Cap 17 Hidratos

7/25/2019 Cap 17 Hidratos

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


2/15

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


3/15

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


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


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


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


7/15

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


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


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


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


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

Cap 17 Hidratos

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Transcript of Cap 17 Hidratos