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

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ECONOMICS 795

Table 17.1.22. Present worth calculation

Net cash ﬂow 141.4 32.7 32.8 32.8 32.9 32.9 32.9 32.9 32.9 32.9 51.5

discounted @ 10%

Discount factor 1.0 0.909 0.826 0.751 0.683 0.621 0.564 0.513 0.467 0.424 .385

Present worth 141.4 29.7 27.1 24.6 22.5 20.4 18.6 16.9 15.4 13.9 19.8

Cumulative

PW 141.4 111.7 84.6 60.0 37.5 17.1 1.5 18.4 33.8 47.7 67.5

Discounted @ 15%

Discount factor 1.0 0.870 0.756 0.658 0.572 0.497 0.432 0.376 0.327 0.284 .247

Present worth 141.4 28.4 24.8 21.6 18.8 16.4 14.2 12.4 10.8 9.3 12.7

Cumulative

PW 141.4 113.0 88.2 66.6 47.8 31.4 17.2 4.8 6.0 15.3 28.0

Discounted @ 25%

Discount factor 1.0 0.800 0.640 0.512 0.410 0.328 0.262 0.210 0.168 0.134 .107

Present worth 141.4 26.2 21.0 16.8 13.5 10.8 8.6 6.9 5.5 4.4 5.5

Cumulative

PW 141.4 115.2 94.2 77.4 63.9 53.1 44.5 37.6 32.1 27.7 22.2

Discounted @ 30%F

Discount factor 1.0 0.769 0.592 0.455 0.350 0.269 0.207 0.159 0.123 0.094 .073

Present worth 141.4 25.1 19.4 14.9 11.5 8.9 6.8 5.2 4.0 3.1 3.8

Cumulative

PW 141.4 116.3 96.9 82.0 70.5 61.6 54.8 49.6 45.6 42.5 38.7

The ﬁrst objective using this technique is to build a mathematical model that fully

deﬁnes the problem. The main model constitutes the “objective function.”The purpose

of linear programming is to either maximize or minimize the objective function (e.g.,

maximize overall proﬁt or minimize overall costs) subject to a number of constraints.

The model itself will consist of many submodels which will be inter-related by linear

equations. In the case of an oil reﬁnery conﬁguration study the data for the model

development will include:

All the processing plants to be considered

The yield from each plant

The product and feed stream properties

All the possible routing of the streams

All the possible ﬁnal blending recipes

Utility and operating requirements for each process on a unit throughput basis

Investment cost for each process

These data are coded and the coded items used in equations that represent all

the relationships to one another. The coded data items are shown graphically in

Figure 17.1.11. The relationship equations representing physical and ﬁnancial data

are input to the computer program. Proprietary sub routines included in the program

resolve nonlinear relationships to linear. Such routines as the DCF calculation are

796 CHAPTER 17

Figure 17.1.11. An example of a mathematical model used in linear

program.

ECONOMICS 797

Figure 17.1.11. (Cont.)

798 CHAPTER 17

Figure 17.1.11. (Cont.)

ECONOMICS 799

also added into the main program. The computer solves the many hundreds of equa-

tions meeting the problem premises and any other applied constraints. The selected

conﬁgurations are optimized within the program and the ﬁnal printout shows product

quantities, product quality, stream ﬂows, unit capacities, investment costs, operating

cost, and the ROI. The graphical representation of the model, part of which is shown

here as Figure 17.1.11, contains 511 variables and over 400 equations. By present

linear program standards this is a family small model.

17.1.3 Executing an approved project

This part of the chapter deals with the activities that are carried out by the reﬁnery in

installing a new or revamped facility which has been approved by the mechanism de-

ﬁned in Part 2 of this Chapter. Much of the initial work is carried out by the company’s

development engineers. Later, during the early part of the work being executed in a

selected engineering ofﬁce, the company will set up a project team headed by a project

manager to monitor and oversee that the company’s interests and requirements are met.

Developing the duty speciﬁcation

Among the ﬁrst activities to initiate a reﬁnery project is the development of the “Project

Duty Speciﬁcation”. The process major input to this most important document is

supplied by the company’s process development engineers.

As soon as a project and the funds have been approved for implementation work must

begin to appoint an engineering and construction contractor to do the work. Before

this can be accomplished however it is necessary to complete the following:

1.0 Prepare the duty speciﬁcation for the plant.

2.0 Develop the contractor’s scope of work.

3.0 Develop the contractual terms and the contract itself.

4.0 Assemble the document inviting contractor bids.

5.0 Decide on the selection procedure for the contractor appointment.

Normally the operating company will assemble a team of people who will be

dedicated to carry out these activities. This team will usually be headed by a member

of the company’s middle management who will be responsible for these precontract

activities. The team itself will consist of senior personnel from its engineering

disciplines, and other parties such as the company’s purchasing and legal departments.

The development engineer will almost invariably participate in some or all of the ac-

tivities listed above. His/her main participation however will be in the development of

the duty speciﬁcation. The importance of this document is that it forms the basis of the

800 CHAPTER 17

selected contractor’s scope of work. It also establishes the standard of quality required

by the operating company as the client. A well drafted project duty speciﬁcation will

contain at least the following sections and information:

The process speciﬁcation

General design criteria

Any preliminary ﬂow sheets (duly labeled “Preliminary”)

Utilities speciﬁcation

Basis for economic evaluations

Materials of construction

Equipment standards required

Instrument standards required

These are now discussed in the following paragraphs:

The process speciﬁcation

This document is developed entirely by the senior development engineer assigned to

the project team. It gives in precise terms, the plant required, the number of units,

its throughput, the product yields and quality, the required test standards, and any

salient process requirements. An example of a typical process speciﬁcation is given

in Appendix 17.8.

General design criteria

This section of the project duty speciﬁcation is usually compiled by the reﬁnery’s

project engineers assigned to the team. Some input is required from the development

engineer to ensure that technical documents and data developed by others will conform

with the company’s usual format. This section, as the name implies, supplies the

general data associated with any work done in the company’s plant site. The following

topics make up this section:

Scope

This is a brief statement covering the objective of the project. This is followed by a

list of the company’s standards that are to be used in the implementation of the work.

Climatic data

A list of the following data is given here. These are the data that will be used in the

various speciﬁcations and calculations developed during the course of the project:

Dry and wet bulb temperatures

Winter design dry bulb temperature

Temperature extremes

Barometric pressure

ECONOMICS 801

General design considerations

This section lists all of the legislative criteria associated with the building of an

industrial plant in the area. It will include the environmental requirements, and safety

and quality standards to be adhered to by contractors and licensor.

Units of measurement

The units of measurement that will be used in all calculations, ﬂow sheets, and

speciﬁcations are given in this item. It begins by stating the units in terms of:

English units

or SI units

or any others

The next section then deﬁnes these units in more detail such as for example:

Linear—Millimeters (mm)

Mass—Kilograms (kg)

Flow gas—Normal cubic meters per hour (Nm

/h).

Flow liquid—(large) cubic meters per hour (m

/hr).

—(small) liters per minute (l/min).

There follows then a list of conversion units that should be used on the project.

Engineering flow diagrams

Many companies include those ﬂow diagrams they have developed during the ap-

propriation stage of the project. These are usually the process ﬂow diagram and the

mechanical ﬂow diagram. The inclusion of these adds to the description of the work

scope by supporting the process speciﬁcation. In most contracts however the client

company will expect the contractor and/or the licensor to take responsibility for the

process performance and to guarantee it. Under this circumstance these engineering

diagrams are released into the project duty speciﬁcation as “Preliminary”issues. The

contractor is expected to check and, if necessary, revise them. Thereafter, the contrac-

tor must accept responsibility for the technical content of the diagrams as a basis for

future normal design development.

Utilities speciﬁcation

Full details of the utility streams available in the clients plant is given in this item. This

item is generally prepared by the process engineers and must include data concerning

all steam and condensate systems, water systems, air, and fuel. Such data will include

at least the temperature and pressure of these streams. In the case of circulating

systems such as fuel oil and cooling water both supply and return conditions will be

required. Raw water available should also have a complete analysis of its impurities.

The analysis of the normal fuel oil and fuel gas should be included. These would

contain at least the following:

802 CHAPTER 17

Fuel oil: Supply temperature.

Sg @ supply temp

Visc @ 120

◦

Fcs

Visc @ 210

◦

Fcs

Supply and return pressure.

Fuel gas: Supply temperature and pressure.

Molecular weight.

Approximate molar analysis.

The dew point of instrument air required at the air supply header must be stated. If the

client has any preference as to the type of desiccant to be used in the drying of the air

this too must be stated.

All other preferences or standards that the client company requires to be utilized in

the utilities design must be given in this section. For example most companies have

a standard burner control system for their plants. This should be fully described here

together with some appropriate sketches.

The electrical engineers usually add details of the plant power supply and distribution

in this section. If some existing switch boards and substations are to be utilized this

should be noted together with a list of drawings that should be referenced.

Other systems, although not strictly utilities, may be included here. Among these

would be:

The ﬁre main

The ﬂare(s)

Water efﬂuent treating and disposal

Other environmental protection systems

Boiler blow-down systems

Basis for economic evaluations

The process engineer completes this section with the criteria used in the evaluation

studies and the appropriation design. The section should begin with a statement that

contractors are encouraged to review all ﬂow and equipment arrangements so that

all possible alternatives are considered. The incremental cost of any alternative ar-

rangements must however yield a minimum of the company’s stipulated return on

investment. This return on investment is stated here.

This section continues with the detail costs of labor, utilities, feedstock, and products

to be used for any such economic analysis.

ECONOMICS 803

Materials of construction

This section is usually compiled by the team’s project engineers with some input by

the process engineers. It should begin with a paragraph on the references to be used

in selecting the materials used in handling the corrosive streams. For example:

Selection of steels exposed to hydrogen service shall be based on “API-941.”

Corrosion allowances for steels subject to hydrogen sulﬁde environment shall be in

accordance with “Computer correlations to estimate corrosion of steels”by Couper

and Gorman, NACE paper No 67.

The next paragraph or sub section should detail the client’s requirement for corrosion

allowances in terms of equipment life. For example:

Equipment Life in years

Columns, drums, and reactors 20

Heat exchanger-shell 20

Heat exchanger-tubes 5

Pumps 10

Compressors 10

Heater tubes 100,000 hours.

Atmospheric tankage 20

Piping material 10

These data may also be supplemented by a table giving the company’s accepted

minimum corrosion allowance in mm or ins (mils) for the various equipment items.

Equipment standards to be used for sizing and design

There follows in this section data to be used for the sizing and the specifying of the

major equipment items, which are:

Vessels and columns

Heat exchangers

Pumps

Compressors

Heaters

Examples of such data included in this section are as follows:

Vessels and columns

Columns to be sized on a speciﬁed type of tray (e.g. valve trays).

Design for a percentage of ﬂood (usually 80%).

Design pressure and temperature criteria for all vessels.

Minimum diameter for trayed columns.

804 CHAPTER 17

Heat exchangers

Design standards to be used (e.g. TEMA, ASME, etc.).

Rating procedures.

General design criteria (e.g. use of ﬁxed and ﬂoating head tube bundles, expansion

bellows, tube side ﬂows where special considerations are required, use of kettle or

thermo-syphon reboilers etc.).

Tube sizes and pitch relative to fouling factors.

Table of fouling factors to be used.

Allowable velocities and pressure drops.

Approach temperatures to be used.

Design temperature and pressures.

Pumps

The design standard to which the pumps are to be designed. For example API 610

“Centrifugal Pumps for General Reﬁnery Services.”

Rated capacity of the pump over normal. For example:

Service Rated Capacity over Normal %.

Flow control Level or temp control

Feed 5 10

Product 10 15

o/hd reﬂux. 15 20

Inter reﬂux. 20 20

Reboiler feed 15 20

1. Sparing requirement.

2. NPSH calculation criteria—Source pressure

Rate for suction line losses

Level for static head. (e.g. bottom tangent line for columns)

Pump centre line etc.

3. Pump selection preferences

4. Minimum ﬂow criteria

5. Casing design conditions

6. Type of drivers

7. Piping hook-ups (company standard)