Pressurized saturated steam

Moist-heat sterilization is achieved when a suitable combination of temperature and humidity can be introduced (or indirectly generated) at the level of the micro-organisms to be inactivated. The classic way to achieve this is by means of pressurized saturated steam at the temperature of 121°C (250°F). However, other sterilizing media (e.g., superheated water or a steam-air mixture) are also frequently used to obviate certain problems that pure steam may pose. Sometimes the load is rotated inside the chamber of the sterilizer to achieve particular results. 

Current pharmaceutical production practice uses substantially three moist-heat sterilization processes 1) pressurized saturated steam 2) superheated water and 3) steam-air mixture. Process 1 is the traditional multipurpose process, which obviously uses pure pressurized saturated steam as sterilizing medium. Processes 2 and 3 are so-called counterpressure processes they were introduced in pharmaceutical production practice approximately 20 years ago and, respectively, use a spray superheated water and a homogeneous... 

This is certainly the most widely used and most versatile moist-heat sterilization method. Accordingly, it is widely used not only for sterilization of pharmaceutical products but also for laboratory and hospital sterilization and for the treatment of medical devices. Nonetheless, it has significant limitations, especially in pharmaceutical use, which are described later. The sterilizing medium is obviously pure pressurized saturated steam. The word saturated means that the steam is in thermodynamic equilibrium with its liquid form (water) at the temperature being considered. 

The samples were loaded into the autoclave and treated with a standard autoclave procedure for solutions (15 min at 121°C, 29 psi pressure, saturated steam). 

Consider a small industrial boiler system generating low-pressure saturated steam. The boiler drum receives preheated feedwater that is required to maintain the water level in the drum between 30 and 40% of its total height. There is a visual level indicator (LI) on the drum in addition to a level indicator controller (LIC) that is connected to a level indicator control valve (LICV) in the feedwater line. Natural gas fuel is fired below the boiler drum. The fuel line contains a pressure control valve (PCV) that is connected to a pressure controller (PC) located in the steam line in and leaving the drum. 

In smaller installations, steaming may be done in the kiln but, in larger installations, a separate chamber supplied with low-pressure, saturated steam is used for this purpose. Such chambers are not normally supplied with fans, and... 

Effect of steam pressure. Using higher pressure, saturated steam increases the AT, which decreases the size and cost of the evaporator. However, high-pressure steam is more costly and also is often more valuable as a source of power elsewhere. Hence, overall economic balances are really needed to determine the optimum steam pressures. 

Another option for consideration is reheat turbines HP steam turbine can exhaust medium pressure saturated steam, which can be reheated by hot process gases if available and more power can be generated. 

Many industrial processes in the chemical industry and elsewhere require large amormts of low-pressure saturated steam to heat the materials being processed. For instance, crude oil is made to boil in... 

The reaction heat of the ammonia formation is large. For example, producing one kmol of ammonia at 450°C will give out 54.5 MJ of reaction heat which is equal to the heat of low-pressure saturated steam of 1.44t/t. Actually, the reaction heat cannot be recovered fully because of the temperatme difference losses among the heat-recovery equipments where only 50%-90% of reaction heat can usually be recovered. 

In smaller installations, steaming may be done in the kiln but, in larger installations, a separate chamber supplied with low-pressure, saturated steam is used for this purpose. Such chambers are not normally supplied with fans, and stratification of the steam and air can be a problem. Lumber that is high-temperature dried should be cooled so that the wood temperature falls to about 70 to 95°C to enable the wood to pick up moisture. The steaming induces a reversal of the moisture content profile through the wood, with concomitant reduction in the residual stresses. 

The crude raw gas leaving the gasifier at 2,300-2,700°F contains a small quantify of unburned carbon and a significant portion of molten ash. Depending on the end use, this gas stream would either be directly quenched in water (to cool the gas and remove solidified ash particles) or would be cooled in radiant/convection boilers for sensible heat recovery (via high-pressure saturated steam generation) prior to water scrubbing. 

---