James W. Walcott Robert G. Gorgol. HPD Incorporated. Naperville, Illinois

Transcription

1 AN EVAPORATON/CRYSTALLZA - P3dF. ROUTE TO ZERO DSCHARGE POWER GENERATON James W. Walcott Robert G. Gorgol of HPD ncorporated Naperville, llinois PREPARED FOR PRESENTATON AT THE AlChE DENVER SUMMER NATONAL MEETNG, AUGUST, 1988

2 1 2 Table of Contents ntroduction System Description Brine Concentrator Crystallizer/Centrige o 2.3 Control , Operation vs. Design Conclusion

3 1 ntroduction A new 425 MW, coal fired power generating plant recently went through a successful start-up in the southeastern part of the United States. t was designed and is operating as a "zero discharge" plant. A natural draft cooling tower utilizes secondary treated municipal waste water as make-up water. Residual organic and inorganic components are concentrated during cycles through the cooling tower and a blowdown stream must be removed to prevent further build-up of components that will affect the cooling tower efficiency and life. At design rate, a blowdown stream of approximately 300 gpm is produced. -;L31;2,0~~ This paper presents the reliable and economical use of evaporation and crystallization to recover clean, reusable water and to minimize disposal costs by producing a small volume of dry solids from the large volume waste water stream. The installation of the cooling tower blowdown system includes two 300 gpm trains, each consisting of a falling film evaporator with mechanical vapor recompression as an energy source, a forced circulation crystallizer constructed of high alloy material using mechanical vapor recompression as an energy source, and a centrifuge to separate the crystallized waste for disposal. -1 -

4 2 System Description The Cooling Tower Blowdown Treatment (CTBT) System consists of two identical Trains of equipment, the A and B Trains, with each train utilizing a Brine Concentrator System and a Crystallizer System. The Brine Concentrator System uses an HPD Preheat Failing Film (PF) Evaporator as the Brine Concentrator with ancillary operations including feed filtration, feed preheat, feed deaeration and sludge (seed) recycle. The crystallizer system contains an HPD crystallizer with entrainment separator and a centrifuge. Both the BC Evaporators and the crystallizers utilize mechanical vapor recompression (MVR). n each train the operation and control of the Evaporator and Crystallizer are totally independent of each other. The Cooling Tower Blowdown Treatment System (CTBT) has been designed for the feed water conditions shown in Table

5 Table 1 : Feed Water Conditions Variable Typical Minimum Maximum Calcium, ppm as CaC Magnesium, ppm as CaCO3 Sodium, ppm as CaC03 Sulfate, ppm as CaCO3 Chloride, ppm as CaC03 Alkalinity, ppm as CaC03 Silica, ppm as Si Ooo Suspended Solids, ppm Total Dissolved Solids, ppm 50 3Ooo PH 8.5

6 2.1 Brine Concentrator Each falling film cooling water blowdown evaporator system as depicted in Figure 1, is used with mechanical vapor recompression to reduce the waste stream from 300 GPM to V4qO approximately 6 GPM. GTD) The waste water feed to the evaporator has a high scaling tendency due to the presence of various carbonates, sulfates and silica. The feed is pretreated by the addition of H2SO4 or NaOH for ph adjustment ( range) for better metal protection and to keep the metal hydroxides in solution. Anti-scaling agents (organic phosphate or hexameta phosphate) and/or anti-foaming agents (high temperature formulated silicone based) are also added to the feed. The falling film evaporator design encompasses parameters to minimize the scaling of heat transfer surfaces and maximize energy efficiency. From the feed pretreatment tank and prior to entering the evaporator, the waste stream passes through filters to remove solid particles. This will minimize scaling and solid build-up of the plate type heat exchanger and the stripper packing. After filtration, the feed passes through a plate type heat exchanger where it is heated by the dean condensate returned from the evaporator. A plate type heat exchanger will provide better heat transfer coefficients and better heat recovery than a shell and tube type heat exchanger. The feed is further treated by deaeration in order to remove air and gases contained in the feed, minimizing noncondensibles build-up in the evaporator heat exchanger shell side. The HPD PH falling film evaporator is shown in the attached diagram (Figure 2). -4-

7 PF EVAPORATOe EXTERNAL* PLENUM STEAM+ HEATER< J "E * CONDENSATE& VAPOR BODY Figure 2

8 The recirculated liquor is pumped from the bottom of the vapor body through the tube bundle section to the top liquor box. The very low velocity gradient of the liquor at this point helps in distributing evenly the liquor over the distribution tray located just above the top tubesheet. A detail of the top liquor box is shown on Figure 3.

9 DSTRBUTON PLATE DETAL DSTRBUTON- PLATE HOLES t DSTRBUTON PLATE \TOP TUBE SHEET t FALLNG LQUOR Figure 3 The liquor is then distributed to the tubes in the falling film section and a liquor/vapor mixture exists at the bottom of these tubes. Liquor and vapors are separated in the vapor body with the liquor being collected in the lower portion while the vapors pass through the entrainment separator prior to exiting the effect. The liquor is then recirculated via the circulation pump for a subsequent cycle. The advantage of this design is its simplicity, in terms of piping, control and maintenance. The circulation piping is minimized as it consists of a downcomer from the vapor body to the circulation pump and then a return pipe back to the vapor body. This is in direct contrast

10 with other falling film designs where a large circulation pipe has to run externally up to the top liquor box. Aside from reduced capital costs the PF evaporator minimizes radiation losses and pump head requirements through the circulation piping. Steam or vapor is distributed to the unit by way of an external plenum which wraps around the heat exchange bundle. The vapors, thus introduced from all directions, sweep through the heat exchanger bundle providing even distribution. Noncondensible gases are efficiently removed from the system utilizing HPD s special baffling. Main plant steam is often utilized to provide heat for evaporation. Vapors from the evaporator are then condensed in a condenser by using cooling water. For the quantities of water to be evaporated in most cooling tower blowdown process waste streams, the energy costs required to produce the necessary steam cannot be justified. However, the use of mechanical vapor recompression, utilizing approximately 88 KwHr/l,OOO gallons feed, makes evaporation a feasible solution to reduce the volume of waste. The vapors generated by evaporation pass through an entrainment separation step before going to the mechanical compressor. The risk of corrosion on erosion of the compressor can be virtually eliminated by providing a properly sized vapor body that has both the proper vapor release velocities and propre height for entrainment separation. A double stage entrainment separator is provided that includes a Chevron type separator which remove the bulk of liquid entrainment. This type of separator is rugged and not subject to plugging with entrained solids. The second stage of entrainment separation is a mesh pad type separator that further reduces entrainment levels to a minimum obtainable level. With this combined entrainment separation design, the compressor is fully protected from liquid carryover. -8-

11 The evaporated vapors are then compressed in the mechanical compressor to a pressure that will provide a heat source on the shell side of the falling film evaporator a high enough temperature to promote heat transfer. The required compression ratio is governmed by the boiling point rise of the concentrated solution and by the driving force necessary to transfer heat. The falling film design is chosen for this application because of its ability to provide relatively high heat transfer rates with low temperature driving forces of 8-10 degrees F. A portion of the concentrated recirculation flow containing primarily sodium sulfate and sodium chloride crystals (approximately 15% undissolved solids) to a hydrocyclone. The cyclone underflow containing approximately 30% undissolved solids is mixed with the feed solution in the feed tank. The purpose of adding solids to the feed is to provide crystal seeding of the evaporator. Seeding the feed with undissolved solids provides a "sludge recycle" that will assist in minimizing scaling of the heat transfer surfaces. Sufficient solids are added by proper sizing of the hydrocyclone. This will ensure that any solids liberated by supersaturation will deposit on the larger available crystal area of the seed crystals rather than on the heat transfer surfaces. During initial start-up of the system, seed crystals are provided from an external source. A portion of the overftow from the cydone is sent back to the concentrated side, while the other portion is mixed with the concentrated slurry overflow from the evaporator prior to being sent to the waste slurry tank. The MVR falling film evaporator recovers 97% of the waste stream as clean water.

12 2.2 Crystalizer/Centrifuge Further volume reduction is by utilizing a forced circulation type crystallizer as depicted in Figure 4. This system consists basically of a vapor body, external heat exchanger, recirculation pump, recirculation and vapor piping, entrainment separator and condenser. This type of system is specifically designed to concentrate waste streams to high percent solids without significant scaling and operating disturbances. The feed enters the suction side of the recirculation pump and passes through the heat exchanger, prior to entering the vapor body where vapors are separated from the liquor and supersaturation is released. The vapor leaving the crystallizer vessel passes through an entrainment separator and compressor. Concentrated liquor is recirculated from the vapor body retention chamber through a two-pass, shell and tube heat exchanger and back to the vapor body. The heat that is added to the recirculating liquor in the heater is released by vaporization of water in the crystallizer vessel. An important feature of this type of system that enhances it s capability to prevent scaling of the heat exchanger, isthe suppression of boiling in the two-pass heater. This suppression is usually achieved by providing a static pressure above the heater equivalent to the steam temperature in the heat exchanger shell. This design approach will eliminate bulk liquor boiling due to temperature rise through the heat exchanger as well as film boiling at the tube wall

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14 Generally, when concentrating waste streams to high percent solids, two inch diameter tubes are utilized. The large diameter tubes provide turbulent flow at lower tube velocities and minimize the possibility of plugging. The shell side of the heat exchanger is vented continuously to avoid the build-up of noncondensibles. This feature minimized the required steam pressure by allowing full use of the available heat transfer surface. This will assist with the suppression of film boiling at the heat exchanger tube walls. The crystallizer vessel provides retention volume and time for the recirculating liquor. This time is necessary to assure separation of vapors from the liquor and full release of supersaturation. The recirculation rate is partially determined by the necessary tube velocities to enhance heat transfer and prevent scaling. A portion of the crystals are recirculated through the heat exchanger, thus, providing a "sludge recycle". As in the brine concentrator, this provision will further minimize the tendency of scaling on heat transfer surfaces. The recirculation pump matches the crystallizer system in terms of rate and total dynamic head. Normally, it is possible to include a low head, high volume pump that will operate at relatively low speeds. This will provide long life for rotating parts and will minimize maintenance requirements. The forced circulation crystallizer reduces the system waste from 6 GPM at a concentration 4032 of 15% total solids to approximately 2.0 GPM, of final waste slurry at a concentration of 44% total solids (CF = 2.9) and produces 5.8 GPM of reusable dean condensate. The overall water removal is increased to approximately 98-99%.

15 Finally, the waste stream is further reduced by centrifuging the solids from the liquor that is received back to the crystallizer. A Sharpies basket-type only is utilized with all wetted parts constructed of 316L stainless steel materials. Final solids moisture content is slightly higher than design. Diagram for 80% solids. Actual operation is providing 75% solids. This is due primarily to the higher TOC level than expected that created a higher viscosity liquor that makes separation more difficult. 2.3 Control A unique feature of this waste brine concentration system or CTBT system is that it is totally automatic, or computer controlled. After manual operation has been started, the system may be converted over to the automatic mode, under which the computer (Gould Modicon 584L) will monitor and control the system. There is a separate Modican PC for Train A and a separate Modicon for Train 6. Since the evaporator in each Train may be operated independent of the crystallizer in each Train, it is therefore possible that the evaporator from one Train can be operated with the Crystallizer of the other Train

16 3 Operation vs. Design The cooling tower blowdown system has been operating for several months. The following table provides a comparison of operation and design parameters. Table 2: Operation and Design Parameters tem Design Operation Minimum Feed Flow 150 gpm 160/165 gpm Condensate Quality < 10 mg/l <1/<1 mg/l e 81 5 mg/l (crystallizer) Solid Product 80%?4/69.6% Recovery /98% Power 93.7 kwh/ 80.2/87.1 kwh/ lo00 gal feed lo00 gal feed The solid product is not as dry as expected due to unexpected high TOC levels. However, the solid product is easily handled.

17 4 Conclusion The combination of evaporation and crystallization successfully reduces the volume of waste for landfill. Although capital and operating costs are high, it is proven technology that is reliable and efficient. Future cooling tower blowdown systems may include other membrane type technologies in combination with evaporation and crystallization that will substantially reduce both initial capital and operating costs. -14-

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