ropert ie s. This
behavior of
hydro
carbons as
a
f amily has
been
veri-
fied. The
presence
of non-hydro
carbons such
as
carbon
dioxide,
nitrogen an d hydrogen sulphide re-
duces th e
accuracy
of these correla
tions
as these
gases
do no t belong
to
th e
hydrocarbon
family.
The Kellogg
fugacity
charts 1)
an d
th e
NGAA
charts
2) have
found
wide use fo r
predicting
equil-
i br ium const an ts
fo r natural
gas
systems. The NGAA
charts
which
are
contained in
th e Natural Gaso-
l ine Supply Men s Association
Data
Book, were used fo r calculations in
1) Benedict, M.,
al
Vapor
Liquid Equilibrium in Mixtures
of
Light
Hydrocarbons, M.
W.
Kel logg Company .
New
York;
Chem. Eng . Progr .,
46
(3):
20
1950).
2)
Natural
Gasoline
Supply Men s
Association,
Engineering Data
Book
1957).
o o l i l < oM o oNM NO
OC lli lMOOOOO
C icicicicicicicici
o
The
dew point s
of pipeline
gases
depend to
a l ar ge e xte nt
on
th e
p ropane and heavier
hydrocarbons.
Th e propane-plus
content
of
most
pipeline gases
is
quite small
an d
th e
methods
of analysis ordinari ly
used
are least accu rate for
these
components. This
is
illustrated by
Table I
which
reports th e
results
of
th e
analysis of
th e s ame
ga s by
eight
different
laboratories.
The
analyses
appear
reasonably
consist-
en t
bu t the variat ion in th e relative
amounts of propane and higher
hydrocarbons
i s s uf fi ci en t
to cause
a considerable difference in calcu-
lated
dew point
temperature. This
will be d iscusse d
further.
Addition
of air to
the g as
sample
or th e condensation of heavy hydro
carbons
from a gas will result in
inaccurate
analysis.
Th e
ga s
may
be
contaminated by
ai r if
th e
s ampl e con ta in er is no t
properly
purged.
The
condensat ion of heavy
hydrocarbons may
occu r i f
th e
ga s
is cooled below
i ts dew poi nt d ur in g
th e
expan sion o f th e gas from a
high pressure
pipeline to
a
low pres
s ure co ntain er o r f rom a
high pres
sure
bomb to
a low p re ssur e gas
intake of an analysis system.
Hydrocarbon ga s analyses ar e
usual ly ob ta ine d by chr omat og ra
phic methods.
Special
care
should
be
taken
to ensure
proper
sampling
of th e gas.
EQUILIBRIUM RATIO K
Ideally K
values should
be deter
mined
exper imen ta ll y f or t he sys-
tem under
conside ra ti on. Thi s
is
seldom
pract ical and fo r most cal-
culations
th e eng in ee r must r esor t
to generalized empirical correla
tions.
Correla t ions based on
data
f rom sy stems sim ila r
with respect
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CALCULATED
RESULTS
The ana lyses an d properties of 10
different Alberta gases
are present
ed in Table
II ,
duplicate analyses
being
given
fo r several.
Dew
points
were calculated from
these analyses
an d
NGAA
K
data
at a
converg
ence pressure of 5000 psi. The re
sults at pressures from 100 to 800
psia
appear
in
Table
III.
The
var
iation in calculated dew point
temperature
for the
duplicate
an
alyses ranges from 2 °F to 25°F.
this investigation.
These
charts are
based on th e assumption that th e
K value
fo r
a component in a ga s
is a f unct ion o f th e
pressure,
tem
perature an d
th e
convergence pres
sure only.
The convergence pressure is em
ployed as an approximate measure
of the composi tion of th e gas.
The
calculation procedures fo r
th e
con
vergence
pressure are descr ibed in
the NGSMA Data Book and
th e
Handbook
of Natural Gas Engin
eering
by
Katz and Associates (31.
The accuracy of dew point
calcu
l at ions depends
on
al l of
th e
fac
tors discussed
above. With
a preci
sion analysis
an d
suitably
chosen
K values
calculated
dew point s fo r
uncontaminated gases
may
be ex
pected
to
check measured
values
within about 5°F.
Normal
com
mercial gas
analyses
may
lead to
much greater discrepancies an d
if
inappropriate
K
values are used
differences
between
calcu la ted and
measured
dew points may easily
reach 40° or 50°F.
The theoret ical
correlation is
based on
th e
fol lowing considera
tions. i Th e d ew poin t
tempera
ture
of
a ga s is a f unct ion o f the
pressure
an d
ga s
composition. (il)
The equ il ib ri um constants
are
a
f unct ion o f th e
pressure , tempera
ture and
composition.
Oii)
For
natural
gas systems the
propane
plus content of th e natural gas
might be expec ted t o have
th e dom
inant effec t on the
hydrocarbon
dew
point. Ov)
I f
th e value of K
is
selected fo r each component at
some
a rb itr ar y ave rage
pressure
and t empe ra tu re ,
th e dew point
temperature might be considered to
CORRELATION
(3)
Katz,
D. L.,
al.
Handbook
of Natural Gas
Engineering,
McGraw-Hill,
1959.
Technology, Spring, 962 lg ry
11
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TABLE III.
CALCULATED
HYDROCARBON
DEW
POINT
TEMPERATURES
OF.
Wetness
Gas Parameter
Pressure, Psia
No.
W
100 300
500
600
800
241
0.231
-7O
-49 -42 -38.5
-36
242
0.427
-45
-24.5
-18 -16
-15
20 0.353
-63 -37 -27
-2.4
-21.5
23 0.317
-63 -40
-31
-29
-26
1 0.302
-66 -43 -34
-32
-29
15
0.238 72 -44.5 -40.5
-38
-35
18
1
0.582
-45 -18
8
- 6
- 3.5
b
0.804
-35
- 4
8
13
17
331
0693
-43
-11
1 5
10
332
0.623
-48
-15.5 2
2
8
361
0.484
-53
-24
-12
8
3.5
22
0.634
-42
-14
3
0
4
362
0.698 ·34
- 8.5
2
5
8.5
Convergence
Pressure
5000
psi.
NGAA K values used;
K fo r C
n
used fo r C
n
be
a
f unct ion of
th e pressure
and
a wetness parameter, W, defined by
Equation 2 .
The values of
Wand
dew
point
temperatures at 100, 300,500, 600
and 800 psia, were calculated fo r
gas samples 24, 242 20, 23 1, 15, 18,
31, 33. 332, 36,
22
and
362.
The cal
culated d ata ar e tabulated
in
Table
III
0.135
0.050
0.034
0,013
0.0096
0.00335
0.009
ydrocarbon
Propane _
i-Butane _
r ~ u t n e _
i-Pentane
n-Pentane
_
Hexane .__
Heptane _
TABLE IV.
Note:
A
is
the
equilibrium
constant
K, at oa 500 psia
an d
a
convergence pressure of
5000
psi.
Journal of anadian Petroleum
ACKNOWLEDGMENT
The res earch reported here w as
undertaken at th e re ques t
of
and
under
the
financial support of Al
berta
Gas
Trunk Line Company
Limited.
The authors
wish to acknowledge
th e enthusiastic co-operation
of
Mr.
E. V.
Hunt,
Mr.
C. T. McCa ll
an d
Mr. J . Bul ley, al l of th e Alberta Gas
Trunk
Line Company .
omenclature
A: Equil ibr ium rat ion , K, at OaF
and 500
psia
fo r a
hydrocarbon
mixture
of
5000 ps i
convergence
pressure.
Ct, C2
-
C6:
methane, ethane - -
hexane.
K:
Equilibrium ration
for the
hy
drocarbon at pressure P and
temperature T.
P:
pressure, psia.
T:
dew
point
temperature, OF.
W: wetness
parameter defined by
Equation 2 .
x:
mole
fraction
in
the liquid
phase.
y:
mole
fraction in the vapor
phase.
CONCLUSION
A correlation
based on theoretical
considerations
ha s been developed
for the rapid estimation
of
th e
hydrocarbon
dew point temperature
of c le an
pipeline natural
gases.
The
correlation results
in
dew points
which check those determined by
conventional calculations
to
within
about 4°F. The correlation, like
conventional calculations, does
not
app ly to ga ses contamin ated
with
hydrate
depressants or compressor
oil.
The
final theoretical correla
t ion which
is a
smoothed cross p lo t
of
Figure I
is presented
in
Figure
II This permi ts t he
direct est ima
tion of
the
dew poi nt
temperature
at any pressure f rom a calculation
of th e wetness parameter.
a
more accur at e
cal cu la ti on o f
the
dew
point
is
required,
th e calculated
dew
point
f rom th e
correlation
may
be used
as a
first
estimate
of
the
dew
point i n conven ti onal calcula
tions.
Th e cor re la tion e limina tes
th e
normal
t ime-consuming tr ia l .
and-error
calculations.
By
way
of example,
the dew
point a t
568 psia
of
th e ga s
report
ed in
Table
I
was
calculated assum
ing the analysis
from
Laboratory
B.
Conventional calculations
gave
a
dew point of +13°F. The
wet
ness parameter for the ga s was
determined
as 0.9 5
and the
dew
point i ndi ca ted by
Figure
is
+ 16°F. Other comparisons show
a
check between convent iona l
cal
culations
an d
that from
Figure
of
a
F in t he pressure
range
of
300
to
800
psia.
This
accuracy
refers
to th e agreement obtained
from th e cor re la ti on a s compa red
with the
conventional
calculations,
not
th e
agreement
between calcu
la te d and a ctu al
dew
points.)
represented by t he s tr aig ht lines
except
at 100
psia
where a
scatter
ing up to about lOa i s observed.
3)
K at arbit·
rary
constant
P and T
«2)
K
C
n
W
1
A
C3
where
C
n
is th e heaviest hydrocar
bon present in
th e
gas.
For
th e
calcu la tions o f th e wet
ness parameter, W, values
of
K at
oaF
and 500 psia were selected.
These
v alu es of K are called vari
able A and are
tabulated
in
Table
IV for
reference.
Equation
2
may now
be w rit te n
as :
The
dew po int temperatures are
shown fo r various
constant
pres
sures as a function
of
t he wetness
parameter, W, in
Figure I. Straight
line relationships
were
observed on
semi-logarithmic co-ordinates fo r
each pressure. The data are
well
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9
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p s i a
3
~ 0 J : : : O ~
t
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;
.
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k
W
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I
, -
w
1.0
. , : .,.,, 800
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0.8
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«
0.6
LEGEND
£l.
0.5
CALCULATED
DATA
J)
0.4
PRESSURE
SYMBOL
J)
psio)
W
0.3
100 -
b
Z
30 0
-
i
I
500 -
•
w
0.2
600
- - -
t
800 - - -
-70 -60 -50 -40 -30
-20 -10
0 10 20 30 40
50 60
DEW POINT TEMPER TURE
igur
800
700
600
500
c:i
400
200
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w
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WETNESS
PARAMETER, W
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I :
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DE W
POINT TEMPERATURE ,OF
igur
Technology Spring 1962, Calgary
13
