Encyclopedia of Environmental Science and Engineering, Volume I and II

FOSSIL FUEL CLEANING PROCESSES 413

In addition, during the course of desulfurization, non-sulfur

containing molecules may be hydrogenated and in some cases

cracked.

The fl ow design of hydrosulfurization process systems

is relatively simple. Preheated oil and hydrogen under pres-

sure are contacted with catalyst. The effl uent from the reac-

tor is passed to one or more separators to remove most of the

effl uent hydrogen and light hydrocarbon gases produced in the

operation. These gases are generally recycles with or without

prior removal of light hydrocarbons by absorption. The separa-

tor liquids may be stripped, rerun or otherwise treated to obtain

hydrogen sulfi de free products of the desired boiling range.

Except in the case of residuum processing, plant design

options are few in number and relatively simple. For exam-

ple, in the processing of distillates, correlation systems have

been developed which relate degree of desulfurization to

about three parameters which defi ne the charge stock, reac-

tor temperature, temperature, pressure, feed space velocity,

hydrogen rate and a catalyst activity parameter.

When residuum stocks are considered, however, general-

izations are not so easily made. The wide variance in resid-

uum properties (i.e., atmospheric or vacuum type, viscosity,

Conradson carbon content, metal content and the paraffi nic

or aromatic nature of residuum) makes each case a special

one as far as process design. Catalyst poisoning due to metals

deposition on the catalyst surface can reduce overall desulfur-

ization yields. Catalyst must then be regenerated or replaced,

thus adding to overall cost of the particular system employed.

An alternative to desulfurization exists, that being the

use of natural low sulfur fuel oils. They may be used alone

or in blends with higher sulfur content material. The major

source of low sulfur fuel oil is North African crudes, princi-

pally from Libya and Nigeria, and some Far Eastern crude

from Sumatra. Fuels made from these crudes will meet even

very low sulfur regulations calling for 0.5% sulfur or less.

However, the highly waxy nature of these paraffi nic materi-

als makes handling diffi cult and costly. Therefore, the blend

becomes a more palatable course of action.

Blends of natural low-sulfur fuels oils with other high

sulfur fuel oils will be adequate in some cases to meet

more moderate sulfur regulations. The fuel oil fractions of

North African crudes contain about 0.3% S. Thus signifi -

cant amounts of higher sulfur fuel oils can be added to make

blends calling for 1–2% sulfur. These blends have physical

properties which obviate the need for specialized handling

(a must for existing industrial installations).

Before delving into specifi c desulfurization technology

and applications, pertinent terms will be defi ned. Figure 1

FUEL OIL

NAPHTHALEN

NO. 6 FUEL OIL

BENZENE

TOLUENE

E F G H I

L

ATM GAS OIL H 2

KEROSENE

PREMIUM GASOLINE

RES. GASOLINE

BUTANE

NAT GASO

C D

CRUDE A NAPHTHA B

LIGHT ENDS

LIGHT REFORMATE

HEAVY REFORMATE

GENERAL FLOWSHEET - CRUDE

OIL PROCESSING

LEGEND

A - CRUDE DISTILLATION

B - CATALYTIC REFORMER

C - BTX EXTRACTION

D - GASOLINE POOL

E - PYROLYSIS

F

G

H

I - HYDRODEALKYLATION

HYDROTREATER TA R

ALKYL NAPHTHALENE

FIGURE 1

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