Density Measurement in the Hydrocracker

Photo by Johannes Plenio on Unsplash

 

The hydrocracking process is a key method in modern refinery technology, used to convert heavy petroleum fractions into high-quality, more readily usable products such as diesel, kerosene, and naphtha. Unlike thermal cracking, the splitting of long hydrocarbon chains occurs under high hydrogen pressure (typically 100–200 bar) and at temperatures of 350–450 °C in the presence of a catalyst. The process combines chemical reactions such as hydrogenation, cracking, and isomerization to increase both yield and product quality.

 

Importance of Density Measurement in Refinery Technology

The density of a medium is a crucial process parameter in hydrocracking, as it directly correlates with the composition and the quality level of the products. Density fluctuations provide early indications of changes in the reaction process, catalyst activity, or contamination. Precise and continuous density measurement enables:

• the optimization of the process through real-time monitoring of conversion rates,

• the increase in product yield through targeted control of product separation, and

• the ensuring of plant safety by detecting undesirable reactions at an early stage.

In a complex and potentially hazardous environment such as the hydrocracker, reliable density measurement is thus an essential component of process control and quality assurance.

The aim of this article is to highlight the role of density measurement in the hydrocracking process, analyze the technical requirements and challenges, and present suitable measurement technologies with a focus on Coriolis flow meters. Practical examples will demonstrate how the selection and integration of appropriate sensors can contribute to efficiency improvement, product enhancement, and plant safety.

→ How the Hydrocracker Works

Overview: Reaction Conditions

The hydrocracker is a catalytically supported high-pressure reactor that is charged with hydrogen to convert difficult-to-process hydrocarbon fractions into lighter and more valuable products. The reactor conditions are extreme and place high demands on materials and measuring technology:

The combination of high temperature, high pressure, and a reactive medium makes the hydrocracker one of the most demanding processes in refinery technology.

 

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Process Effects on the Medium

During the cracking process, large molecules are broken down into smaller ones. This molecular transformation results in a measurable change in the medium's density:

• Density decrease: As the cracking reaction progresses, density drops because heavy molecules are transformed into lighter fractions.

• Indicator function: Density thus serves as an indirect measurement of reaction progress and end-product quality.

• Layer formation: After the reaction, different fractions may separate in separators due to density differences (e.g., light fractions at the top, heavy oil at the bottom), making density a central control parameter.

 

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Typical Feedstocks

Hydrocrackers primarily process high-boiling, sulfur-rich, and difficult-to-process fractions that result from atmospheric or vacuum distillation:

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These substances require robust and highly accurate sensors, as even small density changes can indicate significant process variations.

 

→ Why Is Density Measurement Critical in Hydrocracking?

Continuous density monitoring is a key component of process control in hydrocracking, as it not only provides insights into product quality but also contributes to the safety and efficiency of plant operation. Unlike lab-based analyses, inline density measurement allows for immediate reaction to changing process conditions.

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Process Control and Quality Monitoring

The density of the process medium is closely related to the API gravity, an internationally standardized measure of a petroleum product’s lightness. The lower the density, the higher the API value – an indicator of product quality and marketability.

• Online density measurement provides real-time data on reaction depth

• Fluctuations in density trends indicate catalyst aging, mixing errors, or incomplete reactions

• Supports targeted reactor parameter control (e.g., temperature profile, residence time)

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Safety

Hydrocracking involves potentially critical operating conditions, such as H₂ enrichment, overheating, or reaction blockage. A sudden increase in density can indicate coke formation, catalyst fouling, or a drop in temperature.

• Early warning system for undesired side reactions

• Supports automated emergency shutdown in case of deviations

• Helps prevent explosive conditions

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Optimizing Hydrogen Supply and Cracking Rate

The cracking rate directly depends on the availability and excess of hydrogen. Density measurement enables a demand-driven regulation:

• Avoiding overdosing (which increases operating costs)

• Ensuring reaction stability

• Improving efficiency by adjusting crack depth while maintaining consistently high product quality

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Importance of Density in Product Separation

After the reaction, the products are separated into fractions through several stages, including pre-separators, fractionators, and aftercoolers. Density serves as a key separation parameter in this process.

 

→ Challenges in Measurement

Density measurement in hydrocrackers poses unique challenges for instrument manufacturers and plant operators. In addition to the critical importance of the measured value itself, extreme process conditions make precise and long-term stable measurements difficult.

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Extreme Process Conditions

Measurement devices must operate continuously and reliably in an environment that exceeds the capabilities of many standard technologies:

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Example Scenario 1: Sulfur-Rich Residues in the Hot Separator

In a hydrocracker bypass, a partial stream is extracted from the hot separator to analyze residues. The density fluctuates strongly depending on the depth of the reaction. The medium contains hydrogen sulfide (H₂S) and is hotter than 200 °C.

Challenge:

• Chemically aggressive mixture attacks seals and metallic sensor surfaces.

• Conventional measuring devices show drift or failure due to corrosion.

 

Solution:

• Use of a Coriolis meter with a Hastelloy C22 measuring tube and PTFE-coated seals.

• Installation in an Ex-d housing with thermal decoupling via extended neck.

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Example Scenario 2: Limited Space at Reactor Inlet

At the inlet of a hydrocracker reactor, there is very little space between two fittings. A new density measurement point is to be added to better monitor the hydrogen content in the inlet mixture.

Challenge:

• Insufficient space for traditional inline density meters with long inlet and outlet runs.

• Vibrations from adjacent pumps impair measurement stability.

 

Solution:

• Use of a compact Coriolis flow meter with short fittings and vibration-insensitive dual-tube design.

• Integration into the existing Profibus system with real-time density output.

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Additional Challenges:

• Maintenance & Calibration: Built-in sensors are often hard to access; calibration must be done online (using reference values) or via smart diagnostic functions.

• Long-Term Stability: Mechanical stress, pressure changes, and temperature shocks can cause premature aging in standard sensors.

• Signal Processing: For fast process fluctuations, measurements must be not only robust but also dynamically fast (typically <1 second).

 

→ Comparison of Measurement Technologies

Several technologies are available for density measurement in hydrocrackers. The selection depends on factors such as temperature, pressure, media properties, hazardous area requirements, and spatial constraints. Below is a comparison of the three most relevant methods: Coriolis, ultrasonic, and nucleonic density measurement:

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1. Coriolis Meters (e.g. Siemens SITRANS FC430)

Coriolis technology is considered the gold standard for combined mass flow and density measurement. It is based on the analysis of vibrations in a flow-through measuring tube—depending on the medium's density, the moment of inertia changes, altering the phase shift of the tube vibration.

Advantages:

• Direct, high-precision density measurement (accuracy up to ±0.0005 g/cm³)

• Robust design, optionally with Hastelloy, Duplex, or titanium

• Compact devices like the SITRANS FC430 are space-saving and Ex-certified

• No moving parts → low maintenance

• Integrated temperature measurement and diagnostic functions

• Suitable for pressures up to >400 bar

Limitations:

• Medium must flow continuously (batch operation limited)

• Higher initial costs, but economically viable in the long term

Application Recommendation:

✅ Ideal for critical process points in hydrocrackers (e.g. reactor inlet, product separation)

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2. Ultrasonic Density Measurement

In the ultrasonic method, the transit time of an acoustic signal through the medium is measured. Based on known sound velocities, density can be calculated indirectly.

Advantages:

• Non-contact measurement possible (with clamp-on version)

• Low installation costs

• Well-suited for lower temperatures (<150 °C) and moderate pressure ranges

Limitations:

• Sensitive to gas bubbles, deposits, and vibrations

• Temperature- and pressure-dependent – calibration is complex

• Limited material options

• No integrated mass flow measurement

Application Recommendation:
⚠️ Only partially suitable for core hydrocracker areas – better used in downstream systems or where requirements are lower

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3. Nucleonic Density Measurement

This method uses a radioactive source (e.g., Cesium-137) whose radiation intensity is attenuated by the medium. The degree of attenuation allows the density to be derived.

Advantages:

• Non-contact, continuous measurement, even under extreme conditions

• Independent of pressure, temperature, viscosity, or conductivity

• Ideal for media containing solids or gas bubbles

Limitations:

• Radiation protection requirements, regulatory approval needed

• Extensive documentation, special maintenance and disposal

• Higher operating costs due to safety regulations

Application Recommendation:

✅ Suitable for special applications (e.g., residues with coke or tar content)

Not first choice for standard fluids in reactor zones

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Technology Comparison Table

→ Selection Criteria for Suitable Sensors

Selecting an appropriate density sensor for use in a hydrocracker requires a comprehensive consideration of process, safety, and installation conditions. In addition to physical requirements, legal regulations and economic factors also play a key role.

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1. Process Conditions (Pressure, Temperature)

The extreme operating conditions in a hydrocracker demand high mechanical and thermal resilience from measuring devices:

• Pressure range: Sensors must operate reliably at up to 200 bar (sometimes more), both statically and dynamically.

• Temperature resistance: Near the reactor, temperatures > 400 °C are common and require thermal decoupling or special materials.

• Pressure/temperature fluctuations: Rapid changes from switching processes or cleaning cycles must be tolerated mechanically.

Tip: Look for certification according to PED, ASME, or AD-2000 for pressure equipment.

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2. Medium (Chemical, Physical)

The composition of the medium determines the requirements for materials, sensor principle, and signal evaluation:

• Chemical aggressiveness: Media containing H₂S, NH₃, aromatic hydrocarbons or acids attack standard materials. High-alloy materials such as Hastelloy, titanium, or duplex stainless steel are often required.

• Viscosity: Heavy fractions or bitumen-containing residues require sensors that deliver reliable readings even with high flow resistance.

• Multiphase flows: Gas bubbles or solid particles interfere with many measurement principles – robust technologies (e.g., Coriolis or nucleonic) are needed here.

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3. Piping (Installation Location, Nominal Size)

Available installation space and pipe dimensions limit sensor selection:

• Nominal size: Sensors must fit into existing pipelines – typically DN25 to DN100 in crackers.

• Mounting position: Is horizontal or vertical installation possible? Are there sufficient inlet and outlet lengths?

• Vibration and temperature decoupling: Especially for sensitive sensors (e.g., ultrasonic), mechanical isolation must be ensured.

• Bypass solution: If inline measurement is not feasible, a sensor can be installed in a temperature-controlled bypass pipe.

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4. Certifications (e.g., ATEX Zone 1/2)

The hydrocracker area is generally classified as a hazardous (explosive) environment. Measuring devices used here must have the appropriate Ex certifications:

• • ATEX / IECEx: Mandatory for installations in Europe or worldwide

  • • Zone 0: Permanent explosive atmosphere → intrinsically safe design required

    • Zone 1: Frequent explosive atmosphere → flameproof enclosure, protection type Ex d

    • Zone 2: Occasional explosive atmosphere → standard devices with protective concept possible

      Zone classification: Selection is based on the assigned zone:

Note: Accessories such as cables, housings, and junction boxes must also be Ex-certified.

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5. Maintenance & Calibration

In continuous operation, sensors must be reliable, long-term stable, and low-maintenance:

• Self-diagnosis functions: Modern sensors like the SITRANS FC430 offer integrated monitoring of sensor status, vibration behavior, and signal quality.

• Calibration: For density measurement, a one-time factory calibration is usually sufficient, provided no media-related deposits or drift occur.

• Remote functionality: Ideally, sensors can be diagnosed and configured remotely via HART, Profibus, or Modbus – without on-site access.

• Cleaning: If cleaning intervals are necessary, the sensor should be flushable or CIP-capable.

 

→ Practical Example: Use of a Coriolis Meter in a Hydrocracker

Application Summary

A two-stage hydrocracker is operated at a mid-sized refinery in Central Europe to convert heavy vacuum gas oil into diesel and kerosene. The plant repeatedly struggled to detect reaction fluctuations early, leading to product deviations and increased rework in the fractionation column.

Specifically, in the reactor feed there was no continuous measurement value to monitor the feedstock composition. Lab-based density measurements were too slow to effectively influence process control. The goal was therefore to integrate an inline-capable real-time density measurement solution.

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Selection Decision

After evaluating different technologies, the engineering team opted to use a Coriolis flowmeter of the type Siemens SITRANS FC430 for the following reasons:

• Direct density measurement with high accuracy (±0.0005 g/cm³)

• Compact design, ideal for the tight piping section between mixer and reactor

• Pressure resistance up to 400 bar at temperatures up to 200 °C

• Material design in Hastelloy C-22, resistant to H₂S and other aggressive components

• Ex certification according to ATEX Zone 1, flameproof enclosure (Ex d)

• Integrated Profibus interface for connection to the control system

Installation was done without modifying the piping, by integrating it into an existing bypass system with process cooling.

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Results

After 12 months of operation, the refinery documented the following results:

• Stable and reliable measurements across the entire operating range, even with varying feed compositions

• Significant improvement in reaction control: Density changes at the inlet correlate precisely with product quality at the outlet – adjustments to hydrogen feed and temperature control are now made proactively

• Reduction in product variation by ~15%, less rework in the fractionation column

• Low maintenance effort: no calibration drift, no visible sensor wear

• Thanks to integrated diagnostic functions, an impending blockage effect in the feed was detected early and resolved

 

→ Conclusion

Summary of Key Points

Density measurement is a critical control parameter in hydrocrackers, directly linked to product quality, reaction control, and plant safety. Due to the extreme process conditions (high pressure, high temperatures, chemically aggressive media), standard measurement methods are often unsuitable.

Coriolis meters, such as the Siemens SITRANS FC430, offer a reliable, direct, and highly accurate solution for simultaneous measurement of density, mass flow, and temperature – even under the harshest conditions. Compared to alternative methods such as ultrasonic or nucleonic measurement, Coriolis sensors are often easier to integrate, require less maintenance, and involve fewer regulatory hurdles.

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Future Trends: Smart Sensing and Digitalization

As the process industry becomes increasingly digital (keyword: Industry 4.0), density sensors are also becoming smarter and more connected:

• Real-time diagnostics: Sensors detect drift, deposits, or mechanical issues at an early stage

• Connectivity via HART, Profibus, or Ethernet/IP enables remote configuration and maintenance

• Predictive maintenance: Trends in temperature, signal quality, and vibration patterns allow failure prediction before it occurs

• Data analysis: Continuous density profiles serve as input for automated process optimization

Sensors are increasingly becoming active data sources that contribute to efficiency improvements and operational safety far beyond their core measurement function.

 

Recommendation for Plant Operators

The following applies to plant operators in the refinery sector:

✅ Density measurement should no longer be performed exclusively in the laboratory – inline measurements offer significant added value for control and quality.
✅ Coriolis flow meters are the technically and economically best solution in most applications.
✅ The selection of the appropriate sensor should be process-specific – a structured checklist helps guide the decision-making process.
✅ Those who invest early in digitally networked sensor technology will benefit in the long term from higher availability, lower maintenance costs, and transparent process control.

 

→ Appendix: API Conversion and Typical Crude Oil Densities

What is API Gravity?

API gravity is a measure developed by the American Petroleum Institute to describe the density of crude and refined oil products. It allows better comparability between crude oil types and is widely used in refineries for quality classification.

• High API value → light crude oil (lower density, more valuable)
• Low API value → heavy crude oil (higher density, more difficult to process)

Example Conversion Table:

Density [g/cm³]	API Gravity	Crude Type / Notes
0.970	~10.0	Very heavy crude oil (e.g., Bitumen)
0.930	~20.5	Heavy oil (Fuel Oil, DAO)
0.880	~29.3	Standard Vacuum Gas Oil (VGO)
0.850	~35.6	Medium crude oils (e.g., Brent)
0.820	~41.9	Light crude oil (e.g., West Texas)
0.780	~51.5	Condensates / Light gasoline fraction

 

Relevance for Density Measurement

• Accurate on-site density measurement enables the direct calculation of the API value.
• The API value is a key control parameter in refineries for product quality, pricing, and taxation.
• In automated systems, the API value from a Coriolis flow meter can be directly integrated into the process control system.