Autorefrigerating

The minimum design temperature should be the lowest temperature to which the tank will be subjected, taking into consideration the minimum temperature of material entering the tank, the minimum temperature to which the material may be autorefrigerated by rapid evaporation of low-boiling hquids or mechanically refrigerated, and the minimum ambient temperature of the area where the tank is located. API 620 provides for installations in areas where the lowest recorded one-day mean temperature is —50°F. 

Dense Gas Dispersion A dense gas is defined as any gas whose density is greater than the density of the ambient air through which it is being dispersed. This result can be due to a gas with a molecular weight greater than that of air, or a gas with a low temperature due to autorefrigeration during release, or other processes. 

Overpressme which may occur at normal or below normal pressures, as a result of reduced allowable stresses at higher than design temperatures, are also evaluated and appropriate protective features applied in the design. For example, such conditions may result from chemical reactions, startup or upset conditions. Likewise, low metal temperature must be considered, such as from autorefrigeration, to make sure that brittle fracture conditions do not develop. 

Autorefrigeration if it affects materials of construction of the collection system. 

Design Temperature of Outlet Piping - The design temperature of outlet piping from PR valves discharging to the atmosphere is normally ambient. However, autorefrigeration and need for brittle-fracture-resistant materials or thermal expansion forces should be examined if the release pipe is unusually long. 

The blowdown drum design temperature is set by the extremes of emergency operating temperature which can result from any of the streams tied into it. If materials are handled at temperatures below 15 C, or if they can autorefrigerate to below 15 C, a minimum design temperature must also be specified. 

For services where the heating coil may be exposed to cold or autorefrigerated liquids, the design should be such as to prevent blockage by freezing of steam condensate. The following methods are available to achieve this ... 

Combinations of water and cold or autorefrigerating hydrocarbons which could result in freezing problems. 

To remain a liquid at a reasonably low pressure, liquefied natural gas (LNG) must be maintained at below at least -117°F. Insulated storage tanks alone cannot maintain these very cold temperatures. LNG is stored at its boiling point to take advantage of autorefrigeration. Just as the temperature of water does not rise above its boiling point (212°F) with increased heat (it is cooled by evaporation), LNG is kept near its boiling point if kept at a constant pres-... 

There is a middle steady state, but it is metastable. The reaction will tend toward either the upper or lower steady states, and a control system is needed to maintain operation around the metastable point. For the styrene polymerization, a common industrial practice is to operate at the metastable point, with temperature control through autorefrigeration (cooling by boiling). A combination of feed preheating and jacket heating ensures that the uncontrolled reaction would tend toward the upper, runaway condition. However,... 

Use temperature-control techniques that inherently scale as S e.g., cold feed to a CSTR, or autorefrigeration. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

AUTOREFRIGERATED REACTOR OF LUYBEN DYNAMIC STABILITY ANALYSIS... 

The purchaser shall specify the minimum design metal temperature the pump will be subjected to in service. This temperature shall be used to establish impact test requirements. Normally, this will be the lower of the minimum surrounding ambient temperature or minimum liquid pumping temperature. However, the purchaser may specify a minimum design metal temperature based on pump liquid properties, such as autorefrigeration at reduced pressures. 

Figure 6. Autorefrigerated Integral Reactor-Settler System...

Of course, combinations and variations of these systems are in use however, those outlined represent some of the more common installations. All of these systems can be used for sulfuric acid alkylation but only the types using tube-bundle refrigeration can be used with hydrofluoric acid since its high volatility precludes the use of an autorefrigerated system. 

In one continuous method (17), the waxy oil charge—for example, in the case of a topped crude—is diluted with an equal volume of propane and the mixture is chilled to about —25° F. by means of heat exchange with cold filtrate in double-pipe scraped-surface equipment. This chilled mixture is then mixed with propane which has been chilled by autorefrigeration to —45° F. to bring the ratio of propane to oil to about 5 to 1. Ninety per cent of the refrigeration required is accomplished in this manner the remaining 10% is obtained by evaporating propane from the diluted mixture. 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 22.1, the refrigerant is chilled from 100 to 40°F. Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

Time-tank jet Butylene Autorefrigeration Effl. flash Propylene Butylene Amylene... 

There are some basic factors in alkylation which should be considered in addition to the catalyst, feed stock, and normal operating variables. Although thermal alkylation can be accomplished without the presence of a catalyst, essentially all commercial processes today employ a catalyst. The alkylation reactions considered here normally take place in the liquid state. Sufficient pressure is therefore required to insure that the hydrocarbons in the reactor remain in the liquid state. This is only partially true for the autorefrigeration process, where cooling of the reaction zone takes place as a result of vaporizing some of the light hydrocarbons in the reactor. The ethylbenzene reaction is also an exception, since it normally takes place in the partial vapor state, especially when the feed contains considerable ethane. The alkylation reaction is exothermic, and this heat of reaction must be removed from the reaction zone. This heat can be removed by cooling coils, by vaporization of hydrocarbons, or in the effluent stream. 

The sulfuric acid process was developed by several companies and is available for licensing by these companies. The first sulfuric acid units were of the jet design with time tanks equipped with baffles for reactors. The M. W. Kellogg Company developments in the sulfuric acid process include the cascade reactor, which uses autorefrigeration. 

The Stratford Engineering Company furnished many alkylation reactors and were therefore interested in efficient mixing and heat transfer in this equipment. They subsequently developed a very efficient reactor, which makes part of the recycle by a flashing operation and is applicable to both hydrofluoric acid and sulfuric acid alkylation. This flashing differs from autorefrigeration in that the effluent stream is flashed after the reaction zone instead of in the reaction zone. 

Kellogg Sulfuric Acid Design Using Autorefrigeration (8)... 

Fig. 7. Kellogg cascade type of sulfuric acid alkylation for aviation alkylate using autorefrigeration.

The effluent flash system of refrigeration differs from autorefrigeration in that the vaporization of the reactant hydrocarbons takes place after the hydrocarbons have left the reaction zone instead of taking place in the reaction zone. The flashed material contains a relatively high percentage of isobutane, which is condensed by compression and cooling and returned to the reactor as isobutane recycle. Care must be taken to limit the amount of normal butane in the reactor, however otherwise, the purity of the flashed vapors will contain excessive normal butane and will reduce the effectiveness of this recycle stream. 

The final heat removal scheme discussed here is called autorefrigeration or evaporative cooling (Fig. 1.10/). The pressure in the reactor is adjusted so that the liquid can boil if... 

The third item is other ways to increase heat transfer area. They include circulating reactor liquid through an external heat exchanger or autorefrigeration. The dynamics of both of these alternatives will be studied in Chapter 3. As earlier mentioned, for a jacketed reactor, the heat transfer area can be increased by using larger aspect ratios. 

---