Contaminated condensate reuse

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

Surface Condensers Surface condensers (indirect-contact condensers) are used extensively in the chemical-process industiy. They are employed in the air-poUution-equipment industry for recoveiy, control, and/or removal of trace impurities or contaminants. In the surface type, coolant does not contact the vapor condensate. There are various types of surface condensers including the shell-and-tube, fin-fan, finned-hairpin, finned-tube-section, ana tubular. The use of surface condensers has several advantages. Salable condensate can be recovered. If water is used for coolant, it can be reused, or the condenser may be air-cooled when water is not available. Also, surface condensers require less water and produce 10 to 20 times less condensate. Their disadvantage is that they are usually more expensive and require more maintenance than the contac t type. 

Condensate contaminated by process leaks is unsuitable for reuse as... 

Within any particular facility, steam is expected to be delivered to various points of use safely and at controlled temperatures and pressures through relatively long-lasting distribution networks. Where the steam is not excessively contaminated or directly consumed in a process as live steam, it should be condensed, collected, and returned to the boiler for reuse. Thus, the post-boiler section of a boiler plant essentially relates to the systems concerned with steam distribution and condensate return (CR). 

A typical evaporative recovery system consists of an evaporator, a feed pump, and a heat exchanger. Plating solution or rinsewater containing dilute plating chemicals is circulated through the evaporator. The water evaporates and concentrates the plating chemicals for reuse. In open evaporator systems, the water evaporates and mixes with air and is released to the atmosphere. It may be necessary to vent the contaminated airstream to a ventilation/scrubber treatment system prior to release. In enclosed evaporators the water is condensed from the air and can be reused in rinses, which further increases savings. Water reuse is preferred whenever possible. 

Barometric condenser systems can be a major source of contamination in plant effluents and can cause a particularly difficult problem by producing a high-volume, dilute waste stream [8]. Water reduction can be achieved by replacing barometric condensers with surface condensers. Vacuum pumps can replace steam jet eductors. Reboilers can be used instead of live steam reactor and floor washwater, surface runoff, scrubber effluents, and vacuum seal water can be reused. 

The CleanSoil process is an ex situ treatment technology that uses steam to remove hydrocarbons and chlorinated solvents from contaminated soils. The steam vaporizes the contaminants from the soil and carries them to a condenser for recovery. The water is converted back into steam and reused in the system. The remaining vapors pass through an activated carbon filter and are released into the atmosphere. The technology has been applied full-scale at multiple sites and is commercially available. 

In most cases, condensate does not require treatment prior to reuse. Makeup water is added directly to the condensate to form boiler feedwater. In some cases, however, especially where steam is used in industrial processes, the steam condensate is contaminated by corrosion products or by the inleakage of cooling water or substances used in the process. Hence steps must be taken to reduce corrosion or to remove the undesirable substances before the condensate is recycled to the boiler as feedwater. 

Steam st ripping — This technique has long been used on sour water and process condensate in refineries. Steam stripping is also becoming more attractive in other industries as the demand for water conservation increases. In a number of process plants, high-quality process condensates are recovered, stripped, and reused. Effluent oil concentrations of less than 50 ppm are achievable in stripping columns but actual concentration depends on influent concentration, tower design and contaminant type. 

In the production process of an ammonia plant, a lot of process condensate is produced everyday. Usually, this kind of process condensate contains many contaminants, such as ammonia, carbon dioxide, methanol etc, which can pollute the environment if emitted directly. However, several techniques have been developed to remove the pollutants, such as stripping with natural gas and medium pressure steam, which can be applied one at a time or together, so that the recovery condensate can be reused as boiler feed water or as water makeup for other process operating units. It is very important for the plant to design a process which can solve the problem with minimum cost. This is the work that can be solved by the synthesis of MEN. 

The process condensate data is derived from the process of a Synthetic Ammonia in which 100,000 kg -hr" of process condensate is produced. All the related stream data are shown in Table 1 and Table 2. The condensate from the process consists of four contaminants mainly, which are to be removed away before reused by the boiler. There are several possible alternative processes to be selected, one is using natural gas to remove the contaminants, other is using medium pressure steam, and still others are employing both processes (in series or in parallel). 

The sour-water stripper removes contaminants from condensed water and wash water used in various process units. The water may then be reused, for example, as desalter wash water or to water-wash light gas streams. In this process, the contaminants such as H2S are removed by stream stripping of the water. Figure 9 is a general diagram of a sour-water stripper. 

Caustic is the most valuable as well as the most hazardous of the latter group of contaminants. Most plants have more or less elaborate schemes for recovery of caustic evaporator condensate and its use in the process. This is especially true in diaphragm-cell plants with their high evaporative load, and the reuse of evaporator process condensate is the subject of Section 12.4.3.2. Hydrogen gas will also carry a certain amount of caustic entrained from the cells. Much of this is removed in cooling the gas. 

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