Feature
Acid plant debottlenecking strategies By: Guy Cooper, P. Eng.; Dr. Andres Mahecha-Botero, P. Eng.; Dr. Werner Vorster, P. Eng.; Neal Londry, E.I.T.; NORAM Engineering and Constructors Ltd., Vancouver, Canada.
Take advantage of your assets
NORAM receives many requests for strategies to increase the capacity of an acid plant. Each plant has a unique set of debottlenecking solutions which can result in capacity increases between 10-30% or even higher. Compared to the cost of a new acid plant, a plant upgrade taking maximum advantage of existing assets is a much more economical approach. And if equipment needs to be replaced for maintenance reasons, so much the better, because the new equipment can be sized for the target capacity.
Play to your (SO2) strengths
One of the first things we ask of a client who is looking for more capacity is to provide us with the current operating data, including SO2 concentrations. A plant operating with an SO2 strength to the first converter pass of, say, 10%, could achieve a 15% increase in capacity by increasing the gas strength to 11.5% SO2. Of course, emissions and heat exchange equipment will need to be assessed and may need upgrades/replacement. Cesiumpromoted and recently introduced highactivity vanadium catalysts make it possible to accommodate increased SO2 loadings within the existing converter. Heat removal from the sulfuric acid loop can be handled with acid side upgrades. More on that later.
Checking the pressure (drops)
We also ask for recent pressure surveys. Routine pressure surveys (say, monthly) are one of the best ways to track plant performance and equipment condition. Typically, measurements are taken across all gas-side equipment with a digital manometer and recorded with the production rate, blower rate/rpm, and the SO2 strength. In one plant, we found an extremely high pressure drop across a cold gas exchanger due to fouling and the design. A replacement cold exchanger with a hot sweep feature (see Fig. 1) designed for low pressure drop contributed to that plant achieving and maintaining a 20% increase in capacity. In another plant, we identified high pressure drops in the towers and addressed this by upgrading to a low-pressure drop packing (see Fig. 2). Replacing high pressure drop equipment, even if more capacity is not immediately required, will often improve overall plant performance and lower blower energy costs. PAGE 24
Fig. 2: NORAM HP™ Low Pressure Drop Saddle.
Metallurgical acid plant opportunities
Temperature limits for catalysts restrict SO2 concentration to the first pass to a maximum of about 12% with standard catalyst, and 13-14% with cesiumpromoted catalyst operating at a lower inlet gas temperature. Smelters with oxygen enrichment may produce an SO2 gas with up to 30% strength prior to air dilution upstream of the acid plant. NORAM has designed pre-converter systems for such plants, which take a portion of the high strength gas flow, dilute it, then convert two thirds of the SO2 to SO3 which is then removed as acid. The low-concentration SO2 residue gas is mixed with the highstrength gas upstream of the existing acid plant. The preconverter system uses standard processing steps found in every acid plant, including a converter, a gasto-gas heat exchanger, and an absorbing tower. NORAM has designed pre-converter systems to increase capacity by 20-35%.
Fig. 1: NORAM Cold Exchanger with Hot Sweep.
Blower enhancements
One may be tempted to go out and purchase a bigger blower to increase gas flow through the plant. A word of caution here: pressure drop increases with the square of the flow. So a 20% increase in flow results in a 44% increase in pressure drop corresponding to much larger energy requirements. A large pressure increase may cause mechanical challenges with downstream equipment, in addition to the significant price tag for a full-flow, highhead 200” W.C. (5,000 mm) blower and drivers. However, there are some blower techniques that we use for modest increases in flow and capacity. For a sulfur burning plant with the blower taking suction on the dry tower, rerouting some ducting can allow the blower to take suction on the air filter and then discharge the air into the dry tower. This arrangement has the advantage of cooler air going into the blower, allowing more air flow for the same horsepower and an increased compression ratio (discharge/
suction pressure), which also improves performance. Flow and corresponding capacity increase of 3-7% are possible for this ducting reconfiguration. There is a slight loss of energy efficiency per ton of sulfur burned because the blower’s heat of compression now is removed by the dry tower acid cooler instead of the waste heat boiler, but the increased production actually results in a net increase in steam produced compared to that produced before the upgrade. Booster blowers designed for full gas flow and low head, less than 50” W.C. (1250 mm), are sometimes used for increased capacity by supplementing a main blower. They can be located either downstream of an interpass tower (stiffening of the candle housing shell may be necessary) or take suction on the air filter and supplement a blower taking suction on the dry tower. The increased capacity offsets the increased energy operating cost of a second blower.
High pressure drop sulfur furnace and boiler?
For plants where the sulfur furnace and boiler experience a high pressure drop, we have the solution for you: A furnace and boiler bypass system. With this ducting arrangement, a slip stream of cold air bypasses the sulfur furnace (increasing the gas strength and furnace temperature), mixes with the hot furnace gas that bypasses the waste heat boiler, and combines with the main process stream downstream of the boiler. A schematic is shown in Fig. 3. This simple arrangement gives several benefits. First off, there is a reduced pressure drop which varies to the second power with flow. For example, a 10% bypass results in a 21% reduction in the total furnace/blower pressure drop. If you have a total furnace and boiler pressure drop of 20” W.C. (500 mm), there would be a pressure reduction of 4.2” W.C. (105 mm) for the 10% bypass. For a 2,400 TPD acid plant, this could add 35 TPD of production. The reduced flow to the sulfur furnaces increases the residence time, which improves sulfur combustion and reduces the chances of uncombusted sulfur. The higher temperature improves the thermal driving force (LMTD) of the boiler, resulting in better heat transfer. The furnace refractory needs to be checked that it can accommodate the higher furnace temperatures. And as a final benefit, this arrangement may permit replacement of the maintenance-prone jug valve controlling the hot furnace gas split with a stainlessSulfuric Acid Today • Fall/Winter 2021
