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Title:
PLANT AND PROCESS FOR PRODUCTION OF A DRIED PRODUCT FROM A HUMID PRODUCT
Document Type and Number:
WIPO Patent Application WO/2018/109144
Kind Code:
A1
Abstract:
Disclosed is a plant for production of a dried product from a humid product, comprising a drying apparatus having an inlet for a humid product to be dried and an inlet for a stream of superheated steam, and an outlet for the dried product and an outlet for a stream of saturated steam, a first compressor having an inlet configured to receiving a first part of the stream of saturated steam from the outlet of the drying apparatus, and an outlet for the stream of compressed steam, a second compressor having an inlet con- figured to receiving a first part of the stream of compressed steam from the outlet of the first compressor, a first condenser having an inlet configured to receiving the second part of the stream of saturated steam from the outlet of the drying apparatus and an outlet for superheated stream of steam, and an inlet configured to receiving the second part of the stream of compressed steam from the outlet of the first compressor and an outlet for condensate, and a second condenser having an inlet configured to receiving the stream of superheated steam from the outlet of the first condenser and an outlet for the stream of further superheated steam, and an inlet configured to receiving the stream of compressed steam from the outlet of the second compressor and an outlet for condensate, wherein the inlet for a stream of superheated steam of the drying apparatus is configured for receiving the stream of superheated steam from the outlet of the second condenser. The plant has the advantage of being able efficiently to recover the energy in a stream of saturated steam leaving the drying apparatus.

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Inventors:
HANSEN, Niels (Bagergade 3C, 5700 Svendborg, 5700, DK)
Application Number:
EP2017/082952
Publication Date:
June 21, 2018
Filing Date:
December 15, 2017
Export Citation:
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Assignee:
HSL ENERGY HOLDING APS (Søhesten 2 C, 2635 Ishøj, 2635, DK)
International Classes:
F26B21/04; F26B23/00
Foreign References:
EP0079523A11983-05-25
EP2511637A12012-10-17
US4242808A1981-01-06
Attorney, Agent or Firm:
NORDIC PATENT SERVICE A/S (Bredgade 30, Copenhagen K, DK-1260, DK)
Download PDF:
Claims:
P A T E N T C L A I M S

A plant for production of a dried product from a humid product, comprising

a. A drying apparatus having an inlet for a humid product to be dried and an inlet for a stream of superheated steam, and an outlet for the dried product and an outlet for a stream of saturated steam,

b. A first compressor having an inlet configured to receiving a first part of the stream of saturated steam from the outlet of the drying apparatus, and an outlet for the stream of compressed steam,

c. A second compressor having an inlet configured to receiving a first part of the stream of compressed steam from the outlet of the first compressor,

d. A first condenser having an inlet configured to receiving the second part of the stream of saturated steam from the outlet of the drying apparatus and an outlet for superheated stream of steam, and an inlet configured to receiving the second part of the stream of compressed steam from the outlet of the first compressor and an outlet for condensate, and

e. A second condenser having an inlet configured to receiving the stream of superheated steam from the outlet of the first condenser and an outlet for the stream of further superheated steam, and an inlet configured to receiving the stream of compressed steam from the outlet of the second compressor and an outlet for condensate,

wherein the inlet for a stream of superheated steam of the drying apparatus is configured for receiving the stream of superheated steam from the outlet of the second condenser.

The plant according to claim 1, wherein one or more intermediate compressor(s) is(are) present, wherein an intermediate compressor has an inlet configured for receiving a first part of a stream of compressed steam from the outlet of the first compressor and an outlet for the stream of compressed steam configured for delivering to the inlet of a further intermediate compressor or the inlet of the second compressor.

3. The plant according to claim 1 or 2, wherein one or more intermediate condenser(s) is(are) present, wherein an intermediate condenser has an inlet configured to receiving the stream of superheated steam from the outlet of the first condenser and an outlet for the stream of further superheated steam configured to delivery to the inlet of a further intermediate condenser or the inlet of the second condenser, and an in- let configured to receiving the stream of compressed steam from the outlet of the intermediate compressor and an outlet for condensate.

4. The plant according to any of the claims 1 to 3, wherein a preheater is provided upstream of the drying apparatus, said preheater being connected, directly or indirectly, to the outlet for condensate from the first, intermediate or second condenser.

5. The plant according to any of the claims 1 to 4, wherein first, second, and any intermediate compressors share a common shaft, optionally interrupted by a gearing.

6. The plant according to any of the claims 1 to 5, wherein the first, sec- ond, and any intermediate compressors are driven by the same driver.

7. The plant according to any of the claims 1 to 6, wherein the drying apparatus is selected among the group comprising a spray drying apparatus, a fluid bed dryer, a tray dryer, a roller dryer, a drum dryer, a pneumatic dryer, a rotary drier, a bin drying apparatus, a tunnel dryer, and belt drying apparatus.

8. The plant according to anyone of the claims 1 to 7, wherein the drying apparatus is divided into two or more section, wherein a first drying section has an inlet for superheated steam connected to the outlet for superheated steam of the first condenser and a second drying section has an inlet for superheated steam connected to the outlet for superheated steam of the second condenser.

9. The plant according to claim 8, wherein the drying apparatus is continuous mesh-belt drying apparatus divided into two or more drying zones.

10. The plant according to any of the claims 1 to 9, further comprising a boiler configured for boiling water to steam, a heat exchanger for superheating the steam, and an expander for transferring energy from the superheated steam to a shaft.

11. The plant according to claim 10, wherein the boiler is heated by ex- haust gas from a gas motor or a gas turbine.

12. The plant according to claims 10 or 11, wherein the heat exchanger is configured for cooling a stream of steam from a compressor, while the steam from the boiler is heated.

13. The plant according to any of the claims 10 to 12, wherein the steam from the boiler is heated in two sequential heat exchangers, while compressed steam from first and the second compressor are cooled.

14. A process for the production of a dried product from a humid product comprising the steps of:

a. Treating a humid product with a stream of superheated steam to obtain a dried product and a stream of saturated steam, b. Compressing a first part of the saturated stream of steam in a first compressor,

c. Compressing a first part of the compressed stream of steam from the first compressor in a second compressor,

d. Superheating the second part of the saturated stream of steam in a first condenser, which condenses the second part of the compressed stream of steam from the first compressor, e. Further superheating the stream of steam from the first condenser in a second condenser, which condenses the compressed stream of steam from the second compressor,

f. Delivering the superheated stream of steam from step e. to step a.

15. The process according to claim 14, wherein the stream of superheated steam delivered to the inlet of the drying apparatus has a temperature of 125°C or above.

16. The process according to claims 14 or 15, wherein the temperature difference between the outlet for the stream of superheated steam in the first and the outlet for the stream of superheated steam in the second condenser is 30°C or less.

17. Use of the plant according to anyone of the claims 1 to 13 for drying woodchips.

Description:
Title

Plant and process for production of a dried product from a humid product Introduction

The present invention relates to a plant and a process for production of a dried product from a humid product.

Background art

Hot humid air from drying plants is traditionally discharged to the ambient air, optionally after particles have been removed . However, the hot humid air contains energy, which may be used for various purposes, including heating of a steam which may be used for drying the humid product.

A number of prior art documents suggest using the energy in the humid air for reheating the drying air used for drying the wet product. Thus, FR2347087 discloses a method for drying humid air using a number of heat pumps. The humid air from a drying plant is first conveyed to two or more evaporators in which the humid air progressively is cooled and a refrigerant is evaporated . The evaporated refrigerant from each evaporator is compressed and delivered to two or more condensers. Condensed water is discharged from the cooled humid air and the air is subsequently heated in the two or more condensers that receive the evaporated refrigerant. The heated air is returned to the drying plant for drying the wet product.

FR2341342 discloses a plant for drying humid air. Initially, the humid air is cooled in a heat exchanger and subsequently further cooled in an evaporator. The condensate is collected and the cooled air is conveyed to the heat exchanger for heating before it is further heated in a condenser that receives the refrigerant from the evaporator after it has been compressed. The heated air is returned to the drying plant.

WO 2004/046629 discloses a method for drying of a wet product in two or more steps. In a first step the wet product is dried by a hot drying gas. The resulting humid gas is subsequently condensed and water in a closed loop is heated . The heated water is used to heat drying air to be used in a subsequent drying step.

Other prior art documents have suggested in addition to reheating the drying air also to heat water in a district heating system. Thus, EP 129631 relates to a plant for production of hot water for district heating systems and cooling or condensing of steam. The steam is used in a fluid bed that dries biofuels, such as peats, and two streams of exhaust steams streams are produced . A top stream from the fluid bed is cleaned in a cyclone and subsequently conveyed to a condenser that produces a condensate and heated water in a district heating system. The water of the district heating system is further heated by a second steam stream from the fluid bed in a heat exchanger.

WO 82/02939 shows a method for reducing the heat loss in a drying plant for paper production. The paper is heated by a drum, which uses steam for the heating . The humid air from the drying plant is treated by several heat pumps. In a first heating pump, the humid air is initially cooled by an evaporator, which evaporates a refrigerant. The refrigerant is subsequently compressed and condensed in a condenser, which evaporates condensate from the drum. The evaporated condensate or steam is reintroduced in to the drum for heating purposes. The cooled humid air from the first heating pump is subsequently further cooled by a second heat pump, which heats process water. In a final heat pump the humid air from the second heat pump is fur- ther cooled and water for room heating is heated.

EP 2511637 discloses a method and an apparatus for drying particulate materials, such as calcium carbonate. The drying apparatus comprises a drying chamber having an inlet for which particulate material, an outlet for dried particulate matter, an inlet for superheated steam and an outlet for saturated steam. The apparatus also comprises a single compressor having an inlet for a part of the saturated steam and an outlet for compressed steam. Furthermore, the apparatus comprises a single condenser having an inlet for the remainder of the saturated steam and an outlet for superheated steam, an inlet for the compressed steam and an outlet for the condensate.

The present invention provides an improved plant and process for production of a dried product. The invention makes use of two or more compressors and two or more condensers combined in a certain way to obtain a high energy efficiency. Summary of the invention

The present invention relates to a plant for production of a dried product from a humid product, comprising

a. A drying apparatus having an inlet for a humid product to be dried and an inlet for a stream of superheated steam, and an outlet for the dried product and an outlet for a stream of saturated steam,

b. A first compressor having an inlet configured to receiving a first part of the stream of saturated steam from the outlet of the drying apparatus, and an outlet for the stream of compressed steam,

c. A second compressor having an inlet configured to receiving a first part of the stream of compressed steam from the outlet of the first compressor,

d. A first condenser having an inlet configured to receiving the second part of the stream of saturated steam from the outlet of the drying apparatus and an outlet for superheated stream of steam, and an inlet configured to receiving the second part of the stream of compressed steam from the outlet of the first compressor and an outlet for condensate, and

e. A second condenser having an inlet configured to receiving the stream of superheated steam from the outlet of the first condenser and an outlet for the stream of further superheated steam, and an inlet configured to receiving the stream of compressed steam from the outlet of the second compressor and an outlet for condensate,

wherein the inlet for a stream of superheated steam of the drying apparatus is configured to receiving the stream of superheated steam from the outlet of the second condenser.

The invention also relates to a process for the production of a dried product from a humid product comprising the steps of:

a. Treating a humid product with a stream of superheated steam to obtain a dried product and a stream of saturated steam, b. Compressing a first part of the saturated stream of steam in a first compressor, c. Compressing a first part of the compressed stream of steam from the first compressor in a second compressor, d. Superheating the second part of the saturated stream of steam in a first condenser, which condenses the second part of the compressed stream of steam from the first compressor, e. Further superheating the stream of steam from the first condenser in a second condenser, which condenses the compressed stream of steam from the second compressor,

f. Delivering the superheated stream of steam from step e. to step a.

According to the present invention the steam usually discarded to the ambient air is used as a source of energy for drying the humid product. In this way a substantial amount of the energy in the previous discarded steam is recovered. The plant in addition makes use compressors, which are provided power from a driver. The compression in two steps decreases the difference in pressure needed for supplying the condensers with sufficient condensing energy, thereby increasing the efficiency of the plant. Furthermore, the superheating in two steps performed by the condensers also increases efficiency due to the decreased difference in temperature in each step.

As used in the context of the present invention as disclosed in the present description and claims, the term "saturated steam" or any derived forms thereof is to be interpreted as a steam having a relative humidity at or above 90%, such as above 95% and suitably above 98%. In some occasions, the steam may be super-saturated, i.e. the steam is a mixture of steam and water droplets. In most instances, however, the term "saturated steam" is to be understood as steam having a relative humidity close to 100%.

As used in the context of the present invention as disclosed in the present description and claims, the term "superheated steam" or any derived forms thereof is to be interpreted as a steam being at a temperature higher than its vaporization point (dew point) at the pressure, where the temperature is measured . Thus, a superheated steam can therefore evaporate water in a humid product without condensing to water.

In some embodiments of the plant and process it may be advanta- geously to use more than two compressors. Thus, one or more intermediate compressor(s) may be present, wherein an intermediate compressor has an inlet configured for receiving a first part of a stream of compressed steam from the outlet of the first compressor and an outlet for the stream of compressed steam configured for delivering to the inlet of a further intermediate compressor or the inlet of the second compressor.

The presence of three or more compressors further improves the energy efficiency due to possibility of decreasing the pressure difference provided by the condensers in each step.

In some embodiments of the plant and process it may be advanta- geously to use more than two condensers. Thus, one or more intermediate condenser(s) may be present, wherein an intermediate condenser has an inlet configured to receiving the stream of superheated steam from the outlet of the first condenser and an outlet for the stream of further superheated steam configured to delivery to the inlet of a further intermediate condenser or the inlet of the second condenser, and an inlet configured to receiving the stream of compressed steam from the outlet of the intermediate compressor and an outlet for condensate.

The presence of three or more condensers further improves the energy efficiency due to possibility of decreasing the temperature difference provided by the condensers in each step. In a certain embodiment of the invention, the plant and the process is provided with one or more intermediate condenser(s) as well as one or more compressor(s) to make use of the advantage of both a reduced difference in pressure in the compressor section and a reduced difference in temperature in the condenser section.

In some embodiments of the invention a different number of compressors and condensers are used . As an example, the plant and the process may be provided with three compressors and two condensers. In this configuration a part of the compressed stream of steam from the first or the intermediate compressor is not directed to a condenser. Instead the entire amount of compressed stream of steam from the first or intermediate compressor is forwarded to the subsequent compressor.

The condensate produced by the first, second, and any intermediate condensers is hot. Under atmospheric conditions the temperature of the condensate will be around 100°C. The thermal energy in the condensate may be used to heat the humid product before it enters the drying apparatus. Thus, in an aspect of the invention the plant and the process in further provided with a preheater upstream of the drying apparatus, said preheater being connected, directly or indirectly, to the outlet for condensate from the first, intermediate or second condenser.

The compressor used in the present invention may be of the reciprocating, rotary screw or rotary centrifugal type. Generally, it is desired that the compressors are of the rotary centrifugal type. The compressor may be driven by a conventional driver such as an internal combustion engine, a gas turbine, a gas engine, a steam turbine and/or an electric motor.

The advantages of using the conventional combustion drivers is that the plant may be completely independent of existing infra structure as it can run on its own means as long as it is supplied with fuel . The advantage of using an electric motor as a driver is that the exhaust emission may be virtually zero, which may be desired in population areas. A further advantage of using electric motors is that electricity in certain areas may be abundant and cheap, making it favorable to use electricity to drive the heating process. Areas with abundant electricity will typically be areas with access to hydro, geo or nuclear electricity. In an embodiment the compressor may be driven by a combination of an electric motor and one of the other mentioned convention- al motors, so that the electric motor can be operated as a back-up to the conventional motor.

The compressors may be driven individually or they may be driven co-axially by a driver. In a preferred embodiment the first, second, and any intermediate compressors are driven co-axially by the same driver.

The advantages of driving two or more of the compressors co-axially is that the heat pump may be driven by a single or few drivers, which reduce the maintenance cost of running the plant. This embodiment may be realized by integrating the compressors in a multi-stage compressor, wherein each impeller can be considered as one of the compressors. Thus, in a suitable embodiment of the invention the first, second, and any intermediate compressors share a common shaft, optionally interrupted by a gearing, transmission or clutch .

The drying apparatus used in the present invention may be selected among a variety of known apparatuses, i.e. the drying apparatus may be se- lected among the group comprising a spray drying apparatus, a fluid bed dryer, a tray dryer, a roller dryer, a drum dryer, a pneumatic dryer, a rotary drier, a bin drying apparatus, a tunnel dryer, and belt drying apparatus.

In tray dryers, the humid product is spread out, generally quite thinly, on trays in which the drying takes place. Heating is performed by a steam current sweeping across the trays or through perforation of the bottom of the trays.

Tunnel dryers may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapors removed. In most cases, the humid product can move through the dryer either parallel or counter current to the steam flow. The tunnel dryer may be compartmented and cross-flow may be used .

In roller or drum dryers the humid product is spread over the surface of a heated drum. The drum rotates, with the humid product being applied to the drum at one part of the cycle. The humid product remains on the drum surface for the greater part of the rotation, during which time the drying takes place, and is then scraped off.

In a fluid bed dryer, the humid product is maintained suspended against gravity in an upward-flowing stream of steam. There may also be a horizontal steam flow helping to convey the food through the dryer. Heat is transferred from the steam to the humid product mostly by convection.

In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of steam. Steam and solids may move in parallel or counterflow. Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

In a pneumatic dryer, the humid particles are conveyed rapidly in a steam stream, the velocity and turbulence of the stream maintaining the particles in suspension. The steam accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried product is separated, the dry material passes out as product and the moist remainder is recirculated for further drying.

In rotary dryers the humid product is contained in a horizontal in- dined cylinder through which it travels, being heated either by steam flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through .

In trough dryers the materials to be dried are contained in a trough- shaped conveyor belt, made from mesh, and air is blown through the bed of material . The movement of the conveyor continually turns over the material, exposing fresh surfaces to the hot steam .

In bin dryers, the humid material is contained in a bin with a perfo- rated bottom through which warm steam is blown vertically upwards, passing through the material and so drying it.

In belt dryers the humid product is spread as a thin layer on a horizontal mesh or solid belt and air passes through or over the material . In most cases the belt is moving, though in some designs the belt is stationary and the material is transported by scrapers.

In a certain embodiment of the invention the drying apparatus is divided into two or more sections, wherein a first drying section has an inlet for superheated steam connected to the outlet for superheated steam of the first condenser and a second drying section has an inlet for superheated steam connected to the outlet for superheated steam of the second condenser.

The application of two or more sections in the drying apparatus allows for heating of the humid product in a steps with varying exposure to the superheated steam .

When drying a product, it is often suitable to dry at different temper- atures depending on the content of water in the product to be dried . In one way of using the present invention it is generally preferred to dry at a higher temperature when the product contains the highest amount of water. When the product is partially dried it is subjected to a lower drying temperature. The lower drying temperature in later drying stages may be used to prevent heat damages. The higher temperature in the first section may be obtained by directing the superheated steam from the second condenser to the inlet for the first section and the relative lower temperature in the second section may be obtained by directing part of the stream of superheated steam from the first condenser to the inlet of the second section .

In another embodiment it is desired to use a steam with a lower temperature in the first section and a steam with a higher temperature in the second section. Thus, in the first section when the humid product has the highest humidity, a stream of steam with a relative low heating capacity is used. In a certain aspect of the invention, the steam for the first section is obtained from the outlet of the first condenser. In the second section the product is partly dried and it may advantageous to use a stream of steam with a higher heating capacity for obtaining a product with a low residual humidity. In a certain aspect of the invention, the steam for the second section is obtained from the outlet of the second condenser.

The invention also comprises the use of one or more intermediate drying sections. These intermediate sections may be fed with superheated steam from either the first, the second, and any intermediate condensers. Optionally, the streams of the first, the second, and any intermediate condensers may be mixed before entering into the first, second or intermediate drying sections.

In a preferred aspect of the invention, the drying apparatus is a continuous mesh-belt drying apparatus divided into a first, a second and optionally intermediate drying zones. The humid product may be applied on the mesh-belt in an even layer and transported by the mesh-belt through the various drying zones. In each drying zone, steam is directed through the mesh and allowed to interact with the humid product. During the interaction, the humidity of the product to be dried is decreased and the relative humidity of the steam is increased . To obtain a high efficiency it is desired that the steam leaving each drying zone is saturated with humidity, i.e. the relative humidity (RH) is at or close to 100%. However, in some instances it may be desired to operate at a lower relative humidity of the leaving steam, such as relative humidity of 90%. In another aspect of the invention, the drying apparatus is a fluid bed divided into a first, a second and optionally intermediate drying zones.

The dried product may be potato starch, grains, minerals, phosphates, peat, wood chips, timber, cellulose pulp, hay, veneer, foodstuff, sludge, bark, enzymes, pharmaceuticals, instant coffee, etc.

In an aspect of the process according to the present invention the stream of superheated steam delivered to the inlet of the drying apparatus or the second section of the drying apparatus if two or more sections are used, has a temperature of 125°C or above. Usually, the temperature of the superheated steam is not above 250°C due to process economics and the risk of heat damaging the product to be dried.

In an aspect of the process according to the present invention, the temperature difference between the outlet for the stream of superheated steam in the first and the outlet for the stream of superheated steam in the second condenser or any intermediate condenser is 30°C or less. A relatively low temperature difference ensures an effective use of the energy.

In a certain aspect of the invention the plant and further comprising a boiler configured for boiling water to steam, a heat exchanger for superheating the steam, and an expander for transferring energy from the superheated steam to a shaft. The boiler contains or is supplied with water from various sources including the condensate from either the first, the second or both condensers. The steam generated in the boiler is saturated and is sup- plied to at least one heat exchanger for superheating thereof. The superheated steam is supplied to the expander for conversion to shaft power. Subsequently, the stream is condensed in one or more of the condenser(s). By recovering the heating in the stream from the compressor(s) in a stream of steam from the boiler, further energy may be extracted from the overall pro- cess.

The boiler may be heated by an independent energy source or, alternatively and preferably, by a heat stream of the plant. When a gas motor or a gas turbine is used as the driver, the exhaust gas may be used to heat the water in the boiler. Furthermore, the cooling medium form a gas motor may be used to preheat the water or condensate before it enters the boiler.

In a certain embodiment of the invention, the heat exchanger is configured for cooling a stream of steam from a compressor, while the steam from the boiler is heated. One or more heat exchanger(s) may be connected to a stream of compressed steam from one or more of the first, the second and any intermediate compressors, thereby cooling the compressed stream of steam from the compressors(s) and heating the steam from the boiler.

In a preferred embodiment, the steam from the boiler is heated in two or more sequential heat exchangers, while compressed steam from first, intermediate, and second compressor is cooled . Thus, the saturated steam from the boiler is suitably heated in a first heat exchanger by exchanging heat with the stream of steam from the first compressor, followed by further heating in an intermediate heat exchanger, if present, by exchanging heat with the stream of steam from any intermediate compressor, and finally by heating in a second heat exchanger by exchanging heat with the stream of steam from the second compressor.

The pressure and temperature of the superheated stream of steam is decreased in the expander in a process in which energy is transferred to the shaft. Preferably, the expander is provided on the same shaft as the first and second expander to assist the driver in driving the compressors. The decom- pressed steam from the expander is subsequently condensed in the first and/or second condenser. Preferably, the steam from the expander is condensed in the first condenser to recover more heat energy.

Summary of the drawings

The present invention will now be described in greater detail based on preferred embodiments with reference to the drawings on which :

Figure 1 discloses a depiction of an embodiment of the invention according to claim 1.

Figure 2 shows the embodiment of figure 1 further provided a third compressor and a third condenser.

Figure 3 illustrate an embodiment in which the drying apparatus is a spray drier.

Figure 4 shows a further development of the embodiment shown in figure 3 further provided with a fluid bed.

Figure 5 discloses an embodiment in which the drying apparatus is a continuous mesh-melt drying apparatus divided into 3 drying zones.

Figure 6 shows an embodiment in which an expander assists the diver in driving the compressors.

Detailed description

Figure 1 discloses a depiction of an embodiment of the invention according to which a humid product (HP), such as humid woodchips, are dried . The plant comprises a drying apparatus 10, which receives the humid product at inlet 11. In addition, the drying apparatus receives superheated steam at inlet 12. The superheated steam interacts with the humid product in the drying zone 13, whereby heat is transferred from the superheated steam to the humid product. In consequence, the humidity of the humid products is lowered by the evaporation of vapour. The vapour is adopted in the steam thereby increasing the relative humidity and lowering the temperature of the superheated steam . After a treatment of the humid product has occur for some time the steam becomes saturated, i.e. the relative humidity is 95- 100% .

The drying apparatus is also equipped with an outlet for the dried product (DP) 14 and an outlet for the stream of saturated steam 15. A part of the stream of saturated steam is delivered to the inlet 24 of a first compressor 21. The compressor 21 compresses the steam thereby increasing the pressure and temperature thereof. The compressed steam exists the compressor at outlet 25. A part of the compressed steam leaving the first com- pressor is entered into a second compressor 22 at inlet 26. In the second compressor the steam is further compressed to elevate the temperature and pressure of the steam . The stream of further compressed steam exists compressor 22 at outlet 27.

The second part of the stream of saturated steam from the outlet 15 of the drying apparatus 10 is transferred to a first condenser 31 and enters the condenser at inlet 34. The first condenser is also delivered a part of the compressed steam from the outlet 25 of the first compressor. The compressed steam from the first compressor enters the first condenser at inlet 35. In the first condenser, the compressed steam is condensed to a conden- sate, which leaves the first condenser at outlet 36. The condensing of the compressed steam evolves heat, which is transferred to the stream of steam that entered at inlet 34. The heated steam leaves the first condenser at outlet 37. As the steam leaving the condenser is heated above its saturation temperature, it is generally referred to herein as superheated steam.

The superheated steam from the first condenser is supplied to the second condenser 32 and enters at inlet 42. The second condenser is also delivered the compressed steam from the outlet 27 of the second compressor. The compressed steam from the second compressor enters the second condenser at inlet 43. In the second condenser, the compressed steam is condensed to a condensate, which leaves the second condenser at outlet 44. The condensing of the compressed steam evolves heat, which is transferred to the stream of steam that entered at inlet 42. The heated steam leaves the second condenser at outlet 45. As the steam leaving the condenser is heated above its saturation temperature, it is generally referred to herein as a su- perheated steam.

The superheated steam exiting the second condenser at outlet 45 enters the drying apparatus 10 at inlet 12. In the drying apparatus, the superheated steam and the product to be dried is contacted so that the heat transfer can occur efficiently.

The first and the second compressors are linked by a shaft 50, which furthermore links to the driver M. In this configuration a single driver is able to drive two or more compressors.

The embodiment shown in Figure 1 will now be illustrated by a specific embodiment. A batch of woodchips to be dried are introduced in a drying apparatus and provided on a mesh. The superheated steam is entered into the drying apparatus at a temperature of 140°C. The superheated steam is allowed to interact with the woodchips until the steam get saturated. The saturated steam leaves the drying apparatus at ambient pressure and a temperature of 100°C.

A first part of the saturated stream of steam from the drying apparatus is introduced in the first condenser. A second part of the saturated stream of steam from the drying apparatus is entered into the first compressor, in which the compression results in a heating to a temperature of 125°C. A part of the stream of compressed steam is introduced into the first conden- ser, in which it is condensed to a condensate. The heat evolved by the condensation process is transferred to the first part of the saturated stream from the drying apparatus, thereby super heating the stream of steam to 120°C.

Another part of the compressed stream of steam exiting the first compressor is introduced into a second compressor. The second compressor increases the pressure and the resulting temperature becomes 145°C. The partly superheated stream of steam from the first condenser and the stream of compressed steam from the second compressor is entered into the second condenser at the respective inlets. In the second condenser, the stream of partly superheated steam is further heated to 140°C and the stream of steam from the second compressor is condensed to a condensate. The stream of superheated steam from the second condenser is subsequently introduced in the drying apparatus at a temperature of 140°C. The condensates from the first and the second condenser leaving the condenser have a temperature of 105°C and 125°C, respectively. The streams of con- densates are used for pre-heating the woodchips before they are introduced into the drying apparatus.

Figure 2 shows an embodiment using an intermediate compressor and an intermediate condenser. In addition to the embodiment of figure 1, the partly superheated stream of steam exiting the first condenser 31 at out- let 37 is introduced into the intermediate condenser 33 and inlet 52. Furthermore, a part of the stream of partly compressed steam exiting outlet 25 of the first compressor is introduced into an intermediate compressor 23. In the intermediate compressor the steam is further compressed to elevate the temperature and pressure of the steam . The stream of further compressed steam exists compressor 23 at outlet 29. A part of the steam from compressor 23 is delivered to the second compressor 22 and another part is forwarded to the intermediate compressor 33.

The intermediate condenser is delivered the compressed steam from the outlet 29 of the intermediate compressor 23. The compressed steam from the intermediate compressor enters the intermediate condenser at inlet 53. In the intermediate condenser, the compressed steam is condensed to a condensate, which leaves the intermediate condenser at outlet 54. The condensing of the compressed steam evolves heat, which is transferred to the stream of steam that entered at inlet 52. The heated steam leaves the intermediate condenser at outlet 45.

The superheated steam exiting the intermediate condenser at outlet 55 enters the second condenser 32 at inlet 42. By introducing an intermediate step, the efficiency of the system is further improved as the temperature and pressure difference in each step can be reduced . It will be understood that further intermediate condensers and compressor may be introduced into the plant to further improve the efficiency.

Figure 3 illustrate an embodiment in which the drying apparatus is a spray drier. Referring to the description of the embodiment of figure 1, the humid product (HP) is fed to the spray drier by a suitable pump (not shown) and sprayed at nozzle 16. The nozzle may be any kind of conventional nozzle, such as pressure nozzle, two-fluid nozzle and three-fluid nozzle. The specific type of nozzle used is selected in accordance with the feed to be sprayed and the desired product. The drying steam is obtained from the outlet 45 of the second condenser 32 and introduced co-currently with the direction of the spray. The steam for drying the sprayed droplets may be introduced into the spraying chamber in a number of ways, usually through an annular opening in the top of the spray drying chamber. The annular opening is generally positioned around the nozzle to obtain an even distribution of drying steam around the sprayed droplet. The annular opening will ensure that the drying steam interacts essentially uniformly with the droplets. In the body of the drying chamber, the droplets and the drying steam further interacts, thereby increasing the humidity of the steam and drying the droplets to particles or agglomerates. The obtained particles or agglomerates leaves the spray drying apparatus at outlet 14. If desired or necessary, the particles may be cooled or further dried in a downstream process.

The saturated steam from the spray drying apparatus exits at the outlet 15. A part of the stream of steam is introduced into the first compressor and another part of the stream of saturated steam is introduced to the first condenser, as explained in further detail in the description of the embod- iment shown in figure 1.

Figure 4 shows a further development of the embodiment shown in figure 3. According to this embodiment, the dried particles or agglomerates from the outlet 15 is further dried in a fluid bed 60. The particles from the spray drying apparatus enters the fluid bed at inlet 61. Part of the superheat- ed steam from outlet 65 of condenser 32 is directed to inlet 62. The stream of superheated steam is allowed to interact with the partly dried particles or agglomerates entered at inlet 62. Due to the interaction, the particles or agglomerates are further dried in fluid bed 60 and in consequence the humidity of the steam is increased. The essentially saturated drying steam exits the fluid bed at outlet 65 and is subsequently directed to inlet 34 of the first condenser and treated as described in more detailed in figure 1 and figure 3.

Figure 5 discloses an embodiment in which the drying apparatus is a continuous mesh-melt drying apparatus divided into 3 drying zones. Figure 5 may be considered a further development of the embodiment shown in figure 2, and reference is made to the description of Figure 2 and 1 for a full de- scription of the embodiment according to figure 5.

The humid product (HP) enters on a continuous mesh-belt at the left side of the drying apparatus into a first drying zone section 13'. The first drying zone section receives a part of the partly superheated stream of steam from the outlet 37 of the first condenser 31. In the first drying zone section, the humidity of the product to be dried is relatively high and is therefore subjected to the only partly superheated stream of steam from the first condenser. After treatment in the first zone the partly dried humid product is transported on the mesh-belt to an intermediate drying zone section 13". In the intermediate drying zone section, the humidity of the product to be dried is relatively high and is therefore subjected to the only partly superheated stream of steam from the intermediate condenser. After treatment in the intermediate zone the partly dried humid product is transported on the mesh- belt to a second drying zone section 13"'. In the second drying zone section, the humidity of the product to be dried is relatively low and is subjected to the final treatment by a superheated stream of steam from the second condenser 32.

The treatment of the humid product in two or more stage increases the efficiency. As each subsequent stage furthermore receives superheated steam with an increased temperature, the efficiency is further improved .

The saturated streams of steams from the drying zone section 13', 13", and 13"' leaves the continuous mesh-belt apparatus at the respective outlets 15', 15", and 15"'. The streams of steams from the outlets 15', 15", and 15"' are collected and a first part is directed to the for compressor 21 and another part is directed to the first condenser 31. A suitable valve may be applied to regulate the amount of steam that is distribute the condenser and the compressor.

Figure 6 discloses a further development of the plant, which is the subject matter of Figure 1. The plant of figure 1 further comprises a boiler 70, expander 80, and heat exchanger section 90 comprising two heat exchanger for heating the steam in two steps.

The condensate exiting at 44 is transferred by a pump 72 to the boiler 70. The condensate is heated by an exhaust gas (EG), which enters at the inlet 71 and exits at the outlet 75. The exhaust gas may emanate from the gas motor M . In this way, energy from the exhaust gas otherwise dis- carded to the atmosphere, is recovered . The steam produced in the boiler 70 escapes at outlet 73 and is transferred to a first heat exchanger 91. The saturated steam from the boiler is heat exchanged with the compressed steam from the first compressor 21 by entering the saturated steam from the boiler 70 at the inlet 93 and exiting the now superheated steam at outlet 94. The compressed steam from compressor 21 enters the first heat exchanger at inlet 95 and exists as a cooled stream of steam at outlet 96. The cooled stream of steam is directed to the first condenser and is treated as described above for figure 1.

The partly superheated steam exiting at outlet 94 is conveyed to the second heat exchanger 92 for further superheating of the steam, whereas the steam from the second compressor 22 is cooled . The steam from the first heat exchanger enters the second heat exchanger at inlet 97 and exits at outlet 98, whereas the steam from the second compressor enters the second heat exchanger at inlet 99 and exits at outlet 100. The now cooled compressed steam from the second compressor is directed to the second condenser and treated as described above for figure 1. The now superheated steam leaving the second heat exchanger at outlet 98 enters the expander 80 at inlet 81. The expander is provided on the same axis as the first and the sec- ond compressor, and thus assist the driver M in driving the compressors.

The steam leaves the expander 80 at outlet 82 and is mixed with the stream of steam from the first compressor before the combined stream enters the first condenser at inlet 35. Usually, the pressure of the stream of steam from the outlet 82 is adjusted to the same or similar level as the pres- sure of the steam from the first compressor. If a difference in pressure exist at suitable valve may assure proper mixing of the streams.

Reference numbers on the drawings:

Drying apparatus

Inlet for humid product

Inlet for super heated steam

Drying zone

Outlet for dried product

Outlet for stream of saturated steam

Nozzle for spraying humid product

Compressor section

First compressor

Second compressor

Intermediate compressor

Inlet for stream of saturated steam

Outlet for from first compressor

Inlet for stream of steam to the second compressor

Outlet for from second compressor

Inlet for stream of steam to the intermediate compressor

Outlet for from intermediate compressor

Condenser section

First condenser

Second condenser

Intermediate condenser

Inlet for stream of saturated steam

Inlet for compressed stream of steam

Outlet for condensate from first condenser

Outlet for stream of partly superheated steam

Inlet for stream of partly superheated steam

Inlet for compressed stream of steam

Outlet for condensate from second condenser

Outlet for stream of superheated steam

Inlet for stream of partly superheated steam

Inlet for compressed stream of steam

Outlet for condensate from intermediate condenser

Outlet for stream of partly superheated steam

Fluid bed

Inlet of partly dried product to fluid bed

Inlet of superheated steam to fluid bed

Outlet of dried product from fluid bed

Outlet of stream of saturated steam from fluid bed

Boiler

Inlet for the boiler

Pump for raising the pressure in the boiler

Steam outlet from the boiler

Inlet for exhaust gas 75 Outlet for the exhaust gas

80 Expander

81 Inlet to expander

82 Outlet from the expander

90 Heat exchanger section

91 First heat exchanger

92 Second heat exchanger

93 Inlet of the steam from boiler to first heat exchanger

94 Outlet of the steam from first heat exchanger

95 Inlet of compressed steam to first heat exchanger

96 Outlet of cooled steam from first heat exchanger

97 Inlet of steam to second heat exchanger

98 Outlet of steam from second heat exchanger

99 Inlet of compressed steam to second heat exchanger

100 Outlet of cooled steam from second heat exchanger

EG Exhaust gas

HP Humid Product

DP Dried Product

M Driver