Novel Solvent Selection and Solvent Stripping for CO2 Capture From Power Plants

Novel Solvent Selection and Solvent Stripping for CO₂ Capture from Power Plants

Juan Salazar and Urmila Diwekar

Center for Uncertain Systems: Tools for Optimization & Management

Vishwamitra Research Institute

Clarendon Hills, IL 60514

E-mail: urmila@vri-custom.org; Tel: 630-886-3047

And

Kevin Joback

Molecular Knowledge Systems, Inc.

Bedford, NH  03110-0755

And

Abhoyjit S. Bhown

Electric Power Research Institute, Inc.

Palo Alto, CA 94304

This paper presents a new approach to solvent selection for post-combustion CO₂ capture from fossil fuel fired power plants.  Two main questions arise in a solvent capture process: how to select an effective separating agent and how to design and synthesize this separation process. In previous studies, few solvents were considered for CO₂ capture. Further, researchers arrived at these solvents using experimental methods with limited capacity to screen a large number possible candidates.  In this work, we are using computer aided molecular design (CAMD) method to obtain a large number of possible solvent candidates. These candidates are then evaluated by integrating the solvent selection and CO₂ capture process to ensure improved energy, environmental, and economic performance. In this paper we present the first attempt at using group contribution based CAMD to derive new solvents for CO₂ capture.  

CO₂ absorption in a solvent-based capture process is essentially based on the reversible, selective nature of the chemical reaction between the liquid solvent and CO₂ in the flue gas. The process flow diagram shown in Figure 1 has been widely used in literature.  Flue gas from the boiler is brought into contact with the solvent in the absorber after the removal of impurities such as NOx, SOx and particulate matter using processes that also cool the flue gas.  A blower is used to compensate the pressure drop experienced in the absorber. In the absorber, CO₂ selectively absorbs into the solvent, and flue gas leaving the absorber is relatively free of CO₂. The CO₂-rich solvent from the absorber is then pumped to the top of a stripper (or regeneration vessel), via a heat exchanger. Solvent is regenerated in the stripper at elevated temperatures and near atmospheric pressure. The desorption heat required for removing the absorbed CO₂ is provided to the reboiler section of the stripper.  The CO₂-lean solvent, containing far less CO₂ is then pumped back to the absorber via a lean-rich heat exchanger to cool it to the operating temperature of the absorber.  We used this process diagram to evaluate new solvents.

Figure  SEQ Figure \* ARABIC \s 1 1
Base Absorption Process Flowsheet for CO₂ Capture 

 In our CAMD approach, we concentrated on amines.  In order to use group contribution methods, at first we analyzed the equilibrium performance for these new amines for absorption (CO₂ capture) and for regeneration (solvent stripping).   The CAMD resulted in a list that included a group of more than 50 alkyl alcohol amines whose CO₂ solubility properties have not been experimentally determined as far as we know.  We evaluated these solvents using ASPEN Plus models and group contribution methods like UNIFAC to evaluate the nonideality.  Figure 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300320033003000320037003400390030000000 shows the preliminary results of these solvents. The figure shows the energy requirement versus boiling point for 18 solvents simulated both as ideal an nonideal solvents.   In general, we observe the nonideality related to these solvents reduces the energy consumption.   We will be generating and evaluating additional solvents in order to determine optimum solvents.

Figure  SEQ Figure \* ARABIC \s 1 2
Energy Requirements for the New Solvents