Thermodynamical aspects of ocean sequestration by pptfiles


									Chemical & Bioengineering

Thermodynamical aspects of ocean sequestration

Wolfgang Arlt
Chair of Separation Science & Technology
Prof. Arlt

Friedrich-Alexander-University Erlangen-Nuremberg


capture of CO2 in electric power stations fired with fossile energy


Prof. Arlt


efficiency drop
• Nat. gas CC: 56% => 47% • Coal fired: • Gasif. CC: 46% => 33% 46%=> 38%

captured CO2
(370 => 60 g CO2/kWh) (720 => 150 g CO2/kWh) (710 => 130 g CO2/kWh)

CC: combined cycle

It is the task of thermodynamicists to reduce this drop of efficency.

Lysen, Utrecht

CO2 capture in power plants reduces efficiency

Prof. Arlt


Natural dissolution in the sea water
The sink for CO2 are mainly the oceans, absorbing CO2 in a buffered system. The surface is affected most.


Prof. Arlt


Alkalinity there are two definitions for alkalinity: 3- dimensional dynamical-statistical atmosphere model - The alkalinity of seawater is the sum of the concentrations of anions that accept protons at theof 10° (2°) in latitude and 51° (4°) in longitude •spatial resolution pH of seawater. •multibasin zonally averaged ocean model - Another definition for the alkalinity is that it is the difference between the •sea ice model which operates with latitudional resolution of 2,5° concentrations of total cation and total anion that do NOT exchange H+ in the •terrestrial vegetation model pH range of seawater. (Lawrence Livermore National Lab model) Ocean Biogeochemistry: biogeochemical tracers considered in the model
•phosphate PO42- and oxygen O2 •alkalinity ALK •dissolved inorganic carbon DIC •two types of dissolved organic carbon DOC

advective, diffusive and convective transports for every tracer at the sea surface DIC and O2 are exchanged with the atmosphere what is simulated by climate models like CLIMBER-2?

Prof. Arlt


Units are Pg for carbon storages and Pg/yr for carbon fluxes The oceanic and terrestrial carbon storages are 38320 and 1930 Pg C oceanic and terrestrial net primary productivities are similar 44 and 55 PgC/yr respectively


P = 1015; NPP = net primary production; POC =particulate organic carbon Scheme of the global carbon cycle in preindustrial steady state
Prof. Arlt


driving force: partial pressure of CO2 in the air and in the surface water = Henry Law
gas exchange coefficient estimated to 0,06 mol m-2 yr-1 10-6 atm-1 [Siegenthaler 1986] PCO2 in the surface water is calculated from surface temperature, salinity, DIC and ALK in accordance with Millero [1995] dissolution of CaCO3 Biological processes in the euphotic zone (the upper 100 meters) are explicitly resolved using the model for plankton dynamics. Availability of nitrate and in some places of iron (IronEx) biological productivity

It was proposed by John Martin, researcher at Moss Landing Marine Laboratories (MLML), that adding iron to the oceanic waters would increase plankton growth to a significant degree. In 1993 and 1995, researchers at MLML conducted research experiments called IronEx I and Brovkin IronEx II in which they tested this hypothesis.

. thermodynamics of dissolution in the sea water

Prof. Arlt


Downwelling and upwelling currents are produced by density differences in sea water, e.g.

•at the north pole pure water ice is produced what causes an increase of density in the remaining sea water.
•in Gibraltar, denser sea water from the „cooking pot“ Mediterranean flows into the Atlantic ocean.

Flows can be very high.

Diffusion is important but does not really contribute to mixing.

mixing effects with deep water

Prof. Arlt


The vertical tracer mixing coefficient varies from 0.2 cm2/s [Wickett 2003]

1 cm2/s
0,1-0,3 cm2/s

[Radhakrishnan 2003]
[Wong, 1996]

... at the surface to 10.0 cm2/s at depth. Since model momentum mixing (viscous) coefficients are larger than physically justifiable, smallest value is used, what is relatively free of numerically induced noise. [Wickett 2003]

uncertainties in diffusion

Prof. Arlt


Man-made, two phase dissolution in the sea water
•impact on microorganism
•available data and modelling


Prof. Arlt


1. inject liquid CO2 into liquid seawater (plume, dissolution of drops)

2. inject gaseous CO2 into liquid seawater
(plume, dissolution of bubbles) 3. dispose adsorbed CO2 into liquid seawater 4. single phase, saturated at the land

how CO2 is injected in the ocean

Prof. Arlt


deep sea

flat water
CO 2 gas

liquid CO2

liquid CO2

200 ~ 400 m
1 phase or 2 phases

1000 ~ 3000 m

>3400 m CO 2 -hydrate

liquid CO2

Where to leave the CO2: ocean (the natural option)

Prof. Arlt


The uplift velocity is a function of the viscosity and density of the sea water and the size of the bubbles. The density of seawater is increased by dissolved CO2.
The size of the bubbles is extended by decreasing pressure and reduced by dissolved gas. So a gas transfer coefficient must be known.

airlift principle by Kajishima [1997]

Prof. Arlt


There are 2 principal topics:
•thermodynamics of hydrate formation •dynamics of hydrate formation

for the dynamics the following data is needed 1. diffusion coefficient of CO2 in seawater (assumed to be 2x10-9 m2s-1) 2. diffusion of water thru the hydrate film (1,1 x 10-12 m2/s) [Rad..]

hydrate formation (Radhakrishnan,Kajishima)

Prof. Arlt


The CO2 may form a hydration shell from a symmetrical dodecahedral arrangement of 18 water molecules where each CO2 oxygen atom is hydrogen bonded to three water molecules.

Clathrates exist near 0°-+10°C and higher pressures 40-1000 bar

Clathrates are known between water and different gases, among them methane. Clathrate
Prof. Arlt


impact of dissolved CO2 on marine organisms


Prof. Arlt


consider the marine organism as a membrane

CO2 affects •the transport of respiratory gases in the organism

•the formation of the calcareous skeleton


Prof. Arlt


Thermodynamic data in literature


Prof. Arlt


[Roy, 1995 /Subow, 1931] define sea water as compound NaCl MgCl2 MgSO4 CaCl2 KCl NaHCO3 NaBr conc in g/ kg seawater 26,518 2,447 3,305 1,141 0,725 0,202 0,083

Approximation of data is made by an empirically extended virial equation by Zhenhao Duan.

. what is seawater?

Prof. Arlt


The CO2-solubility in pure water is known from 0,05 to 30 MPa and from 0 to 100°C. Some data are on seawater.
Clatharates are formed below 10°C and 4,4 MPa. [Diamond 2003]

Liquid densities of CO2/pure water 4 to 20°C by [Teng 1997], for water/CO2/NaCl at more than 130°C by Gehring [1986] We did not check for viscosities.

thermodynamic data

Prof. Arlt




10°C 0°C


Approximate density of sea water

(Brewer, Science 1999)

Densities of pure liquid and solid Carbon Dioxide

Prof. Arlt


density of water + CO2 density of sea water + CO2 solubility of CO2 in water at 5°C X solubility of CO2 in sea water at 5°C X pH of CO2-water/seawater X impact of pH-change on marine life deep sea current X surface current X diffusion X biological conversion of CO2 X alternative power processes X effective absorption procedures 10->1 vol% total energy balance

known X

unknown X X X


What is known?

Prof. Arlt


„Everything“ is in the ocean.

CO2-hydrogencarbonate-carbonate equilibrium .

Prof. Arlt


equilibrium of carbonate salts
Prof. Arlt


These data vary with water depth, too. Fluctuation in pH about 0,15.

pH and alkalinity of surface water in the Pacific

J.P.Riley, G.Skirrow: Chemical Oceanography, Vol. 2, page 145
Prof. Arlt


alkalinity as a function of locus and depth

Prof. Arlt


„Sea water“ is a different liquid a different locations and different depths. A flexible model is needed.

Most thermodynamical data for carbon dioxide and water is known, but few is available for sea water. Small changes in density are important for downwelling currents. This density must be known precisely as a function of CO2 concentration. The thermodynamical model is more or less the semi-empirical EoS by Duan, what only reflects phase equilibrium data. A more general model is needed.

Most transport data is interpolated and approximated. This is especially true for hydrate formation and plume.

Résumé .

Prof. Arlt


„If you want toSchiff bauen wdon‘t trom m le nicht die M ännerto divide en, um W enn D u ein build a ship, illst, drum the men together zusa m m the die A rbeit einzuteilen, sondern lehre Sie them to yearn for the last and work and give orders. Instead, teach die Sehnsucht nach dem sea. M eer.“ endless w eiten Antoine E xupéry A ntoine de Saintde Saint Exupéry
courtesy: BASF

The end.

Prof. Arlt


Arlt, not Saint Exupéry

Prof. Arlt


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