Modeling benzene emissions reductions in condensation tanks

According to the Canadian Association of Petroleum Producers (CAPP), there are over 2,500 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. 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 (a field test which directly measures before- and after- condenser volume and composition) 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. 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 which includes a condenser can vary with changes in operating parameters and ambient conditions. The figure shows how some operating parameters, such as use of stripping gas, can have a large impact on benzene emissions reduction.

Figure 2. Changes of benzene emissions as a function of expected changes in operating/ambient conditions

Thermodynamic model 

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

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

Generic Tank

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

TankSafe

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

Web application

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: Enter the facility information (name, location, ambient temperature, and wind velocity).

Step 2: Specify the tank type/Pipe details.

Step 3: Input still overhead information (pressure, temperature, mass flow rate, composition), from 3rd party simulator.

Step 4: Review the data and submit the data.

Step 5: View and Print the results 

Conclusions

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 ambient data for BC and SK; also we are currently working with the AER to gain acceptance of this Application. It has been made available to industry through PTAC to assist in efforts to continue reducing emissions of benzene from natural gas dehydration plants. Please visit http://condenser.ptac.org for more information, and to use the application.

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