Butane and Heavier

The hydrocarbons butane and larger (and larger ) are less volatile than acid gas. 

Methane is more volatile than acid gas. The presence of methane in an acid gas mixture tends to increase the bubble point pressure. 

Ethane has approximately the same volatility as carbon dioxide. 

Hydrocarbons laiger than propane (butane, pentane, hexane, etc.) are less volatile than add gas. The presence of these heavier hydrocarbons tend to decrease the bubble point pressure. 

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When gases are rich in ethane, propane, butane and heavier hydrocarbons and there is a local market for such products it may be economic to recover these condensable components. Natural gas liquids can be recovered in a number of ways, some of which have already been described in the previous section. However to maximise recovery of the individual NGL components, gas would have to be processed in a fractionation plant. 

TOSCO tar oils have high viscosity and may not be transported by conventional pipelines. Heating values of product gas on a dry, acid gas-free basis are in the natural gas range if butanes and heavier components are included. 

In general, light ends towers are named after the light key. Thus, a depropanizer will take overhead the propane and lighter in the feed, and will have butane and heavier as the bottoms product. 

Butane and heavier bottoms from the depropanizer flow to the debutanizer where the C4 stream (almost entirely olefins and diolefins) is taken overhead and sent to butadiene and isobutylene recovery facilities. 

The term gas processing is used to refer to the removing of ethane, propane, butane, and heavier components from a gas stream. They may be fractionated and sold as pure components, or they may be combined and sold as a natural gas liquids mix, or NGL. 

To limit the scope of this paper we may define natural gas liquids as liquid hydrocarbons normally recovered and recoverable from natural gas by physical processes only. Customarily these include propane, butane, and heavier hydrocarbons, but to a limited extent ethane is also recovered as a natural gas liquid. 

FIG. 14-11 Graphical design method for multicomponent systems absorption of butane and heavier components in a solute-free lean oil. 

A hydrocarbon feed gas is to be treated in an existing four-theoretical-tray absorber to remove butane and heavier components. The recovery specification for the key component, butane, is 75 percent. The composition of the exit gas from the absorber and the required liquid-to-gas ratio are to be estimated. The feed-gas composition and the equilibrium K values for each component at the temperature of the (solute-free) lean oil are presented in the following table ... 

Feedstocks for various industrial pyrolysis units are natural gas liquids (ethane, propane, and n-butane) and heavier petroleum materials such as naphthas, gas oils, or even whole crude oils. In the United States, ethane and propane are the favored feedstocks due, in large part, to the availability of relatively cheap natural gas in Canada and the Arctic regions of North America this natural gas contains significant amounts of ethane and propane. Europe has lesser amounts of ethane and propane naphthas obtained from petroleum crude oil are favored in much of Europe. The prices of natural gas and crude oil influence the choice of the feedstock, operating conditions, and selection of a specific pyrolysis system. 

Combining refrigeration with oil absorption allowed for additional liquid recoveries and greater economy of operation. Using temperatures as low as -40°F (-40°C), refrigerated oil absorption plants were capable of recovering 70 percent or more of the propane present in the natural gas stream. Recoveries of the butane and heavier constituents were essentially 100 percent. 

In the gas phase, methane and ethane reduce the density of an acid gas mixture. Propane has an approximately neutral effect - in H2S rich mixtures it tends to reduce the density and in C02-rich mixtures it has little effect. Hydrocarbons butane and heavier increase the density of the acid gas mixture. 

The composition of coking gases is similar to that of cracked gases, but coking gases are richer in butane and heavier constituents. 

The composition of natural gas varies considerably from one country to another and from one well to another in the same locale. The representative composition of natural gas for various countries is given in Table 6.2. Some extreme values are riot shown and include 70% CO2 in some gas in Indonesia and 40% CaHs for some Siberian gas. Dry natural gas is gas freed of liquids of butanes and heavier hydrocarbons which are called natural gas liquids. 

Because of the complexity of the mechanism of the low- and medium-temperature gas-phase oxidation of rich butane—oxygen mixtures at elevated pressures, there are no works on a detailed kinetic modeling of the process. The mechanism of the low- and medium-temperature oxidation of butane and heavier alkanes at atmospheric pressure has been studied more extensively in connection with the modeling of the self-ignition processes in engines [253—255], to mention only a few. Processes at elevated pressures 9 < P < 11 atm (700 < T < 900 K), but for almost stoichiometric mixtures, 0.8 <

work experimentally identified 22 molecular intermediate products, which gives a clear idea about the level of complexity of the problem. 

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