Carbon dioxide liquid phase partial pressure

The dehydration of ammonium carbamate is appreciable only at temperatures above the melting point (about 150°C) and this reaction can only proceed if the combined partial pressure of ammonia and carbon dioxide exceeds the dissociation pressure of the ammonium carbamate (about 100 atmospheres at 160°C and about 300 atmospheres at 200°C). Thus commercial processes are operated in the liquid phase at 160—220°C and 180—350 atmospheres. Generally, a stoichiometric excess of ammonia is employed, molar ratios of up to 6 1 being used. The dehydration of ammonium carbamate to urea proceeds to about 50—65% in most processes. The reactor effluent therefore consists of urea, water, ammonium carbamate and the excess of ammonia. Various techniques are used for separating the components. In one process the effluent is let down in pressure and heated at about 155°C to decompose the carbamate into ammonia and carbon dioxide. The gases are removed and cooled. All the carbon dioxide present reacts with the stoichiometric amount of ammonia to re-form carbamate, which is then dissolved in a small quantity of water and returned to the reactor. The remaining ammonia is liquefled and recycled to the reactor. Fresh make-up ammonia and carbon dioxide are also introduced into the reactor. Removal of ammonium carbamate and ammonia from the reactor effluent leaves an aqueous solution of urea. The solution is partially evaporated and then urea is isolated by recrystallization. Ammonium carbamate is very corrosive and at one time it was necessary to use silver-lined equipment but now satisfactory alloy steel plant is available. Urea is a white crystalline solid, m.p. 133°C. 

The dew point must be warmer than -56.6°C to permit use of liquid carbon dioxide absorbent because pure liquid carbon dioxide cannot exist below the triple point. The carbon dioxide partial pressure, i.e., gas phase CO2 mol fraction times total pressure, of synthesis gas mixtures with -56.6°C dew points is plotted versus synthesis gas pressure in Figure 4. Increasing the H2 CO ratio at fixed total pressure decreases the carbon dioxide partial pressure required for a -56.6°C dew point. Liquid carbon dioxide can be used to absorb sulfur molecules for any combination of gas pressure and carbon dioxide partial pressure which lies above the curves of Figure 4. 

There is an abundance of experimental gas partial pressures for gas hydrate equilibria across a broad range of temperatures (Fig. 3.10 Sloan 1998). The lower temperature limit in our model database for these systems is 180 K (Fig. 3.10) because this is the lower limit of our model s ability to estimate aw (Fig. 3.1, Eq. 3.11), which is needed to calculate the solubility product of gas hydrates (Eq. 3.36). In our model, the upper temperature limit for methane hydrate is at 298 K (25 °C), which is the upper temperature limit for FREZCHEM the upper temperature limit for carbon dioxide hydrate is at 283K (10 °C), which is the temperature where liquid C02(l) becomes the thermodynamically stable phase. 

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

The computer simulation program which was available for miscible flood simulation is the Todd, Dietrich Qiase Multiflood Simulator (28). This simulator provides for seven components, of which the third is expected to be carbon dioxide and the seventh water. The third component is allowed to dissolve in the water in accordance with the partial pressure of the third component in the non-aqueous phase or pdiases. It is typically expected that the first two components will be gas components, while the fourth, fifth, and sixth will be oil components. There is provision for limited solubility of the sixth component in the non-aqueous liquid p ase, so that under specified conditions of mol fraction of other components (such as carbon dioxide) the solubility of the sixth component is reduced and some of that component may be precipitated or adsorbed in the pore space. It is possible to make the solubility of the sixth component a function of the amount of precipitated or adsorbed component six within each grid block of the mathematical model of the reservoir. This implies, conversely, a dependence of the amount adsorbed or precipitated on the concentration (mol fraction) of the sixth component in the liquid non-aqueous j ase, hence it is possible to use an adsorption isotherm to determine the degree of adsorption. 

Gas Composition. To remove the restriction of constant dissolved carbon dioxide concentration, it is necessary to consider the interactions between the gas and liquid phases of the reactor. At equilibrium the dissolved carbon dioxide concentration can be related to the partial pressure of carbon dioxide in the gas phase by Henry s law as shown in Equation 23. 

Gas Flow Rate. The model can be extended to consider all five desired variables, and the restriction of a constant partial pressure of carbon dioxide can be removed by developing material balances for carbon dioxide in both the liquid and gas phases. The material balance on dissolved carbon dioxide is shown in Equation 25. Rb is the rate of production of carbon dioxide from the substrate by the methane bacteria and Rc is the rate of production of carbon dioxide from bicarbonate. The reaction of substrate and bicarbonate to produce carbon dioxide is given in Equation 28. 

In Fig. 9.1, we plot the variation of the partial pressure of carbon dioxide with the mole fraction of the gas dissolved in a liquid mixture of carbon dioxide and hexane. When. 77 7,. = 0, the system consists solely of hexane, and the partial pressure of carbon dioxide is zero. The vapor space consists entirely of hexane at its vapor pressure. As the concentration of carbon dioxide dissolved in the liquid phase increases, the partial pressure of carbon dioxide in the vapor phase also increases. However, for a co2 > 0-7, the liquid phase no longer exists above this mole fraction, the system is a one phase vapor (or gas) mixture. 

Figure 9.1 Variation of the partial pressure of component carbon dioxide in the vapor phase with its mole fraction in the coexisting liquid phase mixture of carbon dioxide and hexane at 393.15 K. The solid line and filled symbols denote the actual variation of the partial pressure. The dotted line is Henry s law. Data taken from YH Li, KH Dillard, and RL Robinson, J. Chem. Eng. Data. 26, 53 (1981).

The decomposition of calcium carbonate (Eq. 13.2-3), or any other reaction in which the reaction products and reactants do not mix in the gas or liquid phase, represents a, fundamentally different situation from that just considered, and such a reaction may go to completion. To see why this occurs, consider the reaction of Eq. 13.2-3 in a constant temperature and constant pressure reaction vessel, and let Ncaco2,o md Nqo-,0 represent the number of moles of calcium carbonate and carbon dioxide, respectively, before the decomposition has started. Also, since none of the species in the reaction mixes with the others, we use pure component molar than partial molar Gibbs energies in the analysis. Then,... 

Experimental procedure. The autoclave is partially filled with a clear solution of PPE in toluene and closed. The pressure is raised linearly from atmospheric to the desired final pressure by supplying carbon dioxide to the bottom of the precipitator (V-1 open). The CO2 bubbles through the solution, where it is absorbed by the liquid phase. Close to the final pressure the liquid level begins to rise rapidly and the clear... 

The solubilities of solids and liquids are not appreciably affected by pressure, whereas the solubility of a gas in any solvent is increased as the partial pressure of the gas above the solvent increases. We can understand the effect of pressure on gas solubility by considering Figure 13.12, which shows carbon dioxide gas distributed between the gas and solution phases. When equilibrium is established, the rate at which gas molecules enter the solution equals the rate at which solute molecules escape from the solution to enter the gas phase. The equal number of up and down arrows in the left container in Figure 13.12 represent these opposing processes. 

Figure 16.9 Carbonated beverages. Bottling under a high carbon dioxide partial pressure increases the solubility of the gas in the liquid solution. When you open the bottle, the partial pressure of carbon dioxide above the solution drops to become equal to atmospheric Pcoj> dramatically decreasing the solubility of the gas, which escapes from the solution in the form of gas-phase bubbles that consist of carbon dioxide molecules (and a relatively small quantity of water molecules). 

Figure 1.10 Pressure/composition diagram for carbon dioxide//i-hexadecane (a Class B1 system) at 26.5°C, showing the very restricted range over which liquid CO2 is partially miscible with the -hexadecane. Vapour/liquid equilibria have not been investigated in detail in the immediate vicinity of the vapour pressure for pure CO2 but probably take the form shown in the inset, the mole fraction of CO2 at the point Q being about 0.98. The three phase pressure (P ") is probably about 2 bar less than the CO2 vapour pressure (P°). Subscripts 1 and 2 denote hexadecane-rich and...

Physical Solution. In this type of process the component being absorbed is more soluble in the liquid absorbent than other components of the gas stream, but does not react chemically with the absorbent. The equilibrium concentration of the absorbate in the liquid phase is strongly dependent on the partial pressure in the gas phase. An example is the absorption of hydrogen sulfide and carbon dioxide in the dimethyl ether of polyethylene glycol (Selexol Process). Relatively simple analytical techniques have been developed for designing systems of this type. 

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