Optimization of ethylene in the processing of hydrocarbons
The measurement of moisture or humidity is very important in most industries, including the petrochemicals sector. Excess moisture in instrument air can cause pipes to rust or pneumatic equipment to malfunction.
The measurement of moisture or humidity is very important in most industries, including the petrochemicals sector. Excess moisture in instrument air can cause pipes to rust or pneumatic equipment to malfunction. Controlling the moisture concentration in process solvents helps maintain quality and prevents the formation of corrosive acids (e.g., hydrochloric acid). High moisture levels can impact the energy content of natural gas, so it is critical for many process applications to have a reliable moisture measurement system in place to ensure smooth operations.
The ethylene process
Moisture measurement is essential in the optimization of ethylene production. Ethylene is the world’s most produced chemical, and its largest uses are for polyethylene, vinyl chloride monomer (ultimately polyvinyl chloride) and ethylene glycol.
Ethylene can be obtained through steam cracking, using a hydrocarbon feedstock such as ethane, naphtha or gasoil. Ethane feedstock is considered an ideal feedstock, since it has a higher selectivity and yield of ethylene than propane, butane or heavier feedstocks (like naphtha or gasoil). This results in simpler processing and reduced capital costs.
Ethane is diluted with steam and heated to 759°C–950°C (1,398°F–1,742°F) in a furnace. Steam cracking is an endothermic process that breaks up large molecules into smaller ones. The steam cracking effluent is sent to different separation and purification units, which allows for the recovery of ethylene and other products, such as methane, ethane, propane and propylene, which are also cracking remnants.
After ethylene recovery, a purification process is necessary to obtain the desired ethylene quality. One of the main objectives of the purification process is to reduce the presence of water necessitated by the handling of ethylene at cryogenic temperatures. Ethylene must be pressurized and cooled to obtain the liquid ethylene that facilitates transportation in specially built tank cars to storage tanks, or through pipelines to different plants. A compression and cooling process is needed to obtain cryogenic ethylene. Moisture in ethylene forms hydrates that are likely to block exchangers and lines or reduce flow performance, causing significant and costly operational issues.
Moisture measurement
Previous technologies used for moisture measurement in ethylene gas were sensitive to external temperature and process pressure changes and were not sensitive enough to small variations in the process that the operators required. The evaluation of a quartz crystal microbalance (QCM) trace moisture analyzer in ethylene gas was undertaken by the co-author’s company. The primary objectives included seeing how this technology responded to external temperature changes and testing the sensitivity of the analyzer to small variations in the process.
Given the variations in ambient temperature that some sites have throughout the year (–10°C to 50°C) and during a typical day (some day/night variations are greater than 10°C), the decision was made to verify this analyzer after it was installed, and to observe the performance in actual ethylene process gas that has very low concentrations of moisture. The variations in outside ambient temperature and process pressure were first analyzed to ensure that process moisture was not affected. Then, the temperature and known process changes were compared with the moisture measurements.
Sampling conditions
The sampling point was located at the outlet of the two ethylene dryer columns. Low-level moisture measurement best practices were followed by using 0.125-in. electropolished stainless-steel sample lines with electrical heat tracing, minimizing the distance (5.5 m) between the sample tap and the analyzer. Increasing the speed of response by reducing the sample volume in the sample conditioning system was executed by reducing the sample pressure to the required analyzer’s sample inlet pressure as close to the sample tap (30 psig, 0.5 m) as possible. Characteristics of the analyzer included the following:
- Analyzer measuring range: 0.02 ppmv–100 ppmv by volume
- Moisture generator: 1 ppmv nominal
- Lower detectable limit: 0.02 ppmv
- Set output range: 0 ppmv–1 ppmv
- Accuracy: +/– 0.02 ppmv or +/– 10% of reading, whichever is greater
- Analyzer sample flow: 150 cm3/min.
Outside temperature variations: Ambient temperature vs. ethylene temperature variations
The objective of this project was to observe how the outside ambient temperature (OAT) affects the temperature of the ethylene in the process. This analysis helped to determine if there was a large absorption or desorption of moisture due to changes in ambient temperature during the day.
The temperature sensor instrument used for the measurement was at the inlet of both dryers. The ambient temperature vs. the ethylene temperature variations is shown in FIG. 1. This period was taken as an example due to the significant day/night variation in OAT (from 0°C–14°C), with an average day/night variation of 8°C. Two observations were made in the ethylene temperature:
FIG. 1. Ambient temperature vs. ethylene temperature variations. The black line represents the OAT trend, and the green line is the process temperature at the input of both dryers.
- Quick variations (average 0.3°C) due to analog signal noise and process temperature variations
- Slow day/night oscillations related to OAT.
The relationship between day/night temperature and ethylene temperature depends on the rate and amount of variation in the outside temperature; however, the variation is less than 1°C on the ethylene temperature for every 10°C OAT.
It can be concluded that adsorption or desorption in this part of the process due to day/night OAT variations would not have a significant effect.
Ambient temperature vs. process pressure variations
The ambient temperature vs. the process pressure variations is shown in FIG. 2. The process pressure remains constant during the day/night. Therefore, process pressure is not influenced by the OAT. In conclusion, process pressure does not affect the measurements of the analyzer.
FIG. 2. Ambient temperature vs. process pressure variations. The black line represents the OAT trend, and the brown line is the process pressure at the input of both dryers.
Moisture measurement variations regarding ambient outside temperature: Winter trend
QCM moisture measurement vs. OAT is shown in FIG. 3. In this winter period, the OAT normally stays low and stable. The trend period shown in FIG. 3 was selected for its large day/night oscillation between 0°C and 13°C, with an average day/night variation of 11°C.
FIG. 3. Winter season trend. The black line represents the OAT, while the red line is the moisture analyzer measurement. The OAT range is –5°C–15°C, while the analyzer measurement ranges from 0 ppmv–0.5 ppmv.
Despite the outside temperature variations, the measurement remains at values of 0.03 ppmv–0.07 ppmv. Twelve hours are indicated on the trend to show the peaks and valleys of the OAT.
It can be concluded that there was no evident relationship between the outside temperature and the moisture measurements. Moisture was at the lower end of the expected values.
Summer season trend
In this summer period, the OAT ranged between 20°C and 40°C, with an average day/night variation of 14°C (FIG. 4). The measurement remained at 0.01 ppmv–0.08 ppmv. A small inverse relationship between the OAT and moisture measurements can be seen when OAT varies widely. Despite high variations in the OAT, moisture remained within the expected values.
FIG. 4. Summer season trend. The black line represents the OAT, while the red line is the moisture analyzer measurement. The OAT range is 20°C–40°C, while the analyzer measurement ranges from 0 ppmv–0.5 ppmv.
Measurement variations regarding dryer swapping
The following analysis used small process variations to check the sensitivity of the analyzer. Using the QCM analyzer, small variations in moisture could be measured during dryer swaps.
In the two-column dryer dehydration system (FIG. 5), one dryer dehydrates the ethylene while the other is in regeneration. When one dryer becomes saturated with water vapor, it is swapped with the other dryer. The saturated dryer is then regenerated. The moisture analyzer was installed at the outlet of both dryers.
FIG. 5. A typical drying process system using two dryer columns.
Moisture measurements related to process dryer swapping. A small variation in the dryer’s moisture measurement (a drop of 0.03 ppmv) can be seen at each dryer swap (FIG. 6). This variation was expected, and it confirmed the sensitivity of this analyzer to process changes. The trend shown in FIG. 7 extended two drying cycles to observe in more detail the moisture variations after each dryer swap.
FIG. 6. Moisture measurement related to process dryer swapping. The red line indicates the moisture analyzer measurement, while the yellow lines indicate the moment of the dryer swap.
FIG. 7. Moisture measurements relate to process dryer swapping (two drying cycles). The red line is the moisture analyzer measurement, while the yellow lines indicate the moment of the dryer swap.
Takeaway
QCM moisture technology has negligible influence due to the OAT. Some variations can be seen (no more than 0.02 ppmv) with day/night temperature changes (5°C–10°C). The lower detectable limit for this analyzer is 0.02 ppmv, which is consistent with what was observed.
In relation to the detection of low moisture values, this analyzer has proven to be very sensitive to small variations in the process. With the analyzer’s internal National Institute of Standards and Technology (NIST) traceable calibration/verification system, it was possible to validate the analyzer’s performance during testing. Because of this operation, the detection of moisture changes during dryer swapping has been transformed into valuable information for monitoring dryer performance and verifying ethylene quality. QCM technology has been proven to be suitable for this type of process. HP
The Authors
Hecker, H. - DOW Chemical Co., Buenos Aires, Argentina
Herman Hecker is an Electromechanical Technician with a background in electronics and computers. He has worked with analyzers at Dow Chemical for 22 yr and is responsible for supporting analytical projects and improvements in Latin America.
Kim, J. - AMETEK Process Instruments, Newark, Delaware
Jung-Il Kim is a Product Manager with a background in electrical engineering. He has worked at AMETEK Process Instruments for 18 yr and is responsible for the moisture measurement product line.
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