1. Introduction
Among the various chemical reactions occurring in industrial reactors, the use of catalytic gas-solid reactions is widespread. A packed bed reactor is commonly used for these types of reactions. Conventional packed bed reactors are associated with various difficulties and disadvantages, including pressure drop, intra particle diffusion limitations, flow channeling, and heat transfer limitations [1, 2]. Structured catalyst reactors are frequently used to address these challenges. These structured catalyst reactors are commonly utilized when there is a need for controlled endothermic or exothermic reactions. However, existing structured catalyst reactors, while demonstrating higher performance in comparison to the packed bed reactors, still have a number of drawbacks, including high cost, weight, thermal resistance, and heat management, among others [3, 4]. Important products of the Fischer-Tropsch reaction include gaseous hydrocarbons, such as lower olefins, paraffins, or alcohols, and liquid hydrocarbons, such as higher olefins, paraffins and alcohols [5, 6]. The Fischer-Tropsch reaction is highly exothermic and therefore effective heat transfer and temperature controls are important prerequisites for the successful operation. Furthermore, the reaction operability range, with pressures between 1-30 bar and temperatures ranging from 200 °C to 350 °C, requires an additional control to prevent formation of local hot spots responsible for the deterioration of the catalyst [7, 8]. Thus, there is a need for very efficient heat transfer during the reaction to prevent metallic catalyst deactivation and formation of undesirable products. Accordingly, there remains a need for a reactor that provides an efficient control over endothermic and exothermic chemical reactions, such as those carried out in the presence of a catalyst. For example, there is a need for a reactor for the Fischer-Tropsch catalytic processing of the synthesis gas that ensures very efficient heat transfer during the catalytic reaction.
Natural gas and methane, a major constituent of natural gas, are difficult to economically transport and are not easily converted into liquid fuels or chemicals that are more readily contained and transported. To facilitate transport, methane is typically converted to synthesis gas which is an intermediate in the conversion of methane to liquid fuels, methanol or other chemicals [9, 10]. Synthesis gas is a mixture of hydrogen and carbon monoxide with a hydrogen to carbon monoxide molar ratio of from about 0.6 to about 6. One chemical reaction effective to convert methane to synthesis gas is steam reforming. Methane is reacted with steam and endothermically converted to a mixture of hydrogen and carbon monoxide [11, 12]. The heat energy to sustain the endothermic reaction is generated by the external combustion of fuel. A second chemical reaction effective to convert methane to synthesis gas is partial oxidation. Methane is reacted with oxygen in an exothermic reaction [13, 14]. Synthesis gas can be produced by combined partial oxidation and steam reforming. The synthesis gas is then converted to liquids by the Fischer-Tropsch process or can be converted to methanol by commercial processes [15, 16]. In the primary steam reforming of fluid hydrocarbons, such as natural gas or methane, the feed material and steam are passed through catalyst-containing vertically hanging reformer tubes maintained at an elevated temperature by radiant heat transfer and by contact with combustion gases in the furnace portion of the tubular reactor. The hot reformer tube effluent may be passed to a waste heat recovery zone for the generation of steam that can be used in the steam reforming operations. Conventional primary steam reforming operations are commonly carried out at temperatures of from about 750 °C to about 850 °C or above [17, 18]. The primary steam reforming is a highly endothermic reaction, and the large amounts of required heat are typically provided by combusting external fuel at close to atmospheric pressures in the reforming furnace. Consequently, the reformer tubes are generally made of high alloy, expensive materials having a limited operating life under such extreme conditions [19, 20]. The reaction temperatures existing inside the reformer tubes are generally lower than about 850 °C so that the effluent gas recovered from the primary reformer typically contains 2-6 percent methane [21, 22]. The effluent from primary reforming is sometimes passed to a secondary reforming zone in which unconverted methane present in the reformed gas mixture is catalytically reacted with air, oxygen or other suitable oxygen-containing gas.
Large quantities of hydrogen, or of an ammonia syngas mixture of hydrogen and nitrogen, are produced either by such steam reforming operations or by partial oxidation reactions. Partial oxidation, like secondary reforming, is an exothermic, autothermal, internal combustion process [23, 24]. While secondary reforming is also a catalytic process, however, the various known partial oxidation processes employ non-catalytic reactions, and thus operate at higher reaction temperatures on the order of about 1300 °C. The significant advantages obtainable by the use of secondary reforming, or by the use of partial oxidation processing, are off-set to some extent by the need to compress the oxygen-containing gas to the desired reaction pressure or higher [25, 26]. Another disadvantage of secondary reforming and of partial oxidation processing is that part of the feed gas is combusted to carbon dioxide and water instead of to desired product. As a result, more natural gas or other feed gas is required to produce a given amount of hydrogen or synthesis gas, although the autothermic processes do not require any fuel. By contrast, the fuel consumption rate for primary reforming is typically between 30 percent and 50 percent of the feed rate [27, 28]. Despite such efforts to improve steam reforming operations, it will be appreciated that there remains a desire to achieve lower steam and fuel requirements and higher thermal efficiencies in such operations [29, 30]. In addition, improved mechanical designs are also desired to reduce the size of the overall reforming systems employed and to achieve other useful purposes, such as a reduction for the thermal stresses to which the primary reformer tubes are subjected [31, 32]. It is also desired to carry out steam reforming operations at higher pressures, as in the range of 20-100 Bar. Such desired improvements also relate to the integration of primary and secondary reforming operations, so as to obtain the benefits of secondary reforming while achieving a more efficient overall reforming operation than has heretofore been possible.
Currently, endothermic reactions performed in microreactors are driven using heat from an external source, such as the effluent from an external combustor. In doing so, the temperature of the gas stream providing the heat is limited by constraints imposed by the materials of construction. Practically, this means that tile effluent from an external combustor must be diluted with cool gas to bring the gas temperature down to meet material temperature constraints. This increases the total gas flow rate, raising blower and compressor costs. Moreover, heating the gas stream externally introduces heat losses and expensive high temperature materials to connect the combustor to the microreactor. On the other hand, an integrated combustor can produce heat for the reaction in close proximity to the reaction zone, thus reducing heat losses and increasing efficiency. Because traditional combustion catalysts are riot stable at high temperatures due to noble metal sintering, the integrated combustor must transfer heat at a rate sufficient to keep local temperatures at the catalyst surface below this level or risk rapid catalyst deactivation. The present study is focused primarily upon the designs and operations of heat integrated reactors for thermochemically producing hydrogen from methanol by steam reforming. A symmetry boundary condition is used to model half of each system where symmetry exists. Computations are performed using grids with varying nodal densities to determine the optimum node spacing and density that would give the desired accuracy and minimize computation time. The final grid density is determined when the centerline profiles of temperature and species concentration do not show obvious difference. The second-order upwind scheme is used to discretize the mathematical model, and the semi-implicit method for pressure-linked equations algorithm is employed to solve for the pressure and velocity fields. The simulation convergence is judged upon the residuals of all governing equations. The present study aims to provide a fundamental understanding of the designs and operations of heat integrated reactors for thermochemically producing hydrogen from methanol by steam reforming. Particular emphasis is placed upon the effect of various factors on the thermochemical steam reforming processes in heat integrated reactors.