The ethylene molecule consists of a double bond between carbon atoms and four hydrogen atoms bonded, two to each carbon atom. Per the Wikipedia Ethylene entry, it:
…is widely used in the chemical industry, and its worldwide production (over 150 million tonnes in 2016) exceeds that of any other organic compound. Much of this production goes toward polyethylene, a widely used plastic containing polymer chains of ethylene units in various chain lengths.
Th growth of natural gas production from the shale gas formations in the United States has led a rapid increase in global ethylene production. Per a recent Petrochemical Update post:
The ACC [American Chemistry Council] estimates there are 310 projects currently under construction or planned and $185 billion in potential capital investment as of mid- 2017, up from the 97 projects and $72 billion in Mach 2013.
In a Hydrocarbon Processing magazine article, Optimizing ethylene production with laser technology, Emerson’s Amanda Gogates and Jeff Gunnell describe how laser gas analyzer technology is being used in ethylene production.
These analyzers are being applied to improve production throughput, reduce energy consumption and help to meet ethylene purity specifications. They open noting that gas chromatographs (GCs):
…have been the measuring instrument of choice for all areas of ethylene production. From feed qualification, through the cracking furnaces and purification train, to the final product certification of ethylene delivered via ship or pipeline, a typical plant may have 40–50 GCs. In many of these areas, the GC remains the best analytic choice.
GCs remain the online analyzer of choice on the “hot end” of ethylene production, but:
…laser technology has quietly slipped into petrochemical manufacturing as an analytical option, and it has now advanced to a point where it provides a real improvement in terms of speed, precision, reliability and cost in critical areas of the purification train and product certification.
In the cracking process of converting ethane to ethylene, over-cracking leads to the production of acetylene and catalytic beds are used to convert it back to ethylene.
Precise and rapid control of the catalyst activity is vital to maximize the ethylene produced. If the catalyst is not active enough, then not all of the acetylene will be converted into ethylene. Conversely, if the catalyst is too active, then some of the ethylene could be converted back to ethane.
Amanda and Jeff explain:
It is essential to look for acetylene breakthrough at the outlet of the converters to avoid process excursions downstream. Most importantly, measurement of the outlet must be done quickly and with a very low limit of detection.
With typical purity requirements of 99.99% pure ethylene, ethylene fractionation towers perform this purification.
Operating the tower efficiently offers considerable economic advantages in reducing product giveaway, thereby minimizing energy usage and avoiding ethylene recycle. Measuring the C1 and C2 molecules, as well as CO and carbon dioxide (CO2), allows the tower operation to be fine-tuned for maximum efficiency and ensures that ethylene production is on-spec.
Laser gas analyzer technology found in products such as Rosemount Quantum Cascade Laser Analyzers. These analyzers:
…can combine up to six quantum cascade lasers (QCLs) and tunable diode lasers (TDLs) in a single system for multiple gas measurements…
…between six and 12 highly varied measurements can be made in a single analyzer. When GCs are used, multiple instruments are required; therefore, the use of lasers can save capital expenditure costs. An advanced signal processing procedure enables real-time validation of measurements and greatly reduces the need for calibrations, reducing ongoing operational costs.
Based on the fast response times required in controlling the ethylene process:
…the sample flows through a measurement cell where laser beams continuously analyze the gas. The response time is typically less than 10 sec to achieve 90% of a step change. The output is effectively continuous and in real time.
They summed up the QCL/TDL analyzers’ advantages:
This speed of the laser measurement is helpful in the process tower. However, it makes a huge difference in acetylene control, where it can quickly detect a process upset that might otherwise cost hundreds of thousands of dollars per hour to correct.
Read the article for more on the performance improvements provided with the addition of laser gas analyzer technology. You can also connect and interact with other analyzer experts in the Analytical group in the Emerson Exchange 365 community.