Applications for

isomerization processes

2

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under Patent, Copyright and Designs cannot be assumed.

3

Introduction

Isomerization converts normal paraffins to iso-

parrafins. Isomerization of light naphtha (C5-C

6) can

raise its octane rating 15 to 20 numbers and so help

the gasoline blending pool meet octane targets. As

environmental constraints on lead and aromatics

tighten, the demand for isomerization capacity is

increasing.

Isomerization of n-butane to i-butane is

necessary for further processing to alkylate and

MTBE, again as gasoline pool blending stock.

Isomerization plant operation is exceptionally

sensitive to the presence of impurities which can

reduce unit gross margins if not carefully controlled.

Johnson Matthey’s PURASPECJM

TM Processes with

fixed beds of catalysts or absorbents, are used by

many refineries to remove these impurities and so

increase refinery margins.

Isomerization Processes

Figure 1 shows a typical flowsheet for both

C4

and C5-C

6 isomerization. The principles

are similar for both. The stabilizer bottoms

can be separated into normal and isoparrafin

components via fractionation or molecular

sieve separation to obtain recycle of

the normal paraffins and hence

higher final product octane. (One

C5-C

6 design has separation

of the iso/normal paraffins

upstream of the stabilizer).

Lead-isomerization reactor

Gas

To fuel

Product to

storage or to

Iso/normals

separation for

recycle of

normals

Fresh / Spent

caustic

Makeup H2

C4 or

C5-6

feed

Caustic scrubberLight-Ends

Stripper

Tall-isomerization reactor

Organic chloride

makeup

Fig. 1 Paraffin isomerization process

The majority of units use a platinum-alumina catalyst

promoted with an organic chloriding agent. Some C5-C

6

units use a non-chloride activated zeolitic catalyst. These

catalysts are more resistant to poisons but operate at higher

temperatures and so give a lower conversion per pass which

necessitates use of a normal paraffin recycle scheme if high

product octane is to be achieved, see Figures 2 and 3.

Impurities in Isomerization Plants

Feed impurities may be present in the hydrocarbon

feed and/or the hydrogen feed to the unit. Sulfur and

nitrogen are temporary poisons whilst the others in Table

1 are usually permanent. Oxygenates include ethers and

alcohols and may be present for example in butane from

an MTBE unit

Poisoning of the isomerization catalyst reduces its

activity and a point will be reached where the catalyst,

especially in the lead reactor, needs to be replaced. This

importance of minimizing ingress of poisons can be seen

by the wide variance in bed life that refineries experience.

Where there is poor control, especially over sulfur and

oxygenates, bed lives can be less than one year. If impurity

slip to the unit is tightly controlled bed lives of more than

four years are possible. This has a big impact on plant

availability and catalyst replacement costs.

Some units do have catalyst regeneration facilities

These can help recover some of the lost activity from

poisoning. They are not common because of the extra

capital expenditure involved.

4

Stream Contaminant Effect Limits

Feed Sulfur Increasedhydrocracking and coking on catalyst 0.5 ppmw

Organic nitrogen Reduction of isomerization activity; 0.5 ppmwammonium chloride deposits

Water Increased coking; high light end production; 0.5 ppmwlow conversion

Oxygenates Are converted to water in the process and then 0.5 ppmwaffect as above

CO & CO2

Reduction of isomerization activity 0.1 ppmv

Metals Diminish metals function activity 0

Olefins Increase coke formation 0.1 vol %

Fluorides Catalyst poison and destroy feed pretreatment adsorbents 0.1 ppmw

Products Chlorides Corrosion and fouling; destroy mol sieves on separation 0.1 ppmwand recycle step

80

75

70

65

45

35

25

15

Platium-chloride

Zeolite

Platium-chloride

Zeolite

Temperature oF

2,2

-DM

B /P

6%

IC5

/P5%

200 300 400 500

Catalyst operating temperature oF

Eq

uili

bri

um

oct

an

e (

rese

arc

h c

lea

r)

90

88

86

84

82

80

200 400 600

With normal paraffin recycle

Once-though

isomerization

Fig. 3 C5-C

6 Isomerization equilibrium ratios (ref 2)Fig. 2 Maximum RON at thermodynamic equilibrium

for a typical C5-C

6 feed (ref 1)

Table1 Summary of feed contaminants

Johnson Matthey's PURASPECJM

Processes are used to

remove impurities from feed and product streams. Figure 4

shows typical locations where these impurity removal steps

can be located

SULFUR

Sulfur, and organic nitrogen, in light naphtha are usually

removed by hydrotreating. This will remove most of the

sulfur and normal sulfur specs will be <1 ppmw. Upsets

in the hydrotreater can lead to sulfur slip, panticularly of

H2S from hydrotreater stripper upsets. Mercaptan sulfur

can be present because of recombination reactions exit

the hydrotreater reactor towards end of run as reactor

temperatures are raised. See Case Study 3.

Butane feed streams are normally desulfurized by a

caustic wash system. This will remove most of the sulfur

present although there will still be some slip of trace sulfur

and carbonyl sulfide, (COS) will not be effectively removed.

Thus both C4 and C

5,C

6 units can suffer from trace sulfur

slip. Many operators address this problem by installing on

the hydrocarbon feed a fixed bed of PURASPECJM

high

activity non-regenerable sulfur absorbent for selective

removal of sulfur species. This bed removes the trace sulfur

present in the hydrocarbon and also removes any peaks of

sulfur which are present due to upstream unit upsets.

WATER & OXYGENATES

hese are removed using fixed beds of adsorbents,

usually based on aluminas and/or molecular sieves. The

adsorbents are regenerable, therefore two vessels are

normally used. One vessel is on-line whilst the other is

being regenerated using hot gas. Johnson Matthey can

supply these adsorbents

CO & CO2

These can be present in the hydrogen feed gas

depending on its source. Johnson Matthey can supply

catalytic processes using PURASPECJM

technology for

removal of these compounds in the gas. See Case Study 2.

OLEFINS

For C5-C

6 streams the hydrotreater should remove

these. For C4 streams. Johnson Matthey can provide

selective hydrogenation processes to remove unsaturates.

FLUORIDES

These can be selectively removed by chemical reaction

using a PURASPECJM

fixed bed non-regenerable absorbent.

CHLORIDES

For chloride-activated isomerization catalysts the

chloride loading in the fuel gas product is relatively high

and the normal practise is to use a caustic scrubber to

remove the chlorides. In locations where it is critical to

have negligible chlorides in this gas at all times, operators

may install a fixed bed of PURASPECJM

non-regenerable

chloride absorbent for final polishing of the gas. See Case

Study 1.

Where molecular sieves are being used on the isomerate

product for iso/normal separation they are protected using

a fixed bed of PURASPECJM

non-regenerable chloride

absorbent. This guard bed can operate at either ambient or

elevated temperatures.

Removal of Impurities

5

6

Case Study 1

A European refinery operating a C5-C

6 chloride isomerization

unit wished to send the product offgas to their hydrogen plant

as part of an offgas rationalization project. This gas was cleaned

of the chlorides by a caustic scrubber. Unfortunately the caustic

scrubber was not effective at removing all the chloride (their

main problem was unreliable control of the caustic make-up)

and this chloride offgas was acting as a poison in the hydrogen

plant. The refinery installed a bed of PURASPECJM

chloride

absorbent for selective removal of the chlorides from the

offgas. Chloride contamination of the hydrogen plant is now no

longer a problem.

Case Study 2

A European refinery building a C4 isomerization plant

needed a source of clean hydrogen to feed the unit. The

site hydrogen supply contained too much CO and CO2. A

PURASPECJM

process for catalytic removal of the CO/CO2 was

designed by Johnson Matthey and then supplied as a small skid

mounted unit to the refinery.

Lead-isomerization reactor

Gas

To fuel

Product to

storage or to

Iso/normals

separation for

recycle of

normals

Fresh / Spent

caustic

Makeup H2

C4 or

C5-6

feed

Cautic scrubberLight-Ends

Stripper

Tall-isomerization reactor

Organic chloride

makeup

Design conditions are:

Flow 72,000 scf/day (80 Nm3/hr)

96.0Raw gas

Composition

0.2

H2

0.2

CH4

3.6

CO

CO2

%v/v

Figure 4 Potential Locations for PURASPECJM

Processes

Case Study 3

An American refinery operating a C5-C

6 isomerization

unit was experiencing problems with short bed life and low

octane product.

Its isomerization unit takes light naphtha feed which

has first been hydrotreated. This naphtha hydrotreating

unit (NHT) was successful at removing the bulk of sulfur

contamination from the feedstock. However the refinery

still found slip of sulfur from the NHT stripping column at

levels of about 0.5 ppmw.

Refinery personnel believed this slip of low levels of sulfur

was the cause of poor run length on the isomerization unit.

Typically the precious metal catalyst on the isomerization

unit was only running for 4 to 5 months before product

octane started to fall from about 84 to 81. At this point the

unit was uneconomic to run and it was necessary to do a

hot hydrogen strip of the catalyst to regenerate it. Obviously

this regeneration step resulted in lost production and extra

costs for regeneration activities.

The Refinery installed a PURASPECJM

Process Reactor

on the isomerization unit downstream of the NHT to act as

a trace sulfur removal guard on the C5-C

6 feed, see Figure

5. The improvement in performance of the isomerization

unit has been dramatic. Since installation the unit has not

needed a single hot hydrogen strip.

In addition to significant improvements on the

isomerization unit the refinery personnel have also found

that the PURASPECJM

sulfur guard allows them to run their

hydrotreating catalyst for longer and so make cost savings

on this unit by requiring less frequent hydrotreating catalyst

changeouts. They are able to run longer than before

because, although the hydrotreating catalyst performance

declines slightly with time, they are able to accommodate

its increased slip of sulfur into the product because this is

totally "mopped-up" by the PURASPECJM

guard. Before

installation of the PURASPECJM

sulfur guard they were

changing NHT catalyst when its life reached 200 Bbl

naphtha feed/lb of catalyst. With the PURASPECJM

sulfur

guard now installed they have been able to achieve a NHT

catalyst life >500 Bbl/lb.

Disposal of spent catalysts

Johnson Matthey offers the services of its Catalyst

Care Program for the environmentally correct disposal

of all spent PURASPECJM

catalysts and absorbents. This

service uses registered recycling facilities and provides a

certification of consumption upon completion.

7

Light

Naphtha

Hydrogen

PURASPECJM

600

ReactorStripper

Isomerization

Unit

NHT

Hydro-

Treater

References1. A Hennico& J-P Cariou, Hydrocarbon International 1991 p 68-72

2. M E Reno et al, Hydrocarbon International 1991 p 73-78

Figure 5 PURASPECJM

process reactor on isomerization unit

Typical operating conditions are:

Feed rate

Temperature

Pressure

Inlet sulfur

Exit sulfur

4,000 Bbl/day

220 °F (104°C)

125 psig (8.5 barg)

0.5 ppmw

zero

For further information on Johnson Matthey, please contact your local sales representative or visit our website. PURASPEC is a trademark of the Johnson Matthey group of companies.

1570JM/1116/0/PT2016