Method For Providing A Homogeneous Mixture Of Liquid Fuels And Oxidants For Use In A Catalytic Reactor

PATENT
Method For Providing A Homogeneous Mixture Of Liquid Fuels And Oxidants For Use In A Catalytic Reactor
Pub. No.: WO/2012/005644
International Application No.: PCT/SE2010/051220
Publication Date:  12.01.2012
Applicants:
REFORMTECH SWEDEN AB [SE/SE]; Järnvägsgatan 5 S-172 75 Sundbyberg (SE)
Inventors:
LINDSTRÖM, Bård; (SE).
HAGSTRÖM, Daniel; (SE)
Abstract:
The method according to the invention in its broadest aspect is a method for generating a homogeneous mixture of oxidant(s) and vaporized fuel for a catalytic process in a reactor having a reaction space. In particular it comprises a method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6), comprising the steps of providing a fuel having a boiling point range; providing one or more oxidants and introducing said oxidants in said reaction space (8; 14); heating the oxidant(s) to a temperature above the boiling point range of the fuel, and introducing the fuel into the oxidant(s) in said reaction space; wherein the reaction space is restricted in size such that the dwelling time of the fuel and oxidant mixture is below the explosion limit of the fuel, is outside the cool flame regime, and is not below 25 ms.

Background
Conventional processes for upgrading or for converting a fuel, such as flame combustion, are related with high emissions, high reaction temperatures and complex system designs. The complex designs also entails use of expensive materials rendering the material costs of the equipment used in these processes unduly high.
By replacing a flame combustion system with a catalytic combustion system it is possible to eliminate the emissions of nitrous oxides and particulates as well as to reduce cost and complexity of a system through the use of lower operation temperatures and reduction of the number of parts in the system.
Catalytic reactor systems are also easily modified for use in alternative applications. A catalytic combustor can for example be modified to a catalytic hydrogen generator (reformer), by simply changing the catalyst in the reactor and adjusting the operating conditions, thereby enabling the development of a single technology platform for many applications.
Industrial catalytic processes are today mainly focused on using gaseous fuels
(Methane, LPG) or single component fuels (methanol, ethanol) as these are easily gasified and can be treated as a gas in the mixing chamber. Gaseous fuels are commonly employed as they can readily be mixed with oxidants at low
temperatures and present a low risk for damaging the catalyst through droplet contamination or soot formation on the catalyst surface.
The ability to create a homogeneous of a liquid fuel(s) and oxidant(s) is a critical requirement for any catalytic reactor as the failure to accomplish such a mixture will lead to poor conversion of the fuel in the catalyst as well as hotspots that can lead to thermal degradation of the system and shorten the life time of the products. The fact that the today available fuels, such as diesel, cannot simply be heated into a vapour makes the task of designing a catalytic reactor for such a fuel much more complex than for a gaseous fuel that can simply be mixed a low temperatures in a static mixing equipment.
There are several methods that have been employed in catalytic reactors to realize a mixture between a liquid fuel(s) and oxidant(s) . The related systems all have a common method of operation:
1. The fuel is atomized at elevated pressure
2. The fuel is introduced into a high temperature oxidation gas (or gasses),
which results in complete or partial evaporation of the fuel
3. The mixture is then delayed so that complete mixing can be achieved before the fuel -oxidant mixture is introduced to the catalyst
In US-4,302, 177 there is described how a pre-heated fuel is mixed with, and evaporated in a hot oxidant gas stream to produce a mixture of evaporated fuel and oxidant. The concept of direct fuel evaporation in a hot oxidant and allowing the fuel to be slowly mixed with gas is a proven method when using steam as the oxidant, in for example a hydrogen reforming system.
However when using a logistical fuel such as diesel or gasoline in reforming (or any other oxidation process), air is required as part of the process mixture, and while the introduction of fuel into hot air in the reactor facilitates the vaporization of the fuel , it also initiates chemical reactions that lead to autoignition of the fuel-air mixture.. Autoignition of the fuel causes hot flame combustion that leads to high emissions as well as the risk for thermal degradation of the catalyst and reactor housing.
Kohne et al, proposed using the cool flame as method for preparing mixtures for catalytic reactors, USP 6,793,693. The concept of using the cool flames to prepare fuel oxidant mixtures have however been shown to have several drawbacks:
1. The cool flame reactions are precursors to complete combustion and the long delay times, from 25 up to the preferred 500 ms increase the risk of autoignition of the fuel oxidant mixture through the formation of the OH radical at 700 K
2. In low oxygen containing systems (such as reformers) the cool flame
consumes only the alkanes, leaving the more complex aromatics that require oxygen for conversion in a oxygen depleted environment that cannot be converted in the catalyst, resulting in a high fuel slip from the reactor - that can damage e.g. a fuel cell utilizing the fuel, and poison the environment
3. The flame velocity of the cool flame, due to the prolonged dwelling time, often exceeds the gas velocity at low loads, increasing the risk for flashback explosions that can damage the catalyst and the reactor housing
4. The radical precursors in the cool flame have been found to initiate fuel polymerization at the walls and at the fuel nozzle
Summary of the Invention
The object of the present invention is to overcome the drawbacks connected with prior art systems and methods.
Thus, the inventors have devised a method of operating a catalytic reactor as defined in claim 1.
The method according to the invention in its broadest aspect is a method for generating a homogeneous mixture of oxidant(s) and vaporized fuel for a catalytic process in a reactor having a reaction space. In partiular it comprises a method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6), comprising the steps of providing a fuel having a boiling point range; providing one or more oxidants and introducing said oxidants in said reaction space (8; 14);
heating the oxidant(s) to a temperature above the boiling point range of the fuel, and introducing the fuel into the oxidant(s) in said reaction space; wherein the reaction space is restricted in size such that the dwelling time of the fuel and oxidant mixture is below the explosion limit of the fuel, is outside the cool flame regime, and is not below 25 ms.
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