Feed and Condensate System

The secondary system group includes the turbine, main steam, main feed, and condenser systems. 

The AP1000 has two steam generators. These extract heat from the reactor coolant by boiling feed water to produce steam. The feed water is supplied from the feed and condensate system the steam is taken away by the main feed system the cooled reactor coolant is returned to the reactor. 

The following design requirement for the feed and condensate system supports safe operation of the plant imder normal conditions ... 

The gas feed and mixing system consists mainly of glass flowmeters or electronic mass flowmeters connected to gas bottles. For reactants that are in liquid state at room conditions (e. g. methanol) a saturator is normally used through which helium is sparged and then mixed with the other reactants. In this case all lines connected to the reactor are heated (e.g. at 150°C) to avoid condensation in the lines. In certain cases the gases from the bottles should be pretreated in order to avoid contamination of the catalyst. For example, a... 

When condensate returns to a boiler, it may be contaminated by pipeline corrosion or with process materials. Likely sources of the latter include exchanger leakage and poorly installed process tie-ins. Carbon dioxide and oxygen are the chief causes of corrosion in steam and condensate systems. Section 12.4.3.3 discusses the treatment of boiler feed water to remove these contaminants, as well as hardness. 

Capital costs for vapor compression systems are usually higher than for multiple-effect systems, because they are usually designed for a lower temperature difference across the evaporator. As a result, greater heat transfer surface is required. In addition, the compressor and drive are relatively expensive. Equipment costs are affected by variables such as feed properties, materials of construction, available energy supply, and calandria type. Expected maintenance and necessary spare parts must be also evaluated. Heat exchangers must be provided to exchange heat between feed and condensate and between feed and product. Other costs that must be considered, but are less easily defined, are the penalty for interrupted operation during repairs and the part load response. 

There is some disagreement as to the desirable feeding point for film-forming inhibitors. All inhibitor suppliers say that the materials can be fed directly to the steam and condensate systems. Some suppliers recommend adding the inhibitor to the feed water or directly to the boiler and say that the inhibitor will evaporate with the steam and condensate in a thin, continuous film. However, most of the commercially available filming inhibitors are formulated produets, each component having a somewhat different volatihty (and solubility) and, therefore, the preferred point of addition should be the steam header. 

The commercial production equipment consists of a furnace, heat-exchanger tubes, a fractionating column packed with Rachig rings, a KCl feed, a waste removal system, and a vapor condensing system (Fig. 1). 

Intermediate Condenser. As shown in Figure 3, an intermediate condenser forces the operating line closer to the equiUbrium line, thus reducing the inherent inefficiencies in the tower. Using intermediate condensers and reboilers, it is possible to raise the efficiency above that for a simple reboder—condenser system, particularly when the feed composition is far from 50 50 in a binary mixture. 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

Multiple-Effect Evaporators A number of approximate methods have been published for estimating performance and heating-surface requirements of a multiple-effect evaporator [Coates and Pressburg, Chem. Eng., 67(6), 157 (1960) Coates, Chem. Eng. Prog., 45, 25 (1949) and Ray and Carnahan, Trans. Am. Inst. Chem. Eng., 41, 253 (1945)]. However, because of the wide variety of methods of feeding and the added complication of feed heaters and condensate flash systems, the only certain way of determining performance is by detailed heat and material balances. Algebraic soluflons may be used, but if more than a few effects are involved, trial-and-error methods are usually quicker. These frequently involve trial-and-error within trial-and-error solutions. Usually, if condensate flash systems or feed heaters are involved, it is best to start at the first effect. The basic steps in the calculation are then as follows ... 

These applications have considerably more stringent performance requirements than any other application. Circulating water pumps, boiler feed pumps, forced-draught (FD) and induced-draught (ID) fans, pulverizers (ball mills) and condensate pumps are components in a thermal power station that may require extra safety in a standard motor to make it able to fulfil these requirements and withstand abnormal service conditions and system disturbances. Abnormal operating conditions may be one or more of the following ... 

Instead of using conventional refrigeration and condensers to convert incoming gas into LNG, the system was designed to feed 2,340 Nm / day (86,800 scfd) into each of two expanders. Both are connected in parallel, with compressors attached at the back end. The compressors are connected in series to boost a single stream of returning gas. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Relieving vapors from various pressure-relief and depressuring valves in the system must be collected in individual flare headers that should be appropriately located near each process area. Subheaders must be interconnected to a main flare header which feeds to a knock- out drum and disposal system. Condensates that are carried over by vapors are separated in the knock-out drum. The vapors that exit the vessel go to the flare stack where they are burned. 

Condensate The water returning from the steam user(s). Feedwater The water entering the boiler feed heating system, which will normally be a blend of treated water and condensate. 

Because of their compact size, packaged vertical boilers can be custom-designed as a complete boiler plant system and simply shipped to the customer on a steel skid or platform. This type of system may comprise a dual boiler arrangement, with a pretreatment unit (water-softening and chemical-feed system), boiler blowdown and condensate return facilities, and also possibly a dual stack containing an economizer. This type of packaged system may reach 85% GCV efficiency. 

When produced from natural gas the synthesis gas will be impure, containing up to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate are favoured by high pressure. The conversion is low, about 15 per cent and so, after removal of the ammonia produced, the gas is recycled to the converter inlet. A typical process would consist of a converter (reactor) operating at 350 bar a refrigerated system to condense out the ammonia product from the recycle loop and compressors to compress the feed and recycle gas. A purge is taken from the recycle loop to keep the inert concentration in the recycle gas at an acceptable level. 

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