Three stage single pass drying apparatus for particulate materials

A single pass, multiple stage, rotary drum heat exchange dryer (22) is provided for drying products such as distillers grains and includes a tubular shell (64) with a moist product inlet (66), an opposed dried product outlet (70), and an internal drying chamber (78). The chamber (78) includes a convection drying first stage (80), and conductive drying final curing stage (82) an intermediate stage (84); the intermediate stage (84) is subdivided into a plurality of contiguous drying zones (86-92). The zones (86-92) include individual flighting assemblies (164) which are of substantially the same density and heat transfer ratios.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is broadly concerned with high density, multiple stage, single pass rotary drum dryers especially useful for the high-efficiency drying of moisture-laden products. More particularly, the invention is concerned with such dryers which include an initial, primarily convection drying stage, a final, primarily conductive drying stage, and an intermediate multiple-zone stage where both convective and conductive drying occurs. The individual zones within the intermediate stage are equipped with internal flighting designed so as to provide a substantially uniform heat transfer ratio (the total zone heat transferring surface area divided by the zone volume) along the length of the intermediate stage.

Description of the Prior Art

Drying of large volumes of fragmented fibrous materials has long been carried out in heat exchangers consisting of one or more elongated, generally horizontally oriented drums. Hot gases are caused to flow through each to remove moisture from the material by heat exchange between the hot gases and the fibrous product. Generally, a burner is disposed to direct hot products of combustion directly into the inlet of the drum which also receives the moisture-bearing material to be dried. However, advantage has also been taken of other sources of waste heat. After removal of the requisite amount of moisture from the material, the dried product is directed into a collector or other receiving means at the outlet of the heat exchange drum. A blower or equivalent device is provided to accomplish the required rate of flow of hot gases through the drum heat exchanger.

Three pass dryers have been used in the past which include a single rotatable drum with concentric stages arranged so that the material being dried traverses the drum in a serpentine fashion. Three pass dryers are relatively expensive but have been used primarily because of the decreased product residence time necessary to obtain adequate drying, while minimizing ground space in the drying plant. A limiting factor in the use of three pass dryers has been the restricted inlet opening of the concentrically arranged drying zones, thus resulting in a fairly severe heat transfer in the first pass. High temperatures have been tolerated in the first pass of the three pass dryers in connection with the drying of alfalfa because the product typically is introduced into the three-pass dryer at a moisture level of about 80%. The latent heat transfer that occurs in the first pass thereby protects the product notwithstanding the high-temperature level that exists in the first pass drying zone.

In the case of prior single pass dryers, efforts to increase the airflow velocity simply resulted in excessive blowing of the material out of the dryer and resulting inadequate product retention time. A by-product of the decreased retention time was a lessening of the Δ T between the inlet and outlet temperatures of the dryer. Even at air velocities of no more than about 500 feet per minute, the resulting discharge temperature on most products was found to be in the range of 300° F. to 350° F. Single pass dryers, as contrasted with three pass dryers, are particularly useful for drying temperature-sensitive products that either have a substantially lower initial moisture content than relatively wet alfalfa, as for example about 30%, or that are blended with previously dried material to bring the moisture content of the product entering the inlet of the dryer to about that moisture level. The single pass dryer may be operated at a substantially higher throughput than a three-pass dryer. In addition, high temperature levels in the initial drying stage are avoided as occurs in the first pass of a three pass dryer.

U.S. Pat. No. 4,193,208 illustrates a single pass dryer having inwardly extending internal flighting within the drum which caused the material conveyed through the dryer to be lifted up and then dropped back into the hot gas stream, rather than simply resting at the bottom of the drum as it was rotated. The secondary flighting in the central part of the drum was provided to enhance heat exchange between the hot gases directed through the drum and the product to be dried. In order to prevent hot gases from being blown directly through the dryer from one end to the other, single pass dryers have included transverse plates in the drum to obstruct the flow of hot gases therethrough. The net result of such constructions was to decrease the capacity of the dryer while at the same time interfering with uniform temperature control and preventing maintenance of constant material flow rates through the dryer.

U.S. Pat. No. 5,157,849 illustrates and describes an improved single pass dryer having circumferentially spaced, inwardly directed, product conveying and showering conductive and convective heat transfer flights extending inwardly toward the center of the drum where the total surface area of the flights is at least as about as large as the total heat transfer surfaces of the products to be dried at maximum throughput capacity. The flighting design of the '849 patent leaves a flight-free central showering zone of a size to permit heat exchange and conveyance of material along the length of the dryer at a predetermined rate, and establishes a specific range of diameter ratio between the diameter of the drum and the diameter of the internal cylindrical flight-free central product showering zone.

U.S. Pat. No. 6,584,699, which is incorporated by reference herein in its entirety, illustrates a single pass dryer comprising three drying stages: a convection drying first stage, an intermediate stage, and a conductive drying final curing stage. The intermediate stage is subdivided into a plurality of contiguous drying zones having fighting assemblies of increasing density and progressively increasing heat transfer ratios. It was discovered that such dryers present difficulties in handling high-density moist particulate materials, such as distillers grains mixed with condensed distillers solubles (also known as “syrup”). The addition of syrup to the wet distillers grains has a significant effect on how the particulate product flow through the dryer. It has been discovered that the syrup inhibits the “showering” effect of the particulate material as it flows through the dryer and causes more of the product to stay in the outer periphery of the drum as opposed to being more evenly distributed toward the center. This reduces dry efficiency and necessitates a reduction in product throughput in order to obtain a finished product with the desired moisture characteristics at the dryer outlet.

SUMMARY OF THE INVENTION

The present invention provides an improved single pass drum dryer exhibiting enhanced drying efficiencies, particularly when drying high-density, high-moisture content materials. Broadly speaking, the drum dryer of the invention includes an elongated, hollow drum having a moist product inlet and a spaced dried product outlet, with a drying chamber between the inlet and the outlet. Flighting is provided within the drum which effectively separates the drying chamber into a plurality of drying stages, including a first stage adjacent the inlet, a final stage adjacent the outlet, and at least one intermediate stage between the first and final stages. The intermediate stage includes a plurality of drying zones arranged in successive order, from a point proximal to the first stage and extending towards the final stage. Each of the zones is configured with internal flighting having heat transfer surfaces that define a predetermined ratio calculated by dividing the total heat transferring surface area within the zone by the volume of the zone. The flighting is arranged so that the heat transfer ratio of at least two of the zones, and preferably all of the zones, is approximately the same. In one particular embodiment, the at least two zones have a heat transfer ratio of about 2.25 to about 3.25 ft−1.

The preferred design of dryers in accordance with the invention is that the intermediate stage zones are arranged in contiguous relationship, with the first zone being contiguous with the first stage and the last zone being contiguous with the final dryer stage. The number of intermediate stage zones is variable, but usually ranges from 2-8, with four zones being most preferred.

The intermediate stage zones are advantageously equipped with heat transfer flighting which presents a series of inwardly extending, circumferentially spaced apart metallic heat transfer panels, with the number of panels in at least two of the zones, and preferably all of the zones, being essentially the same. In practice, the panels are supported on corresponding strut elements coupled to the inner surface of the drum; these strut elements support L- and Z-shaped members which cooperatively define the individual panels.

The final stage of the preferred dryer has a heat transfer ratio smaller than the heat transfer ratio of any of the intermediate stage zones, and is preferably designed as a curing chamber of the type described in U.S. Pat. No. 5,157,849, incorporated by reference herein in its entirety.

In operation, initially moist product (e.g., distillers grain, bakery wastes, alfalfa, peat moss, wood materials or similar particulates) is introduced into the dryer inlet along with heated air during rotation of the drum. In one particular embodiment, the moist product comprises a mixture of distillers grains and condensed distillers solubles. Preferably, the moist product comprises from about 20% to about 50% by weight of the condensed distillers solubles and from about 50% to about 80% by weight of the moist distillers grains. Most preferably, the moist product comprises about 40% condensed distillers solubles and about 60% moist distillers grains. Typically, the moisture content of the incoming product would range from about 30-80% by weight, more preferably from about 50% to about 75% by weight, and most preferably from about 60% to about 70% by weight. The inlet air temperature is generally from about 600-1800° F.; where distillers grain products are being dried, the temperature would be normally be from about 550-700° F. Airflow rates through the dryer would commonly range from about 60,000 CFM to about 180,000 CFM, or higher.

As the product is advanced along the length of the drum by virtue of drum rotation and passage of air therethrough, it is progressively dried. At the same time, the air temperature decreases along the drum length. In the distillers grain example, the air would have a temperature of around 450° F. as it enters the intermediate stage, and a temperature of about 225-250° F. into the third stage. The exiting air would have a temperature on the order of 190° F. In the first stage, product drying is primarily from convective heat transfer, while in the second stage a combination of convection and conductive drying is carried out in the final stage, almost all of the product drying is accomplished by conduction.

By configuring at least two, and preferably all, drying zones of the intermediate stage to have essentially the same flighting configuration and heat transfer ratio, the present invention overcomes the problem of inefficient showering effect that was exhibited with previous dryer designs having progressively increasing flight density within the intermediate stage drying zone. Surprisingly, it was discovered that not only can sufficient drying of the material be accomplished in this manner, but that air flow rates through the dryer are not sacrificed, and in some instances, are actually capable of being increased while still maintaining the desired air discharge temperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, and in particularFIG. 1, an overall product drying assembly20in accordance with the invention includes a rotary drum dryer22adapted to receive and dry a particulate material, with a furnace24and blending chamber26adjacent the inlet of the dryer22, and a cooling drum28at the outlet end of the drum for receiving and cooling dried product. The assembly20further includes an air-handling unit30, including a primary fan32, recycle collector34, discharge collector36, dual inlet centrifugal separator38, and ducting40interconnecting the collectors34-38and fan32. An optional return air conduit42extends from the top of recycle collector34to the inlet of furnace24and has an intermediate blending air conduit44leading to chamber26. A pair of tandem-mounted product recycle screw conveyors46and48extend along the length of drum22from the outlet end thereof to a product input conveyor50, and receive output from the collectors34and36. Similarly, a dried product screw conveyor52extends from the outlet end of the dryer22to cooling drum28. The furnace24is equipped with a gas-fired burner54as well as a gas recycle conduit56from blending chamber26. The latter may include a boiler gas recycle duct58as shown. Air discharge from the assembly20is provided via discharge duct60coupled to collector36.

During use of the assembly20, the dryer22is rotated (typically at a speed of from about 3-12 rpm) by means of trunnion drive62, while heated air is delivered to the input end of the drum by means of furnace24, blending chamber26and air handling unit30. Initially, moist product is delivered to conveyor46by conventional means (not shown), with a predetermined portion of partially dried product being transferred by conveyors46,48from the outlet end of the dryer back to conveyor46for recycling through the dryer. The air-handling unit30serves to move air throughout the assembly20, with exhaust through duct60and product dropout through the collectors34-36-38, as will be understood by those skilled in the art.

The drum dryer22includes an elongated, circulated in cross-section tubular metallic shell64presenting an inlet66defined by inwardly extending, flanged circular wall68, and an outlet70formed by a flanged, tapered segment72of the shell64. It will be observed that the inlet66and outlet70are essentially concentric and in opposed relationship. A pair of trunnion tracks74,76are secured to the outer surface of shell64and engage corresponding trunnion wheel assemblies.

Referring toFIG. 2, it will be seen that the interior of drum dryer22is provided with differently configured heat transfer flighting along the length thereof between inlet66and outlet70, effectively forming an internal drying chamber78presenting a first stage80(Dryer Stage I), a final stage82(Dryer Stage III), and an intermediate stage84(Dryer Stage II). The intermediate stage84is in turn subdivided into four contiguous drying zones86(Zone I),88(Zone II),90(Zone III), and92(Zone IV), with the first zone86being contiguous with first stage80and fourth zone92contiguous with final stage82.

As shown inFIGS. 3 and 8, the first stage80is equipped with flighting broadly referred to by numeral94comprising a total of five adjacent, axially spaced apart rows96-104of flighting elements. Each of the rows96-104is made up of a plurality of identical, circumferentially spaced apart L-shaped flighting members106, each presenting a first leg108secured to the inner surface of shell64by welding or the like, and a transverse leg110in spaced relationship from the shell64. As best seen inFIGS. 3 and 10, the adjacent transverse legs110in each of the flighting rows96-104are interconnected by elongated metallic straps112. It will also be seen that the fighting members106of each of the rows96-104are circumferentially offset from the fighting members in adjacent rows. In the illustrated embodiment, each successive row98-104is offset 5° from the preceding row.

The final stage82is in effect a curing stage for the product prior to exiting from the dryer22, and is described in U.S. Pat. No. 5,157,849 incorporated by reference herein. This stage is equipped with an inner set of three sector plate assemblies114-118, an intermediate, inwardly extending annular wall120, a further set of six sector plate assemblies122-132and a final sector plate assembly134. Each of the sector plate assemblies114-118and122-132are identical and include (seeFIG. 5) a plurality of circumferentially arranged, somewhat trapezoidal plates136each presenting an arcuate outer margin138secured by welding or the like to the inner face of shell64, a complemental, arcuate inner margin140and a pair of side margins142,144which diverge from the ends of inner margin140to the ends of outer margin138. The plates136are arranged in close proximity at their respective outer margins138thereby defining a series of substantially V-shaped passageways146between adjacent pairs of the plates136. Adjacent ones of the sector plate assemblies114-118and122-132are offset from each other so that the V-shaped passageways146formed by each of the sector plate assemblies are likewise offset as depicted inFIG. 5. Finally, the stage82has a plurality of elongated, axially extending vanes148secured to the interface of shell64. The final sector plate assembly134is depicted inFIG. 6and is made up of a series of circumferentially arranged sector plates150each presenting an outer margin152secured to shell64, inner margin154and side margins156,158. A shallow V-shaped groove160is formed at the center of each plate150as illustrated.

A series of circumferentially spaced lifter plates162are located between the outer surface of sector plate assembly134and the inner face of shell segment72. The plates162extend from the main body of shell64to a point adjacent the outlet70.

The intermediate stage84is designed so that the heat transfer ratio defined thereby remains substantially constant from the inlet end of the stage adjacent first stage80to the outlet end of the stage adjacent final stage82. This is accomplished by providing uniform construction and density of the fighting component within each of the zones86-92. Unlike the dryer embodiment illustrated in U.S. Pat. No. 6,584,699, the fighting density and heat transfer ratio does not progressively increase from the inlet to first zone86to the outlet of the final zone92of intermediate stage84. In a preferred embodiment, the heat transfer ratio within each of the zones is from about 2.25-3.25 ft−1.

Referring toFIG. 4, the fighting assembly164includes six V-frame assemblies166,168,170,172,174, and176spaced about the interior of the shell64with intermediate L members178within each V-frame assembly and between the respective assemblies.

In more detail, each V-frame assembly166-176includes two aligned strut units180and182(seeFIGS. 7aand 7b), with each strut unit made up of a pair of strut tubes184and186. The strut tubes184and186are secured to the inner face of shell64by means of wed brackets188and extend inwardly in a radial direction to an apex190. Generally, trapezoidal gusset plates192interconnect the inner end of tube184and an inboard portion of tube186. The inner ends of tubes184from adjacent V-frame assemblies are interconnected by additional trapezoidal gusset plates194. The aligned strut tubes184,186of each strut tube unit180,182support elongated, metallic heat transfer plates196, i.e., the plates196bridge the aligned tubes184and the aligned tubes186. The plates196include an outermost, somewhat L-shaped plate198having a laterally extending segment200and a short, transverse segment202. The L-shaped plate198is secured to the outboard ends of the aligned tubes184,186by welding using clips204. In addition, the plates196include a series of generally Z-shaped intermediate plates206supported on the aligned tubes186,186. Specifically, each of the Z-shaped plates206includes a central planar segment208, an aperture, outboard transverse segment210, and an inboard transverse segment212. Each segment210has a pair of spaced-apart openings214formed therein which are adapted to receive the respective tubes184or186. During construction, a series of the Z-shaped plates206are slid onto the aligned tubes184,186so that the plates206are in abutting contact, and these are welded in place to the strut tubes. The outermost L-shaped plate198is then positioned on the outer ends of the aligned struts and secured in place via welding and the clips204. At this point, the end most brackets188are welded to the strut tubes permitting the entire V-frame to be secured to shell64. In preferred practice, the plates198and206extend the full width of the zone and may be of any desired length, e.g., 8 feet. In addition, the plates196may be spaced from each other in various increments. It is preferable, though, for the spacing between plates196to be uniform. In certain embodiments, the spacing between plates196is from about 6 to about 10 inches, and more preferably, about 8 inches.

It will thus be appreciated that the flighting assembly164presents a total of twelve generally radially oriented, spaced apart, essentially continuous heat exchange panels defined by the plates196which extend the full length of the zone.

Drying assembly20is particularly well suited for drying high-density particulate materials without sacrificing material through-put times as compared with conventional single-pass dryers that employs an intermediate stage having drying zones with progressively increasing heat transfer ratios. As disclosed in U.S. Pat. No. 6,584,699, a drum dryer having a twelve-foot diameter, that is about 44 to about 58 feet in length, and comprising an intermediate stage with drying zones of progressively increasing heat transfer ratios may be operated at air flow velocities of 100,000 to 180,000 CFM. However, the dryer disclosed in the '699 patent is not capable of maintaining these high air flow velocities when drying distillers grains mixed with significant quantities of condensed distillers solubles (“syrup”). The wet distillers grains being fed to the dryer generally have a density of approximately 47.0 lb/ft3. However, the syrup portion is much denser having a density of approximately 68 lb/ft3. Both the syrup and wet distillers grains comprise approximately 68% moisture when fed to the dryer. It was discovered that the syrup inhibits the “showering” effect of the particulate material as it flows through the dryer and causes more of the product to stay in the outer periphery of the drum as opposed to being more evenly distributed toward the center. Therefore, in order to obtain a finished product of the desired moisture content, typically about 10% by weight, the residence time of the product in the dryer must be increased and/or recycle of dried product into the dryer must be increased. Either way, the product rate declines accordingly.

It was discovered that the problem with prior dryer designs could be overcome by replacing the flighting of the drying zones of the intermediate stage that have progressively increasing heat transfer ratios with uniformly configured flighting that results in a substantially constant heat transfer ratio across at least two, but preferably all, drying zones of the intermediate stage.

In certain embodiments, the temperature of the gases introduced into the inlet66of the dryer22may range from 500° F. to as much as 1,800° F. In the case of products to be dried that contain a protein and/or fat content that is to be protected against excessive temperatures, the inlet temperature of the drying airstream is usually recommended to be less than about 700° F., and especially between about 550° F. to about 700° F. Under these conditions the dryer22can process wet particulate matter that has a total moisture content of up to 70% or 75% by weight. Preferably, in embodiments in which the material being dried comprises a mixture of distillers grains and syrup, the moisture level of the material introduced into the process is from about 50% to about 75% by weight, from about 60% to about 70% by weight, or about 65% to about 68% by weight. The temperature in the outlet of the drum dryer22, in the case of a 700° F. inlet temperature, will be no more than about 180° F. to about 200° F., when the drum is rotated from 4 to 12 rpms and usually about 6 rpms.

Although a preferred drum dryer22in accordance with this invention contains 12 radial flighting arms as illustrated in the drawings, it is to be understood that other numbers of radial flighting arms, e.g., 8, 10, 14, 16, etc., may be used. When wet material having an initial moisture content of about 68% is introduced into the inlet66of drum dryer22at a preferred drying gas inlet temperature of about 700° F. and the inlet air velocity is of the order of 165,000 CFM, the temperature of the material entering the intermediate stage84will generally be about 400° F. to 450° F. The temperature of the material entering the curing or final stage82will be about 225° F. to 270° F., and the outlet temperature will be from about 180° F. to 200° F. The air volume out of the outlet70of the drum dryer22will nominally be about 125,000 CFM. Most importantly, the temperature of the heat transfer media or air/water vapor mixture as it is conveyed through the first, second and third drying zones86-92of intermediate stage84decreases relatively uniformly, and is consecutively lowered about 60° F. through each stage.

As material dries along the length of a single pass dryer, the particles tend to accelerate as the moisture content decreases and the particles become lighter, even though there is some decrease in velocity of the air flow. It is to be recognized that material being dried is initially carried by the surfaces of the radial flighting in each of the zones86-92until such time as the material may fall from the flighting surface as a result of gravity. Thus, material falls from a respective radial flight surface twice during each rotation of the drum.

In the drying zones having 12 radial flights, the material during each 180° of rotation of the drum will fall a distance that averages approximately ⅙ of the diameter of the drum. Accordingly, conductive heat transfer and convective heat transfer tends to remain substantially the same throughout the length of the intermediate stage86. The residence time of the material decreases somewhat in each successive zone due to the tendency for the velocity of the material to increase along the length of the dryer as the particles dry out.

The drum dryer22of assembly20is particularly useful for drying products that have a relatively high-fat content, as for example distillers grain that is generally known as DDGS. Other materials that may beneficially be dried in assembly20include hydrolyzed feather meal, potato waste, high-fat bakery feed or fish meal which has very fragile oils. In some instances, a proportion of the dried material out-feed from drum dryer22will be recycled back to the inlet of the dryer for blending with the moist product to provide the desired inlet moisture content for the feed material. In certain embodiments in which the dryer22is used to process meal products such as DDGS, corn gluten feed, hydrolyzed feather meal, and municipal sludge system performance can be improved by recycling at least 60%, at least 70%, or at least 80%, and preferably from about 60% to about 90%, from about 65% to about 85%, or from about 70% to about 80% of the dried material to the dryer inlet. In particular, the recycled product is mixed with the wet matter that is being fed to the dryer in order to provide a moist feed product Recycling the dried product increases the surface area of the product in the drying process making for a vastly improved heat transfer rate. The formula for the heat transfer rate is Q=hA (T-t), where h is the film coefficient, A is the surface area of the product being dried, T is the hot gas temperature, and t is the product temperature.

EXAMPLE

In this Example, two processes for drying distillers grain from an ethanol plant were simulated and compared. Case A involves the use of a three-stage dryer400having an intermediate stage with zones of progressively increasing flighting density constructed per U.S. Pat. No. 6,584,699. Case B utilizes an otherwise identical system, except that dryer400comprises an intermediate stage with zones having the same heat transfer ratio. The drying process set up (FIG. 9) is similar to that illustrated in FIG. 3 of U.S. Pat. No. 7,654,011, incorporated by reference in its entirety.

Generally, moist distillers grains are fed to dryer400by line300at 65% moisture content. The dried distillers grains exit dryer400by line302at 12% moisture. A portion of the product in line302is recycled to the dryer inlet by line328where it is combined with the moist feed to form a combined stream329. The non-recycled portion of the dried product is directed via line303to a cooling vessel408where additional moisture is removed. The finished product is recovered from cooling vessel408by line330. Drying air provided to dryer400by line318, which is heated in heat exchanger406. Dryer off-gas is removed from dryer400by line320. A majority of the dryer off-gas is recycled to heat exchanger406by line322. The air provided to the hot side of heat exchanger406is provided primarily by a gas-fired heater402, which comprises a furnace, mixing chamber, and thermal oxidizer. Preheated combustion air is provided to heater402by line304. Natural gas fuel is provided to heater402by line306. In addition, dryer off-gas is also provided to heater402, and specifically to the mixing chamber, by line326. The hot air from heater402is directed to a tempering chamber404by line308, where it is mixed with a portion of cooled gas from heat exchanger406via line314. The tempered air is directed to heat exchanger406by line310. Cooled gas exits heat exchanger406via line312, the majority of which, represented by line316, is used to preheat the combustion air within preheater407.

The results of each simulation are provided in Tables 1, 2 and 3 below.

As can be seen from the data, the original progressively increasing drying zones dryer of Case A, processing 51,429 lb/hr, requires 41,581,338 BTU/hr, or an energy consumption of 1342 BTU/lb of water evaporated. The dryer of Case B, having substantially uniform drying zones, processing 77,143 lb/hr, requires 61,624,183 BTU/hr, or energy consumption of 1326 BTU/lb of water evaporated. Thus, the data demonstrates that the dryer of Case B permits a higher product throughput and higher air flow rates through the dryer, while drying more efficiently that the dryer of Case A and while maintaining the same desired air discharge temperature. Moreover, because the dryer of Case B accommodates a larger mass flow rate of material, a larger recycle mass flow rate can also be used (i.e., about 50% greater than Case A). This increase in recycle mass flow rate results in a correspondingly greater heat transfer rate (Q) due to the additional surface area provided by the extra recycled material.