study the effect of ph and concentration of glycol

STUDY THE EFFECT OF PH AND CONCENTRATION OF GLYCOL SOLUTION IN THE DEHYDRATION OF NATURAL GAS +

دراسة تأثير الاس الهيدروجيني وتركيز محلول الجلايكول وعوامل اخرى على عملية تجفيف الغاز

الطبيعي

Rifat M.Dakhil *

Abstract: The paper presents the effect of pH and concentration of glycol solution and other general operating problems and some important trouble shootings which are concerning and involved with the operation and performance of the different equipment of the gas dehydration plant by using glycol system. The trouble shooting deal and diagnosis of the problems encountered during plant operation with the main equipment like High Dew Point of Sales Gas, Low Glycol pH (Less than 6.5), and Lean Glycol Low Concentration. Also many tips and recommendations regarding the operation performance of the dehydration process has been added.The work included three Libyan gas fields ( North Zelten, Meghil, and Sorra plants). Keywords: Gas dehydration, Glycol system, Trouble shooting

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اول البحث تأثير الأس الهيدروجيني ألحامضي وتركيز محلول الجلايكول وارتباطهم بالظروف التشغيلية يتن

إضافة إلى عرض بعض المشاكل التشغيلية التي ترتبط بهذه المتغيرات وانجاز . الأخرى لوحدة معالجة الغاز الطبيعي

إضافة . لغاز الطبيعي باستخدام محلول الجلايكولالوحدات المختلفة المتواجدة في وحدة إزالة محتوى الرطوبة من ا

إلى التعامل وتشخيص بعض المشاكل التشغيلية في الوحدات المختلفة خلال عملية التشغيل مثل نقطة الندى المرتفعة

والتركيز المنخفض لمحلول ) ٦،٥اقل من ( الأس الهيدروجيني المنخفض لمحلول الجلايكول ، للغاز المباع

إضافة إلى عرض بعض الملاحظات والاقتراحات المتعلقة بالإنجاز التشغيلي لعملية إزالة محتوى . لمستخدمالجلايكول ا

البحث تناول ثلاثة حقول غازية مختلفة هي حقل زلطن الشمالي وحقل الصرة وحقل المقيل .الرطوبة من الغاز الطبيعي

.الليبية Introduction

Moisture content of natural gas is found in most produced natural gas. Some of this moisture

is called 'FREE' water, (liquid phase) and may be removed by passing the gas through a

+ Received on �� cce��ed on �� .

* Lecturer /Technical College / Basrah

separator or scrubber. Water cause problems by plugging transmission lines, also damaging

equipment[1].

1- GENERAL DEHYDRATION PROCESS

The presence of water vapor in natural gas can lead to many problems. The

dehydration process of natural gas is therefore carried out for the following reasons: -

-Water vapor reduces the ability of gas to flow in the flow lines and process systems.

-Water vapor causes corrosion in lines and equipment.

-At low temperature, water vapor forms hydrates - complicated molecules of hydrocarbon

liquid and water, causing blockage of lines and equipment.

Water vapor must therefore be removed from the gas stream[3].

Natural gas may contain from 0.15 to 4.5 kilograms of water vapor per one thousand cubic

meter of gas produced (0.15 to 4.5 kg.water/mcm), depending on the temperature and pressure

of the natural gas; the warmer the gas, the more water vapor it will contain.

Tri-Ethylene Glycol (TEG) dehydration systems are the most common means used for the

process, this is practically approved. When lean TEG is brought into contact with wet natural

gas, it absorbs the water vapor from the gas stream[4,5].

Fig.1 Complete Glycol Unit - Absorption & Regeneration

2- MAIN OPERATING VARIABLES AND LIMITS

In order to understand the operating mechanism of the glycol dehydration process, it is

necessary to consider and understand the effect of the following four major variables[6]

Drawn By: Dr.Rifat M. DAKHIL

1)Temperature The temperature of the lean glycol entering the top tray of the contactor tower should be 5.5

to 8.5 °C above the temperature of the gas to be treated. If the glycol temperature is too much

higher than the gas temperature, the glycol will tend to foam and be carried out of the

contactor tower with the gas[7].

2) Pressure At constant temperature, the lower the pressure, the higher the water content of the inlet gas.

Other than affecting the water content of the inlet gas stream, pressure has very little effect on

the mechanics of glycol dehydration.

3) Glycol circulation flow-rate Determining the proper glycol circulation rate is not an easy task due to several

limitations and considerations involved. There are many factors that must be considered, but,

for simplicity, over a normal pressure range up to 8000 kpa, about 55 to 95 kg of glycol must

be circulated for every kg of water removed at 13°C dew point depression.

4) Glycol concentration Since the main objective of natural gas dehydration is maximum dew point

depression, relatively high glycol concentrations are used. The usual practice is to introduce,

at the top of the glycol contactor tower, a solution of regenerated glycol with a concentration

ranging from 97 to 99 %, and to remove the solution from the base of the contactor tower at a

glycol concentration of 80 to 90 %[8]

Fig. 2 Dew point Depression [6] Fig.3 Natural Gas Water Vapor Content [6]

OPER�TING PROBLEMS �ND GLYCOL C�RE

Most operating and technical problems usually occur when the circulating glycol

solution gets dirty. In order to get a long, trouble-free life with the glycol system, it's

necessary and very important to recognize these problems and know how to prevent them.

1- Glycol pH control

The glycol pH should be checked periodically and kept on the basic side by neutralizing the

acidic compounds with borax, Ethanol-amines or other suitable alkaline chemicals to maintain

the pH at7.5 to 8. A glycol solution that is alkaline pH greater than 9, tends to foam and

emulsify .

2- TROUBLE SHOOTING Problems and Possible Solutions[6,11,12]

Problem 1. High Dew Point - Sales Gas

POSSIBLE CAUSE POSSIBLE SOLUTION

1. Low glycol concentration.

2. Glycol Circulation Rate too low.

3. Absorber inlet gas flow rate too high.

Gas velocity causing poor contact in

the trays.

4. Glycol Foaming causing poor gas and

glycol contact.

5. Dirty contacting devices in absorber.

Check the glycol concentration and the

regeneration system - adjust if and where

necessary.

Increase circulation rate and observe effect.

Check Sales Gas flow rate and decrease if

possible. Check for gas to flare downstream

of absorber.

Check glycol pH and adjust as required.

or add anti-foam agent and observe effect.

This can only be checked out by shutdown

and inspection.

Problem 2. Low Glycol pH (Less than 6.5)


1. Build up of organic acids due to

overheating.

Check the reboiler temperature and adjust if

required.

Have the glycol composition checked by the

laboratory.

Add the required amount of MEA or other

alkaline as per the procedure.

Problem 3. Lean Glycol - Low Concentration


1. Reboiler temperature too low.

2. Poor stripping in the still column

and/or the reboiler stripping section.

If all other factors are normal, then

suspect fouling of the contacting devices.

3. Circulation rate too high causing

insufficient residence time in the

regeneration system.

Check the reboiler temperature and adjust

Increase the stripping gas flow rate and await

effects.

Check of the contacting devices will require

shut down and inspection.

Decrease circulation rate and await effects.

EXPERIMETAL WORK: 1- Glycol pH Control :

It is important to control the glycol pH within a certain specified value to get optimum water

removal and protect the equipment from corrosion. Too high a pH (more than the specified

values) will tend to increase the glycol foaming , hence more glycol losses as well as less

intimate contact between the gas stream and the absorbent agent (glycol)[6].

.

The recommended pH values for glycol are given in the table below :

Glycol Type Required pH values Lean 7.0 - 7.5

Rich 5.5 (minimum)

Di-ethanol Amine (DEA) is added to the glycol to bring its pH to a higher value .

The following data represents actual pH figures collected at various dates for 3 Gas plants in

Libya :

i. Zelten gas plant (South Unit) - 26 August 2007 to 6 October 2007

ii. Meghil gas plant - 3 December 2007 to 18 November 2007

iii.Sorra gas plant - 25 April 2007 to 18 November 2007

2- Glycol Concentration :

The glycol concentration refers to the percentage of water in the glycol and it is regarded as

an important process variable which has to be monitored and controlled at all times .

Usually, the manufacturer gives the operating glycol circulation rate as that which is required

to remove 16 kg of water vapor per one million standard cubic meter of gas at given process

conditions in the absorber (contactor) .

Table (1) Samples for Rich and Lean Glycol pH, including calculated pH depression and absorbed water

percentage for Zelten Gas Plant.

Sample NO.

pH Rich G.

pH Lean G. pH Depression Absorbed H2O %

1 6.7 7.3 0.6 1.7

2 5.9 6.6 0.7 1.9

3 6.6 7.3 0.7 1.9

4 6.6 7.3 0.7 1.9

5 6.6 7.4 0.8 2.2

6 6.4 7.3 0.9 2.5

7 6.5 7.4 0.9 2.5

8 6.6 7.5 0.9 2.5

9 6.0 6.9 0.9 2.5

10 5.6 6.6 1.0 2.8

11 6.3 7.3 1.0 2.8

12 6.1 7.2 1.1 3.0

13 6.1 7.2 1.1 3.0

14 6.4 7.6 1.2 3.3

15 6.0 7.2 1.2 3.3

16 5.9 7.3 1.4 3.9

17 6.0 7.5 1.5 4.1

18 5.7 7.3 1.6 4.4

19 5.9 7.7 1.8 5.0

20 5.8 7.8 2.0 5.5

Table (2) Samples for Rich and Lean Glycol pH, including calculated pH depression and absorbed water

percentage for Meghil Gas Plant

Sample NO.

pH Rich G.

pH Lean G. pH Depression Absorbed

H2O%

1 5.9 6.4 0.5 1.4

2 6.4 7.0 0.6 1.7

3 5.2 5.8 0.6 1.7

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Table (3) Samples for Rich and Lean Glycol pH, including calculated pH depression and absorbed water percentage for Sorra Gas Plant

Sample NO.

pH Rich G.

pH Lean G. pH Depression Absorbed

H2O%

1 6.7 6.8 0.1 0.2

2 5.4 5.5 0.1 0.2

3 5.0 5.3 0.3 0.5

4 5.4 5.7 0.3 0.5

5 5.8 6.7 0.9 1.5

6 6.1 7.0 0.9 1.8

7 6.2 7.1 0.9 1.8

8 6.4 7.5 1.1 1.8

9 6.0 7.1 1.1 1.8

10 6.0 7.2 1.2 2.0

11 6.2 7.4 1.2 2.0

12 6.1 7.3 1.2 2.0

13 5.8 7.1 1.3 2.2

14 5.7 7.1 1.4 2.3

15 6.2 7.8 1.6 2.6

These data are plotted in the following bar charts which clearly indicate that the pH is kept

under close control within the required limits

Fig4 The effect of pH values for Lean and Rich glycol on dehydration of Zelten Plant

Fig5 The effect of pH values for Lean and Rich glycol on dehydration of Meghil Plant

Fig.6 The effect of pH values for Lean and Rich glycol on dehydration of Sorra Plant

Fig7 The effect of pH depression and absorbed H2O % values on dehydration of Zelten Plant

Fig.8 The effect of pH depression and absorbed H2O % values on dehydration of Meghil Plant

Fig.9 The effect of pH depression and absorbed H2O % values on dehydration of Sorra Plant

Fig10 Effect of pH depression on absorbed H2O % in a dehydration of Zelten Plant

Fig.11 Effect of pH depression on absorbed H2O % in a dehydration of Meghil Plant

Fig.12 Effect of pH depression on absorbed H2O % in a dehydration of Sorra Plant

DISCUSSION AND CONCLUSION: It is important to note that the more the lean glycol concentration, the greater the dew point

depression of the process gas being treated ". As it shown from the Figs.2.1, 2.2 , and 2.3,

the relation between the pH depression for the different samples of the three gas plants are

slightly difference, which means that this depression depends on the original lean glycol

concentration and also affected by the plant capacity and other operational factors.

It was highly recommended to take in consideration the pH and concentration of the glycol

solution of the dehydration process which plays an important part in reducing the water

content of the treated gas and to keep these with the recommended value in order to achieve

the high possible performance.. The effect of the glycol concentration on the performance of

the plant is depend on the capacity of the plant, This drop in performance may not be too

important in small plants but becomes quite significant in large capacity plants. The Figs. 2.7,

2.8, and 2.9 which represents the relation between the pH depression and the amount of

absorbed water for the different gas plants, this shows that the amount of absorbed related

directly with pH depression, so in order to achieve more absorbed water it is recommended to

keep the pH depression as high as possible. To establish a trouble shooting guide seems to be very important to be adapted as a training

course for the process engineers and plant operators in order to understand the effect of the

different process variables and their fluctuations effect on the process and to take the correct

decision in case of any malfunction with the process. This study help the process engineers

and operators for keeping their gas dehydration plant in a safe and good working conditions REFERENCES:

1. John J.Carroll, Natural gas hydrates, a guide for engineers, 2nd edt.2002

2. Perry and Chilton, Chemical Engineering Handbook, sec.14-6, 14-7, 6th edt.1984.

3.Harold O. Ebeling, Latoka Engineering l.L.C, Tulsa, Reduce Emissions and operating costs

with appropriate Glycol selection, proceeding of the 77th Annual convention 1996,

p.1,4.

4. Vincente N. Hernandez Valencia, Design Glycol units for maximum efficiency, Bryan

research and engineering, Proceeding 71st Annual convention, Tulsa, OK.

Processors Association 1992, P.310, 313.

5. Kirck / Othner, Encyclopaedia of Chem.Tech., dehydration by glycols,

3rd edt. 1978, Vol.4, p.702.

6. Zelten Operating Manual, Sirte Oil Company. Esso Standard Libya Ltd, dehydration of

natural gas, 1981.

7. M.A.Huffmaster, Laurance Reid gas conditioning, Conference 2004, p.8

8. Saied Mokhatab, William Poe, James Speight, Hand book of Natural Gas Transmission and

Processing, Chapter 9, Sept.2006

9. Kohl, A. and Riesenfeld, Gas Purification, Gulf Publishing Company, Houston 1985

10. Kimberly C. Covington, John T. Collie Bryan, Research of engineering, Bryan, Texas.

Proceeding of the 78th GPA Annual Convention 1999, p.48.

11. Meghil Operating Manual, Sirte Oil Company. Esso Standard Libya Ltd, 1982.

12. Sorra Operating Manual, Sirte Oil Company. Esso Standard Libya Ltd, 1982.

study the effect of ph and concentration of glycol

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