is introduced at only one point at the top of a packed bed, it will tend to remain well segregated from the vapour as it flows down the column. It naturally takes (or even creates) the flow path of least resistance. Likewise, the path of least resistance for the vapour is where there is little or no confronting liquid counterflow. As liquid and vapour flow through the column, they tend to segregate even after great pains have been taken to ensure these phases were introduced with very uniform distribu- tion. This is why liquid redistributors can be such a critical part of a packed column.4 There are numerous causes of maldistributed flows. These include: Introducing vapour at high velocity through a single feed pipe at the base of a large diameter column (causing mald - istributed vapour). Careless dumping of random packing into the column, leading to large voids. Using packing that is too large for the diameter of the column, causing preferential flows through the high void - age areas adjacent to the column walls. The conventional guideline that D/d >8 gives the maximum packing size (d) relative to the column diameter (D) to ensure negligible wall flow is, in our experience, optimistic – a value of 12-15 is minimal for surety. Wrong distributor type for the liquid flow rate. Poorly designed liquid distribution.4 Out-of-level distributor. Lack of liquid redistributors (dumped packings) or wall wipers (structured packings). Packed bed too deep (single beds should not exceed L/D = 15-20 without liquid redistribution).
Gas and solvent conditions for case study
Tail gas
Solvent
Flow
10 MMSFCD
150 USgpm
Pressure (psig) Temperature (F) MDEA (wt%)
1
1
100
110
—
45
CO 2 H2S
5 mol%
0.005 Loading 0.001 Loading
1.5 mol% 93.5 mol% Saturated
N 2
—
H2 O
Remainder
Table 1
Case study The TGTU absorber that forms the basis for this study con - tains 15ft of IMTP-40 random packing and uses 45 wt% methyldiethanolamine (MDEA) solvent. Other data are shown in Table 1 . This packing depth maximises CO2 slip at almost 93% while removing H2S to 85 ppmv. Although a depth of 40ft will remove H2S to 31 ppmv, it will lower CO2 slip to only 82%. However, whether 15 or 40ft of packing are used, the conclusions reached in this study are qualita- tively unaffected. To handle the flows, the column needs to be about 5.3ft in diameter (65% flood). With the bed depth (15ft) being less than three times the column diameter, no redistributor is needed. Two scenarios are considered: in the first, varying but specific degrees of excess gas flow rate are assumed to pass through part of the cross-sectional area of the absorber and a deficit through the remainder. Here, two cases of gas maldistribution are considered: one in which half the column area carries the excess, and the other
3
psig F mol % MMSCFD
P T
0.9
To incinerator
110.4
1
4.34
Carbon dioxide
Solvent
Flow
7.2
psig F mol % MMSCFD
P T
0.9
Hydrogen sulphide
ppmv 32.93
Absorber 5.312’ diam DP = 0.130 psi
110.9
10.0 4.37
Carbon dioxide
Flow
psig F mol % MMSCFD
P T
0.9
Hydrogen sulphide
ppmv 85.01
109.4
Tail gas
4.4
Carbon dioxide
11
2
Parallel absorber 1 4.600’ diam 3/4 area DP = 0.1 28 psi
To regeneration
4
Flow
10.0
Hydrogen sulphide
ppmv 3057.72
psig F Loading USgal/min
P T
1.0
Original absorber
108.7 0.017 149.7
Carbon dioxide
psig F Loading USgal/min
P T
1.0
15
Flow
109.6 0.015 144.3
12
Hydrogen sulphide 0.052 Loading
7
Carbon dioxide
Incineration
Flow
9
Hydrogen sulphide 0.040 loading
psig F mol % MMSCFD
P T
0.9
106.7
12
2.7 4.5
Carbon dioxide
14
Regen.
Flow
psig F Loading USgal/min
P T
1.0
Hydrogen sulphide 10997.3 ppmw
16
109.3 0.016 149.8
Carbon dioxide
Divider
Flow
Solvent
13
Hydrogen sulphide 0.041 loading
8
5
Parallel absorber 2 2.656’ diam 1/4 area DP = 0.128 psi
3.7% of liquid total ow
psig F Loading USgal/min
P T
1.0
103.8 0.046
2.5% more vapour ow to 1/4 of column area. 2.5% less vapour ow to 3/4 of column area.
Carbon dioxide
6
Tail gas
Flow
5.5
Hydrogen sulphide 0.085 loading
Divider
10
27.5% of gas ow 2.5% excess ow
13
Figure 1 Parallel column model for simulation with 2.5% excess gas flow to ¼ of absorber area
23
Gas 2025
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