SECTION III.IV: Downstream Operations: Petroleum Refining of Crude Oil Into Useful Products (Three Parts) and Oil and Gas Transportation (One Chapter)

'I Q Crude Oil

Crude Oil Refining—Part 1

• 19.1 OIL FRACTIONATION (DISTILLATION)
• 19.1.1 Overview

Petroleum is of little use when it first comes from the ground. It is a raw material, much as newly fallen trees are raw materials for furniture, construction, etc. Thus, crude oil must be put through a series of processes to be converted into the hundreds of finished oil products derived from it. These processes, collectively, are known as refining. However, by today’s technological standards, the term refining is a misnomer. In the early petroleum industry, the refining process involved nothing more than the use of a crude still (pipe still) that produced useful oil products by physical separation only.

Currently the expression crude oil processing is more appropriate, since more than 85% of petroleum products are produced by processes involving chemical changes along with the basic physical separation.

The first step in refining is distillation. This step roughly separates the molecules in crude according to their size and weight. The process is analogous to taking a barrel of gravel containing stones of many different sizes and running the gravel through a series of screens to sift out first the small stones, next those slightly larger, and so on up to the very largest stones. As applied to crude oil, the distillation process “sifts out” progressively such components as gas, gasoline, kerosene, home heating oil, lubricating oils, heavy fuel oils, and asphalt.

The oil refining process is the central activity of downstream oil and gas operations. The Energy Information Administration (EIA) explains the breakdown this way: "Inside the distillation units, the liquids and vapors separate into petroleum components called fractions according to their weight and boiling point. Heavy fractions are on the bottom, and light fractions are on the top”.

The objective of petroleum refining processes to transform crude oil into useful products such as liquefied petroleum gas (LPG), gasoline or petrol, kerosene, jet fuel, diesel oil, and fuel oils.

The basic aspects of current refining operations, involving physical separation are presented in this chapter along with the application of economic techniques and analysis to many problems encountered in the petroleum industry.

The physical separation of crude oil into valuable products (cuts) is highlighted. Crude oil separation is accomplished in two consecutive steps: first by fractionating the total crude oil at essentially atmospheric pressure; then feeding the bottom residue from the atmospheric tower to a second fractionator, operating at high vacuum. Types of oil refineries and their classification are given.

Economic analysis is presented for the refining operations in various ways to determine the most economical refining scheme to find out, for example, whether to use new or existing equipment.

19.1.2 Illustrative Example

As we have seen the objective of petroleum refining processes to transform crude oil into useful products such as LPG, gasoline or petrol, kerosene, jet fuel, diesel oil, and fuel oils.

The oil refining process is the central activity of downstream oil and gas operations. The EIA explains the breakdown this way.

Crude oil is to be fractionated into straight-run products such as gasoline, gas oil, and others. It is heated first by heat exchangers and then desalted. Its temperature is raised next using fire heaters, before it is introduced to the fractionation tower. This process is illustrated as shown in the following sketch in Figure 19.1. The next step in this process is to explain how and why the crude oil is separated into products?

Well, in the transformation of raw materials (crude oily, and in the presence of energy (heat) to produce finished products, three modes of transfer are encountered in this process. They are known as:

FIGURE 19.1 Illustrative example for a refining distillation operation.

• 1. Momentum Transfer (fluid flow), using a pump.
• 2. Heat Transfer of oil, using heat exchangers and a furnace.
• 3. Mass Transfer through the distillation column that leads to the separation of crude oil into different cuts (transfer is due to molecular diffusion of the components that separates the light from heavy). The physical operations (known as unit operations) and shown above in the example are: fluid flow, heat transfer, and distillation. Unit operations deal chiefly with the transfer of energy and the transfer, separation, and conditioning of materials by physical means.

At this stage, two basic questions arise:

• 1. What is the mechanism(s) underlying this process?
• 2. How and where it takes place?

The answer to the first question deals with theory of transfer or transport, as explained, within the boundaries of our system. For the second question, it is the combined effect of Momentum, Heat and Mass (MHM) that is responsible for the physical changes that took place in the distillation column to produce the finished products.

To make the story complete in the above example, it will be assumed, that the gasoline exit the distillation column is introduced into what is known as a “Reforming Unit”, in order to obtain a higher grade gasoline. This reforming process represents a typical example of a chemical conversion or chemical reaction process-known as Unit Process—where the hydrocarbons undergo molecular changes and rearrangement leading to high octane gasoline. Unit processes involve primarily the conversion of materials by means of chemical reactions. Again, it should be pointed out that the three modes of transfer, MHM, take place as well for operations involving chemical reactions, or chemical changes.

In general, refineries rely on four major chemical processing operations in addition to the backbone physical operation of fractional distillation, in order to alter the ratios of the different fractions. These are normally called the Five Pillars of petroleum refining:

Pillar 1: Fractional distillation

Pillar 2: Cracking

Pillar 3: Unification (alkylation)

Pillar 4: Alteration (catalytic reforming)

Pillar 5: Hydroprocessing

19.1.3 Refinery Design as a Chemical Plant

The following one-page summary represents an avenue what is involved to design a refinery guided by the basic principles of plant design taught in chemical engineering.

Refinery design follows the outline of Figure 19.2 as shown:

FIGURE 19.2 A design of a refinery follows the same aspects as plant design.

The design of a chemical plant (which is a refinery for our case) would normally go through the following steps: ^•

Inception of an idea (e.g., to produce a product).

• • Find out if it is feasible to build a plant (technical and economic feasibility study).
• • Carry out “Process Design” which involves three basic stages:

a. Draw a qualitative block diagram based on a written description for the selected process.

b. Carry out basic calculations using M.B. & E.B. (Material Balance & Energy Balance) to come up with a quantitative block diagram. Material balance is the basis of process design.

c. Determine the size and capacity of equipment (equipment sizing).

• • Do cost estimation for the capital investment of the plant.
• • Carry out profitability analysis for the project.
• • When it comes to computer applications, spread sheet software has become indispensable tool in plant design, because of the availability of personal computers, ease of use, and adaptability to many types of problems. On the other hand, many programs are available for the design of individual units of chemical process units. The Chemical Aids for Chemical Education (CACHE) Corporation makes available several programs mainly for educational use.

Apart from the engineering principles considered in the plant design, there are other important functions and items to be considered regarding safety, health, loss prevention, plant location, plant layout, and others.

A brief summary is given as follows:

• • Health & Safety Hazards: One should consider toxicity of materials and frequency of exposure, fire, and explosion hazarads.
• • Loss Prevention: HAZOP study.
• • Environmental Protection and Pollution Control: This includes: air, water, solid wastes, thermal effects, noise effects, and others.
• • For Plant Location: Primary factors and specific factors, both are to be considered.
• • For Plant Layout: Optimum arrangement of equipment within a given area is a strategic factor.
• • Plant Operation and Control: Designer should be aw'are of:

a. Instrumentation

b. Maintenance

c. Utilities

d. Structural design

e. Storage

f. Materials handling; pipes and pumps

g. Patents aspects

Once crude oil is produced, exit oil well, it goes through the following treating operations before distillation (Figure 19.3).

If the mixture to be separated is a homogenous, single-phase solution, a second phase must generally be formed before separation takes place. This is an “interphase'” operation, which involves the transfer of mass from one phase to another. This second phase is introduced by two methods:

a. By adding or removing energy: Energy Separating Agent (ESA), e.g., distillation.

b. By introducing a solvent: Mass Separating Agent, e.g., absorption.

FIGURE 19.3 Surface petroleum operations for crude oil exit the well.

“Intraphase” separation, on the other hand, implies separation of components within a phase, such as diffusion through inert barriers or membranes. These are rate-governed operations.

Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapor pressure characteristics of liquid mixtures.

For separation to take place, say by distillation, the selection of an exploitable chemical or physical property difference is very important. Factors influencing this are:

a. The physical property itself.

b. The magnitude of the property difference.

c. The amount of material to be distilled.

d. The relative properties of different species

• 19.1.4 Different Types of Distillation Methods
• 1. Flash—Vapors are kept in intimate contact with the liquid.
• 2. Simple—Vapors are withdrawn as quickly as they are formed to be condensed by a condenser.

These methods are illustrated in Figure 19.4.

19.1.5 Design Aspects

There are two main factors that govern the design of equipment in diffusional operations:

FIGURE 19.4 Classification of distillation method.

a. The thermodynamic equilibrium distribution of the components between the phases.

b. The rate of movement, diffusion rate, from one phase to the other.

Main factors to be considered in the design of finite-stage columns, other than calculating the number of the theoretical stages (plates) required for a given separation are the following:

a. Column diameter

b. Tray efficiency

c. Pressure drop across the tray

It should be pointed out that the number of plates in a column is a function of the degree of separation required, i.e.

On the other hand, the diameter of a column is a function of the charge input to the column or capacity, i.e.

19.1.6 Operating Pressure

The primary physical separation process, which is used in almost every stage while processing the crude oil, is fractional distillation, as explained above. The distillation

TABLE 19.1

Three Systems of Oil Fractionation Wrt Operating Pressure

operation can take place at atmospheric pressure, under vacuum, or under high- operating pressure. The three operations are common in the oil refining industry. For example, crude oil fractionation is always accomplished at atmospheric pressure (slightly higher), topped crude oil (fuel oil residue) is distilled under vacuum, while the stabilization of straight-run gasoline utilizes high-pressure fractionators or stabilizers. A comparison between these three systems of fractionation is shown in Table 19.1, which shows the technical merits and economic implications of each system.

19.1.6.1 Distillation Models

Distillation models are based on three pillars:

• • Laws of conservation of mass and energy
• • The concept of ideal stage

Rault’s law and Henery’s law used (for ideal case) to describe the tendency of escape for vapor/liquid at equilibrium

19.1.7 Types of Refineries and Economic Analysis

Depending on the type of crude oil used, the processes selected, and the products needed, as well as the economic considerations involved, refineries can have different classifications, as shown in Figure 19.5. The products that dictate the design of a fuel refinery or conventional refinery are relatively few in number but are produced

FIGURE 19.5 This illustratres these types of refinery.

in large quantities, such as gasoline, jet fuels, and diesel fuels. The number of products, however, increases with the degree of complexity of a fuel refinery, which varies from simple to complex or to fully integrated.

A simple refinery consists mainly of a crude oil atmospheric distillation unit, stabilization splitter unit, catalytic reforming plant, and product-treating facilities. Products are limited: LPG. gasoline, kerosene, gas oil, diesel oil, and fuel oil. A complex refinery will employ additional physical separation units (such as vacuum distillation) and a number of chemical conversion processes, including hydrocatalytic cracking, polymerization, alkylation, and others. The fully integrated refinery will provide other processes and operations necessary to produce practically all types of petroleum products, including lubrication oils, waxes, asphalts, and many others.

A chemical refinery, on the other hand, is a special case of the conventional oil refinery in which the emphasis is on manufacture of olefins and aromatics from crude oil. A chemical refinery can be defined as one that includes an olefin complex for the pyrolysis of petroleum fractions (e.g., C₂H₆ to C₂H₄). It must not produce motor gasolines; that is, it is a non-fuel-producing refinery. In other words, the purpose of chemical refining is to convert the whole crude oil directly into chemical feedstocks.

An example is the heavy oil cracking (HOC) process, in which the atmospheric residuum is catalytically cracked directly into lighter products. Chemical refining is an economically attractive venture for large chemical companies that can penetrate the market by selling large quantities of olefins and aromatics.

Economic analysis is used in refining to determine the most economical refining operations, to determine whether to use new or existing equipment, etc. Economic analysis, including cost analysis, is complicated in a refinery because an operation in a refinery with lower operating costs is not necessarily the most desirable procedure, and similarly, an operation giving higher yields, or production rates, is not necessarily a more economical one. A highest yield with lowest cost is what the refiner would like to achieve. Economic analysis is further complicated by the fact that several hundred different products may be produced from one basic raw material, crude oil.

There are also other complications. The basic crude may consist of a number of different crudes that have considerably different characteristics and different selling.

• 19.2 ECONOMIC BALANCE FOR DISTILLATION SCHEMES
• 19.2.1 Economic Balance

Economic balance in refining operations means that costs are balanced with revenue, inputs w'ith outputs and crudes with refined products. The object is to find the combination of least cost with the “’greatest” contribution.

There are two corollaries of great significance to the oil refiner that follow from the principle of diminishing productivity: namely, the principle of variable proportion, and the principle of least-cost combination.

The principle of variable proportion enters into all decisions relative to combining economic factors (inputs) for full production. In chemistry, we know' that elements combine in definite proportions. For instance, the combination of 2 atoms of hydrogen w'ith l atom of oxygen will produce 1 molecule of water: H₂ + О —> H_:0. No other combination of hydrogen atoms and oxygen atoms w ill produce w'ater. What is true in this instance is also true in all other chemical combinations, and in oil production as well. In other words, a law of definite proportions governs the combination of the various chemical elements and the various factors of production, such as amount of labor, materials consumed, and capital in a plant investment.

Economic balance applies to both physical operations (unit operations) and chemical conversion processes. It may involve a design problem or may address a processing operation or a separation step. In other words, economic balance may refer to the period before installation of equipment, in which case it consists of a study of costs and values received on design of equipment, or the period after installation of equipment, in which case it is a study of costs and values received on processing operations. The latter means on one hand an economic balancing of costs against optimum yield or optimum recovery, and on the other hand, elimination of as much waste as possible.

19.2.2 Economic Balance in Design

Design of equipment for process operations is a complex problem because of the many variables involved and the fact that broad generalizations about these variables cannot be made. Economic balance is not discussed in detail here, as much of it is beyond the scope of this book. A number of cases of economic balance in design, however, will be discussed. ^[1]

costs are reduced, but at the same time there is an increase in fixed costs when an increasing number of effects are used. So selection of which number of effects will balance direct costs is desirable.

• • Economic balance in vessel design may involve specific design problems, such as heating and cooling, catalyst distribution, design of pressure vessels for minimum cost, etc.
• • Economic balance in fluid flow involves the study of costs in which such direct costs as power costs for pressure drop and repairs, as well as fixed costs of pipe, fittings, and installation, are related to size of pipe. For example, power costs decrease as pipe size increases, and total costs are at a minimum point at some optimum pipe size.
• • Economic balance in heat transfer requires an understanding of how fixed costs vary, with a selected common variable used as a basis for analysis. Variable costs must also be related to this same variable. Thus, both fixed costs and variable costs are required for economic balance.

In any study of either design or operations, only the variable cost often referred to as direct costs, which are affected by variations in operation is included.

The following case study stresses the role of an economic balance in design in many applications throughout the processing of crude oil, which may involve the transfer of material, heat, or mass with or without chemical conversions.

19.2.3 Case Study 1: Optimum Reflux Ratio

In designing a bubble plate distillation column, the design engineer must calculate:

• 1. The number of plates
• 2. The optimum reflux ratio
• 3. The diameter of the column

It is well established that if the reflux ratio is increased from its minimum value, R„„ the number of plates would be decreased to attain the same desired separation. This means lower fixed costs for the column. The other extreme limit for the reflux could be reached by further increase in R with corresponding decrease in the number of trays until the total reflux, R,, is reached (case of minimum number of trays, NJ. Attention is now directed to the effect on the diameter of the column of increasing the reflux ratio, that is, increasing vapor load.

As R increases, the vapor load inside the column increases; consequently, the diameter of the column must be increased to attain the same vapor velocity. A point is reached where the increase in column diameter is more rapid than the decrease in the number of trays. Hence, the only way to determine the optimum conditions of reflux ratio that will result in the right number of trays for the corresponding column diameter is to use economic balance. For different variable reflux ratios, the corresponding annual fixed costs and operating costs must be combined and plotted versus the reflux ratio.

FIGURE 19.6 A plot of the total annual costs versus the reflux ratio.

(Source: Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers, McGraw-Hill International, 1981.)

Annual fixed costs are defined as the annual depreciation costs for the column, the reboiler and the condenser, where the cost of a column for a given diameter equals the cost per plate of this particular diameter times the number of plates. Therefore, the operating cost equals the cost of the steam plus the cost of cooling water. Figure 19.6 illustrates how to obtain the optimum reflux ratio (a design parameter) by minimizing the total annual costs of the distillation column.

19.2.4 Case Study 2: Economic Balance in Yield and Recovery

Principles of economic balance must be applied to different processes in the oil refinery for the purpose of determining how variations in yield, as affected by design or operation will produce maximum profit. The effect of changing the crude feed and refined oil product compositions on the overall profit for a refinery process can best be illustrated, in most cases, as follow:

A typical study of economic balance in yield and recovery reveals that obtaining a higher-grade product from a fixed amount of given feed means an increase in variable costs because of costs of increased processing. The final refined oil product, of course, has a higher value, but for some product grades, the costs may equal the selling price, with the result that it becomes uneconomical to exceed that particular “’specification”.

At some optimum grade of a product, however, a maximum gross profit, or difference between the sales dollars curve and the total costs curve, may be obtained per barrel of pure material (crude) in the feedstock.

In general, capacity is reduced as grade is increased, with the result that the maximum profit per barrel of pure material (crude) may not correspond to the maximum annual profit.

Although graphic analysis is the best procedure to use for such problems, there are also some useful mathematical relations. For example, if D is total refined product, F is total feed (crude), and Y is a conversion factor relating to feed (crude) and product (refined), then, under physical operations,

or,

or recovery in percent form. Also, if fixed costs are constant for a given process, then fixed costs will be constant for a given value of F or total feed (crude). However, as is usually the case, equipment costs will be higher for a higher-grade product, with the result that the annual fixed cost per unit of refined product increases.

For a given crude feed rate, raw material costs are constant, but refinery processing costs usually increase for a higher-grade product to give a variable cost curve that also increases.

The value of the finished product, like that of fixed costs and variable costs per unit of refined oil, will vary with the grade of product.

Figure 19.7 is a typical economic chart with curves illustrating economic balance curves in a refinery. Recovery, or ratio of output to input, in the oil refinery is greater than recovery in the oil fields. Note that to make a profit the refiner must stick to the product grades marked between A and В on Figure 19.7.

19.2.5 Selected Case Study 3: Crude Oil Desalter

The salt content of a Middle-Eastern crude oil (API gravity 24.2) was found to be 60 PTB. In order to ship and market this oil, it is necessary to install a desalting unit in the field, which will reduce the salt content to 15 PTB. This upgrading in the quality of oil in terms of an acceptable PTB could realize a possible saving of 0.1 $/bbl in the shipping cost of the oil.

Assume the following:

The crude oil desalter has a design capacity of 120,000 bbl/day.

The current capital investment of the desalting unit is estimated to be $3.0 million plus another $2.0 million for storage tanks and other facilities.

Service life of equipment is 10 years with negligible salvage value, while the operating factor = 0.95.

The total operating expenses of the desalter are estimated to be $10/1,000 bbl.

FIGURE 19.7 Economic level of refined oil production from a given feed of crude oil.

The annual maintenance expenses are 10% of the total capital investment. Evaluate the economic merits of the desalter by calculating, the ROI and payout period (P.P.).

SOLUTION

The total annual cost is the sum of the annual operating expenses plus annual depreciation costs. Assuming straight line depreciation:

the simultaneous solution of equilibrium relationships (VLE) and the operating line; where the operating line is used to compute the composition of one of the two streams passing each other for two consecutive plates, while the equilibrium relationship is used to compute the composition of either the vapor or liquid (in equilibrium) on the same plate.

19.3.2 Case Study 4: Plate-to-Plate Calculations (Case of Rectification Column)

This example is an oversimplified one for illustrative purposes.

Given:

• 1. Derivation of the Operating line: y_n+1 = [R/R + 1] X_n + [1/R + 1] X_D Equation 19.1 R is reflux ratio (R.R.) = L/V, X_D is the composition of the overhead product.
• 2. Equilibrium Data: X_n = Y„/[Y_n + a (1 - Y„)] Equation 19.2.

Statement of the Example:

Given: 40 mols/hr of feed (vapor) that contains 20% hexane and 80% octane entering the bottom plate, where D = 5 mols/hr, X_D = 0.9, R.R. = 7, a = 6. Find the number of theoretical trays, N.

1^st: Numerical Solution

Steps: ¹

• 1. The liquid composition leaving the partial condenser (plate number 0) is in equilibrium with the vapor[top product] and is calculated by Equation 19.2. Hence, X_reflux (leaving plate 0) = 0.9/0.9 + 6( 1-0.9) = 0.6.
• 2. Y, is calculated using Equation 19.1, substituting for R = 7. X = 0.6, and X_n = 0.9. we get Y, = 0.637.
• 3. Get the equilibrium composition of the liquid on the same tray, X, = 0.226.
• 4. Again, using Equation 19.1, get y₂ = 0.3.
• 5. Next, get X, = 0.066, which is the bottom product leaving the column, call it x_w.

FIGURE 19.8 Solution of problem—Case study 4.

Finally, we make overall M.B., and C.M.B. (Component Material Balance): (40) (0.2) = (5)(0.9) + (35)X,..

Solve for X*. = 0.1.

Therefore, 2 plates plus the condenser, make a total of 3 theoretical plates.

• 2^nd: Using Excel
• 19.3.3 Case Study 5: Plate-to-plate Calculations by Excel (Case of Stripping Column) Solution is illustrated in Figure 19.8

A liquid mixture at the boiling point consists of 70 mole% Benzene and 30 mole% Toluene is fed to a stripping column. Pressure is taken 1 atm. Feed rate is 400 kg mole/hr. Stripping operation is carried out to achieve a bottom product W = 60 kg mole/hr. that contains no more than 2 mole% Benzene.

Solve the problem using Excel, in order to determine the number of theoretical trays, N required to obtain the desired specifications of the bottom product W. Use a_AB relative volatility for Benzene/Toluene, where a_AB = K_Benzene/K_Toluene = P^HB/ P°_T = 2.45

SOLUTION

The number of trays required to reach a bottom product, exit the stripping column is found to be around 11 trays, as seen next; that corresponds tol.9 mole% benzene.

FIGURE 19.9 Solution of problem—Case study 5.

CASE STUDY: RECOVERY OF BUTANE USING LEAN OIL EXTRACTION

Objective:

This case study was presented as a senior project to students studying chemical engineering at KFUPM.

Case Study:

Associated natural gas is passed through an absorption unit to recover heavier hydrocarbons (butane plus), which can be sold for a value of $7.5/gal. Calculations show that the minimum total cost for the recovery and the extraction of the butanes in the plant is estimated to be $1,2/gal of butane recovered. Other additional costs for processing the absorbing oil used in the recovery are estimated to be $27/million gal of the lean oil circulated.

The engineering group in the plant developed the following empirical relationship for the rate (R) of the absorber oil used as a function of the rate of butane produced (P):

• 1. Compute the optimum butane recovery P_a, and the optimum circulating oil rate, P„ applicable to this plant.
• 2. What is the value of P at which the process of recovery breaks even?

SOLUTION

d/dp(profit) = 6.3 - 1.3(0.108) P^{0 3}; setting this derivative equal to zero:

At the break-even point, profit = 0

CASE STUDY: UTILIZATION OF NATURAL GAS RECOVERED FROM GAS PLANT

This case study fits Chapter 19.

Objective:

To investigate the economics of utilizing natural gas as a fuel for heating crude oil.

Process Description:

Natural gas is recovered from gas-oil separator plant (GOSP) using an absorber de-ethanizer system, along with an amine treating unit and a gas dryer to have available desulfurized gas that can be used or sold as a fuel gas.

Given: The total cost for the recovery of this gas is estimated to be $0.75/ MCF (Million Cubic Feet). It has been suggested to use this gas as a fuel for heating 5000 bbl/day of 40° API crude oil from 80°F to 250°F:

Find:

• 1. The cost of heating the crude oil using this gas.
• 2. Compare it with the cost of heating using fuel oil at $2.2/MMBtu.
• 3. Do you recommend change in operation to use the fuel gas as a heating fuel instead of using the fuel oil?

SOLUTION

The heat duty required is calculated using the well-known equation: Q = m cp Д T

Assuming the heating value of the gas is 960 Btu/ft³ and the heat efficiency is 60%; then the fuel gas consumption will be 221,700 ftVday.

A daily savings in the cost of fuel of about $300 is realized if the change to fuel gas takes place. One has to consider other economic factors in making this analysis. The capital cost involved in changing the burner system has to be considered.

CASE 2: HOW TO CONTROL THE CO, SPECS IN THE SWEET GAS? (DISCUSSION TYPE)

Methyl diethanolamine (MDEA) has become the amine molecule chosen to remove hydrogen sulfide, carbon dioxide, and other contaminants from hydrocarbon streams. Amine formulations based on MDEA can significantly reduce the costs of acid gas treating.

CASE STUDY: RECOVERY OF BUTANE USING LEAN OIL EXTRACTION

This case study fits Chapter 19.

Objective:

This case study was presented as a senior project to students studying chemical engineering at KFUPM, Dhahran, Saudi Arabia.

Case Study

Associated natural gas is passed through an absorption unit to recover heavier hydrocarbons (butane plus), which can be sold for a value of $7.5/gal. Calculations show that the minimum total cost for the recovery and the extraction of the butanes in the plant is estimated to be $1.2/gal of butane recovered. Other additional costs for processing the absorbing oil used in the recovery are estimated to be $27/million gal of the lean oil circulated.

The engineering group in the plant developed the following empirical relationship for the rate (R) of the absorber oil used as a function of the rate of butane produced (Py.

R, millions of gal/hr = 0.004 P¹ where P is in gal/hr

• 1. Compute the optimum butane recovery P,„ and the optimum circulating oil rate, R„ applicable to this plant.
• 2. What is the value of P at which the process of recovery breaks even?

SOLUTION

d/dp(profit) = 6.3 - 1.3(0.108) P⁰³; setting this derivative equal to zero:

At the break-even point, profit = 0

2f) Crude Oil

• [1] Economic balance in evaporation is a problem of determining the most economical number of effects to use in a multiple-effect evaporation operation.There is economy in increasing the amount of steam used because direct