THERMAL CRACKING OF A HYDROCARBON FEED

CRAQUAGE THERMIQUE D'UNE ALIMENTATION D'HYDROCARBURES

English Abstract

Thermal cracking of a hydrocarbon feed comprising heating the feed in a
furnace (1); supplying the feed to a reaction chamber (15);
separating the stream from the reaction chamber (15) into a light products
stream and into a heavy products stream; and supplying the heavy
products stream to a vacuum distillation column (30) to separate the heavy
products stream into fractions, wherein supplying the heavy
products stream to the vacuum distillation column comprises passing the heavy
product stream through a flow control valve (36), passing
the heavy products stream upwards through a stand-pipe (38) having such a
length that the fluid pressure at the end of the stand-pipe (38) is
such that vaporization at its bottom end is suppressed, and subsequently
passing the heavy products stream through a transfer conduit (40)
into the vacuum distillation column (30), the transfer conduit (40) having
such a configuration that the fluid pressure at its outlet matches
the fluid pressure in the vacuum distillation column (30).

Claims

-6-
CLAIMS
1. Process of thermal cracking of a hydrocarbon feed comprising
heating the feed at a pressure in the range of from 0.2 to 5 MPa
and at a temperature in the range of from 390 to 530 °C; supplying
the feed to a reaction chamber; separating the stream from the
reaction chamber into a light products stream and into a heavy
products stream; and supplying the heavy products stream to a
vacuum distillation column to separate the heavy products stream
into fractions at a pressure in the range of from 1 to 10 kPa and
at a temperature in the range of from 320 to 400 °C, wherein
supplying the heavy products stream to the vacuum distillation
column comprises passing the heavy product stream through a
restricted opening, passing the heavy products stream upwards
through a stand-pipe having such a length that the fluid pressure
at the end of the stand-pipe is such that vaporization at its
bottom end is suppressed, and subsequently passing the heavy
products stream through a transfer conduit debouching into the
vacuum distillation column, the transfer conduit having such a
configuration that the fluid pressure at its outlet matches the
fluid pressure in the vacuum distillation column.
2. Process according to claim 1, wherein the transfer conduit
includes a plurality of straight sections joined by curved
connectors.
3. Process according to claim 1 or 2, wherein the stream from the
reaction chamber is separated in a cyclone separator in a light
products stream and a heavy products stream.
4. Process according to claim 1 or 2, wherein the stream from the
reaction chamber is separated in a distillation column in a light
products stream and a heavy products stream.

Description

'O 94/21749 21 ~ gy G 5 PCT/EP94100895
1 -
THERMAL CRACKING OF A HYDROCARBON FEED
The present invention relates to thermal cracking of a
hydrocarbon feed as described in The Petroleum Handbook, 6th
Edition, Elsevier, 1983, pages 279-281.
Thermal cracking of a hydrocarbon feed comprises heating the
feed at a pressure in the range of from 0.2 to 5 MPa and at a
temperature in the range of from 390 to 530 °C; supplying the feed
to a reaction chamber; separating the stream from the reaction
chamber into a light products stream and into a heavy products
stream; and supplying the heavy products stream to a vacuum
distillation column to separate the heavy products stream into
fractions at a pressure in the range of from 1 to 10 kPa and at a
temperature in the range of from 320 to 400 °C.
In the known process a conduit is used to pass the heavy
products stream to the vacuum distillation column. As the pressure
of the heavy products stream is larger than the pressure in the
vacuum distillation column, the pressure drop along the conduit
should equal the pressure difference between the pressure of the
heavy products stream and the pressure in the vacuum distillation
column. One way of obtaining this rather high pressure drop along
the conduit is to provide the conduit with a restricted opening,
for example in the form of a flow control valve. However, when the
heavy products stream is throttled over a flow control valve rapid
vaporization occurs. This rapid vaporization is accompanied by the
formation of very small liquid droplets which cannot easily be
handled in the vacuum distillation column.
The present invention provides a simple way to suppress
vaporization over the restricted opening.
To this end the process of thermal cracking of a hydrocarbon
feed according to the present invention comprises heating the feed
at a pressure in the range of from 0.2 to 5 MPa and at a
temperature in the range of from 390 to 530 °C; supplying the .feed

q,
WO 94/21749 P'CTlEP94/0089
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to a reaction chamber; separating the stream from the reaction
chamber into a light products stream and into a heavy products
stream; and supplying the heavy products stream to a vacuum
distillation column to separate the heavy products stream into
fractions at a pressure in the range of from 1 to 10 kPa and at a
temperature in the range of from 320 to 400 °C, wherein supplying
the heavy products stream to the vacuum distillation column
comprises passing the heavy product stream through a restricted
opening, passing the heavy products stream upwards through a
stand-pipe having such a length that the fluid pressure at the end
of the stand-pipe is such that vaporization at its bottom end is
suppressed, and subsequently passing the heavy products stream
through a transfer conduit debouching into the vacuum distillation
column, the transfer conduit having such a configuration that the
fluid pressure at its outlet matches the fluid pressure in the
vacuum distillation column.
The expression "configuration of the transfer conduit' in the
specification and in the claims is used to refer to the features of
the transfer conduit that contribute to the pressure drop along the
transfer conduit, such as the dimensions of the transfer conduit
and other flow resistances such as bends or U-turns in the transfer
conduit.
The invention will now be described by way of example in more
detail with reference to the accompanying drawings, wherein
Figure 1 shows schematically a line-up for the process
according to the present invention; and
Figure 2 shows schematically an alternative line-up for the
process according to the present invention.
Reference is made to Figure 1. The line-up for the process
according to the present invention includes a furnace 1 provided
with a burner 2 and a stack 3 and a heater tube 4 arranged in the
furnace 1. The heater tube 4 is connected to an inlet conduit 7 and
an outlet conduit 9.
The outlet conduit 9 is connected to the inlet of a reaction
chamber 15. The reaction chamber is more clearly described in

'O 94/21749 PCT/EP94/00895
_ 3 _
European patent No. 7 656. The outlet of the reaction chamber 15 is
connected by conduit 17 to a separation device in the form of
cyclonic separator 20.
The cyclonic separator 20 has two outlets, an outlet 21 for
light products and an outlet 24 for heavy products. The outlet 21
' for light products is connected by a conduit (not shown) to a
device (not shown) for further treating the light products. The
outlet 24 for heavy products is connected by means of a conduit
system 26 to a vacuum distillation column 30.
The vacuum distillation column 30 has an outlet conduit 31
which is connected to a vacuum pump (not shown), a residue outlet
conduit 32 for heavy products and an outlet conduit 33 for an
intermediate product fraction. Devices for providing a suitable
reflux stream to the top part of the vacuum distillation column 30
and stream of stripping medium to the bottom part of the vacuum
distillation column 30 are not shown.
The conduit system 26 includes a conduit section 35 in which a
restricted opening in the form of flow control valve 36 is
arranged, a stand-pipe 38 joined to the conduit section 35 and a
transfer conduit 40 debouching into the vacuum distillation
column 30.
The length of the stand-pipe 38 is so determined that, during
normal operation, the fluid pressure at the bottom end of the
stand-pipe 38 is such that vaporization of the fluid at its bottom
end is suppressed. The expression "suppressing the vaporization of
the fluid" is used in the specification and in the claims to
indicate that at most a small amount of liquid is vaporized (less
than about 5% by volume).
The configuration of the transfer conduit 40 is so determined
that, during normal operation, the fluid pressure in the transfer
conduit 40 at its outlet end 41 matches the fluid pressure in the
vacuum distillation column 30. This implies that the transfer
conduit 40 is so designed that the friction experienced by the
fluid flowing through the transfer conduit 40 equals the difference
in pressure at the outlet of the stand-pipe 38 and in the vacuum

WO 94/21749 PCT/EP94/0089:
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distillation column 30. In this case the transfer conduit 40
includes several straight lines 43 joined by means of curved
connectors in the form of U-shaped connectors 44.
During normal operation, 3 000 ton/day of a hydrocarbon feed is
continuously supplied to the furnace 1 at a pressure of 3 MPa, the
feed is heated at a temperature of 450 °C in the furnace 1. Then
heated feed is supplied to the reaction chamber 15 where the feed
is allowed to crack. The stream of products from the reaction
chamber 15 is separated in cyclonic separator 20 into 900 ton/day
of light products removed through outlet 21 and into 2 100 ton/day
of heavy products removed through outlet 24. The heavy products are
supplied through the conduit system 26 to the vacuum distillation
column 30 which operates at a pressure of 5 kPa and at a
temperature of 380 °C. In the vacuum distillation column 30 the
heavy products stream is separated into 200 ton/day of a gaseous
fraction removed through outlet conduit 31, 400 ton/day of
intermediate fraction removed through outlet conduit 33, and
1 500 ton/day of a residue removed through residue outlet
conduit 32.
The pressure drop over the flow control valve 36 is 150 kPa.
The length of the stand-pipe 38 is 15 m, the presence of liquid
in the stand-pipe 38 prevents vaporization of liquid at the
downstream end of the flow control valve 36.
The length of the transfer conduit 40 is 70 m, the average
internal diameter of the transfer conduit 40 is 50 cm and the
transfer conduit includes four U-shaped connectors 44. The fluid
pressure at the inlet of the transfer conduit 40 is 60 kPa and the
fluid pressure at the outlet of it matches the fluid pressure in
the vacuum distillation column 30.
Omitting the stand-pipe 38 would result in vaporization at or
near the flow control valve 36, which vaporization is uncontrolled
and is accompanied by the formation of extremely small liquid
droplets.
Reference is now made to Figure 2 showing an alternative
line-up for the process according to the present invention. Parts

O 94/21749 PCT/EP94/00895
_ 5 _
of the line-up which a similar to the line-up as described with
reference to Figure 1 have got the same reference numeral.
In the line-up of Figure 2 the stream from the reaction
chamber 15 is separated in a distillation column 50 into a light
products stream removed through outlet 51 and into a heavy products
stream removed through outlet 52.
The heavy products stream is then supplied through conduit
system 26 to the vacuum distillation column 30.
Devices for providing a suitable reflex stream to the top part
of the distillation column 50 and stream of stripping medium to the
bottom part of the distillation column 50 are not shown.
Curved connectors 44 may have a U-shape as shown in Figures 1
and 2, or the curved connectors may have an L-shape.
The presence of liquid in the stand-pipe prevents vaporization
of liquid at the downstream end of the restricted opening. Omitting
the stand-pipe would result in vaporization at or near the
restricted opening, which vaporization is uncontrolled and is
accompanied by the formation of very small liquid droplets. In the
vacuum distillation column these small fine liquid droplets cannot
be separated from the gas stream so that the liquid droplets are
entrained with the fraction leaving the vacuum distillation column
through the outlet conduit for the intermediate fraction or with
the gaseous stream leaving the top of the vacuum distillation
column through outlet conduit. This entrainment adversely affects
the separation efficiency of the vacuum distillation column.
The stand-pipe 38 as described with reference to Figures 1 and
2 is a vertical pipe, it will be understood that the stand-pipe can
also be a slanted pipe provided that there is, during normal
operation, sufficient liquid in the stand-pipe to prevent
vaporization.

Representative Drawing