Reducing Hydrocarbon Partial Pressure

One day my mother served me a bowl of mushroom soup which I didn t want to eat. I disliked mushroom soup, but I was a practical child. It would serve no purpose to tell my mother I hated the taste of mushrooms because she would say, I ve spent all day cooking. You re not going outside till you eat that soup. So I said, Mom, the soup is too hot. I ll burn my tongue. And she said, Norman, blow across the soup to cool it off. While I knew this would cool the soup, I really didn t like mushrooms. So I responded, Mom, why will blowing across the soup cool it off How does that work  

At this point your typical mother would slap the kid in the head and say, Children in Europe are starving (this was in 1947 now European children are overweight). Shut up and eat your soup. But not my mother. Norman, blowing across the soup blows away the molecules of steam covering the top of the soup. This makes room for more molecules of water to escape from the surface of the soup in the form of steam. When the molecules of water are changed into molecules 

The temperature of the soup is called sensible heat. When you blow across the soup, you re helping the sensible heat content of the soup to be converted to latent heat of evaporation of the soup. And that s why the soup cools. But your breath simply acts as a carrier— to carry away the molecules of steam covering the surface of the soup.  

And Mom said, Norman, in effect, your breath is reducing the partial pressure of steam in contact with the soup. For every one weight percent of evaporation, the soup will cool by 10° .  

If my mother had served me a hydrocarbon soup, then for every one weight percent of evaporation, the soup would have cooled by 2°F. Then she would have said the carrier gas or stripping steam would be reducing the hydrocarbon partial pressure. 

I have designed process equipment where the carrier medium is the air. Sometimes we use nitrogen or hydrogen. But mainly we use steam because it s cheap and condensable. We use steam in  

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Steam is used as the diluent to reduce hydrocarbon partial pressure and hence to improve selectivity toward olefins. Steam also gasifies (via oxidation) part of the coke deposited on the tubular reactor wall producing CO and CO2 and hence promotes coke removal from the inner surfaces of the coils. 

Improved catalyst/oil mixing achieved by good atomizing nozzles results in less thermal cracking and yields that are more selective to gasoline. The presence of the large quantities of diluent in the riser (the steam) reduces hydrocarbon partial pressure and hence reduces coke make (see Table 2). 

An increased stripping steam rate will decrease product draw tray temperatures due to the reduced hydrocarbon partial pressures. 

The Phillips Steam Active Reforming (STAR) process catalyticaHy converts isobutane to isobutylene. The reaction is carried out with steam in tubes that are packed with catalyst and located in a furnace. The catalyst is a soHd, particulate noble metal. The presence of steam diluent reduces the partial pressure of the hydrocarbons and hydrogen present, thus shifting the equHibrium conditions for this system toward greater conversions. 

Advanced Cracking Reactor. The selectivity to olefins is increased by reducing the residence time. This requires high temperature or reduction of the hydrocarbon partial pressure. An advanced cracking reactor (ACR) was developed jointly by Union Carbide with Kureha Chemical Industry and Chiyoda Chemical Constmction Co. (72). A schematic of this reactor is shown in Figure 6. The key to this process is high temperature, short residence time, and low hydrocarbon partial pressure. Superheated steam is used as the heat carrier to provide the heat of reaction. The burning of fuel... 

The injection of superheated steam at the base of the vacuum fractionator column further reduces the partial pressure of the hydrocarbons in the tower, facilitating vaporization and separation. The heavier fractions from the vacuum distillation column are processed downstream into more valuable products through either cracking or coking operations. 

Tlie feedstock is mixed witli steam before entering tlie pyrolysis reactors. Steam reduces tlie hydrocarbon partial pressure, acts as a heat transfer... 

The reaction is highly endothermic, so it is favored at higher temperatures and lower pressures. Superheated steam is used to reduce the partial pressure of the reacting hydrocarbons (in this reaction, ethane). Superheated steam also reduces carbon deposits that are formed by the pyrolysis of hydrocarbons at high temperatures. For example, pyrolysis of ethane produces carbon and hydrogen ... 

A higher steam/hydrocarhon ratio favors olefin formation. Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0.2-1 for ethane and approximately 1-1.2 for liquid feeds. 

The desorption using a nonadsorbing medium such as nitrogen equals a pressure swing process because the purge gas reduces the partial pressure of the n-alkane and works as a vacuum. This variant also is only suitable for the isolation of n-alkanes from low molecular weight hydrocarbon mixtures like gasoline fractions. 

Many processes in a refinery use steam as a stripping medium in distillation and as a diluent to reduce the hydrocarbon partial pressure in catalytic or thermal cracking [37]. The steam is eventually condensed as a liquid effluent commonly referred to as sour or foul water. The two most prevalent pollutants found in sour water are H2S and NH3 resulting from the destmction of organic sulfur and nitrogen compounds during desulfurization, denitrification, and hydrotreating. Phenols and cyanides also may be present in sour water. 

A steam stripper, as shown in Fig. 10.1, works in the same way. The diesel-oil product drawn from the fractionator column is contaminated with gasoline. The stripping steam mixes with the diesel-oil product on the trays inside the stripper tower. The steam reduces the hydrocarbon partial pressure and thus allows more gasoline to vaporize and to escape from the liquid phase into the vapor phase. The heat of vaporization of the gasoline cannot come from the steam, because the steam (at 300°F) is colder than the diesel oil (at 500°F). The heat of vaporization must come from the diesel-oil product itself. 

The heat required for the cracking reaction is brought in by the effluent itself from the cracking heater, as well as by the superheated steam, which is heated in the convection section of the cracking heater and blown into the reactor bottom. The superheated steam reduces the partial pressure of the hydrocarbons in... 

In practice, industrial processes operate in the presence of catalysts, at above 600 C with a large adduct of steam, whose effect is to reduce the partial pressure of the hydrocarbons and also to slow down the formation of coke. Depending on the extent of this coking, the process may require operation in cles, with a frequency proportional to the amount of coke deposited. Table 6.1 gives typic examples of operating conditions and results obtained with several catalysts. 

The effects of hydrocarbon partial pressure and residence time distribution are easily incorporated into the reaction model, when the reaction kinetics are mathematically described. Figures 3 and 4 show the examples of simulated results by use of the model. They clearly show that the effect of hydrocarbon partial pressure on the product distribution is larger than generally recognized and low pressure is preferable to obtain high liquid yield and that residence time distribution should be controlled as narrow as possible to reduce coke precursor(Ql) in the residual component. 

This is an endothermic conversion, which takes place in the gas phase between 150 and 300 0 (preferably at about 275 0), at a pressure as low as possible, but sufficient to recover the isobutene in the liquid phase by cooling with water, namely about 0.6I06 Pa absolute. To avoid dehydration side reactions, operations are conducted in the presence of steam, with a typical H20/MTBE mole ratio at the reactor inlet of 5/1. As in the steam cracking of hydrocarbons, this procedure serves to reduce the partial pressure of the components and to facilitate the production of isobutene and methanol. 

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