PREPARING SOLUBILITY DATA FOR USE
BY THE GAS PROCESSING INDUSTRY:
UPDATING KEY RESOURCES
Darryl Mamrosh and Kevin Fisher
Trimeric Corporation
Jeff Matthews
URS Corporation
Mapping Midstream’s Future
2012 GPA Convention
Background
• GPA sponsored a number of projects for the
collection of VLE and LLE data to describe
solubilities of interest to gas processors:
– Methanol in natural gas and liquid phases
– CO2 and H2S in TEG and EG
– Hydrocarbons (BTEX) in Amines & Glycols
• GPSA Engineering Data Book (EDB)
– These topics in EDB have not been updated to the
most recent data
2
Background
• This project: create models, graphs, etc., as
updated content for EDB, and make the data
readily usable to GPA members
• Projects 975-5, 975-7, 975-8
• This work focuses on simple, generalized
representations of data
– Use for quick process calculations
3
Summary
• Topics will be covered separately
– Solubility of methanol in natural gas and liquids
– CO2 and H2S solubility in glycols
– Hydrocarbon solubility in amines & glycols
• To Include
– Review of current EDB (12th) content
– Treatment of new data
– Using the information
• Not all is covered: The paper available on-line
4
Solubility: What does it mean?
• Concentration, at specified conditions of
equilibrium, of a solute dissolved in a solvent.
• In gas processing:
– Concentration of a solute in a gas in equilibrium
with a liquid phase
– Concentration of a light solute in a liquid in
equilibrium with a gas phase
– Concentration of a solute in a liquid in equilibrium
with another liquid phase
• Always refers to an equilibrium condition
5
Approach
1. Use common terminology and units for
expressing solubility concentrations
2. When feasible use same formats as in
existing EDB content
3. When appropriate, create a mathematical
model of data. If not, create a graph and/or
table.
•
Models should be easy to use (not EOS for simulation
software)
6
Approach
4. Generalize the data and simplify
– Ignore minor variables
– Group data that are similar
7
Solubility of methanol in natural gas
and HC liquids: GPA Project 975-7
Importance of Data
• Methanol used in hydrate inhibition, dehydration,
sweetening
• In Hydrate inhibition:
– Predict loss of injected methanol in natural gas phase
– Predict loss of injected methanol in liquid hydrocarbon
phase (if present)
 Material balance calculations used to predict methanol
injection rate required to prevent hydrates
8
Degr. F
Pressure (psia)
Solubility of
Methanol in
the Natural Gas
Phase.
EDB Graph
(12th)
Lb MeOH per MMSCF Gas
per wt% MeOH in Aq Phase
9
Compare with
new data.
Red & Green
lines are traces
of current
figure 20-65.
Red and Green
Points are
expt’l data
10
Solubility of methanol in natural
gas: Treatment of new data
• Variables:
– Methanol content of aqueous phase
– Temperature
– Pressure
• Consistent simple model not developed
• Conclusion:
Produce update of EDB Figure 20-65
11
12
Solubility of methanol in
hydrocarbon liquids (975-7)
Mol% MeOH in HC Liquid
Wt% MeOH in
Aq Liquid
Temperature (F)
13
Solubility of methanol in
hydrocarbon liquids: Treatment of Data
• Variables:
–
–
–
–
Methanol content of aqueous phase
Temperature
Composition of HC liquid phase
Pressure – small impact (ignored)
• Express in terms of distribution ratio:
14
Solubility of methanol in
hydrocarbon liquids: Treatment of Data
Liquid hydrocarbon phase composition
– Typical hydrocarbon components had minor effect
on solubility
– Toluene had a significant effect
• Implies that all aromatics will probably be signficant
15
Methanol
solubility in
liquid
hydrocarbons:
Proposed
representation
of data.
Groups all
hydrocarbons
except toluene
(aromatics).
Figure 2. Liquid-Liquid Methanol Distribution Ratios.
1000
100
No Toluene
28-33mol% Toluene
10
50-70 mol% Toluene
70-80 mol% Toluene
Distribution of methanol between aqueous
and hydrocarbon phases, data from various
sources. Hydrocarbon phases includes various
alkane and cycloalkane compounds. Data
shows the variation of distribution with
changes in the amount of toluene in the
hydrocarbon phase.
1
-50
-30
-10
10
30
50
70
90
110
130
Temperature (oF)
16
Solubility of CO2 and H2S in
TEG & EG: Project 975-8
Importance of Data
• H2S and CO2 in gases being dehydrated can dissolve
in the solvent under pressure and then be released
to the gas phase during flash and regeneration steps
• Important for a few reasons:
–
–
–
–
Product Quality
Environmental
Safety
Design of equipment (e.g., flash drum)
17
Current EDB
Content.
Solubility in TEG
as function of
temperature
and H2S or CO2
partial pressure
Solubility
defined in
terms of std vol
of gas per gallon
solvent
18
Compare with
new data.
Purple points
are expt’l data
(scf / gallon) at
the T & Pi of
the red data
point.
19
Solubility of CO2 and H2S in
TEG & EG: Treatment of Data
• Modeling versus graphing
– The new data was not amenable to parametric
graphing
– Various mathematic models were attempted
• Final model based on one used by source data
authors
20
Solubility of CO2 and H2S in
TEG & EG: Treatment of Data
•
•
•
•
•
•
•
Pi is the partial pressure of the acid gas (component i: CO2 or H2S)
Pi = yi P
yi is the mole fraction of acid gas in the vapor phase
xi is the mole fraction of the acid gas in the liquid phase
T is the absolute temperature
xH2O is the mole fraction of water in the liquid phase
P is the absolute pressure
Different A, B, C, D constants for each of the four systems based on data fit
xi can be used to calculate the solubility of CO2 or H2S in units
of std vol of gas per volume of solvent
21
Figure 9. Solubility of H2S in TEG: Correlation of Reference 2 data with
the Model-Generated and Process Simulation Software-Generated Data.
Correlation
plot of model
to data
Simulation
software
VLE method:
Peng-Robinson
Model or Simulator Predicted H2S concentration in Liquid (mole frac)
0.35
0.30
0.25
0.20
Model
Simulator
0.15
SourceData
0.10
0.05
0.00
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Source Data H2S concentration in Liquid (mole frac)
22
FIGURE 10. Approximate Solubility of CO2 in Triethylene Glycol at 50 psia
vs. Temperature, H2S Content of Gas Phase, and Water Content of TEG
CO2 Solubility, scf CO2 / gallon TEG solution
1
0.1
0.01
0.001
60
80
100
120
140
Temperature, °F
160
180
200
23
Solubility of CO2 and H2S in
TEG & EG
Uses of Data
• Charts and models can be used to estimate
the equilibrium content of gases in glycols
– Can be used to estimate the uptake of CO2 and
H2S in TEG contactor, and the subsequent release
to the gas phase in the flash and regeneration
• Can also use as a check of other models (e.g.,
simulations)
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Solubility of Hydrocarbons in
Glycols and Amines
• Project 975-5
• Vapor-liquid and liquid-liquid equilbria
• Data collection focused on BTEX compounds,
and the differences between similar aromatic
and non-aromatics
25
Solubility of Hydrocarbons in
Glycols and Amines
Importance of Data
• Predict hydrocarbon absorption from gas
during amine and glycol treating
• Vaporization during flash and regen
– Equipment design
– Product loss
– Environmental (VOC, BTEX)
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Solubility of Hydrocarbons in
Glycols
• VLE data for BTEX distribution between TEG
and gas phase
– Bulk gas phase: methane
– Data taken at conditions typical of TEG dehy
regenerator, contactor, and flash drum
– Use equilibrium ratios (K values) to represent data
Ki = yi / xi
27
Solubility of Hydrocarbons in
Glycols
Figure 4.3. Equilibrium Ratios for BTEX in TEG
Typical Contactor Conditions
0.1
DATA FROM RR-131.
PRESSURE: 1000 psia
WATER CONCENTRATION IS FOR
GLYCOL LIQUID PHASE
0.09
Benzene, 5 w% Water
0.08
Equilibrium Ratio yi/xi
0.07
Toluene, 5 w% Water
Benzene, 1 w% Water
0.06
Benzene, 0 w% Water
Ethyl Bz, 5 w% Water
0.05
Toluene, 1 w% Water
Toluene, 0 w% Water
o-Xylene, 5 w% Water
0.04
EthylBz, 1 w% Water
0.03
Ethyl Bz, 0 w% Water
o-Xylene, 1 w% Water
0.02
o-Xylene, 0 w% Water
0.01
0
70
80
90
100
Temperature (°F)
110
120
130
28
Solubility of Hydrocarbons in
Amines
• Two data groups:
– LLE measurements “solubility limit”
– VLE measurements “subsaturation solubility”
• Majority of data taken via LLE measurements
– Can be used directly for binary interaction
parameters
– Maybe not as directly useful for most gas treating
calculations
29
Solubility of Hydrocarbons in
Amines
• Solubility limit data (LLE)
– Solubility in terms of SCF vapor / gallon solution
– Variables: Temperature, amine, hydrocarbon, water
content of amine
– Little variation with pressure above solubility limit
• Solubility limit data can be modeled
S is the solubility limit (SCF/gallon sol’n), W is the amine
concentration, A, B, C values for each amine – hydrocarbon
pair
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Solubility of Hydrocarbons in
Amines
Figure 4.7. Solubility Limit of Benzene in Amine Solutions.
Liquid-Liquid Solubility Data and Model.
Solubility Limit of Toluene (scf Vapor / Gallon Solution)
10
DATA POINTS FROM RR-180 and TP-29.
LINES GENERATE USING EQN. 1.
PRESSURE RANGE OF DATA: 72 - 732 PSIA
70 wt% DGA
50 wt% MDEA
46 w% DGA
35 wt% DGA
25 wt% MDEA
1
0.1
50
70
90
110
130
150
170
190
210
230
250
Temperature °F
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Solubility of Hydrocarbons in
Amines
• VLE data (subsaturation)
– Less data available
– Represented as equilibrium ratio (K) values
• Rough approximation: base subsaturation
solubility on solubility limit data
32
xi,s Solubility Limit
Generic graph showing solubility limit & subsat
Pi* Saturation Pressure
Concentration of hydrocarbon in amine (aqueous) phase
Figure 16. General Approximation
of Hydrocarbon Solubility.
Solubility
of Hydrocarbons
in
Glycols and Amines
One Liquid
Phase
(Aqueous)
Two Liquid
Phases
Constant Temperature
Partial Pressure of Hydrocarbon
33
Solubility of Hydrocarbons in
Amines
Table 4.3. Experimental K values compared with Estimations from Equation 4.6
Hydrocarbon Amine Solution Temperature Pressure K
K
(F)
(psia) Experimental Eq. 4.6
73
15.6
12.9
Toluene
50 w% MDEA
140
Toluene
50 w% MDEA
140
1019
3.9
0.9
Cyclohexane 50 w% MDEA
140
73
316.6
296
16.9
Benzene
50 w% MDEA
140
73
19.7
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Conclusions
• See reports (presentation and Research
Reports) for:
– More data and details, graphs
– Discussion of general trends
• Research reports contain reference for the
source data
• Updated content submitted for consideration
for next version of EDB
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