preparing solubility data for use by the gas processing...

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

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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

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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)

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Approach

4. Generalize the data and simplify – Ignore minor variables – Group data that are similar

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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

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9 Lb MeOH per MMSCF Gas per wt% MeOH in Aq Phase

Pres

sure

(psi

a)

Degr. F

Solubility of Methanol in the Natural Gas Phase. EDB Graph (12th)

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Compare with new data. Red & Green lines are traces of current figure 20-65. Red and Green Points are expt’l data

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

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Solubility of methanol in hydrocarbon liquids (975-7)

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Mol

% M

eOH

in H

C Li

quid

Temperature (F)

Wt% MeOH in Aq Liquid

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:

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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

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Methanol solubility in liquid hydrocarbons: Proposed representation of data. Groups all hydrocarbons except toluene (aromatics).

1

10

100

1000

-50 -30 -10 10 30 50 70 90 110 130

Temperature (oF)

Figure 2. Liquid-Liquid Methanol Distribution Ratios.

No Toluene

28-33mol% Toluene

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.

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)

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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

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Compare with new data. Purple points are expt’l data (scf / gallon) at the T & Pi of the red data point.

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

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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

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0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Mod

el o

r Sim

ulat

or P

redi

cted

H2S

con

cent

ratio

n in

Liq

uid

(mol

e fr

ac)

Source Data H2S concentration in Liquid (mole frac)

Figure 9. Solubility of H2S in TEG: Correlation of Reference 2 data with the Model-Generated and Process Simulation Software-Generated Data.

Model

Simulator

SourceData

Correlation plot of model to data Simulation software VLE method: Peng-Robinson

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0.001

0.01

0.1

1

60 80 100 120 140 160 180 200

CO2

Solu

bilit

y, s

cf C

O2

/ ga

llon

TEG

sol

utio

n

Temperature, °F

FIGURE 10. Approximate Solubility of CO2 in Triethylene Glycol at 50 psia vs. Temperature, H2S Content of Gas Phase, and Water Content of TEG

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

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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

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Solubility of Hydrocarbons in Glycols

Benzene, 0 w% Water

Toluene, 0 w% Water

Ethyl Bz, 0 w% Water

o-Xylene, 0 w% Water

Benzene, 5 w% Water

Toluene, 5 w% Water

Ethyl Bz, 5 w% Water


Benzene, 1 w% Water

Toluene, 1 w% Water

EthylBz, 1 w% Water


0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

70 80 90 100 110 120 130

Equi

libriu

m R

atio

y i/x

i

Temperature (°F)

Figure 4.3. Equilibrium Ratios for BTEX in TEG Typical Contactor Conditions

DATA FROM RR-131. PRESSURE: 1000 psiaWATER CONCENTRATION IS FOR GLYCOL LIQUID PHASE

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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

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• 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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0.1

1

10

50 70 90 110 130 150 170 190 210 230 250

Solu

bilit

y Li

mit

of T

olue

ne (s

cf V

apor

/ G

allo

n So

luti

on)

Temperature °F

Figure 4.7. Solubility Limit of Benzene in Amine Solutions. Liquid-Liquid Solubility Data and Model.

70 wt% DGA

50 wt% MDEA

46 w% DGA

35 wt% DGA

25 wt% MDEA

DATA POINTS FROM RR-180 and TP-29.LINES GENERATE USING EQN. 1.PRESSURE RANGE OF DATA: 72 - 732 PSIA

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• VLE data (subsaturation)

– Less data available – Represented as equilibrium ratio (K) values

• Rough approximation: base subsaturation solubility on solubility limit data

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Generic graph showing solubility limit & subsat

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Conc

entr

atio

n of

hyd

roca

rbon

in a

min

e (a

queo

us)

phas

e

Partial Pressure of Hydrocarbon

Figure 16. General Approximation of Hydrocarbon Solubility.

Constant Temperature

xi,s Solubility Limit

P i*

Satu

ratio

nPr

essu

re

Two Liquid Phases

One Liquid Phase (Aqueous)


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Hydrocarbon Amine Solution Temperature Pressure K K(F) (psia) Experimental Eq. 4.6

Toluene 50 w% MDEA 140 73 15.6 12.9Toluene 50 w% MDEA 140 1019 3.9 0.9Cyclohexane 50 w% MDEA 140 73 316.6 296Benzene 50 w% MDEA 140 73 19.7 16.9

Table 4.3. Experimental K values compared with Estimations from Equation 4.6

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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