bài giảng quá trình chưng cất

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Chapter
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Introduction

Hydrotreating processes aim at the removal of impurities such
as sulfur and nitrogen from distillate fuels—naphtha, kerosene,
and diesel—by treating the feed with hydrogen at elevated
temperature and pressure in the presence of a catalyst.

Hydrotreating has been extended in recent years to
atmospheric resids to reduce the sulfur and metal content of
resids for producing low-sulfur fuel oils.

The operating conditions of treatment are a function of type
of feed and the desulfur- ization levels desired in the treated
product.
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Introduction

The feed types considered here are: Naphtha, Kerosene, Gas
oils, Atmospheric resids or reduced crudes.

The principal impurities to be removed are: Sulfur, Nitrogen,
Oxygen, Olefins, Metals.

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Basic Reactions in Hydrotreating
HC - CH
CH
3
- HC
CH
S
+ 4H
2
C
5
H
12
+ H
2
S
Methyl thiophene
n-pentane

Desulfurization
C
5
H
12
+ H
2
S
Amyl mercaptan

n-pentane
CH
3
- CH
2
- CH
2
- CH
2
- CH
2
- SH + H
2
CH
3
- CH
2
- CH
2
-S - S - CH
2
- CH
2
- CH
3
+ 3H
2
2C
3
H

8
+ 2H
2
S
Dipropyl disulfide
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Basic Reactions in Hydrotreating

Denitrification
HC - CH
CH
3
- HC
CH
NH
+ 4H
2
C
5
H
12
+ NH
3
N
+ 5H
2
+ C
3
H

8
+ NH
3
Methyl pyrrol
Quinoline
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Basic Reactions in Hydrotreating

HYDROCARBON SATURATION
R - CH = CH
2
+ H
2
RCH
2
CH
3
+ H
2
+ H
2
O
R - OH + H
2
OH
RH + H
2
O


OXYGEN REMOVAL
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Basic Reactions in Hydrotreating


Metals

The metals contained in the naphtha feed are arsenic, lead,
and to a lesser degree copper and nickel, which damage the
reforming catalyst permanently.

Vacuum gas oils and resid feeds can contain a significant
amount of vanadium and nickel.

During the hydrotreating process, the compounds that contain
these metals are destroyed and the metals get deposited on
the hydrotreating catalyst.
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Catalysts


Cobalt (molybdenum on alumina support). This is the general
catalyst for HDS service.

Nickel (molybdenum on alumina support). This is used
especially for denitrification.

Cobalt (molybdenum and other metals on neutral support).

The neutral support prevents the polymerization of the olefins.
This catalyst is employed in conjunction with other catalysts for
olefinic feeds.
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Operating Variables
Temperature
The HDS reactions are favored by an increase in temperature,
but at the same time, high temperature causes coking
reactions, diminishing the activity of the catalyst.
Pressure
The increase in partial pressure of hydrogen increases the HDS
rate and diminishes the coke deposits on the catalyst, thereby
reducing the catalyst fouling rate and increasing the catalyst
life.
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Space Velocity

The liquid hourly space velocity (LHSV) is defined as

HDS reactions are favored by a reduction in VVH. The rate of
desulfurization is a function of (PPH
2
/VVH) or the ratio of
partial pressure of hydrogen in the reactor to liquid hourly
space velocity.
Operating Variables

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Recycle rate

In an HDS process, the hydrogen separated in a high-pressure
(HP) separator drum is recycled to the reactor via a recycle
compressor and furnace. This stream joins the incoming fresh
feed, which has been heated in the feed furnace.
Operating Variables