taking about gas dehydration using solid bed

1. Gas Dehydration
Using Solid Bed Dehydration
By Engineer
Mohammed Bedair Yossof
Graduated from Faculty of Petroleum
& Mining Engineering
Suez Canal University
E-mail : bakar_zezo99@yahoo.com

2. CONTENTS
What is gas dehydration ?
2. Water content calculations
3. Solid bed dehydration
process
4. Solved example about it
1.

3. WHAT’S THIS ?
This is the process of removing water
vapor
from a gas stream to lower it’s ‘’ Dew
point’’


The gas sales contracts specify a
maximum value for the amount of
water vapor allowable in the gas .

4. EX : AT 1000 PSI
Place
Value
lb / MMscf
Southern U.S.
7
Dew point
temperature
°F
32
Northern U.s
4
20
Canada
2:4
Zero

5. WATER CONTENT DETERMINATION
For sweet gas containing over 70 % methane
and small amounts of ‘’heavy ends ‘’
 The Mc Ketta-Wehe pressure –temperature
correlation ,as shown can be used
 Ex :
 Assume Molecular weight of 26 that is in
equilibrium with 3 % brine at a pressure of
3000 psia an temperature of 150 F


7. SO FROM FIGURE 1
At 3000 psia and 150 F water content
= 104 lb water per MMscf of wet gas
The correction for salinity is 0.93 and for
molecular weight is 0.98
Then the total water content
= 104*0.93*0.98
=94.8 lb / MMscf

8. CORRECTION FOR ACID GAS
 Correction
for acid gas should be
made when the gas stream contains
more than 5 % CO2 and /or H2S
 EX :
 Assume the previous gas example
contain 15 % H2S

11. SOLUTION
From figure 3 , the water content of H2S is
400 lb / MMscf
 So the effective water content of the
stream is equal to

=(0.85*94.8)+(0.15*400)

= 141 lb / MMscf


13. SOLID BED DEHYDRATION
Solid bed dehydration system work on the
principle of adsorption .
 Adsorption involves a form of adhesion
between the surface of the solid desiccant
and the water vapor in the gas .
 The water forms an extremely thin film
that is held to the desiccant surface by
the forces of attraction , but there is no
chemical reaction .


14. The desiccant is a solid , granulated
drying or dehydrating medium with an
extremely large effective surface area
per unit weight because of a multitude
of microscopic pores and capillary
openings .
 A typical desiccant might have as
much as 4 million square feet of
surface area per pound
 The initial cost for a solid bed
dehydration unit generally exceeds that
of a glycol unit .


15. However the dry bed has the advantage
of producing very law dew points
,which are required for cryogenic gas
plants
 Cryogenics : Is the study of the
production of very low temperature
materials (below −43°C) and the
behavior of materials at those
temperatures .
 Disadvantages are that it is a batch
process , there is a relatively high
pressure drop through the system , and
the desiccant are sensitive to
poisoning with liquids or other


16. PROCESS DESCRIPTION
1.
2.
3.
4.
5.
6.
Inlet gas separator .
Two or more adsorption towers (contactors
)filled with a solid desiccant .
A high –temp heater to provide hot regeneration
gas to reactive the desiccant in the towers
A regeneration gas cooler to condense water
from the hot regeneration gas .
A regeneration gas separator to remove the
condensed water from the regeneration gas .
Piping , manifolds , switching valves and
controls to direct and control the flow of gases
according to the process requirements .

18. DESIGN CONSIDERATION


1- Temperature
Adsorption plant operation is very
sensitive to the temperature of the
incoming gas , Generally , the adsorption
efficiency decrease as the temperature
increases
The maximum hot regeneration gas
temp. depends on the type of
contaminants.

19. If a wet gas is used to cool the
desiccant , the cooling cycle should be
terminated when the desiccant bed
reaches a temperature of approximately
215 °F .Additional cooling may causes
water to be adsorbed from the wet gas
and preload the desiccant bed before
the next adsorption cycle
2- PRESSURE
 The adsorption capacity of a dry bed
unit decreases as the pressure is
lowered


20. 


3- CYCLE TIME
Most adsorbers operate on a fixed
cycle time and , frequently , cycle time
is set for the worst conditions
However , the adsorbent capacity is
not fixed value ; it decline with usage
4- GAS VELOCITIES
Generally , as the gas velocity during
the drying cycle decreases ,the ability
of the drying desiccant to dehydrate
the gas increases

21. MAXIMUM SUPERFICIAL VELOCITY FT/MIN

22. 
Low velocity require towers with large
cross sectional areas to handle a given
gas flow
d² = 3600*(Qg*T*Z) / (P*Vm)
d
 Qg
 T
 Z
 P
 Vm

= vessel internal diameter , in
= gas flow rate ,MMscf
= gas temp , R
= compressibility factor
= gas pressure , psia
= gas superficial velocity , ft/min

23. 5- Bed height to Diameter Ratio
A bed height to diameter ratio (L/D) of more than 2.5 is
desiable
6- PRESSURE DROP
∆P / L =B *μ* Vm + ρ *C* Vm²






∆P = pressure drop , psi
L = length of bed , ft
μ = gas viscosity , cp
Vm = gas superficial velocity , ft/min
ρ = gas density , lb/ft³
Note , pressure drop greater than approximately 8
psi are not recommended

24. B and C are constants given by
Practical Type
B
C
⅛ - in . Bead
0.0560
0.0000889
⅛ - in . extrudate
0.0722
0.000124
⅟16 - in . Bead
0.152
0.000136
16 - in . extrudate
⅟
0.238
0.000210

25. 7- Moisture content of the inlet gas
 How much lb of water vapor per
MMscf , and to how much you want to
be .
8- DESICCANT SELECTION
 No desiccant is best for all
applications
 The most common desiccant used is
the Molecular sieves as
 It less affected by temperature
increases (capacity decrease )
 Less contaminated with liquid

26. Desiccant disadvantages
 Molecular sieves and alumina gels acts
as a catalyst with H2S to form COS ,
when the bed is regenerated , sulfur
remains and plugs up the spaces
 Silica gels will shatter in the presence
of free water and are chemically
attacked by many corrosion inhibitors

27. PROPERTIES OF COMMERCIAL DESICCANTS
Molecular
sieves
Silica gel
Sorbeads ‘’R’’
Design loading
( % weight of
water / weight
bed )
8-10
6-7
6-7
Regeneration
temperature
(°F)
450-550
350
300-500
Bulk density
Lb/ ft³
40-46
45
49
Specific heat
Btu / lb-°F
.25
.22
.25

28. EXAMPLE OF DRY DESICCANT DESIGN
Feed rate
50 MMscfd
 Molecular weight of gas
17.4
 Gas density
1.7 lb/ft³
 Operating temperature
110 °F
 Operating pressure
600 psia
 Inlet dew point
100 °F
(equivalent to 90 lb H2O / MMscf )
 Desired outlet dew point
1 ppm H2O


29. SOLUTION
Assume an 8-hours on stream cycle with
6 hours regeneration
 Water absorbed
=(8/24)*50 MMscfd * 90 lb/MMscf
=1,500 lb H2O/ Cycle
 Loading
Use sorbeads as a desiccant and use
design loading = 6%


30. 1500 lb H2O
.06 lb H2O / lb desiccant

= 25,000 lb desiccant per
bed

25,000 lb desiccant per bed
49 lb desiccant / ft³
= 510 ft³ per bed

31. Tower sizing
Assume Z = 1
From chart max Vm = 55 ft / min

d²= 3600*(50*570*1)/(55*600)
d = 55.7 in = 4.65 ft
The bed height is :
L= 510 ft³ / ((π*4.65²)/4) ft²
L= 30 ft

32. The pressure drop assume ⅛ - in .bead
and μ=0.01 cp
∆P =
[(0.056*0.01*55)+(0.00009*1.7*55²)]*30
∆P = 14.8 psia > 8 psi
 this is more than the recommended 8
psi
 Choose dia. Of 5 ft 6 in
Vm = 39.2 ft/min
L = 21.5 ft/min
∆P = 5.5 psi < 8 psi accepted


33. 

Leaving 6 ft above and blow the bed , so
the total length including the space to
remove the desiccant and refill would be
about 28 ft
So L/D = 28/5.5 = 5 >2.5 accepted
Regeneration heat requirement
Assume the bed and the tower is heated
to 350 °F , so the average temperature will
be (350+110)/2 = 230 °F
The approximate weight of the 5.5 ft *28 ft
tower is 53,000 lb including the shell ,
head , nozzels and support for the
desiccant


35. Q = m Cp ∆T
Heating requirement /cycle
Desiccant
Tower
Desorb water
25000 lb*(350-110)*0. 25 = 1,500,000 Btu
53000 lb*(350-110)*0. 12 = 1,520,000 Btu
1,500 lb *1,100 Btu/lb
1500 lb*(230-110)*1
= 1,650,000 Btu
= 200,000 Btu
4,870,000 Btu
+10% for heat losses ,etc
490,000 Btu
total heat = 5,360,000 Btu / cycle



0.12 specific heat of steel
The number 1100 Btu/lb is the heat of water desorption ,
value supplied by the desiccant manufacturer
The majority of the water will adsorb at the average
temperature . This heat requirement represent the sensible
heat require to raise the temperature of the water to the
adsorption temperature

37. Cooling requirement / cycle
Desiccant 25000 lb*(350-110)*.25 = 1,500,000 Btu
Tower
53000 lb*(350-110)*.12 = 1,520,000
Btu
3,020,000
Btu
+ 10 % for non-uniform cooling ,etc
300,000
Btu
Total cooling heat =3,320,000 Btu /cycle
 Regeneration gas heater
Assume the inlet temperature of the regeneration
gas is 400 F , The initial outlet temp. of the bed
will be temperature of 110 F , outlet temp will be
the designed value of 350 F , So the average
outlet temperature will be (350+110)/2 =230 F


39. 
Then the volume of the gas required for the
heating will be V heating
5,360,000 Btu/cycle
(400-230) °F*0.64 Btu/lb/°F
V heating = 49,400 lb/cycle
QH is then :
QH =49,400*(400-110) °F*0.62
=8,900,000 Btu/cycle
The regeneration gas heater load
For design , add 25 % for heat losses and non-uniform
flow . Assuming a 3-hours heating cycle , the
regenerator gas heater must be sized for
QH=8,900,00*1.25/3 =3,710,000 Btu/hr
0.62 →specific heat of gas at average
temperature

41. 
Regeneration gas cooler

The regeneration gas cooling load is
calculated assuming that all the adsorbed
water is condensed during a ⅟2 hr of the 3
hrs cooling cycle
Regeneration gas
49,400(230-110)*0.61/3 = 1,205,000
Btu/hr
Water
Btu/hr
1,500*(1,157-78) / 0.5
= 3,237,000
4,442,000
Btu/hr
+10 % losses
Btu/hr
444,000
Total cooling load Qc =4,886,000 Btu/hr

43. 
Cooling cycle
V cooling
=
3,320,000 Btu/cycle
(230-110)°F*0.59 Btu/lb/°F
V cooling
= 46,900 lb/cycle
0.59 → Specific heat at average
temperature

44.  References
Chapter 8
Gas dehydration
Book Gas Reservoirs

45. THE END