ANAEROBIC DIGESTER MIXING SYSTEMS. ENVE 737 Anaerobic Biotechnology for Bio-energy Production

Transcription

1 ANAEROBIC DIGESTER MIXING SYSTEMS ENVE 737 Anaerbic Bitechnlgy fr Bi-energy Prductin

2 AD MIXING SYSTEMS Natural mixing ccurs in ADs with; rise f gas bubbles thermal cnvectin currents created by additin f heat These levels f mixing is inadequate Auxiliary mixing is required What is adequate mixing? The unifrmity f slids cncentratins in any pint within the digester will nt vary by mre than 10%, except fr depsits n the tank bttm (cntaining curse sand) and flating scum (cntaining lw density material).

3 Objectives f mixing Prvide cntact between the active bimass and feed Prvide physical, chemical & bilgical unifrmity within AD Distribute rganics & dilute inhibitry substances within AD Utilize the digester vlume effectively Prevent stratificatin and temperature gradients Minimize the frmatin f a scum layer and grit settling

4 Rules-f-thumb in sizing AD systems Digester Vlume Turnver Time (DVTT) is a measure f anticipated mixing capacity f the digester DVTT = Tank vlume/pump capacity Hydraulic Retentin Time is an indicatr f mean reactin time HRT = Tank vlume/sludge flw rate Unit Pwer (UP) and G-value quantify pump capacity and nrmalize mixing intensity based n the flw prperties f the sludge UP = Pump hrsepwer/tank vlume/1000 G = Pump pwer/tank vlume/sludge viscsity Merney RN, Clrad PE (2009) CFD simulatin f mechanical draft tube mixing in anaerbic digester tanks, Water Research, 43(4);

5 Rules-f-thumb in sizing AD systems Desirable values DVTT = h HRT = days UP = Hp/1000 ft 3 G = s 1 Merney RN, Clrad PE (2009) CFD simulatin f mechanical draft tube mixing in anaerbic digester tanks, Water Research, 43(4);

6 "G" value A thery presented by Cpper & Tekippe (1982) fr determining prper mixing levels n the basis f Rt-Mean-Square Velcity Gradient (VGT r G Value). G = Rt-Mean-Square Velcity Gradient, s -1 W = Pwer Dissipated per unit vlume, J/m 3.s (r W/m 3 ) E = Rate f Wrk n energy transfer (pwer), J/s (r W) V = Vlume f Reactr, m 3 μ = Abslute viscsity f liquid, kg/m.s Cper AB, Tekippe RJ (1982) Current anaerbic digester mixing practices, 55th Annual WPCF Cnference, St. Luis, Missuri (1982)

7 "G" value Fr a gas mixing system, the rate f wrk impsed upn the system can be calculated as fllws: E = 2.40 P 1 Q ln(p 2 /P 1 ) Where: Q = Inlet Gas flw, m 3 /min P 1 = abslute pressure at the liquid surface, atm P 2 = abslute pressure at the depth f gas injectin, atm Cper AB, Tekippe RJ (1982) Current anaerbic digester mixing practices, 55th Annual WPCF Cnference, St. Luis, Missuri (1982)

8 AD MIXING SYSTEMS Types f mixing typically used in ADs; 1. Lng shafted paddle mixing 2. Pumped (jet) mixing 3. Gas mixing 4. Wall munted draft-tube mixing 5. Submersible mixers

9 1. Lng shafted paddle mixing There are tw versins available: Side entry (shaft ges frm the side twards centre) Tp entry Side entry mixers are nly suitable fr relatively small ADs Tp entry mixers are used when tank height>> diameter Nrmal peratin fr bth types wuld be intermittent, i.e fixed perids f several minutes every hur

10 1. Lng shafted paddle mixing Gas tight wall bushings take the shaft f mixer. Lnger end (inside) facing dwnwards twards the middle f the digester hlds 1 r 2 prpellers. Mtr, gearbx and all electrical cnnectins are lcated utside the AD.

11 Advantages/Disadvantages Advantages A reasnably lw pwer cnsumptin mtr is used Mtr is munted externally giving easy access N regular servicing is required (nly greasing etc.) Slw speed, large bladed units minimizes risk f mechanical failure Disadvantages Stratificatin n sme substrates may ccur Cstly t install because ADs has t remain gas/liquid tight If fails then ttal shutdwn is required t remve the unit r carry ut repairs.

12 2. Pumped mixing One r mre internally r externally munted pumps deliver a recirculated flw int a series f nzzles. N in-basin mving parts. Vaughan s Rtamix system; flr munted nzzles fed by a Vaughan chpper pump

13 2. Pumped mixing Munted at grund level and fixed t the utside f tank. Large diameter nzzles are installed externally thrugh the tank walls. Shrt pipe run resulting in minimal frictin lsses and maximum energy imparted t fluid. Hidrstal s Externally Munted Pump Mixing Systems Screw centrifugal impeller pump

14 Advantages/Disadvantages Advantages Pump unit is munted externally s regular service is facilitated. Externally installed nzzles may be remved withut entering the digester Disadvantages Nzzles may becme blcked ver time which will degrade mixing perfrmance and increase pwer csts. Internal nzzles may nt be cleaned withut empting the digester.

15 3. Gas mixing Lances Gas mixing invlves recirculating a fractin f the digester gas thrugh the digesting sludge via a cmpressr and a series f lances and nzzles. Sequencing the gas flw t varius pints causes mixing actin t be distributed thrughut the tank. Siemens - Pearth gas mixing

16 3. Gas mixing Bttm diffusers Bttm diffuser mixing systems include diffusers n the flr. All diffusers receive and discharge equal amunts f cmpressed gas, creating a rising gas clumn. Gas flw must be checked peridically, because diffusers are prne t plugging. Plugging can be cleared by directing entire gas flw thrugh affected diffuser r by flushing it with high-pressure water

17 3. Gas mixing - Bubble gun system Re-circulated gas is cntinuusly fed t bubble generatr and intermittently discharged int stack pipe as a large pistn bubble. Pistn bubble fills the entire crss sectin f pipe, driving ut liquid as it rises and creating a siphn. As ne bubble leaves stack pipe at the tp, anther enters frm generatr fr bth cntinuus mixing and preventin f slids settling. Large bubbles burst as they leave liquid surface, creating substantial turbulence that prevents scum buildup. Cannn Gas Bubble Mixing Technlgy

18 3. Gas mixing Gas lifter Gas is injected int a vertical tube via lances typically belw the midpint f draft tube As the gas is released, it carries slids upward thrugh draft tube, drawing in mre slids at the base f tube. Slids that leave the tp f tube flw away radially. Large tanks are equipped with multiple draft tubes; smaller vessels typically cntain a single tube lcated in the center.

19 Advantages/Disadvantages Advantages The cmpressrs are munted externally as is mst f the pipewrk, easing the maintenance requirements. With sme systems, a separate mixing chamber is prvided which is accessible frm the utside f the tank fr maintenance. Disadvantages Initial purchase cst is slightly mre expensive than ther systems But whle life csts are usually the lwest.

20 4. Wall munted draft-tube mixing Munted n the wall with a shrt shaft and high speed impeller External draft tube Rf munted draft tube

21 4. Wall munted draft-tube mixing External draft tube Rf munted Rf munted Rf munted Type I Type II Type III

22 Advantages/Disadvantages Advantages Majr cmpnents are munted externally with autmatic ilers and grease applicatrs. Submerged unit requires nly a limited amunt f maintenance every tw t three years. N entry int the tank is required at any time as the whle unit is remved by a very simple guide system. Disadvantages As the mixer psitin is fixed, there is n rm fr adjustment and the psitin needs t be crrect fr the mixer t perate efficiently.

23 5. Submersible mixers Similar t submersible pumps except that the pump head is replaced by a nrmally quite slw running prpeller. The unit is munted n guide rails thrugh the rf f the tank. They can be electrically r hydraulically pwered and are usually perated cntinuusly. Amaprp submersible mixers - KSB

24 5. Submersible mixers Typical ragged up submersible mixer, which has t be lifted ut f the tank fr cleaning

25 Advantages/Disadvantages Advantages Lw cst in terms f initial purchase cst. Very efficient and cntrllable mixing with the pssibility f raising and lwering mixer. Sme systems are available with a gas tight chamber at the tp f the guide rail which allws the submersible mixer t be remved withut the ttal lss f gas prductin. Disadvantages Regular servicing must be cmpleted t prevent breakdwns. Seals can be a weakness n these units.

26 Innvative mixing systems Landia GasMix TM