the venezuelan gas processors association xviii international … · the venezuelan gas processors...

Operational Improvements in Gas Processing

XVIII Gas Convention, AVPG, Caracas, Venezuela, May 27 - 29 th, 2008

THE VENEZUELAN GAS PROCESSORS ASSOCIATION

XVIII INTERNATIONAL GAS CONVENTION

TECHNICAL WORK

IMPACT OF THE VARIATION OF THE COMPOSITION TO THE EXIT OF A SYSTEM OF REGENERATION, IN THE DESIGN OF A SYSTEM OF BTEX'S CONTROL, OF A PLANT OF DEHYDRATION OF NATURAL GAS

FATIMA DA SILVA

GUSTAVO SUCRE



INDEX

1. INTRODUCTION ………………………………………………………… 4

2. GENERAL CONSIDERATIONS 5

2.1. Parameters for the design of incinerators for the control of emission of BTEX and VOC. ..........................................................................5

2.2. Common configuration of incinerator for Gas and Liquiqs..............6

2.3. Influence of the composition of the gas to incinerating, in the definition of the design parameters. .............................................................7

3. RESULTS ……………………………………………………………………… 9

3.1. Description of the study.......................................................................9

3.2. Composition of the studied gases ....................................................10

3.3. Process Simulation ............................................................................11

3.4. Combustion Chamber Dimensions. ..................................................14

3.4.1. Dimensions of the Chamber Combustion 1. ..................................17



4. CONCLUSIONS ……………………………………………..………. 20

5. REFERENCE …………………………………………………. 21



1. Introduction

Natural gas obtained from wells typically contains water vapor along with other

hydrocarbons which are considered impurities of the natural gas. These

impurities, particularly water, can cause many problems in pipeline and

processing equipment. For this reason, it has long been a common practice to

treat the natural gas to remove these impurities. Removing water vapor helps to

prevent clogging of the pipeline with liquid water and also helps prevent corrosion

of the pipelines and hydrates formation.

The most commonly used method of removing water from the natural gas is

through glycol dehydration. Removing the water in the glycol typically entails

heating the glycol to between 350° F. and 400° F. at atmospheric pressure. This

vaporizes the water to leave a pure stream of glycol for reuse. However, other

hydrocarbon impurities are also removed from the glycol, and simply heating the

glycol in a traditional atmospheric pressure reboiler also vaporizes those

hydrocarbons, which are thus released into the atmosphere. This presents an

environmental problem.

The aromatic compounds are transferred from natural gas into the liquid phase

under (glycol) under absorber conditions of moderate temperature and high

pressure, they transfer into the vapor phase in the glycol regenerator under

conditions of high temperature and low pressure.

BTEX components in feed gas to dehydrators range from 0,01 to 0,1 mol %

(1,2). These aromatics are highly soluble in triethylene glycol. BTEX compounds

exit the regenerator with still vent vapors and can be emitted to the atmosphere

or further processed through an emissions control technology. (3,4)

Of the industry producers that are employing BTEX control technology, use a

vapor recovery system (condensation) to control emissions or incineration

systems to destroy all organic compounds vented from the glycol purifier.



The condensation of water and heavy hydrocarbons using either air, water, or

glycol as the cooling agent, followed by three phase separation.

The products of separation are condensable hydrocarbons, water in its liquid

form, and non-condensable gases. Because separation occurs at atmospheric

pressure, all three phases must be pumped or compressed to storage tanks and

the gas system. The incineration systems do not recover any hydrocarbons for

sales. (5, 6)

The still vent stream is typically 50 wt % water, with the balance being

hydrocarbons along with small quantities of carbon dioxide, nitrogen, and

hydrogen sulfide, if present in the processed gas.

The Clean Air Act limits the amounts of heavy hydrocarbons (Volatile Organic

Compounds /VOC) which may be emitted from a facility to 250 tons per year.

Aromatic compounds such as benzene, toluene, ethylbenzene and xylene

(BTEX) are limited to 25 tons per year of total aromatics and 10 tons per year of

any individual aromatics. (7,8)

In this work is evaluated the impact of the variation the composition of the still

steam In the operation of an incinerator used as method of controlling emissions

of a dehydration plant.

2. General considerations

2.1. Parameters for the design of incinerators for the control of emission of BTEX and VOC.

In the design of an incinerator there are born in mind certain factors that will

depend directly on the composition of the gas current to treating. The combustion

is the principal process in these equipments, which is not more that a chemical

reaction that is accompanied of great heat production, which generates high

temperatures that facilitate, in presence of oxygen, a rapid oxidation of the

present hydrocarbons in the feed gas to the incinerator.



2.2. Common configuration of incinerator for Gas and Liquids

Fig. 1 Configuration of incinerator

In the previous figure it is shown the typical arrangement of the incinerators of

Gas and Liquids, this made up of two important areas; burner and mixer, in which

is carried out the mixture of the necessary air for the combustion and the flame

takes place. The second are the combustion chamber in which the products are

incinerated to control, in their dimensioning it influences the time of residence a

lot (8,9).

In order that the combustion process takes place, it is necessary that

hydrocarbons are oxidized and therefore requires the presence of oxygen which

is supplied by the addition of air. The combustion requires specific amounts of

air, being the case of combustion theory when the exact amount of air is provided

to oxidize all the components in the exhaust gases there is no presence of

oxygen, there is the case of combustions with defect air, in this case the air

supplied to the air is less theoretical and finally combustion with excess oxygen

in which adds a porcentage above the theoretical air, this is the most common

case in combustion processes, in fact, in reality you can not have a single

burning with the amount of air theoretical, always should exist excess of air.



Principally to design a gas incinerator there must be considered 4 parameters,

which are listed:

• Temperature of incineration.

• Time of residence.

• Turbulence

• Composition of the gas to incinerating.

All these conditions are related but the composition of the gas plays the principal

role because in principle it will define the other conditions.

2.3. Influence of the composition of the gas to incinerating, in the definition of the design parameters.

Since design parameters defined the temperature of incineration, time of

residence, turbulence and composition of the gas to incinerating, the latter affects

the establishment of the previous ones, and these they all at the same time

depend on the process of combustion.

According to the present components in the gas, they will be needed different

times of residence, major or minor magnitudes of temperature and different

quantities of air of combustion.

If only exists volatile hydrocarbons the gas stream, there are defined

temperatures of incineration of 850 °C for times of residences > 0,7 seconds

which are sufficient to allow the complete oxidation of the hydrocarbons.

When in the gas stream exists fluorados, chlorinated and sulphurous compounds

or metals, is necessary high values of temperatures and very much major times

of residence. These values are 1200 °C and times of residence of 1,5 to 2

seconds, for guarantee percentages of elimination of 99,99 %.

The composition of the gas to incinerating also defines the air quantity that is

needed to allow that the reaction of oxidation should be carried out, which is



governed by relations stechiometrics that give the volume or air mass needed in

the process of combustion.

After knowing the components of the gas stream are definite the minimal values

of temperature and time of residence, which together with the volumes of the

gases let estimate the internal size that must have of the equipment. The

temperature also determines the selection of materials and the wall thicknesses

of the incinerator.

While the temperature and time of residence be major it will turn out the

equipment to be more robust and need of specials materials producing very high

amounts of investment.

The process of combustion needs that the mixture combustible and comburente

(air) is homogeneous, this is obtained by turbulence in the boarders of the

equipment before the mixture catches fire. In order that the combustion is carried

out it is necessary to take the mixture to temperatures superior to that of

inflammation temperature. In order that it is kept, the heating power of the fuel

must be such, that the heat build-up allows that the temperature of combustion

always should be superior to the temperature of inflammation. If the composition

of the gas stream is such that has a low heating power, which does not allow to

support the combustion, must be supplied additional fuel to reach the sufficient

heating power that supports the temperature of the flame and at the same time

that of oxidation of the components to incinerating, this implies major costs of

investment and major operative expenses if is necessary additional fuel.

The composition of the gas to incinerating, defines the principal design

parameters, the heating power indicates if additional fuel is needed and this

information is sufficient to define the dimension of the incinerator

To achieve the elimination of benzene, ethylbenzene, xylenes and toluenes

(BETX), it is necessary to achieve minimal temperatures of 850 °C and time of

residence> 0,8 seconds. If in the course of the operation of the incinerator, the



composition of the gas changes in such a way that the generated heating power

does not manage to support the combustion, or the combustion is incomplete,

they will present in the out gases presence of black smokes products of not burnt

hydrocarbons and emission to the atmosphere of pollutant substances,

definitively badly functioning of the equipment.

If for the opposite, the variation in the composition generates an increase in the

heating power, major quantities of air will be needed for the combustion and if in

the process of design this possibility was not born in mind, the flame will go out

for lack of oxygen or it is possible to generate incomplete combustions

generating presence of soot, black smokes, emission of pollutants to the

atmosphere, (10,11), etc.

3. Results

3.1. Description of the study

In order to verify the above, three streams of gas were analyzed, which were

measured at the entrance of an incinerator to control emission of BTEX in which

there was observed malfunctioning by generating gases of black exhaust and

continuous explosions. This study is aimed to identify the possible causes of the

malfunction and at the same time to evaluate the impact of variation in the

composition of the inlet gas, dimensioning and operation of incinerators of gases

in order to control emission of BTEX.



3.2. Composition of the studied gases In the table 1 appears the composition of the gases, object of this study.

Table 1. Composition of the gases Component % molar 1 2 3 N2 0,52 1,73 0,33 CO2 4,23 3,59 5,23 Methane 33,42 31,04 46,09 Ethane 5,23 4,31 6,42 Propane 2,02 1,94 2,45 Butanes 2,57 2,32 3,11 Pentanes 2,95 2,27 3,56 Hexanes 2,73 2,58 3,35 Benzene 2,95 2,70 4,31 Heptanes 2,13 0,77 3,15 Toluene 3,23 0,81 4,72 Octanes 2,25 2,54 2,31 Ethylbenzene 0,31 0,13 0,44 Xylenes 1,82 0,68 2,53 Nonanes 1,67 0,63 2,48 Decanes 0,73 0,25 1,32 Undecanes 1,10 0,07 1,93 Water 30,14 41,62 6,27 Flows [Ft/min] 5050 5050 5050

The temperature and currents pressure are 400 ° F and 1.1 atm.

In order to study each one of the currents, it was modelled through the software

PRO II V7.0 processes simulation. The simulation consisted in modelling the

combustion process to ensure the removal of VOCs and by studying the exhaust

gases and with the volume of the same to be able to estimate the internal

dimensions that should have the combustion chamber, so that to ensure the



residence time defined and verify the variation of the same with the different

streams studied.

3.3. Process Simulation

The simulation carried out consisted in modelling the combustion processing, for

that three lines with the streams were modelled S1, S2 and S3, which were

introduced to a conversion reactor R1, R2 and R3, respectively, these were

configured so that the occurrence of combustion equations introduced were

maximize. To each reactor lines for the supply of air were introduced, in order to

provide the necessary amount of oxygen for combustion of hydrocarbons, the

lines were named as Air1, Aire2, and Aire3 and coincide with the number of

reactors. As follows the PFD scheme of the simulation.

.

For the simulation of combustion reactions, in reactors, certain conditions were

accounted, which are listed as follows:

• The combustion process will be 100% theoretical, i.e., it will burn all

the hydrocarbon oil and only get as product CO2 and H2O.

• Air supply will be the stoichiometric, i.e., the burning will be without

excess of air, so that the presence of Oxygen in the exhaust gases is

not expected.



• The composition of the flow (currents) of air is 79% nitrogen and 21%

oxygen.

• The air supply was assumed at 35 ° C and 1.1 Atm.

For combustion reactions the oxidation stoichiometrics equations were defined of

each one of the hydrocarbons present in streams in study, the same are shown

below:

Combustión Equation:

• CH4 + 2O2 CO2 + 2H2O Methane

• C2H6 + 3.5O2 2CO2 + 3H2O Ethane

• C3H8 + 5O2 3CO2 + 4H2O Propane

• C4H8 + 6O2 4CO2 + 4H2O Butane

• C5H10 + 7.5O2 5CO2 + 5H2O Pentane

• C6H14 + 9.5O2 6CO2 + 7H2O Hexane

• C6H6 + 7.5O2 6CO2 + 4H2O Benzene

• C7H16 + 11 O2 7CO2 + 8H2O Heptane

• C7H8 + 9 O2 7CO2 + 4H2O Toluene

• C8H18 + 15.5 O2 8CO2 + 9H2O Octane

• C8H10 + 10.5 O2 8CO2 + 5H2O Ehylbenzene

• C8H10 + 10.5 O2 8CO2 + 5H2O Xilene

• C9H20 + 14 O2 9CO2 + 10H2O Nonane

• C10H22 + 15.5 O2 10CO2 + 11H2O Decane

• C11H24 + 17 O2 11CO2 + 12H2O Undecane



The equations shown were placed as instruction to occur in each of the reactors

in conjunction with the air supplied, there were several calculations in which each

was increasing the air flow until to get the stoichiometric relation of combustion

theory, detecting this when the outflows (S4, S5, S6) of the reactor were

removed all hydrocarbons and the same time there was no presence of oxygen.

The next image shows the flowchart of the modelling process.

Process Combustion Flow Diagram

As a result of the simulation, after having optimized the amount of air, the

following table summarizes is shown:



Inflows and Output Reactors Combustion

Table 2. Summary of the simulation

As it can be seen in the flow S4, S5 and S6 all components Volatile Organic

Compounds (VOCs) present in inflows S1, S2 and S3, were removed leaving

only exhaust products such as CO2, H2O and N2 which are typical products of

the combustion process theoretical.

3.4. Combustion Chamber Dimensions.

To measure the combustion chamber, it is necessary to know the time of

residence that the combustion gases should remain in that space and flow that

for this will pass. For incinerators that will be handling VOCs and where will not

exist, sulphurous, chlorinated or metal products, length of residence



Suppose a combustion chamber of cylindrical section, and shall set the

transverse area and the length of residence at least 1.5 seconds, so that the flow

rates obtained from the simulation, the length of the equipment can be calculate

and verify if this varies for different flow inlet gas compositions. In the table 3 are

the properties and quantities of the input and output flows.

As the gas flow is known, which is one of the results of the simulation performed,

and the flow rate is the product of the speed of flow through the area, if this area

is know, the speed can be obtained which is a factor that allows to know the

length of the combustion chamber.

The sizing is based on the speed with which the gas will travel within the

combustion chamber, as the speed is the ratio of the distance in time which runs

the same, if the time and speed are known, the distance travelled could be

known, this approach is based on the estimate.

recommended round between 1 to 2 seconds and the temperature of the

combustion chamber must be at least 800 °C, with these conditions are

guaranteed the removal of VOCs.

tVdtdV ⋅=⇒=

AQVAVQ =⇒⋅=



Table 3. Inlet and outlet flows properties



In the table above can be observed properties of the flow as: upper and lower

calorific power, compressibility factor, stream flow conditions and standard

conditions of the process, temperatures, actual rate (Q), etc.

It is quite important to check the temperature of the outflows reactors (S4, S5,

S6) are hanging out the 2000 ° C, this is the approximate value that will take the

flame and the exhaust gases to the compositions evaluated. Also significant

value is the calorific value of the flows S1, S2 and S3, these vary between 2000

and 2500 Btu/ft3, which is a value that the highest calorific value of methane

(1000 Btu/ft3), which indicates that the flow S1, S2 and S3, they have a fair

amount of calorie to maintain and ensure the continuous and complete

combustion which does not require the addition of extra fuel.

With the flow of process conditions, the minimum internal dimensions should be

estimated the combustion chamber for each one of the outflows S4, S5 and S6.

3.4.1. Dimensions of the Chamber Combustion 1.

Flows S4 Q=564.846,44 ft3/min

The diameter of the equipment should be fixed to 1,5 m. d=4,92 ft

The transversal area is:

201,194

)92,4(4

ftftdA =⋅=⋅= ππ22

With this area and the flow we obtain the speed:

segftft

ft

ft

AQV 25,8min13,713.29

01,19min44,846.564

2

3

====

At multiplying the speed by the time length is obtained:

mftsegsegfttVd 77,3375,125,125,8 ==⋅=⋅=



For the current S1 when incinerating it the escape gases (current S4), they

require some diameter dimensions 1,5 m and longitude of 3,77 m so that these

they remain for 1,5 seconds in the combustion camera.


Flow S5 Q=381.207,56 ft3/min

The diameter was fixed at 1,5 m=4,92 ft. d=4,92 ft


201,194

)92,4(4



segftft

ft

ft

AQV 57,5min00,053.20

01,19min56,207.381

2

3

====

At multiplying the speed by the time length is obtained

mftsegsegfttVd 55,236,85,157,5 ==⋅=⋅=





Flow S6 Q=783.496,56 ft3/min

The diameter was fixed at 1,5 m=4,92 ft. d=4,92 ft


201,194

)92,4(4





segftft

ft

ft

AQV 45,11min97,214.41

01,19min56,496.783

2

3

====

At multiplying the speed by the time length is obtained:

mftsegsegfttVd 24,5175,175,145,11 ==⋅=⋅=




The table 4, summarize the dimensions of the incinerator for three cases.

Table 4. Dimensions Summary

INCINERATOR DIAMETER [m] LONG [m]

1 1,50 3,77

2 1,50 2,55

3 1,50 5,24



4. Conclusions

• It could be proven that when designing incinerator equipment for the

control of BTEX emissions, it is extremely important to know the

composition and the possible variation of the same so as to give the

most appropriate dimensions to the equipment.

• By varying the composition of the gas stream to be treated in an

incinerator of Gas, not only can be required to vary the dimensions of the

equipment, if not it will be required to vary the operations conditions.

• The dimensions of the equipment should be dimensioning for the waited

case where the composition shows the sizes of larger dimensions and

compensate for the excess of size with a control system that register on-

line the composition and by means of this, supplies the amount of air

required for a proper combustion.

• For the studied case, the possible problem with the malfunctioning

incinerator where the currents S1, S2 and S3 were measured should be

related to the air supply system necessary for the combustion, since the

composition of flows have enough calorific power to maintain

combustion. Another possible option is that the dimensions that it was

built are lower than those required and thus gases do not meet the

minimum residence time which generates the waste in to the atmosphere

of VOC combustibles not burned.



5. Reference 1. Brad Johnson, PanCanadian Petroleum Ltd, Gary Webster,Canadian

Association of Petroleum Producers, report on Glycol Dehydrator

Emissions Study, June 1995.

2. Carl W. Fitz, The University of Oklahoma, R.A. Hubbard, John M.

Campbell & Co, “Quick manual calculation estimates amount of benzene

absorbed in glycol dehydrator”, Oil and gas Journal, November 23,1987,

pg. 72

3. Michael W. Hlavinka, Vicent N. Hernandez-Valencia, & Jerry A. Bullen,

Bryan Research & Engineering, Inc, “Influence of Process Operations on

VOC and BTEX Emissions from Glycol Dehydration Units”, Procedings of

Seventy Second GPA Annual Convention.

4. Thompson, P.A., Cunningham, I.A., Berry, C.A., and Evans, J.M., "PC

program estimates BTEX, VOC emissions," OGJ, June 14, 1993, pp. 36-

41.

5. Rueter, C.O., Wessels, J.K., and Hurtado, M.L., Paper No. GRI 94/0156,

GRI Glycol Dehydrator/Gas Processing Air Toxics Conference, June

1994.

6. Graham, J.F., Krenek, M.R., Maxson, D.J., Pierson, J.A., and Thompson,

J.L., Final Report No. GRI 94/0099, "Natural Gas Dehydration: Status

and Trends," January 1994.

7. Fitzsimons, G., and Viconovic, G. "Status of U.S. Environmental

Protection Agency Activities on Oil and Gas Production MACT Standard

Development," GRI Glycol Dehydrator/Gas Processing Air Toxics

Conference, Austin, Texas, April 1994.

8. Charles, E., and Robert, E. “The John Zink Combustion Handbook”, John

Zink Company LLC, Tulsa, Oklahoma.



9. Martinez, I. “Termodinámica Básica y Aplicada”, Ed. Dossat, Madrid

1992.

10. Brizuela, E and Romano, D. “Combustión”, Departamento de Ingeniería

Mecánica y Naval Faculta de Ingeniería, UBA 2003.

11. A compendium of tecnologies used in the treatment of hazardous waste,

US EPA 1987.

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