In the process of sulfuric acid production, the fluid flow rate is a seemingly insignificant parameter but it is crucial for the safety, efficiency and lifespan of the equipment. It should neither be too fast nor too slow - the precise control of this "degree" directly determines whether the equipment can achieve long-term, low-cost and high-efficiency stable operation.

1. Sulfuric acid pipelines

Concentrated sulfuric acid has extremely strong corrosiveness, but the corrosion rate of it on carbon steel and other materials is not constant. Under suitable conditions, concentrated sulfuric acid can form a dense iron sulfate protective film on the inner wall of the pipeline, providing protection. However, this protective film is very fragile - it is affected by factors such as fluid temperature, flow rate, and oxygen content.

If the flow rate is too low, local corrosion and accumulation occur, manifested as local perforation and leakage at the bottom and low-angle elbows of the pipeline. Therefore, the flow rate of sulfuric acid pipelines should not be lower than 0.3 m/s.

If the flow rate is too high, erosion and scouring occur, the protective film is damaged, and corrosion accelerates. Therefore, for carbon steel pipelines transporting concentrated sulfuric acid, the flow rate generally should not exceed 1.0 m/s; for stainless steel pipelines (such as 304, 316L), the flow rate can be appropriately increased, but generally it is recommended to control within 1.0 to 2.0 m/s; for media containing solid particles, the flow rate even needs to be controlled below 1.5 m/s to prevent particles from intensifying erosion.

In addition, in key equipment such as acid distributors, excessive flow rate will also cause severe impact on the liquid level, resulting in fluctuation and uneven acid distribution, directly affecting the absorption efficiency.

2. Heat exchangers

In heat exchangers and other equipment, the uniformity of fluid velocity distribution is the core factor determining the heat transfer efficiency. CFD simulation results show that by optimizing the design, the internal velocity field of the heat exchanger can be made more uniform, and the heat transfer effect can reach 2 to 3 times that of the traditional design.

If the flow rate is too slow, insufficient turbulence is formed in the pipe, the boundary layer thickens, and the total heat transfer coefficient decreases, resulting in a decrease in heat transfer efficiency.

If the flow rate is too high, the gas residence time is shortened, and the gas has not completed heat exchange before flowing out of the heat exchanger, resulting in insufficient effective heat exchange time; although the convective heat transfer coefficient increases, the medium does not have time to transfer heat, and the outlet temperature does not reach the target, and the unit air volume heat transfer efficiency decreases. Resistance almost increases by 1.8 to 2 times as the gas velocity increases, and if the gas velocity is increased by 20%, the resistance increases by about 40%, and the SO2 blower current actually becomes electricity cost. The excessive velocity in the shell side can also induce fluid-induced vibration in the tube bundle, causing wear and fatigue fracture at the perforated plate holes.

Recommended range: The gas-side fluid velocity between the tubes and the tube bundle is generally 6 to 9 m/s; for the acid side and water side, the values are determined based on the corrosion, scaling and pressure drop boundaries of each medium, with the acid side typically at 1 to 2 m/s, and the water side approximately 0.5 to 1.5 m/s, as per the design of the equipment manufacturer. Using efficient structures such as scaled tubes and hollow rings can compress the heat exchange area by 30% to 50% under the same load.

3. Absorption towers

The gas velocity of dryers and first and second absorption towers, going upward is flooding and acid mist, going downward is absorption rate and investment. It adheres to the narrow gate between the absorption efficiency and environmental protection red line.

If the flow rate is too slow, the gas phase turbulence weakens, the gas film thickens, the mass transfer coefficient decreases, and SO3 cannot be "caught" in time and passes through the tower; low gas velocity often accompanies the expansion of tower diameter, if the spray volume does not keep up, the packing is not evenly wetted, the effective mass transfer area does not increase but decreases, and the absorption efficiency of SO3 decreases. The mist catcher is also "not allowed to be slow" - the wire mesh mist catcher captures droplets by inertial collision, and when the gas velocity is too low, the droplets have insufficient inertia and directly pass through the mesh.

If the flow rate is too high, when the operating gas velocity reaches the flooding point gas velocity, the liquid holdup rises sharply, the pressure drop increases sharply, and the liquid cannot be discharged, resulting in flooding; the acid absorption tower usually takes 50% to 80% of the flooding point gas velocity, while the flooding point gas velocity decreases with the reduction of spray density and increases with the increase of acid temperature. Therefore, "high gas velocity + low spray density" is the most dangerous working condition combination. Even if there is no flooding, the amount of entrained mist will increase sharply. When the gas velocity is too high, the liquid droplets are blown apart and break, and then re-enter the gas flow as smaller droplets, resulting in a decrease in the deswelling efficiency instead of an increase.

The recommended operating gas velocity for fiber deswimmers is 0.1 - 0.16 m/s (one suction), and for wire mesh deswimmers, it is 1.5 - 3 m/s.

The recommended empty tower gas velocity for drying towers is 0.7 - 1.0 m/s, and for absorption towers, it is 0.8 - 1.5 m/s.

4. Converter

The converter is the only place where "velocity" is directly converted into "time". The residence time τ ≈ the void volume of the catalyst bed / the gas volume flow under the operating condition. It is the reciprocal of the space velocity: for each increase of one step in the space velocity, the residence time is compressed by one step.

If the velocity is too fast, the gas passes through the bed, and SO2 has no time to complete the oxidation on the active sites, resulting in a low conversion rate and SO2 exceeding the limit in the tail gas. Even worse, the bed is "damaged": when the apparent gas velocity exceeds the starting critical value of the catalyst particles, the particles are carried by the gas flow to migrate and wear against each other, the bed integrity is destroyed, the catalyst is pulverized, and the pressure drop rises sharply; while the pressure drop increases according to ΔP ≈ ΔP0·(W/W₀)^1.8, the resistance rises and further worsens the working condition, forming a "high gas velocity - pulverization - high pressure drop" vicious cycle.

If the velocity is too slow, the converter relies on the bed's own resistance to "even out" the uneven inlet gas: if the gas velocity is too low and the bed pressure drop is too high, the distribution pressure drop accounts for less than the required proportion, the gas distribution is extremely sensitive to the inlet structure, cross-flow, dead zones, and short circuits occur simultaneously, some catalysts "do not have enough food" and some "are overloaded". At the same time, the gas volume passing through per unit time is locked, and the production capacity is wasted in vain. At low gas velocities, reaction exothermism is prone to accumulate locally in the bed. The residence time must simultaneously fall within the "enough for reaction and able to dissipate heat" window. The recommended bed velocity of the converter is controlled at 0.7 - 1 m/s.