SPRING 2019 - Anand Bhartianandbharti.co.in/pre/L20.pdf · Two visbreaking processes are...

transcript

SPRING 2019

CL 4003 PETROCHEMICALS AND REFINERY ENGINEERING

Lecture 20

Department of Chemical Engineering

Birla Institute of Technology Mesra, Ranchi1

Isomerization of Light Naphtha

✓ Isomerization is the process in which light straight chain

paraffins of low RON (C6 and C5) are transformed with proper

catalyst into branched chains with the same carbon number

and high octane numbers.

✓ The hydrotreated naphtha is fractionated into heavy naphtha

between 90-190 °C which is used as a feed to the reforming

unit. Light naphtha C5-80 °C is used as a feed to the

isomerization unit.

Isomerization

There are two reasons for this fractionation:

✓ The first is that light hydrocarbons tend to hydrocrack in the

reformer.

✓ The second is that C6 hydrocarbons tend to form benzene in the

reformer. Gasoline specifications require a very low value of

benzene due to its carcinogenic effect .

Isomerization

✓ The isomerization reactions are slightly exothermic and the

reactor works in the equilibrium mode.

✓ There is no change in the number of moles and thus the

reaction is not affected by pressure change.

✓ Better conversions are achieved at lower temperature.

✓ Operating the reactor at 130 °C will give good results.

Thermodynamics of Isomerization

Thermodynamic equilibrium with and without

recycling normal paraffin

✓ Isomerization is a reversible and slightly exothermic reaction:

✓ The conversion to iso-paraffin is not complete since the reaction

is equilibrium conversion limited.

✓ It does not depend on pressure, but it can be increased by

lowering the temperature.

✓ However operating at low temperatures will decrease the

reaction rate. For this reason a very active catalyst must be

used. 7

Isomerization Reactions

There are two types of isomerization catalysts:

✓ the standard Pt/chlorinated alumina with high chlorine

content, which is considered quite active, and

✓ the Pt/zeolite catalyst.

Isomerization Catalysts

✓ This bi-functional nature catalyst consists of highly chlorinated

alumina (8–15 wt% Cl2) responsible for the acidic function of the

catalyst.

✓ Platinum is deposited (0.3–0.5 wt%) on the alumina matrix.

✓ Platinum in the presence of hydrogen will prevent coke

deposition, thus ensuring high catalyst activity.

✓ The reaction is performed at low temperature at about 130 °C to

improve the equilibrium yield and to lower chlorine elution.

Standard Isomerization Catalyst

✓ The standard isomerization catalyst is sensitive to impurities

such as water and sulphur traces which will poison the catalyst

and lower its activity.

✓ For this reason, the feed must be hydrotreated before

isomerization.

✓ The pressure of the hydrogen in the reactor will result in the

elution of chlorine from the catalyst as hydrogen chloride.

✓ For all these reasons, the zeolite catalyst, which is resistant to

impurities, was developed. 10

Standard Isomerization Catalyst

✓ Zeolites are crystallized silico-aluminates that are used to give

an acidic function to the catalyst.

✓ Metallic particles of platinum are impregnated on the surface of

zeolites and act as hydrogen transfer centres.

✓ The zeolite catalyst can resist impurities and does not require

feed pre-treatment, but it does have lower activity and thus the

reaction must be performed at a higher temperature of 250 °C.

Zeolite Catalyst

✓ The reformate yield from light naphtha isomerization is usually

very high (>97 wt%).

Isomerization Yields

Light naphtha with a specific gravity of 0.724 is used as a feed to

the isomerization unit at a rate of 100 m3/h. Find the product

composition.

Example

Solution

Thermal cracking process

Visbreaking

Delayed coking

Fluid coking and Flexicoking

Thermal cracking process

✓ Thermal cracking is the cracking of heavy residues under severe

thermal conditions.

✓ The liquid products of this process are highly olefinic, aromatic

and have high sulphur content.

✓ They require hydrogen treatment to improve their properties.

✓ Coking is the process of carbon rejection from the heavy

residues producing lighter components lower in sulphur, since

most of the sulphur is retained in the coke.

Introduction

✓ The thermal treatment of hydrocarbons follows a free radical

mechanism where cracking reactions take place in the initiation

✓ The reactions in the final step result in the formation of heavy

fractions and products like coke.

Introduction

Visbreaking

Visbreaking is a well-established noncatalytic thermal process that

converts atmospheric or vacuum residues to gas, naphtha, gas oil,

and tar (75–85% cracked material of lower viscosity that can be

used as fuel oil).

Introduction

The conversion of these residues is accomplished by heating the

residue material to high temperatures in a furnace. The material is

passed through a soaking zone, located either in the heater or in

an external drum, under proper temperature and pressure

constraints so as to produce the desired products. The heater

effluent is then quenched with a quenching medium to stop the

reaction.

Introduction

With refineries today processing heavier crudes and having a

greater demand for distillate products, visbreaking offers a low-

cost conversion capability to produce incremental gas and

distillate products while simultaneously reducing fuel oil viscosity.

Introduction

The feed to visbreaker can be either

❑ Atmospheric residue (AR)

❑ Vacuum residue (VR)

Feed Sources

✓ The main reaction in visbreaking is thermal cracking of heavy

hydrocarbons.

✓ The cracking of resin will result in precipitation of asphaltene

forming deposits in the furnace and will also produce unstable

fuel oil.

✓ The cracking severity or conversion is limited by the storage

stability of the final residual fuel.

Visbreaking Reactions

✓ Paraffinic side chain breaking which will also lower the pour

point;

✓ Cracking of naphthens rings at temperature above 482 °C;

✓ Coke formation by polymerization, condensation,

dehydrogenation and dealkylation.

Possible reactions in visbreaking

Four products are produced in the visbreaking process:

✓ gases (𝐶4− ),

✓ Naphtha C5 - 166 °C,

✓ gas oil 166–350 °C and

✓ residue or tar 350 °C.

Visbreaking results in an increase of API of 2–5 for the vacuum

residue feed and a reduction of viscosity of 25–75%.

Product Yield and Properties

Typical yields of visbreaking process

Visbreaking of Kuwait residue

There are two types of visbreakers:

✓ coil visbreaking, in which thermal cracking occurs in the coil of

the furnace, and

✓ the soak visbreaker, in which cracking occurs in a soak drum.

Process Description

Coil Visbreaker

✓ Vacuum or atmospheric residue feedstock is heated and then

mildly cracked in the visbreaker furnace.

✓ Reaction temperatures range from 450 to 480 °C, and operating

pressures vary from as low as 3 bar to as high as 10 bar.

✓ Visbroken products are immediately quenched to stop the

cracking reaction.

✓ The quenching step is essential to prevent coking in the

fractionation tower. The gas oil and the visbreaker residue are

most commonly used as quenching streams. 30

Coil Visbreaker

✓ Steam is injected into each heater coil to maintain the required

minimum velocity and residence time and to suppress the

formation of coke in the heater tubes. After leaving the heater

soaking zone, the effluent is quenched with a quenching

medium to stop the reaction and is sent to the visbreaker

fractionator for separation.

Coil Visbreaker

✓ After quenching, the effluent is directed to the lower section of

the fractionator where it is flashed.

✓ The fractionator separates the products into gas, gasoline, gas

oil and visbreaker tar (residue).

✓ The gas oil withdrawn from the fractionator is steam-stripped to

remove volatile components and then blended with the

visbreaker bottoms or routed for further processing, such as

hydrotreating, catalytic cracking or hydrocracking.

Coil Visbreaker

✓ The un-stabilized naphtha and fuel gas, recovered as overhead

products, are treated and then used as feedstock for catalytic

reforming, blended into finished products or sent to the fuel

system.

✓ The visbreaker bottoms are withdrawn from the fractionator,

heat exchanged with the visbreaker feedstock, mixed with

stripped gas oil (optional) and routed to storage.

Coil Visbreaker

Soaker Visbreaker

✓ According to the fundamentals of thermal cracking technology,

the conversion is mainly a function of two operating

parameters, temperature and residence time.

✓ Coil cracking is described as a high temperature, short

residence time route whereas soaker cracking is a low

temperature, long residence time route.

✓ The yields achieved by both options are in principle the same,

as are also the properties of the products.

Soaker Visbreaker

✓ Both process configurations have their advantages and

applications.

✓ Coil cracking yields a slightly more stable visbreaker products,

which are important for some feedstocks and applications. It is

generally more flexible and allows the production of heavy cuts,

boiling in the vacuum gas oil range.

✓ Soaker cracking usually requires less capital investment,

consumes less fuel and has longer on-stream times.

Soaker Visbreaker

✓ Two visbreaking processes are commercially available. The first

process is the coil, or furnace, type.

✓ The coil process achieves conversion by high-temperature

cracking within a dedicated soaking coil in the furnace. With

conversion primarily achieved as a result of temperature and

residence time, coil visbreaking is described as a high-

temperature, short-residence-time route.

COIL VERSUS SOAKER DESIGN

✓ With the coil-type design, decoking of the heater tubes is

accomplished more easily by the use of steam-air decoking.

✓ Coil-type cracking heater produces a stable fuel oil. A stable

visbroken product is particularly important to refiners who do

not have many options in blending stocks.

✓ The alternative soaker process achieves some conversion within

the heater. However, the majority of the conversion occurs in a

reaction vessel or soaker which holds the two phase effluent at

an elevated temperature for a predetermined length of time.

✓ Soaker visbreaking is described as a low-temperature, high-

residence-time route.

✓ By providing the residence time required to achieve the desired

reaction, the soaker drum design allows the heater to operate at

a lower outlet temperature. This lower heater outlet temperature

results in lower fuel cost.

✓ The main disadvantage is the decoking operation of the heater

and soaker drum. Although decoking requirements of the

soaker drum design are not as frequent as those of the coil-type

design, the soaker design requires more equipment for coke

removal and handling.40

✓ The customary practice of removing coke from a drum is to cut

it out with high-pressure water. This procedure produces a

significant amount of coke-laden water which needs to be

handled, filtered, and then recycled for use again.

✓ Unlike delayed cokers, visbreakers do not normally include the

facilities required to handle coke-laden water. The cost of these

facilities can be justified for a coker, where coke cutting occurs

every day. However, because of the relatively infrequent

decoking operation associated with a visbreaker, this cost

cannot be justified. 41

✓ In order to maintain a desired degree of conversion, it is

necessary to stop the reaction at the heater outlet by

quenching.

✓ Quenching not only stops the conversion reaction to produce

the desired results, but will also prevent production of an

unstable bottoms product.

REACTION PRODUCT QUENCHING

✓ Quenching can be accomplished by using different mediums.

The most frequently used quenching mediums are gas oil,

residue, or a combination of both.

✓ Gas oil is the most prevalent medium used for reaction

quenching. The gas oil quench works primarily by vaporization

and therefore requires a smaller amount of material to stop the

conversion reaction than a residue quench. The gas oil quench

promotes additional mixing and achieves thermal equilibrium

rapidly.

✓ The residue quench operates solely by sensible heat transfer

rather than the latent heat transfer of the gas oil quench.

✓ The gas oil quench is a clean quench and thus minimizes the

degree of unit fouling.

✓ It is believed that the use of a residue quench gives way to

fouling in the transfer line and fractionator. Also the visbreaker

bottoms circuit, from which the residue quench originates, is in

itself subject to fouling.

Thermal cracking mechanism

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