CHAPTER 2

Isomerization Technique

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 CHAPTER 2

ISOMERIZATION TECHNIQUE

2.1UOP‘S PENEX PROCESS:

The Penex process has served as the primary isomerization technology for upgrading

C5/C6 light straight-run naphtha feeds since UOP introduced it in 1958. This process has

a wide range of operating configurations for optimum design flexibility and feedstock

processing capabilities.

The Penex process is a fixed-bed procedure that uses high activity chloride-promoted

catalysts to isomerize C5/C6 paraffins to higher octane branched components. The

reaction is conducted in the presence of a minor amount of hydrogen. Even though the

chloride is converted to hydrogen chloride, carbon steel construction is used successfully

because of the dry environment. For typical C5/C6 feeds, equilibrium will limit the

product to 83 to 86 RON (Research Octane Number) on a single hydrocarbon pass basis.

The operating conditions are such that promote isomerization and minimize

hydrocracking. Operating conditions are not severe, as reflected by moderate operating

pressure, low temperature, and low hydrogen partial pressure requirements.

Ideally, this isomerization catalyst would convert all the feed paraffins to the high octane-

number branched structures: normal pentane (nC5) to isopentane (iC5) and normal

hexane (nC6) to 2,2- and 2,3-dimethylbutane. The reaction is controlled by a

thermodynamic equilibrium that is more favorable at low temperature.

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Equipments Used in Penex Process:

• Methanator feed effluent exchanger

• Methanator feed steam exchanger

• Methanator

• Methanator knockout drum

• Make-up gas dryers (2 in number)

• Liquid feed dryer (2 in number)

•Regenerant super heater

•Regenerant evaporator

•Liquid feed surge drum

• Charge pump (2 in number)

• Chloride drum

•Chloride injection pump (2 in number)

• Combine feed exchanger (3 in number)

• Reactors (2 in number)

• Stabilizer column

• Stabilizer re-boiler

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 • Stabilizer overhead air cooler

• Stabilizer overhead trim cooler

• Stabilizer overhead separator

• Stabilizer reflux pump (2 in number)

• Net gas scrubber

• Caustic circulation pump (2 in number)

• Caustic tank

• Water circulation pump (2 in number)

• Water Tank

• Water injection pump (2 in number)

Operation and Operating Conditions of some Penex Process

Equipment:

2.1.1 Liquid Feed Driers Operation:

Hydro treated SR light naphtha at temperature 45 0C & pressure 4.5 kg/cm2 is passed

through driers to control moisture at 1.0 ppmw in the feed. Drying medium is the

molecular sieves. There are two drier, one remain in operation while the other is on

regeneration. Isomerate is used as regenerant. Dry liquid feed is collected in feed surge

drum. Molecular sieves are regenerated by isomerate & there replacement depends on the

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efficiency or after period of four year.

2.1.2 Make Up Gas Driers Operation:

Make up gas is dry by passing into dryers. Molecular sieves used as drying agents. Dry

gas is control at moisture < 1.0 ppmv. Before drying of gas CO & CO2 isremoved from

the makeup gas. It is accomplished by passing the gas through Methanator. CO & CO2

are converted into methane in presence of Nickel oxide catalyst. Nickel catalyst cannot be

regenerated. It is replaced totally; its life is 4-5 years. Temperature & pressure of

Methanator is maintained 220 °C & 27 kg/cm2.

2.1.3 Reactor Operations:

Combine liquid feed & make up gas is heated in pre-heat exchangers & chloride is

injected before entering the reactors. The Reactor System is typically designed to operate

at a minimum pressure of 31.6 Kg/cm2 (g). Lead reactor inlet temperatures range from

131°C to 200°C and lag reactor inlet temperatures range from 142°C to 186°C.

H2/HCBN mole ratio is maintained as 0.20 at reactors inlet & 0.05 at reactors outlet.

2.1.4 Stabilizer Operation:

Reactor effluents passed through stabilizer where lighter gases & propane is separated

from the isomerate. Stabilizer column is operated at temperature 145 °C & pressure 18.0

kg/cm2.

2.1.5 Stabilizer Net Gas Scrubber Operation:

The purpose of net gas scrubbers is that to neutralize the net gas prior sending to fuel gas

header with caustic (strength is 10%wt). Operating parameters of net scrubbers is that

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pressure is 6.5 kg/cm2 and temperature is 45 °C.

The most common Penex process is Hydrocarbon Once-Through Penex process.

Figure 2.1 Hydrocarbon Once-Through Penex Process:

2.1.6.1 Process Description:

Hydrogen Once-Through Penex process flow scheme results in a substantial saving in

capital equipment and utility costs by eliminating product separator and recycle gas

compressor.

Light naphtha feed is charged to one of the two dryer vessels. These vessels are filled

with molecular sieves, which remove water and protect the catalyst. After mixing with

makeup hydrogen, the feed is heat-exchanged against reactor effluent. It then enters a

charge heater before entering the reactors.

Typically, two reactors in series are used to achieve high on-stream efficiency. The

catalyst can be replaced in one reactor while operation continues in the other. One

characteristic of the process is that catalyst deactivation begins at the inlet of the first

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Reactor and proceeds slowly as a rather sharp front downward through the bed. The

adverse effect that such deactivation can have on unit on-stream efficiency is avoided by

installing two reactors in series. Each reactor contains 50% of the total required catalyst.

Piping and valving are arranged to permit isolation of the reactor containing the spent

catalyst while the second reactor remains in operation. After the spent catalyst has been

replaced, the relative processing positions of the two reactors are reversed. During the

short time when one reactor is off-line for catalyst replacement, the second reactor is fully

capable of maintaining continuous operation at design throughput, yield, and conversion.

The reactor effluent flows to stabilizer after passing through the heat exchanger. The

stabilizer overhead vapors are caustic scrubbed for removal of the HCl formed from

organic chloride added to the reactor feed to maintain catalyst activity. After scrubbing,

the overhead gas then flows to fuel. The stabilized, isomerized liquid product from the

bottom of the column then passes to gasoline blending.

The Penex process (see below) uses the most active chlorided-alumina catalyst and

operates in the range 120-180°C. overall catalyst cost. At lower LHSVs, more catalyst is

loaded resulting in a longer residence time. As a result lower temperature Operation is

possible, resulting in a higher octane product.

System pressure is another variable and is considered in conjunction with the hydrogen

flow rate to the reactor. Chlorided-alumina is more active at higher pressures. It requires

only a slight excess over stoichiometric hydrogen, since the catalyst does not produce

coke. A Penex unit operates at about 30 to 32 bar with once-through hydrogen.

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 Figure 2.2 (Block flow diagram of ―one through process)

To achieve higher octane, UOP offers several schemes in which lower octane components

are separated and recycled back to LHSV is set during the design phase of any

isomerization project and reflects the compromise between residence time and the

reactors. These recycle modes of operation can lead to product octane as high as 93 RON.

2.1.7 Penex Process/DIH (De-isoHexanizer): This flow scheme is same as Penex

Process with an addition of deisohexanizer column to recycle the methylpentanes, n-

hexane, and some C6 cyclics. It is the lowest cost option of the recycle flow schemes &

provide high octane isomerate

product, especially on C6 rich feed.

Figure 2.3 (Block flow diagram of process with DIH)

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2.1.8 Penex Process With Recycle And Fractionation (DIP/Penex

Process/DIH):

Separation and recycle of unconverted normal C5 and C6 paraffins and low octane C6

isoparaffins back to the reactor, produce a higher octane product. The most common flow

scheme uses a deisohexanizer (DIH) column to recycle methylpentanes, n-hexane, and

some C6 cyclics. It is the lowest capital cost option of the recycle flow schemes and

provides a higher octane isomerate product, especially on C6 rich feeds.

In the Penex/DIH process the stabilized isomerate is charged to a DIH column producing

an overhead product containing all the C5 and dimethylbutanes. Normal hexane and some

of the methylpentanes are taken as a side-cut and recycled back to the reactors. The small

amount of bottoms (C7+ and some C6 cyclics) can be sent to gasoline blending or to a

reformer

The addition of a deisopentanizer (DIP) or a super DIH will achieve the highest octane

from a fractionation hydrocarbon recycle flow scheme. In this scheme, both low octane

C5 and normal and isoparaffin C6 are recycled to the Penex reactors

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 Figure 2.4 (DIP-Penex-DIH)

2.1.9 Penex/Molex Process:

Figure 2.5 (Penex/Molex Process flow scheme)

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2.2 PROCESS DESCRIPTION:

This flow scheme uses Molex technology for the economic separation and recycle of n-

paraffin from the reactor effluent.

The Molex process is an adsorptive separation method that utilizes molecular sieves for

the separation of n-paraffins from branched and cyclichydrocarbons. The separation is

effected in the liquid phase under isothermal conditions according to the principles of the

UOP Sorbex separations technology. Because the separation takes place in the liquid

phase, heating, cooling and power requirements are remarkably low.

Sorbex is the name applied to a particular technique developed by UOP for separating a

component or group of components from a mixture in the liquid phase by selective

adsorption on a solid adsorbent.

Sorbex is a simulated moving bed adsorption process operating with all process streams

in the liquid phase and at constant temperature within the adsorbent bed. Feed is

introduced and components are adsorbed and separated from each other within the bed. A

separate liquid of different boiling point referred to as ‗desorbent‘ is used todisplace the

feed components from the pores of the adsorbent. Two liquid streams emerge from the

bed – an extract and a raffinate stream, both diluted with desorbent. The desorbent is

removed from both product streams by fractionation and is recycled to the system.

The adsorbent is fixed while the liquid streams flow down through the bed. A shift in the

positions of liquid feed and withdrawal, in the direction of fluid flow through the bed,

simulates the movement of solid in the opposite direction. It is, of course, impossible to

move the liquid feed and withdrawal points continuously. However, approximately the

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same effect can be produced by providing multiple Liquid access lines to the bed, and

periodically switching each net stream to the next adjacent line. A liquid circulating pump

is provided to pump liquid from the bottom outlet to the top inlet of the adsorbent

chamber. A fluid-directing device, known as a ‗rotary valve‘, is also provided.

Figure 2.6 (BFD of Molex Process)

2.2.1 Operating Conditions Of Molex Process:

Molex unit involves three processes.

a) Adsorption

b) Purification

c) Desorption

2.2.1.1 Adsorption Operation:

The adsorbent employed in Molex is a specially prepared molecular sieve with selective

pores. Molex feed enter the adsorbent chamber via rotary valve. Adsorbent chamber is

operated at pressure 15 kg/cm2.

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2.2.1.2 Purification Operation:

Non-normal paraffins (iso-paraffins) are removed from the adsorption chamber &

purified in raffinate column. Operating temperature & pressure of raffinate column is 125

°C & 13.0 kg/cm2.

2.2.1.3 Desorption:

Operating temperature & pressure of the extract column 130 °C & 16.0 kg/cm2.

2.2.2 Penex-Plus Technology

The performance of the Penex process can be compromised when processing this

feedstock because benzene hydrogenation is a highly exothermic reaction. The heat

generated by the benzene hydrogenation reaction can cause the reactors to operate at

conditions that are less favorable for octane upgrading. For these applications, UOP offers

the Penex-Plus Technology.

It includes two reactor sections. The first section is designed to saturate the benzene to

cyclohexane. The second section is designed to isomerize the feed for an overall octane

increase. Each reactor is operated at conditions that favor the intended reactions for

maximum conversion.

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2.3 OCTANE COMPARISON FOR DIFFERENT PROCESSES:

Figure 2.7 (Octane Comparison for different Processes) (Feed RON = 60 to 70)

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