cap 17 hidratos
TRANSCRIPT
-
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1/15
4
74
G
as
Hy
drate Formatlon
Gas hydr
a
tes behave
a
s s
olution
s o
f
gases in crystalline solids rather
than
a
s ch
e
mical
c
ompoun
ds
Th
e main fram
ew
ork o
f
the hydrate
c
ryst
a
l
is formed with w
ate
r
molec
ul
e
s . The hydr
ocarbo
n
mo
le
cules o
cc
upy void
spa
c
e
s
w i
thin the l
a
tti
c
e o
f
water mo
lecule
s .
Hydrate
formati
on is physical
ra
the
r
than
c
h
em
ical in nature. Appar
-
e
ntly, no strong chemical bonds
ar
e formed between the hy
droc
arbon
a
nd
wa
ter
molecule
s.
A
c
tually
,
th
e
h
y
d
roc
arbon
mole
c
ule
s ar
e
fr
ee to ro
ta
t
e
w ith
in
th
e void
spaces
.
Toe
wa
ter
fr
ame
w
ork
s
eem
s
ice
-
like be
ca
use o
f
comparable
h
eats
of
forrnation.
H
oweve
r,
the crysta
l l
attice is somew
h
at different t
h
an
ic
e
s
ince an ice l
a
ttice
pro
vide
s
no space for
e
ven the
s
m
al
lest
hydrocar
bon
mol
ec
ule
Two types
of
hydrate crystal lattice
s
are know
n.
Each contains void
space
s of
two differe
nt
size
s .
One lattice has voids s
i
zed to accept small
molecules su
ch
as methane and l
ar
ge
r
m
o
l
e
cul
es such as propane
a
t a
r
a
tio
o
f
ab
o
ut
tw
o
s
mall
m
olecules
to
on
e
l
ar
ge
m
olecule
The
oth
e
r
lattice accepts methane mol
e
cules and medium-sized m olec
ule
s--such as
ethan
e
-at
a ratio o
f
about three mediums to one small.
Although gas hy
drat
es
a
ppear to
be
solid solutions rather th
a
n
chemical compounds
,
a specific number o
f
water mole
c
ules is associated
H
ydrocarbon gas and liquid w
ater
c
omb
i
n
e to fo
nn
solid
s
resembling
wet snow at
temperatures
so
me
what
abo
ve the temperature
at
w
h
ich
wate
r
solid
ifi
es. The
s
e
soli
ds are called They are one of a
form
of
cornplexes
know
n as Th
i
s phenomenon particul
ar
ly
interests those in the
petrol
eum industry because
th
ese s
olid
s
ca
n form at
te
m
peratures and pressures n
ormal
ly
enco
untered in producing
an
d
tran
sporting n
a
tura
l
gases.
Gas Hydrates
17
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Conditions For Gas Hydrate Formation
The most important consid
e
ration in hydrate formation is that liquid
water must be present for hydrate to form Even with liquid water present
a
metastable equilibrium
c
an
e
xist b
e
tween water and gas at conditions of
with each gas
mol
ecule. This dueto the framework o
f
the crystal. The
ratio depends primarily on the size
of
the gas molecule.
Figure 17-1 shows a number of gase
s
which form hydrate
s
, along
with the maximum length of each molecule in angstrom units and the
approximate ratio
of
water molecules to ga
s m
o
le
cules in the resulting
crystals. Not only the size o
f
the ga
s m
olecule but also its isomeric
configuration control whether or not hydrate will form. For example, iso-
butane readily fo
r
ros hydrate, but the
longe
r n-butane
m
olecule forros
hydra
te only at temperature
s s
lightly above the
fre
ezing point
of
water.
Yet, in the presence
of
other g
as
es with smaller molecules
,
the n-butane
molecules readily enter the h
ydra
te. This
indic
ates that n-butane mole-
cules occupy the larger
v
oids the crystal lattice once h
y
drate formation
begins.
Fig. 17-1.
Handbook of Natural
Eng
i
neering
u
'4
'-
:~~#'$'.#
.#
i ' -~ 1
-7//#/
$#////#n'#/#/#r.
Dl1
C
M
t1
1
U\8 i
(t ,He.
bo
C.
l flll
. ,~
. .
. . . . . .
475
-
7/25/2019 Cap 17 Hidratos
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pressure and temperature for which
hy
drate forrn
a
tion could occur. But,
once seed crystals are forme
d
,
hy
dration
occur
s rea
dil
y.
Seed crystals be gin to forrn at
temperatur
es
3 t
o
O
F
lower than the
hydrate-forming
temperat
ures dis
cuss
ed late
r
in
t
his chapter
.
Or, at a
given temperature
,
seed cry
s
tals start forming at 300 or more psi above
hydrate-forming
pressure.
However
,
dust or rust particles may act like
seed crystals in initiating
h
ydrate formation
.
A portian
of
the ph
as
e diagram for a mixture of water and a light
hydrocarbon is given in
Fi
g
ur
e 1
7
-2. The line Q C separates the region
in which liquid water and
h
y
drocarbon
g
as
ex
is
t from the region in which
liquid water and hydrocarbon liquid exist. None
of
the phases is pure; ali
contain slight amounts
of
the other substance ac
cordin
g to their mutual
solubility.
Line Q C is parallel to and slightly
abov
e the vapor-pressure line for
the pure hydrocarbon. The dashed Ii ne to the left
of
point Q is simply an
extension
of
the vapor-pressure Iine
of
the
hy
drocarbon. Point C is the
three-phase critica point at which the
properti
es of the hydrocarbon gas
and liquid merge to form a
s
in
g
le
hydro
carbon phase in equilibrium with
liquid water.
The line Q1 Q 2 separates the area in
whi
ch liquid water and hydrocar-
bon gas exist from the
are
a in w
hi
ch liquid water and
hy
drate e
x
ist. This
line represents the conditions at w
hi
ch
gas
and liquid water combine to
form hydrate.
Fig.
H y
iNnpefl;iu
e
PETROLEUM FLUIDS
76
-
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4/15
Fig. 17-3.
Handbook
of Natural Eng
ineerng
20
100
60
200
i
800
2000
4000
1 0 ,000
8000
6000
80 90
0 40 .50 60 70
_/
, , , .
/
- ~
1
1
:..,
-
..-
: . . . - -
1
, 1
/
..,,/ , , .
--
1
/
- -
' ' -
r
- -
/
-
40
Point
Q
2
i
s a quadruple
poin
t. At Q 2, four phases are in
equil i
brium:
Iiquid water, hydrocarbon liquid,
hy
dr
o
carbon gas, and solid h
yd
rate.
To
e
almo
st
v
ertical line
extendin
g
fr
o
m point
separate
s
th
e
ar
e
a
o
f
liquid water and hydrocarbon Iiquid from the area of liquid water
a
nd
hy
d
ra
te
.
Q which occurs at approximately 32F, is also a quadruple
p
oi
n
t
re
presenting the point at whi
c
h ice
,
hydrate, liquid water, and hyd
roc
ar-
b
o
n gas
ex
ist in equilibrium. The vertical line extending from point
separates the area far hydrate and liquid
wat
er from the area far hy
dra
te
and ice.
T
he
lin
e
of
major intere
st
on
this
phase diagram is the Ii
n
e Q
Q
w
hi
c
h
r
e
p
r
esents the equilibrium
betw
ee
n
h
y
drocarbon
ga
s
,
liquid
water
,
and hydrate.
4
77
-
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5/15
Bureau of
Min
es
M
on
ograph
8
lO
1
00
'
1
000
IOO
1
500
'lY
/
-
V
, , ,
V :
/
# .
~
1 1 ~
'o'
-7
,
,o
. .
Figure
17
-3 shows the ~ lines for the five h
y
d
r
ate-forming
hydrocarbon cons
t
ituents of natural
gas
. Portio
n
s of the
vapor-pressur
e
lines of the hydrocarbons and parts
of
the vertical li
n
es from Q and Q
also are shown
.
Pressures and
tempe
ra
tures along
the~
line for each
hydrocarbon indicate c
on
ditions for which
h
ydra
t
e
formatio
n occurs
fo
r
mixtures of that hydrocarbon with li
qu
id water. Methane forms
hy
drate
at much higher
temperature
s than the larger hydrocarbons. However
,
the
pressure required for hydrate formation becomes significantly lower as
m o
l
ecular size
in
creases
.
Figure
17
-
4
gives a
compariso
n of the hy
drate-f
ormation
lines
of
methane and propane w i
t
h the hydrate-formation lines for mixtures of
th
es
e two
h
y
drocarb
o
ns.
3
Note
th
at
v
ery sm al
l
quantities
o
f the larger
hydrocarbon in the
mixtur
e
s
c
au
s
e large
re
ducti
o
ns
i
n the
pre
ssure
s
required to initiate hydrate forma
ti
on .
A mixture of
4 0
%
m eth
ane and
60
% propane forms
hy
drate at
pressures nearly as low as pure propane, but hydr
at
e
form
s
a
t much
higher temp
e
ratures
t
han for pure propane. Thus, mixtures of methane
and l
ar
ger hydrocarbons retain the high
h
ydr
ate-forming tempe
ratur
es
o
f
meth
a
ne and approach the
low
er hy
drate-f
orming
pres
sures of th
e
larger
molecules .
This fact
i
s confirmed
b
y the data
giv
en in Figure
17-5
1
This
fi
g
ure
shows
th
e
h
ydrate-forming
c
ond
iti
ons fo r s
ev
eral n
a
tural g
a
ses
al
ong
PETROLEUM FLUIDS
78
-
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6/15
with the hydrate-
for
m
ati
on line fo
r
met
hane .
Toe pr
e
ssures at which
hydrate formation occurs are
s
ignificantly low
e
r for the natural gases
than for me
than
e.
Th
e natural gases w ith high
e
r specific gravitie
s
(that
is , gases with more o
f
the
heav
ier hydro
c
arbons) form hydra
tes
at much
lower pres
s
ure
s
than the lighter gases.
The data are incomplete at the higher
te
mperatures, but appar
e
ntly all
the gases form hy
dra
tes at
temp
eratures som e
w
hat
a
bov
e
70F.
Figure
17-6
was
d
e
v
e
l
o
p
ed
fro
m
th
e
da
ta of
Fi
g
ure
1
7
-5
.
4
Fig
ure
1
7
-6 is
a correl
a
tion of hy
d
rate
-formin
g
co
ndition
s
fo
r
natural gases
with various
s
pecific
gra
v
ities, Th
is
fig
ure can be used to estmate the
condition
s
under which hy
d
r
a
tes
w ill
fo
rm
Toe
re sul
ting hydrate
formation
conditions m
u
s
t u
s
ed with
cau ti
on be
caus
e
th
ere
is
a great
discrepancy bet
w
een the
li
m
it
ed publi
sh
ed d
a
ta
a
nd the
c
orr
e
lation
shown in Figure
1
7
-6. To
e
differe
nces
hy
drate-
forming
pr
ess
ures
Fig.
17-5.
A
Hendbook
of Natural Gas Engneering
40
90
00 60 70
Temperoture, deg F
30
2
00
40
0
300
600
~
1
000
~ 800
a.
0
20
0
0
; ;
c..
4000
3000
~ I J V
V
~
I
~
I
/ ~
/
/ /J
/
V
~ ~
f
V
~
~
~
~
10,000
8000
6
000
-
7/25/2019 Cap 17 Hidratos
7/15
EXAMPLE
17
-1:
7
between
th
e
avail
able data
an
d the co
rre
lation in
Fi
g
ur
e 1 7
-
6
ran
ge from
about
1
0% at the higher specific grav
iti
es to as
mu
ch as 35% at
th
e low er
specific g
rav
itie
s.
G i
ve
n a
va
lu
e of
pr
ess
ure,
th
e
erro
r
i
n
th
e
estmate of
hydrate-formi
ng
temp
erature
co
uld be 5F or
m
o
re .
Figure
17 -6
is for sweet gases.
T
he pr
ese
nce
of
hydrogen
s
ulfide and
carbon dioxide
shift
s
th
e
lin
es to
th
e
righ
t. Thi
s results i
n an
in
crease in
the hydrate temper
a
ture for a given pr
e
ssure or a d
ec
r
ea
se in hydrate
pressure for a given t
e
mpera
ture
.
Trans.,
160,
80
0 60 70
f
4000
1/)
(L
00
J
'/ji
0
0
//J
c
7
, , , , , ,
,,,
--
00
,r)"
/ /
~7 /
///
0
0
C7m'
- -
h
oc
5 :
q ; / /
W
o
~/
~/
6
tL
- =
-
--- --
1----------
~--
1-------~
--
4
8
o
1 . 1 . . .
iJJ
a:
~ 3
a: 2
iJJ
o:
P
ET
ROL
E
U
M FLUIDS
8
-
7/25/2019 Cap 17 Hidratos
8/15
H andbook of Natural Gas Enginee
r
ing
.ot
o
~14
18
1//
J
lj
2
J I
J
~ : : ;
8
6
'
4
}
< t
V
01~
V
20
lnhibitionof
Gas Hydrate Formation
The p
re se
nce of dissolved
so
lids in the water reduces the te
mperatur
e
s
at which
n
atu
ra
l gases forro
h
ydrates. Figure
1
7-7 gives values to reduce
th
e
t
em
perat
ur
es estimated from Figure
1
7
-6
to account for the e
ff
ect
o
f
d isso
l
v
ed
so l
i
ds
in the
wat
e
r
.
The water which condenses from na
tural
gas at surface conditions is
p
ure w ater. Often an inhibitor is
a
dded to
thi
s
wa
ter to lower the hydrate-
Yes .
Figure
17-6
indicates th
at a
O
.
7 specific
gravi
ty
ga
s
form
s
hydrates at temperatures
b
elow
5
6F at a pressure of 500 psia if
liquid water is present.
4
81
-
7/25/2019 Cap 17 Hidratos
9/15
The pres
en
ce of liquid hydrocarbons with a hydrocarbon gas
al
so
lowers hydrate-forming
temperature s
.
Fi
gure
17-9
indicates this de-
pression
hy
drate-forming
temperatures
. The figure
repre
sents a ga
s
with specific gravity less than 0.6 and is probably not
suffici
entl
y
Prob
abl
y not. The diethylene
gly
col reduces the
hydrate-formin
g
temperature by about
12F
to about 44"F . See Figure
17-8.
EXAMPLE 17-2:
formin
g temperature. The effectivene
s
s of two inhibitors is
sh
own in
Figur
e 1
7-8.
Methanol is
obviousl
y the better inhibitor,
but
di
ethy
lene
glycol,
H O (
C 2H
40)
2H, often is
us
ed because the volatility of
m
ethanol
cau
ses high methanol loss to the ga
s
.
Mono. 8,
Chem
.
Eng. Prog. 52,
324.)
Uqu l d .
30 40
0
0
: .
aol - - - - -- -- - - -- , 1-+- - - - l - - - - -+ -- - 4
:;
s1----1----- '
r
-+---+----+-:r-i
f al-----+
~--+---
',':---+------OO
3000
1
. . . . . . .
e o o o
-
1
\
. . .
1
I O O O
_\
o 1
1'l 7 /
1
/
I
)
~
I
I
J ;..,
lOOO
///
/
ir
. . . - -
. , , . . . . , . .
V
/
2000
r
. . . . . . .
IS
1
./
~
_ J 9 . .
-
//
V
l
-
7/25/2019 Cap 17 Hidratos
14/15
so li
ds). At w hat tempe
ra
ture
s
i
s
hy
dr
ate fo
rm
a tion
poss
ible?
17 -9.
The
gather
ing
sys
tem o
f E
xercise
1
7-8
must
be protected from
hydrate fo rmat
io
n to a temperature o
f
40
"F . Me
thanol
w
ill be
added to
th
e liquid
w
ater.
W hat conce
n
tr
ation m
ust
the methanol
reach to inhibit
hy
dra te
fo
rmation?
17-10. What concentration
o
f di
e
th
y len
e glycol is
r
equired to in
h
ibit
hydrate
fo r
mation
t
o 40
F
E
x
e
rc
i
s
e
17
-
8?
17-11. Tubin
gh
ead con
ditio
ns of a dry gas well are
2 6
00 psia and
l lO F. The gas
h
as s
peci
fic gravity of
0
.600. Sorne water
conden
ses
fro
m the
g
as during
p
rod
u
ction. Gas pressure is
reduc
ed thro
ug
h a choke to 1 200 psia.
I
s
there a
po
ssibility of
hydrate formation?
Fig. 17-13.
Trans.
160,
1 0
ee _
. . . . . . .
/
6
...J
: :
4
3-
-
2
1 . : : :
./
- - - . , . /
487
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7/25/2019 Cap 17 Hidratos
15/15
References
Katz, D
.
L., et
McGraw-Hill Book
C
o., Inc
.
, New
Yo
rk C ity (1 95 9)
.
2. Scauzillo, F.R.:
Inhibi
t
ing H ydr
a
te Fo
rmations
in Hydrocarbon
Gases,
(A ug . 1 956) 3 2 4
-
2 8.
3. Deaton,
W .M.
and Fr
os
t
,
E.M .
,
Jr.:
Monograph
Series,
USBM, (194 6) 8 .
17
-
12.
A
0 .
7
0
s
p
ec
ifi
c
g
ra
v
i
ty gas
i
s e
x
p
a
n
ded
throu
g
h
a c
h
o
ke.
Upstream conditions are 3000
p
sia and
15 0
F.
What is the
lowest pre ss
ur
e
t
o w
h
ich the gas
c
an be e
x
panded prior t
o
onset
of
hy
drate
fo
rmati
on
? W
h
at is t
h
e temperature at this
p
ressure?
Fig. 17-14.
Trans.
1
60
_J
- c
: :
o o o
'
'
1
1 1
~ \
. .
l
1
_/
J
1
1
1 1 ~ 1
:
1 '
::
1
'
1
1
4
000
J
I
EMPERAiU~F
/ I J
1
. ,
1
J
~
V
I
,
,, I
J
J
'0
zooo
"
'
I
J /
"'
- -
V
I
/
e---'
I I
1
1 70
-
1
}
I I
160
-
-
/
, // I
/0
- - -
oo
- -
-
0
s:
:.--
2
:.--
_ _ _ .
7
"
-
/
~
80
V
: : 1 ~
/
60
se
_ _ . ,
'
PET
ROLEUM
FLU
IDS88