Mar-2010

Rethinking refractory design

Near net shape refractory forming technology and its use to create significant process improvement. The vast majority of refractory, by volume, used in the refining and petrochemical sectors is in the form of brick or castable.

Jeffrey J Bolebruch
Blasch Precision Ceramics, Inc

Article Summary

These allow for the greatest flexibility in installation, but greatly limit the ability to develop innovative component and assembly designs that work to the inherent strengths of refractory materials, which can, in turn, provide significant process improvement. One such example is the use of a checkerwall in reaction furnaces to improve various process parameters.

The Claus Sulphur Recovery Process is the primary commercial method of treating the hydrogen sulphide (H2S) generated as a result of the removal of sulphur from crude oil, natural gas, and coal. In the Claus Process, the hydrogen sulphide gas is combusted with air or oxygen in a thermal reactor in order to oxidise it and begin the process of converting the acid gas to sulphur or other commercially saleable products.

The combustion chamber, or thermal reactor, consists of a horizontally arranged pressure vessel ranging in size from a less than a metre in diameter, to ones several metres across and a dozen or more metres long. The reaction furnaces typically have a burner mounted in one end and a shell and tube heat exchanger acting as a waste heat boiler in the other.

The thermal reactor is completely lined with refractory to protect against the effects of temperature and corrosion. This generally consists of a layer of high alumina brick over a layer of insulating backup brick. In addition, the face of the waste heat boiler and the entry to the boiler tubes must be protected as well.

There may often be a refractory wall (or walls) located in the thermal reactor at some point (or points) between the burner and the waste heat boiler that may serve a number of functions, depending on design, such as enhancing mixing (and therefore improving destruction efficiency of unwanted compounds); increasing residence time (in applications utilising multiple solid baffle walls where unused reaction furnace volume can be eliminated); or protecting the tubesheet from radiant heat (or hot spots); depending upon the particular licensor.
 
Checkerwalls/baffle walls
Checkerwalls are generally constructed from standard refractory nine-inch bricks with numerous gaps left in the wall itself, or from cast ceramic cylinders, stacked one atop the other. The intent is for the process gas to flow through the walls. They may often, for a variety of reasons, fall over at some point during the campaign.

 Baffle walls are constructed as solid brick partial walls, obscuring alternating portions of the flow path at different points in the reaction furnace. The intent of these walls is to force process gas to mix by moving it around in the reaction furnace.

While nine-inch bricks work fine for lining vessels, using them to fabricate a wall with something like 50% of the supporting structure removed, or a wall that is only partially anchored in place, can be problematic. Further, in the case of the checkerwall, each unsupported horizontal brick forms a flat span that is then affected by the forces of gravity, and at high temperatures, can creep, or sag. When that occurs, the joints where the bricks come together begin to come apart.

Stacked cylinders may appear to be more stable at first glance, but they are not precision parts, and rather than being stacked, are actually point loaded at the places they happen to touch one another. This can lead to severe point loading conditions and stresses in excess of the ability of the material to support them. Further, cylinders are free to slide against one another, and in the event of a delayed ignition, may be readily blown over.

Missing or damaged checkerwalls or baffle walls can impede the flow of process gas, or otherwise negatively affect the process parameters of the Claus unit, or of whatever type of incinerator in which it is used.

The Blasch HexWall Checkerwall was designed in response to requests from a number of users of the Blasch prefired hex head or square head ferrules for WHB tubesheet protection, and was intended to eliminate the problem of creep and instability due to the presence of unsupported spans; to ensure the complete support of, and positive engagement between, individual blocks; and to provide for 360-degree anchoring. The original HexWall design was intended to be a simple, stable wall that would act as a checkerwall, but was upgraded along the way, as a result of additional input from the field, to provide for mixing enhancement and to provide the same functionality as a series of solid baffle walls.

It is important to remember that refractory does not perform nearly as well in tension as it does in compression, so that the presence of unsupported spans should be avoided if at all possible, and, ideally, each individual piece should be fully supported by the ones under it. Further, with respect to checkerwalls, the possibility of delayed ignition, and resulting overpressure, dictates that enough mechanical engagement should be present to ensure that the blocks cannot move laterally against one another.

These blocks may then be stacked tightly, one atop the other, until a wall is formed. Please note that each block is supported on each of their six sides, and the open area of each is an arch, which puts the refractory into compression, where it is up to five times stronger than in tension.

This design, for all its engineering detail, is actually quite flexible, simply requiring a larger or smaller number of blocks, depending on the diameter of the vessel. A subsequent modification was made to include half blocks so that a manway may be added, if desired, although since the wall is laid up without mortar between the blocks, it may be disassembled for maintenance events, and reused, if desired.  

Results from the field have been exceptional, with no catastrophic wall failures to date, even in units as large as 10 feet in diameter, and running in oxygen enriched conditions that have driven temperatures above 1,700°C.