FR2507169A1 - PROCESS FOR CATALYTICALLY CRACKING NATURAL GAS - Google Patents

Abstract

THE INVENTION CONCERNS A PROCESS FOR THE PRODUCTION OF A GAS MIXTURE INTENDED FOR THE SYNTHESIS OF METHANOL, A PROCESS WHICH IS BASED ON THE CATALYTIC CRACKING OF NATURAL GAS UNDER PRESSURE AND AT HIGH TEMPERATURE. ACCORDING TO THIS PROCESS THE CRACKING OF NATURAL GAS IS CARRIED OUT IN TWO STEPS. IN THE FIRST, THE NATURAL GAS IS CRACKED, AT A PRESSURE OF 35 TO 55 BAR AND AT A TEMPERATURE OF 650 TO 800C, UP TO A RESIDUAL CH CONTENT OF 13 TO 20 AND, IN THE SECOND, THE CRACKING IS COMPLETED, AFTER ADDITION OF OXYGEN, AT A TEMPERATURE OF 900 TO 1200C.

Description

The present invention relates to a method for

  catalytic cracking of natural gas for the purpose of

  a synthesis gas for the preparation of methanol.

  Natural gas is today an important raw material for the preparation of synthesis gas. According to the chemical formula of methanol, that is to say CH 3 OH, it is necessary to synthesize H 2,

  CO and CO 2 The synthesis gas has the stoichiometric composition

  exactly when the following condition is met: H 2 Co 2

at __ = 2

CO + Co 2

  Theoretically, the CO 2 content could also

  be zero, in which case we would have H 2

___ = 2.

  While in hydrocarbons such as naphtha (heavy gasoline) the ratio H / C is equal to 2, in the catalytic cracking of natural gas,

  has a high content of CH 4, a gas whose composition

  corresponds to a value of the ratio: H 2 Co 2 CO + Ca 2 can be up to 3, which is to say that there is too much hydrogen for the synthesis of methanol One of the means to which we can have recourse to remedy this unfavorable situation consists in bringing out from time to time a

  important part of the gas of the circuit of the synthesis of metha-

  It is only in this way that we can oppose the enrichment

  The expanded gas that is removed from the methanol synthesis circuit is used as the heating gas for catalytic cracking in the steam reforming furnace, as described in CA 786,393 or US Pat. as shown in Figure 2 at

  page 30 of Chemical Economy and Engineering Review, Septem-

  1971, Volume 3, Number 9 (Number 41) This way of

  to maintain the functioning of the synthesis of methanol

  the advantage of the simplicity of the produc-

  gas, but it has the disadvantage of producing too much heat

  which must also be conducted through the whole

  of the installation to then be relaxed almost to the

  atmospheric pressure so that it can be burned later

  as a flue gas in the reforming furnace.

  According to another known method which is described in Chemical Economy and Engineering Review, September 1971, Volume 3, number 9 (number 41), page 21, the adjustment is carried out

  optimum H / C ratio for the synthesis of methanol by

  killing a parallel conversion of CO: more precisely, a portion of the crude gas from catalytic cracking in a single step, a raw gas that contains CO and is rich in

  H 2, is treated in a CO-generating step in which

  the carbon monoxide is converted into CO 2 and H 2 Then, in a step of subsequent absorption of carbon dioxide,

  H 2 is separated from C 02 Hydrogen can be used from another

  side while CO 2 is sent into the main gas stream.

  to adjust the optimum H / C ratio.

  The known processes give rise, during catalytic cracking, to an excessive amount of hydrogen,

  which constitutes an unnecessary burden The construction of

  increased size in proportion to this hydro-

  gene-charge also leads, for large installations, to the limits of the technical possibilities of

  consumption of natural gas and energy per unit of metha-

they are excessive.

  Another possibility for improving the unfavorable H 2 / CO ratio in the catalytic cracking of CH 4 consists of

  to add CO 2 directly to the stream of synthesis gas

  This is how it is known (see Petroleum & Petrochemical, August 1973, Volume 13, Number 8, page 74, Figure 2) to take CO 2 on the flue gas from the cracking plant, by means of of a chemical wash, to compress this CO 2 and to add it to the synthesis gas To isolate CO 2 from the flue gas, which is not under pressure, is an expensive operation both from the

  energy view only from the point of view of investments.

  It was therefore necessary to find a means of avoiding the drawbacks of known methods: it was in these terms that the technical problem solved by the

present invention.

  The subject of the invention is in fact a method

  for the production of syngas for the preparation of

  methanol production, by catalytic cracking of natural gas under pressure and at elevated temperature, characterized in that the cracking of the natural gas is carried out in two stages,

  more precisely, in the first step, the crack is

  at a pressure of 35 to 55 bar and at a temperature of 650 to 800 ° C to a residual CH 4 content of 13 to

  % and, in the second stage, the cracking

  dual by adding oxygen at a temperature of 900 to 1200 ° C,

in known manner.

  The technical advantage of the process according to the

  This is because the optimal H / C ratio for methanol synthesis is already adjusted in the plant.

  catalytic cracking No "hydrogen-charge" is formed.

  This avoids excessive energy expenditure for the compressor.

  of this hydrogen-charge and the consumption of

natural gas.

  In addition all the parts of the installation are dimensioned, in volume, only for the gas flow

really necessary.

  Here is a numerical example that illustrates the

transferred from the invention.

  Quantity of natural gas to be cracked: 181 285 Nm 3 / h Analysis of natural gas: CH 4 98%

N 2 2%

Pressure: 51.8 bar

Temperature: 92 OC.

  Volume flow at the reformer inlet Analysis: Pressure: Temperature: primary: 607 666 Nm 3 / h

CH 4 29.24%

N 2 0.60%

H 20 O 70.16%

bar

510 C.

  Volume flow at the reformer outlet Analysis: Pressure: Temperature: primary: 730 726 Nm 3 / h

C 2.78%

002 6.51%

H 2 30.90%

CH 4 15.02%

N 2 0.50%

1120 44.29%

42 bar

720 C.

  Volume flow at the inlet of the secondary reformer: Gas, gas at the

out of the refor-

primary mender.

  Oxygen: 67,539 Nm 3 / h Purity: 99,5% Pressure: Temperature: 41,5 bar

 C

  Flow rate at the outlet of the synthesis gas reformer product: Analysis: Pressure: Temperature: secondary, ie 927,590 Nm 3 / h Co 12.06%

   2 6.10%

H 2 43.48%

CH 4 0.85%

N 2 0.42%

H 20 37.09%

, 5 bar

1000 C.

  For dry synthesis gas the following composition is obtained by calculation:

CO 19.17%

Co 2 9.70%

H 2 69.11%

CH 4 1.35%

N 2 0.67%

We thus obtain the following report

H 2 C 02

at = 2.06.

CO + Co 2

  The value thus obtained is a calculated value

  which can be lowered to 2.0 when the operating values

  relating to the raw material, the pressure and the

  temperature are slightly different.

  In the process of the invention, there is no need

  take the known, costly measures to adjust the ratio

port H 2 / CO necessary.

2507169.

Claims (1)

CLAIM   Process for producing a synthesis gas of   for the synthesis of methanol, by catalytic cracking of   natural gas at a pressure above atmospheric pressure   and at a high temperature, characterized in that: a) the cracking of the natural gas is carried out in two stages, as follows: b) the cracking is carried out in the first stage under a pressure of 35 to 55 bar and at a temperature of 650 to 8000 C, to a residual CH 4 content of 13 to 20% and c) the residual cracking is carried out, in the second stage, by addition of oxygen, at a temperature of 900 to 12000 C, in known manner.