Cooling water makeup

This system also is a good illustration of the improvement in dynamic performance that cascade control can provide in some systems. As we will show quantitatively in Chap. 11, the closedloop time constant of the reactor temperature will be smaller when the cascade system is used than when reactor temperature sets the cooling water makeup valve directly. Therefore performance has been improved by using cascade control. 

In modern high-pressure systems, blowdown water is normally of better quality than the water supply. This is because plant intake water is treated using clarification, filtration, lime/lime soda softening, ion exchange, evaporation, and in a few cases reverse osmosis to produce makeup for the boiler feedwater. The high-quality blowdown water is often reused within the plant for cooling water makeup or it is recycled through the water treatment and used as boiler feedwater. 

The chemical composition of the cooling water makeup supply used in the plant determines the choice of the cycles of concentration. Some of the important constituents that must be controlled in the tower are calcium, magnesium, silica, carbonate, bicarbonate and sulfate ions. Alkalinity levels are regulated by the addition of acid or alkali to achieve the desired pH. When adding H2S04 (sulfuric acid) for pH control, it should be assured that calcium sulfate solubility limits are not exceeded (see Chapter 8). 

The paper provides an overall cooling system model called Unimod. The model is applied to present and future cooling system requirements, and eliminates plant trials of treatment chemicals. At a midwest refinery petrochemical plant, oil leaks, the use of four alternating water sources, and entrained solids were causing heat transfer losses and unscheduled shutdowns. In less than an hour, the model identified the best of the available water sources and blends, and recommended a treatment program for use before each water change. This enabled the refinery to increase heat transfer by 20%, eliminate unscheduled shutdowns, and with treatment, use the plant wastewater safely for all cooling water makeup. 

Surface waters such as rivers, streams, and lakes are often of variable quality due to seasonal changes. As with city water, the permutations of dissolved impurities and pH vary considerably depending on geographic location, but often the most important factors affecting cooling water makeup quality are the biological loading and the suspended solids content of the water. 

Water used for general industrial and cooling water makeup purposes will commonly contain a wide variety of dissolved minerals, including readily measurable quantities of most or all of the following ... 

Cooling water makeup should contain less than 5 ppm SS. If the level of suspended solids is considerably higher, say over 20 ppm SS, pretreatment by the use of polyelectrolytes or filtration is recommended. If the SS level is perhaps 5 to 10 ppm or if there is air-blown dust entering a cooling system, a sidestream sand filter or self-cleaning filter will be of benefit. In cases of high SS, chemical polymeric dispersants may be suitable as a total replacement for the sidestream filter but preferably will be used in tandem with a filter. 

Typically, for cooling water makeup, the filtration rates are 4 to 6 gpm/sq ft of media surface area, with backwash rates of 10 to 12 gpm/sq ft for a period of 5 to 8 minutes. When the filter is used for removing suspended solids from recirculating cooling water, the filtration rate may be as high as 10 to 15 gpm/sq ft and the backwash rate 15 to 20 gpm/sq ft. 

Assuming an overall heat transfer coefficient of 851 W K-1 m-2, the required temperature differential between the reactor and the jacket is only 2.9 K, giving a jacket temperature of 330 K. If the supply cooling water temperature is 294 K, the cooling water makeup flowrate is 5.43 g/s. 

Cooling water makeup normally is not highly treated water, and so it adds to the dissolved solids disposal burden. It also contains treating chemicals such as corrosion... 

Gasification processes purify and recycle raw process liquids, and the net process water effluent is normally only a blowdown. Like cooling-water blowdown, the process water blowdown is necessary to avoid buildup of salts and other dissolved minerals. This net process water effluent is usually higher quality than the cooling-water blowdown. In fact, some plants use the process water effluent as part of the cooling-water makeup (10). 

The secondary controller responds rapidly to any temperature disturbances in the reactor feed line and provides improved reactor temperature control. As discussed in Ref. 7, a similar cascade control scheme can be implemented in the case where the reactor is jacketed and the reactor temperature is controlled by manipulating the cooling medium (typically water) inlet stream. In this case the additional measurement is the temperature of the jacket, which is compared with a set point provided by the master reactor temperature controller. The resulting error signal is the input to the controller for the cooling water makeup. 

Natural 7.8 Gcal (LHV)/t Electrical power, 74 kWh/t Cooling water makeup, 8.16 m3/t Boiler feedwater makeup, 0.85 m3/t Catalyst cost, 1.60/t... 

The point is that adequate cooling-water makeup was provided throughout the incident, and additional alternatives could have been used to provide makeup water with the reactor at either high or low pressure. 

The fresh C5 stream containing the reactive isoamylenes and the chemically inert other C5 components is fed into the reactor on flow control. The methanol fed to the prereactor is ratioed to the fresh feed flowrate. The exit temperamre of the reactor is controlled using a ternperamre/temperamre cascade structure. The reactor effluent temperamre controller changes the setpoint of the circulating cooling water temperamre controller, which manipulates the cooling water makeup valve (see Fig. 14.7). 

Eig. 1. Monopressure process using catalytic NO abatement, where BEW = boiler feed water, CH = high level compression, CM = medium level compression, CW = cooling water, and D = makeup driver, EX = expander, and E = filter. 

Precipitation softening processes are used to reduce raw water hardness, alkalinity, siHca, and other constituents. This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. Waters with moderate to high hardness and alkalinity concentrations (150—500 ppm as CaCO ) are often treated in this fashion. 

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

Removal of Particulate Matter. The amount of particulate entering a cooling system with the makeup water can be reduced by filtration and/or sedimentation processes. Particulate removal can also be accompHshed by filtration of recirculating cooling water. These methods do not remove all of the suspended matter from the cooling water. The level of fouling experienced is influenced by the effectiveness of the particular removal scheme employed, the water velocities in the process equipment, and the cycles of concentration maintained in the cooling tower. 

Water reuse is practiced at several plants in this industry. Water that may be reused for such purposes as rinse water, makeup water, and cleanup water includes air conditioning water, acid treatment rinsewater, and noncontact cooling water. Reuse of acid rinsewater in alkaline rinses has been demonstrated at many electroplating plants. 

It works on any type of water-using operation (e.g. firewater makeup, cooling tower makeup, and so on). 

At the heart of the UOP HF alkylation unit is a vertical reactor-heat exchanger, shown in Fig. 14. The isobutane-alkene mixture enters the shell of the reactor through several nozzles, and HF enters at the bottom of the reactor. The reaction heat is removed by cooling water, which flows through cooling coils inside the reactor. After phase separation in the settler, the acid is recycled to the reactor. The hydrocarbon phase together with a slipstream of used acid and makeup isobutane is sent to the isostripper , where the alkylate product, n-butane, and isobutane are separated. The isobutane is recycled to the reactor. During normal... 

Alternative 1 consists of preliminary treatment for heavy metals removal with the primary concern being iron removal (Figure 8.3). The levels of iron observed in the groundwater at this site would be very detrimental to the downstream treatment processes. This pretreated water would then be used for cooling tower makeup water followed by biological treatment. This approach would be the easiest and cheapest alternative. This combined process should provide effective removal of BTEX. 

A reactor is cooled by a circulating jacket water system. A double cascade reacidr temperature control to jacket temperature control to makeup cooling water flow control is employed. 

Reactor temperature transmitter range SO-250 F Circulating jacket water temperature transmitter range 50-1 SO F Makeup cooling water flow transmitter range 0-250 gpm (orifice plate + diflerential pressure transmitter) >... 

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