chemical engineering

Thursday, September 10, 2009

How to Increase Mass Flow Across RO When Choked Flow Already Occured ?

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Restriction orifice (RO) is widely used to in Oil and Gas, Refinery and Petrochemical chemical plant. Simple search on internet lead you to plenty of articles related to functionality, calculation and specification of restriction orifice. "Restrcition Orifice Used in Many Applications in Different Manners" brought out a few applications of RO in the industries. Earlier post "A refresh to Process Engineer on few phenomenons in restriction orifice" has summarized a few concepts that a process engineer may needs to understand for restriction orifice :

Restriction orifice is used to limit flow to required or expected flowrate with the available differential pressure across.
Vena contracta (VC) present just short distance downstream of restriction orifice
Maximum velocity and minimum pressure at vena contracta (VC)
Choked flow occurred when velocity at vena contracta (VC) reach sonic velocity (Ma=1). The corresponding downstream pressure (P2) at choked condition called critical pressure (Pc).
Increase in upstream pressure (P1) will increase mass flow passing the restriction orifice but velocity at VC still maintaining at Ma=1

A young engineer asked... If compressible fluid composition and upstream pressure are fixed and choked flow already occurred, should there be other way to increase the mass flow passing the RO ?

For compressible fluid, mass flow passing through an fixed bore size RO is a function of

composition ( k, MW, ...)
driving force (Differential pressure) across RO (P1-P2)
density (function of P1, MW, z, T1)

If composition and upstream pressure (P1) are fixed,

composition (and therefore k, MW...) remain unchanged
reducing P2 lead to higher P1-P2 which will result higher mass flow passing RO. Mass flow increase will continue until until P2 reach critical pressure of fluid (Pc). Sonic velocity (Mach no =1) occurred at vena contrata, some distance downstream of RO. When P2 lower than Pc, further reduction of P2 will not result any mass flow increase.
reduce upstream temperature (T1) will results higher density and higher mass flow passing RO.

Thus, one of the way to increase mass flow through RO when gas composition and upstream pressure are fixed and choked flow already occurred is dropping upstream temperature (T1).

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Wednesday, September 9, 2009

FREE Chemical Engineering Digital Issue for Sept 2009

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FREE Chemical Engineering Digital Issue for Sept 2009 has just been released !

Chemical Engineering Magazine as just released Sept 2009 issue. If you are the subscriber of Chemical Engineering, you should have received similar notification.

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Interesting articles for this month :

Strategies For Water Reuse
Membrane technologies increase the sustainability of industrial processes by enabling large-scale water reuse

Measuring Dust and Fines in Polymer Pellets
The ability to carry out such measurements can help operators improve quality control, assess
equipment performance and optimize the process

FAYF - Heat Transfer - Design II
This one-page guide outlines considerations for designing a heat-transfer system

Multivariable Predictive Control : The Scope is Wider Than You Think
With tighter integration between process units and more aggressive optimization goals, this technique is gaining attention throughout the CPI as an alternative to PID control

CSTR Design for Reversible Reactions
A design approach for continuous stirred-tank reactorsis outlined for three cases of second order reactions

CPVC Piping in Chemical Environments : Evaluating the Safety Record
No torches, fewer burn hazards and outstanding fire characteristics make CPVC a safe, effective alternative for industrial piping

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TIPS
If you are subscriber, you may access previous digital releases. Learn more in "How to Access Previous Chemical Engineering Digital Issue".

If you yet to be subscriber of Chemical Engineering, requested your FREE subscription via this link (click HERE). Prior to fill-up the form, read "Tips on Succession in FREE Subscription".

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Saturday, September 5, 2009

Relate LFL to MOC

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Minimum oxygen concentration (MOC) is minimum quantity of oxygen required present in hydrocarbon mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form. More discussion in "Minimum Oxygen Concentration (MOC) for Flare Purge". A mixture is combustible / flammable if and only if the hydrocarbon composition is within mixture LFL/LEL and UFL/UEL as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T One shall remember, as the operating pressure (P) and temperature (T) change (from reference P & T, the mixture LFL/LEL and UFL/UEL at P & T will change accordingly. This post will discuss the relationship between MOC with LFL/LEL.

LFL Relate to MOC
LFL is minimum hydrocarbon (HC) concentration in air which a mixture will burn when an ignition source is present. LFL can be written as follow

MOC is minimum oxygen present in hydrcarbon mixture which a mixture will burn. MOC can be written as follow

Combining [1] & [2],

Combustion of Hydrocarbon

Above equation may be used to estimate MOC if you know the LFL of hydrocarbon.

Example
1) A Ethane (C₂) having LFL of 3.0 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of Ethane.

Combustion of C₂H₆,

C₂H₆ + d.O₂ ==> 2CO₂ + 3H₂O

a = 2
b = 6
c = 0
d = 2 + 6 /4 - 0/2 = 3.5

MOC = 3.5 x 3.0 = 10.5 Vol%, close to 11.2 vol% in literature.

2) A n-butane (nC₄) having LFL of 1.86 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of n-butane.

Combustion of C₄H₁₀,

C₄H₁₀ + d.O₂ ==> 4CO₂ + 5H₂O

a = 4
b = 10
c = 0
d = 4 + 10 /4 - 0/2 = 6.5

MOC = 6.5 x 1.86 = 12.1 Vol%. Close to 12.3 vol% in literature.

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Wednesday, September 2, 2009

Estimate Mixture Flammability & Explosivity At Operating P & T

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Earlier post "Estimate Mixture Flammability & Explosivity At Reference P & T" discussed about the Lower flammable limit (LFL) or Lower Explosive Limit (LEL) and Upper flammable limit (UFL) or Upper Explosive Limit (UEL) for single component fluid and mixture at reference pressure (P_ref) and temperature (T_ref). This post will discuss the way to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T) .

Temperature & Pressure Corrected LFL/LEL & UFL/UEL
Below are two equations may be used to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T).

One shall take note that these equations are used for single component. For a mixtures, the LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) will be calculated using following equation (as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T":

Calculation Steps
2 steps in determining mixtures LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) .
(i) Estimate LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) for every component in a mixture
(ii) Estimate mixture LFL/LEL and UFL/UEL

Example
A mixture contains of Methane, Ethane and Propane with volume% of 20%, 20% and 60%. Estimate UEL at (i) 20 degC & 101.325 Pa, (ii) 70 degC & 3 MPa.

Data
Methane (C1)
UEL_C1,20C,1ATM = 15.0%, E_{C1,combustion} = 212.79 kcal/mole

Ethane (C2)
UEL_C2,20C,1ATM = 12.4%, E_{C2,combustion} = 372.81 kcal/mole

Propane (C3)
UEL_C3,20C,1ATM = 10.1%, E_{C3,combustion} = 526.74 kcal/mole

Output
(i) UEL_Mix at 20 degC & 101.325 Pa

UEL_Mix = 1 / [ 0.2/15 + 0.2 /12.4 + 0.6 / 10.1 ]

UEL_Mix = 11.25 vol% at 20 degC & 101.325 kPaA

(ii) UEL_Mix at 70 degC & 3 MPa
UEL_C1,7oC,3MPa
= 15 x [1+0.75(70-20)/212.79]
+ 20.6 x [Log10(3)+1]
= 48.07 vol%

UEL_C2,7oC,3MPa
= 12.4 x [1+0.75(70-20)/372.81]
+ 20.6 x [Log10(3)+1]
= 44.08 vol%

UEL_{C3,7oC, 3MPa}
= 10.1 x [1+0.75(70-20)/526.74]
+ 20.6 x [Log10(3)+1]
= 41.25 vol%

UEL_Mix,70C,3MPa = 1 / [ 0.2/48.07 + 0.2 /44.08 + 0.6 / 41.25 ]

UEL_Mix,70C,3MPa = 43.02 vol% at 70 degC & 3 MPaA

Monday, August 31, 2009

Estimate Mixture Flammability & Explosivity At Reference P & T

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Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present. Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present. Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.

Component LEL & UEL
LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas	(LFL/LEL) (%)	(UEL/UFL) (%)
Acetaldehyde	4	60
Acetone	2.6	12.8
Acetylene	2.5	81
Ammonia	15	28
Arsine	5.1	78
Benzene	1.35	6.65
n-Butane	1.86	8.41
iso-Butane	1.80	8.44
iso-Butene	1.8	9.0
Butylene	1.98	9.65
Carbon Disulfide	1.3	50
Carbon Monoxide	12	75
Cyclohexane	1.3	8
Cyclopropane	2.4	10.4
Dimethyl Ether	3.4	27
Diethyl Ether	1.9	36
Ethane	3	12.4
Ethylene	2.75	28.6
Ethylene Oxide	3.6	100
Ethyl Alcohol	3.3	19
Ethyl Chloride	3.8	15.4
Fuel Oil No.1	0.7	5
Hydrogen	4	75
Isobutane	1.8	9.6
Isopropyl Alcohol	2	12
Gasoline	1.4	7.6
Kerosine	0.7	5
Methane	5	15
Methyl Alcohol	6.7	36
Methyl Chloride	10.7	17.4
Methyl Ethyl Ketone	1.8	10
Naphthalene	0.9	5.9
n-Heptane	1.0	6.0
n-Hexane	1.25	7.0
n-Pentene	1.65	7.7
Neopentane	1.38	7.22
Neohexane	1.19	7.58
n-Octane	0.95	3.20
iso-Octane	0.79	5.94
n-Pentane	1.4	7.8
iso-Pentane	1.32	9.16
Propane	2.1	10.1
Propylene	2.0	11.1
Silane	1.5	98
Styrene	1.1	6.1
Toluene	1.27	6.75
Triptane	1.08	6.69
p-Xylene	1.0	6.0

Note : The limits indicated are for component and air at 20^oC and atmospheric pressure.

Mixture LFL/LEL & UFL/UEL

A mixture is combustible / flammable within mixture LFL/LEL and UFL/UEL. Common units for both limits is mole (or volume) percent fuel in air [moles fuel/(moles fuel + moles air)]. A mixture LFL/LEL and UFL/UEL limits can be calculated using the equations first proposed by Le Chatelier in 1891 :

Example
A vapor contains of 20 vol% of Methane (C1), 20 vol% of Ethane (C2) and 60 vol% of Propane (C3). Find LEL of this mixture at 20 degC and Atmospheric pressure (101325 kPaA).

LEL_C1 = 5 vol% at 20 degC & 101.325 kPaA
LEL_C2 = 3 vol% at 20 degC & 101.325 kPaA
LEL_C3 = 2.1 vol% at 20 degC & 101.325 kPaA

LEL_Mix = 1 / [ 0.2/5 + 0.2 / 3 + 0.6 / 2.1 ]

LEL_Mix = 2.55 vol% at 20 degC & 101.325 kPaA

Above LEL may be linked to MOC as discussed in "Minimum Oxygen Concentration (MOC) for Flare Purge". Vapor mixture flammability & explosivity at Operating P & T discussed in this post.

Saturday, August 29, 2009

Minimum Oxygen Concentration (MOC) for Flare Purge

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A fire is formed or a flame can sustains and propagate, three main elements shall present as defined in well known FIRE triangle. There are combustible material (or fuel), oxygen (O2) and heat. See following image. One additional characteristic shall also present is the potential of chain reaction to maintain continuous combustion before any of these three elements is removed. This speed of chain reactions define if a mixture is combustible (slow) or explosive (fast).

Minimum Oxygen Concentration (MOC)
Oxygen is common obtained from atmosphere. Standard air at Mean Sea Level (MSL) contains 20.95 vol% Oxygen (O2), 78.08 vol% Nitrogen (N2), 0.038 vol% Carbin Dioxide (CO2) and others inert gas i.e. Neon, Xenon, etc. Although Oxygen is the major component in generating fire, there is still a minimum oxygen concentration (MOC) required present in combustible mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form.

Following MOC for some hydrocarbon common found in oil and gas plant.

Component	MOC (Vol% O2)
Hydrogen	4.0
Acetylene	6.2
Methane (C1)	12.0
Ethane (C2)	11.2
Ethylene (C2=)	9.9
Propane (C3)	11.6
Propylene (C3=)	11.5
Butane (C4)	12.3
1-Butene (C4=)	11.0
Pentane (C5)	11.8
Hexane (C6)	11.8
Benzene (Bz)	11.5
Carbon Disulfide	5.0

Base on this principle, a flare header is purged with hydrocarbon (i.e. fuel gas ) to evacuate air that ingressed via flare tip and stack in order to ensure quantity of oxygen level in the flare system is always below minimum oxygen concentration (MOC).

MOC For Flare Purge
From above table, you may noticed that MOC for majority of components are equal to or more than 10 vol% except Hydrogen (H2), Acethylene and Carbon Disulfide (CS2). It is recommended MOC of 6 vol% for flare purging design with 4 vol% as design margin. One shall remember plant releasing large amount of Hydrogen shall use lower MOC with margin i.e. 2 vol%. One of the example is Hydrogenation unit in Refinery plant.

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Monday, August 24, 2009

Emerging Technologies to Monetize Small Natural Gas Resources

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Simple update to IEM Members or those who are practicing Engineering in Malaysia...

The number of large natural gas fields for mankind’s exploitation is reducing as we go into the future. Future natural gas fields are relatively smaller, scattered in various geographical locations and contains higher acidic components. As the world’s energy demand continues, there is an urgent need to develop emerging technologies to monetize small natural gas resources around the globe which could not be economically carried out now.

The emerging technologies have to be safe, economical and more robust than presently available technologies for successful and meaningful gas production. This presentation will share several emerging technologies currently being studied and developed by the oil and gas industry for such purpose.

A talk on "Emerging Technologies to Monetize Small Natural Gas Resources", organized by Chemical Engineering Technical Division, IEM has been scheduled.

Date : 10 October 2009 (Saturday)
Time : 9.00 am – 11.00 am (Refreshment would be served at 11.00 am)
Venue : C&S Lecture Room, 2nd Floor, Wisma IEM, Petaling Jaya

Speaker : Engr. Dr. Chan Tuck Leong

*Any queries, please contact sec@iem.org.my.