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Jul-2013

Putting a better spin on combustion

How the incorporation of a mixing checkerwall can increase acid gas enrichment plant capacity, eliminating the need for additional train

Jeff Bolebruch, Blasch Precision Ceramics
Meng-Hung, Chen CPC Corporation
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Article Summary
Operators of large refractory lined combustion chambers, whether used as primary reaction furnaces, thermal oxidisers in tail gas units, or miscellaneous incinerators, all rely on stable, consistent operation, and the efficiency of the combustion reaction taking place within the vessel to sustain economical operation, or to keep other areas of their plants operating normally.

Often, however, these pieces of equipment can be subject to variable feeds, periodic downstream upsets, and ignition problems; stable consistent operation alone can be a challenge. Add to that the limitations imposed by reaction kinetics, compounded by the penalties of inefficient combustion, the result easily leads to very costly consequences.

Frequently, some type of structure will be placed in the furnace in an attempt to influence process efficiency by creating specific conditions in certain parts of the furnace, or some type of mixing downstream. These structures may be constructed from standard firebricks with every other brick removed, or dry stacked cast refractory cylinders.

The results, however, are generally short of optimal, both in terms of mechanical stability, and in terms of process impact.

Mechanical considerations
Stability of these large, partially open walls can be problematic. The effects of high temperature creep and delayed ignition can each negatively impact the stability of these walls in two very different, but equally catastrophic, ways. Consequently, the goal is to address stability issues with a design that would be refractory friendly; stable within the plane of the wall during times of high temperature excursions and thermal cycling, and resistant to lateral movement axial to the reactor during times of upset.

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of stress, which is exacerbated in high temperature applications where the material is close to its melting point.1 Creep may be mitigated through the use of appropriate structural design. Using arches in place of unsupported flat spans will ensure that the material goes into compression as it deflects, not into tension.2 The Blasch VectorWall mixing checkerwalls are constructed from a series of hexagonal blocks that stack together and remain fully supported on all six surfaces, while the openings are round, and form a series of arches on the upper surfaces.

A delayed ignition occurs when a greater than necessary volume of a combustible gas accumulates in a confined space prior to ignition. A resulting pressure wave then impacts the wall, which is typically located just downstream of the burner. This force can easily blow structurally questionable walls over. Stacked cylinder walls, for example, are loosely stacked in the furnace and quite susceptible to this. The Blasch VectorWall mixing checkerwalls are formed with a series of tabs and slots on each of the six external surfaces that mechanically engage to resist lateral movement.

Process improvement
Combustion efficiency is often defined in terms of the ‘3 T’s’: Time, temperature, and turbulence. Time refers to the optimal amount of time that the reactants need together in order to completely react. This is not necessarily the amount of time in the furnace, but the amount of time properly mixed so that the required reactions can proceed. Temperature refers to the optimal temperature to drive the reaction taking place, and the assumption is that this temperature will exist consistently throughout the furnace. Turbulence refers to the mixing that must occur to put the reactants in intimate contact and when and how completely it occurs.

In light of this, once the stability of the HexWalls was demonstrated, inquiries began to come in regarding the potential for process improvement and how the blocks that form the HexWall might be configured to impact that. It was determined that the addition of a directional ‘vectoring’ tile at the outlet end of the block could change the direction of the gas exiting the wall, and by installing them onto all of the blocks in the wall and orienting them so that they acted in unison, a large vortex could be generated. It was felt that this type of flow would provide good mixing and a consistently long path length, and should improve combustion efficiency. Initial installations bore this out, but soon questions arose as to what an optimal mixing solution for any of a number of different furnace configurations might look like.

This begs the critical mixing related question. What is the optimal mixing scheme in this or other types of reaction furnaces, and what is the mechanism for determining it?

Residence time distribution
The residence time distribution of a reactor can be used to compare its behaviour to that of two ideal reactor model, the plug flow reactor (PFR) and the continuous stirred tank reactor (CSTR), otherwise known as a mixed flow or back flow reactor. This characteristic is important in order to calculate the performance of a reaction with known kinetics. The residence time distribution of a chemical reactor is a probability distribution function that describes the amount of time a fluid element could spend inside the reactor. Chemical engineers use the residence time distribution to characterize the mixing and flow within reactors and to compare the behaviour of real reactors to their ideal models. This is useful, not only for troubleshooting existing reactors, but in estimating the yield of a given reaction and designing future reactors.3

Fluid going through a plug flow reactor may be modelled as flowing through the reactor as a series of infinitely thin coherent ‘plugs’, each with a uniform composition, traveling in the axial direction of the reactor, with each plug having a different composition from the ones before and after it. The key assumption is that as a plug flows through a PFR, the fluid is perfectly mixed in the radial direction but not in the axial direction (forwards or backwards). As it flows down the tubular PFR, the residence time of the plug is a function of its position in the reactor. This is more appropriate in processes that process homogeneous feeds.4
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