Ethylene limiting oxygen concentrations

Ethylene oxidation was studied on 8 mm diameter catalyst pellets. The adiabatic temperature rise was limited to 667 K by the oxygen concentration of the feed. With the inlet temperature at 521 K in SS and the feed at po2, o=T238 atm, the discharge temperature was 559 K, and exit Po =1.187 atm. The observed temperature profiles are shown on Figure 7.4.4 at various time intervals. The 61 cm long section was filled with catalyst. 

To complete the picture w7e should investigate if there are more dominant variables in the system. We have already touched on this issue in the section on reaction rates. There we noted that the acetic acid concentration to the reactor is not dominant. We can also argue that the ethylene partial pressure is not likely to be a dominant variable since ethylene enters the reactor in large excess. However, oxygen is the limiting component and it plays a role in the main reaction as wrell as in the side reaction. Oxygen therefore affects the economic objectives and is considered dominant. Feedback control of the oxygen concentration to the reactor is necessary if we wrant complete control of the unit. 

The ethylene oxidation rate is proportionhUo the oxygen concentration. This means that the air-to-ethylene ratio has a predominant influence on the conversion and yield. For practical purposes, however, the optimal ethylene concentration is determined by the flammability limits of the mixtures with oxygen or air, and by the olefin loss in tbe off-gases. 

FIGURE 1. Induction of nitrogenase activity in Phormidium foveolarum at low light intensity (1 W/m ). Cell density was 40 ul packed cell volume/ ml culture suspension chlorophyll 73 hg/ml. (A-A) hydrogen, (A-4) ethylene, (o—O)carbon dioxide, and ( r— ) oxygen content in the assay vessels, as referred to ml cell suspension. Ethylene was not measurable throughout the assay time due to limited acetylene concentrations (10% v/v) present at start. 

Ethylene oxide is produced by adding ethylene, oxygen, a methane diluent, and recycled carbon dioxide to a continuous reactor. Gaseous compositions are controlled carefully to keep the concentrations outside the explosion limits. 

Several processes based on air or oxygen have been developed.890-895 Oxidation with air (260-280°C) or oxygen (230°C) is carried out at about 15-25 atm at a limited conversion (about 10-15%) to achieve the highest selectivity.896-898 High-purity, sulfur-free ethylene is required to avoid poisoning of the catalyst. Ethylene concentration is about 20-30 vol% or 5 vol% when oxygen or air, respectively, is used as oxidants. The main byproducts are C02 and H20, and a very small amount of acetaldehyde is formed via isomerization of ethylene oxide. Selectivity to ethylene oxide is 65-75% (air process) or 70-80% (02 process).867... 

In most industrially relevant reacting systems, one main reaction typically makes the desired products and several side reactions make byproducts. The specific rate of production or consumption of a particular component in such a reaction set depends upon the stoichiometry and the rates. For example, assume that the main reaction for making vinyl acetate, Eq. (4.4.1, proceeds with a rate r< (mol/L s) and that the side reaction, Eq. (4.8), proceeds with rate r2 (mol/L s). Then the net consumption of ethylene is (-l)r1 - (-1 )r2 (mol/L s). Similarly, the net consumption of oxygen is (-0.5)fi + (— 3)r2, and the net production of water is (l)r-, + (2)ra. For a given chemistry (stoichiometry), our ability to control the production or consumption of any one component in the reactor is thus limited to how well we can influence the various rates. This boils down to manipulating the reactor temperature and/ or the concentrations of the dominant components. Occasionally, the reaction volume for liquid-phase reactions or the pressure for gas-phase reactions can also be manipulated for overall production control. These are the fundamentals of reactor control. 

The nature of intermediates formed in diffusion flames is similar to the premixed ones, albeit differences in the contacting pattern. In Fig. 11, the species concentration profiles in a laminar ethylene diffusion flame front are presented. The fuel and oxygen diffuse toward each other undergoing virtual annihilation within the flame zone concomitant with the establishment of a peak temperature of about 1600°C. Because premixed systems provide a better control of combustor temperature, and many practical combustion devices operate under diffusion limited conditions, considerable effort has been expended to ensure the rapid mixing of fuel and oxygen in combustion chambers and approach premixed conditions. 

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