According to the Canadian Association of Petroleum Producers (CAPP), there are about 4,000 operating glycol dehydration units in Western Canada, and benzene emissions from these units are a major concern. The Alberta Energy Regulator (AER) regulates the maximum allowable benzene emissions from these units through Directive 39. In January 2013, AER introduced new regulations to reduce benzene emissions from most dehydration units to less than 1.0 tonnes/year by 2018. AER currently accepts process simulation predictions (HYSYS, GlyCalc, ProMax) for the entire facility, except for condensers when used as emissions control technology. Total Capture Testing is currently the only option acceptable to the AER for claiming benzene emissions reductions due to a condenser.
The Alberta Upstream Petroleum Research Fund launched a project with Process Ecology Inc. with the objective to develop a software tool which could be used to accurately estimate benzene emissions reductions in condensers.
Figure 1. Process Flow Diagram of a typical dehydration facility
In order to accomplish the stated goal, the following models were developed to capture the performance of condensation tanks:
- Thermodynamic model: To describe vapor-liquid-liquid equilibrium in condensation tanks.
- Property estimation: To estimate mixture properties required as an input for the heat transfer model.
- Heat transfer model: To estimate the heat losses in the upstream pipe and condensation tank based on their configuration and ambient conditions.
- Ambient model: To account for the annual variation of temperatures and wind speeds at specific locations.
The following figure shows how benzene emissions from a typical dehydration facility vary with changes in operating parameters and ambient conditions.
Figure 2. Changes of benzene emissions as a function of expected changes in operating/ambient conditions
The Peng-Robinson equation of state was selected to model the vapor and liquid phase equilibrium. The developed thermodynamic model was validated against Aspen HYSYS using 6 representative still column compositions over the range of expected condenser outlet temperatures. Results of the model are within 10% of the HYSYS flash calculation.
Figure 3. Comparison of HYSYS and developed thermodynamic model
Heat transfer model
The developed heat transfer models include heat transfer from pipe, conventional tank, and TankSafe condenser.
Pipe segment(s) are used to transfer the still vent overhead vapor to the condenser. The pipe would be typically uninsulated to aid in the cooling of the vapor; however the last section may be insulated to avoid freezing in winter conditions. In the software tool, the user can specify pipe diameter and length, and the respective pressure drop and heat loss will be calculated. Figure 4 shows the cross-section of pipe and heat transfer mechanisms implemented in the model.
Figure 4. Cross-section of pipe
Standard storage tanks are used in some units as condensers to reduce benzene emissions. The condenser model for this configuration includes heat transfer coefficients for dry wall, wet wall, roof, and tank bottom, which is shown in Figure 5.
Figure 5. Heat transfer mechanism for a generic tank
A condenser configuration widely used in Western Canada is the “TankSafe” condenser. In this device, vapor is directed to series of baffles and is cooled down with ambient air based on natural convection. For this configuration of condenser, both fluid flow and heat transfer are rigorously modeled in each baffle. Heat transfer coefficients include fluid flow over a flat vertical surface for internal heat transfer along with the correction for non-condensables (which significantly reduce heat transfer coefficients), as well as the thermal conductivity of the wall and outer heat transfer coefficients. Preliminary results show most of the cooling takes place in the first few baffles. Figure 6 (a) shows a typical TankSafe condenser and Figure 6 (b) shows the flow direction and heat transfer area in TankSafe condenser baffles.
Figure 6. (a) TankSafe Condenser (b) Condenser baffle
The software tool has been developed as a web-based application incorporating the described models. The user must follow five simple steps to obtain results:
Step 1: Facility information (name, location, ambient temperature, and wind velocity).
Step 2: Tank type/Pipe details.
Step 3: Still overhead information (pressure, temperature, mass flow rate, composition).
Step 4: Review the data and submit
Step 5: Print the results
An online application has been developed and validated to model benzene emissions reductions in condensation tanks. The model incorporates detailed thermodynamic, heat transfer, and physical property calculations. Future work includes incorporating an ambient air model and solar radiation. This tool will likely be made available to industry to assist in efforts to continue reducing emissions of benzene from natural gas dehydration plants.
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