Simulation of Benfield Process for Co₂ Removal

R W Gaikwad

Department of Chemical Engineering, Pravara Rural Engineering College, Loni, Dist: Ahmednagar (MS)-India-413736.

ABSTRACT

Interest in recovery of carbon dioxide (CO₂) from flue gases is being propelled by multiple factors: the merchant CO₂ market, renewed interest in enhanced oil recovery (EOR), and the desire to reduce greenhouse gas emissions. Benfield process uses hot potassium carbonate solution to absorb CO₂ from the flue gas. It is carried out in a packed absorption column, the absorbed CO₂ in potassium carbonate solution is then removed by distillation and the recovered potassium carbonate solution is recycled. The Benfield process for CO₂ removal has been modeled and simulated using CHEMCAD software package (version 5.1.0 release 2000). The simulation result shows that the CO₂ in the feed stream was almost entirely absorbed in the absorption column with the help of potassium carbonate. The amount of CO₂ was reduced from 117.59 kmol/h (feed stream) to 0.07 kmol/h (purified gas stream).The evolutions of the process parameters were studied during the process. The mathematical model and the simulation results, using CHEMCAD, proved to be a reliable tool for analyzing the process of removing CO₂ from the feed gas.

Keywords: CO₂ removal, Benfield process, potassium carbonate solution, CHEMCAD.

INTRODUCTION:

Many industrial processes must reduce acid gases emissions because of the passage of the Clean Air Act Amendments in 1990. In many processes, especially in waste incineration, the resulting flue gas include acid elements such as CO₂that have to be removed to meet environmental standards. To meet legal limits, it is necessary to reduce the CO₂ content of the flue gas coming from plants.The removal of the toxic acid gases is performed through reaction with alkaline sorbents in wet, semi-dry, and dry processes.Flue gas cleaning is normally done in several steps. Dust and particulates are removed first in cyclones, filters or electrostatic precipitators. Gaseous pollutants such as carbon dioxide (CO₂) is removed in spray towers, scrubbers, where the gas is contacted in counter-current with an absorption liquid. In the pre-scrubbing unit, HCl is removed. Following the pre-scrubbing unit, the main scrubber eliminates SO₂ from flue gas.A typical effect is the variations of scrubber liquid pH when the control system fails to respond in an appropriate way, variations that affect the absorption of pollutants. Scrubber liquid pH is thus an important aspect in flue gas cleaning. Flue gases from various industries like fossil fuel-fired power plants, industrial furnaces, cement plants, engine exhausts, and lime kiln exhausts contain CO₂ as a major pollutant which must be removed before releasing the gas in the atmosphere. Also CO₂ has its own market as it plays key role in places like food industry in carbonated beverages, brewing, and flash drying. Its industrial uses include enhanced oil recovery (EOR), welding, chemical feedstock, inert gas, firefighting, and solvent extraction as a supercritical fluid. It is an essential ingredient in medical oxygen, where in low concentrations it acts as a breathing stimulant. Benfield process is a thermally regenerated cyclical solvent process that uses an activated, inhibited hot potassium carbonate solution to remove CO₂, H₂S and other acid gas components. The Benfield process uses low cost chemicals available on the world-wide market. There are a variety of flow schemes available that permit process optimization and energy reduction with this near- isothermal unit operation. The high temperature operation of the Benfield process prevents hydrocarbon condensation from occurring. Hydrocarbon and synthesis gas losses are minimal due to their low solubility in the Benfield solution. Mostly carbon steel construction is used and the process is oxygen tolerant without solution degradation.

Figure 2. Tray temperature Profile for Absorption Column

The Benfield process can be tailored for either bulk or trace acid gas removal. It is typically used in the applications and markets such as: synthesis gas treating for CO₂ removal in ammonia plants, synthesis gas treating for CO₂removal in direct iron ore reduction plants, natural gas treating to achieve either LNG or pipeline specifications and recycle gas purification in an ethylene oxide facility.¹ developed, a general model for the mass transfer/reaction processes in the carbon dioxide absorber using promoted hot potash.² compared the power plant performance, with special attention to the power output and efficiency penalty, when a MEA scrubbing system and the necessary compression step are integrated with the steam cycle, and to find the scrubber operating conditions that minimize the impact on the power plant operation. A number of studies have employed steady state models of the chemical (or reactive) absorption process at different levels of complexity.³describes the different levels of complexity of these models. At its lowest level of complexity, the chemical reactions of the rate-based model are assumed to be at equilibrium. A more rigorous approach involves the inclusion of an enhancement factor to estimate actual absorption rates (with chemical reactions) from known physical absorption rates. The enhancement factor is calculated based on estimated reaction rates and is best suited for processes involving single irreversible reactions. This approach has been employed by a number of authors in developing steady state absorber and stripper models.^{3, 4, 5}

Table 1. Properties of feed streams

Stream No.	1	9
Stream Name	GAS FEED	Makeup
Temp C	33.0000*	105.0000*
Pres bar	22.2000*	30.0000*
Enth MJ/h	-1.0876E+005	-61568.
Vapor mole fraction	1.0000	0.00000
Ph value	0.0000	6.0870
Ionic strength molal	0.0000	0.0000
Total kmol/h	1725.22	220.04
Total kg/h	39383.00	3964.04
Total std L m3/h	96.30	3.96
Total std V m3/h	38668.52	4931.92
Flowrates in kmol/h
Ethylene	337.92	0.00
Oxygen	78.13	0.00
Carbon Dioxide	117.59	0.00
Water	1.00	220.04
Nitrogen	42.84	0.00
Argon	105.01	0.00
Methane	1036.59	0.00
Ethane	6.15	0.00
K Carbonate	0.00	0.00
H+	0.00	0.00
OH-	0.00	0.00
CO3--	0.00	0.00
HCO3-	0.00	0.00
K+	0.00	0.00

The simulation of the Benefield process was modeled and simulated using ChemCAD software package. As thermodynamic option used for simulation of the plant, liquid phase activity coefficients are calculated by NRTL equation.

Figure 3. Temperature Profile

Modeling And Simulation Of The Benfield Process

The Benfield Process is applicable to the removal of H₂S and CO₂ from gases, such as natural gas, oil refinery off gas, reformed gas, etc. This process can be more efficiently adapted to the requirements for treating a gas by a combination of typical flow types provided for the removal of CO₂ only, or CO₂ and H₂S, and for reducing heat consumption. The features of the process such as effective activator is used to lower the equilibrium partial pressure of acid gases and accelerate the absorption rate. The heat consumption for regeneration of the absorbing liquid is appreciably reduced by the use of a steam ejector. No specific anti-corrosive material in needed for the equipment. The absorbing liquid is nontoxic and no proprietary chemicals are used. The absorbing liquid suffers no degradation and a reclaimer is not necessary. The absorption and regeneration of acid gases in the Benfield Process are based on the following reactions:

K₂CO₃ + CO₂ + H₂O = 2KHCO₃ ……………………     (1)

K₂CO₃ + H₂S = KHS + KHCO₃  ……………………      (2)

Table 2. The properties of output gas flows from first and second scrubbing units

Stream No.	3	5	12
Stream Name	Purge	CO2	Purified gas
Temp C	86.3511	93.6956	83.6994
Pres bar	1.5000	1.3000	21.5000
Enth MJ/h	-594.40	-91608.	-65628.
Vapor mole fraction	1.0000	1.0000	1.0000
Ph value	0.0000	0.0000	0.0000
Ionic strength molal	0.0000	0.0000	0.0000
Total kmol/h	6.54	308.98	1629.72
Total kg/h	149.59	8614.33	34581.89
Total std L m3/h	0.33	9.71	90.22
Total std V m3/h	146.48	6925.29	36527.86
Flowrates in kmol/h
Ethylene	1.97	0.22	335.73
Oxygen	0.11	0.00	78.02
Carbon Dioxide	0.33	117.17	0.07
Water	1.77	191.50	27.76
Nitrogen	0.04	0.00	42.80
Argon	0.26	0.01	104.74
Methane	2.06	0.07	1034.46
Ethane	0.01	0.00	6.14
K Carbonate	0.00	0.00	0.00
H+	0.00	0.00	0.00
OH-	0.00	0.00	0.00
CO3--	0.00	0.00	0.00
HCO3-	0.00	0.00	0.00
K+	0.00	0.00	0.00

Modeling and simulation of the scrubbing unit using ChemCAD, certifies a good pollutant removal at low potassium carbonate consumption.

Figure 4. Equilibrium Curve

The absorption and regeneration of acid gases are conducted in a similar way to that of the conventional amine or carbonate processes. The gas to be treated is fed to the bottom of the absorber and flows countercurrent to the absorbing liquid supplied at the top of the absorber. Acid gases are then absorbed by the absorbing liquid. The liquid that has absorbed the acid gases is preheated and then supplied to the top of the regenerator where the acid gases are stripped by steam for the regeneration of the liquid. The regenerated liquid is precooled and recirculated to the absorber. For simulation the Benefield process the properties of the flue gas are presented in the Table 1 were used.

RESULTS AND DISCUSSIONS:

The scrubbing unit was modeled and simulated using process data presented above. In the both columns, the total vapor flow decrease because of the absorption of acid gas components in the alkaline solution. In the first absorption column, the carbon dioxide content of flue gas is reduced at pH value of 1, and in the second absorption column the hydrogen sulphide are reduced. The variation of Tray temperature Profile for Absorption Column, Temperature Profile and Equilibrium Curve first absorption column are presented in the Figures 2, 3 and 4.

CONCLUSION:

Simulation of the Benefield process was done using ChemCAD software package (version 5.1.0 release 2000). The development of the process parameters were studied during the process. The simulation results were compared with real plant operation data in order to validate the application developed for the process. The mathematical model and the simulation results, using ChemCAD proved to be a reliable tool for analyzing the process of removing carbon dioxide from gaseous emissions.

REFERENCES:

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2. Kenig EY, Schneider R, Górak A., Reactive absorption: optimal process design via optimal modelling. Chem Eng Sci ; 2001;56(2):343–50.

3. Pintola T, Tontiwachukwuthikul P, Meisen A., Simulation of pilot plant and industrial CO2 MEA absorbers. Gas Sep Purif ; 1993;7(1):47–52.

4. Alatiqi I, Sabri MF, Bouhamra W, Alper E., Steady-state rate-based modeling for CO2/amine absorption–desorption systems. Gas Sep Purif ; 1994;8(1):3–11.

5. Al-Baghli NA, Pruess SA, Yesavage VF, Selim MS. A rate-based model for the design of gas absorbers for the removal of CO2 and H2S using aqueous solutions of MEA and DEA. Fluid Phase Equilib; 2001; 185(1–2):31–43.

Received on 15.11.2009

Accepted on 26.11.2009

Research J. Science and Tech. 1(2): Sept –Oct. 2009: 85-87