The following article was selected for presentation at

GLYCOL RECLAIMER

K. Dave Diba, M. Guglielminetti, S.Schiavo COMART Engineering & Contracting

Copyright OMC 2003 This paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 26-28, 2003. It was selected for presentation by the OMC Programme Committee following review of information contained in the abstract submitted by the author. The Paper as presented at OMC 2003 has not been reviewed by the Programme Committee.

ABSTRACT All glycols used for dehydration by absorption in counter current Tri-ethylene glycol (TEG), and Mono-ethylene glycol (MEG) used for glycol injection for hydrate prevention will require reclaiming. The intervals at which reclaiming is required will vary from a month or two for very foul solutions to a year or more for those that are properly conditioned in the course of operation. It should be remembered that glycol solutions are small in quantity compared to the multi-millions cubic meter of gas and condensate that they contact. Contaminants such as salts, lube oil, hydrocarbon condensate, crude oil and corrosion products are present in abundance in liquid phase and in minute “trace” amount in the gas; and they continually plague operators in their glycol systems, and the symptoms of these problems are fouled equipment, foaming in the system resulting in expensive glycol losses, and insufficient process capability. Shutting a plant down to thoroughly clean and repair all the equipment is a periodic necessity and very expensive. MEG catches most contaminants and they accumulate in the plant’s glycol inventory until they create severe operational problems. Most operators drain the contaminated glycol charge and discard it, replacing it with a charge of new glycol. This begins a new cycle that ends in the same way month(s) later. Other plants lose so much contaminated glycol in “normal operations that replacing the losses with new glycol keeps the contamination at a tolerable level due to continuous addition of “makeup” glycol. INTRODUCTION It is the purpose of this paper to review the contaminants, the basic technology available, and the facilities available for reclaiming glycols. Contaminants Principal among these are

1. Organic acids 2. Inorganic acids 3. Iron carbonates and iron sulfides, products of corrosion caused by CO2 and H2S

in the feed gas. 4. Decomposition products from glycol degradation, and heat stable salts. 5. Coke formed by thermal decomposition of heavy hydrocarbons in glycol

emulsion. 6. Crude oil, condensate and compressor lube oil in glycol emulsion. 7. Aromatic hydrocarbons that dissolve in glycols. 8. Salts, and total solids from entrained formation water and condensate.

Special attention must be given to the chemical and physical properties of these contaminants. All reclamation technology must be based on differences in these properties in order to achieve good separation from glycol that carries them in solution or suspension in the reclaimer feed.

RECLAIMER PRINCIPLES

Two basic principles have been applied to glycol reclaiming. The first is electro-dialysis. Since the glycols are non-polar, the feed must be diluted with water to at least 30 WT% or more water to enable the process to operate and transfer the chloride ion through the membrane in exchange for hydroxyl ion. The product, chloride free glycol, must be distilled to remove the excess water. Also In this application careful cleaning of feed is absolutely essential if the membranes are to remain operable. The energy requirement, cleaning and maintenance cost for this method is very high.

The second is vacuum distillation where heat and vacuum are used to boil off all glycol and lighter components, leaving as residue the salts, other heavy high boiling point tars and degradation materials. This paper will be devoted to requirement for complete glycol reclaimer which employs vacuum distillation.

This process comprises three essential operations arranged in the following sequence prior to reclaimer.

1- Removal of all solids. This step is accomplished through a properly designed cartridge

filter(s) in the glycol regeneration skid. 2- Breaking of oil in glycol emulsions and removal of most of aliphatic condensate. This

step is accomplished through a properly designed horizontal three phase separator (AKA flash tank) in the glycol regeneration skid.

3- Distilling off the light ends, water and organic acids. This step is accomplished through a properly designed glycol reboiler, still column, and reflux condenser in the glycol regeneration skid.

The glycol reclamation is carried out on batch or continuous basis; this depends on the amount of impurities added to the glycols during the plant operation. The major contributor of glycol fouling is soluble salts carried into the glycol system. Therefore, gas dehydration by TEG requires less frequent reclamation since very little soluble salt is carried into the TEG dehydration, the presence of acid gases and TEG degradation will determine how often the total TEG solution should be reclaimed (batch); or if the fouling of TEG is very severe due to any or combination of above sources of contaminants, then a slip stream (continuous) of TEG to be reclaimed. Note: Generally the continuous TEG reclamation is carried out only in those gas dehydration

application with high acid gas(s) content. In MEG process for hydrate prevention (wet gas evacuation) the presence of formation water salts, corrosion inhibitors, surfactants, heavy hydrocarbon liquid, and if sour gas(s) are more commonly present; their accumulation in the MEG solution will increase very rapidly; therefore, this necessitate a slip stream (continuous) MEG reclamation. This reclamation rate varies between 3% lean MEG circulation for moderate salts accumulation up to 20% of lean MEG circulation for heavy salts accumulation. RECLAIMER DESIGN The COMART designed reclaimer (please refer to figure 1) is based on feeding the slipstream of regenerated contaminated glycol to the reclaimer with major equipment comprising of:

1- Vacuum Reboiler: The contaminated glycol enters vacuum reboiler; where for TEG operation the normal operating conditions of vacuum reboiler is maintained at 50 to 60mm HG absolute pressure and 204C. For MEG operation the normal operating conditions of

vacuum reboiler is maintained at 80 to 90mm HG absolute pressure and 160C. The dissolved salts and most of the other degradation materials have no impact on the reclaimer vacuum reboiler duty (since they do not vaporize and their additional heating requirement for raising their temperature to the operating temperature of vacuum reboiler is negligible). The heating can be accomplished by direct fired, electrical, or hot oil/steam reboiler.

2- Vacuum Pump: The vacuum is generally maintained by liquid ring pump(s). 3- Vapour Condenser: The overhead vapours (reclaimed glycol and water) are condensed by

a vapour condenser into an accumulator vessel. The condenser can be aerial, liquid cooled, and in colder environment finned pipe alone can accomplish the condensation.

4- Booster Pumps: The condensed vapours from accumulator vessel are pumped back into the suction of main glycol pump on the glycol regeneration skid or to the lean glycol storage tank.

5- Brine/Degradation Materials Accumulator: The accumulated salts and degradation materials are periodically drained into a lower barrel below the vacuum reboiler, from which they are discharged (pumped) into waste storage vessel.

Notes: A- Prior to sending the reclaimed glycol back to the suction of main glycol pump on the

glycol regeneration skid or to the lean glycol storage tank, the pH of the reclaimed glycol should be in the range of 6.5 to 7.5, if it is outside of this range then its pH should be adjustment with a appropriate pH control chemical.

B- Disposal Of Waste: Correct disposal of effluent sludge produced by reclaimer (i.e. salts, glycol degradation materials, high boiling point hydrocarbons, sulfur compounds, etc.) must be considered for environmental conditions with respect to location of disposal sites and the plant location. Generally any solids settled in the bottom of the transportation tank is deposited in a regulated hazardous land fill site. The liquid mixture is incinerated by an authorized Hazardous Waste Incinerator with electrostatic precipitator. The recovered ashes can be sent as a non hazardous waste to any land fill.

CONCLUSION The glycol reclaimers have been used for numerous severe applications with great results in saving for glycol losses, lower maintenance, and meeting process requirement continuously. It has been shown with on-stream reclaimer systems the glycol systems have not been shut down for cleaning or repairs for several years. These units have been designed and built as mobile trailer mounted self contained system for transportation with small truck to different location (Please see attached photograph); or as a dedicated skid mounted plant in one location. The same reclaimer can handle both MEG and TEG (separately) on batch or continuous mode. Few example cases are described below:

1- Coke oven gas dehydration (Indiana, USA) contained high concentration of naphthalene, and high molecular weight hydrocarbon, causing glycol to become very viscous and difficult to flow through the dehydration system and gas dehydration had to be stopped every three weeks intervals for cleaning and changing the glycol. Since the installation of the reclaimer in 1984 no unscheduled stoppage has occurred.

2- Sour gas dehydration (Texas, USA) contained high H2S and CO2 acid gases. Since the reclaimer started running continuously the glycol losses and plant equipment cleanup has been reduced below normal standard.

3- Mobile glycol reclaimer (Pennsylvania, Ohio USA) were used to clean stored contaminated glycol, or used on operating plants. In these mobile units an atmospheric reboiler was “piggy backed” on top of vacuum reboiler for boiling off most of the “Rich” contaminated glycol water and light hydrocarbons prior to entering the vacuum reboiler.

4- Gas dehydration (Offshore Canada) the wet gas evacuation contained high salt content resulting in severe salt contamination in the equipment, in one instance due to some excess water carryover the salt content shut up to above 95000ppmw in glycol; with reclaimer operating and the lean glycol feed containing 95000ppmw salt the reclaimed glycol salt content was below 60ppmw.

ACKNOWLEDGEMENT The late L. S. Reid had worked in the development of this process, portions of the information contained in this paper have been drawn from his work.

1

TRIETHYLENE GLYCOL REGENERATION IN NATURAL GAS DEHYDRATION PLANTS: A STUDY ON THE COLDFINGER

PROCESS

F. Gironi, M. Maschietti, V. Piemonte, Università degli Studi di Roma “La Sapienza”, D. Diba, S. Gallegati, S. Schiavo, Comart SpA, Ravenna, Italy

This paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 28-30, 2007. It was selected for presentation by the OMC 2007 Programme Committee following review of information contained in the abstract submitted by the authors. The Paper as presented at OMC 2007 has not been reviewed by the Programme Committee.

ABSTRACT Natural gas pipeline transportation requires very low water content in the gas stream in order to avoid condensation or hydrate formation. To reach this goal, when triethylene glycol (TEG) is used to dehydrate natural gas, after the absorption step TEG must be regenerated to levels substantially above 98.5-99.0 % by weight available from atmospheric distillation of glycol-water mixtures. In order to regenerate TEG to higher purity levels some of the methods used require a stripping gas, a solvent or to perform the distillation under vacuum. A simpler method to perform a further dehydration of TEG is the use of a water exhauster, known as Coldfinger, where the vapour in equilibrium with the liquid to be dehydrated is continuously condensed and removed. In this work, the Coldfinger apparatus was modelled and a study on the most relevant operating parameters was carried out. A process simulation of a natural gas dehydration plant, provided with a Coldfinger water exhauster for TEG regeneration, was performed on a case study. It was shown that the dehydration process with Coldfinger unit is capable of reaching current water content specifications in a simple and echonomic way.

INTRODUCTION

Natural gas at the producing well contains significant quantities of water vapour. Typically, the gas is water-saturated at the condition of pressure and temperature of the well and a dehydration process is required. In fact, water content must be reduced in order to prevent liquid water condensation and hydrate formation in the pipeline transportation system. Nowadays, typical values of allowable water content in the gas transmission lines range from 70 to 120 mg/Nm3 [1].

Among methods available for natural gas dehydration, absorption by means of triethylene glycol (TEG) is one of the most common. Water removal from the gas stream takes place by means of countercurrent contact between the gas, fed to the bottom of a contactor tower, and TEG, which is a liquid with a great affinity for water, fed at the top of it.

The crucial part of the process is represented by TEG regeneration. If the water-rich TEG is distilled in a simple atmospheric column, TEG can not be regenerated to levels above 98.8-98.9 % by weight. This is caused by the reboiler operating temperature, which can not be fixed at temperature above 204 °C. In fact, this tempe rature must be regarded as an upper limit for TEG processing, because of thermal degradation at higher values [2,3].

In the past these regeneration levels were sufficient because values of allowable water content in the lean gas were higher and regeneration was commonly performed in a simple atmospheric still column. On the other hand, in order to reach current water content specifications, it is necessary to regenerate TEG up to levels substantially above 99.0 % by weight.

2

0

50

100

150

200

250

300

350

400

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Fig 1: Water content in the dehydrated gas as a fun ction of liquid to gas ratio in the absorption column. Case study process parameters: c ontactor theoretical stages 4, contactor pressure 50 bar, wet gas temperature 40 ° C, TEG temperature at the inlet

port 100 °C.

As an example referred to a typical natural gas dehydration case study, Fig. 1 shows water content in the lean gas as a function of the liquid to gas ratio in the absorption column. The curves, obtained by process simulation, refer to four different values of regenerated TEG mass fraction. This case study clearly shows that it is necessary to obtain TEG regeneration levels in the range 99.5-99.9 % by weight in order to dehydrate natural gas to current specifications. Fig. 1 also shows that for low regeneration levels (98.5-99.0 % by weight) a further increase in the liquid to feed ratio in the absorption column has no effect and can not allow to reach water content specifications. At low regeneration levels, also increasing the number of theoretical stages of the contactor does not allow to reach water content specifications.

For these reasons, several alternative regeneration processes (vacuum distillation, stripping gas, azeotropic distillation, etc.) have been proposed in order to enhance TEG regeneration levels. If TEG-water distillation is performed under vacuum, TEG can be regenerated up to 99.9 % by weight, but high operating costs and plant control complexity are serious drawbacks. Another possibility is represented by using some of the dehydrated natural gas as a stripping gas in order to further dehydrate TEG. The contact with lean TEG exiting the still column can take place in the surge drum where TEG make-up is introduced or in an appropriate stripping column. Operating in this way lean TEG purity can be increased above 99.9 % by weight, but the consumption of some of the dehydrated gas represents an echonomic loss. Furthermore, the use of stripping gas causes a considerable increase of the gas send to flare, unless an expensive recovery system is arranged. Purity of lean TEG exiting the still column can also be enhanced by using a circulating solvent (such as heptane or octane) to perform an azeotropic distillation instead of the simple TEG-water distillation. In this way lean TEG concentration can be increased up to 99.99 % by weight but plant complexity and costs are increased because of the need of a three-phase separator at the top of the distillator, the needs of treatments for the oily water discharged from this separator and devices for the solvent circulation line (such as a circulation pump and a heater) [4].

If the required purity of the lean TEG is in the range 99.0–99.9 % by weight, Coldfinger process [5,6] is a valuable alternative. This process is similar to the conventional dehydration process, except for the use of an apparatus, also called water exhauster, where the liquid stream exiting the reboiler of the still column is further dehydrated. Coldfinger process is advantageous because no stripping gas, solvent or further energy consumption are required and because of the simplicity of the water exhauster which can be easily installed as an addition to a conventional plant.

98.5 %

Liquid to gas ratio

99.0 %

99.5 %

99.9 % W

ater

con

cent

ratio

n (m

g/N

m3 )

3

In this work, a study on the performance of the Coldfinger regeneration process, in the context of natural gas dehydration, is carried out and a discussion on the influence of the most relevant process parameters is presented.

THE COLDFINGER WATER EXHAUSTER

In Fig. 2 a schematic representation of the Coldfinger water exhauster is reported. The liquid stream exiting the reboiler of the still column is conveyed to the Coldfinger apparatus where a further dehydration takes place. The liquid flows horizzontally through the lower portion of the apparatus, which is provided with some buffles to maintain the surface of the liquid agitated and turbulent in order to increase the vaporization rate. The liquid can be considered in equilibrium with its vapour, which occupies the remaining part of the vessel. In the upper part of the apparatus, an elongated horizontally disposed condenser (the “cold finger”) occupies a little part of the vapour space and is in contact with the vapour. The cold U tubes of the condenser cause a local condensation of the vapour in close proximity. Because of free convection and local condensation on the tubes, new vapour is drawn from the bulk to the proximity of the condenser, whereas the uncondensed and cooled vapour comes back and mixes again with the bulk. Droplets of condensed liquid gravitate to a collector shaped in a way to allow the liquid to flow and to be removed from the vessel. Removed liquid, which is a TEG-water mixture, can be refluxed to the still column or to its reboiler in order to be distilled again. Since the vapour is richer in water with respect to the liquid, as a result a further dehydration of the liquid flowing in the lower part of the apparatus is obtained. The pressure inside the vessel can be conveniently maintained at values nearly atmospheric by means of a gas regulation line, which can be fed by splitting minimal quantities of lean gas produced in the same dehydration unit.

If the Coldfinger apparatus is fed by a liquid mixture about 98.8-98.9 % TEG by weight, which is the typical concentration of the liquid exiting the still column reboiler at 204 °C and 1.1 bar, TEG purity can be increased up to 99.5-99.9 % by weight operating at atmospheric pressure. Dehydration levels which is possible to obtain in the Coldfinger apparatus basically depend on the rate of heat removal by the condenser and on the rate of hot vapour rising to the proximity of the cold tubes.

Fig 2: Schematic representation of the Coldfinger a pparatus.

4

These parameters depend on the extension of the cold surface of the condenser, the temperature of the refrigerating fluid and on the fluid dynamics of the two phases inside the apparatus.

GAS DEHYDRATION PROCESS WITH COLDFINGER TEG REGENER ATION

A typical flow scheme of the natural gas dehydration process, with TEG regeneration enhanced by the Coldfinger apparatus, is reported in Fig. 3.

Wet gas is fed to the bottom of the absorption column (A), whereas regenerated TEG (13) is fed to the top of it. As a consequence of the contact between gas and TEG, a stream of dry gas is obtained from the top, whereas a stream of water-rich TEG (1) is withdrawn from the bottom of the column and is conveyed to the regeneration section. Since the absorption column operates at high pressure (typically 40-80 bar) and low temperature (typically 20-60 °C), whereas the regeneration section operates at press ure around 1 bar and high temperature (up to 204 °C), water-rich TEG is expande d and heated before entering the still column (S). In order to heat water-rich TEG, heat can be conveniently recovered from the regeneration section. In particular, water-rich TEG after expansion (2) can be pre-heated while removing heat at the Coldfinger tubes and then (3) at the condenser of the still column. Because of heating and reducing pressure, some flash gas is produced and separated from the liquid stream in a flash drum (D). After filtration (F) to remove impurities, water-rich TEG undergoes another pre-heating step in a heat-exchanger (H), recovering heat from lean regenerated TEG (11), and then is fed to the still column.

Water-rich TEG is distilled in the still column. The reboiler (R) can be heated by exhaust gases of the combustion of a small fraction of natural gas produced in the unit and used as a fuel. Reboiler temperature must be limited at 204 °C to avoid TEG thermal degradation. In this condition, TEG mass fraction at the reboiler will be around 98.8-98.9 % by weight. In order to increase TEG mass fraction, this liquid stream is conveyed to the Coldfinger apparatus (C), which can be realized in a separate vessel or in a vessel integrated with the reboiler itself, as in the case of Fig. 3. Heat removal at the condenser placed in the upper part of the Coldfinger apparatus can be conveniently provided by water-rich TEG as explained above.

Fig 3: Typical flow scheme of natural gas dehydrati on with Coldfinger regeneration process.

5

As an alternative, if a greater amount of heat removal is required in order to reach higher regeneration levels, cooling water can be employed to give a lower temperature of the cold tubes. Condensed liquid on the “cold fingers” is reintroduced inside the column. Because of the small quantity of this liquid stream, its inlet point has negligible effect and so it can be conveniently reintroduced inside the reboiler. Regenerated TEG exiting the Coldfinger apparatus is cooled (H) as far as possible before being pumped (P) back to the absorption column.

PROCESS SIMULATION

The process represented in Fig. 3 was studied by means of the process simulator Hysys. Phase equilibria conditions were represented by means of a thermodynamic model based on the Peng-Robinson equation of state. Correct representation of phase equilibria conditions of the binary system TEG-water, especially at high temperature and for mixture composition close to water infinite dilution, proved to be an essential requirement for regeneration process design. A thermodynamic study on TEG-water system was carried out on published data in order to optimize equation parameters. Therefore, to represent binary TEG-water system, simulator data bank was not used. Details on thermodynamic modelling are provided elsewhere [7].

Since commercial process simulators do not provide Coldfinger apparatus by default, an user-defined external routine (Coldfinger routine) was generated in order to include the water exhauster in the whole process scheme. At first, Coldfinger routine was tested separately by considering an inlet liquid at typical reboiler conditions (around 200 °C, 1.1 bar and TEG 98.9 % by weight) and varying the heat removal at the condenser, for three different liquid feed flow rates.

Fig. 4 shows TEG mass fraction in the liquid stream exiting the bottom of the water exhauster as a function of the rate of heat removal. As it was expected, increasing heat removal leads to an increase of liquid dehydration. Furthermore, it can be noticed that the shape of the curves resembles the behaviour of condensation degree, as a function of heat removal, in an isobaric cooling of a vapour mixture with some uncondensable gas. In the first region of the curves, that is for small amounts of heat removal, the slope is quite low, which is to say dehydration is not very effective. Increasing the rate of heat removal a sudden change in the slope of the curves is shown. In this region, condensation degree at the cold tubes increases considerably. As a result, a great amount of water is removed from the system, causing a significant liquid dehydration. At very high heat removal rates, the slope of the curves suddenly decreases again.

99.0

99.1

99.2

99.3

99.4

99.5

99.6

99.7

99.8

10000 60000 110000 160000 210000 260000

Fig 4: Regenerated TEG mass fraction obtained by me ans of the Coldfinger water exhauster as a function of the heat removal. Inlet liquid conditions: 204 °C, 1.1 bar,

TEG mass fraction 98.9 %

L = 5000 kg/h

L = 9000 kg/h

L = 13000 kg/h

Heat removal (kcal/h)

TE

G m

ass

frac

tion

(% b

y w

eigh

t)

6

This is because in this region an almost complete condensation of the vapour in the proximity of the cold tubes is obtained. Since the system contains also small quantities of gas, the uncondensed gas is strongly cooled. When it mixes back with the bulk, it starts to cool the whole system slowing down new vapourization from the liquid. As a consequence, further dehydration is strongly reduced.

After testing the water exhauster alone, the Coldfinger routine was included in the whole process scheme of simulation. Thus, an analysis on the performance of the natural gas dehydration with Coldfinger regeneration process was carried out on a case study. A typical wet gas composition was selected, as reported in Table 1. In some cases, depending on wet gas temperature, a pre-dehydration was obtained by separation of a liquid phase before entering the absorption column. Other process parameters were fixed as follows:

- Total inlet gas flow rate: 100000 kg/h; - Absorption column pressure: 50 bar; - Absorption column theoretical stages: 4 - Still column theoretical stages: 3 - Still column reflux ratio: 0.10

The number of theoretical stages of the absorption column was not increased above the reported value because this parameter has a poor effect on gas dehydration, especially when compared with regenerated TEG purity, which is the key factor of the process. Similarly, the number of theoretical stages at the still column was kept at the reported low value because no further effect on TEG dehydration can be achieved by increasing this parameter. Still column reflux ratio was fixed at the minimal value which allowed to reduce TEG losses around zero (0.001 % by weight on the circulating glycol). As for the absorption column, TEG losses were less than 0.1 % by weight in all simulations.

The process was studied for different wet gas temperatures (from 20 to 60 °C), varying the liquid to gas ratio (L/G) at the absorber column in the range 0.03-0.13, which is to say that circulating TEG was varied from 3000 to 13000 kg/h. Heat removal at the Coldfinger was realized exchanging with TEG from the absorption column, which is the more convenient solution. Continuous lines in Fig. 5 show the results obtained by these simulations. As it was expected, process performance is strongly influenced by the temperature of the wet gas entering the absorption column. In fact, the temperature profile of the absorption column depends primarily on the wet gas temperature, because of the low (L/G) ratio. Furthermore, at higher wet gas temperature, the ratio (L/G) has a greater influence on the dehydration level that is possible to obtain.

Tab. 1: Natural (wet) gas composition selected for the case study.

Total inlet gas flow rate: 100000 kg/h.

Component Weight fraction

Methane 0.3853 Ethane 0.2408

Propane 0.1948 n-Butane 0.0687

Iso-Butane 0.0325 C5 and superior 0.0592

Water 0.0044 Nitrogen 0.0122

Carbon Dioxide 0.0021

7

0

100

200

300

400

500

600

700

800

0.02 0.04 0.06 0.08 0.10 0.12 0.14

Fig 5: Water content in the lean gas as a function of the liquid to feed ratio in the absorption column referred to four different wet ga s temperature. Continuous lines

refer to Coldfinger regeneration process. Dashed li nes refer to regeneration by means of stripping gas.

In the case study under consideration, the use of the Coldfinger apparatus allows to reach typical water content specifications for wet gas temperature up to 50 °C. At lower gas temperatures, the process is more efficient for two reasons: favourable thermodynamic conditions at the absorption column and improvement of Coldfinger regeneration because of the lower temperature of water-rich TEG, which can provide a larger heat removal rate at the “cold fingers”. However, even at 60 °C is possible to str ongly reduce water content in the dehydrated gas stream, down to 200 mg/Nm3 approximately. In this case, a further improvement can be obtained refrigerating the Coldfinger with cold water from an external circuit, in order to reach higher levels of regenerated TEG purity.

Continuous lines in Fig. 6 report natural gas consumption for the dehydration process with Coldifnger regeneration enhancement. Reported values refer to two different wet gas temperatures: 20 and 60 °C.

0

20

40

60

80

100

120

140

160

180

0.02 0.04 0.06 0.08 0.10 0.12 0.14

Fig 6: Comparison of natural gas consumption betwee n Coldfinger regeneration process (continuous lines) and regeneration by mean s of stripping gas (dashed

lines) for two gas temperatures.

Liquid to gas ratio

Wat

er c

once

ntra

tion

(mg/

Nm

3 )

60 °C

50 °C

20 °C

40 °C

20 °C

20 °C

60 °C

60 °C

Liquid to gas ratio

Gas

con

sum

ptio

n (k

g/h)

8

Gas is consumed at the burner of the still column reboiler in order to produce the vapour rising into the column. Values range from around 20 to 70 kg/h, depending on the circulating liquid flow rate. At 20 °C gas consumption is lower b ecause some water is separated before entering the absorption column, thus leading to a smaller boil up at the reboiler.

A classical dehydration process scheme, with TEG regeneration enhanced by using some of the dehydrated gas, was simulated in order to provide a comparison with the Coldfinger process. Process flow scheme is the same as Fig. 3, except for the absence of the Coldfinger, which is substituted by a stripping column fed by some gas splitted from dehydrated gas stream. The number of theoretical stages of the stripping column was fixed at 2, which is a typical value for this process. All the other process parameters were kept equal to the values reported above. In a preliminary set of simulations, stripping gas flow rate was varied until similar water content specifications, with respect to Coldfinger process, were obtained. This similarity was found operating the stripping column with a gas flow rate equal to 100 kg/h. The dependence of water content on liquid to gas ratio for this process is represented by the dashed lines shown in Fig. 5. Operating the stripping column at a constant gas flow rate, a minimum water content was found at high liquid to gas ratio. This is caused by two contrasting effects: increasing the circulating liquid improves the dehydration at the absorption column but regeneration becomes more difficult. This behaviour does not appear with Codfinger process because, when circulating TEG increases, heat removal rate at the “cold fingers” increases as well. A correct design of the Coldfinger can lead to a constant regeneration level while circulating liquid increases.

Dashed lines in Fig. 6 show the consumption of dehydrated gas, with respect to the process with regeneration enhanced by stripping gas. Comparing the two processes, it can be seen that the Coldfinger process is capable of dehydrating natural gas consuming less dry gas, with respect to similar water content specifications. In particular, the gas consumption of the process with stripping gas regeneration resulted to be from 2.5 to 6 times greater than the Coldfinger process.

CONCLUSIONS

A study on the Coldfinger apparatus was carried out in order to underline its capability of further dehydrating TEG from atmospheric distillation. It was demonstrated that Coldfinger unit is capable of increasing TEG mass fraction up to 99.5 –99.8 % by weight, operating at atmospheric pressure. An analysis on the performance of the natural gas dehydration, including Coldfinger regeneration process, was carried out on a case study by means of the process simulator Hysys. It was demonstrated that current water specifications in the dehydrated gas can be reached without adding stripping gas, solvent or increasing energy consumption.

REFERENCES

[1] Huffmaster M.A., “Gas Dehydration Fundamentals”, Laurance Reid Gas Conditioning Conference, 2004 [2] Steele W.V., Chirico R.D., Knipmeyer S.E., Nguyen A., “Vapor Pressure of Acetophenone, 1,2-Butanediol, 1,3-Butanediol, Diethylene Glycol Monopropyl Ether, 1,3-Dimethyladamantane, 2-Ethoxyethyl Acetate, Ethyl Octyl Sulfide, and Pentyl Acetate”, J.Chem.Eng.Data 41 (1996) 1255-1268 [3] Rajeh A.O., Szirtes L., “Thermal Decomposition of Organic Derivatives of Crystalline Zirconium Phosphate – III. Thermal decomposition of dietylene glycol, benzy alcohol and benzylamine intercalates of zirconium phosphate”, J. Thermal Analysis 37 (1991) 777-786 [4] Shell, “Glycol Type Gas-Dehydration Systems”, Technical Manual, 2005 [5] Reid L.S., “Apparatus for dehydrating organic liquids”, US Patent 3589984, 1971 [6] Reid L.S., “Method of removing water from glycol solutions”, US Patent 4332643, 1978 [7] Gironi F., Maschietti M., Piemonte V., "Modelling triethylene glycol-water system for natural gas dehydration", accepted for presentation at the Eight International Conference on Chemical and Process Engineering, Ischia, June 2007.