Fischer-Tropsch process

Details Fischer-Tropsch process Fischer-Tropsch process

2000-06-16

2002-02-19

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

C518S700000

Reexamination Certificate

active

06348510

ABSTRACT:

The present invention relates to an improved process for the Fischer-Tropsch reaction, which essentially consists in a first reaction phase in a gas-liquid-solid fluidized reactor and a second separation phase, at least partial, internal or external, of the solid suspension in the liquid.
The Fischer-Tropsch reaction consists in the production of essentially linear and saturated hydrocarbons, preferably having at least 5 carbon atoms in the molecule, by means of the catalytic hydrogenation of CO, optionally diluted with CO
2
.
The reaction between CO and H
2
is carried out in a gas-liquid-solid fluidized reactor in which the solid, prevalently consisting of particles of catalyst, is suspended by means of the gas stream and liquid stream. The former prevalently consists of reagent species, i.e. CO and H
2
, whereas the latter consists of hydrocarbons produced by the Fischer-Tropsch reaction, optionally at least partially recycled, either from the material liquid under the process conditions, or the relative mixtures.
The gas and liquid, optionally recycled, are fed from the bottom of the column by means of appropriate distributors and the flow-rates of the gas and liquid are such as to guarantee a turbulent flow regime in the column.
In gas-liquid-solid fluidized systems such as that of the Fischer-Tropsch reaction, the flow-rates of the fluids should be such as to guarantee a practically homogeneous suspension of the solid in the whole reaction volume and facilitate the removal of the heat produced by the exothermic reaction, improving the heat exchange between the reaction zone and a suitable exchanger device inserted in the column.
In addition, the solid particles should have dimensions which are sufficiently large as to enable them to be easily separated from the liquid products, but sufficiently small as to render the diffusive intra-particle limitations negligible (unitary particle efficiency) and enable them to be easily fluidized.
The average diameter of the solid particles used in slurry reactors can vary from 1 to 200 &mgr;m, although operating with dimensions of less than 10 &mgr;m makes the separation of the solid from the liquid products extremely expensive.
In the Fischer-Tropsch process, as in all three-phase processes in the presence of catalysts, there is therefore the problem of an optimum particle dimension in both the reaction and separation steps.
As far as the fluidization of the solid particles is concerned, EP-A-450,860 discloses operating in reaction phase with a slurry bubble column under optimum conditions when the following equation is respected:
0.5 (U
s
−U
l
)≦D/H  (1)
wherein U
l
is the circulation velocity of the liquid phase, D is the axial dispersion coefficient of the solid phase, H is the dispersion height (gas+liquid+solid) and U
s
is the settling velocity of the particles defined as follows:
U
s
=
1
18
·
d
p
2
⁢
ρ
s
-
ρ
l
μ
L
·
g
·
f
⁡
(
C
p
)
(
2
)
wherein d
p
is the average particle diameter, &rgr;
s
is the density of the solid, &rgr;
l
is the density of the liquid, &mgr; the viscosity of the liquid, g the gravity acceleration and f(C
p
) represents the hindering function due to the presence of other particles and depending on the volumetric concentration of the particles C
p
.
The description of EP'860, however, is very incomplete and discloses, moreover, the use of particles with very small dimensions, with obvious limits in the solid-liquid separation step. In other words, the technical problem of EP'860 relates only to the reaction phase and not to the whole process, comprising both the reaction and solid-liquid separation.
Above all, EP'860 does not indicate any method or correlation for determining the axial dispersion coefficient of the solid, D (a fundamental parameter in verifying the constraint (1)), neither does it provide any experimental values of D for comparison. In addition, if one succeeds in obtaining a value of D, assuming a dispersion height H=2D/(U
s
−U
l
) (a value which is at the limit of the validity range of (1)), the concentration of the solid proves to decrease from the bottom to the top of the reaction volume by a factor of 7.4. If this height is halved, the reduction factor of the concentration of the solid decreases to 2.4 which however is very high. As mentioned above, on the other hand, an optimum condition for a slurry reactor should comprise a uniform concentration profile in the whole catalyst volume.
EP-A-450,860 also discloses operating according to Stokes' law: it is in fact known in literature that the term,
1
18
·
d
p
2
⁢
ρ
s
-
ρ
l
μ
L
·
g
,
introduced in the definition of U
s
of equation (2), represents the terminal settling velocity of the particle, U
t
, according to Stokes' law. This law (see Perry's Chemical Engineers' Handbook, 6
th
Ed.) is valid in the laminar regime when the Reynolds' particle number Re
p
is less than 0.1. As the Reynolds' number is a function of the properties of the liquid-solid system and of the particle dimensions, once the liquid phase (Fischer-Tropsch synthesis waxes) and type of solid (catalyst for Fischer-Tropsch synthesis, for example Cobalt supported on alumina) have been established, there is a higher limit for the average particle diameter, over which Stokes' law is no longer valid.
As a result EP'860 discloses operating with particle dimensions of over 5 &mgr;m, but not exceeding the limit value of d
p
established by Stokes' law.
For example, considering the data provided in EP'860 for a system consisting of Fischer-Tropsch waxes and Cobalt supported on Titania (&rgr;
l
=0.7 g/cm
3
, &rgr;
s
=2.7 g/cm
3
, &mgr;=1 cP), for Stokes' law to be valid, i.e. Re
p
<0.1, the average particle diameter must be less than 51 &mgr;m (see example 1 of EP'860 for further details).
As is well known to experts in the field, this particle diameter, although excellent for the bubble column in reaction phase, creates drawbacks in the catalyst/liquid separation phase.
A method has now been found for effecting the Fischer-Tropsch process which overcomes the above disadvantages as it allows an optimized operation both in the reaction phase and in the solid-liquid separation phase, without substantially varying the activity of the catalyst.
In accordance with this, the present invention relates to an optimized method for the production of heavy hydrocarbons according to the Fischer-Tropsch process and the relative separation of the above hydrocarbons, starting from mixtures of reagent gases, essentially consisting of CO and H
2
, optionally diluted with CO
2
, in the presence of supported catalysts, which comprises:
(a) feeding the reagent gases into a reactor, preferably from the bottom, so as to obtain a good dispersion of the solid in the liquid phase, in this way at least partially transforming the reagent gases into heavy hydrocarbons, the gas flow-rates being such as to operate under heterogeneous or churn-turbulent flow conditions (i.e. in the presence of a wide size distribution of the bubbles of gas in the column, normally from about 3 mm to about 80 mm);
(b) at least partially recovering the heavy hydrocarbons formed in step (a) by their external or internal separation from the catalytic particles;
the above process being characterized in that in step (a) the reaction takes place:
(1) in the presence of solid particles so that the particle Reynolds' number (Re
p
) is greater than 0.1, preferably from 0.11 to 50, even more preferably from 0.2 to 25, wherein
Re
p
=
d
p
·
ν
·
ρ
l
μ
wherein d
p
is the average particle diameter, v is the relative velocity between particle and liquid, &rgr;
l
is the density of the liquid, &mgr; is the viscosity of the liquid;
(2) maintaining the solid particles suspended at a height H, with such U
s
, U
l
and U
g
values as to have a Bodenstein number Bo
s
≦1, preferably≦0.4

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