Single arrangement vs dual arrangement in sulphuric acid plants

Introduction

In recent years the sulphuric acid industry has seen certain trends that have caused them to adapt to the associated changes. The most predominant trends are the pressures to have improved energy efficiency and to keep production costs low, while battling the overall growing demand for sulphuric acid.

This has lead to plant designers and operators of sulphuric acid plants to go from medium size operations to ultra large scale plants. (For the purposes of this article, “ultra large scale plants” is defined as plants that exceed 4000 MTPD H2SO4.)

In addition to addressing the issues of the aforementioned trends, these ultra large scale plants benefit from having investment cost savings (CAPEX vs. MTPD of H2SO4) and lower operational costs (OPEX).

However, along with these benefits, comes one very important challenge: the impressive equipment scale is already reaching the available single equipment size limitations on some components (e.g. – for boilers, diluters, heat-exchangers, pumps, main blowers and more) and require either dual or special adapted solutions.

Howden has engineered a solution, the SF 22, to overcome these plant design challenges for high efficient operations and scale limitations.

Situational analysis: Single vs. Dual arrangements

As previously stated, plant designers and operations of sulphuric acid plants around the world are facing challenges associated with the recent industry trends; primarily rising energy costs, rising competition and rising demand for sulphuric acid.

All the key components in the acid plant have to adapt to these developments and support the plant designers and customers with suitable solutions.

One of the biggest components that can have the biggest impact is the main blower package. Due to their high reliability and availability, the main blowers are used in an un-spared single blower arrangement which combines simple installation with easy operation and requires low investment costs.

With the increased pressure to expand production capacity; blower suppliers have been successful in improving the maximum flow per frame size. In fact, since the 1970s, capacity has been increased by more than 43%, allowing the customer to reap the benefits associated with increased production.

With the increase in high flow capabilities, the main blower series in acid plants have exceeded the available single equipment size on the main blowers and some other key components including but not limited to boilers, diluters, heat-exchangers, and pumps.

As a result, plants have overcome this design dilemma by either using a single train arrangement wherever technically feasible or using a dual arrangement of two 50% components in parallel for all cases where the components exceed the available single equipment size. (figure 1)

Figure1: Sulfuric Acid Plants Single / Dual Blower Arrangement

Figure 1: Sulfuric Acid Plants Single / Dual Blower Arrangement

While using a dual arrangement may be a temporary fix, it is not the most optimal solution for various reasons. The first being that the dual arrangement requires a complicated piping arrangement including shut-off values, additional monitoring and auxiliary piping in order to ensure a safe operation under all conditions including start-up and shut down procedures. Due to the large scale of equipment, it is vital in acid plants to locate the components as close as possible to minimize the interconnecting piping length, therefore making available space very limited.

Although the dual arrangement is based on two parallel machines of smaller frame-size, the overall space requirement is about 40% higher when compared to a single machine solution. (figure 2)

An additional obstacle of the dual arrangement and increased piping requirements is the pipe work needs to be properly engineered in order to avoid accessibility and maintenance problems.

Figure2: Comparison of Blower Space Requirements

Figure 2: Comparison of Blower Space Requirements

A second advantage of using a single arrangement versus a dual arrangement is the investment costs associated with each. With a dual blower arrangement, the investment costs of each smaller blower are duplicated, along with the additional expenses for the required equipment like valves and auxiliary piping. Therefore, the single unit offers a substantial cost savings.

There are some advantages to using a dual arrangement; the primary being flow redundancy.

If both blowers in a dual arrangement operate synchronously they act as one ‘mega blower.’ While the plant is operating between 60% and 100% plant loads, each individual blower is also operating in its own 60%-100% blower capacity at the same pressure rise but half of the total plant flow.

Things are drastically different if only one blower is in operation. Under this condition, all the flow into the plant has to be transported by the one blower in operation. Even if the blower operates at 120% blower capacity, this is equivalent to only 60% of the plant flow rate. As the plant and the related equipment is designed for the full flow capacity the related pressure drops at 60% plant flow rate is very low. The low pressure rise is compensated with the lower efficiency. Due to this low efficiency operation, the motor power typically is limiting the available flow rate to slightly more than 60% plant flow rate.

Therefore, a benefit to dual arrangement is some redundancy, possibly upwards of 60% plant load.

Based on the above outlined advantages, a single blower arrangement has come to be the preferred choice for sulphuric acid plants. (figure 3)

Figure3: Single vs. Dual Blower Arrangement

Figure 3: Single vs. Dual Blower Arrangement

 

Even with all the advantages and clear preference from the industry, one obstacle remained; for high end plants exceeding 4000 MTPD the single arrangement was not available from the blower market.

Howden has changed that with the SF22.

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