Real Time Measurments of Acetylene in Ethylene Manufacturing Process

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Keywords
Tunable Diode Laser, Wavelength Modulation Spectroscopy, Analyzer, Acetylene, Ethylene, Acetylene Converter

Abstract
The analyzer, based on near infrared Tunable Diode Laser Absorption Spectroscopy (TDLAS), was developed for the measurement of acetylene in process streams. Measurements of acetylene were made as under laboratory conditions and in different locations of the back end acetylene converter of the ethylene plant. In both cases reliable performance was demonstrated over a wide range of analyte concentrations and under a variety of experimental conditions (e.g., sample pressure, temperature, and background gas composition). Field trials were conducted over several months and the performance of the analyzer was compared with two gas chromatographs (GC) measuring acetylene in the same gas stream. A strong correlation of the TDLAS acetylene analyzer reading with gas chromatographs was demonstrated. One of the analyzer configurations for the field trials included simultaneous acetylene measurements in different locations of the converter with a goal to optimize production of ethylene. The acetylene measurements yielded an accuracy of 0.3 ppmv over a concentration range of 0 – 5 ppmv; an accuracy of 40 ppmv was observed for a range of 0 – 3000 ppmv. The advantage of TDLAS acetylene measurements from the view point of the speed of the response in comparison with gas chromatography was also demonstrated.

Introduction
This paper describes the application of TDLAS for measurements of acetylene based on extractive sampling. The principal objective of the work reported here is to characterize new TDLAS-based extractive analyzer with an all-digital protocol for the modulation of the laser drive signal and the demodulation of the detector response. This analyzer was configured for acetylene measurements in the gas stream typical for ethylene plant acetylene converter. Another objective of this work is to present the results of field trials of the acetylene analyzer at different locations of the acetylene converter and to demonstrate the benefits of using TDLAS analyzer for acetylene measurements in comparison with traditionally used gas chromatograph.

Acetylene is a byproduct of modern ethylene production processes and is considered an impurity of the process. Even in small concentrations, acetylene acts as a poison to the catalyst used for production of polyethylene from ethylene. Ideally, the maximum concentration of acetylene in an ethylene final product should be less than 5 ppmv. In modern ethylene production the most common method for acetylene removal is through selective hydrogenation in vapor phase. During the hydrogenation process other chemical reactions can occur in the hydrogenation unit, and as a result, ethylene can be converted to ethane if the reaction goes too far.

It is known that acetylene hydrogenates much faster than ethylene over the same catalyst when both acetylene and ethylene are present. The reason for that is preferential occupation by acetylene of active adsorption sites. This preferential adsorption of acetylene over ethylene assumes that there are sufficient amount of acetylene molecules to cover all active sites to minimize ethylene hydrogenation to ethane. However under conditions of industrial operations at the location near the bottom of the final bed where most of the acetylene molecules are already hydrogenated there is insufficient number of the acetylene molecules to maintain total active site coverage. In this situation carbon monoxide provide a means for blocking and inhibiting the adsorption of ethylene because adsorption rate for carbon monoxide is higher than that of ethylene. These facts are the basis for selective hydrogenation of acetylene to ethylene (1-3). To avoid incomplete conversion of the acetylene or undesired conversion of the ethylene to ethane the optimal conditions for the hydrogenation process may be determined by real time monitoring of acetylene concentrations at different locations of acetylene converter. Traditionally, gas chromatography has been used to provide acetylene monitoring in the ethylene production process. However despite the high sensitivity, which can result in the ability to measure sub ppmv levels of acetylene, gas chromatography’s disadvantage is its cycle time requirement resulting in low speed of the response. The analysis time for a GC usually takes several minutes. TDLAS-based measurements provide real time monitoring with data acquisition rate of few seconds. It was also found (4) that during plant start up and other unconditional events the concentration of acetylene can suddenly reach very high levels. As was experimentally demonstrated in the environment of the ethylene plant during these fast acetylene fluctuations, the gas chromatograph either misses the event entirely or report it with delay after completion. It is critical to provide real time monitoring of the acetylene hydrogenation process at the required concentration levels and to control the time of the hydrogenation process.

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