Improving energy efficiency is fundamental to reducing carbon dioxide emissions and achieving sustainability goals. Michael Machuca and Jimmie Soderstrom from Emerson provide practical and actionable ways to make improvements without the need for major capital investments.
Energy efficiency measures are a cost-effective way for chemical plants to decrease their carbon footprint and increase profitability. Effective process automation is essential to improving energy efficiency. Monitoring relevant variables enables measurable improvements. Installing intelligent field devices and analysing data helps operations teams implement corrective action and revised control strategies. These projects deliver significant results without major capital investment.
Around 30% of the chemical industry’s fuel consumption is due to fired heaters. Optimising the fuel-to-air ratio through automation can increase combustion efficiency, but it can be difficult to maintain a consistent ratio due to varying fuel composition.
Proper balancing of the burner results in maximum heat recovery with minimal emissions. Fuel gas supply can be controlled through volumetric flow or pressure control, but these methods may be insufficient for ensuring safety, fuel efficiency, and environmental compliance when there is variability in the system.
Monitoring combustion efficiency by measuring oxygen in flue gas or in the bridge wall of a fired heater using Rosemount™ In Situ Oxygen Analysers can help avoid high levels of O2 that can negatively impact thermal efficiency and environmental compliance.
When operating heaters, high O2 levels provide an added margin of safety, but impact efficiency and environmental compliance, causing increased NOx emissions and difficulty in meeting permit requirements. Low O2 levels increase the risk of sub-stoichiometric combustion, leading to heater tripping or damage in extreme cases.
Sub-stoichiometric conditions can arise when the fuel composition suddenly changes to a richer blend with a higher heating value, requiring more oxygen. Feed-forward control facilitates fuel composition changes, with mass flow meters preferred for accurate measurements.
The heating value of the hydrocarbons in the gas correlates more closely on a mass basis than on a volumetric basis. Hydrogen (H2) is the exception to the relatively consistent energy value.
On a mass basis, it has twice the energy content of methane. However, H2 is so light compared to the hydrocarbons that it doesn’t impact the overall heating value of the gas significantly on an energy/mass basis, thereby furthering the case for mass flow measurement. Even when hydrogen is blended with a hydrocarbon fuel gas mix, mass-based control schemes are superior to volumetric-based schemes.
Fuel flow measurement
When fuel flow is measured in kilograms per hour, it becomes easier to maintain a consistent energy flow. This is particularly helpful in situations where the gas mixture is highly variable.
Air flow control also becomes more stable and easier to balance when stoichiometric air flow matches the energy content rather than volume. For instance, Micro Motion™ flow meters can measure mass directly, which lowers the variability in the fuel-to-air ratio caused by changes in fuel gas heat value. These flowmeters are available in a wide range of sizes and feature a wide turn-down ratio to accommodate varying operating conditions. By using mass flow measurements to keep burner control more stable, you can save €250,000 to €1 million per year in fuel costs.
Heat exchanger efficiency
Optimal control of heat exchangers can improve energy efficiency, but fouling often causes inefficient operations, especially in processes where deposits build up on coils and tubes. Heat exchanger fouling contributes 1-2.5% of global CO2 emissions. Despite the seriousness of the issue, many chemical plants still rely on manual inspections and schedule-based cleaning, as only a few heat exchangers have advanced instrumentation.
To address fouling and maintain efficient operation of heat exchangers, there is a need for instrumentation to measure temperature, pressure, flow and conductivity. WirelessHART® transmitters can simplify and reduce the cost of adding new devices. A supporting software application is used to monitor and evaluate heat exchanger performance.
Data from the instruments and known process fluid characteristics (heat of vaporisation, inlet/outlet vapour fraction, etc.) enables actionable information to be provided to support maintenance planning, reducing energy losses and maintenance costs with a ROI in as little as six months.
Steam distribution systems
Steam generation accounts for 40% to 50% of a chemical plant’s energy budget and improving efficiency can boost sustainability and profitability. Better measurements are needed to establish a baseline for reducing losses from steam distribution. However, the mass imbalance of a steam network often makes it difficult to assess accurately.
Multivariable vortex flowmeters or Annubar flowmeters can reduce measurement uncertainty. Where it is difficult to add new measurements, installing wireless flow meters in strategic locations can help identify steam inefficient use cases, allowing for prompt attention to underperforming systems for a faster return on investment.
Monitoring corrosion and erosion in steam systems allows for early detection of equipment deterioration, optimising maintenance planning and preventing losses. Wireless non-intrusive corrosion and erosion sensors are easy to install and maintain, with associated software providing real-time data on pipework condition.
Steam traps
Improving steam trap effectiveness is a significant opportunity for energy savings. Once steam traps are a few years old, they can become maintenance headaches, with up to 30% malfunctioning at any given time.
A faulty steam trap can waste thousands of euros worth of energy over a year. With a wireless acoustic transmitter mounted on the inlet pipe, the ultrasonic noise and temperature of each trap can be measured and analysed using an algorithm. Dashboards display each trap’s status and the estimated energy and cost losses. Maintenance personnel can identify which traps need attention and prevent small problems from becoming costly issues.
Boilers and steam production
Steam generation typically accounts for 40-50% of a chemical plant’s energy budget. Improving boiler performance can lead to significant cost savings. As with fired heaters, combustion and emissions control are important, but boilers also have unique requirements. For instance, the water in a boiler is recirculated to increase efficiency. However, impurities in the water can build up and deposit on internal surfaces, reducing heat transfer. Conductivity and pH sensors can monitor condensate and make-up water to prevent mineral deposits and corrosion. Such analysis can also detect leaks in heat exchangers adding other contaminants to the circulation.
Large chemical plant boilers use a drum to circulate boiling water and collect steam. It is crucial to control the level in the drum to avoid sending water into the steam line. They will also trip if it gets too low as this could cause the boiler to run dry.
Deviations of just 5 – 10 cm can cause the boiler to trip, leading to potential hazards. However, maintaining the critical level is challenging due to turbulence and variable density. The Rosemount 5300 Level Transmitter is a guided wave radar capable of solving this problem since radar level technology is immune to density, temperatures and pressure variations. It has a self-diagnostic capability that can evaluate changes in the vapor space dielectric constant caused by saturated steam, reducing level measurement errors to 2% even in turbulent conditions.
Distillation column control
Distillation columns are very energy intensive. Additional instrumentation and improved utilisation of installed devices supported by well-designed analytics and corresponding control strategies can help solve common problems and reduce overall energy consumption.
For example, pressure transmitters with advanced diagnostics can quickly detect early column flooding and tray malfunctions. Coriolis flowmeters directly measure the mass flow of gas streams, liquid streams, and liquid density for quick reactions to process changes for improved product quality control.
Multivariable flowmeters provide data needed for fully compensated mass and energy flow calculations for tighter control of critical process parameters. A field-mountable gas analyser or chromatograph can measure overhead compositions quickly. Localised installation reduces operating costs while providing data for closed-loop control, improving quality, increasing product recovery, and reducing energy consumption.
Conclusion
These measurement solutions require minimal investment. For instance, adding an acoustic transmitter to a steam trap costs only a few thousand euros. Yet, the improvements captured deliver a positive return on investment in just a few weeks or months.
The advantage here is the incremental nature of the improvements and their cumulative effects towards energy savings, reduced costs, and sustainability. Initial successes can fund additional upgrades, with the momentum required for continued success. If one heat exchanger and two or three steam traps can be retrofitted with flowmeters and acoustic instruments each week, a facility could be transformed in a year.
