White Paper - Process NMR Control for Strategic Refinery Operations

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John Edwards

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White Paper - Process NMR and Strategic Refinery Operations

STRATEGIC REFINERY OPERATION USING NMR-ENHANCED ADVANCED PROCESS CONTROL

INTRODUCTION – NMR-Enhanced Advanced Process Control and Optimization

Advanced process control (APC) schemes frequently require near real-time stream composition information to make adjustments to controls – the faster and more reliably the better. To obtain these crucial measurements of process performance, refiners have been deploying GC, boiling point, RVP, cloud point, octane, and numerous other process analyzers. Nuclear Magnetic Resonance (NMR) spectroscopy is rapidly emerging as one of the most versatile and cost-effective technologies for process analysis.

NMR offers tremendous analytical measurement flexibility, non-invasive sampling, rapid and precise analysis, high system availability with low maintenance requirements. Figure 1 identifies potential application sites for NMR-enhanced APC systems in the refinery.

How NMR Spectroscopy Works
During an NMR analysis, a sample stream is passed through a precisely controlled external magnetic field, which brings the magnetic moments of all its protons (hydrogen atoms) into alignment with the homogeneous magnetic field. To take a reading, an NMR analyzer transmits a pulse of 60 MHz radio frequency (RF) energy, through a tuned circuit coil outside of the sample piping, through the sample. The magnetic field component of the radio frequency energy perturbs the magnetic moments of the various protons off their aligned axes. The amount of deflection and the subsequent recovery time will vary according to the length of the applied pulse. When the RF pulse is turned off the proton magnetic moments will return to alignment with the external magnetic field. As the magnetic moments precess back to equilibrium, protons with different chemical environments generate alternating currents at different frequencies in the NMR irradiation coil. These currents represent protons is different molecular environments and the magnitude of the current is proportional to the amount of that chemical proton type in the sample. Fourier transformation of the raw signal generated in the coil yields a spectrum of peaks where each peak represents a proton in a unique chemical/molecular environment. In the course of one minute the analyzer averages multiple pulses into a spectrum that reveals hydrocarbon chemical make-up, and their relative concentrations.

This NMR spectrum can also be correlated with physical properties other than the chemical composition, enabling determination of multiple parameters from a single spectrum. And since NMR is not an optical technology, the analysis is essentially independent of sample state (e.g., solid, gas, or liquid) or physical condition. Small particulates or bubbles, for example, have little or no effect on the analysis. The sample passes through the magnetic field in a small tube, untouched and unchanged in any way, and is returned to the process downstream.

Refinery Control and Optimization by Process NMR Technology
Advanced process control, utilizing technology ranging from simple multivariable control to model-based predictive control (MPC) and rigorous on-line modeling, generally requires near real-time stream quality information. The exceptional availability of the NMR analyzer enables this information to be supplied reliably for process control while the technology ensures accuracy and repeatability. Because this analyzer can be applied to numerous component quality measurements, a single analyzer can often alleviate the need for multiple analyzers to satisfy an APC application.

Recognizing that NMR technology is a winner for the petroleum refining industry, teaming the analyzer with appropriate advanced process control tools and control systems is a logical next step. Thus, configurations of the process NMR analyzer with optimization/control software, control system hardware, and engineering services can help resolve the more costly refinery process control and optimization problems. NMR applications developed by Process NMR Associates have been utilized with and without integration with advanced control are (and Figure 1):

· Crude Oil Blending
· Atmospheric Crude Oil Distillation
· Fluid Catalytic Cracking
· Sulfuric Acid Alkylation
· Gasoline Blending
· Diesel/Distillate Blending

Managing Crude Transitions
Traditionally, many refineries were built on the premise that supply crude would always come from a specific sources. Supply varied little, and setpoints could be operated adequately with laboratory analysis and predictive control. Today’s market is quite different as may refineries have or are moving to a “tanker refinery strategy” in which open market crudes are being utilized for their economic benefit potential. Competitive pressure to maximize profitability is driving refiners to find new ways to leverage low-cost crude feeds. They are buying more crude on the spot market, and this crude usually differs significantly from the design-crude used when the refinery was built. Managing these variations profitably requires daily revision of production schedules and continuous profitability optimization.

Figure 2 shows how variations in crude feed quality affect production of low-value atmospheric residue. As the graph shows, a feed change from typical Syrian Light crude to an Iranian Heavy, unaccompanied by a corresponding change in the process conditions of the crude unit, will increase production of low-value atmospheric residue by about 10 percent.

Variations in crude quality can affect cut point optimization, product quality control, feed rate maximization, and energy consumption, while also violating process equipment constraints. Without process control compensation for a crude transition, the process will experience an upset and become both less efficient and less profitable.

There are several options in managing the transition of crude feeds, although all options are typically not available at each refinery. Crude oil blending is very advantageous to those refiners receiving constant supplies from fields through pipelines or those with large tank farms. Refiners not so lucky must battle unit upset when transitions occur, unless they are made aware of a pending transition and have the capability to minimize the effects through process control.

Crude Oil Blending
The capability of accurately monitoring crude compositions enables precise blending of crude feeds. This means that the refiner can blend less expensive heavy, sour crudes with more expensive light, sweet crudes to achieve desired properties while maximizing profitability.

A crude oil blending system is shown in Figure 3. A digital blending control system integrated with a refinery information management system provides crude blend planning functionality that downloads total flow requirements, ratio limits for the crude blend components, and product quality constraints. These settings are based on refinery models that define optimal utilization of distillation and downstream units for various crude types.

Depending on physical location requirements, one or more NMR analyzers are applied to the blended crude stream and to the crude component streams. The NMR analyzer measures essential qualities such as API gravity or density, true boiling point /ASTM distillation, initial and final boiling point, and water content.

Operating the refinery at optimal and constant crude composition can generate savings for major refineries on the order of 2% to 3% of the operating margin of the whole refinery. By controlled blending of crudes one can achieve:

Improved distillation unit throughput. Constant attention to the distillation quality of the crude loads the crude distillation unit and all the other downstream units consistently. This allows refiners to operate their crude distillation unit closer to its limits, which increases throughput.

Improved refinery throughput. If the throughput of any refinery unit is limited, a constant and optimal distillation curve for the crude oil can push all units to their limit simultaneously. This maximizes throughput for the overall refinery.

Improved performance of downstream units. Specific characteristics of the crude will also influence performance of some of the downstream units. Changes in the ratio of paraffins to aromatics in crude, for example, will impact/affect the benzene, toluene, and xylene output of catalytic reformers.

Improved product quality and reduced energy costs. Stability of the crude composition also eliminates one of the major disturbance factors in a refinery, resulting in more stable operation. This contributes positively to overall quality, fosters efficient energy consumption, and improves equipment reliability.

Improved management of crude changes. Maintaining optimum and constant crude quality and composition enables more efficient management of changes in crude.

Atmospheric Crude Oil Distillation
In the past, refiners would manage the transition from one crude to another by manual adjustment of various controlled variables for a given time, thus relying upon prior crude transition experience in order to minimize process upset. By using NMR-enhanced control and process optimization, however, the refiner can follow the transition from one crude to another in real-time and adjust parameters as needed to maintain maximized profit. The result can be dramatic savings per crude transition, since the typical 4 – 8 hour upset due to a transition is essentially eliminated.

Figure 4 shows how NMR measurements would be deployed in an atmospheric crude oil distillation unit application. Because NMR technology can also monitor the distillate streams as well as crude feed, the cost benefits are substantial. It can replace complex traditional physical property and laboratory analyzers as well.

The crude feed analysis supplies crude characterization information to enable feed transition compensation. Advanced Process Control and Optimization can maintain unit operation at optimum between crude transitions. Common advanced control targets include:

· Maximizing unit throughput up to equipment constraints

· Maintaining product quality while maximizing yield of most valuable products

· Maximizing preheat train, pumparound, and fired heater heat transfer efficiencies

Fluid Catalytic Cracking
The fluid catalytic cracking unit (FCCU) is one of the most important units in the refinery. Few FCCUs have real-time process optimization implemented, since feeds typically have been measurable only in the laboratory. These measurements take many hours, with reports available only once or twice a day. Even the measurement of PIONA (paraffins, isoparaffins, olefins, naphthenes, and aromatics) and the distillation properties of the rundowns are difficult to achieve on-line. NMR now provides a means of obtaining these measurements near real-time, thus enabling significant economic benefit to the refiner through APC and process optimization.

Like the atmospheric crude distillation units, the FCCU has been built on the supposition that the feed composition will remain near design specifications. In today’s economic climate, this is no longer true. Using NMR to characterize the feed and coupling it to the an Optimizer helps optimize the process in the following ways:

1. If the feed has changed, the NMR analyzer provides near real-time data on changing feed properties to enable the most economical conversion of the available feed.

2. If the feed remains unchanged, the on-line analysis of the feed enables the operator to run the process closer to equipment constraints; for example, near the limits of the LPG compressor at the back end of the process. This increases the throughput of the unit at very little additional cost.

3. As with feed transitions to a crude distillation unit, APC and optimization can maintain FCCU operation at optimum. The main fractionator overhead and sidecut product draw stream quality measurements provided by NMR can be integrated into APC models to monitor process operation performance and supply control feedback information.

FCCU controls and optimization include feed preparation, the reactor/regenerator, the main fractionator, the wet gas compressor, and the downstream gas plant. Typical operating objectives are

· Maximizing unit capacity
· Maintaining product quality while maximizing yields of most valuable products
· Optimizing energy utilization
· Controlling conversion
· Improving safety and reliability via operational stability

Refinery Blending Systems
The movement toward “Clean Fuels” has blend header complexity increasing due to the increasing number of blend components. Diesel and other blended fuels are also subjected to more severe blending requirements in order to comply with environmental mandates. As a result, refiners are compelled to evaluate the effectiveness of their blending operations and are adding or improving blend optimization to boost profitability. Common blending operation targets are:

· to reduce reblends and improve profitability
· to meet product specifications while conforming to environmental requirements
· to enhance effective inventory capability
· to lower risk of missed export schedules
· to improve refinery planning/scheduling accuracy

To realize the greatest profitability in refinery blending operations, a blend optimization system is used to provide management of the component and product tanks, blend header, on-line and laboratory analytical systems, and planning/scheduling activities. Such systems facilitate production of blended products with a high degree of precision to meet specifications while minimizing quality giveaway, maximizing the use of the lowest cost components in the blend, increasing the flexibility of the tank farm operation, and minimizing the frequency of reblends. A flexible objective function permits component cost, inventory constraints, or product specification to direct the optimizer.

By coupling blend optimization with near real-time component stream and blended product chemical quality information from NMR (Figures 7 and 8), enables multivariable analyzer-directed control including:

· Feedforward control for component quality variations
· Feedback control for product quality variations
· Quality integration of product and component tanks
· Projected product qualities at the blend header

Actual process manipulations are made by existing digital blend controllers or a Digital Blending System (DBS). DBS features include uniform ramping, continuous pacing, analyzer trim, temperature-compensated flow measurement, and flexible loop configurations. It may be configured to include an automated procedure for manipulating the equipment involved in blending, transfer, flushing, and pigging operations.

In summary, NMR offers optimal integration with advanced process control and optimization throughout the refinery.

For more information on this topic please contact:

John Edwards

Manager, Process and Analytical NMR Services

Process NMR Associates LLC,

87A Sand Pit Rd

Danbury, CT 06810, USA

Tel: (203) 744-5905