Jan-2014

Field experience with a Claus furnace checker wall

A new design of Claus furnace checker wall delivers improved mechanical performance and process flow characteristics.

JEFFREY BOLEBRUCH, Blasch Precision Ceramics
MOSSAED Y AL-AWWAD, Saudi Aramco
MENG-HUNG CHEN, CPC Corporation

Article Summary

Depending on the licensor, there are a number of different architectures that may be placed in the Claus furnace to influence performance. Each has its pros and cons. Regardless of design, mechanical stability has traditionally been problematic.

Blasch was tasked to develop a checker wall that would be easy to install, mechanically stable under all conditions, allow for design with varying degrees of open area, and could incorporate the use of an integral manway, if desired. Concerns at this point were strictly mechanical; there were no requests for mixing or flow management. The goal was simply to put in a checker wall that could survive a campaign intact.

The result was the Blasch HexWall checker wall. The blocks are designed to be stacked dry and are mechanically engaged through a series of tabs and slots, so the wall is quite stable, even when several metres in diameter. No mortar is used between the blocks, allowing the assembly to accommodate the thermally driven expansion and contraction that comes with reaction chamber operation. The blocks may also be reused.

The initial checker wall blocks were designed to be 9in wide, the same as a row of bricks in the hotface. As larger furnaces were encountered, additional designs utilising longer blocks were developed so as to retain a reasonable aspect ratio of wall width to height. The largest walls (>5m) utilise 18in wide blocks.

The walls may either be erected into a slot in the existing lining, or installed against the hot face brick with a course of brick on either side to lock it in. The interlocking nature of the blocks makes provisions for expansion management straightforward and effective. The openings in the blocks are round, so that any portion of the block not directly supported by other blocks is in the form of an arch, and, as such, has greater mechanical stability than a flat span.

Over the next several years, as the stability and durability of the checker walls was proven out, some operators, who previously spent their time rebuilding walls, began to question what they were actually designed to do — other than remain standing for an entire campaign.

After considerable consultation with process licensors, engineering companies, end users, and miscellaneous stakeholders, it became apparent that almost everyone agreed that some combination of time, temperature and turbulence were the variables that played the largest roles on the process constituents. That makes sense, as the ‘three Ts’ of reaction kinetics, and their effects on process efficiency, are well documented.

Blasch was determined to find a way to use the HexWall checker wall to influence these parameters. We selected turbulence, thinking that if we could improve mixing in the furnace, we could improve efficiency, but soon learned that they are all inter related, and we ended up learning a lot more about residence time, residence time distribution, and plug flow vs stirred tank design along the way.

A vectoring hood was developed that would fit into the outlet ends of the existing blocks, and could be oriented and secured in the field in such a way as to redirect, or ‘vector’, the downstream flow allowing each individual block in the assembly to contribute to the creation of the overall desired flow.

The vortex was initially selected because it was felt that this configuration made the best use of the furnace volume and created a flow pattern that yielded a long path length with a very tight residence time distribution. This variation of the HexWall checker wall was christened the VectorWall.

There are now nearly two dozen VectorWalls installed worldwide, as well as approximately 100 of the earlier HexWalls. Highlighted below is a pair of VectorWall installations where it was possible to contrast stability related details from before and after the installation

Field experience with the VectorWall checker wall: mechanical
One of the larger diameter VectorWalls done to date is installed in Saudi Aramco’s Berri Gas Plant, in its SRU200. This portion of the article is excerpted from a paper written by Mossaed Al-Awwad, Heat Transfer Engineer for the Aramco Consulting Services Department (CSD). Blasch gratefully acknowledges Saudi Aramco for the following data.

Saudi Aramco’s Berri Gas Plant
The current refractory system used on the internal reaction furnace lining in the Saudi Aramco sulphur recovery unit (SRU) is a dual layer of fire bricks as a hot face layer and a castable layer as back-up with stainless steel anchors to support the castable refractory. Part of this refractory system’s internal features is a checker wall that must be incorporated into the refractory hot face lining design. The existing checker wall system that was used for over 20 years frequently failed due to improper design and installation techniques. This article addresses the experience of a checker wall refractory system, the modified system and its impact on the reaction furnace operation.1

Refractory lining description
The normal furnace operating temperature at Saudi Aramco SRU units is 1800-2000°F (980–1090°C); however, the hot face material must be capable of withstanding temperatures of 2500-3000°F (1370–1650°C) which can occur during start-up firing of natural gas. All SRU reaction furnace linings should utilise at least 90% alumina hot face material.

Brick linings at the hot face layer are more durable than castable or plastic ram materials. The initial installation of brick may be slightly more expensive and require a highly skilled installer, but brick normally provides a longer lining life and requires less maintenance. Part of this refractory system’s internal lining is a checker wall that is made of a high density alumina brick system that has good high temperature strength to sustain the harsh environment.