RU2808457C2 - Method for suppressing soot formation in autothermal reforming and partial oxidation reactors - Google Patents

Abstract

FIELD: autothermal reforming; partial oxidation reactors.

SUBSTANCE: method for suppressing soot formation during partial oxidation of a hydrocarbon-containing gaseous feedstock in the presence of steam and in an autothermal reforming (ATR) or partial oxidation (PO) reactor is disclosed. The method includes adding carbon dioxide gas to a hydrocarbon-containing gaseous feedstock prior to feeding it into a reactor. The minimum steam to carbon ratio for the soot-free mode of the partial oxidation process is also determined in accordance with the following relationship: , where (S/C)_lim is the minimum steam to carbon ratio for the soot-free process mode performed with gaseous feedstock without the addition of CO2; (S/C)_{lim, CO2} ^_ the minimum steam to carbon ratio for the soot-free process operation performed with the addition of CO2 to the gaseous feed; (m_CO2/m_C) ^_ the ratio of the moles of CO2 added to the feedstock to the moles of carbon contained in the feedstock before adding CO2; ς is in the range from 0.4 to 0.6. In addition, a method is disclosed for determining the minimum steam to carbon ratio required for the soot-free process of the partial oxidation of a hydrocarbon-containing gaseous feedstock and a method for controlling the partial oxidation process in an ATR or CHO reactor.

EFFECT: creation of an improved reactor with lower soot formation compared to reactors corresponding to the state of the art, for a given set of operating parameters, such as reaction pressure and temperature, steam and oxygen consumption, pressure drop of the supplied fuel and oxidizer flows.

8 cl, 2 dwg

Description

Field of technology to which the invention relates

The present invention relates to autothermal reforming and partial oxidation reactors. In particular, the invention relates to a method for suppressing soot formation in an autothermal reforming reactor or in a partial oxidation reactor.

State of the art

Autothermal reforming (ATR) and partial oxidation (PO) reactors are widely used in the synthesis gas production field.

The ATP reactor can be used as a separate unit for producing synthesis gas or used for secondary reforming after primary steam reforming in a fired furnace. The ATR reactor, into which air, including oxygen-enriched air, is supplied, is often called a secondary reformer. In contrast, PO systems are an alternative technology used to convert preheated hydrocarbon gas and oxidizer.

Autothermal reforming occurs in the presence of a catalyst, while partial oxidation occurs in the absence of the latter. The temperature can range from approximately 1000 to 1300°C for ATP (at the entrance to the catalyst bed) and be even higher (1300°C or more) for PO. The pressure is usually in the range of 10 to 100 bar.

ATP and PO reactors include a burner, which is usually installed at the top of the reactor vessel above the reaction chamber. The reaction chamber contains, in the case of ATP, a catalyst. The burner ensures mixing of gaseous fuel and gaseous oxidizer. The fuel may be a preheated or partially reformed hydrocarbon with a certain amount of steam already contained in the fuel or added to it. The oxidizing agent is usually air, including air enriched with oxygen, or essentially pure oxygen with the possible addition of steam. The hydrocarbon may be, for example, natural gas. In a known configuration, the burner includes a round pipe with an oxidizer, located coaxially with an external pipe with fuel.

The use of ATP and PO systems is of particular interest in the field of conversion of the original hydrocarbon into partially oxidized CO-containing synthesis gas, for example a mixture of H2 and CO. To produce such gas, both ATP and PO can be used.

CO-containing gas still has significant calorific value and can be used as a fuel; H2 and CO gases also have a variety of uses in the chemical industry, for example as feedstocks for the synthesis of various products including, among others, ammonia and methanol.

The production of CO-containing gas from hydrocarbon feedstocks requires, however, hypostoichiometric combustion conditions, which can lead to undesirable soot formation. Soot formation is accompanied by a number of negative phenomena: loss of carbon raw materials that are not properly converted into CO, contamination and blockage of pipelines, and the need for periodic expensive cleaning and disposal of polluting and potentially carcinogenic substances.

Soot formation is a complex process influenced by several parameters.

It is known, for example, that soot formation is influenced by the ratios of steam to carbon and oxygen to carbon: the higher these two ratios, the lower the soot formation. However, increasing the steam to carbon ratio and/or the oxygen to carbon ratio to suppress soot formation has the disadvantage of increasing the consumption of steam or oxygen, which are valuable raw materials.

It is also known that soot formation is reduced by intensive mixing of fuel and oxidizer. Therefore, a number of prior art solutions attempt to reduce soot by imparting high acceleration and/or swirl to one or both of the fuel and oxidizer streams. Other technical solutions move away from the usual coaxial flow configuration in an attempt to improve mixing, for example by orienting the fuel flow in a direction perpendicular to the direction of the oxidizer flow. These, and indeed all technologies based on high speeds, swirlers and changing the direction of flows, have the disadvantage that they have a significant pressure loss.

For example, WO2016/198245 describes a method for determining temperature, pressure and steam to carbon ratio for soot-free ATP operation.

Thus, the need to reduce soot formation forces the introduction of certain operating parameters, such as a large excess of steam/oxygen or high speed and turbulence of flows, which inevitably entails a number of negative phenomena.

Therefore, there is still a need for an improved burner design for ATP or PO processes that reduces soot formation and provides more favorable process conditions, thereby minimizing the above disadvantages.

Disclosure of the Invention

The purpose of the invention is to solve the above problem and create an improved reactor with lower soot formation compared to reactors corresponding to the prior art, for a given set of operating parameters, such as reaction pressure and temperature, steam and oxygen flow, pressure drop of the fuel and oxidizer feed streams.

The invention is based on the idea that carbon dioxide added to the gaseous feedstock of the partial oxidation process acts as a soot suppressant. The object of the invention is achieved by a method for suppressing soot formation during the partial oxidation of a hydrocarbon-containing gaseous feedstock in the presence of steam and in an ATP or CHO reactor, which includes adding carbon dioxide gas to the gaseous hydrocarbon-containing feedstock before entering the reactor. The preferred distinctive features of the invention are given in the dependent claims.

In particular, carbon dioxide is preferably added in such an amount that the ratio _mCO2 / _mC is at least 0.25, where _mCO2 is the moles of carbon dioxide added and _mC is the moles of carbon contained in the feed before the addition of CO2. This ratio is preferably in the range from 0.25 to 2, more preferably from 0.25 to 1.

Carbon dioxide may be added to the gaseous hydrocarbon-containing feedstock before or after the addition of steam.

The hydrocarbon-containing feedstock is preferably natural gas or methane-containing feedstock.

Another object of the invention is a method for determining the minimum steam to carbon ratio for a soot-free process for the partial oxidation of a hydrocarbon-containing gaseous feedstock in accordance with the claims.

The minimum steam to carbon ratio mentioned is also called the critical steam to carbon ratio. This ratio is usually considered as the ratio of the onset of soot formation for a given set of process parameters. These process parameters may include pressure, temperature, feed composition, and oxygen to carbon ratio, among others.

In particular, if the steam to carbon ratio exceeds a critical value, then the process is effectively soot-free, whereas below the critical value, soot formation becomes noticeable and increases rapidly as this ratio decreases further.

The term "soot-free oxidation process" refers to a process in which soot formation is considered to be negligible. Typically, this is the case when the soot content of the oxidation process effluent does not exceed 5 parts per billion (ppb) by volume or even less (eg, 2 ppb or less).

In one embodiment of the invention, the minimum steam to carbon ratio (critical ratio) in the presence of CO2 added to the steam is calculated by adjusting this minimum ratio in the absence of added CO2, with other process parameters remaining constant.

Then the minimum steam to carbon ratio when adding CO2 is calculated as:

Where:

- (S/C) _{lim, CO2} ^_ critical steam to carbon ratio of the process carried out with the first gaseous feed, which is a feedstock containing hydrocarbon and steam, and added CO2,

- (S/C) _lim is the critical steam to carbon ratio of a given process performed with the second feedstock, which is the feedstock without added CO2,

- m _CO2 - moles of CO2 added to the first gaseous feed,

- m _C - moles of carbon contained in the first gaseous feed before adding CO2,

- ς is in the range from 0.4 to 0.6.

Equation (1) was derived by the applicant experimentally. This equation shows that for a given set of parameters, adding CO2 to the feed stream reduces the steam to carbon ratio critical for soot formation. Therefore, for example, for a given set of process parameters, adding CO2 as a soot suppressant allows the amount of steam to be reduced and operations to be carried out at a lower steam to carbon ratio without soot formation. Reducing the amount of steam is an advantage because steam is expensive in terms of energy and increases the volumetric flow rate and therefore the size of the equipment.

The above method for calculating the minimum steam to carbon ratio can be applied to control the partial oxidation process. Therefore, another object of the invention is a method for controlling a partial oxidation process in which the steam to carbon ratio is maintained above the minimum value calculated by formula (1) above.

The advantages of the invention are illustrated by means of FIGS. 1 and fig. 2, which shows:

fig. 1 is a graph of soot concentration versus steam to carbon ratio for a partial oxidation process performed using methane (CH4) as fuel at a pressure of 15 bar and an oxygen to carbon ratio (O2/C) of 0.5,

fig. 2 is a graph of soot concentration versus steam to carbon ratio for the partial oxidation process shown in FIG. 1, with an oxygen to carbon ratio (02/C) of 0.6.

In fig. 1 curve I refers to the base case without adding CO2 to the feedstock, and curve II refers to the same process as in the base case, but with the addition of CO2 to the feedstock gas in an amount for which m _CO2 /m _C is 0, 25.

In fig. 2, curve I refers to the base case without adding CO2 to the feedstock, curve II refers to the addition of CO2, for which m _CO2 /m _C is equal to 0.25, and curve III for which m _CO2 / m _C is equal to 0.66.

All curves of the graphs in Fig. 1 and fig. 2 show that in the basic version, soot formation begins at relatively higher S/C values. In other words, the addition of CO2 reduces the S/C ratio critical for soot formation, meaning the process can run without soot formation at a lower S/C to save steam. Fig. 2 shows that adding more carbon dioxide further reduces the critical S/C ratio, as can be seen by comparing curves II and III.

Claims (20)

1. A method for suppressing soot formation during partial oxidation of a hydrocarbon-containing gaseous feedstock in the presence of steam and in an autothermal reforming (ATR) or partial oxidation (PO) reactor, including the addition of carbon dioxide gas to the hydrocarbon-containing gaseous feedstock before entering the reactor and the step of determining the minimum ratio of steam to carbon for the soot-free mode of the partial oxidation process in accordance with the following relationship: , Where: (S/C) _lim is the minimum steam to carbon ratio for the soot-free process operating with gaseous feedstock without the addition of CO ₂ ; (S/C) _{lim, CO2} ^_ the minimum ratio of steam to carbon for the soot-free mode of the process performed with the addition of CO ₂ to the gaseous feed; (m _CO2 /m _C ) ^_ the ratio of the moles of CO ₂ added to the feedstock to the moles of carbon contained in the feedstock before the addition of CO ₂ ; ς ranges from 0.4 to 0.6. 2. The method of claim 1, wherein the ratio of _mCO2 / _mC moles of carbon dioxide added to moles of carbon contained in the feedstock before _CO2 addition is at least 0.25. 3. The method according to claim 2, wherein said _mCO2 / _mC ratio is in the range from 0.25 to 2, preferably from 0.25 to 1. 4. Method according to one of paragraphs. 1-3, in which CO ₂ is added to the hydrocarbon-containing gaseous feed before or after the addition of steam. 5. The method according to claim 1, in which the concentration by volume of soot in the effluent of the partial oxidation process does not exceed 5 ppb. 6. A method for determining the minimum steam to carbon ratio required for the soot-free mode of a partial oxidation process of a hydrocarbon-containing gaseous feedstock, in which said partial oxidation process occurs in the presence of steam in an ATR or CHO reactor, and carbon dioxide is added to said gaseous feedstock before entering the reactor, wherein the minimum steam to carbon ratio for soot-free operation in the presence of added CO ₂ is determined as: , Where: (S/C) _lim is the minimum steam to carbon ratio for the soot-free process operating with gaseous feedstock without the addition of CO ₂ ; (S/C) _{lim, CO2} ^_ the minimum ratio of steam to carbon for the soot-free mode of the process performed with the addition of CO ₂ to the gaseous feed; (m _CO2 /m _C ) the ratio of the moles of CO ₂ added to the feedstock to the moles of carbon contained in the feedstock before the addition of CO ₂ ; ς ranges from 0.4 to 0.6. 7. The method according to claim 6, in which the concentration by volume of soot in the effluent of the partial oxidation process does not exceed 5 ppb. 8. A method for controlling the partial oxidation process in an ATR or PO reactor, occurring in the presence of steam and with the addition of carbon dioxide to the hydrocarbon-containing gaseous feedstock of the process, including determining the minimum ratio of steam to carbon for the soot-free mode by the method according to claim 6 or 7.