Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing

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PETROLEUM PRODUCTS AND A REFINERY CONFIGURATION 69

Table 2.12. Oxygenates commonly used in gasoline (subject to phase-out in the U.S.A.)

Oxygen Water

Name Formula RON RVP psig %wt solubility %

∗

Methyl tertiary butyl

ether (MTBE)

(CH

)

COCH

110–112 8 18 4.3

Ethyl tertiary butyl

ether (ETBE)

(CH

)

COC

110–112 4 16 1.2

Tertiary amyl methyl

ether (TAME)

(CH

)

)COCH

103–105 4 16 1.2

Ethanol C

OH 112–115 18 35 100

∗

Wt % soluble in water.

See also Part 3 of this chapter.

Using the oxygenates. Oxygenates were used originally simply as a additive to im-

prove octane number. However, because of their oxygen content they are now added

also ton reduce the carbon monoxide and hydrocarbon in the emission gases. There

are a number of oxygenates now used in gasoline manufacture some of the more

common are given in Table 2.12.

The EPA have established limits for the use of each oxygenates in gasoline blends.

For example, MTBE may be blended up to 15 %vol subject to an overall limit of

2.7 %wt oxygen content. The role of MTBE and other oxygenates in the U.S. was

discontinued in 2002 after it was discovered that underground storage tanks had not

been upgraded to retain reformulated gasoline. As a consequence tanks were leaking

gasoline that contained MTBE into the ground and drinking water systems.

Diesel fuel

The policies in place to control, or at least to endeavor to control, emissions from

motive fuels has moved from gasoline engines to diesel engines. Similar to the gasoline

fuels diesel has also changed in composition to meet these environmental constraints

while still maintaining a required performance standard. These changes have imposed

a major impact on the reﬁning industry at a time when the demand for diesel is

increasing. Again as in the case of the gasoline fuel reformulated diesel fuel becomes

part of the modern diesel speciﬁcation and composition. These are described and

discussed in the following sections.

The diesel engine

Before dealing with the diesel fuel it may be advantageous to review brieﬂy the

principle of the diesel engine itself, diesel engines have historically been preferred in

70 CHAPTER 2

many applications because of their simplicity of design, power, durability, and higher

fuel efﬁciency. Diesel engines ﬁt into three categories and these are:

Low speed engines

Medium speed engines

High speed engines

Low speed engines. These operate at less than 300 revs per min (rpm), and are used

for applications that require sustained heavy loads at constant engine speed. Examples

are the main propulsion engines in marine vessels and engines used in electric power

generation.

Medium speed engines. These operate at speeds between 300 and 1,000 rpm, and are

used for applications with fairly high loads, and relatively constant speeds. Engines

used in auxiliary power plants on marine vessels, and in smaller power generation

plants are examples of this type of diesel engine.

High speed engines. Operate at speeds above 1000 rpm, and are designed for frequent

and wide variations in load and speed. These are the type of engines used for road

transport and Diesel locomotives.

Almost all diesel engines use a standard four stroke design which are, intake, com-

pression, power, and exhaust. During the intakes stroke, air alone enters the cylinder,

during the compression stroke the air is compressed by the upward movement of the

piston. The ﬁnal temperature and pressure reached in the cylinder is a function of

the compression ratio, engine speed, and engine design. Pressures of 450 psig, and

temperatures of 500

◦

F are typical. Shortly before the end of the compression stroke,

one or more jets of fuel are injected into the cylinder. The fuel pump introducing these

jets is usually cam driven type and operates at pressures between 1,800 and 30,000

psig. The fuel does not ignite immediately. There is a delay period when the fuel

droplets vaporize and reach their ignition temperature. The extent of this time delay is

affected by the design of the engine, the temperatures of the inlet air and fuel, and the

degree to which the injected fuel is atomized on entering the cylinder. This ignition

delay must be kept as short as possible to avoid accumulation of fuel in the cylinder

before ignition. Such a situation causes diesel knock when this large quantity of fuel

detonates.

After ignition, fuel injection continues for a portion of the power stroke. This fuel

burns almost instantaneously with the remaining air and combustion products in the

cylinder. This more controlled burning period is referred to as diffusion combustion.

Fuel injection is stopped part way through the power stroke. Finally the exhaust stroke

purges the combustion products from the cylinder, and the cycle begins again.

PETROLEUM PRODUCTS AND A REFINERY CONFIGURATION 71

Design improvements to the basic diesel engine

Fuel injection improvements. Most diesel engines built in the 1980s were equipped

with indirect fuel injection system (IDI). Modern engines are equipped with the

direct fuel injection (DI) which is designed for the geometry of the combustion

chamber, and is signiﬁcantly more efﬁcient. (Note: This can well be compared with

the direct injection concept in gasoline engines which was introduced around this time

also).

Four valve cylinder heads. This promotes better movement of both combustion air

and exhaust gases. This causes high turbulence of the fuel and air in the combustion

chamber resulting in good distribution of the fuel and increased ﬂame speed which

reduces the ignition delay time.

Electronically controlled fuel injection timing. This allows for the injection of a small

quantity of fuel before the main charge into combustion chamber. The result is a

quieter combustion minimizing the amount of premixed combustion and maximizing

the more controlled diffusion combustion.

Increasing the amount of air introduced into the cylinder. Using a Turbocharger,

which is an air compressor driven by a turbine using exhaust gas energy, the mass of

air introduced into the cylinder is increased. This enables the engine to burn more

fuel thus increasing its power output. Using this concept the power output for a diesel

engine can be increased by as much as 50% for the same engine displacement.

The parameters of diesel fuel

The following are the major parameters in meeting the diesel fuel speciﬁcation with

respect to engine performance or emission or both:

The cetane number

Aromatic content

Density

Sulfur content

Distillation

Viscosity

Cloud and pour points

Flash point

Most of these parameters have been deﬁned elsewhere in this book. This section then

deals with the effect these parameters have on the performance of the diesel engine or

the emission of undesirable components from the engine or both. Where the deﬁnition

72 CHAPTER 2

of the parameter is not dealt with in detail elsewhere in the book, then deﬁnition is

included here.

Cetane number. This is the result of an engine test that compares the ignition delay

for a fuel. For this test two reference fuels are chosen. The ﬁrst is normal cetane

) and the second is an isomer of cetane which is heptamethylnonane. The normal

cetane is arbitrarily given the cetane number of 100, while the isomer as the second

reference fuel is assigned a cetane number of 15. The fuel being tested is run in a

standard test engine. The cetane number is derived by comparing the ignition delay

of the test diesel with a blend of the two reference fuels. The cetane number is then

calculated using the equation:

Cetane Number = % normal cetane + 0.15 ×% heptamethylnonane.

Higher cetane numbers indicates that the fuel has a shorter ignition delay. The higher

the cetane number also results in less CO and unburnt hydrocarbons in the engine

emission gases. This has a greater effect in the older diesel engine. Modern engines

are equipped with retarded ignition timing and increasing the cetane number has a

smaller effect on these more modern engines.

Aromatics. The aromatic content of diesel fuel can be measured for single ring aro-

matics, multi-ring or poly-aromatic hydrocarbons (PAH). Some studies show that

reducing the aromatics results in the reduction of all regulated emissions, but other

studies have indicated that the reduction of emissions of unburned hydrocarbons,

, and particulates can only be achieved by reducing multi-ring aromatics.

Density. As density is a measure of the mass per unit volume, diesel fuels of low

density require a longer injection time to deliver the same mass of fuel into the

cylinder. The longer the injection time the lower is the peak temperatures which, in

turn, results in lower No

formation. At high loads and engine speeds, the longer

injection interval causes some incomplete combustion, resulting in a high emission

of unburned hydrocarbons and CO. When the load is being increased however the

lower density fuel results in less over-fueling, which actually decreases the emission

of particulates, hydrocarbons, and CO.

Sulfur. The sulfur in diesel fuel is burned to SO

, a portion of which is further oxidized

to sulfates. This binds with water to form a portion of the particulate matter. Because

only a small percent of the total sulfur in fuel is oxidized to sulfates the contribution

of sulfates to the total particulates is quite small. However if an oxidation catalyst

is used to reduce emission of hydrocarbons, CO, and particulate matter a signiﬁcant

amount of the SO

is converted to sulfates and consequently making a signiﬁcant

contribution to the particulates in the emission gases.

PETROLEUM PRODUCTS AND A REFINERY CONFIGURATION 73

Distillation. The distillation range of diesel fuel has a signiﬁcant inﬂuence on engine

performance. This is especially so in medium and high speed engines. If the fuel is too

volatile the engine loses power and efﬁciency because of vapor lock in the fuel system

or poor droplet penetration into the cylinder. On the other hand if the volatility of the

fuel is too low, the engine will lose power and efﬁciency as a result of poor atomization

of the fuel. Both the front end and the back end of the distillation are important. If the

10 %vol point is too high, the engine will have difﬁculty starting. A low 50% point

reduces particulate emissions and odor. Because heavier molecules are more difﬁcult

to burn, both soot and the soluble organic fraction (SOF) of the particulate emissions

are increased if the 90% point is too high, the emission of unburned hydrocarbons

will also increase.

Viscosity. Fuel viscosity has an important effect on the fuel pump and injector system.

The shape of the fuel spray is affected by viscosity. If this is too high, the fuel will

not be properly atomized into the cylinder, which will result in poor combustion, loss

of power, and efﬁciency, with an increase in CO and hydrocarbon emission. Another

effect of poor atomizing of the fuel will be to allow the fuel to impinge on the cylinder

walls, and remove the lubricating ﬁlm. This results in excessive wear and increase of

hydrocarbon emissions.

If the viscosity of the fuel is too low, the injection spray is too soft and will not

penetrate far enough into the cylinder. A loss of power and efﬁciency will occur due

to this. Where the fuel system is also lubricated by the fuel, as is the case in some

engine designs, increased wear to the system will result through low fuel viscosity.

Cloud and pour point. Cold weather performance of diesel fuel is a key considera-

tion for users. Actual speciﬁcations for cold ﬂow properties are based on expected

temperature extremes and different test methods are used in different parts of the

world. In the United States cloud point is used as an indicator of the cold ﬂow

properties of the fuel. Cloud point is the temperature at which wax begins to pre-

cipitate out of the fuel. The longer parafﬁn molecules in the fuel precipitate as a wax

when the temperature falls below the cloud point. This wax clogs unheated fuel lines

and ﬁlters. The more parafﬁnic a fuel the higher will be its cloud point. In some

parts of the world, pour point is used as an indication of the lowest temperature at

which a fuel can be pumped. Pour points are generally 4–5

◦

C lower than the cloud

point.

Flash point. The ﬂash point of a fuel is the temperature that the fuel must be heated

to produce an ignitable mixture of vapor and air above the surface of the liquid. This

property is only important for safe handling and storage of the fuel. It has no effect

on the performance of the fuel or its emission properties. If the ﬂash point is too low,

ﬁre or explosion could occur when it is handled.

74 CHAPTER 2

Meeting the diesel fuel parameters

There are various solutions for meeting the parameters of diesel fuel in the design

of the fuel. These solutions range from fractionation, adding improvers, and more

complex hydro-processing. Using modern developments in hydro-processing it is

possible to convert low-grade blend stocks, such as FCCU or thermal cracker prod-

uct streams to good diesel precursors. Among the more important parameters to be

met both from the performance and the emission aspects of the fuel are discussed

below.

Increasing cetane value. The simplest way to improve cetane number is to use an

appropriate ignition improvement additive. These are mainly alkyl nitrates. The ef-

fectiveness of cetane improvement additives tend to be linear with addition rate, how-

ever the improvement can vary with different diesel blend stock. Parafﬁns which

already have high cetane value respond best to the additives. Aromatics on the

other hand, which have a low cetane number have a poorer response. About 500

wppm of standard alkyl nitrate will increase the cetane value by 2–5 numbers.

Another way to increase cetane number is to modify the hydrocarbon type in the

diesel blend. Figure 2.2 shows the relationship between molecular types and cetane

number.

Straight chain parafﬁn molecules have the highest cetane numbers while multiple

ring aromatics have the lowest cetane number improvement also occurs during the

hydro-treating of the gas oil fractions used as blend components in the diesel prod-

uct. During the hydro-treating to remove the organic sulfur and nitrogen species

found in these untreated gas oil fractions, the molecules containing these items are

opened and saturated to form parafﬁns and naphthenes. At the same time some of the

Figure 2.2. Cetane number of hydrocarbon types.

PETROLEUM PRODUCTS AND A REFINERY CONFIGURATION 75

aromatic molecules contained in the gas oils are also opened and saturated increas-

ing the parafﬁn/naphthene content of the hydro-treated product. As a result of this

ring opening and saturation a cetane number improvement of between 3 and 5 num-

bers can be expected. Hydro-cracking of heavy gas oil fractions, and hydro-treating

of thermal cracked gas oils and FCC gas oils signiﬁcantly increase product cetane

numbers.

Meeting sulfur content. Sulfur content of 0.05 %wt is now quite a common require-

ment for diesel fuel. In most cases this level of sulfur can only be met by extensive

hydro-treating or hydro-cracking to remove the sulfur from the middle distillates that

will be used to blend the diesel product. Some reﬁneries select their crude slate with

low sulfur crudes to reduce the hydro-treating demand. The impact on reﬁners to meet

this parameter extends beyond the severity of hydro-treating that may exist. It impacts

on the availability of hydrogen in the front end to the capacity for handling the sour

gases (H

S in particular) in the back end of the process. It is likely that the legislative

sulfur content in diesel may be reduced still further in the near future. This makes the

selection between crude slate demand for low sulfur crude and a modiﬁed reﬁnery

conﬁguration and operation a critical one.

Reducing aromatic content. With the increasing demand for diesel fuel, particularly

in North America and Europe, reﬁners will need to use streams normally routed to

heating oils of fuel as components for the diesel product blend. These streams are

of lower quality and in particular contain higher quantity of aromatics that will be

acceptable as diesel blend stocks without severe treating to improve their blending

characteristics. While high severity hydro-treating to remove sulfur and nitrogen helps

in reducing aromatics in acceptable diesel blend stocks, it will not be sufﬁcient to

improve the quality of the heating oil and fuel oil blend stocks to meet the diesel pool

requirement. Hydro treating using nickel molybdenum catalyst (de-aromatization

process) is being considered for the purpose of upgrading these poorer blend stocks.

This de-aromatization process consists of two stages. The ﬁrst is processing over a

conventional Co–Mo hydro-treating catalyst to remove sulfur and then to process over

the Ni–Mo catalyst. The nickel catalyst is poisoned by sulfur, thus the two stages. The

economics of using the de-aromatization process must be carefully evaluated because

the metal content of the nickel catalyst represents a signiﬁcant investment. The process

however operates at a higher space velocity and lower pressure than the conventional

Co–Mo hydro-treating process.

Improvement of cold ﬂow properties. Problems with meeting the cold ﬂow properties

of diesel fuel are associated with the presence of straight chain parafﬁns. Although

these have higher cetane numbers they pose cold ﬂow limitations because they tend to

precipitate as a wax at low temperatures. The simplest and probably the best solution

to this cold ﬂow property of these parafﬁns is the use of additives. These additives are

however costly ranging from below $50 per thousand bbls for pour point reduction

76 CHAPTER 2

to as high as $250 per thousand bbls for cloud point reduction (these are year 2000

dollars). Another alternative is to reduce the diesel FBP and add kerosene. This

action removes the heavier parafﬁnic molecules, which tend to precipitate at higher

temperature. Reducing the cut point from 700

◦

Fto640

◦

F reduces the yield of diesel

by 6 %vol on crude. Solvent dewaxing is another option. In this process the wax

forming components are selectively removed from the diesel product. This is only

considered however for stocks from high parafﬁn crudes. Selective hydrocracking of

the straight chain parafﬁns to smaller parafﬁn molecules is an option. This however

is accompanied by a reduction in yield and cetane number. Finally isomerization of

the straight chain parafﬁns to branched parafﬁns minimizes cetane and yield loss is

a viable option. However these isomerization catalysts contain noble metals and are

therefore quite expensive and require the two stage process, desulfurization followed

by the isomerization stage.

2.4 A reﬁnery process conﬁguration development

There are many steps necessary to complete a reﬁning company’s marketing strategy

and production objectives. These steps may culminate in building new processing

facilities or revamping existing ones. The course of action that will ultimately be

adopted will, almost invariably, be decided on an economic basis reﬂecting the com-

pany’s future proﬁtability picture. Among the most important of these steps however

will be the development of several reﬁning scenarios and processing conﬁgurations.

A process conﬁguration and its accompanying Block Flow Diagram is a blue print

and a basis for economic study and decision making. The ﬁnal and accepted conﬁgu-

ration will also be the foundation for the design of new facilities or the revamping of

existing ones. The block ﬂow diagram shows the calculated results of process plant

design capacity, the quantity of process streams to and from each process plant, the

sequence of the plants to one another in the reﬁning scheme, and ﬁnally the blending

recipes (streams and their quantities that make up the ﬁnished product blends). Such

a development requires the in-depth knowledge of the reﬁning industry and relies,

in no small measure, on the expertise of those companies that license processes and

their technology.

The conﬁguration example described here would be one of many that would be

examined meeting the production and economic objectives of a particular reﬁnery

operation. In this example a reﬁning company wishes to examine a new ‘Grass Roots’

conﬁguration that will produce the following products from a medium Middle East

crude, whose assay TBP curve is given as Figure 2.1.

The product speciﬁcations are given in the respective text, and the production limits

are as follows:

PETROLEUM PRODUCTS AND A REFINERY CONFIGURATION 77

Crude feed 50,000 BPCD

LPG 3,289 BPCD (Max)

Gasoline 22,170 BPCD (Max)

20,390 BPCD (Min)

Kerosene No target

Auto diesel 8,830 BPCD (Max)

3,000 BPCD (Min)

Gas oil 8,553 BPCD (Max)

2,630 BPCD (Min)

Marine diesel 3,000 BPCD (Max)

2,300 BPCD (Min)

Heavy fuel 8,500 BPCD (Max)

6,500 BPCD (Min)

Bunker fuel No restrictions

A solution

For this conﬁguration the following processes will be examined:

Atmos crude dist unit (CDU)

Vacuum dist unit (VDU)

Light ends dist units

Naphtha hydrotreater (Nap Hds)

Catalytic reformer

Lt gas oil hydrotreater (LGO Hds)

Hy gas oil hydrotreater (HGO Hds)

Fluid catalytic cracker (FCCU)

Isom/alkylation plant

Thermal cracker (TC)

Gas treating and sulfur recovery processes

The stream quantities to each of these units will be developed as follows:

The atmospheric crude distillation unit. The crude is split to satisfy the quantities and

distillation characteristics of the ﬁnished products required. For this example these

are as follows:

Gas to C

1.71 %vol on crude

to 210

◦

F 10.44 %vol on crude Light naphtha (LSR)

210 to 380

◦

F 14.20 %vol on crude Heavy naphtha (HSR)

380 to 520

◦

F 9.09 %vol on crude Kerosene

520 to 650

◦

F 13.83 %vol on crude Diesel

650

◦

F+ 50.73 %vol on crude Residue (feed to crude vacuum unit)

78 CHAPTER 2

Table 2.13. ASTM and TBP of the crude distillate cuts

to 210

◦

F 210 to 380

◦

F 380 to 520

◦

F 520 to 650

◦

ASTM

◦

F TBP

◦

F ASTM

◦

F TBP

◦

F ASTM

◦

F TBP

◦

F ASTM

◦

F TBP

◦

IBP 90 30 198 132 363 310 451 392

10 %vol 115 78 235 196 387 357 507 480

30 135 116 260 242 407 395 533 527

50 148 140 280 276 424 425 551 558

70 162 163 301 308 440 451 567 583

90 182 191 332 349 462 481 593 618

FBP 225 238 390 412 493 515 622 650

Then from the cut points corresponding to the stream’s volume % on crude the fol-

lowing ASTM and TBP of the distillate streams leaving the unit are calculated. The

ASTM (Starting at the C

+ level) are developed using the 50% point of each cut and

the 70% point converted to ASTM (using the Edmister correlation) and a straight line

drawn through them to give the ASTM distillation. These are then converted to TBP

(again using the Edmister correlation) and are shown in Table 2.13.

These curves are given in Figure 2.3.

From these curves the table below gives a component break down of the distillate

streams (Table 2.14).

The properties of these distillate cuts are determined using the component mid vol% on

crude multiplied by its volume on the respective cut or by use of indices as previously

described.

The following table gives the component properties from the assay data (Table 2.15).

Using the methods described earlier in this chapter the products from the atmospheric

crude unit including their salient properties are as follows (Table 2.16).

The vacuum crude unit, VDU. The feed to the vacuum distillation unit will be the

atmospheric residue from the CDU. This stream will be treated in the same manner

as the distillate streams shown above for the CDU. Thus the distillate streams and the

vacuum residue stream including their salient properties are shown in Table 2.17.

The product Streams from this unit are:

LVGO (Light Vacuum Gas Oil)—650 to 750

◦

F (49.27–60.12 %vol on crude)

HVGO (Heavy Vacuum Gas oil)—750 to 930

◦

F (60.12–78.67 %vol on crude)

Vacuum Residue—+930

◦

F (21.33 %vol on crude)