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

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1126 CHAPTER 19

provide for the cracking and isomerization and the metal function, which provide for

the oleﬁn formation and hydrogenation. The cracking reaction requires heat while the

hydrogenation reaction generates heat. Overall, there is heat release in hydrocracking,

and just like in treating, it is a function of the hydrogen consumption (the higher the

consumption, the more important the exotherm). Generally, the hydrogen consump-

tion in hydrocracking (including the pre-treating section) is 1,200–2,400 SCFB/wt%

change (200–420 Nm

/wt% change) resulting in a typical heat release of 50–100

Btu/Scf H

(2.1–4.2 kcal/m

) which translates into a temperature increase of about

0.065

◦

F/Scf H

consumed (0.006

◦

C/Nm

). This includes the heat release gen-

erated in the treating section. In general, the hydrocracking reaction starts with the

generation of an oleﬁn or cycleoleﬁn on a metal site on the catalyst. Next, an acid site

adds a proton to the oleﬁn or cyclo-oleﬁn to produce a carbenium ion. The carbenium

ion cracks to a smaller carbenium ion and a smaller oleﬁn. These products are the

primary hydrocracking products. These primary products can react further to produce

still smaller secondary hydrocracking products. Some of these reactions are given in

Figure 19.D.4.

Full details and discussion on Flow schemes and hydrocracking reactions are given

in Chapter 7.

(D) Olefin hydrogenation

=C-CH

Metal

-CH-CH

R-CH

-CH

-C-CH

R-CH

+ CH

= C-CH

Acid

(B) Formation of tertiary carbenium ions

R-CH =CH-CH -CH

Acid

R-CH

-CH

-C-CH

(A) Formation of olefins

R-CH

-CH

-CH-CH

Metal

R-CH =CH-CH-CH

Figure 19.D.4. Hydrocracking reactions of parafﬁns.

A DICTIONARY OF TERMS AND EXPRESSIONS 1127

Distillation

Part 1: Laboratory distillation tests

The ASTM distillation curve. While the TBP curve is not produced on a routine basis

the ASTM distillation curves are. Rarely however is an ASTM curve conducted on

the whole crude. This type of distillation curve is used however on a routine basis for

plant and product quality control. This test is carried out on crude oil fractions using

a simple apparatus designed to boil the test liquid and to condense the vapors as

they are produced. Vapor temperatures are noted as the distillation proceeds and are

plotted against the distillate recovered. Because only one equilibrium stage is used

and no reﬂux is returned, the separation of components is poor and mixtures are

distilled. Thus the initial boiling point for ASTM is higher than the corresponding

TBP point and the ﬁnal boiling point of the ASTM is lower than that for the TBP

curve. There is a correlation between the ASTM and the TBP curve, and this is

dealt with later in Chapter 1 of this Handbook.

The true boiling point curve (TBP). This is a plot of the boiling points of almost

pure components, contained in the crude oil or fractions of the crude oil. In earlier

times this curve was produced in the laboratory using complex batch distillation

apparatus of a hundred or more equilibrium stages and a very high reﬂux ratio.

Nowadays this curve is produced by mass spectrometry techniques much quicker

and more accurate than by batch distillation.

The equilibrium vaporization curve (EFV). The EFV curve of an oil is determined

in a laboratory using an apparatus that conﬁnes liquid and vapor together until the

required degree of vaporization is achieved. The percentage vaporized is plotted

against temperature for several runs to produce the EFV curve. Separation is poorer

for this type of distillation than for an ASTM or TBP. Therefore the initial boiling

point will be higher for the EFV than for the ASTM. The ﬁnal boiling point of the

EFV will be lower than that for the ASTM. This test is rarely done but the EFV

curve is calculated either from a TBP curve or an ASTM curve. These methods are

given in Chapters 1 and 3.

Part 2: The distillation processes

Next to heat exchange the unit operation of distillation is the most utilized operation in

oil reﬁning. The separation of products by distillation fall into three major categories.

These are:

Total vapor condensation such as the atmospheric crude distillation

Vacuum distillation processes. Such as the vacuum distillation of atmospheric

residue

Light ends distillation

1128 CHAPTER 19

Total vapor condensation processes. The best example of this type of distillation

units in modern reﬁning is the atmospheric crude distillation unit. In these type of

units the feed is heated and vaporized to a temperature above the total product cut

point. This mixed vapor/liquid is produced by an external heater (and heat exchanger

system) and the mixed stream ﬂashed in the lower section of a tower. The vapor

rises in the tower and condensed by cooled reﬂux stream at various stages up the

tower according to the various distillate boiling points. Full details of this type of

distillation is given in Chapter 3 of this book. Similar distillation systems are also

used in the primary separation of Fluid Catalytic Cracking Unit efﬂuent, and the

reactor efﬂuent from hydrocrackers.

Vacuum distillation processes. These processes are designed to operate as total

vapor condensation similar to the atmospheric units. However the feed to these

units is usually heavy residual oils, which if heated to vaporize the product distillate

required at atmospheric pressure (or near atmospheric pressure) would cause the

feed to crack and coke. The system is therefore set at low vacuum pressure so

that the vaporizing temperature is well below the feed’s cracking properties. The

distillates are produced in the same way as that for the atmospheric units. That is by

selective cooled reﬂux streams. Full details of a crude oil vacuum process are also

given in Chapter 3 of this book. Vacuum distillation is also used for the distillation

of light residues from hydrocrackers to produce light vacuum distillates and heavy

residuum for further thermal cracking.

Light ends distillation. The most common of this type of distillation is the crude

unit light ends process. The feed to this unit is the overhead distillate from the

atmospheric crude unit. This feed may also include overhead distillate from other

processes (such as the catalytic reformer). The product from the light end s process is

the production of light and heavy naphtha, butane LPG, propane LPG, and reﬁnery

gas. These processes are discussed fully in Chapter 4 of this book.

Drums

Drums may be horizontal or vertical vessels. Generally drums do not contain complex

internals such as fractionating trays or packing as in the case of towers. They are used

however for removing material from a bulk material stream and often use simple

bafﬂe plates or wire mesh to maximize efﬁciency in achieving this. Drums are used

in a process principally for:

Removing liquid droplets from a gas stream (knockout pot), or separating vapor

and liquid streams

Separating a light from a heavy liquid stream (separators)

Surge drums to provide suitable liquid hold up time within a process

To reduce pulsation in the case of reciprocating compressors

Drums are also used as small intermediate storage vessels in a process.

A DICTIONARY OF TERMS AND EXPRESSIONS 1129

Vapor disengaging drums

One of the most common example of the use of a drum for the disengaging of vapor

from a liquid stream is the steam drum of a boiler or a waste heat steam generator.

Here the water is circulated through a heater where it is risen to its boiling point

temperature and then routed to a disengaging drum. Steam is ﬂashed off in this drum

to be separated from the liquid by its superﬁcial velocity across the area above the

water level in the drum. The steam is then routed to a super-heater and thus to the steam

main. The performance of the steam super-heater depends on receiving fairly “dry”

saturated steam. That is steam containing little or no water droplets. The separation

mechanism of the steam drum is therefore critical.

The design of a vapor disengaging drum depends on the velocity of the vapor and the

area of disengagement. This is expressed by the equation:

= 0.157



− ρ

where

= Critical velocity of vapor in ft/sec.

= Density of liquid phase in lbs/cuft.

= Density of vapor phase in lbs/cuft.

The area used for calculating the linear velocity of the vapor is:

The vertical cross sectional area above the high liquid level in a horizontal drum

The horizontal area of the drum in the case of vertical drums

The allowable vapor velocity may exceed the critical, and normally design veloc-

ities will vary between 80% and 170% of critical. The use of crinkled wire mesh

screens (CWMS) screens are an effective entrainment separators and are often used

in separator drums for that purpose.

Liquid separation drums

The design of a drum to perform this duty is based on one of the following laws of

settling:

Stoke’s law

V = 8.3 ×10

S

When the Re number is < 2.0.

1130 CHAPTER 19

Intermediate law

V = 1.04 ×10

1.14

S

0.71

0.29

× µ

When the Re number is 2–500.

Newton’s law

V = 2.05 ×10



d · S



1/2

When the Re number is > 500,

where

Re number =

10.7 × d

V = Settling rate in ins per minute.

d = Droplet diameter in ins.

S = Droplet speciﬁc gravity.

= Continuous phase speciﬁc gravity.

S = Speciﬁc gravity differential between the two phases.

µ = Viscosity of the continuous phase in cps.

The following may be used as a guide to estimating droplet size:

Lighter Phase. Heavy Phase. Minimum Droplet Size.

0.850 SG and lighter. water 0.008 ins.

Heavier than 0.850. water 0.005 ins.

The holdup time required for settling is the vertical distance in the drum allocated to

settling divided by the settling rate.

Settling bafﬂes, are often used to reduce the holdup time and the height of the liquid

level.

Surge drums

This type of drum, the calculation of holdup time and surge control is used in the

control the movement of liquids from one process to another. It’s purpose is to smooth

out the ﬂow to meet the process criteria. For example, when a liquid feed leaves the

ﬁrst process item (such as an overhead reﬂux drum) under level control a surge drum

may be used to collect the feed and deliver it under ﬂow control to the next process.

This is particularly desirable if the feed enters a ﬁred heater as the ﬁrst item of the

second process.

A DICTIONARY OF TERMS AND EXPRESSIONS 1131

Pulsation drums or pots

This type of drum is used almost entirely in the operation of a reciprocating com-

pressors. It is used to smooth out the compressed gas ﬂow leaving the compressor

cylinders.

Economic evaluations

Economic evaluation is used for most aspects of the reﬁnery planning, and its op-

eration. The methods used in this evaluation may differ from company to company,

but the end product must reﬂect the proﬁtability of the present and often the fu-

ture proﬁtability of a proposed venture or operation. This measure is reﬂected in

terms of the Return on Investment of the item. This is described and discussed in

Chapter 8.

The following deﬁnitions and items are used in the economic evaluation exercises:

Ad volorum tax. This is the ﬁxed tax levied in most countries payable to local mu-

nicipal authorities, provincial, or state authorities to cover property tax, municipal

service costs etc.

Calculation of cash ﬂow method. There are several methods of assessing proﬁtability

based on discounted cash ﬂow (DCF), but the most reliable yardstick is a return on

investment method using the Present Worth (or Net Present Value) concept. This

concept equates the present value of a future cash ﬂow as a product of the present

interest value factor and the future cash ﬂow. Based on this concept, the Return on

Investment is that Interest value or Discount Factor which forces the Cumulative

Present Worth value to Zero over the economic life of the project.

Net investment costs. The Net Investment for the project includes the capital cost of

the plant, which is subject to depreciation, and the Associated Costs, which may not

be subject to depreciation.

The capital cost of the plant is the contractor’s selling price for the engineering,

equipment, materials, and the construction of the facilities. In a process study using

a DCF return on investment calculation, the capital cost should be an estimate with

an accuracy at least that based on an equipment factored type.

1132 CHAPTER 19

The associated costs include the following elements:

Any licensor’s paid up royalties

Cost of land

First inventory of chemicals and catalyst

Cost of any additional utilities or offsite facilities incurred by the project

Change in feed and product inventory

Working capital

Capitalized construction period loan interest

Construction period. This is the period before year 0 during which the plant is con-

structed and commissioned. Assume this period is 3 years, this is designated as end of

year –2, –1, 0. During this period, the construction company will receive incremental

payments of the total capital cost of the plant with ﬁnal payment at the end of year 0.

The construction cost may be paid from the company’s equity alone or from equity

and an agreed loan or entirely from a loan. In the case of a loan to satisfy this debt, the

payment of the loan interest commences in this period. The interest payment over this

period, however, is usually capitalized and paid over the economic life of the project.

Depreciation. Part of the cost to a project or venture which is considered as a de-

ductible from the gross proﬁt for tax purposes is the depreciation of the plant value.

This is calculated over the PLANT LIFE as the plant capital cost divided by the plant

life. The term Plant Life is the predicted life of the facility before it has to be disman-

tled and sold for scrap. Usually this is set at 20 years and indeed all speciﬁcations

relating to engineering and design of the facilities will carry this requirement. So all

material and design criteria, such as corrosion allowances, associated with the plant

will meet this plant life parameter.

Discounted cash ﬂow, deﬁnition of. The development of a DCF return on investment

is a combined effort between the Technical Disciplines and the company’s Finance

Specialist. The engineers provide the technical input to the work such as operating

costs, type of plants, construction schedules, and cost, yield and reﬁnery fence product

prices, and the like. While the ﬁnancial specialist provides the ﬁnancial data based

on statutory and company policies, such as the form of depreciation, tax exemptions,

tax credits (if any), items forming part of the company’s ﬁnancial strategy, etc. The

calculation itself is in two parts, which are:

Calculation of Cash Flow

Present Worth Calculation. (or any other method for calculating the Return on

Investment the Company may use)

Economic life. This is the number of years over which the project is expected to

yield the projected proﬁt and pay for its installation. These are the number of years

A DICTIONARY OF TERMS AND EXPRESSIONS 1133

starting at year 0 which indicates the end of construction and the commissioning of

the facilities. The last year (usually year 10) is the year in which all loans and other

project costs are repaid and the “Terminal Investment”released.

Plant Operating Cost. This includes the cost of utilities used in the process, such as

power, steam, and fuel. It also includes the cost of plant personnel in salaries, burdens,

and indirects and the cost of chemicals and catalyst used.

Maintenance. There are two kinds of maintenance costs included in this item These

are the preventive maintenance carried out on a routine basis, and those costs

associated with incidental breakdowns and repair.

Loan repayment. The loan principal is paid back in equal increments over the

economic life of the plant. This item includes the payback increments and the

associated interest on a declining basis.

Net cash ﬂow. This item is calculated for each year of the project’s economic life. It

commences with year 0 with the net investment shown as a negative net cash ﬂow

item. Then for each successive year until the end of the last year of the economic

life, the net cash ﬂow is calculated as the sum of Proﬁt After Tax PLUS Deprecia-

tion.

The depreciation is added here because it is not really a cost to the project. It is a

“Book”cost only and is used speciﬁcally for tax calculation.

The cash ﬂow item for the last year of the economic life must now include the

“Terminal Investment Item”. This item is the sum of the net scrap value of the plant

(scrap value less cost of dismantling), the estimated value of the land and the Working

Capital initially used as part of the “Associated Costs”.

Thus, the ﬁnal cash ﬂow item will be the sum of Proﬁt After Tax, plus Depreciation,

plus Terminal Investment.

With the Net Cash Flow in place, the second part of the calculation which is the

determination of the Return on Investment for the project can be carried out.

Pay out time. The pay out time for a project or any venture is given by the expre-

ssion:

Net Investment Cost

Net Income after taxes + depreciation

= Years

Simple return on investment. This calculation is used for screening purposes of several

options that are apparent in any venture. It does not consider construction time or the

1134 CHAPTER 19

discounting of the cash ﬂow over the economic life of a plant. Its deﬁnition is given

by the equation:

Net Income after Taxes

Net Investment Cost

× 100 = %ROI

Where

Net Investment is Capital Cost plus Associated Cost.

Net Income is Gross income less operating cost, depreciation, and tax.

Revenue. This is income received for the sale of the product(s). This is calculated from

projected process yields of products multiplied by the market price of the products.

A market survey should already have been completed to ensure that the additional

products generated by the project are in demand and the price is in an acceptable

range. Later, a sensitivity analysis of the DCF return on investment may be conducted

changing the revenue recovered by price escalation or other means.

Taxable income. Taxable income is revenue less operating cost, depreciation, and Ad

Valorem Tax. This of course is simply put as in most countries, states, or provinces

there will probably be certain local tax relief principals and tax credits that will affect

the ﬁnal taxable income ﬁgure. The company’s ﬁnancial specialist will be in the

position to apply these where necessary.

Tax. This is quite simply the tax rate applied to the “Taxable Income”ﬁgure. This

will vary from location to location but will be taken as one rate over the economic life

of the project for the purpose of a process study, unless there are legislative changes

already in place.

Working capital. In all capital projects there is initially a ﬁnancial loss when the

company has to purchase land, equipment, pay contractors to erect equipment, and

the like. To do this not unlike most private individuals, companies may need to

borrow money, on which of coarse they have to pay interest. In addition to the cost

of construction, the company must keep in hand some capital by which it can buy

feedstock, chemicals, and pay the salary of its staff during the commissioning and

the initial operation of the plant. During this initial operation, the working capital

must also be considered as debt. At the end of this period of the project initiation, the

operation must recover the working capital and make its prescribed proﬁt.

Edmister correlations

A series of correlations which relates an ASTM distillation test to the TBP

of a petroleum cut is presented in W. Edmister’s publication titled “Applied

A DICTIONARY OF TERMS AND EXPRESSIONS 1135

Thermodynamics”. The correlation relating to the ASTM and TBP curves from this

book is used in this Handbook and is given in Chapter 1. Similar correlations have

also been prepared by Edmister for the relation between TBP and the equilibrium

ﬂash vapor curve (EFV).

Efﬂuent water

This item is described and discussed in Chapter 14. Pollutants in waterways from

reﬁneries are listed in the following table.

Process Waste water Air

Atmospheric and

vacuum distillation

Sour water (NH

Desalter water Spent caustic

Process area waste water (pump glands,

area drains etc)

Furnace

Flue gases—SO

Thermal cracking

Delayed coking

Sour water

Decking water (Oil)

Process area waste water

Furnace

Flue gases

Fluid cat cracking

Unsat gas plant

Sour water (NH

S, Phenols)

Spent caustic

Process area waste water.

Furnace

Flue gases SO

,CO

particulates.

Hydrocracker

hydrogen plant

Sour water (inc Phenols)

Process area waste water

Furnace

Flue gases SO

Sat gas plant alkylation Spent caustic

Process area waste water

Nil

Naphtha hydrotreater

Cat reformer

Sour water

Process area waste water

Furnace

Flue gases

Sulfur plant Nil Incinerator Flue gas—SO

Hydrocarbons—Flare

Tankage area Tank dike area drains Non contaminated

rain runoff

Tank vents Hydrocarbons.

The most undesirable pollutants in aqueous waste streams are:

Those that deplete the dissolved oxygen content of the waterways into which they

discharge.

Those contaminants that are toxic to all forms of life, such as arsenic, cyanide,

mercury, and the like.

Those contaminants that impart undesirable tastes and odors to streams and other

waterways into which they discharge.

Oxygen depletion occurs by the introduction of one or more of the following oxidizable

contaminants entering the waterway: