Predictive and Preventive Maintenance

Practical Advice for Predictive and Preventive Maintenance

With shrinking budgets and fewer resources, plant management must find ways to improve equipment reliability while optimizing cost efficiencies, says Bart Winters, senior marketing manager, Reliability Solutions, Honeywell Process Solutions. A key factor to ensuring equipment reliability is to focus on predictive and preventive maintenance, he says. Following, according to Winters, are crucial things to consider for predictive and preventive maintenance:

• Start at the beginning. Develop overall condition/performance assessment of plant wide assets
• Know your assets. Understand the current state of each asset and its condition
• Develop collaboration. Create a partnership between operations, maintenance and suppliers for quicker resolution of issues
• Prevent surprises. Reduce equipment damage by operating in the design envelope
• Performance and condition monitoring. Monitor instruments and the control system, rotating equipment and fixed equipment to predict behavior and detect conditions before failure
• Early event detection. Use performance-to-failure model to detect issues early
• Ongoing maintenance. Implement a continuous improvement process to ensure optimum asset performance

"Consider that just one percent improvement of overall equipment effectiveness at a plant can equal roughly $1 million in savings, and it’s easy to see that sites embracing proactive and predictive maintenance, including best-in-class work practices and technology, can improve operational performance, asset reliability, manage costs and minimize downtime," says Winters.

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Mass Balance

A mass balance (also called a material balance) is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique. The exact Conservation law used in the analysis of the system depends on the context of the problem but all revolve around mass conservation, i.e. that matter cannot disappear or be created spontaneously.

Mass balances are used widely in engineering and environmental analyses. For example mass balance theory is used to design chemical reactors, analyse alternative processes to produce chemicals as well as in pollution dispersion models and other models of physical systems. Closely related and complementary analysis techniques include the population balance, energy balance and the somewhat more complex entropy balance. These techniques are required for thorough design and analysis of systems such as the refrigeration cycle.

In environmental monitoring the term budget calculations is used to describe mass balance equations where they are used to evaluate the monitoring data (comparing input and output, etc.) In biology the dynamic energy budget theory for metabolic organisation makes explicit use of time, mass and energy balances.

Introduction

The general form quoted for a mass balance is The mass that enters a system must, by conservation of mass, either leave the system or accumulate within the system .

Mathematically the mass balance for a system without a chemical reaction is as follows:

Input = Output + Accumulation

In the absence of a chemical reaction the amount of any chemical species flowing in and out will be the same; This gives rise to an equation for each species in the system. However if this is not the case then the mass balance equation must be amended to allow for the generation or depletion of each chemical species. Note that the one term (depletion or generation) is used in the equation, which will be negative for depletion and positive for generation. This modified equation can be used not only for reactive systems, but for population balances such as occur in particle mechanics problems. The amended equation is given below; Note that it simplifies to the earlier equation in the case that the generation term is zero.

Input + Generation = Output + Accumulation

• In the absence of a nuclear reaction the number of atoms flowing in and out are the same, even in the presence of a chemical reaction
• To perform a balance the boundaries of the system must be well defined
• Mass balances can be taken over physical systems at multiple scales.
• Mass balances can be simplified with the assumption of steady state, where the accumulation term is zero

Illustrative example

Diagram showing clarifier example

At this point a simple example shall be given for illustrative purposes. Consider the situation whereby a slurry is flowing into a settling tank to remove the solids in the tank, solids are collected at the bottom by means of a conveyor belt partially submerged in the tank, water exits via an overflow outlet.

In this example we shall consider there to be two species, solids and water. The species are concentrated in each of the output streams, that is to say that the water to solid ratio at the water overflow outlet is higher than at the slurry inlet and the solids concentration at the exit of the conveyor belt is higher than that at the slurry inlet.

Assumptions

• Steady state
• Non-reactive system

Analysis

The slurry inlet composition has been measured by sampling the inlet and has a composition (by mass) of 50% solid and 50% water, with a mass flow of 100 kg per minute, the tank is assumed to be operating at steady state, and as such accumulation is zero, so input and output must be equal for both the solids and water. If we know that the removal efficiency for the slurry tank is 60%, then the water outlet will contain 20Kg/min of solids (40% times 100Kg/min times 50% solids). If we measure the flow rate of combined solids and water the water outlet to be 60Kg per minute then the amount of water exiting via the conveyor belt is 10Kg/min. This allows us to completely determine how the mass has been distributed in the system with only limited information and using the mass balance relations across the system boundaries

Mass Feedback (Recycle)

Cooling towers are a good example of a recycle system

Mass balances can be performed across systems which have cyclic flows. In these systems output streams are fed back into the input of a unit for often for further reprocessing.

Such systems are common in grinding circuits, where materials are crushed then sieved to only allow a particular size of particle out of the circuit and the larger particles are returned to the grinder. However recycle flows are by no means restricted to solid mechanics operations, they are used in liquid and gas flows as well. One such example is in cooling towers, where water is pumped through the cooling tower many times, with only a small quantity of water drawn off at each pass (to prevent solids build up) until it has either evaporated or exited with the drawn off water.

The use of the recycle aids in increasing overall conversion of input products, which is useful for low per-pass conversion processes, for example the Haber process.

Differential Mass balances

A mass balance can also be taken differentially. The concept is the same as for a large mass balance, however it is performed in the context of a limiting system (for example, one can consider the limiting case in time or, more commonly, volume). The use of a differential mass balance is to generate differential equations that can be used to provide an understanding and effective modelling tool for the target system.

The differential mass balance is usually solved in two steps, firstly a set of governing differential equations must be obtained, and then these equations must be solved, either analytically or, for less tractable problems, numerically.

A good example of the applications of differential mass balance are shown in the following systems:

1. Ideal (stirred) Batch reactor
2. Ideal tank reactor, also named Continuous Stirred Tank Reactor (CSTR)
3. Ideal Plug Flow Reactor (PFR)

Ideal Batch reactor

A closed system. Many chemistry textbooks implicitly assumes that the studied system can be described as a batch reactor when they write about reaction kinetics and chemical equilibrium The mass balance for a substance A becomes

IN + PROD = OUT + ACC

where r_A denotes the rate at which substance A is produced, V is the volume (which may be constant or not), n_A the number of moles (n) of substance A.

In a fed-batch reactor some reactants/ingredients are added continuously or in pulses (compare making porridge by either first blending all ingredients and the let it boil, which can be described as a batch reactor, or by first mixing only water and salt and making that boil before the other ingredients are added, which can be described as a fed-batch reactor). Mass balances for fed-batch reactors become a bit more complicated.

Reactive Example

In this example we will use the law of mass action to derive the expression for a chemical equilibrium constant.

Assume we have a closed reactor in which the following liquid phase reversible reaction occurs:

The mass balance for substance A becomes

IN + PROD = OUT + ACC

As we have a liquid phase reaction we can (usually) assume a constant volume and since n_A = V * C_A we get

or

In many text books this is given as the definition of reaction rate without specifying the implicit assumption that we are talking about reaction rate in a closed system with only one reaction. This is an unfortunate mistake that has confused many students over the years.

According to the law of mass action the forward reaction rate can be written as

r₁ = k₁[A]^a[B]^b

and the backward reaction rate as

r _{− 1} = k _{− 1}1[C]^c[D]^d

The rate at which substance A is produced is thus

r_A = r _{− 1} − r₁

and since, at equilibrium, the concentration of A is constant we get

or, rearranged

Ideal tank reactor/Continuously stirred tank reactor

An open system. A lake can be regarded as a tank reactor and lakes with long turnover times (e.g. with a low flux to volume ratio) can for many purposes be regarded as continuously stirred (e.g. homogeneous in all respects). The mass balance becomes

IN + PROD = OUT + ACC

where Q_0 and Q denote the volumetric flow in and out of the system respectively and C_A_0 and C_A the concentration of A in the inflow and outflow respective. In an open system we can never reach a chemical equilibrium. We can, however, reach a steady state where all state variables (temperature, concentrations etc.) remain constant (ACC = 0)

Ideal Plug Flow Reactor (PFR)

An open system with no mixing along the reactor but perfect mixing across the reactor. Often used for systems like rivers and water pipes if the flow is turbulent. When a mass balance is made for a tube, one first considers an infinitesimal part of the tube and make a mass balance over that using the ideal tank reactor model. That mass balance is then integrated over the entire reactor volume to obtain:

In numeric solutions, e.g. when using computers, the ideal tube is often translated to a series of tank reactors, as it can be shown that a PFR is equivalent to an infinite number of stirred tanks in series, but the latter is often easier to analyze, especially at steady state.

More complex problems

In reality, reactors are often non-ideal, in which combinations of the reactor models above are used to describe the system. Not only chemical reaction rates, but also mass transfer rates may be important in the mathematical description of a system, especially in heterogeneous systems.

As the chemical reaction rate depends on temperature it is often necessary to make both an energy balance (often a heat balance rather than a full fledged energy balance) as well as mass balances to fully describe the system. A different reactor models might be needed for the energy balance: A system that is closed with respect to mass might be open with respect to energy e.g. since heat may enter the system through conduction.

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Ultra-Low Sulfur Diesel (ULSD)

As the EPA mandate for 100% conversion by 2010 to ultra-low sulfur diesel (ULSD) fuel approaches, stakeholders are on track to resolve or prevent issues that may arise, according to Michael Harrigan of Zen Fuels, LLC, in Ann Arbor, Michigan. “The awareness exists,” says Harrigan, “of what some of the potential issues might be as the oil companies actually produce this fuel.” He cautions that ultra-low sulfur diesel must be forthcoming in order to meet the particulate standards because sulfur fosters the production in the particulate of soot during the combustion process. Key learnings from the run-up to the 80% mandate have proved beneficial in more ways than were anticipated. “We’ve been through the whole business of removing the sulfur and having lubricity issues,” says Harrigan, “but as I understand it, that’s been pretty well addressed, and because of the lubricity-enhancing qualities of biodiesel, or fatty-acid methyl esters, its use provides a kind of a win-win for the biodiesel groups in having at least two or three percent biodiesel in all of the diesel fuels.”

Issues of ultra-low sulfur in gasoline have created different challenges. In the production of the fuel, the process of sulfur-compound removal left behind extremely aggressive, highly chemically active sulfur that attacked fuel-indication systems. “There were numerous instances around the United States,” says Harrigan, “of the fuel-level sensors inside the fuel tank being contaminated and ceasing to function.” The aggressive sulfur attacked the variable resistance cards and produced a deceptively high resistance. “As luck would have it,” Harrigan notes, “high resistance would show up on the gas gauge as a full tank. So people would be fooled into believing they had a lot of fuel, and then eventually, they would run out.” Harrigan is confident, however, that the industry understands these threats and is equipped with solutions. Standards and ASTM qualification tests have been developed to address concerns as the refineries change over to producing ultra-low sulfur fuels. “I think there’s adequate industry awareness of the potential issues,” says Harrigan, “and also awareness of the preventive techniques and corrective actions that are necessary for things to happen smoothly for the diesel-fuel-using industry, the customers.”

An issue that may not have been fully thought out by those in the industry relates to the handling of the ULSD. “They think of it in terms of refining it or of burning it in the combustion chamber of the engine, not what happens in between.” Yet, with a note of optimism, Harrigan adds, “I think that we’re ready. I think that we understand the fuel, with this aggressive sulfur, and that steps have to be taken in the design of the components in the system that could be affected to make them more robust against those types of issues.” In addition, he adds, “We’re still counting on the oil industry to maintain the quality of the fuel coming in.” He anticipates only minimal problems and perhaps the occasional quality breakthrough as the refineries become accustomed to producing the fuel on a day-in-and-day-out basis.

Difficulties might arise in terms of handling if the levels of biodiesel in USLD rise. “I don’t believe anyone has any real concerns about the fuel at the two or three percent level, but the biodiesel industry is pushing for 20% fatty-acid methyl ester in the fuel.” Though the issue of higher percentages of biodiesel—the nature of the biodiesel and how its chemistry affects materials in the system—is being actively studied, past examples can demonstrate what happens when caution is abandoned. According to Harrigan, “there have been attempts to rush this into the marketplace with some pretty public embarrassments to the biodiesel industry when things didn’t work out as they intended, especially in the wintertime.” To avoid such incidents from occurring in the future, the industry needs a proactive approach. “It remains for the industry to police itself,” Harrigan concludes, “or run the risk of there being some government regulations added to keep things running smoothly.”

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Distillation theoretical plate

A theoretical plate in many chemical engineering separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Those zones or stages may also be referred to as a theoretical tray or an equilibrium stage. The performance of many separation processes depends on having a series of equilibrium stages and is enhanced by providing more such stages. In other words, having more theoretical plates increases the efficacy of the separation process be it either a distillation, absorption, chromatographic, adsorption or similar process.

Applications

The concept of theoretical plates and trays or equilibrium stages is used in the design of many different types of separation.

Distillation columns

The concept of theoretical plates in designing distillation processes has been discussed in many reference texts. Any physical device that provides good contact between the vapor and liquid phases present in industrial-scale distillation columns or laboratory-scale glassware distillation columns constitutes a "plate" or "tray". Since an actual, physical plate is rarely a 100% efficient equilibrium stage, the number of actual plates is more than the required theoretical plates.

where:

• N_a = the number of actual, physical plates or trays
• N_t = the number of theoretical plates or trays
• E = the plate or tray efficiency

So-called bubble-cap or valve-cap trays are examples of the vapor and liquid contact devices used in industrial distillation columns. The trays or plates used in industrial distillation columns are fabricated of circular steel plates and usually installed inside the column at intervals of about 60 to 75 cm (24 to 30 inches) up the height of the column. That spacing is chosen primarily for ease of installation and ease of access for future repair or maintenance.

Bubble-cap trays in an indutrial distillation column.

For example, a very simple tray would be a perforated tray. The desired vapor and liquid contacting would occur as the vapor flowing upwards through the perforations would contact the liquid flowing downwards through the perforations. In current modern practice, as shown in the adjacent diagram, better contacting is achieved by installing bubble-caps or valve caps located at each perforation to promote the formation of vapor bubbles flowing through a thin layer of liquid maintained by a weir on each tray.

To design a distillation unit or a similar chemical process, the number of theoretical trays or plates (that is, hypothetical equilibrium stages), N_t, required in the process should be determined, taking into account a likely range of feedstock composition and the desired degree of separation of the components in the output fractions. In industrial continuous fractionating columns, N_t is determined by starting at either the top or bottom of the column and calculating material balances, heat balances and equilibrium flash vaporizations for each of the succession of equilibrium stages until the desired end product composition is achieved. The calculation process requires the availability of a great deal of vapor-liquid equilibrium data for the components present in the distillation feed, and the calculation procedure can be very complex.

In an industrial distillation column, the N _t required to achieve a given separation also depends upon the amount of reflux used. Using more reflux decreases the number of plates required and using less reflux increases the number of plates required. Hence, the calculation of N_t is usually repeated at various reflux rates. N_t is then divided by the tray efficiency, E, to determine the actual number of trays or physical plates, N_a, needed in the distillation column. The final design choice of the number of trays to be installed in a distillation column is then selected based upon an economic balance between the cost of additional trays and the cost of using a higher reflux rate.

There is a very important distinction between the theoretical plate terminology used in discussing conventional distillation trays and the theoretical plate terminology used in the discussions below of packed bed distillation or absorption or in chromatography or other applications. The theoretical plate in conventional distillation trays has no height. It is simply a hypothetical equilibrium stage. However, the theoretical plate in packed beds, chromatography and other applications is defined as having a height.

Distillation and absorption packed beds

Distillation and absorption separation processes using packed beds for vapor and liquid contacting have an equivalent concept referred to as the plate height or the height equivalent to a theoretical plate (HETP). HETP arises from the same concept of equilibrium stages as does the theoretical plate and is numerically equal to the absorption bed length divided by the number of theoretical plates in the absorption bed (and in practice is measured in this way).

where:

• N_t = the number of theoretical plates (also called the "plate count")
• H = the total bed height
• HETP = the height equivalent to a theoretical plate

The material in packed beds can either be random dumped packing (1-3" wide) such as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapors contact the wetted surface, where mass transfer takes place.

Chromatographic processes

The theoretical plate concept was also adapted for chromatographic processes by the British chemists and Nobel Prize winners, Martin and Synge. The IUPAC's Gold Book provides a definition of the number of theoretical plates in a chromatography column.

The same equation applies in chromatography processes as for the processes, namely:

where:

• N_t = the number of theoretical plates (also called the "plate count")
• H = the total column length
• HETP = the height equivalent to a theoretical plate

Other applications

The concept of theoretical plates or trays applies to other processes as well, such as capillary electrophoresis and some types of adsorption.

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McCabe-Thiele method

The McCabe-Thiele method is a graphical approach, published by McCabe and Thiele in 1925, considered to be the simplest and most instructive method for the analysis of binary distillation. This method uses the fact that the composition at each theoretical plate (or equilibrium stage) is completely determined by the mole fraction of one of the two components.

The McCabe-Thiele method is based on the assumption of constant molar overflow which requires that:

• the molal heats of vaporization of the feed components are equal
• for every mole of liquid vaporized, a mole of vapor is condensed
• heat effects such as heats of solution and heat transfer to and from the distillation column are negligible.

Construction and use of the McCabe-Thiele diagram

Before starting the construction and use of a McCabe-Thiele diagram for the distillation of a binary feed, the vapor-liquid equilibrium (VLE) data must be obtained for the lower-boiling component of the feed.

Typical McCabe-Thiele diagram for distillation of a binary feed

The first step is to draw equal sized vertical and horizontal axes of a graph. Each axis should be marked off from 0.0 to 1.0 in tenths, as shown in Figure 1. The horizontal axis will be for the mole fraction (denoted by x) of the lower-boiling feed component in the liquid phase. The vertical axis will be for the mole fraction (denoted by y) of the lower-boiling feed component in the vapor phase.

The next step is to draw a straight 45 degree line from the point where x and y both equal 0.0 to the point where x and y both equal 1.0, which is the x = y line in Figure 1. Then draw the equilibrium line (also seen in Figure 1) using the VLE data points of the lower boiling component, representing the equilibrium vapor phase compositions for each value of liquid phase composition. Also draw the vertical red lines (from the horizontal axis up to the x = y line) for the feed and for the desired compositions of the top distillate product and the corresponding bottoms product.

The next step is to draw the green operating line for the rectifying section of the distillation column (i.e., the section above the feed inlet). Starting at the intersection of the red distillate composition line and the x = y line, draw the rectifying operating line at a downward slope of L / (D + L) where L is the molar flow rate of reflux and D is the molar flow rate of the distillate product. The slope is defined as Δy/Δx. For example, in Figure 1, assuming the molar flow rate of the reflux L is 1000 moles per hour and the molar flow rate of the distillate D is 590 moles per hour, then the downward slope of the rectifying operating line is 1000 / (590 + 1000) = 0.63 which means that the y-coordinate of any point on the line decreases 0.63 units for each unit that the x-coordinate decreases.

The next step is to draw the blue q-line (seen in Figure 1) from the x = y line so that it intersects the rectifying operating line.

The parameter q is the mole fraction of liquid in the feed and the slope of the q-line is q / (q - 1). For example, if the feed is a saturated liquid it has no vapor, thus q = 1 and the slope of the q-line is infinite which means the line is vertical. As another example, if the feed is all saturated vapor, q = 0 and the slope of the q-line is 0 which means that the line is horizontal.

Some example q-line slopes are presented in Figure 2. As can be seen now, the typical McCabe-Thiele diagram in Figure 1 uses a q-line representing a partially vaporized feed.

Next, as shown in Figure 1, draw the purple operating line for the stripping section of the distillation column (i.e., the section below the feed inlet). Starting at the intersection of the red bottoms composition line and the x = y line, draw the stripping section operating line up to the point where the blue q-line intersects the green operating line of the rectifying section operating line.

Finally, as exemplified in Figure 1, draw the steps between operating lines and the equilibrium line and then count them. Those steps represent the theoretical plates (or equilibrium stages). The required number of theoretical plates is 6 for the binary distillation depicted in Figure 1.

Note that using colored lines is not required and only used here to make the methodology easier to describe.

In continuous distillation with varying reflux ratio, the mole fraction of the lighter component in the top part of the distillation column will decrease as the reflux ratio decreases. Each new reflux ratio will alter the slope of the rectifying section operating line.

When the assumption of constant molar overflow is not valid, the operating lines will not be straight. Using mass and enthalpy balances in addition to vapor-liquid equilibrium data and enthalpy-concentration data, operating lines can be constructed based on Ponchon-Savarit's method.

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What is Asphalt?

Properties

Asphalt is water repellant due to the bitumen which is part of the asphalt conglomerate.

The physical properties are dependent on the temperature of the material, as at high temperatures, asphalt has a very distinctive viscous behaviour, which becomes more and more elastic with dropping temperatures.

As the physical properties are mainly dependent on the used aggregates, they can vary in a large range. Typical values are:

• heat conductivity: 0.8 - 1.2 W/mK
• specific heat capacity: 850 - 1050 J/kgK

Usage

The main use of asphalt is the creation of pavements like roads or airfields. Due to it's physical properties, it can be used as a robust basement as well as a high-load surface.

Manufacturing

1. The manufacturing of asphalt is done in several steps:
2. predose: Depending on the asphalt recipe, the single aggregate components are weighed. Usually this is done using a belt weigher, as the material has to reach the next step in a continuous flow
3. drying: In a rotation drying drum, the material is dried in a temperature between 140 and 190 °C. After the drying, the components are sifted and divided in several silos.
4. weighing: As the drying can change the previous grain-size distribution curve, a final weighing has to take place before adding the single components to the mixer. This also allows for quick changing of the recipe. The binder is kept in heated tanks and dosed measuring the rate of flow.
5. mixing: The mixer can be working either continuously or at intervals. The continuously working mixer is specially used for very high volumes as it can not be adapt quickly to changing volume and recipe needs. The interval mixer is fed with the aggregates using a charging screw, and synchrounously, the binder is injected into the mixing chamber.
6. dispatching: The mixed material is stored in a heated silo which usually has several chambers to store different recipes.

Transport

Asphalt has to reach the construction site with a reasonably high temperature as it can not be used anymore when it's too cold due to its physical properties. The transportation has to be set up in a way to ensure the right temperatures until the construction site. On short distances between the asphalt plant and the construction site, usual trucks can be used. If the ambient temperature is too low or the the construction site is too far away and it's not ensured that the temperature can be kept until the site, special trucks have to be used. The simplest way to keep up the temperature is to cover the truck with a tarpaulin.

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Hydrodesulfurization

Hydrodesulfurization (HDS) or Hydrotreater is a catalytic chemical process widely used to remove sulfur compounds from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils. The purpose of removing the sulfur is to reduce the sulfur dioxide emissions that result from using those fuels in automotive vehicles, aircraft, railroad locomotives, ships, gas or oil burning power plants, residential and industrial furnaces, and other forms of fuel combustion.

Another important reason for removing sulfur from the intermediate product naphtha streams within a petroleum refinery is that sulfur, even in extremely low concentrations, poisons the noble metal catalysts (platinum and rhenium) in the catalytic reforming units that are subsequently used to upgrade the octane rating of the naphtha streams.

The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting hydrogen sulfide gas. In petroleum refineries, the hydrogen sulfide gas is then subsequently converted into byproduct elemental sulfur. In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from petroleum refining and natural gas processing plants.

History

Although reactions involving catalytic hydrogenation of organic substances were known prior to 1897, the property of finely divided nickel to catalyze the fixation of hydrogen on hydrocarbon double bonds (e.g., (ethylene, benzene) was discovered by the French chemist, Paul Sabatier. Thus, he found that unsaturated hydrocarbons in the vapor phase could be converted into saturated hydrocarbons by using hydrogen and a catalytic metal. His work was the foundation of the modern catalytic hydrogenation process.

Soon after Sabatier's work, a German chemist, Wilhelm Normann, found that catalytic hydrogenation could be used to convert unsaturated fatty acids or glycerides in the liquid phase into saturated ones. He was awarded a patent in Germany in 1902 and in Britain in 1903, which was the beginning of what is now a worldwide industry.

In the mid-1950's, the first noble metal catalytic reforming process (the Platformer process) was commercialized. At the same time, the catalytic hydrodesulfurization of the naphtha feed to such reformers was also commercialized. In the decades that followed, various proprietary catalytic hydrodesulfurization processes such as the one depicted in the schematic flow diagram below have been commercialized. Currently, virtually all of the petroleum refineries world-wide have one or more HDS units.

By 2006, miniature HDS units had been developed for treating jet fuel to produce clean feedstock for synthesizing hydrogen for a fuel cell. By 2007, this had been integrated into an operating 5kW fuel cell generation system.

The process chemistry

Hydrogenation is a class of chemical reactions that results in the addition of hydrogen. Hydrogenolysis is a type of hydrogenation and results in the cleavage of the C-X chemical bond, where C is a carbon atom and X is a sulfur (S), nitrogen (N) or oxygen (O) atom. The net result of a hydrogenolysis reaction is the formation of C-H and H-X chemical bonds. Thus, hydrodesulfurization is a hydrogenolysis reaction. Using ethanethiol (C₂H₅SH), a sulfur compound present in some petroleum refining streams, as an example, the hydrodesulfurization reaction can be simply expressed as:

Ethanethiol + Hydrogen → Ethane + Hydrogen Sulfide

C₂H₅SH + H₂ → C₂H₆ + H₂S

Catalysts

Many metals catalyse the HDS reaction, but it is those at the middle of the transition metal series that are most active. Ruthenium disulfide appears to be the single most active catalyst, but binary combinations of cobalt and molybdenum are also highly active.

In practice, most HDS units in petroelum refineries use catalysts based on cobalt-modified molybdenum disulfide (MoS₂) together with smaller amounts of other metals. Aside from the cobalt-modified MoS₂ catalysts, nickel and tungsten are also used, depending on the nature of the feed. For example, nickle-wolfram (Ni-W) catalysts are more effective for hydrodenitrification (HDN).

Metal sulfides are "supported" on materials with high surface areas. A typical support for HDS catalyst is alumina. The support allows the more expensive catalyst to be more widely distributed, giving rise to a larger fraction of the MoS₂ that is catalytically active.

Process description

In an industrial hydrodesulfurization unit, such as in a refinery, the hydrodesulfurization reaction takes place in a fixed-bed reactor at elevated temperatures ranging from 300 to 400 °C and elevated [pressures ranging from 30 to 130 atmospheres of absolute pressure, typically in the presence of a catalyst consisting of an alumina base impregnated with cobalt and molybdenum.

The image below is a schematic depiction of the equipment and the process flow streams in a typical refinery HDS unit.

Schematic diagram of a typical Hydrodesulfurization (HDS) unit in a petroleum refinery

The liquid feed (at the bottom left in the diagram) is pumped up to the required elevated pressure and is joined by a stream of hydrogen-rich recycle gas. The resulting liquid-gas mixture is preheated by flowing through a heat exchanger. The preheated feed then flows through a fired heater where the feed mixture is totally vaporized and heated to the required elevated temperature before entering the reactor and flowing through a fixed-bed of catalyst where the hydrodesulfurization reaction takes place.

The hot reaction products are partially cooled by flowing through the heat exchanger where the reactor feed was preheated and then flows through a water-cooled heat exchanger before it flows through the pressure controller (PC) and undergoes a pressure reduction down to about 3 to 5 atmospheres. The resulting mixture of liquid and gas enters the gas separator vessel at about 35 °C and 3 to 5 atmospheres of absolute pressure.

Most of the hydrogen-rich gas from the gas separator vessel is recycle gas which is routed through an amine contactor for removal of the reaction product hydrogen sulfide gas (H₂S) that it contains. The H₂S-free hydrogen-rich gas is then recycled back for reuse in the reactor section. Any excess gas from the gas separator vessel joins the sour gas (i.e., gas containing H₂S) from the stripping of the reaction product liquid.

The liquid from the gas separator vessel is routed through a reboiled stripper distillation tower. The bottoms product from the stripper is the final desulfurized liquid product from hydrodesulfurization unit.

The overhead sour gas from the stripper contains hydrogen, methane, ethane, hydrogen sulfide, propane and perhaps some butane and heavier components (i.e., higher molecular weight components). That sour gas is sent to the refinery's central gas processing plant for removal of the hydrogen sulfide in the refinery's main amine gas treating unit and through a series of distillation towers for recovery of propane, butane and pentane or heavier components. The residual hydrogen, methane, ethane and some propane is used as refinery fuel gas. The hydrogen sulfide removed and recovered by the amine gas treating unit is subsequently converted to elemental sulfur in a Claus process unit.

Note that the above description assumes that the HDS unit feed contains no olefins. If the feed does contain olefins (for example, the feed is a naphtha derived from a refinery fluid catalytic cracker (FCC) unit), then the overhead gas from the HDS stripper may also contain some ethene, propene, butenes and pentenes or heavier components.

It should also be noted that the amine solution to and from the recycle gas contactor comes from and is returned to the refinery's main amine gas treating unit.

Sulfur compounds in refinery HDS feedstocks

The refinery HDS feedstocks (naphtha, kerosene, diesel oil and heavier oils) contain a wide range of organic sulfur compounds, including thiols, thiophenes, organic sulfides and disulfides, and many others. These organic sulfur compounds are products of the degradation of sulfur during the natural formation of the petroleum crude oil.

When the HDS process is used to desulfurize a refinery naphtha, it is necessary to remove the the total sulfur down to the parts per million range or lower in order to prevent poisoning the noble metal catalysts in the subsequent catalytic reforming of the naphthas.

When the process is used for desulfurizing diesel oils, the latest environmental regulations in the United States, Europe and elsewhere requiring what is referred to as ultra-low sulfur diesel (ULSD) requires that very deep hydrodesulfurization is needed. In the early 2000's, the governmental regulatory limits for highway vehicle diesel was within the range of 300 to 500 ppm by weight of total sulfur. As of 2006, the total sulfur limit for highway diesel is in the range of 15 to 30 ppm by weight.

Other uses

The basic hydrogenolysis reaction has a number of uses other than hydrodesulfurization.

Hydrodenitrogenation

The hydrogenolysis reaction is also used to reduce the nitrogen content of a petroleum stream and, in that case, is referred to Hydrodenitrogenation (HDN). The process flow scheme is the same as for an HDS unit. In fact, units may be designed for both hydrodesulfurization and hydrodenitrogenation.

Hydrodenitrogenation of pyridine

Using pyridine (C₅H₅N), a nitrogen compound present in some petroleum fractionation products, as an example, the hydrodenitrogenation reaction has been postulated as occurring in three steps:

Pyridine + Hydrogen	→	Piperdine + Hydrogen	→	Amylamine + Hydrogen	→	Pentane + Ammonia
C5H5N + 5H2	→	C5H11N + 2H2	→	C5H11NH2 + H2	→	C5H12 + NH3

and the overall reaction may be simply expressed as:

Pyridine + Hydrogen	→	Pentane + Ammonia
C5H5N + 5H2	→	C5H12 + NH3

Saturation of olefins

The hydrogenolysis reaction may also be used to saturate or convert olefins (alkenes) into paraffins (alkanes). The process used is the same as for an HDS unit.

As an example, the saturation of the olefin, pentene, can be simply expressed as:

Pentene + Hydrogen	→	Pentane
C5H10 + H2	→	C5H12

Some hydrogenolysis units within a petroleum refinery or a petrochemical plant may be used solely for the saturation of olefins or they may be used for simultaneously desulfurizing as well as denitrogenating and saturating olefins to some extent.

Hydrogenation in the food industry

The food industry uses hydrogenation to completely or partially saturate the unsaturated fatty acids in liquid vegetable fats and oils to convert them into solid or semi-solid fats, such as those in margarine and shortening.

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History of the petroleum industry and refining

Brief history of the petroleum industry and petroleum refining

Prior to the 19th century, petroleum was known and utilized in various fashions in Babylon, Egypt, China, Persia, Rome and Azerbaijan. However, the modern history of the petroleum industry is said to have begun in 1846 when Abraham Gessner of Nova Scotia, Canada discovered how to produce kerosene from coal. Shortly thereafter, in 1854, Ignacy Lukasiewicz began producing kerosene from hand-dug oil wells near the town of Krosno, now in Poland. The first large petroleum refinery was built in Ploesti, Romania in 1856 using the abundant oil available in Romania.

In North America, the first oil well was drilled in 1858 by James Miller Williams in Ontario, Canada. In the United States, the petroleum industry began in 1859 when Edwin Drake found oil near Titusville, Pennsylvania. The industry grew slowly in the 1800s, primarily producing kerosene for oil lamps. In the early 1900's, the introduction of the internal combustion engine and its use in automobiles created a market for gasoline that was the impetus for fairly rapid growth of the petroleum industry. The early finds of petroleum like those in Ontario and Pennsylvania were soon outstripped by large oil "booms" in Oklahoma, Texas and California.

Prior to World War II in the early 1940s, most petroleum refineries in the United States consisted simply of crude oil distillation units (often referred to as atmospheric crude oil distillation units). Some refineries also had vacuum distillation units as well as thermal cracking units such as visbreakers (viscosity breakers, units to lower the viscosity of the oil). All of the many other refining processes discussed below were developed during the war or within a few years after the war. They became commercially available within 5 to 10 years after the war ended and the worldwide petroleum industry experienced very rapid growth. The driving force for that growth in technology and in the number and size of refineries worldwide was the growing demand for automotive gasoline and aircraft fuel.

In the United States, for various complex economic reasons, the construction of new refineries came to a virtual stop in about the 1980's. However, many of the existing refineries in the United States have revamped many of their units and/or constructed add-on units in order to: increase their crude oil processing capacity, increase the octane rating of their product gasoline, lower the sulfur content of their diesel fuel and home heating fuels to comply with environmental regulations and comply with environmental air pollution and water pollution requirements.

Processing units used in refineries

• Crude Oil Distillation unit: Distills the incoming crude oil into various fractions for further processing in other units.
• Vacuum Distillation unit: Further distills the residue oil from the bottom of the crude oil distillation unit. The vacuum distillation is performed at a pressure well below atmospheric pressure.
• Naphtha Hydrotreater unit: Uses hydrogen to desulfurize the naphtha fraction from the crude oil distillation or other units within the refinery.
• Catalytic Reforming unit: Converts the desulfurized naphtha molecules into higher-octane molecules to produce reformate, which is a component of the end-product gasoline or petrol.
• Alkylation unit: Converts isobutane and butylenes into alkylate, which is a very high-octane component of the end-product gasoline or petrol.
• Isomerization unit: Converts linear molecules such as normal pentane into higher-octane branched molecules for blending into the end-product gasoline. Also used to convert linear normal butane into isobutane for use in the alkylation unit.
• Distillate Hydrotreater unit: Uses hydrogen to desulfurize some of the other distilled fractions from the crude oil distillation unit (such as diesel oil).
• Merox (mercaptan oxidizer) or similar units: Desulfurize LPG, kerosene or jet fuel by oxidizing undesired mercaptans to organic disulfides.
• Amine gas treater, Claus unit, and tail gas treatment for converting hydrogen sulfide gas from the hydrotreaters into end-product elemental sulfur. The large majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from petroleum refining and natural gas processing plants.
• Fluid Catalytic Cracking (FCC) unit: Upgrades the heavier, higher-boiling fractions from the the crude oil distillation by converting them into lighter and lower boiling, more valuable products.
• Hydrocracker unit: Uses hydrogen to upgrade heavier fractions from the crude oil distillation and the vacuum distillation units into lighter, more valuable products.
• Visbreaker unit upgrades heavy residual oils from the vacuum distillation unit by thermally cracking them into lighter, more valuable reduced viscosity products.
• Delayed coking and Fluid coker units: Convert very heavy residual oils into end-product petroleum coke as well as naphtha and diesel oil by-products.

Auxiliary facilities required in refineries

• Steam reformer unit: Converts natural gas into hydrogen for the hydrotreaters and/or the hydrocracker.
• Sour water stripper unit: Uses steam to remove hydrogen sulfide gas from various wastewater streams for subsequent conversion into end-product sulfur in the Claus unit.
• Utility units such as cooling towers for furnishing circulating cooling water, steam generators, instrument air systems for pneumatically operated control valves and an electrical substation.
• Wastewater collection and treating systems consisting of API separators, dissolved air flotation (DAF) units and some type of further treatment (such as an activated sludge biotreater) to make the wastewaters suitable for reuse or for disposal.
• Liquified gas (LPG) storage vessels for propane and similar gaseous fuels at a pressure sufficient to maintain them in liquid form. These are usually spherical vessels or bullets (horizontal vessels with rounded ends).
• Storage tanks for crude oil and finished products, usually vertical, cylindrical vessels with some sort of vapor emission control and surrounded by an earthen berm to contain liquid spills.

The crude oil distillation unit

The crude oil distillation unit (CDU) is the first processing unit in virtually all petroleum refineries. The CDU distills the incoming crude oil into various fractions of different boiling ranges, each of which are then processed further in the other refinery processing units. The CDU is often referred to as the atmospheric distillation unit because it operates at slightly above atmospheric pressure.

Below is a schematic flow diagram of a typical crude oil distillation unit. The incoming crude oil is preheated by exchanging heat with some of the hot, distilled fractions and other streams. It is then desalted to remove inorganic salts (primarily sodium chloride).

Following the desalter, the crude oil is further heated by exchanging heat with some of the hot, distilled fractions and other streams. It is then heated in a fuel-fired furnace (fired heater) to a temperature of about 398 °C and routed into the bottom of the distillation unit.

The cooling and condensing of the distillation tower overhead is provided partially by exchanging heat with the incoming crude oil and partially by either an air-cooled or water-cooled condenser. Additional heat is removed from the distillation column by a pumparound system as shown in the diagram below.

As shown in the flow diagram, the overhead distillate fraction from the distillation column is naphtha. The fractions removed from the side of the distillation column at various points between the column top and bottom are called sidecuts. Each of the sidecuts (i.e., the kerosene, light gas oil and heavy gas oil) is cooled by exchanging heat with the incoming crude oil. All of the fractions (i.e., the overhead naphtha, the sidecuts and the bottom residue) are sent to intermediate storage tanks before being processed further.

Schematic flow diagram of a typical crude oil distillation unit.

Flow diagram of a typical petroleum refinery

The image below is a schematic flow diagram of a typical petroleum refinery that depicts the various refining processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end-products.

The diagram depicts only one of the literally hundreds of different oil refinery configurations. The diagram also does not include any of the usual refinery facilities providing utilities such as steam, cooling water, and electric power as well as storage tanks for crude oil feedstock and for intermediate products and end products.

A schematic flow diagram of a typical petroleum refinery.

Refining end-products

The primary end-products produced in petroleum refining may be grouped into four categories: light distillates, middle distillates, heavy distillates and others.

Light distillates

• Liquid petroleum gas (LPG)
• Gasoline (also known as petrol)
• Kerosene
• Jet fuel and other aircraft fuel

Middle distillates

• Automotive and railroad diesel fuels
• Residential heating fuel
• Other light fuel oils

Heavy distillates

• Heavy fuel oils
• Bunker fuel oil and other residual fuel oils

Others

Many of these are not produced in all petroleum refineries.

• Specialty petroleum naphthas
• Specialty solvents
• Elemental sulfur (and sometimes sulfuric acid)
• Petrochemical feedstocks
• Asphalt and tar
• Petroleum coke
• Lubricating oils
• Waxes and greases
• Transformer and cable oils
• Carbon black

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