A NOVEL LAYOUT FOR INTER-BED COOLING IN SULFURIC ACID PLANTS

Abstract

In a converter for the catalytic oxidation of SO.sub.2 to SO.sub.3 in a sulfuric acid plant, which comprises a boiler section for the cooling of process gas between catalytic layers (beds), one or more water tube boilers (inter-bed boilers) having horizontal or slightly sloped tubes are used to cool the process gas between the catalytic layers (beds) in the converter. Each water tube boiler is provided with a process gas side bypass to control the temperature to the down-stream catalyst layer.

Claims

1. A converter for the catalytic oxidation of SO.sub.2 to SO.sub.3 in a sulfuric acid plant, said converter comprising a boiler section for the cooling of process gas between catalytic layers (beds), wherein one or more water tube boilers (inter-bed boilers) are used to cool the process gas between the catalytic layers (beds) in the SO.sub.2 converter of the plant, and wherein each water tube boiler is provided with a process gas side bypass to control the temperature to the downstream catalyst layer.

2. Converter according to claim 1, wherein each inter-bed boiler has horizontal or slightly sloped tubes, and the gas flow is vertical.

3. Converter according to claim 2, wherein the tube slope is in the range 0 to 15 from horizontal.

4. Converter according to claim 2, wherein the tubes have one or more passes.

5. Converter according to claim 1, wherein the inter-bed boiler has vertical tubes, and the gas flow is horizontal.

6. Converter according to claim 1, wherein the tubes of the water tube boilers are bare, fitted with fins or have a combination of finned and bare tubes.

7. Converter according to claim 1, wherein at least one inter-bed boiler is placed within the converter shell.

8. Converter according to claim 1, wherein at least one inter-bed boiler is placed outside the converter shell.

9. Converter according to claim 1, comprising a damper on the cooled process gas line between the inter-bed boiler outlet and the mixing point with hot bypassed process gas.

10. Converter according to claim 1, comprising a damper for the process gas going to the inter-bed boiler, said damper being located between the outlet of the upper catalyst layer and the inlet to the inter-bed boiler.

11. Converter according to claim 1, comprising a mixer downstream the mixing point of hot bypassed and cooled process gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The figures show an SO.sub.2 converter in which the inter-bed cooling is carried out by an inter-bed boiler and a hot bypass followed by a mixing device. More specifically,

[0043] FIGS. 1A-1C show shows an inter-bed boiler having horizontal or slightly sloped tubes, and the gas flow is vertical in downwards direction, and

[0044] FIGS. 2A-2C show an inter-bed boiler having vertical tubes, and the gas flow is horizontal.

DETAILED DESCRIPTION

[0045] In FIG. 1, the inter-bed boiler layout with horizontal (or slightly sloped) tubes is shown in two side views (A and B) and as top view (C). Unconverted process gas comprising SO.sub.2 enters the SO.sub.2 converter (1) through the inlet nozzle (2) located at the top of the converter.

[0046] The process gas flows downwards through the upper catalyst layer (3), where a fraction of the SO.sub.2 is converted into SO.sub.3, increasing the process gas temperature. The hot, partly converted process gas flows down to the inter-bed cooling section, where a horizontal water tube boiler (4) is located. To increase the process gas velocity around the inter-bed boiler tubes, the cross sectional area for process gas flow is significantly decreased.

[0047] The hot process gas passing through the inter-bed boiler is cooled to a temperature below the inlet temperature to the downstream catalyst layer (9). To reach the desired catalyst inlet temperature, a fraction of the hot, partly converted process gas is bypassed the inter-bed boiler by passing through the hot bypass damper (6) via the hot gas line (5) to the mixing chamber (8), where the hot process gas is mixed with the cooled process gas. To enhance the controllability of the streams being cooled or bypassed, the cooled gas is also provided with a damper (7). This damper can either be located upstream or downstream of the inter-bed boiler, depending on the actual layout of the inter-bed cooling section. The damper positions are controlled from the outside of the converter via a long rod between the damper plate and the actuator. An active sealing (e.g. purging air) between rod and converter shell is required to avoid process gas escaping to the atmosphere. In the figure, the mixing chamber (8) is a duct located at the periphery of the converter shell, such that the total height of the inter-bed cooling section is minimized. The well mixed, partly converted process gas leaves the outlet of the mixing chamber and flows down to the lower catalyst layer for further SO.sub.2 conversion.

[0048] Normally the process gas distributes evenly across the entire cross section of the lower catalyst layer within the first 10-20 cm of the catalyst layer. Alternatively, distributor plates, guide vanes etc. can be installed between the mixer outlet and the lower catalyst layer.

[0049] In FIG. 2, also shown in two side views (A and B) and as top view (C), the inter-bed boiler has vertical tubes, and the gas flow is horizontal. The advantage of vertical tubes in the boiler is that the risk of steam pocket formation and thus metal overheating is avoided, independent of the flow regimes inside the boiler tubes. The disadvantage is a significantly higher inter bed boiler, requiring more converter shell material and thus increasing the total cost of the solution.

[0050] As in FIG. 1, the unconverted process gas enters the SO.sub.2 converter (1) via the inlet nozzle (2) and is partly converted in the upper catalyst layer (3). The inter-bed boiler (4) is vertically oriented, and the process gas damper (7) is most conveniently located in the hot section. The bypass damper (6) passes hot process gas via the hot gas line (5) to the mixing point (8) of the cooled and uncooled process gas. In this layout, the mixing chamber/duct is vertical and connected to the converter shell, but it could also be a circular duct around the periphery of the shell as in FIG. 1. The well mixed, partly converted process gas leaves the mixing chamber and flows down to the lower catalyst layer (9).

[0051] The process gas bypass and the mixer can be made in a compact design to fit internally in any SO.sub.2 converter. The mixer can have many specific layouts, such as a curved duct with or without internal structures to increase turbulence. The mixer can also be located outside the SO.sub.2 converter shell, but that is not recommended as it increases the risk of cooling the mixing chamber surfaces to a temperature below the sulfuric acid dew point with the consequence of sulfuric acid condensation and rapid corrosion.

[0052] The use of water tube boilers for inter-bed cooling in a WSA plant provides a reduction in plant cost and complexity compared to the traditional steam superheaters used for inter-bed cooling. In order to control the gas temperature outlet from the inter-bed boiler, a by-pass is required on the gas side, and in order to minimize temperature variation at the inlet to the lower catalyst layer, an efficient mixer is required.