Post on 27-Jul-2020
Cyclic Separation: New trays and Experimental Results
EFCE WP Fluid SeparationsTechnical Meeting #58, May 12, 2016
Copenhagen, Denmark
Abildskov J., Bay Jørgensen S.CAPEC‐PROCESS
Department of Chemical and Biochemical EngineeringTechnical University of Denmark
Outline
Cycled Trays Operation‐ Theoretical Performance: Lewis Case II analogy
‐ Early realizations; Simultaneous Draining
‐ Simultaneous Draining; Problems
‐ Ideas to approach theoretical performance
New Principles of Design and Operation
New Trays and Sequential Tray Draining
New Realization
Results
Future Directions
Conclusions
Other Explorations
• AIChE Spring Meeting 2016
• C. Knoesche, J. Paschold, S. Buetehorn(BASF):
• Paper 87b: “Cyclic Operation of Distillation Columns”
• Maleta CD
Theoretical Performance:
Lewis Case II Analogy
Theoretical Performance:
Lewis Case II Analogy
• Performance:
• (Convenience of) realization:
CASE ”I” ”II” ”III”
εtray/εpoint 1.7 2 1.5
”II” > ”I” > ”III”
”III” > ”I” > ”II”
“ … (Lewis’ case III) …. is probably the most commonin conventional operation because of designers’ understandablereluctance to install downcomers in the configuration of case II”
(Sommerfeld et al., 1966)
εpoint → 1
Theoretical Performance:Lewis Case II Analogy;
Conventional Operation with Downcomers
yn-1(z)
yn (z)
z=0
xn(0) = xn+1(z0)
xn(z0)
z=z0
z: horizontal tray positionz0: length of liquid flow
x n(z)
z
xn(0) = xn+1(z0)
xn(z0) = xn‐1(0)
xn(0) = xn+1(z0)
xn(z0) = xn‐1(0)
z00
→ Liquid in down‐comer from n+1 enters plate n at z = 0: xn(0) = xn+1(z0)
→ No liquid back‐mixing; progressive depletion of volatile component by contact with vapor from plate n – 1
→ z = z0: xn(z0) is discharged to n – 1
Theoretical Performance:Lewis Case II Analogy;
Cyclic Operation without downcomers
x n(t)
t
xnL = xn+1
V
xnV = xn‐1
L
τ0
yn-1(t)
yn (t)
t=0
xnL =
xn+1V
xnV =
xn-1L
t=τ
Independent variable: tTime length: VFP = τ
→ No liquid phaseenrichment on plate nby liquid flowing down→ No liquid back‐mixing in time domain→ Depletion of volatile component occurs monotonically with time→ Perfectly mixed plate→ Amount of material dropped during LFP is exactly equal to one plate holdup
Theo
reticalPerform
ance:
Lewis Case II analogy
‐Sommerfeld(1966)
z: horizontaltray position
t: time of vaporflow period ofduration τ
x n(z)
z
xn(0) = xn+1(z0)
xn(z0) = xn‐1(0)
xn(0) = xn+1(z0)
xn(z0) = xn‐1(0)
x n(t)
t
xnL = xn+1
V
xnV = xn‐1
L
zn
τ
0
0
Early realizations; Simultaneous Draining• Three 1961 papers (Cannon)
• Exactly one plate holdup must be dropped during LFP
• Basis of the generation: Simultaneous Draining
• TP = VFP + LFP
• Modeling: McWhirter (1963)
Sommerfeld et al. (1966)
Chien et al. (1966)
Robinson/Engel (1967) Vapor Flow Period (VFP):
VF, VR, VT and VB are closed. VV is open.
Liquid Flow Period (LFP):
VT, VB, VF, VR are opened. VV is closed.
Simultaneous Draining; Problems
• Back‐Mixing
• Non‐uniform draining of traysSchrodt V. N. et al., 1967. Chem. Eng. Sci., 22, 759
Gerster J.A., Scull H.M., 1970. AIChE J. , 16, 108.
Larsen J., Kümmel M., 1979. Chem. Eng. Sci., 34, 455.
• Pressure Dynamics
• Liquid oscillations on trays
Approaches to Theoretical Performance‐ Improved Liquid Flow Control
• Matsubara• Collect liquid in a store, prior to transfer to the tray below.
• Vapor transferred in tubes from tray to tray
• Furzer (TD) Time delay trays• Gutter channels to delay the liquid draining
• Baffles to avoid oscillations
• Hentrich/Vogelpohl (1980)• 1st approach: Liquid was led into a storage tube; when all trays were empty, the liquid was released into the next tray. This was accomplished using a central rod in the column (EXPENSIVE).
• 2nd approach: Sieve tray with packing material below, to make the liquid drain more slowly during the LFP
• Baron: Stepwise Periodic Operations
• Maleta CD (21st century): Sluice chambers
New Principles of Design/Operation
Tray Design Principle: The periodic cycled separation tray should be constructed such that:
1) All the liquid leaves the tray and enters the tray below without being mixed with any other liquid during the liquid drainage
2) Vapor flow can continue during liquid drainage
Operational Principle for Sequential Tray Draining:To minimize mixing with liquid from other trays enforce a special
operational principle for Sequential Tray Draining:
The trays are emptied sequentially rather than simultaneously, while the vapor flow is maintained
throughout
New Realization; Trays
Cyclically Operated Perforated Sheets (COPS)
Layer of structured packing for liquid oscillation damping
Sulzer MellapakPacking
Pressure Control Valves
Sheets
Closed Open (for drainage)
When tray is closed: Free area due to perforation is 6%When tray is open: Free area near 100%
New Realization; Sequential Tray Draining
Sequential tray drainage operation for a column with three trays. When the top tray isempty (d) a load of liquid is fed to the top tray (e). The column remains in state (e) forthe remainder (if any) of the cycling period time until a new liquid drainage sequenceis initiated at the start of the next period (a) – (c).
PH3
PH2
PH1
P3
T6P4
On/off valveColumn
ProductTank
FeedTank
PumpsSide streamoutlet
Outlet
MeasurementsT1,T3: dry thermomentertemperatureT2, T4: wet thermometertemperatureT5: humidifier temperatureT6: column temperatureP1, P2: pressure difference across orificeP3, P4: pressure difference across top trayPH1, PH2, PH3: places for pH measurements
Outlet
Throttle
Humidifier
Hea
t Exchanger
PumpCentrifugal Blower
Orifice
P1 P2
T2T1
T4T3
drain
Ventilation
Steaminlet
Wateroutlet
Manual valve
Automatic valve
T5
WaterFeed
ProductReservoir
Control: G <‐ (P1,P2) <‐ Frequency ConverterT6 <‐ T5 <‐ T4 (Cascade)
Results; Point Efficiency, εt
• Strip NH3 from water (Gerster/Scull, 1970)
• Tray molar balance + Murphree expression:→ Slope of pH versus t gives εt
• Assume: Only NH3 transfer
• CO2 Controversy
y = 0.0295x + 0.7588
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
00 20 40 60 80 100
ε t
H/G (seconds)
Efficiency vs. Hold up/Vapor Flow
Efficiency vs. Hold up/VaporFlow
Linear (Efficiency vs. Holdup/Vapor Flow)
in out
t t
t
dxH = G(y ‐ y )dt
dx G dx G= ‐ε m x + ε m x = 0
dt H dt H
Hε = logc(t = 0) ‐ logc(t) ln10
Gmt
2H= pH(t = 0) ‐pH(t) ln10Gmt
G,yin(t)
G,yout (t)
H, x(t)
Results; Column Efficiency,
Experimental Procedure
id idp c
N NCE = , CE =
3 3
f p
p f
x (1 ‐ x )S =
x (1 ‐ x )
f
pid
1 x 1ln ︵1- ︶ +λ x λ
N =lnλ
mG, λ =L
G
L
L ρFP =
G ρ
Measure(xf, xp) by
titration
ObtainStationaryPeriodicState
Set (L,G)
16656.695‐
Tm = 0.073×10
(Gerster,1958)
Results; Separation Factors
Stream Limits
G (mol/s) 5‐10
L (mol/s) 0‐20
0
5
10
15
20
25
30
35
40
1.E‐03 1.E‐02 1.E‐01 1.E+00
Separation Factor, S
FP
Sieve Trays Periodic Trays Packings (Sulzer BX gauze)
Results; Column Efficiencies
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.E‐03 1.E‐02 1.E‐01 1.E+00
Column Efficiency
FP
Sieve Trays Periodic Trays
Results; Influence on Tray Efficiency of Flows
y = 0.0055x + 0.7921
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
0 10 20 30 40 50 60 70
Column Efficiency, C
E p
H/G (seconds)
CEp vs. H/G
Column Efficiency vs. H/G
Linear (Column Efficiency vs. H/G)
No points at H/G > 70 …….!
Results; Influence on Column Efficiency of Period Lengths
• Maximum?
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 50 100 150 200 250
CE p
= Nid / 3
TP (seconds)
CEp vs. TP
TP: Total Period Duration
f
pid
id
1 x 1ln ︵1- ︶ +λ x λ
N =ln ︵λ ︶
λ= mG / LExperimental data make N go thru extremum
6.9 < G(mol/s) < 8.42 < L (mol/s) < 8.8
Why optimum TP?
0.E+00
1.E‐04
2.E‐04
3.E‐04
4.E‐04
5.E‐04
6.E‐04
0.E+00 5.E‐04 1.E‐03
y
x
Henry's law
1.1666
Equilibrium Curve:y* = x∙mOperating Line:y = (x‐xp)∙(L/G)
TP↑ ~ L↓~ (L/G)↓ ~ Nid ↓
BUT if
xp ↓ Nid ↑
This may compensate, if xpgets sufficiently low. Thisseems to be the case at low TP values, but only up to 80 seconds.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 50 100 150 200 250
CE p
= Nid / 3
TP (seconds)
TP/CEp
Blue: G = 7dP = 50
Red: G = 8.3dP = 60
Further Work
• Operation at H/G beyond 70 seconds?
Latest Results
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
00 20 40 60 80 100
CE p
= Nid/3
H/G
Spring 2016
Previous
Linear (Spring 2016)
Conclusions
• Presented:
– New Tray Design (Principle): COPS
– New Operational Principle: Sequential Draining
– Steady Operation: Avoiding Pressure Dynamics and Liquid (Back) Mixing
• Future Directions:
– Operation at higher (periodic) tray efficiencies