Gas equilibrated

The influence of the pre-expansion temperature of the equilibrated gas-saturated solution on the particle size was studied over a pressure range from 80-200 bar. The temperature is represented as the temperature-difference between the saturation temperature before expansion and the melting point of the substance. The results are shown in Figure 9.8-9. 

Given an equilibrated gas-liquid system containing only a single condensable component A, a correlation for pX(T), and any two of the variables y, (mole fraction of A in the gas phase), temperature, and total pressure, calculate the third variable using Raoult s law. 

Equation 6.3-1 is a limiting case of Raoult s law, which will be introduced in a more general context in Section 6.4. It is the fundamental relation used in the analysis of equilibrated gas-liquid systems containing one condensable component. A wide variety of problems occur in... 

Virtually all laboratories report 5 0 activities (not concentrations) for water samples. The 5 H of waters may be reported in either concentration or activity 5 values, depending on the method used for preparing the samples for analysis. Methods that involve quantitative conversion of the H in H2O to H2 produce 5c values. Methods that analyze H2O by equilibrating it with H2 (or with CO2), and then analyzing the equilibrated gas for isotopic composition produce 5a values. Equilibrate in this case means letting the liquid and gas reach isotopic equilibrium at a constant, known temperature. 5 C, 5 N, and 5 " S preparation methods do not involve equilibration and, hence, these are 5c values. To avoid confusion, laboratories and research papers should always report the method used. 

On linear heating, CF from this study initiates at 400 - 420°C and proceeds to 670°C (Tm), at which point CF changes to GAS. The reverse transition of GAS to CF takes place at T , during linear cooling. Thus, the principle of equilibrated gas [1] can be applied to our system and T, can be taken as a thermodynamic equilibrium temperature. 

Such thermodynamic conclusions are only relevant when the system is completely at equilibrium for reactions (4), (5), and, say, (9), but in an open system, such as a catalyst zone in a reformer where the gas is not yet at equilibrium, reaction between the components of that non-equilibrated gas can produce carbon even when the equilibrated gas shows no affinity for carbon formation. This is particularly so when higher hydrocarbons are involved and reaction (7) is possible. Whether carbon is deposited in that zone depends upon the kinetics of the carbon-forming and carbon-removing reactions, which can be influenced... 

With the syringe tip upward, the remaining gas is carefully and completely discharged by moving the solution to the syringe tip, and 25 mL of fresh gas is added. Equilibration is repeated as many times as required for the specific application. If some water should be accidentally lost, it is necessary only to proportionately reduce the equilibrating gas volume. Temperature must be kept constant during successive equilibrations. 

If samples are always of the same composition (e.g., fresh water or seawater) from a given location, the k s need be measured on only a few samples thereafter, a single equilibration suffices. The concentration of each compound in the first equilibration gas-phase times (1 -f- k) divided by k gives the concentration in the original sample. 

If needed, increased sensitivity can be obtained for trimethylben-zenes, and the range can be extended to include the naphthalenes if the equilibrated gas is analyzed by temperature-programmed gas chromatography. Figure 3a shows an isothermal run of hydrocarbons dissolved in seawater from an excess of a Murban crude oil. Figure 3b shows a temperature-programmed run on a similar water sample. 

A mixture of metallic nickel and nickel oxide was treated with a CO2/CO gas mixture at temperatures ranging from 853 to 1289 K. The partial pressure of CO in the equilibrated gas mixture was measured by means of a C tracer method after the carbon dioxide had been removed by a liquid nitrogen trap. The authors found for the temperature dependence of the partial pressure ratio logio(p Q / ) = - 2381 (K/7) - 0.025. From this... 

At a given temperature and for a given hydrocarbon feed, carbon will be formed below a critical steam to carbon ratio (15), the carbon limit A in Fig. 5. It can be shown that this critical steam to carbon ratio increases with temperature. By promotion of the catalyst, it is possible to push this limit to the thermodynamic carbon limit B reflecting the principle of equilibrated gas (4,15) ... 

Iron has also been reacted with equilibrated gas mixtures in the CO-CO2-SO2-N2 system and it was found that gas mixtures with the same SO2 partial pressure reacted at the same rates for a given temperature. This result confirms Hatley and Birks assumption that the SO2 species reacts directly with the metal it does not dissociate first to provide O2 and S2 at the metal surface as the reactive species. The direct reaction of the SO2 molecule with the metal is implied in Figure 7.8 and has been demonstrated subsequently by several authors. ... 

The principle of equilibrated gas is justified by the low effectiveness factor of the reforming reaction, which implies that the gas inside most of the catalyst particle is nearly at thermodynamic equilibrium. The data in Table 5.4 supports this assumption. A series of TGA tests were carried out to determine the critical H2O/CH4 for onset of carbon formation [389], The results shown in the table support the principle of equilibrated gas . The calculated affinity for carbon formation (-AG°) from the equilibrated gas (approximately 4 kJ/mol) from graphite data is comparable with the deviation to be expected from the effect of the whisker structure. 

Table 5.4 Carbon formation and equilibrated gas [389]. Thermogravimetric studies at 1 bar abs. CO2/CH4. The CH4 flow was increased stepwise until on-set of carbon formation. Then the CH4 flow was decreased for removal of carbon. (H20/C)exit was determined by interpolation.

Figure 5.15 Carbon limit diagram. Principle of equilibrated gas. Carbon is formed for conditions to the left of the curves. Curves 1 and 2 represent graphite data and whisker carbon (Appendix 2), respectively. The dotted hnes show feed gas compositions leading to product gas with indicated H2/CO ratios [152]. Reproduced with the permission of...

Figure 5.17 Principle of equilibrated gas. Carbon limit temperatures and Ni-crystal size [382]. Conditions H20/CFLt=1.8, C02/CH =2.2, P=18 bar abs.

The effect of particle size on carbon limits from principle of equilibrated gas is also illustrated in Figure 5.17, representing conditions for an industrial oxo-syngas plant [382]. Graphite data predicts carbon formation, whereas carbon-free operation was obtained with a catalyst with nickel particles less than 250 run. 

The principle of equilibrated gas is no law of nature. It is possible to break the thermodynamic limit. This can be done by using noble metals [396] or by using a sulphur passivated catalyst as practiced in the SPARG process [390] (refer to Section 5.5). Examples are shown in Figure 2.18. 

The prineiple of equilibrated gas predicts conditions where carbon formation is expected (except for noble metals and SPARG). It does not guarantee that carbon is not formed if the principle predicts no potential in the equilibrated gas. 

One example is the formation of carbon in high flux reformers [389] operating far from thermodynamic carbon limits. It means that methane may decompose to carbon instead of reacting with steam to form the required syngas in spite of no potential for carbon in the equilibrated gas. This is of course not possible in a closed system, but in an open system carbon may be stable in a steady state and the accumulation of carbon may continue [389], This risk may be assessed by the so-called criteria of actual gas, which for the methane decomposition reaction as in Equation (5.5) can be written as ... 

At equilibrium with a given composition at the gas phase (not necessarily an equilibrated gas), the carbon activity ao.eq is expressed by the ratio between Kp qr and Q for the reaction ... 

A no potential for carbon in actual gas. A critical steam-to-carbon ratio (carbon formation on Ni). B thermodynamic limit potential for carbon in equilibrated gas. 

The higher hydrocarbons may lead to carbon formation by all three mechanisms (Table 5.1). Thermod5mamics will predict carbon formation as long as the higher hydrocarbons are present. Carbon may be stable in a steady state in spite of the principle of equilibrated gas (refer to Section 5.2.4). 

The risk of carbon (or gum) formation from higher hydroearbons can be analysed separately from the potential for carbon predicted by the principle of equilibrated gas . 

A pilot test was carried out with naphtha as feed. The critical H20/CnHm was assessed to be around 1.3. As shown in Figure 5.33, it was possible to operate at a H2O/CnHm=1.0 in the feed gas by using a hot (ejector) recycle, which brought H20/CnHm up to 1.6. The principle of equilibrated gas predicts carbon-free operation above O/C=0.9. 

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