1. Introduction
Heterogeneous
vapor-phase reactions are used to produce many large volume organic and
inorganic chemicals. Examples of vapor-phase processes include
oxidation, ammoxidation, and oxychlorination of olefins, alkanes, and
inorganic species to yield a variety of chemicals [1, 2]. In
commercial processes, these vapor-phase reactions are conducted in
fixed-bed or fluidized-bed reactors [3, 4]. Commercial versions of
vapor-phase processes typically possess at least one characteristic,
namely a dilute reaction stream [1, 2]. The actual volume fraction
of the process stream entering the reactor that is converted to products
is typically small. The remainder of the stream consists of inert and
unconverted reactants [3, 4]. Consequently, process equipment
associated with reactor process stream, the recycle stream, and product
recovery, and purification streams must be sized to accommodate the
combined flow of reacting and nonreacting species. The excess diluent
represents a process ”inefficiency” that adds to fixed capital and
operating costs associated with the processes.
The requirement for excess diluent arises from several process
constraints inherent in conventional fixed-bed and fluidized-bed
vapor-phase reactor operations [5, 6]. These constraints include the
following critical factors: limitations due to inadequate rates of heat
transfer to remove heat of reaction from catalyst particles, limitations
due to conversion-selectivity trade-offs, and limitations in feed
compositions due to flammability and explosion hazards [7, 8].
Vapor-phase heterogeneous oxidations are typically highly exothermic
reactions [9, 10]. Reaction rates are limited by the rate at which
heat can be removed from the solid catalyst. Rapid reaction rates or
poor rates of heat transfer may lead to generation of catalyst hot
spots, which reduce product yields and catalyst lifetimes. Frequently,
in heterogeneous vapor-phase oxidations, as the extent of reaction
increases, the catalyst selectivity to the desired product decreases
[11, 12]. Loss in selectivity is often due to side and secondary
reactions forming undesirable by-products, for example, carbon dioxide.
In economic terms, these trade-offs balance effectiveness of raw
materials utilization and, in some cases, the size of recycle streams
[9, 10]. Flammable compositions of mixtures containing oxygen and
hydrocarbons or other combustibles are demarcated by upper and lower
flammability limits [11, 12]. Fixed-bed vapor-phase oxidation
processes are typically designed to ensure that the compositions of all
process streams lie outside the flammable region to avoid explosion
hazards.
These limitations are particularly important in heterogeneous fixed-bed
processes. In heterogeneous fluidized-bed processes, heat removal from
the catalyst is much improved over fixed-bed designs, and explosion
risks are considerably reduced by separating the organic and air feed
streams and operating without recycle [13, 14]. Fluidized-bed
processes typically run at very high conversion of organic reactant and
relatively low conversion of oxygen [15, 16]. However, like their
fixed-bed counterparts, they suffer from excess inert in the reactor
process stream [17, 18]. Since oxygen efficiency is sacrificed to
achieve high conversion of organic reactant, these heterogeneous
processes cannot tolerate the relatively high cost of the needed oxygen,
and instead use feed air as the oxygen source [19, 20]. A new
reactor development that eliminates heat transfer limitations, reduces
or eliminates the need for excess inert, and ensures safe operation
could provide significant savings in the capital equipment and operating
costs associated with chemical manufacture.
There are two classes relating to vapor phase reactions, one related to
conventional two-phase reactor systems, and the other related to
three-phase reactor systems. The two-phase systems cannot fully address
all three issues raised above. In general, two-phase reactor designs
solve the problems of flammability and heat removal by dilution of the
reaction stream [21, 22]. By utilizing inert components or operating
at low conversions with large recycle streams, the composition of the
reaction stream can be maintained outside the flammable region [21,
22]. This dilution using inert and unconverted reactants in the
recycle stream also serves to mitigate catalyst heating effects by
reducing the heat of reaction per unit volume of feed. Heat transfer
constraints can also be significantly reduced by utilizing fluidized-bed
designs in preference to fixed-bed designs [23, 24]. Smaller
catalyst particle size and higher solid-vapor heat transfer coefficients
associated with fluidized-bed designs both contribute to the improved
performance. However, trade-offs also pertain with respect to fluidized
bed designs [23, 24]. Back-mixing, which can lead to reduced
catalyst selectivity, often results in lower reaction yields and higher
rates of production of undesired reaction products.
Three-phase reactors offer significantly better heat transfer than is
obtainable using two-phase reactors [25, 26]. Higher rates of heat
removal are achieved by contacting the catalyst directly with a liquid
solvent. Three-phase reactors come in several forms: fixed beds, either
trickle-bed or bubble-bed reactors, depending on whether the vapor phase
is continuous or not; ebullated bed reactors, the three-phase equivalent
to a fluidized-bed reactor; and slurry reactors in which the catalyst
and vapor phases are dispersed in the liquid phase, either with or
without forced convection [27, 28]. The predominant application of
three-phase reactors has been to liquid-phase hydrogenations, although
some liquid-phase oxidations have also been proposed [25, 26]. In
three-phase systems, the distinction between vapor-phase and
liquid-phase heterogeneous catalysis is not entirely obvious since both
phases coexist with the solid catalyst [27, 28]. It is possible,
however, to distinguish between vapor-phase and liquid-phase catalytic
processes in these three-phase systems, based on the mass transfer
mechanisms and the resultant reaction rates [29, 30]. If the
vapor-phase reactants must diffuse through the liquid to reach the
active catalyst sites, the overall reaction rates will be much slower
than if mass transfer proceeds through the vapor phase [31, 32].
Thus, liquid-phase processes are characterized by much lower space
velocities and slower reaction rates than vapor-phase processes.
The three-phase, liquid-phase oxidation processes remove the heat
transfer limitations, and in many cases, provide yields and selectivity
superior to commercially practiced technology [33, 34]. They have
failed to achieve commercial significance because, as liquid-phase
processes, they do not generate commercially acceptable rates of
reaction [35, 36]. Reaction rates comparable to vapor-phase
processes have been achieved recently in three-phase reactors by using
non-wetted support material [37, 38]. On such supports, the solvent
does not capillary condense in the pores of the support, and the
reactants have direct access to the active catalyst sites by vapor-phase
transport without the additional mass transfer resistances associated
with absorption and diffusion in the liquid phase [39, 40]. The
three-phase reactor processes provide better heat transfer than
conventional vapor-phase processes due to the higher heat capacity of
the liquid phase [41, 42]. They also provide for in situ product
recovery in the liquid phase [43, 44]. However, there are two
significant problems with current three-phase reactor approaches as
applied to vapor phase heterogeneous catalysis.
The first problem in inherent in trickle-bed designs utilizing
hydrophobic catalyst supports in vapor-phase oxidation processes [45,
46]. The catalyst support must be hydrophobic to ensure vapor-phase
reaction rates. It must also be intimately contacted with liquid to
remove the heat and recover reaction products. For example, in
trickle-bed reactors, the support catalysts are bonded to ceramic
packing [47, 48]. A solvent flows through the column with the gas
and contacts the catalyst and removes the oxidation product and the heat
of reaction. However, if the support is hydrophobic, liquid flow will
tend to be characterized by liquid rivulets with large portions of the
catalyst bed remaining dry. Consequently, heat and mass transfer rates
will be low, resulting in catalyst overheating and inefficient product
recovery. The second serious limitation of this approach is that such
trickle-bed processes are co-continuous in the liquid and vapor phases
[49, 50]. Consequently, there is significant risk of explosion if
operating within the flammability limits. To eliminate explosion
hazards, such processes, use excess reactant or inert species to dilute
the reactive species and minimize the risk of explosion [51, 52].
There remains, therefore, a need for further development in three-phase
reactor processes and systems. Such development will desirably overcome
the two significant problems referred to above with respect to
vapor-phase heterogeneous catalysis.
The present study aims to provide an improved reactor system and process
for the carrying out of vapor phase heterogeneous reactions. The present
study relates to plurality of small diameter, high aspect ratio,
multi-pass tubular catalytic steam reforming reactors loaded with rolled
catalyst inserts positioned in a non-vertical parallel configuration
thermally coupled with a heat source such as oxidation reactors. This
unique arrangement allows rolled catalyst inserts to be self-supported
on the reactor wall, efficient heat transfer from the reactor wall to
the reactor interior, improved gas mixing within reactor interior while
achieving lower pressure drop than known particulate catalysts. In
reactors being provided with a bed of highly active steam reforming
catalyst, the temperature at the catalyst surface in the upper portion
of the bed is, thereby, considerably lower than the temperature of the
oxidation gas traversing it and deposition of solids takes place
typically in the uppermost layers of the catalyst bed. Deposition of
solids is, therefore, concentrated substantially to a thin layer in the
uppermost portion of the catalyst bed and causes restriction of gas
passage in this layer leading to heterogeneous flow distribution in
subjacent layers of the bed and eventually to detrimental channeling
through the catalyst bed. The effect of temperature on the methanol mole
fraction and effective factor is investigated for a microchannel
methanol steam reformer with different shapes of the cross section of
the process microchannel. Particular emphasis is placed upon the heat
and mass characteristics involved in vapor phase heterogeneous reaction
processes in methanol steam reformers.