Reconciling Mass and Energy Balances in an Ethylene Complex is the name of a presentation recently given by Emerson’s Patrick Truesdale. The venue was the 2009 AIChE annual meeting in Nashville, Tennessee. Patrick is a senior consultant on the Industry Solutions team and has a wealth of experience in the refining and petrochemical industries.
Patrick began his presentation by identifying some of the challenges ethylene producers face, especially in mature plants found in North America, Latin America, and Europe. Plants (brownfield) built decades ago were typically sized around 300 KTA (kilo tons per annum). Today’s newer, greenfield plants found mostly in the Asia Pacific, Middle East, and Africa world areas are typically larger than 1000 KTA.
With the run up in energy prices over the last several years, the older, brownfield plants were not instrumented with mass measurements to do mass and energy balances. Ethylene plants have also been focusing their capital expenditures to meet higher quality specifications, increase capacity, address new regulatory compliance, and process cheaper feedstocks. For many ethylene producers, the plant staff has been stretched and unable to focus on improving energy efficiency.
Patrick described one of the common key performance indicators (KPIs), loss opportunities. He defined the gain/loss as open inventory plus receipts minus shipments minus closing inventory. Receipts include the fuel consumed in the production process and shipments include the fuel produced by the process. These gains and losses can be accountable and unaccountable. Examples of accountable losses include what is flared and coke produced. Examples of unaccountable losses include leaks and theft.
Process energy usage is the largest controllable cost in most ethylene plants and requires a focused effort to improve this gain/loss KPI. Patrick outlined a process that begins with defining the process boundaries. It starts with the feeds, steam, and fuel gas to the fired heater as well as the distillation tower feeds, tops, and bottoms. Next, look at the cracked gas compressor turbo machinery, cryogenic distillation, and hydrogenation reactor. Temperature and pressure measurements around these boundaries provide the inputs for mass and energy balance calculations.
The second step is to identify custody transfer boundaries, which includes the raw material production and supply at the load port, the unload port at the ethylene complex, the load port for ethylene at the complex, and finally the unload port at the customer site. The bill of lading (BOL) combined with the custody transfer metering equipment measure transportation losses from each load to unload handoff.
The third step is to design mass and energy balances to account for losses and consumption in the production process. This includes measurements at the inventory holding tanks, tank-to-tank transfers, tank to unit transfers, and process nodes, as well as incoming receipts and outgoing shipments. Also, measurements need to be in place for any intercompany transfers of materials and energy.
The fourth is to survey the existing measurement systems. From Patrick’s experience KPIs associated with mass loss are directly based on the quality of instrumentation and procedures: poor results–1.5-2.5% mass losses, average results–0.7-1.5% mass losses, good results–less than 0.5% mass losses, and pacesetters–less than 0.2% mass losses. Patrick notes there are a wide variety of flow measurement technologies from which to choose including Coriolis, differential pressure, magmeter, positive displacement, turbine, ultrasonic, and vortex. They each have strengths and weaknesses with respect to pressure drop, accuracy, required maintenance, slurry flow tolerance, viscosity tolerance, fluid conductivity, and process intrusiveness. In slides 18-22, Patrick provides some selection considerations.
Once the measurements are in place, the fifth step is to develop mass and energy balance models. Measurements should be validated, with errors detected and corrected to fix gross and bias errors. Random errors are reconciled through a simultaneous solution of the equations for:
- flows, inventories, and material transfers mass and volume balancing
- flows, inventories simple and multi-phase component balancing
- enthalpy, power, heat exchanger, and steam energy balancing
Patrick’s final step is to define the business process to identify KPI reporting, the KPI dashboard, and the processes to implement continuous improvement. Having the metrics for when the plant is both operating smoothly and having problems helps provide the feedback to solve the operational issues more quickly and return to more energy efficient, smooth operations.
The KPIs for optimized energy usage and loss reduction are interrelated and the need to address grows with global competitive pressures, higher energy costs, and increasing regulations.