Cracking the code: thermal insulation in ethylene plants

Figure 1. Operating temperatures in different industrial facilities.

Another reason is that ethylene is difficult to store and transport, so crackers tend to be co-located with downstream chemical producers (e.g., polyethylene, PVC, styrene), necessitating long runs of insulated pipe. It is no wonder that nearly 3% of the energy required to produce ethylene is lost through an insulated surface.¹ This is the ideal performance for a brand new system, but an existing plant – of which there are nearly 300 globally – can lose more than twice that, along with the commensurate carbon dioxide (CO₂) emissions.

Roughly 25% of the insulated surface area in an ethane cracker is for cold or cryogenic service (see Figure 2), with the remainder being for hot work (i.e., above-ambient service temperatures). This article will look at the root causes of degradation for both types of insulation, diagnostic indicators to look for, and ways to improve thermal performance in ethylene service.

Figure 2. Insulation in a typical 1 million tpy ethylene plant.

Hot insulation

Insulators speak of the three horsemen of insulation damage – heat, water, and mechanical abuse – but a more accurate census might be one horseman and its two attendants. While heat and mechanical abuse can as much as halve insulation resistance through sagging, crushing and reduced thickness, this effect is highly localised, and is barely felt at the system level. Their more serious offence is ushering water into the insulation space by opening up seams in the jacketing (see Figure 3). From these breaches, water spreads via both liquid- and gas-phase transport until even areas that are far away from the damage become wet. While water does not ‘want’ to remain inside a hot insulation system – it is always in the process of vaporising or draining away – the local environment usually provides frequent replenishment via precipitation, deluge testing, or cooling tower drift. In this way, old, hot insulation gradually enters a quasi-equilibrium state where moisture in-flows and out-flows balance over time to produce an average degree-of-wetness across the entire system.

Severe insulation wetness can increase apparent thermal conductivity, and therefore heat loss, by a factor of 20 or more. While that degree-of-wetness will decay with distance away from a jacketing breach, water’s influence can reach for tens or even hundreds of meters. Aspen Aerogels has conducted dozens of kilometer-scale steam system audits around the world, using plant process data (inlet and outlet temperatures, pressures, and flow rates) to diagnose insulation underperformance. Averaged over the entire hot insulation system, a doubling of heat loss after 5 – 10 years in service should be expected for water-absorbent insulation materials such as mineral wool, calcium silicate, and fibreglass.

Figure 3. Degraded hot insulation.

That loss factor, if applied to a 1 million tpy ethylene plant, would cost US$3.8 million/yr in unplanned fuel usage.² For a facility that likely spent less than US$10 million on its entire inventory of hot insulation material, a US$3.8 million annual loss from underperformance is a bitter pill to swallow. This scenario is like buying the material again every two-and-a-half years, and that is on top of the extra 40 tpy of CO₂ emissions such underperformance would generate.

One other thing that has been learned from these audits is that even catastrophically-failed insulation systems are not easily diagnosed. To all outward appearances, the insulation will be mostly intact, with perhaps a few damaged or missing sections. Infrared scans do not tell us much either, as they are defeated by most metallic jacketing systems and, even when not, the data is sparse and requires too much post-processing to be broadly useful. Process monitoring of end-to-end temperature drop only works on a small subset of the 100+ km of piping in these plants and, even then, is rarely done. The incremental demand that insulation places on steam and electrical utilities can easily be obscured by the normal ebb and flow of plant life. Systemic failure of hot insulation lives beneath the noise floor; its effects are felt, but rarely identified ...

This article was written by John Williams, Aspen Aerogels, USA.

This article was originally published in the July 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/18072023/cracking-the-code-thermal-insulation-in-ethylene-plants/