Chemical & Petrochemical Applications

Heavy Fuel Oil Blending with Pour-Point Depressant

Process Description

The refinery produces a heavy fuel oil (HFO) grade for marine and industrial boilers. The base stock has a kinematic viscosity of 500 cP at 50 °C and a pour point of 24 °C — borderline for winter shipping. To meet a customer specification of pour point ≤ 9 °C, the refinery injects an ethylene-vinyl acetate (EVA) pour-point depressant at 800 ppm into the transfer line between the hydrotreater and the storage header. Flow rate is 50 m³/h, line pressure is 8 bar, and operating temperature is 60 °C.

The dosage ratio is 1:1250 — 50 m³/h of base stock against 40 L/h of neat additive. The additive is dosed through a positive-displacement pump from a heated day tank, then fed into the suction side of the transfer pump. The challenge is twofold: the additive is highly viscous (8,000 cP at 60 °C) and has a strong tendency to form a separate phase in the low-shear region of the pipe, and the base stock cools rapidly after the hydrotreater, increasing viscosity and reducing the molecular mobility of the EVA.

Challenge

The original installation used a four-element SV-type mixer in DN150. While adequate for moderate-viscosity blending, the SV geometry did not impose enough shear to disperse the viscous additive into the heavy base stock. Periodic samples at the storage header showed additive concentration swinging from 400 ppm to 1,400 ppm, with a coefficient of variation of 28%. Worse, undispersed additive occasionally accumulated in the bottom of the transfer line, where the cooler wall temperature (45 °C) caused localized solidification. The result was a pipeline blockage once every six to eight weeks, requiring steam-out and a four-hour production interruption.

Solution

We replaced the SV mixer with an SK-type mixer in DN150 with 12 elements, installed vertically immediately downstream of the additive injection point and 2 meters upstream of the transfer pump. The SK geometry's multi-strand construction creates alternating high-shear and low-shear zones as the fluid passes through each element. The high-shear zones break up the additive stream into thin films and droplets, while the low-shear zones allow those droplets to merge and distribute across the pipe cross-section.

The mixer housing is carbon steel with a 3 mm SS316L internal liner at the wetted surfaces — adequate for the hydrocarbon service and more economical than a full alloy build. The elements are SS316L, welded to a central removal rod that allows extraction through the top flange for inspection without breaking the line. A heating jacket is fitted to the housing to maintain the additive and base stock above 55 °C during low-flow conditions.

Results

Since commissioning, the refinery has operated for 18 months without a single pipeline blockage. Weekly samples at the storage header now show additive concentration within 720-880 ppm (CoV 4.2%), well within the customer's specification of 700-900 ppm. The pour point of the finished HFO has stabilized at 6-8 °C, and the refinery has reduced its PPD consumption by 6% because the more uniform dispersion achieves the target pour point with less additive. The capital cost of the mixer was recovered in 14 weeks through the avoided shutdowns and the additive savings alone.

Continuous Acid-Base Neutralization Loop

Process Description

The plant generates 15 m³/h of acidic process wastewater from a chlor-alkali byproduct stream, with a typical pH of 2.0 ± 0.4 and a chloride concentration of 8,000 mg/L. The wastewater must be neutralized to pH 7.0 ± 0.5 before discharge to the on-site biological treatment plant. The neutralization reagent is 30% NaOH, dosed via a magnetic-drive dosing pump. The plant operates 24/7 with daily wastewater flow variations of ±20%.

The previous installation was a 2 m³ agitated neutralization tank with an 18-minute residence time. The tank was stirred by a 5.5 kW paddle agitator and instrumented with a single pH probe in the outlet. Because the tank had a long residence time relative to the dosing response, any correction in NaOH flow took 5-8 minutes to show up at the probe — long enough for pH overshoots to push the outlet beyond the discharge limit.

Challenge

The legacy stirred tank presented two operational problems. First, the long residence time created control-loop instability. Operators reported pH excursions to 9-10 occurring two to three times per day, each lasting 3-6 minutes. Most of these excursions fell within the integrated daily limit, but several triggered non-compliance notifications to the local environmental authority. Second, the tank's shaft seal was a known source of chloride-induced pitting and had been replaced three times in four years, each time requiring a 12-hour shutdown and a 30,000 RMB service call.

The plant's process engineering team evaluated three options: replace the tank agitator with a larger, variable-frequency unit; add a buffer tank upstream of the existing neutralizer; or replace the tank with a continuous inline mixer. The first two options preserved the existing pH probe location and the existing dosing control loop but did not address the seal reliability issue. The inline mixer option required moving the pH probe and reconfiguring the control loop, but eliminated the moving parts entirely.

Solution

We supplied an SV-type mixer in DN80, with 6 elements, in PP (polypropylene) housing for chloride resistance. The mixer was flanged into the existing transfer line, immediately downstream of the NaOH injection quill. The total wetted length of the mixer is 480 mm, giving a residence time of 0.45 seconds at the design flow. The pressure drop at 15 m³/h is 0.35 bar — acceptable within the 1.5 bar available pressure budget for the transfer line.

Critical to success was the relocation of the pH probe. The probe was moved from the outlet of the old tank to a tee 800 mm downstream of the mixer outlet. This 800 mm straight pipe run allows the neutralized stream to develop a uniform pH before measurement, while keeping the total loop delay (mixer + probe) under 2 seconds. The dosing pump is now controlled by a PID loop with proportional band 50% and integral time 8 seconds — far faster than the old loop could respond.

Results

In the 12 months since start-up, the discharge pH has held within 6.9-7.1 (the analytical method uncertainty is ±0.1), with no recorded excursions above 7.5 or below 6.5. Daily average NaOH consumption has fallen by 8% because the tighter control loop eliminates the over-shoot that previously wasted reagent. The plant has eliminated its quarterly shaft-seal replacement, removing one scheduled shutdown per year. The 220,000 RMB cost of the inline mixer was recovered in 9 months through chemical savings and avoided maintenance.

Asphalt Conditioning with SBS Polymer Modifier

Process Description

The terminal produces polymer-modified asphalt (PMA) for high-performance road paving. The base asphalt (penetration grade 60/70) is heated to 180 °C and blended with 4.5% styrene-butadiene-styrene (SBS) block copolymer to produce a modified binder with enhanced elasticity and temperature susceptibility. The blend is then stored at 170 °C and dispatched to the asphalt plant at 30-35 m³/h peak flow.

The base asphalt viscosity at 180 °C is approximately 200 cP, but the addition of 4.5% SBS raises the blend viscosity to 10,000 cP and fundamentally changes the rheology — the blend is now non-Newtonian and shear-thinning. The SBS polymer must be uniformly dispersed into the asphalt matrix; any region with locally high SBS concentration will form gel particles that are not homogeneous with the surrounding matrix and that cause problems in storage (phase separation) and on the road (variable pavement performance).

Challenge

The original installation used a high-shear rotor-stator mill downstream of a 30 m³ agitated holding tank. While the mill could break up SBS pellets and produce a uniform dispersion, the throughput was limited to 18 m³/h — below the terminal's dispatch capacity — and the mill's mechanical seals required replacement every six weeks. The high-shear energy also raised the blend temperature by 6-8 °C, requiring additional cooling capacity downstream.

Additionally, the storage tank accumulated a "heel" of high-polymer-concentration material at the bottom. When the next batch was started, the heel partially dissolved into the new batch and produced erratic penetration values for the first 4-6 hours of operation. This heel problem forced the terminal to drain and clean the tank every three weeks, generating 800 kg of off-spec material each time.

Solution

We supplied an SK-type mixer in DN100 with 8 elements, fitted with a thermal oil heating jacket to maintain the blend above 175 °C throughout the mixing section. The mixer is installed in a vertical orientation, with the SBS injection point at the bottom inlet and the homogenized blend exiting from the top outlet. The SBS is fed as a 25% masterbatch in oil (viscosity 3,000 cP at 180 °C), pre-heated in a small agitated feed tank, and dosed into the base asphalt through a heated injection lance.

The mixer's heating jacket is supplied from the same thermal oil circuit that heats the storage tanks, at 200 °C inlet and 185 °C outlet. The jacket covers the entire housing and the first three elements, where the viscosity gradient is highest and the heat loss through the pipe wall would otherwise be significant. The remaining five elements run on the heat already absorbed by the fluid in the jacketed section, which is sufficient because the high-shear zones in those elements generate additional viscous dissipation.

Results

The terminal has operated the new system for 14 months at peak flows of 32-35 m³/h, well above the 18 m³/h limit of the previous mill. Penetration tests on the finished PMA consistently fall within 55-62 dmm (target range 50-70 dmm), with a CoV of 4.1% across 24 consecutive batches — versus 12% with the mill. Phase separation testing (EN 13399) shows a separation index of 0.6 °C, well within the 2 °C limit for PMA grade. The terminal has eliminated its rotor-stator mill maintenance, removed the 6-week seal replacement cycle, and reduced its storage tank cleaning frequency from every three weeks to every twelve weeks. The capital cost of the mixer and associated piping was 280,000 RMB, recovered in 11 months through maintenance savings and increased dispatch capacity.