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“Thermodyne” under Construction, 2011 Startup U. S. Location

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Example of Indirect Burner (required for “Thermodyne” Use)

(1 required for each burner zone)

Direct Fired “Airdyne” Shown Under Construction (in service at this time)

U. S. Location

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Egg Filler Flats (articles) Entering Dryer at 30%Fiber and 70% Water

The Philosophy of Drying

Merrill is well known for advancing drying technology the world over and has active licenses ongoing. What we are well known for, is the evolution of economical drying from conventional dryers up to both the Airdyne and Thermodyne Technology. The (page 1) bottom picture shows a single Thermodyne under construction. The outward appearances of both styles appear similar. In some parts of the world molded fiber articles are manufactured without the benefit of a dryer. Wet parts from the molder are placed on racks and allowed to dry in the air. What they save in equipment and operating costs they give up in production speed and capacity. Obviously, operations of this sort can only survive where labor costs are exceedingly low. The role of the dryer is to turn liquid water into water vapor in a short period of time, and do it without burning or scorching the product. The tools at our disposal are burners and fans. Burners add the heat necessary to bring the water in the article to the boiling point and evaporate it. Fans deliver that heat from the burners to the surface of the article.

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Dryer “Loading”

To take an article from 30% solids (70% water) to 9% moisture (91% solids) requires removing two kg (pounds) of water for every kg (pound) of oven-dry product. Of the water generally consumed by the plant-wide, far more of it leaves through the exhaust stacks than goes down the drain. If we optimize the wet end tools such as raw materials and drainage aids to increase our wet end consistency to 33% (67% water), that evaporated water–to-product ratio drops to 1.76. The load on the dryer, and the amount of fuel it consumes, is directly related to how much water it needs to evaporate each minute. The actual ratio of course is a variable based on furnish (old news print, office waste, Kraft), plus the dryers “air profile” and “temperature profile”.

Free vs. Bound Water

The designers of Merrill dryers indicate that 75-80% of the water is liberated in the first half of the dryer. Of course that’s a generalization, but it comes from years of observations and actual measurements. When the article first enters the dryer the water is surface, free and bound. That is, it is not bound to the non-water components (like fiber and ash) of the article. Free water (not bound water) requires less energy to evaporate. Bound water has a long list of variables associated with water liberation. When the article is “wet” with water, we can apply higher temperatures because high rates of evaporation keep the article cooled below the point at which it will scorch. When the free water is gone, what is left is bound water. These are the water molecules that interact not just with each other, but also are affected by positive and negative charges on the fibers, fines, and fillers in the article. The various mixtures of cellulose, hemicelluloses, lignin, sugars, starches, silicates, carbonates, and other components in the pulp affect how tightly the water is bound or how easily it is released. At this point in the drying process there is less rapid evaporation and less evaporative cooling. The temperature of the article surface more closely approaches the temperature of the gases hitting it so scorching can happen, especially if some parts of the article are less wet than others. This is why we use a temperature profile with different temperature targets in each dryer zone.

Article Boundary Layer

The fans in the dryer do more than deliver heat to the article; they also impact the boundary layer of the article. Here’s a description of boundary layers: ''The boundary layer is a thin film that forms on the surface of a solid body moving through a viscous fluid, like the wing of an airplane moving through the air. Within the film, velocity increases parabolically, from zero at the solid surface up to the free-stream velocity at the outer edge of the boundary layer. The depth of the layer varies with the smoothness of the surface, the viscosity of the fluid, and the velocity of the airflow.'' Within the boundary layer the water vapor moves more slowly. By impinging the article with high velocity air (greater than 1000 ft/min) we create turbulence that reduces the thickness of that boundary layer. Think of it as scrubbing the water vapor from the surface of the article. The factor that limits the amount of air we can deliver to the articles is their stability on the conveyor surface, plus the passivity that too high a velocity could warp (dent) the article surface. If the articles move around too much (and

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become Frisbees) they can jam up inside the dryer or in the stacker. As the articles dry they become lighter so less air (velocity) can be tolerated closer to the dry end.

What is the Role of Humidity in a Dryer? (Help or hinder drying?)

Everyone who has draped a wet towel over a shower rod knows that it will dry faster if the humidity is low. Dry air can hold more additional water, so water vapor leaves the surface of the towel at a greater rate. Intuitively, we compare our experience with the wet towel to what’s happening in the dryer and conclude that the more we exhaust the water-saturated air, the faster the product will dry. However, that is not the case. Inside the dryer the temperature is above the boiling point. Humidity becomes irrelevant. Unless the containing vessel is pressurized like a boiler, water above 100o

C must exist as a gas. In fact, there are advantages to operating at high humidity. Water vapor is a better heat transfer medium than is dry air. Water vapor is also less dense than dry air, so it is more easily moved with fans. Exhausting a dryer too much just means replacing hot gases with cold gases and increasing the work load on the burners.

Sensible and Latent Heat Definition

Place a pan of water over a flame and the temperature will slowly rise. We are increasing the sensible heat of the water. Once the boiling point has been achieved the temperature no longer goes up. Heat from the flame is still being added to the water, but it is now changing the water from a liquid to a gas. We can’t sense a change in the temperature, but the water vapor contains more energy than did the liquid water. This is the latent heat of vaporization and it equals 970 Btu per pound of water. If we are able to divert that water vapor and cool it until it condenses, the latent heat can be recovered. That is exactly what we do in the heat reclaim tower. When we condense the water vapor in the exhaust by cooling it to below the dew point, the latent heat in the water vapor is released. Merrill Thermodyne dryers were designed with heat exchangers so that the relatively dry burner exhaust gases do not enter the tunnel and reduce the water vapor concentration. Gases exhausted to the tower are very high in water vapor, which means they have a high dew point and the latent heat is easily recovered using white water. We also recover the condensed water, which reduces our need for additional makeup water.

Some Recent Advances in Dryer Design

Before any of our customers can fire up a new dryer, all states (at least in America) require emissions data for the new dryer. Our staff is fully capable of supporting our customers on these issues at the beginning of a project. VOCs, NOx, CO, CO2 are all generated from natural gas combustion. Most of the VOC's generated come from the process of drying paper. The amount depends on the nature of the raw materials and the temperatures to which they are exposed. If one is using old newspapers or other Groundwood types, the paper (furnish or stock) contains all the materials that were originally in the tree. This includes pitch and resins. These materials are, to one degree or another, are evaporated in the heat of the dryer. Subsequently, when they cool down they condense. If the condensation takes place inside the dryer or

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dryer stacks, we see this as a “wood creosote-like material” that burns furiously when ignited. This phenomenon is most noticeable in the exhaust stacks and the heat recovery if present. If the condensation takes place in the atmosphere it appears as smoke, quantified as “percent opacity”. This too has environmental issues. Handling these problems could be difficult and expensive. Many customers fight opacity problems, and many will have visits from their local clean air agency. “Wood creosote” fires in the dryers, can easily destroy exhaust stacks AND cyclone type/water coil type heat recovery equipment. To reduce this phenomenon you incorporate other furnishes made from chemical pulp and the problems became much less severe. The resins and pitch are dissolved in the Kraft digestion process and removed from the product. Also, with better drainage and raw materials you can reduce dryer temperatures (directly save on Btu/ton), so less of the remaining volatile organics were evaporated. In addition, some states also have an additional list of toxic air pollutants that have to be reviewed and some can be generated through combustion. Any improvement in combustion efficiency (e.g. ensuring that the organic components of the natural gas as completely converted to CO2 and water) will also

reduce emissions of VOC and the associated toxic air pollutants. CO is reduced by increasing combustion efficiency or efficacy, but NOx is generated from both combustion of the fuel and from oxidation of the air so the combustion temperature can drive the NOx generation up even as combustion becomes more efficient. There is a fine balance between efficiency and flame temperature in order to minimize the NOx and CO (and therefore VOC, toxics) emissions. By implementing flue gas recirculation or “FGR” on the Eclipse indirect heaters, we are able to maintain consistent CO levels and a high level of efficiency while reducing NOx. Using a state of

the art Siemens LMV ratio control system allowed flexible and precise control of the air, gas, and FGR flows into the Eclipse RatioAir burner. The Siemens LMV with FGR and Eclipse burner gave the ability to significantly reduce the NOx output from the combustion system without compromising CO emissions or efficiency

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So What is this New Piezometric Ring All About

What does this do for my drying problems? By utilizing the Piezometric Rings as shown at the (left) and adding the required

instrument readouts, we can read the AFCFM locally or transmit (0-10V, 4-20ma) the actual ACFM per zone, for

heat recovery, and/or limit any of the exhaust. The actual operating ACFM, utilizing the zone wet air density, you

can arrive at a figure of ±5% for accuracy and a higher level of repeatability based on the calibration and instruments used. Why is this important? It clearly speeds up the supervisor/operators work when drying loads change. Assume you are making four cup carriers one day. You recorded the

ACFM per zone, operating temperature, and make note of it because when you come back later to the same load, the results are clearly repeatable. So now you shift over to making egg filler flats or egg cartons. Wouldn’t it be nice to have an operating record there too? Drying capacity is clearly a function of the base dryer design, the operating ACFM profile (per zone), and the temperature profile (per zone). Air ratio to the 0.6 power and temperature ratio to the 0.7 power becomes the change in “drying capacity”, assuming other variables like entering and leaving article moisture remains steady. But even it does not remain steady, having the air and temperature profile will assist in dryer “load” changes.

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Where Are The Big Bucks on Energy Savings?

Right here, that’s where.

The following discussion and drawings covers the design background for “high vapor content” exhaust from a dryer (up to 100% saturated steam loading) for both the Airdyne and Thermodyne series. The higher the vapor content (i.e. airstream wet bulb), the higher the value (temperature) of heat recovery will be. This discussion applies only to Merrill designed condenser. In all instances the height (dwell time) and circumference (dryer exhaust volume) will change.

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Example Plate Type Condenser Standing Beside Typical Dryer

Summary

The whitewater condenser contacts superheated steam against falling (white) water. The cooler water condenses the steam. The design basis uses 150 gpm of water being heated from approximately 125ºF to exhaust wet bulb ºF (that’s why you want a Thermodyne). Sufficient steam-air mixture to accomplish this is assumed. The steam inlet conditions normally are 400ºF with an approximate mix of two pounds of steam per one pound of air, or even better. Countercurrent contacting reduces the steam-air mixture to an outlet temperature of approximately 135ºF and increases the water temperature to the design value of 175ºF or higher. Merrill will provide flow ranges for each dryer as ordered. For this reason and other experiences we have seen, we do not recommend multiple dryers with one condenser.

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The condenser has four distinct zones for heat transfer and water vapor condensation plus a top mounted mist reduction zone to reduce duct opacity of exhaust going to atmosphere:

1. Desuperheating 2. Condensation 3. Barometric spray condensation 4. Mist reduction

Clean makeup water at approximately one gpm is injected into the makeup line. The nozzles for this service should be fine mist atomization nozzles wall mounted on the gas duct (Spraying Systems Fogjet® nozzle or equivalent, 3 x ¼LND-SS 22).

Desuperheating Section

The condensation section takes a water shower across multiple segmental baffle trays. The vapor is forced to pass through eight water curtains. Heat transfer occurs in each water curtain. Tray spacing drops from bottom-to-top to maintain gas velocities and keep heat transfer coefficients up. Additionally these trays remain free of fiber pluggage and scour any wood creosote vapor that may be present causing duct fires. Condensation with this design is far superior to centrifugal cyclone style with the additional dwell time for water droplets to obtain the highest level of heat transfer.

Condensation Section

A water spray above the top baffle plate distributes water to the baffle plates. The spray cone also accomplishes the last major heat transfer in the condenser.

Barometric Spray Condensation

A chevron mist eliminator is used to reduce water carry-over to either the induced draft fan or forced draft fan (either way). A water wash of approximately one gpm is placed on top of the mist eliminator to wash any solids or deposits back down to the condensation section. The nozzle for this service should be a low-pressure drop and large-drop diameter nozzle. A Spraying Systems FullJet® ¼ GANV-SS316 11 or Bete 3/8 MP125 nozzle or equivalent is recommended for this service. These nozzles will pass the required flow rate at approximately 7 psi pressure drop and produce large size droplets.

Mist Reduction (important for opacity)

Heat transfer contactors of this type in similar services are relatively rare. Little data are available to evaluate them. Every effort has been made to provide a prudent design that will meet the design objectives. However, the stated operating ranges make this a severe service for any equipment. Details on these issues are included in separate sections on the discussion on heat transfer limitations.

Design Concerns, Flexibility, and Alternate Equipment

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Control and Operation

The target objective is to maintain a water outlet temperature of approximately 175ºF (depending on dilution capabilities) during normal operation. During startup conditions, the water outlet temperature may be allowed to vary to lower temperatures. The bulk water temperature should not be allowed above 175ºF in order to prevent temperature damage to the paper fibers in the whitewater. Whitewater enters the unit at a rate set by flow control. This rate will vary depending upon the heat makeup required by the whitewater system. A bypass control from the whitewater discharge line adds extra whitewater to the net incoming flow to satisfy the water rate requirement of the spray distributor. In normal operation, the spray distributor requires 150-160 gpm at flowing conditions at an inlet pressure of 25 psig to the spray header. This is equivalent to approximately 33 psig at grade. A local pressure gauge reading is available downstream of the flow junction between the net whitewater in and the recycle whitewater. The gauge should be located as close as possible to the condenser. If located at grade, its location should be at least 10 pipe diameters downstream of the junction and the whitewater flow element. The pressure reading combined with the flow reading allows for monitoring of the condition of the spray header. High pressure drops for a given flow rate indicate plugging and low pressure drops for a given flow rate indicate spray header damage. A temperature control element in the collected whitewater in the pump feed tank controls the steam-air mixture flow rate. Control is achieved via a damper in the line to the exhaust gas induction fan. A local temperature indicator is added to allow for monitoring of the desuperheating effectiveness of the water desuperheater. Flow control of both the demister water and the desuperheater water can be achieved either through direct flow measurement with an orifice meter or it may be calculated from spray nozzle characteristics and a measured water line pressure upstream of the nozzles. Level control on the whitewater collection tanks sets the net flow discharge from the condenser unit.

Design Considerations

Two major design considerations require analysis for a direct contact condenser of this type; first, hydraulic capacity and pressure drop; second, heat transfer capacity.

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Why Baffle Trays

In consideration of the heat transfer questions noted in further discussion, the obvious question comes up about why baffle trays were selected as the desired equipment. Two major reasons support the selection of baffle trays: the presence of “solids” and flexibility required. For heat transfer and capacity, the general order of device selection for this service is (most to least desired):

1. Internal Packing 2. Conventional trays 3. Baffle trays 4. Spray chamber 5. Cyclone wall style

For ability to handle solids the preferred order is: 1. Baffle trays 2. Spray chamber (with large spray nozzles) 3. Conventional trays 4. Packing

For ability to handle large variations in vapor rates, the preferred order is: 1. Packing 2. Spray chamber 3. Baffle trays 4. Conventional trays

Given the overall size constraints for the unit (20’ height max, 6’ diameter max), a spray chamber is not practical. A baffle tray, packed or conventional tray unit is required. Given the necessity to control the vapor rate to set total absorbed heat in the unit and the presence of solids, baffle trays are the best operability choice for the operation. However, there is a possibility that baffle trays may not achieve the desired heat transfer in all operations. If sustained operation is desired in a range of rates they cannot effectively handle, then the next best choice is to use packing. For this reason, man-way access and packed internals support rings that would be required to install packing have been included in the base design. The baffle trays could easily be removed and packing installed in a weekend. The column diameter has sufficient capacity to handle the liquid and vapor rates required without removal of the tray rings used to support the baffle trays.

Baffle Tray Operation and Hydraulics

The above drawing shows a schematic view of baffle trays installed in a condensing tower. Vapor passes through the descending liquid curtain that falls from tray-to-tray. During the

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curtain contact heat and mass transfer occurs. The vapor cools; steam condenses, and water heats, simple. The spray nozzle distributor installed above the baffle trays should deliver the necessary gpm (at flowing conditions) of whitewater at a pressure at a normal pressure drop. Liquid in a baffle tower may either flow over the edge of the tray to the tray below, or may flow through rows of holes close to the edge of the baffle to the tray below. The condenser design uses both approaches. A double or triple row of holes is included at the edge of the baffle. However, the holes have been sized to allow approximately 50% of the design liquid rate of the required gpm through them. The remaining will flow over the weir to the tray below, providing maximum contact with water droplets. To aid in stable operation of the overflow liquid at low flow rates, the outlet weir has been notched in a 90º v-notch pattern with each notch the required depth of the flow rate.

Gas Pressure Drop and Tray Spacing

Tray spacing has been set based on the capacity analysis of Lemieux (1983). Pressure drop calculations modified to use the gas flow velocity through the curtain area rather than through the superficial area. This accounts for the change in tray spacing’s from the 18 inches used by Lemieux in his test unit. The tray spacing of the condenser has also been increased by two inches to allow extra space for the height of the outlet weir and the water crest over the outlet weir. Tray pressure drop also varies between the liquid weir-overflow and liquid hole-overflow designs. The liquid hole-overflow design has a lower gas phase pressure drop due to the gas being able to pass around the liquid and not have to break through a solid liquid curtain. No credit for the hole-overflow pattern of liquid has been taken in the calculations. The hydraulic calculation basis made the conservative assumption that the desuperheater would be out-of-service during standard operation. Pressure drops through the unit and baffle tray spacing used this basis. All these modifications to the vapor pressure drop are in the conservative direction. A total pressure drop of three inches of water is expected for the gas through the condenser. A design value of five inches of water is recommended for blower or fan sizing. Extra pressure drop should also be added for inlet duct and outlet duct losses The foaming tendency of the liquid is not known. If the liquid creates foam due to splash agitation, the capacity of the trays may be lower than expected. Chemicals may be needed to reduce the foam present.

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Heat Transfer

Heat transfer calculations for baffle trays in this type of service are not well understood. Most experimental data are for hydrocarbon systems of for air-water systems at much lower temperatures and different liquid-to-vapor loads. Six methods were checked to determine likely heat transfer coefficients. All six methods are gross extrapolations based on correlated data. The condenser design for this service varies from the data gathered in two major areas: a very high liquid-to-vapor ratio and high temperatures. The values obtained from each method were:

1. Fair 1972 220 btu/hr-ft2

2. Stewart and Huntington 1956 95 -ºF

3. Winkler 1961 75 4. Gautreaux 1958 290 5. Nemunaitis and Eckert 1975 3,300 6. Review of barometric condenser proposal 1,000

Winkler 1961 and Stewart and Hungtinton 1956 appear very low because the data were gathered from oil-oil or oil-air systems. Steam-air-water systems should have much higher values. Fair’s 1972 method claims to be based on the data of Gautreaux 1958. However, the values between them are very different. Fair 1972 gives 220 btu/hr-ft2-ºF and Gautreaux 1958 gives 290 btu/hr-ft2

-ºF. Further investigation of Gautreaux’s report shows that the lowest vapor phase mass velocity in Gautreaux’s data is twice the value in the condenser. Also, the percent of the vapor condensable was much lower. Both of these make extrapolation to the condenser operation here dubious.

The data from Nemunaitis and Eckert 1975, even though for packed beds appear to be the most useful. Plentiful data points across a wide range of operating conditions are shown. Heat transfer coefficients for both humidification and de-humidification operations are shown. Based on using one-third of the Nemunaitis and Eckert values, effective heat transfer coefficients we could expect vary from 1,300 to 3,300 btu/hr-ft2

-ºF.

Due to the uncertainty in the data, provision has been made for converting the contactor to a packed bed design. The packed bed design will certainly transfer the required heat, but will foul much more readily than the baffle tray design. If operating experience shows the packed bed design is necessary, a wash cycle with clean water will be required to rinse the packing before shutting down.

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On-Line Cleaning

No special provisions for on-line cleaning of the equipment have been included. It is not necessary for the baffle tower configuration. For the possible packed configuration, on-line cleaning sprays will not work. A water wash before shutdown using the standard liquid distributor nozzle is the best cleaning method for a packed bed.

Calculations – Process Analysis

Process calculations were carried out using Simulation Science Inc.’s Provision/Pro II Version 5.01 process calculator. Thermodynamic property methods were the system IDEAL method with SRK equation of state enthalpy and PURE method transport properties. Duties were checked against steam table values for duties. Accuracy was 3.3% on desuperheating duty, 0.05% on condensing enthalpy, and 0.27% on latent heat of water. This is more than sufficient for the engineering design. Transport properties were not rigorously checked. However, inspection shows them to be sufficiently good for design purposes. In this system, transport property variations have little to no impact on equipment design. In fact, none of the data reviewed showed any variation for system viscosity or surface tension. The calculations attached from the simulator model include five cases. The design case used is labeled Fifth Option. This case includes the desuperheater, condenser, and barometric condenser sections in one unit.

“Example of Process Conditions: Probable Design Basis” Water Temperature in, ºF 125 Temperature out, ºF 175 Rate, gpm at inlet conditions 150 Steam-Air Mixture Temperature in, ºF 400 Blend, lb steam per lb air 2:1

Process Conditions: Operating Ranges Water Startup temperature in, ºF 70-100 Maximum temperature in, ºF 150 Rate, gpm at conditions 75-150

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Steam-Air Mixture Temperature in, ºF 350-450 Blend, lb steam per lb air 4:1 – 2:1 Expected Pressure Drop Gas side, inches of water 3” expected

Paper Ends -------------------------------------------------------------------------------------------

We gratefully acknowledge the work of those who made this article possible:

Kevin Graber of Eclipse Phone +610-630-2130 KGraber@eclipsenet.com Jon Desmond of Cox Engineering +617-438-6223 JMDesmond@coxeng.com Gary Apel of Michelsen Packaging +509-961-7226 Gary.Apel@trayplant.com Gordon Cheng of ChemicaLogic +781-425-6738 GCheng@chemiocalogic.com Beth Fifield of Spring Environmental +509-534-6417 Beth@SpringEnvirnmental.com Craig DeRoche of Design Engineering +207-793-8246 CSD@pmdesinsulutions.com

By Merrill Air Engineers, Inc. 350 Preble St South Portland, ME 04106-2236 USA Phone +207-767-1223 Fax +207-767-1074 E-Mail DCurry10@GMail.com Web Merrill-Air-Engineers.com