Gasoline Production - أ.د. طارق البحري - Finishing.pdf · Reactors stacked on top of each other (UOP) → MAA refinery. ... Figure 11-2: Catalyst for semi-regenerative

Petroleum Refining – Chapter 11: Gasoline Production

11-1

Chapter 11 Gasoline Production

INTRODUCTION

• Convert SR naphtha to motor gasoline stocks through

1. Reforming

2. Isomerization

• Production of motor gasoline stocks though

1. Alkylation

IsomerizationUnit

Light Naphtha

CrudeCokerHydro/Cat Cracker

High Octane Isomerate

by transforming straight-chain paraffins (low octane) → isomers (high octane)

CatalyticReformer

Heavy Naphtha

CrudeCokerHydro/Cat Cracker

High OctaneReformate

Sold as regular/premium gasoline

AlkylationUnit

Olefins

i-C4Alkylate (high octane)iso-paraffins boiling in the gasoline range .

Catalyst:Sulfuric or

hydrofloric acid

Blended into premium motor gasoline & aviation gasoline

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering

11-2

1. CATALYTIC REFORMING

Introduction

Objective

To increase the octane number of heavy straight run (HSR) naphtha (for motor gasoline

blending) using a catalytic process.

• Catalytic reforming does not change in the IBP and FBP range of naphtha feed.

• Produces large amount of H2 gas that can supplement the refinery H2 system.

• Reforming units can use Platinum (Pt) and Rhenium (Rh) catalysts.

• MAA CCR Platformer uses Platinum (Pt) catalyst.

Capacity

Table 11-1: Catalytic Reforming Units Capacity in Kuwait.

Refinery

Unit

Feed Capacity

(BPD

Reformate

Produced

(BPD)

Octane

Number

(Clear)

MAA CCR Platforming 2X 90

ZOR CCR Platforming

Feed & Product Properties

Table 11-2: Rheniformer Feed and Product Properties.

Fractionator Feed

(total naphtha)

Reformate

API

Sulfur, ppm

N2, ppm

Octane Number (clear)

RVP, psi

Distillation (ºF)

IBP

10%

30

50

70

90

EP

Recovery

65.6

101

4

-

-

126

163

196

221

246

284

316

98

48.6

-

-

95.4

6.2

122

162

217

248

275

313

349

98

Figure 11-1: Catalytic Reforming Unit

Catalytic Reforming Process Description (Figure 11-3)

• The system consists of three reactors, fired heaters, hydrogen recycle system, and

product debutanizing facilities.

• Hydrotreated HSR Naphtha is preheated then vaporized in a fired heater.

• It is mixed with H2 then passed over a catalyst at about 900 ºF and 350 psig in the first

reactor.

• Intermediate heat is provided after first and second reactors because of the

endothermic reactions that reduce the temperature in the reactor.


11-3

• The effluent from the third reactor is cooled, by heating other streams in the unit (for

efficient heat recover) then separated into a liquid product and a H2-rich gas that is

recycled.

• The excess hydrogen which is used as make-up to the refinery hydrotreating system is

bled-off as needed to maintain system pressure.

• A stabilizer is used to remove light gases (C4-) from the reformate product, thus,

control its RVP.

Catalyst & Reactions

• Octane improvement is mainly achieved through

1. Dehydrogenation of naphthenes (low octane) to aromatics (higher octane).

(endothermic)

2. Dehydrocyclization of paraffins (low octane) to aromatics (higher octane).

(slightly endothermic)

3. Isomerization of Paraffins (low octane) to isoparaffins (higher octane).

4. Hydrocracking of heavy paraffins (low octane) to light paraffins (higher octane).

+ 3H2Dehydrogenation

isopropylcyclohexane

ON = 61.1 (clear)

isopropylbenzene

ON = 99.3 (clear)

Toluene

ON = 96.5 (clear)

n-heptane

ON = 0 (clear)

Dehydrocylization4H2+

isomerization

n-octane

ON = -15 (clear)iso-octane

(2,2,4-trimethypentane )

ON = 100 (clear)

n-butane

ON = 89.6 (clear)n-nonane

ON = -20 (clear)

Hydrocracking+

n-pentane

ON = 62.6 (clear)

+ H2


11-4

Continuous Catalytic Reforming (CCR) Platforming. (Figure 11-4)

• The CCR process unit can consist of either

1. Reactors stacked on top of each other (UOP) → MAA refinery.

2. Side-by-side reactors (IFP).

• In both cases the sequence of flow of the reactants is like that shown for the semi-

regenerative system.

• In the (stacked design) CCR Platforming Unit, freshly regenerated catalyst is introduced

in the top of the upper reactor between two concentric perforated cylinders (made from

Johnson screens) and flows by gravity from top to bottom.

• The reactants are introduced on the outside of the outer cylinder and flow radially through

the catalyst to the center of the inner cylinder.

• Partially aged catalyst is removed from the bottom of the lowest reactor and sent to an

external regenerator where the carbon is burned from the catalyst, and the catalyst is

reduced and acidified before being returned to the upper reactor.

Reforming Catalyst

• The reforming catalyst contains platinum supported on chlorinated alumina base.

• In most cases rhenium is combined with platinum to form a more stable catalyst which

permits operation at lower pressures.

• Platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation

reactions and chlorinated alumina provides an acid site for isomerization, cyclization, and

hydrocracking reactions.

• Reforming catalyst activity is a function of surface area, pore volume, and active

platinum and chlorine content.

• Catalyst activity is reduced during operation by coke deposition and chloride loss.

• In a high-pressure process, up to 200 barrels of charge can be processed per pound of

catalyst before regeneration is needed.

• In semi-regenerative reforming process unit, the activity of the catalyst can be restored by

high temperature oxidation of the carbon followed by chlorination and is able to operate

for 6 to 24-month periods between regenerations.

• The activity of the catalyst decreases during the on-stream period and the reaction

temperature is increased as the catalyst ages to maintain the desired operating severity.

• Normally the catalyst can be regenerated in situ at least three times before it must be

replaced and returned to the manufacturer for reclamation.

• Catalyst for fixed-bed reactors is extruded into cylinders 1/32 to 1/16 in. (0.8 to 1.6 mm)

diameter with lengths about 3/16 in. (5 mm).

• In continuous catalytic reforming (CCR) process unit the catalyst is regenerated

continuously online.

• The catalyst for continuous units is spherical with diameters approximately 1/32 to 1/16

in. (0.8 to 1.6 mm).

Figure 11-2: Catalyst for semi-regenerative and continuous regeneration platforming process.


11-5

Figure 11-3: Catalytic Reforming Semi-Regenerative Process simplified


11-6

Figure 11-4: UOP CCR (continuous catalyst regeneration) Platforming Process (MAA Refinery)


11-7

Figure 11-5: Schematic Representation of CCR (continuous catalyst regeneration) Platforming Process & Reactor


11-8

2. ISOMERIZATION

Introduction

• The octane numbers of the (light straight run) LSR naphtha [C5-180⁰F (C5-82⁰C)] can be

improved using isomerization process.

• Isomerization convert normal paraffins to their isomers.

• This results in significant octane increases.

• In once-through isomerization the clear RON of LSR naphtha can be increased from 70 to

about 82–84.

• If the normal components are recycled, the resulting RON will be about 87–93 clear.

• Reaction temperatures of about 200–400⁰F (95–205⁰C) are preferred to higher

temperatures because the equilibrium conversion to isomers is enhanced at the lower

temperatures.

• At these relatively low temperatures a very active catalyst is necessary to provide a

reasonable reaction rate.

• The composition of the reactor products can closely approach chemical equilibrium.

• Following is a simplified conversion summary for a typical LSR Naphtha cut.1

LSR component Feed weight Product weight RONC (unleaded)

Isopentane 22 ↑ 41 92

Normal pentane 33 ↓ 12 62

2,2-Dimethybutane 1 ↑ 15 96

2,3-Dimethybutane 2 ↑ 5 84

2-Methylpentane 12 ↑ 15 74

3-Methylpentane 10 ↓ 7 74

Normal hexane 20 ↓ 5 26

Total 100 100

• If the normal pentane in the reactor product is separated (by fractionation or by vapor

phase adsorption on a molecular sieve bed) and recycled the product RON can be

increased

• Some hydrocracking occurs during the reactions resulting in a loss of gasoline and the

production of light gas. The light gas produced is typically in the range of 1.0 to 4.0 wt%

of the hydrocarbon feed to the reactor.

• A representative flow scheme for an isomerization unit is shown in Figure 11-6

1 The values are on a relative weight basis and do not account for the weight loss resulting from hydrocracking

to molecules lighter than pentane.


11-9

Figure 11-6: UOP H-O-T Penex isomerization unit.

Isomerization Yields

• Isomerization yields vary with feedstock properties and operating severity.

• Typical operating conditions given in Table 11-3.

• Isomerization yield is increased by:

1. High temperature (which increases reaction rate)

2. Low space velocity

3. Low pressure

4. High hydrogen-to-hydrocarbon ratios reduce the hydrocarbon partial pressure and

thus favor the formation of isomers.

• A typical product yield is given in Table 11-4 for 12 psi RVP C5+ isomerate product with

13 RONC and MONC improvement.

Table 11-3: Isomerization unit typical Operating conditions

Reactor temperature 200–400⁰F 95–205⁰C

Pressure

Hydrogen/HC mole ratio

Single-pass LHSV

Liquid product yield

250–500 psig

0.05 : 1

1–2 v/hr/v

>98 wt%

1725–3450 kP

Table 11-4: Isomerization Yields

Component Vol% on feed

C3 0.5

iC4 1.5

nC4 1.0

C5-C7 (Isomerate) 102.0


11-10

Isomerization Reactions

• Isomerization of paraffins and cyclopentanes usually results in a lower octane product

than does conversion to aromatics. However, there is a substantial increase over that of

the un-isomerized materials. These are rapid reactions with small heat effects.

1. Isomerization of normal paraffins to isoparaffins:

Figure 11-7: Isomerization of paraffins in LSR naphtha

Isomerization Catalysts

• The available catalysts used for isomerization contain platinum on various bases.

• Some types of catalysts require the continuous addition of very small amounts of organic

chlorides to maintain high catalyst activities.

• This is converted to hydrogen chloride in the reactor, and consequently the feed to these

units must be free of water and other oxygen sources to avoid catalyst deactivation and

potential corrosion problems.

• A second type of catalyst uses a molecular sieve base and is reported to tolerate feeds

saturated with water at ambient temperature.

• A third type of catalyst contains platinum supported on a novel metal oxide base. This

catalyst has 150⁰F (83⁰C) higher activity than conventional zeolitic isomerization

catalysts and can be regenerated.

• Catalyst life is usually three years or more with all these catalysts.

• An atmosphere of hydrogen is used to minimize carbon deposits on the catalyst, but

hydrogen consumption is negligible.

n-hexane (31 ON) 2-MP (74 ON)

&

3-MP (76 ON)

&

2,3-DMB (105 ON) 2,2-DMB (94 ON)

& &

isopentane (93.5 ON) n-pentane (61.7 ON)


11-11

3. ALKYLATION

Introduction

Objectives

To produce high octane alkylate (96.4 RON min.) suitable for gasoline blending through the

reaction of isobutane with light olefins.

Location: Only at MAA

Capacity: The design capacity is 4,879 BPSD of C4 produced from the MTBE unit.

Types: Two types of Alkylation units exist depending on catalyst type.

1. Sulfuric Acid Alkylation

2. HF acid Alkylation

Reactions

• Olefins (mainly isobutene) react with isobutene in the presence of sulfuric acid

catalyst to form isoparaffins in the gasoline boiling range (mainly 2,2,4-

trimethylpentane) with octane numbers ranging from (93 – 98 RON).

• The reaction is instantaneous and highly exothermic

Feed & Product Properties

• The Alkylation unit olefin feed stream is the C4 raffinate produced at the MTBE unit.

• It represents a suitable Alkylation feed due to its isobutane/isobutene ratio (47 and 39

v% respectively).

• During MTBE shutdown an external source of isobutene is used to maintain the

desired ratio in the feed.

• Isobutane is separated from field butane (containing both n- and iso-butane) using a

deisobutanizer (DIB). The top product is isobutane which is stored in a drum.

propylene isobutane

(gas) (gas)

2,2-dimethylpentane (isoheptane)

(liquid)

2,2,4-trimethylpentane (isooctane)

(liquid)

isobutylene isobutane

(gas) (gas)

+

+

H2SO4

H2SO

4

+ Heat

+ Heat


11-12

Table 11-5: Alkylation Unit Feedstock properties.

A. MTBE Raffinate B. Field Butane C. Sulfuric Acid

Total Sulfur, ppmw

Total oxygenates, ppmw (sum

of DME Methanol and MTBE)

Water, ppmw

20

60

800

11-60

-

-

20

60

800

Component wise Break-up Kg.mole/hr mole%

Ethane

Propane

Isobutene

n-butane

i-pentanes

n-pentanes

C6

ethylene

propylene

avg. C4=

avg. C5=

1,3 butadiene

0.0

0.325

189.315

47.216

5.087

0.085

0.0036

0.0

0.041

174.48

2.622

1.011

-

1

25

72

2

Trace

Strength 98.5 W%

Sp.gr. 1.85

D. Caustic Solution

Table 11-6: Alkylation Unit Product Specifications.

n-butane from field butane deisobutanizer

Max isobutene content

5 wt%

Fuel gas (butane ex-isostripper)

C5+ hydrocarbon

H2SO4 content

Alkali Content

2 wt% max

0

0

Spent sulfuric acid

Strength as H2SO4 (titration)

90 wt%

Alkylate product

RON (clear)

RVP @100ºF

Total sulfur

ASTM Distillation

IBP

50%

90%

EBP

96.4 min

9.0 max

100 ppmw max

102

223

258

401

Process Description

The unit consists of four sections (Figure 11-9)

1. Reaction section.

2. Refregeration section.

3. Reactor effluent treating section.

4. Iso-stripping section.


11-13

Reaction Section:

• The olefin feed, iso-butane feed, and recycled iso-butane stream are first mixed then

cooled to 57 ºF.

• Any condensed water is removed from the cooled hydrocarbon in a coalescer.

• The HC goes to a contactor where the reaction occurs.

• The contactor consists of an impeller for mixing (Figure 11-10), a circulation tube,

and a tube bundle used to remove the heat of reaction.

• Using the impeller to circulate the feed and the acid, creates an emulsion.

• Part of the emulsion is withdrawn and the other is sent to an acid settler where the

acid settles at the bottom by gravity and flows back to the contactor.

• Side reactions in the reaction zone consume the acid, therefore, fresh acid is charged

to the contactor continuously to maintain 90 wt% H2SO4 and equivalent amount of

spent acid is withdrawn.

• The hydrocarbon phase separated from the acid in the settler, flows into the tube side

of the contactor through a pressure control valve which creates a pressure drop of 2–5

psig, flashing (partial vaporizing) the light hydrocarbons. This cools the stream

temperature to 30 ºF, absorbing the heat generated by the alkylation reactions.

Refrigeration Section

• The hydrocarbon stream leaving the tube bundle flows to a suction trap/flash drum

which separates liquid from the vapor.

• The vessel consists of two liquid compartments and a common vapor space.

- The contactor hydrocarbon stream accumulating on one side is pumped to the

effluent treating section.

- The cold condensate mainly isobutene (from the refrigeration section)

accumulating on the other side is sent to the Contactor.

• The common vapor is condensed by cooling in a seawater condenser after

compression to 46 psig.

• Part of the condensate is caustic treated to remove the acidic components and sent to a

C3/C4 splitter. The remaining condensate enters an economizer drum where it is

flashed.

• The vapor flows to the flash drum side of the suction trap/flash drum.

Effluent Treating Section

• The liquid phase of the contactor tube is washed with fresh sulfuric acid (in the acid

wash vessel) and alkaline water (in the alkaline water wash vessel) to remove the

corrosive compounds (esters and acid traces) formed by the reaction of sulfuric acid

with olefins.

Isostripper

• The treated contactor effluent is sent to the isostripper column.

• The overhead iso-butane vapor is recycled to the reaction section.

• The side product (n-butane 67.6 M% purity) is sent either to Mogas (motor gasoline)

or fuel gas blending or both.

• The bottom liquid is the Alkylate product.


11-14

Figure 11-8: Auto-refrigeration Sulfuric Acid Alkylation Unit.


11-15

Figure 11-9: Detailed Sulfuric Acid Alkylation Unit in MAB refinery.


11-16

Figure 11-10: Stratco Sulfuric Acid Contactor in MAB refiner.

Documents

Gasoline Production - أ.د. طارق البحري - Finishing.pdf · Reactors stacked on top of each other (UOP) → MAA refinery. ... Figure 11-2: Catalyst for semi-regenerative