18 Decrease viscosity (increase temperature)

Transcription

1 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk Sustainable Municipal rinking Water Treatment continuing failed Coagulation and Rapid Mix: A search for ^ a basis for rational design Aggregation Intermolecular bonding Mixing Energy issipation Sar and iffusion T quest to improve water treatment: What are our resources? Experience operating water treatment plants Conventional wisdom Predictive performance models for drinking water treatment (Why is this hard?) Rapid mix model Flocculation Sand filters Opportunity! If we can figure out t physics we will be able to create better designs Gravity separation is awesome, but we need lp How could we increase t sedimentation rate of small particles? Increase d (stick particles togetr) Increase g (centrifuge) d g p w Increase density difference t (dissolved air flotation) 8 w ecrease viscosity (increase temperature) Seeking favorable interactions between particles We add a sticky insoluble coagulant that forms nanoparticles We design reactor geometry and flow regimes to create interactions between Nanoparticles of coagulant precipitate issolved organic molecules Inorganic particles (like clay) Organic particles (including pathogens) efinitions Coagulation: T process of adding a sticky solid phase material (adsive nanoparticles) that attacs to t particles so ty can attach to each otr (t topic of tse notes) Flocculation: T process of producing collisions between particles to create flocs (aggregates) (next set of notes) Aluminum Sulfate Cmistry Alum [Al (SO ) *.H O] A widely used coagulant Typically 0 mg/l to 00 mg/l alum is used (0.9 to 9 mg/l as Al) High concentrations (stock solutions) don t precipitate because t ph is low T alum precipitates wn it blends with t water in t water treatment plant T primary reaction produces Al(OH) Al (SO ) + 6H OAl(OH) + 6H + + SO ph = -log[h + ]

2 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk Acid Neutralizing Capacity (ANC or Alkalinity) Requirement Al (SO ) + 6H OAl(OH) + 6H + + SO ANC is measured as mg/l of CaCO How much ANC is consumed by alum? Molecular Formula Alum Al (SO ) *H 0 CaCO Calcium Carbonate Molecular mass 600 g/mol 00 g/mol eq/mol 6 Molecular 00 g/eq 50 g/eq mass/eq Simple guide mg/l Alum consumes 0.5 mg/l Calcium Carbonate ANC This sets t maximum alum dose that can be used for low alkalinity waters Polyaluminum Chloride (PACl) Slowly titrated with a base (in t cmical plant) to produce a meta stable and soluble polymeric aluminum (partially neutralized) Consumes less alkalinity (ANC) Aluminum mass fraction is higr than in alum (no. H O) so t mass of PACl required is less than for alum (0. 0 mg/l as Al) PACl Formula and Basicity [Al n (OH) m Cl n m ] x Basicity: Ratio of hydroxyl equivalents m Basicity to aluminum equivalents n Basicity of would mean that it does not produce any protons wn it dissolves in water 0 means it produces protons per Al (like alum) T lowest basicity commercial PACls are about 0% Most PACls are in t medium to high basicity range (50 70%) Higst stable basicity (8%) is aluminum chlorhydrate (ACH) would be useful for treating water with very low ANC Al concentration (mg/l) 0 AlOH 0. Aluminum Solubility ph AlOH Toretical Al solubility Experimental PACl solubility Minimum EPA MCL imum EPA MCL imum dose Al ph control is critical! Coagulation fails at low ph and high ph because t coagulant becomes too soluble T Conventional Coagulation Hypotsis* T purpose of addition of coagulant cmicals is to neutralize t negative charges on t colloidal particles to prevent those particles from repelling each otr. Coagulants due to tir positive charge attract negatively charged particles in t water. sewagetreatment/coagulation flocculation process/ * This hypotsis has been t explanation of coagulation for close to 00 years. Surface charge of Particles and Natural Organic Matter (NOM) NOM significantly increases t required coagulant dose in some waters T coagulant required increases with NOM concentration Charge density (conventional explanation) Clay: 0.05 to 0.5 eq/mg ( mg/l clay NTU) Fulvic acid 5 to 5 eq/mg C NOM also has a larger surface area* per unit mass than clay particles and thus provides many attachment sites for coagulant nanoparticles

3 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk Electrostatic Charge Neutralization Explains pnomena such as t aggregation of sediment suspensions in estuaries Explains many particle stability experiments with pure systems of nanoparticles May not explain t particle interactions that occur in flocculation using aluminum and iron based coagulants Coagulation: T adsive nanoparticle hypotsis Aluminum coagulants (alum and polyaluminum chloride) produce adsive nanoparticles of precipitated aluminum hydroxide that attach to surfaces including otr nanoparticles T attractive attachment force must be stronger than t polar bond between water molecules so t nanoparticles can push water out of t way Predictive performance model for hydraulic flocculator design with polyaluminum chloride and aluminum sulfate coagulants Karen A. Swetland, Monroe L. Weber-Shirk*, Leonard W. Lion Sticky nanoparticles? Effective flocculation only occurs wn t coagulant is in t solid phase Thus NaOH and Ca(OH) are not effective coagulants because ty are soluble at neutral ph Al(OH) likes to stick to itself and to otr surfaces Why is t solid phase Al(OH) and Al O (OH) 7+ sticky? (Perhaps because it is more polar than water) On Rapid Mixing In summary, little is known about rapid mix, much less any sensitivity to scale. However, t models and data reviewed suggest t need to be on t lookout for certain effects. From what is presently known, it can be speculated that since coagulant precipitation is sensitive to both micro and macromixing, scale up must consider not only energy dissipation rate, but also t reaction injection point and t contacting method. Mixing in Coagulation and Flocculation 99 page 9 Traditional rapid mix units Backmix mechanical reactors In line blenders Hydraulic Jump In line static mixers Traditional esign Conventional design is based on t use of G (velocity gradient) as a design parameter. We don t know what t design objective is for rapid mix and thus it isn t clear what parameter matters G a high degree of turbulence leads to more efficient cmical use for destabilization by adsorption and charge neutralization, wreas destabilization by precipitation is less sensitive to t mixing conditions. (Water Quality Engineering Benjamin and Lawler, 0) In drinking water treatment t coagulant dosages are almost always higr than t solubility limit; perhaps rapid mix isn t important

4 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk P G Q P Q gh P h Qg G h g This is t traditional approach Q Power, ight, and G P G velocity gradient caused by mixing Reactor volume Power required Power required to lift water Equivalent potential energy measured as a ight used for mixing Equivalent potential energy as a function of G G Residence Time (s) Traditional esign Guidelines: Mixing with a Propeller Average values velocity gradient (G) (/s) Energy dissipation rate (W/kg) Equivalent ight (m)* No mention of scale effects G h g from Environmental Engineering: A esign Approach by Sincero and Sincero page 67 * A measure of mechanical energy converted to at Hydraulic Energy Constraint If we use t same amount of mechanical energy in hydraulic water treatment plants as is used in mechanical water treatment plants we will need between 0.8 and 7.5 m of water ight change just to power t rapid mix unit!!!!! Rapid mix is one of t largest energy consumers in mechanical plants We need to be more efficient (and nce smarter) Why might RAPI mixing be necessary? Mix t adsive nanoparticles uniformly with t raw water (definitely important if t flow is split between treatment trains) IF RAPI mixing matters tn tre must be something bad that happens if t mix is SLOW (but maybe it doesn t matter) Self aggregation of nanoparticles into microclusters Non uniform distribution of nanoparticles between particles Not clear if this matters or how to control it Or maybe rapid mix is actually t first stage of flocculation! We will prove later that collision potential (and nce particle aggregation) is proportional to G Rapid mix collision potential can be as great a t collision potential for flocculation So this raises t question Is rapid mix actually t first stage of flocculation? Rapid Mix: From macro to nano scale (in a few seconds?) Length scale Transport Process m Rapid Mix flow dimension Large eddies Small eddies mm Inner iscous scale v kv K kv 50 K m iscous sar Result nm Molecular diffusion Molecular scale

5 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk 5 Three steps for mixing Possible constraints Large scale eddies to mix at t dimension of t reactor (Macro mixing) Generate large eddies Flow expansion with dimensions similar to t dimension of t reactor, pipe, or channel Critical if t flow splits between treatment trains Turbulence to mix down to t scale of t smallest eddies (Micro mixing) Fluid sar and diffusion to transport t coagulant nanoparticles to t particles and pathogens Goals aren t clear Fluid must be fully turbulent to get efficient mixing by eddies Re>0,000 (this seems like a good goal) Uniform distribution of coagulant between particles (all particles get some glue) Constraint isn t obvious Sar/iffusion of nanoparticles from fluid phase to particle surfaces occurs in a reasonable amount of time (ideally completed before flocculation) Might not be possible Turbulence Mixing Energy issipation T turbulent eddies cause stretching and thinning of concentration gradients and shuffle packets of fluid T intensity of t turbulence can be characterized by t rate at which mechanical energy is being lost to trmal energy W J Nm kg mm m Kg s Kg s Kg s skg s How Far Can Turbulence Mix? (iscous inner scale) issipates energy from t mean flow through chaotic eddies and through viscosity wre t kinetic energy is converted to at Turbulence is a great mixer down to about 50x* t Kolmogorov scale * iscosity kills inertia (and eddies)! Kolmogorov length scale K Inner viscous length scale v kv K 50 inner viscous length scale (mm) viscosity, for water is 0-6 m /s kv Eddies cause mixing Sar and diffusion Energy dissipation rate (mw/kg) Mixing below t inner viscous length scale is by sar and diffusion T viscous sar and molecular diffusion processes are both slow Tre is no practical way to speed diffusion A contact chamber might be lpful* Time for dissolved organics to diffuse to coagulant nanoparticles Time for nanoparticles to diffuse to particles and pathogens * Edge of knowledge AguaClara design We have speculated that energy dissipation rate mattered We had an flawed justification for about W/kg We don t currently have a rational basis for design of a rapid mix unit T L/s pilot plant offers us t opportunity to do t needed research.

6 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk 6 T minimum Reynolds number for efficient mixing is 0,000 L Re Figure 5. -fluid concentration in t plane of symmetry of a round turbulent jet. (a) Re.5 0 (0 < z/d j < 5). (b) Re 0 (0 < z/d j < 00). ata from imotakis et al. (98, figures 5 and 9). T Reynolds Number requirement may be a challenge for lab scale flows and irrelevant above 0. L/s Q ReMixing Re h Mixing Q Re g Mixing efinition of Re Head loss for a flow expansion Head loss (m) 0 0. Replace with Q 0 Solve for jet diameter Q Flow rate (L/s) Continuity isclaimer T following analysis is based on t assumption (without basis) that tre may be a goal of achieving a certain energy dissipation rate or velocity gradient. Aggregation of large particles may happen faster than deposition of nanoparticles on particles (flocculation may start well before coagulation ends). A quiescent contact chamber may be more effective because nanoparticles benefit more (than clay particles) from diffusion. How do we generate intense turbulence? We need to be converting mechanical energy (kinetic energy) to trmal energy We want concentrated ad loss! (this shouldn t be too hard to achieve!) Trefore use minor loss (related to a change in flow geometry) ratr than major loss (from sar at t solid boundaries) Almost all minor losses are caused by expansions (We need a flow EXPANSION) Flow Expansion T control volume analysis gave us t total energy loss, but it doesn t give us t energy dissipation rate. T energy dissipation rate varies with location in t expanding jet in Mixing out in A g A Centerline Centerline 50 x Round 0. ε. (W/kg) Minimum x for which this relationship is valid is 7 Round x is distance from t jet origin 6 m 8 s W 70 kg 50 Article x/. (Baldyga et al., 99 Baldyga, J., Bourne, J. R. and Zimmermann, B., 99, Investigation of mixing in jet reactors using fast, competitiveconsecutive reactions. Cm. Engn 0 Sci. 9, 97. T value we will use in our analysis. Furtr work is require to determine t best value of this parameter for different jet geometries.

7 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk Flow rate (L/s) Energy issipation Rate Three orifices, same velocity Round Big eddies create smaller eddies A B C Which jet has t higst energy dissipation rate? Which jet has t higst sar (or velocity gradient)? Which jet has t largest eddies? Which jet will make t smallest eddies first? Which jet will dissipate energy t fastest? Re Higr energy dissipation rates give a smaller Kolmogorov scale! Rapid Mix orifice Orifice iameter to Obtain Target Mixing (Energy issipation Rate, Kolmogorov Length Scale) extra rain plug CC Float Inlet Pin to keep plate in place LFOM Round A A Orifice vc Orifice vc Round Q Substitute for and solve for Orifice Rapid mix orifice Orifice 7 Q Round vc T orifice must be smaller than this to achieve t target energy dissipation rate Rapid Mix Head Loss extra Reduce t required ad loss through t rapid mix for large plant flows extra g Round Round g RoundQ g Round Round 7 Round Rapid mix orifice ad loss (cm) Q This is one of t scale effects for rapid mix Flow rate (L/s) This assumes discharge into a large tank Why? Rapid Mix ad loss 7 RoundQ increases with Q g Round How could we maintain a high energy dissipation rate while reducing t velocity and t ad loss? Round What else is needed? Use a plate with multiple orifices in t rapid mix pipe and add macro mixing upstream Rapid mix orifice ad loss (cm)

8 CEE 50: Sustainable Municipal rinking Water Treatment Monroe Weber-Shirk 8 Hypotsis: Macomixing minor loss coefficient and length scale T eddy velocities are comparable to t mean velocities for a minor loss coefficient of T minimum distance in t direction of flow (L) for mixing over t dimension perpendicular to t direction of flow () would tn be equal to. (L>) This length is t pipe minimum length before t micromixing event (unless t two are combined in a single orifice) extra Mechanized Rapid Mix vs. Hydraulic Rapid Mix ampire loads A 00 L/s AguaClara plant costs approximately $,000,000 After 5 years t electricity cost for mechanized rapid mix would be $0,000 Anotr way to give is to not take T 5 year energy cost for a package plant that uses a total of 00 J/L is.5 million US! Or perhaps just turn off t mixer? J 50 L L 0 9 s 5 yr J Reflections Hydraulic rapid mix should be t default design Research is required with our pilot plant to determine if rapid mix makes a difference or if a quiescent contact chamber would be more beneficial Rapid mix might be t first stage in flocculation