Hydrotreatment

Hydrotreatment belongs to the class of conversions involving reactions with hydrogen. In this section 'hydrotreatment' is limited to hydrogenation and hydrogenolysis reactions in which removal of heteroatoms (in particular S,N) and some hydrogenation of double bonds and aromatic rings take place. In these reactions~in contrast to hydrocracking, in which size reduction is the objective

~ t h e molecular size is not drastically altered. Hydrotreatment as defined here is sometimes called hydropurification.

2 - - C A T A L Y T I C P R O C E S S E S I N I N D U S T R Y

The major objectives are:

- protection of downstream catalysts;

- improvement of gasoline (odour, colour, stability, corrosion);

- protection of the environment.

Typical reactions that occur are"

37

Mercaptanes RSH + H2 ~ RH + H2S

Thiophenes ~ + 3H2 ~ / - ~ + H2S

+ 5H 2 ^-

-Pyridines N + 5H2 ~ C5H12 + NH3

Phenols ~-'-/~~--OH + H2 ~ ~ ' ~ + H2 O

In naphtha reforming, hydrotreatment is always applied to protect the plati- num-containing catalyst against sulphur poisoning. The specification of sulphur content for the feed for the reforming unit is less than I ppm.

The hydrogen used in naphtha hydrotreatment is a by-product of the catalytic reforming. When hydrotreatment of heavy residue is performed, separate H2 production units are often required.

Unstable byproducts are formed in naphtha cracking, and these are hydro- treated in order to increase their stability. One of the important reactions occur- ring is the saturation of diolefins.

Process description

In simple naphtha hydrotreatment the feed is vaporized and led through a fixed bed reactor. When heavier feed stocks are treated, vaporization is not possible and trickle phase operation will be more suitable. In this mode the liquid and gas flow concurrently downwards. Most of the hydrotreatment of heavy residues is performed in trickle bed reactors. The major concern is to guarantee a complete wetting of the catalyst particles, because dry regions will cause extensive coking.

When more extensive hydrogenation occurs (often from the hydrogenation of aromatic rings) interstage cooling is often applied.

38 2 - - CATALYTIC PROCESSES IN INDUSTRY

Typical process conditions are:

Temperature: 350-420~

Pressure: 40-100 bar

LHSV: 1-6 h -1

The severity of the processing conditions depends on the feed; for light petroleum fractions it will be milder than for heavy residues. Moreover, it is common practice to compensate for deactivation of the catalyst by increasing the temperature of the reactor. A simplified flow scheme involving a trickle flow reactor is shown in Fig. 2.5.

h~J,'oge% -_, Q , , h~oga%r~:~=

woter

wotw"

~ P

^woter

product

high pressure low pressure

furnace reactor separator stripper separotor Fig. 2.5. Simplified s c h e m e of h y d r o t r e a t m e n t i n v o l v i n g a trickle flow reactor.

Catalysts

Catalysts are mixed metal sulphides on a carrier. The major examples are sup- ported mixed phases of CoS and

MoS2,

of NiS and

MoS2

and NiS and WS2. There is as yet no full agreement on the structure. For the first catalyst the best description is small

MoS2

clusters with CoS units at the edges. Typical di- mensions of these clusters are I nm and a stacking between I and 3.

Environment

From the point of view of clean technology hydrotreatment is gaining increasing interest. A recent example is diesel fuel. Sulphur-containing compounds in diesel fuel cause serious difficulties in catalytic cleaning of the exhaust gases.

Undoubtedly, in the near future, novel processes will be needed for the deep desulphurization of diesel.

2 m CATALYTIC PROCESSES IN INDUSTRY 39 2.3 TOTAL ISOMERIZATION PROCESS OF PARAFFINS

Overall Reaction

Hydroisomerization of C5 and C 6 paraffins (250~ Pt-H-mordenite as the catalyst) combined with separation of normal and branched paraffins (over zeolite CaA).

Feedstock

Straight r u n C 5 - C 6 paraffins ('light ends') from refinery distillation.

Product Use

As gasoline component with improved octane number, cf. Table 2.1.

Scale

Over 25 plants are operating the total isomerization process (TIP) and this number is growing continuously. Many millions of tons of isomerized paraffins are produced annually by this process.

Process Description

The TIP process is a combination of Shell's Hysomer process and Union Carbide's ISOSIV process. Both processes are zeolite based.

In the Hysomer process (Fig. 2.7) the catalyst is a Pt(O)-loaded zeolite of the H-mordenite type. The temperature applied has a strong bearing on the compo- sition of the equilibrium mixture obtained, see Fig. 2.6 for the hexane isomers: the lower the temperature applicable, the more dimethylbutanes are present. The temperature dependence is understood and it will be clear that a low tempera- ture is favourable for the purpose of the process.

Hydroisomerization is thought to start with dehydrogenation of paraffins, giving carbenium ions and alkenes. Secondary carbenium ions can rearrange to tertiary cations, probably via protonated cyclopropane derivatives. Transfer of hydride towards the tertiary carbenium ions provides the isomerized branched paraffins (see Chapter 4, Section 4.4). A side reaction is C-C scission of carb- enium ions leading to cracked products. Hydrogenolysis on the metal function can also contribute to this undesired reaction. The larger the alkane, the more hydrocracking takes place, which may be the reason that the TIP process so far has focused on the C5 and C6 fractions.

40 2 -- CATALYTIC PROCESSES IN INDUSTRY 60

50-

I/1 40-

IE X JE G)

30-

121 C 0

~ 20

10-

2,2-DMB (92)

2-MP (78)

3-MP (76)

2,3-DMB (104)

~ ~0 ~60 160 260 260 360 a~0 46o 460

Temperature (Celsius)

500

Fig. 2.6. Thermodynamic equilibria for hexane hydroisomerization (RON).

hydrogen

Q

^{recycle 9 0 = _ _ purge}

~ 2 5 0 " C '~ 1 O - 30b or

feed r o d u c t

feed heater reactor separator Fig. 2.7. Diagram of Shell Hysomer process.

It should be noted that H-mordenite itself is also able to isomerize alkanes such as pentane. However, the presence of platinum greatly improves its selec- tivity and stability, as shown in Fig. 2.8. Apparently the hydrogenation function prevents oligomerization etc. leading to coke formation.

In the TIP process the Hysomer process is combined with the ISOSIV process which separates normal paraffins from branched ones by selectively adsorbing the normal fraction into zeolite CaA (pressure swing adsorption). After de-

2 - - CATALYTIC PROCESSES IN INDUSTRY

80 60 z,.O 20 0 IE

>- z o

r -

x \

- \

N N

N \ N N

N

I_ I [ I

0 2 # 6

P t / H - MORDENITE ( SEL 98 % )

H- MORDENITE . J SEL 30-?0 %)

- - - . - . . . . _ . , .

i ! I I

8 10 12 1/.

TIME, h

Fig. 2.8. Influence of metal function in pentane hydroisomerization.

41

- i

Catalysis Separation

Zeolite mordenite

I

! i

._ ~ isomedzation

Normals recycle

.J

"-I

Zeolite 5A

I

!

1 ,

Iso/normal J ~ Separation

n-C5/C6 feed

Fig. 2.9. Paraffin total isomerization process (TIP).

C4_gases

~ ~ . iso-C5/C 6

sorption (by applying vacuum) the normal paraffins are recycled. A schematic view of the TIP process, including a structural picture of the two zeolites in- volved, is given in the Fig. 2.9.

The pore openings of zeolite A just allow normal alkanes to enter the pore sys- tem of the crystal. It might be of interest to have a zeolite available which adsorbs the normals and the mono methyl-branched paraffins and leaves the high octane dimethyl branched compounds unadsorbed.

42 2 - - CATALYTIC PROCESSES IN INDUSTRY

Catalyst and Adsorbent

Mordenite is a versatile zeolite, the Si/A1 ratio of which can easily be changed from 5 to higher values. Its parallel channels having a somewhat elliptical cross section (0.65x0.7 nm) allow all Cs and C6 isomers to enter and to leave. Platinum 2+ Competitive exchange with NH~

is introduced by ion exchange with Pt(NH3) 4 ⁹

(excess) is recommended in order to obtain a high Pt dispersion. A careful calcination follows in which local high temperatures in the zeolite crystal have to be prevented. Finally the Pt catalyst is reduced with hydrogen.

The selective adsorbent CaA is prepared in the sodium form (the detergent zeolite NaA) and subsequently partially exchanged with Ca(II).

2.4 ISOTACTIC POLYPROPYLENE