Vacuum Condensers

Outlet Temperature and Pressure. It is important to have proper subcooling in the vent end of the unit to prevent large amounts of process vapors from going to the vacuum system along with the inerts.

Control. It is necessary to have some over-surface and to have a proper baffling to allow for pressure control during process swings, variable leakage of inerts, etc. One designer adds 50% to the calculated length for the oversurface. The condenser must be considered part of the control system (similar to extra trays in a fractionator to allow for process swings not controlled by conventional instrumentation).


The inerts will “blanket” a portion of the tubes. The blanketed portion has very poor heat transfer. The column pressure is controlled by varying the percentage of the tube surface blanketed. When the desired pressure is exceeded, the vacuum system will suck out more inerts, and lower the percentage of surface blanketed. This will increase cooling and bring the pressure back down to the desired level. The reverse happens if the pressure falls below that desired. This is simply a matter of adjusting the heat transfer coefficient to heat balance the system.

Figure 1 shows typical baffling. The inerts move through the first part of the condenser as directed by the baffles. The inerts then pile up at the outlet end lowering heat transfer as required by the controller. A relatively large section must be covered by more or less stagnant inerts which are sub-cooled before being pulled out as needed. Without proper baffles, the inerts build up in the condensing section and decrease heat transfer until the pressure gets too high. Then the vacuum valve opens wider, pulling process vapor and inerts into the vacuum system. Under these conditions pressure control will be very poor.

Pressure Drop. Baffling must be designed to keep the pressure drop as low as possible. The higher the pressure drop the higher the energy consumption and the harder the job of attaining proper vent end sub-cooling. Pressure drop is lower at the outlet end because of smaller mass flow.

Bypassing. Baffles should prevent bypass of inlet vapor into the vent. This is very important.

Typical Condenser. Figure 1 illustrates an inlet “bathtub” used for low vacuums to limit pressure drop at entrance to exchanger and across first rows of tubes. Note staggered baffle spacing with large spacing at inlet, and the side to side (40% cut) baffles. Enough baffles must be used in the inlet end for minimum tube support. In the last 25% of the outlet end a spacing of 1/10 of a diameter is recommended.

Source: Based on notes provided by Jack Hailer and consultation by Guy Z. Moore while employed at El Paso Products Co.

CHEMICAL REACTION ENGINEERING

Since before recorded history, we have been using chemical processes to prepare food, ferment grain and grapes for beverages, and refine ores into utensils and weapons. Our ancestors used mostly batch processes because scaleup was not an issue when one just wanted to make products for personal consumption.

The throughput for a given equipment size is far superior in continuous reactors, but problems with transients and maintaining quality in continuous equipment mandate serious analysis of reactors to prevent expensive malfunctions. Large equipment also creates hazards that backyard processes do not have to contend with.

Not until the industrial era did people want to make large quantities of products to sell, and only then did the economies of scale create the need for mass production. Not until the twentieth century was continuous processing practiced on a large scale. The first practical considerations of reactor scaleup originated in England and Germany, where the first large-scale chemical plants were constructed and operated, but these were done in a trial-and-error fashion that today would be unacceptable.

The systematic consideration of chemical reactors in the United States originated in the early twentieth century with DuPont in industry and with Walker and his colleagues at MIT, where the idea of reactor “units” arose. The systematic consideration of chemical reactors was begun in the 1930s and 1940s by Damkohler in Germany (reaction and mass transfer), Van Heerden in Holland (temperature variations in reactors), and by Danckwerts and Denbigh in England (mixing, flow patterns, and multiple steady states). However, until the late 1950s the only texts that described chemical reactors considered them through specific industrial examples. Most influential was the series of texts by Hougen and Watson at Wisconsin, which also examined in detail the analysis of kinetic data and its application in reactor design. The notion of mathematical modeling of chemical reactors and the idea that they can be considered in a systematic fashion were developed in the 1950s and 1960s in a series of papers by Amundson and Aris and their students at the University of Minnesota.

In the United States two major textbooks helped define the subject in the early 1960s. The first was a book by Levenspiel that explained the subject pictorially and included a large range of applications, and the second was two short texts by Aris that concisely described the mathematics of chemical reactors. While Levenspiel had fascinating updates in the Omnibook and the Minibook, the most-used chemical reaction engineering texts in the 1980s were those written by Hill and then Fogler, who modified the initial book of Levenspiel, while keeping most of its material and notation.


The major petroleum and chemical companies have been changing rapidly in the 1980s and 1990s to meet the demands of international competition and changing feedstock supplies and prices. These changes have drastically altered the demand for chemical engineers and the skills required of them. Large chemical companies are now looking for people with greater entrepreneurial skills, and the best job opportunities probably lie in smaller, nontraditional companies in which versatility is essential for evaluating and comparing existing processes and designing new processes. The existing and proposed new chemical processes are too complex to be described by existing chemical reaction engineering texts.


The first intent of this text is to update the fundamental principles of the operation of chemical reactors in a brief and logical way. We also intend to keep the text short and cover the fundamentals of reaction engineering as briefly as possible.


Second, we will attempt to describe the chemical reactors and processes in the chemical industry, not by simply adding homework problems with industrially relevant molecules, but by discussing a number of important industrial reaction processes and the reactors being used to carry them out.



Third, we will add brief historical perspectives to the subject so that students can see the context from which ideas arose in the development of modern technology. Further, since the job markets in chemical engineering are changing rapidly, the student may perhaps also be able to see from its history where chemical reaction engineering might be heading and the causes and steps by which it has evolved and will continue to evolve.




Every student who has just read that this course will involve descriptions of industrial process and the history of the chemical process industry is probably already worried about what will be on the tests. Students usually think that problems with numerical answers (5.2 liters and 95% conversion) are somehow easier than anything where memorization is involved. We assure you that most problems will be of the numerical answer type.
However, by the time students become seniors, they usually start to worry (properly) that their jobs will not just involve simple, well-posed problems but rather examination of messy situations where the boss does not know the answer (and sometimes doesn’t understand the problem). You are employed to think about the big picture, and numerical calculations are only occasionally the best way to find solutions. Our major intent in discussing descriptions of processes and history is to help you see the contexts in which we need to consider chemical reactors. Your instructor may ask you to memorize some facts or use facts discussed here to synthesize a process similar to those here. However, even if your instructor is a total wimp, we hope that reading about what makes the world of chemical reaction engineering operate will be both instructive and interesting.

CHEMICAL REACTORS

The chemical reactor is the heart of any chemical process. Chemical processes turn inexpensive chemicals into valuable ones, and chemical engineers are the only people technically trained to understand and handle them. While separation units are usually the largest components of a chemical process, their purpose is to purify raw materials before they enter the chemical reactor and to purify products after they leave the reactor. Here is a very generic flow diagram of a chemical process.

Raw materials from another chemical process or purchased externally must usually be purified to a suitable composition for the reactor to handle. After leaving the reactor, the unconverted reactants, any solvents, and all byproducts must be separated from the desired product before it is sold or used as a reactant in another chemical process. The key component in any process is the chemical reactor; if it can handle impure raw materials or not produce impurities in the product, the savings in a process can be far greater than if we simply build better separation units. In typical chemical processes the capital and operating costs of the reactor may be only 10 to 25% of the total, with separation units dominating the size and cost of the process. Yet the performance of the chemical reactor totally controls the costs and modes of operation of these expensive separation units, and thus the chemical reactor largely controls the overall economics of most processes. Improvements in the reactor usually have enormous impact on upstream and downstream separation processes. Design of chemical reactors is also at the forefront of new chemical technologies. The major challenges in chemical engineering involve


1. Searching for alternate processes to replace old ones,
2. Finding ways to make a product from different feedstocks, or
3. Reducing or eliminating a troublesome byproduct

The search for alternate technologies will certainly proceed unabated into the next century as feedstock economics and product demands change. Environmental regulations create continuous demands to alter chemical processes. As an example, we face an urgent need to reduce the use of chlorine in chemical processes. Such processes (propylene
to propylene oxide, for example) typically produce several pounds of salt (containing considerable water and organic impurities) per pound of organic product that must be disposed of in some fashion. Air and water emission limits exhibit a continual tightening that shows no signs of slowing down despite recent conservative political trends.

Enzyme Fermentation

The term "fermentation" can be used in its original strict meaning (to produce
alcohol from sugar-nothing else) or it can be used more or less broadly. We
will use the modern broad definition:

From the simplest to the most complex, biological processes may be classed as fermentations, elementary physiological processes, and the action of living entities. Further, fermentations can be divided into two broad groups: those promoted and catalyzed by microorganisms or microbes (yeasts, bacteria, algae, molds, protozoa) and those promoted by enzymes (chemicals produced by microorganisms). In general, fermentations are reactions wherein a raw organic feed is converted into product by the action of microbes or by the action of enzymes.

This whole classification is shown in Fig. 27.1.
      Enzyme fermentations can be represented by

Microbial fermentations can be represented by

The key distinction between these two types of fermentation is that in enzyme fermentation the catalytic agent, the enzyme, does not reproduce itself, but acts as an ordinary chemical, while in microbial fermentation the catalytic agent, the cell or microbe, reproduces itself. Within the cells it is the enzyme which catalyses the reaction,

just as in enzyme fermentation; however, in reproducing itself the cell manufactures its own enzyme. In this chapter we introduce enzyme fermentations, in the following chapters we take up microbial fermentations.


source: Octave Levenspiel

Some Recommendation for Process Critical Line

Earlier post “Process Critical Line” has presented a checklist of process critical line. During design phase, these process shall be checked in detail to minimize or avoid problem such as vibration, hammering, capacity reduction, cavitation, etc to occur. This post will further present good engineering practice for process critical line.

Gravity Flow
Any line subject to gravity flow e.g. drain, flare, vent, etc, low pocket shall be avoided. Liquid or solid accumulate in low pocket potentially result corrosion and blockage. Line should be sloped (and/or free draining) from sources to receiver.

Pump Suction
Line to pump suction should be designed to allow self floating as far as possible where lowest liquid level is above the pump highest point.

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Ensure minimum submergence of tank to avoid potential vapor being sucked in to pump suction line due to vortex. If positive submergence is not achievable, installation of vortex breaker is another option. Read more in “Vortex Breaker to Avoid Vapor Entrainment”.

Any high pocket shall be avoided and provision of eccentric reducer at the pump suction to avoid potentially vapor lock prior to pump start-up.

Ensure NPSHa is always higher than NPSHr with a positive margin e.g. 1m for entire operation range (turndown to design capacity) and operation conditions (highest operating temperature). There are 17 Ways to Reduce Likelihood of Pump Cavitation.

Minimize suction length and fitting as much as possible to minimize potential of pump cavitation.

Compressor Suction
Compressor suction knock out drum (KOD) may be equipped with mist eliminator e.g. wiremesh to promote droplet coalescing and separation.

KOD vapor exit nozzle should be designed large enough to minimize exit momentum (rho V2 less than 6000 Pa) in order to minimize reentrainment of coalesced liquid droplet into vapor.

Compressor in general can tolerate small amount of liquid. If absolute no liquid is allowed enter compressor as imposed by compressor manufacturer, one may consider provision of insulation to minimize ambient and JT cooling and heat tracing to compensate heat loss due to above mentioned cooling.

Absolute no low pocket shall present in the compressor suction line as low pocket can accumulate liquid and slug of liquid can cause severe damage to compressor.

May consider a compressor suction strainer for start-up and commissioning. As compressor is sensitive to suction line pressure drop, any additional fitting and device at compressor suction can lead to capacity reduction, installation of suction strainer shall be analyzed in detail during design phase.




Flashing / Two phase Gas-Liquid Flow
Slugging and plugging flow in vertical and horizontal potentially results significant vibration to piping. During process design phase, slugging and plugging flow shall be avoided for entire operating range (turndown to design capacity) and operating conditions.

May consider provision of vapor liquid separation and run separator separate header for vapor and liquid line if slugging / plugging flow is unavoidable. For steam header, provide sufficient steam traps to drain-off condensate and minimize potential of slugging flow.

Extra and strengthen support may be provided to avoid severe vibration and failure on pipe crack.

Liquid-Liquid Coalescer
Vapor generation in liquid-liquid coalescer may accumulate and result under-performed liquid-liquid separation. May consider to provide sufficient static head to suppress vapor generation in liquid-liquid coalescer. It is always recommended to provide a vapor equalization line back to separator to release any vapor form in liquid-liquid coalescer.

Low Pressure Line
Minimizing pressure drop in low pressure line is the key factor to ensure proper performance of system. Minimize line length, fittings, elbow, etc and use of smooth surface pipe e.g. stainless steel may be considered.

Potential Surge Line
Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Therefore proper maintenance of insulation is extremely important in keep steam line from transient surge. Besides, provide sufficient steam trap to eliminate condensate from steam line.

Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Transient surge analysis shall be conducted during design phase to ensure surge is avoided. Slower closure of shutdown valve is one of the key component in minimizing surge in long pipe line. Non-slam check valve on the pump discharge may also assist in minimizing surge in long pipeline with pump. Surge suppression system may be considered in the event surge is unavoidable. One shall take note that provision of pressure relief valve may not help to eliminating surge due to slow response time of PRV.



Pressure Relief Valve Inlet & Outlet
May consider discussion and recommendation in :

• Concerns & Recommendations on PSV INLET line 
• Few Concerns & Recommendations of PSV Discharge Tail pipe

Control Valve & Restriction Orifice
Flow Induced Vibration (FIV) and Acoustic Induced Vibration (AIV) may be studied to identify location of piping which potentially experience high risk of low frequency and high frequency vibration. Minimizing small bore connection may be considered e.g. provision connection with more than 2 inches, avoid using connection smaller than 2 inches. For small bore connection, may consider brazing and extra support to strengthen the connection and avoid pipe cracking.

Anti-cavitation trim could be considered for control valve potentially experience cavitation. Similarly provision of multiple restriction orifice (RO) in series or multi-ported RO may be considered if cavitation occurs in RO.

Related Topic

• Process Critical Line
• Control Valve Cavitation Damage and Solutions
• Problems and Measures for Condensate Recycle Control Valve
• FAQ Related to Control Valves
• Useful Documents Related to Control Valve
• FREE & Reliable Control Valve Sizing Software
• Anti-surge Control (ASC) or Capacity Control (CC) Valve in Vertical Upward Run ?
• Combine Anti-surge control (ASC) & Capacity Control (CC) Functions ?

Understanding Pressure & Measurement

Pressure and measurement can be extremely complex and complicated. However, for most systems it is relatively easy to obtain accurate pressure measurements if the proper techniques are used. 

What is fluid pressure ? Fluid pressure can be understood as the measure of force per-unit-area exerted by a fluid, acting perpendicularly to any surface it contacts (a fluid can be either a gas or liquid, fluid and liquid are not synonymous). The standard SI unit for pressure measurement is the Pascal (Pa) which is equivalent to one newton per square meter (N/m2) or the KiloPascal (kPa) where 1 kPa = 1000 Pa.......

 

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• Definition of Terms Related to PRESSURE
• Oil Characterization Method in HYSYS 
• An Introduction to Oil & Gas Production...
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• Typical Gas Processing Flow Scheme

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Process Critical Line

Process design involve line sizing and pressure profile definition. All line size will be presented in Piping & Instrumentation Diagram (P&ID). Nevertheless, there is no line length, elbow  and elevation define in P&ID. Upon receipt of P&ID, Piping engineer will begin the piping routing activities and assign necessary length, elbow and elevation to the line. This piping routing may not consistent with assumption taken by process engineer during earlier process design. Significant increase in pressure drop, wrong routing of pipe , incorrect sloping, etc could lead to severe vibration, valve chartering, reduced capacity, under-perform equipment, etc. Therefore it is important for a process engineer to identify Process Critical Line for detail isometric checking. Following will tabulate typical line may experience potential problem and required  detail process checking.

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Gravity flow

In general most fluid is transferred by pressure from source to destination during normal operation. Pressure head available at source will overcome frictional loss, velocity head and static head. This allow fluid transfer from low point to elevated point. Typical example is transfer liquid from closed drain drum to production separator with the pressure head develop by a reciprocating pump. This kind of pipe is typically know as pressurized pipe. Nevertheless, there are some fluid is transferred by gravity force (or static head). Typical system is closed drain network, process line designed for gravity transfer, etc. Improper design of gravity flow would lead to reduce or no flow.

Pump Suction

Cavitation is phenomenon cause by bubble generation follow by bubble collapsed. More thorough discussion on cavitation phenomenon, cavitation damages and the way to minimize / avoid cavitation can be found in following post :

• What is pump cavitation ?
• How Pump Cavitation Sound and Looks Like ?
• Why Cavitation is Destructive ?
• Damages by Cavitation 
• Relationship between NPSHa & NPSHr

Typically to minimize / avoid cavitation damage is to ensure Net Positive Suction Head required (NPSHr) by the pump is lower than the NPSH available (NPSHa) by the system itself. Pump suction line size and routing is a dominant factor affects NPSHa. Improper design of pump suction line would lead to severe cavitation, vibration and pump damage.

Centrifugal Compressor Suction

Centrifugal Compressor capacity is subject to designated flow and compressor inlet condition. Any changes in suction condition (e.g. decrease in density) would seriously affect compressor capacity (e.g. decrease in capacity). Improper design of line between Compressor suction Knock-out drum (KOD) and compressor inlet nozzle would lead to high pressure drop, subsequently lower density and capacity decrease. 

Long compressor line (from KOD to compressor) would increase potential of heat loss to ambient (severe during winter time) and results condensation. Present of condensate in vapor to compressor and impinge on compressor impeller when vapor is accelerated potentially damage compressor impeller and severe vibration in compressor.

Flare/Vent Collection Header

Flare / vent collection header has significant impact on built-up backpressure to pressure relief valve. (PRV) Severe pressure drop can lead built-up back pressure exceed it allowable limit e.g. 10% for conventional type PRV. Warm fluid mix with cold fluid in flare header may results two phase gas liquid flow in flare header. Similarly, severe flare header vibration can occur with the present of slugging / plugging flow.  Low point in flare line potentially results liquid accumulation in flare line and corrosion may occur. In the major relief event, high velocity vapor pushing accumulated liquid would results slugging flow in the flare line. Liquid column flowing at vapor velocity knocking of elbow/bend may generate severe vibration.

Flashing / Two phase Gas-Liquid Flow

Liquid at saturation point coming from separator potentially flash and two phase gas liquid flow. Typical flow regime is Bubbly flow. Similarly saturated vapor experience ambient cooling and line frictional loss results condensation and two phase gas liquid flow. Typical flow regime is Mist flow. Both Bubbly and Mist flow are not destructive in nature and properly a normal support would be sufficient. Nevertheless, slugging and plugging flow in vertical and horizontal potential results significant vibration to piping. Extra and strengthen support is required to avoid severe vibration and failure on pipe crack. More discussion on Problems Caused by Two Phase Gas-Liquid Flow.

Liquid-Liquid Coalescer

Saturated liquid from separator feeding liquid-liquid separator, any pressure drop increase potentially lead to vapor accumulation and  under-performed liquid-liquid separation.

Low Pressure Line

Low pressure stream e.g. overhead from amine regeneration column, end flash gas from end flash column, etc is very sensitive to frictional loss.Low pressure here is pressure very close to atmospheric pressure. Any increase in frictional loss will seriously reduce flow through the pipe.

Potential Surge line

Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Piping surge is severe in nature and potentially lead to pipe crack and support failure.

Wet Corrosive Service

Some line is normally flow with vapor contains CO2 & H2S and sulfide stress corrosion cracking (SSCC) and general CO2 corrosion is not expected as only vapor flow. During winter low ambient temperature and under turndown operation, ambient cooling potentially lead to vapor condensation and induced SSCC and general corrosion on under-designed piping. Typical example is Condensate stabilizer overhead. Similarly warm wet flare header is normally dry due to continuous dry gas purging. In the event, PRV passing leaks wet vapor into warm wet header or any PRV open follow by closure of PRVs, wet vapor potentially condensed and accumulate in low point and results general corrosion.

Critical Pressure drop line

Line normally design for low pressure drop, any increase in pressure drop could to capacity reduction and/or under-perform downstream unit. Typical example is high pressure gas feeding liquefaction Main Cryogenic Heat Exchanger. Any reduction in Feed pressure to MCHE would lead to higher heat of vaporization and reduce LNG production.

Pressure Relief Valve Inlet

Under normal design condition, PRV inlet line non-recoverable pressure loss shall be limited to 3% of PRV set pressure  Any significant increase in line length and elbow (due to piping routing) will results non-recoverable pressure loss increase and lead to PRV chattering. 

Pressure Relief Valve Outlet

Upon opening of PRV, instantaneous large gas or vapor passing PRV. High frequency noise is generated results acoustic induced vibration (AIV)  which potentially cause discharge pipe cracking. Instantaneous large gas/vapor flow accelerated from zero velocity to maximum velocity will induced high reaction force to downstream piping. Under-designed pipe may crack on high reaction force.

Control valve and Restriction Orifice

Fluid passing control valve and restriction orifice continuously will generate low frequency noise. This noise wave will be transmitted to downstream piping and result Flow Induced Vibration (FIV) which potentially
leads to pipe cracking in particular at small bore connection to large line

Saturated liquid passing a control valve or restriction orifice, pressure will began to decrease and lowest pressure closed to vena contracta, follow by pressure recovery once is passed the vena contracta. Lowest pressure point could be lower than vapor pressure of fluid. Vapor bubble will begin to form and once fluid passed through the vena contracta, vapor will start to collapse and results jet wave impacting control valve or restriction or piping. Above phenomenon generally known as cavitation which generate severe vibration to the piping.

Any scenario is normally ignore or miss by engineer where control valve downstream piping may not design for occurrence sonic flow downstream piping. This typically occur in line with control valve discharge to flare/vent header. Sonic flow potentially reduce flowing capacity and how reaction force to piping.

Related Topic

• Control Valve Cavitation Damage and Solutions
• Problems and Measures for Condensate Recycle Control Valve
• FAQ Related to Control Valves
• Useful Documents Related to Control Valve
• FREE & Reliable Control Valve Sizing Software
• Anti-surge Control (ASC) or Capacity Control (CC) Valve in Vertical Upward Run ?
• Combine Anti-surge control (ASC) & Capacity Control (CC) Functions ?

Introduction to Oil & Gas Production Presentation

Oil has been used for lighting purposes for many thousand years. In areas where oil is found in shallow reservoirs, seeps of crude oil or gas may naturally develop, and some oil could simply be collected from seepage or tar ponds. Historically, we know of tales of eternal fires where oil and gas seeps would ignite and burn. One example 1000 B.C. is the site where the famous oracle of Delphi would be built, and 500 B.C. Chinese were using natural gas to boil water. But it was not until 1859 that "Colonel" Edwin Drake drilled the first successful oil well, for the sole purpose of finding oil.

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Earlier post "An Introduction to Oil & Gas Production..." has presented a handbook to provide readers with an interested in the oil and gas production industry an overview of the main processes and equipment. This handbook will also provides enough detail to let the engineer get an appreciation of the main characteristics and design issues.,

Similarly, Ta Quoc Dung has made a presentation on "Introduction to Oil & Gas Production". This presentation consists of five (5) chapters.

1. Introduction
2. Process overview
3. Performance of Flowing well
4. Artificial lift
5. Enhanced oil recovery

Going through this presentation, it allows reader

• to have overview of Petroleum Production Technology
• to understand the role of Production Engineer in a Petroleum Operating Company
• to understand production system and its onshore and offshore facilities 
• to understand concept of inflow performance, lift performance and integrated nature
• to understand enhanced oil recovery process

 


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• An Introduction to Oil & Gas Production...
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Classification of Reactions

There are many ways of classifying chemical reactions. In chemical reaction
engineering probably the most useful scheme is the breakdown according to
the number and types of phases involved, the big division being between the
homogeneous and heterogeneous systems. A reaction is homogeneous if it takes
place in one phase alone. A reaction is heterogeneous if it requires the presence
of at least two phases to proceed at the rate that it does. It is immaterial whether
the reaction takes place in one, two, or more phases; at an interface; or whether
the reactants and products are distributed among the phases or are all contained
within a single phase. All that counts is that at least two phases are necessary
for the reaction to proceed as it does.

Sometimes this classification is not clear-cut as with the large class of biological
reactions, the enzyme-substrate reactions. Here the enzyme acts as a catalyst in
the manufacture of proteins and other products. Since enzymes themselves are
highly complicated large-molecular-weight proteins of colloidal size, 10-100 nm,
enzyme-containing solutions represent a gray region between homogeneous and
heterogeneous systems. Other examples for which the distinction between homogeneous
and heterogeneous systems is not sharp are the very rapid chemical
reactions, such as the burning gas flame. Here large nonhomogeneity in composition
and temperature exist. Strictly speaking, then, we do not have a single phase,
for a phase implies uniform temperature, pressure, and composition throughout.
The answer to the question of how to classify these borderline cases is simple.
It depends on how we choose to treat them, and this in turn depends on which description we think is more useful. Thus, only in the context of a given situation
can we decide how best to treat these borderline cases.



Cutting across this classification is the catalytic reaction whose rate is altered
by materials that are neither reactants nor products. These foreign materials,
called catalysts, need not be present in large amounts. Catalysts act somehow as
go-betweens, either hindering or accelerating the reaction process while being
modified relatively slowly if at all.
Table 1.1 shows the classification of chemical reactions according to our scheme
with a few examples of typical reactions for each type.

CE April 2010

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Mechanical Design Aspects for High-Performance Agitated Reactors
An understanding of the mechanical design helps in specifying, maintaining and also revamping agitated reactor systems

FAYF - Measurement Guide for Replacement Seals

This one page reference guide outlines the major steps in measuring machinery for replacement of seals

Full-Length Sleeving for Process Heat Exchanger Tubes
How to calculate the effects of temperature when the sleeves have a higher coefficient of thermal expansion than the installed tubes

A Safety-Centered Approach to Industrial Lighting
The proper design and operation of lighting is essential to ensure plant safety and support good maintenance practices

Solids Processing - Particle Size Measurement
A survey of modern measurement technologies demonstrates how selection criteria vary by application
 

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HP April 2010

REE Hydrocarbon Processing for APRIL 2010 is available now...

Upgrade syngas production
Advancements of synthesis gas processes are key to improved GTL profitability

Plastics enable better automobile designs
High-quality advanced engineered polymers and new molding methods provide advantages in modern vehicle construction and manufacturing processes

Improve inerting practices at your facility
The petrochemical/chemical industry relies on inerting methods to safeguard facilities and maintain product qualities

Upgrade low-value refinery streams into higher-value petrochemicals
New catalytic olefin cracking process yields more propylene over ethylene from stranded refining materials

Update: Spent caustic treatment
Better operating practices and prevention methods reduce problems in handling ‘red oil’

What every manager should know about layers of protection analysis
New methods ‘quantify’ the frequency of risky events in a facility

Looking for improved diesel yields?
Consider using spectro-molecular control to maximize profits

The six sigma green belt training program : An in-depth look
Implementing this program improves competition

Solve liquid-hammer problems
Here are several options

Blower selection for wastewater aeration
Use these guidelines to understand the many factors that differentiate different designs

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MASS AND ENERGY BALANCES

Mass Balances (Sinnott, 1985)

Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. A good understanding of material balance calculations is essential in process design. Here the fundamentals of the subject are covered, using simple examples to illustrate each topic. Practice is needed to develop expertise in handling what can often become very involved calculations. More examples and a more detailed discussion of the subject can be found in the numerous specialist books written on material and energy balance computations.

Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design, to extend the often limited data available from the plant instrumentation, to check instrument calibrations, and to locate sources of material loss.

The general conservation equation for any process system can be written as:

Material out = Material in + Generation - Consumption - Accumulation

For a steady-state process the accumulation term will be zero. Except in nuclear processes, mass is neither generated nor consumed; but if a chemical reaction takes place a particular chemical species may be formed or consumed in the process. If there is no chemical reaction the steady-state balance reduces to

Material out = Material in

A balance equation can be written for each separately identifiable species present, elements, compounds or radicals; and for the total material.

Example

2000 kg of a 5 per cent slurry of calcium hydroxide in water is to be prepared by diluting a 20 per cent slurry. Calculate the quantities required. The percentages are by weight.

Let the unknown quantities of the 20% solution and water be X and Y respectively. Material balance on Ca(OH)2

eq. a
Balance on water:
eq. 2

From equation (a) X = 500 kg
Substituting into equation (b) gives Y = 1500 kg
Chech material balance on total quantity:

X + Y = 2000
500 + 1500 = 2000, correct

to be continued...

source: www.kimyamuhendisi.com

Overview of Chemical Reaction Engineering

Every industrial chemical process is designed to produce economically a desired
product from a variety of starting materials through a succession of treatment
steps. Figure 1.1 shows a typical situation. The raw materials undergo a number
of physical treatment steps to put them in the form in which they can be reacted
chemically. Then they pass through the reactor. The products of the reaction
must then undergo further physical treatment-separations, purifications, etc.-
for the final desired product to be obtained.
Design of equipment for the physical treatment steps is studied in the unit
operations. In this book we are concerned with the chemical treatment step of
a process. Economically this may be an inconsequential unit, perhaps a simple
mixing tank. Frequently, however, the chemical treatment step is the heart of
the process, the thing that makes or breaks the process economically.

Design of the reactor is no routine matter, and many alternatives can be
proposed for a process. In searching for the optimum it is not just the cost of
the reactor that must be minimized. One design may have low reactor cost, but
the materials leaving the unit may be such that their treatment requires a much
higher cost than alternative designs. Hence, the economics of the overall process
must be considered.
Reactor design uses information, knowledge, and experience from a variety
of areas-thermodynamics, chemical kinetics, fluid mechanics, heat transfer,
mass transfer, and economics. Chemical reaction engineering is the synthesis of
all these factors with the aim of properly designing a chemical reactor.
To find what a reactor is able to do we need to know the kinetics, the contacting
pattern and the performance equation. We show this schematically in Fig. 1.2.

Figure 1.1 Typical chemical process.



Figure 1.2 Information needed to predict what a reactor can do.

Much of this book deals with finding the expression to relate input to output
for various kinetics and various contacting patterns, or

output = f [input, kinetics, contacting]

This is called the performance equation. Why is this important? Because with
this expression we can compare different designs and conditions, find which is
best, and then scale up to larger units.



source: Octave Levenspiel

ACTIVITY COEFFICIENTS OF ACIDS, BASES, AND SALTS

This table gives mean activity coefficients at 25°C for molalities in the range 0.1 to 1.0. See the following table for definitions, references, and data
over a wider concentration range.





click image for size large


source: Petr Vany´sek

CO2 - Supercritical fluid...

Carbon Dioxide (CO2) is known as one of industrial waste gas causing global warming...there are number of measures are implemented to reduce global warming. CO2 reduction measures are CO2 capturing, reinjection into aquifer, injection into reservior for well maintenance, gas injection for enhanced oil recovery (EOR), etc. CO2 being captured will be injected in the reservior. The injection can be as high as 300-400 atm which is higher than the critical pressure of CO2. Carbon dioxide is in its supercritical fluid state when both the temperature and pressure equal or exceed the critical point of 31°C and 73 atm (see diagram). In its supercritical state, CO2 has both gas-like and liquid-like qualities, and it is this dual characteristic of supercritical fluids that provides the ideal conditions for extracting compounds with a high degree of recovery in a short period of time. Read more in below...

 

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Suspicious Discrepancy In Supercritical Fluid Relieving Calculation

Pressurized liquid or vapor-liquid in equilibrium or vapor only system during normal operation, when it is expose to external fire attack, heat inputs into vessel (or system) may possibly increase internal pressure and temperature. For system with high design pressure (or Maximum allowable working pressure, MAWP), the pressure relief valve (PRV) protecting the vessel (or system) may have same set pressure as the design pressure (or MAWP). Subject to design code of the vessel, overpressure allowed by code is different from code to code. Typically for ASME unfired vessel section VIII, the maximum allowable overpressure is 10% of set pressure. This will results maximum allowable accumulation pressure (or relieving pressure) reach at 110% of set pressure.

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In some event, the relieving pressure is higher than the fluid critical pressure. For example, a CO2 injection compressor, the injection pressure at is 65 barg. The design pressure may be 72 barg and relieving pressure is approximately 79.2 barg, is higher than CO2 critical pressure of 72.9 barg. The PRV is relieving at supercritical condition.

Conventional method in determining PRV orifice size is presented in API RP-521. Part 1. It considered all “un-wetted” vessels are same regardless the fluid is supercritical, a vapor or a gas. Nevertheless, one shall take note that  this method based on the physical properties of air and the perfect gas laws with no change in fluid temperature. 

• Supercritical fluid may not follow perfect gas law
• Low compressibility of supercritical fluid (e.g. 0.5 to 0.7)
• Change in fluid temperature during relieving

API method may be conservative, there are chattering and oversized PRV and discharge problem. A  rigorous method has been discussed by R.C. DOANE in "Designing for pressure safety valves in supercritical service" published in Hydrocarbon Processing Jan 2010. This method assuming all thermodynamic paths are well defined by a Process Simulator. Thermodynamic path of fluid from operating condition to relieving and drop to back pressure to PRV may be defined by four typical steps.

1. Constant Specific Volume path (Initial to Relieving)
2. Constant Pressure path (Extended relieving)
3. Constant Entropy path (PRV relieving path)

For Step 1, earlier post "Constant Density To Obtain Relieving Condition" has discussed similar subject previously. 

For the step 3, another definition of PRV relieving path can be Isentalpic from relieving to throat follow by isentropic from throat to PRV backpressure as discussed in "Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve".

In the "Designing for pressure safety valves in supercritical service" article, table 1 "Supercritical relief valve sizing example problem—normal butane" has tabulated step-by-step calculation. This table has incorporated equation 1 to equation 10 in this article. The calculation consist of segment 1-to-2, 2-to-3, 3-to-4 and 4-to-5. In recent work in establishing similar task carried out by this example, one suspicious discrepancy is identified. Details as follow.

From segment 1-to-2 to segment 4-to-5, volumetric flow is increased from 1304 ft3/hr to 1315 ft3/hr (segment 2-to-3) and decreased in subsequent segments to 1250 ft3/hr. The volumetric flow is in the range of 1250 to 1315 ft3/hr. HOWEVER, mass flow is increased almost double from 6,374 lb/hr to 13,405 lb/hr (segment 2-to-3) and decreased in similar range (11620 to 12,371 lb/hr). Why there is significant different in segment 1-to-2 compare to other segment ?

Detail checking found that Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition for segment 1-to-2. On the other hand, Mass Flux (G in lb/ft2s) is calculated by dividing Orifice Velocity (V) by Specific Volume (V) at outlet condition for other segments. Different method in calculating Mass Flux has created the discrepancy.

Opinion

As the flow is choked (or highest velocity for subsonic) at the PRV nozzle,  Mass Flux (G in lb/ft2s) should be calculated by dividing Orifice Velocity (v) by Specific Volume (V) at orifice condition.

Well... Above query will be raised and response will be posted once we have received it.

Related Topics

• Tedious & Simple Method In Determining Specific volume For Isentropic Nozzle Flow Mass Flux
• Constant Density To Obtain Relieving Condition
• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• API Std 520 Part 1 Dec 2008 is Released
• API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
• Requirement of overpressure protection devices on system design to PIPING code