Falling film tubular evaporator

TECHNICAL FIELD

The disclosure relates to a falling film tubular evaporator and a process for concentrating a liquid feed.

BACKGROUND

Modern evaporation technology is designed to preserve flavor, aroma, color, and nutritional properties, which can be adversely affected by exposure of solutions to high temperatures. This is in some instances achieved by evaporation under a vacuum in order to ensure a low boiling point.

In the evaporation process, concentration of a product is accomplished by boiling out a solvent, generally water. The recovered end product should have an optimum solids content consistent with desired product quality and operating economics. Evaporation is a unit operation that is used extensively in processing foods, chemicals, pharmaceuticals, fruit juices, dairy products, paper, and pulp, and both malt and grain beverages.

Falling film evaporation is widely applied for evaporation of low-viscosity products such as dairy products, fruit juices, plant extracts, blood plasma, a wide variety of pharmaceutical products, effluents, and many other organic and inorganic products. In some instances, the evaporation is performed to obtain a significant reduction in liquid content prior to a liquid-to-powder drying process. This results in a more efficient drying process with lower energy costs and a reduced environmental footprint.

Generally, falling film evaporation enables minimum flavor impact on sensitive ingredients due to short residence time and high efficiency heat transfer.

Since many pharmaceutical, food, and dairy products are extremely heat sensitive, optimum quality is obtained when processing times and temperatures are kept as low as possible during concentration of the products. The most critical portion in the process occurs during the brief time that the product is in contact with a heat transfer surface which is hotter than the product itself. For this heat sensitive type of application, film evaporators have been found to be ideal for two reasons. First, the product forms a thin film only on the heat transfer surface rather than occupying the entire volume, greatly reducing residence time within the heat exchanger.

Second, a film evaporator can operate with low steam-to-product temperature difference, typically around 6°F (3.5°C). With both the product and heating surfaces close to the same temperature, localized hot spots are minimized.

US 3,412,778 discloses a falling film tubular evaporator having a liquid distributor comprising a horizontally perforated plate.

US 4,094,734 discloses a falling film evaporator in which the distributor arms are formed with a graduated cross-section and the feed tubes have a total cross-sectional area substantially equal to the cross-section of the line through which the viscous solution is pumped so that a substantially constant velocity is maintained throughout the header.

US 4,857,144 discloses an evaporator having vertically mounted elongated tubes. The evaporator comprises an inverted cone mounted as a filler piece on the underside of the top cover and having its apex just slightly spaced from the upper tube sheet.

US 4,925,526 discloses a falling film evaporator having vertical tubes. The evaporator has a mechanism for distributing the material to be evaporated onto the upper ends of the outer tubes in such a way that the material flows down along the tubes and forms a film on the external surface of the tubes.

WO 92/20419 discloses a falling film evaporator provided with a plurality of vertical elongated spaced elements having a substantially U-shaped cross-section and inwardly pointing vertical edges for receiving the evaporated liquid and the vapor. The elements are arranged in a circumferential wall below the tubes within the vapor space spaced from the wall of the vapor space and including a conical bottom. GB154355A relates to concentration of liquids by evaporation in successive stages under the same temperature and pressure in evaporators in which the liquid is circulated in films.

US3875988A discloses a multiple effect evaporator having a plurality of thin-film flow-down type vertical heat exchange tubes the ends of which are connected to partition plates which divide the apparatus into various evaporator chambers.

GB2084885A discloses concentration of an aqueous glycol solution by evaporation of the solution, in the form of a thin film, in a set of multiple-effect vertical film evaporators arranged one above another in a column.

EP2899163A1 discloses a stacked type falling film evaporator.

US4424633A discloses an apparatus for heating and drying articles comprising an evacuable drying chamber for heating therein articles by condensation heat of the vapor of a first liquid supplied to the drying chamber and a thin film evaporator for receiving condensate from the drying chamber in the form of the first liquid and a second liquid having a higher boiling point than the first liquid.

JPS58109102A discloses an evaporator having a liquid supply unit and a liquid drainage portion, each having a partition plate, with the partition plate of the liquid discharge portion having an opening at the center.

US2006260764A1 discloses a plant for tomato juice concentration using a falling-film evaporator with an external sleeve which surrounds a vertical bundle of tubes divided into a plurality of sectors in which the tomato juice circulates in succession.

Evaporation is a highly energy-intensive process widely applied in the dairy, food, and chemical industries. Therefore, approaching evaporation from the viewpoint of economical energy utilization and process effectiveness is essential. It is an object of the present invention to provide an evaporation system that is optimized with regard to operating costs, energy consumption, and capital investment such as but not limited to space, footprint, and transportation. According to an embodiment of the invention, several effects of evaporation are incorporated into a single evaporator unit, allowing for differentiated evaporation criteria. The object of the invention is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the summary, the description, and the figures.

SUMMARY

The invention relates to a falling film tubular evaporator for concentrating a liquid feed comprising a container containing a heating medium compartment, a first liquid feed compartment and a final liquid feed compartment, said first liquid feed compartment comprising a first plurality of vertical tubes being supported by a first top and a first bottom tube plate, an inlet for distributing the liquid feed to the tubes at the top end, whereby a liquid film is formed on the inner side of the tubes and flows down the vertical tubes to the first bottom tube plate, and a first bottom compartment into which the partially concentrated liquid feed is discharged,

said heating medium compartment comprising an inlet for heating medium for heating the liquid feed and an outlet for spent heating medium,

wherein said final liquid feed compartment comprises a final plurality of vertical tubes being supported by a final top and a final bottom tube plate, a final feed inlet for distributing the partially concentrated liquid feed to the tubes at the top end, whereby a liquid film is formed on the inner side of the tubes and flows down the vertical tubes to the final bottom tube plate, a final bottom compartment into which the concentrated liquid feed is discharged, and an outlet for the concentrated liquid feed.

The effect of having a first and a final liquid feed compartment is that differentiated evaporation criteria can be achieved. Thus, initially a liquid feed may be subjected to evaporation using a temperature and/or pressure different from a temperature and/or pressure used in a subsequent liquid feed compartment. Furthermore, capital cost investment and plant floor space is reduced by integrating multiple liquid feed compartments into a single container. In an embodiment of the invention, the container further contains one or more intermediate liquid feed compartments with each compartment comprising an intermediate plurality of vertical tubes being supported by an intermediate top and an intermediate bottom tube plate, an inlet for distribution of the liquid feed to the tubes at the top end, whereby a liquid film is formed on the inner side of the tubes and flows down the vertical tubes to an intermediate bottom tube plate, and an intermediate bottom compartment into which the partially concentrated liquid feed is discharged.

Depending on the product and/or process demands, there is no upper limit to the number of intermediate liquid feed compartments in the container, i.e. splits within the calandria body.

In a certain embodiment, the container comprises two or more heating media compartments having separate inlets for heating medium for heating the liquid feed and outlets for spent heating media. The heating medium is typically steam or any other gas, preferably a condensable gas and more preferably a gas form of the solvent of the evaporated product. However, in certain embodiments of the invention, the heating medium is a liquid, preferably water or oil.

Having two or more heating media compartments, e.g. steam compartments, enables the application of a temperature difference adjusted to the specific need. For heat sensitive products a small temperature difference may be desired between the steam compartment and the corresponding liquid feed compartment. Furthermore, an evaporation performed in multiple steps using a relatively small temperature difference in each stage tends to be more energy-effective than a single stage using a large temperature difference.

If the product and/or process conditions demand, a non-matching number of compartments on the steam side and the product side can be used. In an embodiment of the invention it is possible to obtain more compartments on the steam side than on the product side. In another embodiment more compartments on the product side than on the steam side are used.

In an aspect of the invention, the container of the evaporator is configured to incorporate at least two evaporation effects. In other words, at least two pairs of liquid feed compartments and steam compartments are provided. The effect hereof is to allow for differentiated evaporation criteria concerning temperature of the heating medium (steam) and boiling temperature of the liquid feed, thus resulting in simplification and reduction of the costs, transportation, and/or footprint.

As an example of the use of the present invention, milk may be concentrated up to approx. 40% dry matter at a boiling temperature of 62°C while the final evaporation to approx. 50% dry matter is taking place at 55°C.

For other products one would want a lower temperature at lower concentrations to limit the effect of heat on the product, but the final concentration might need to take place at a higher temperature to prevent components in the product from reaching a concentration above a critical value that might cause deposition or crystallization of the product at an unwanted time.

In an aspect of the invention, the evaporator is configured to employ mechanical vapor recompression and/or thermal vapor recompression for recycling at least a part of the vapor to the inlet for steam. Conservation of energy is one major parameter in the design of an evaporator system. The larger the evaporation duty, the more important it is to conserve energy. In multi-effect evaporation, the steam produced from evaporation in one effect is used to provide the heat to evaporate liquid in a second effect which is maintained at a lower pressure. In some two-effect evaporators it is possible to evaporate approximately 2 kgs of steam from the liquid feed for each kg of steam supply. As the number of effects is increased, the steam economy increases. On some large duties it is economically feasible to utilize as many as seven effects.

Falling film evaporation can employ two vapor recompression techniques, depending on the available energy source: Mechanical Vapor Recompression (MVR) which requires very little or no steam and/or Thermal Vapor Recompression (TVR) using live steam to recompress vapor from one effect to be used as heating medium for a subsequent effect.

MVR delivers substantial operating cost savings in areas with an ample supply of electrical energy. Once an MVR evaporator is running, little or no additional steam is required. In MVR, a high-pressure fan is used to recompress the vapor to a higher pressure, resulting in a rise in temperature. This means that the recompressed vapor can be used as the evaporator heating medium, while the condensate is ideal for preheating of the feed product. The high-pressure fan can be powered by an electric motor, or gas, or steam turbine. The MVR evaporation process is often followed by further concentration in an TVR or forced circulation evaporator.

Thermodynamically, the most efficient technique to evaporate water is to use mechanical vapor thermo-recompression. This process takes the vapor that has been evaporated from the product, compresses the vapor mechanically and then uses the higher pressure vapor in the steam compartment.

The vapor compression is carried out by a radial type fan or a compressor. The fan provides a relatively low compression ratio of, for example, 1:1 to 1:2 which results in high heat transfer surface area but an extremely energy-efficient system. Although higher compression ratios can be achieved with a centrifugal compressor, the fan has become the standard for this type of equipment due to its high reliability, low maintenance cost and generally lower RPM operation.

TVR across one or more effects enhances process cost-effectiveness by using a steam jet compressor to compress part of the vapor discharged from one effect for use as the heating medium for the first effect. Multi-effect evaporation thus enables the regeneration of low-grade heat from the evaporator itself for gentle and effective preheating across liquid feed compartments with a low temperature difference.

When steam is available at pressures in excess of 45 psig (3 barg) and preferably over 100 psig (7 barg), it will often be possible to use TVR. In this operation, a portion of the steam evaporated from the liquid feed is recompressed by a steam jet venturi and returned to the steam chest of the evaporator. A system of this type can provide a 2 to 1 economy or higher depending on the product, the steam pressure, and the number of effects over which TVR is applied. TVR is a relatively inexpensive technique for improving the economy of evaporation.

Suitably, the evaporator of the invention is comprising a partitioning plate extending from the top plates to the upper part of the container and a partitioning plate extending from the bottom plate to the lower part of the container. The partitioning plates split the calandria into two liquid feed compartments. Further partitioning plates may be provided for splitting the container into further liquid feed compartments. The use of partitioning plates allows for a simple but effective way of obtaining compartments.

In some embodiments of the invention a liquid feed compartment is segregated into two or more consecutively connected sections each comprising a plurality of tubes. Usually, the number of tubes decreases in a downstream direction in each section as the liquid evaporates from the liquid feed. The decreasing number of tubes in the downstream direction ensures that a correct wetting rate is obtained. In each segment the partially concentrated liquid feed may be recirculated to obtain correct wetting of the heating surface to avoid fouling or even blockage of the flow path

The wetting rate of every individual tube may be important in preventing product fouling. A suitable but not exclusive way of liquid distribution is based on the static principle with distribution plates built into the top plate or by spray distribution for some products. This ensures constant and even distribution of a thin film over the inside surface of each tube. Thus, in one embodiment of the invention, the evaporator is comprising a distributor plate for distributing feed liquid to the tubes. In another embodiment of the invention, the evaporator comprises spray nozzles for distributing the liquid feed to the tubes.

Spreading of liquid to each tube is sometimes further enhanced by generating flash vapor at this point.

In a certain aspect of the invention, the liquid feed compartments are connected via conduits configured to supply the partially concentrated liquid feed of one compartment to the feed inlet of a consecutive liquid feed compartment. Usually, a pump assists in the transportation of the partially concentrated liquid feed. The conduits may in certain configurations be provided inside the steam compartment(s) to adjust the temperature of the partially concentrated feed liquid before it is liberated or flashed at a position above the top plate. The components of the present invention in contact with the liquid feed are generally prepared of stainless steel, such as 304 or 316 stainless steel. However, other metals can also be used depending on the product. There are occasions in which much more expensive/specialized materials of construction are required, such as 904L, 2205, nickel, hastelloy C, titanium, polymers, composite materials, and even graphite, ceramics, or glass The list is not exclusive and any combinations of afore mentioned construction materials can also be adopted in its construction.

In an embodiment of the invention, the evaporator comprises two or more vapor droplet separators that may be internal or external or a combination of internal and external vapor droplet separators. While the main separation of the mixture of liquid and vapor discharged from the tubes takes place in the bottom compartment of the evaporator body, the effect of the vapor droplet separators is to allow for the separation of droplets, typically water droplets, contained in the vapor from the vapor. The thus obtained droplets may be supplied to the intermediate or final liquid product.

The present invention also relates to a process for concentrating a liquid feed comprising the steps of:

a. supplying the top end of a first plurality of vertical tubes of a first liquid feed compartment provided in a container with a liquid feed for the formation of a film on the inner surface of the tubes,

b. heating the exterior surface of the tubes with a heating medium,

c. allowing the film to flow down to the bottom end of the tubes while partially evaporating the liquid,

d. collecting the mixture of vapor and partially concentrated liquid feed,

e. separating the vapor and partially concentrated liquid feed from each other,

f. supplying the top end of a plurality of vertical tubes of a final liquid feed compartment provided in the container with the separated liquid feed of step e for the formation of an inner film on the inner surface of the tubes,

g. repeating steps b to e for the final liquid feed compartment and

h. collecting the concentrated liquid feed. In a preferred aspect, the falling film tubular film evaporator is used for carrying out the present process.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 discloses a falling film tubular evaporator comprising two liquid feed compartments and a steam compartment.

Fig. 2 discloses a falling film tubular evaporator comprising two liquid feed compartments and two corresponding steam compartments.

Fig. 3 discloses a falling film tubular evaporator comprising two liquid feed compartments and two corresponding steam compartments, the liquid feed compartments being segregated into four sections and two sections, respectively.

DETAILED DESCRIPTION

Fig. 1 shows a simplified scheme of a possible embodiment of the falling film tubular evaporator of the present invention. The evaporator 1 comprises a container 2 containing a steam compartment 31, a first liquid feed compartment 3 and a final liquid feed compartment 4 separated by a partitioning plate 33, the first liquid feed compartment comprising a first plurality of vertical tubes 5 being supported by a first top 6 and a first bottom 7 tube plate, an inlet 10 for distributing the liquid feed to the tubes at the top end, and a first bottom compartment 12. The steam compartment comprises an inlet for steam 8 for heating the liquid feed and an outlet 9 for condensate. The final liquid feed compartment 4 comprises a final plurality of vertical tubes 13 being supported by a final top 14 and a final 15 bottom tube plate, a final feed inlet 18 for distributing the partially concentrated liquid feed to the tubes at the top end, a final bottom compartment 20 into which the concentrated liquid feed is discharged, and an outlet 21 for the concentrated liquid feed.

Liquid feed is first fed to the first liquid feed compartment 3 where the liquid feed is distributed by different means to the top ends of the plurality of tubes 5 comprised by the first liquid feed compartment 3. The liquid feed forms a liquid film on the inner side of the tubes that flows down the vertical tubes to the first bottom tube plate 7. The spreading of liquid to each tube may be enhanced by subjecting the liquid feed to flash, thereby generating flash vapor. During the downward flow of the liquid film on the inner side of the vertical tubes, the liquid film is brought to a boil by steam in the steam compartment 31 condensing on the outer side of the tubes, thereby giving off latent heat through the tube wall to the liquid film. The boiling temperature of the liquid film will depend on the pressure on the inside of the tubes and on the temperature and pressure conditions of the steam in the surrounding steam compartment. As the liquid film is evaporated, vapor fills the center of the tubes, thus accelerating the downward movement of the film and resulting in thinning of the film. The resulting mixture of liquid and vapor from the tubes enters into the first bottom compartment 12, where the liquid is separated from the vapor.

The thus obtained liquid is a partially concentrated form of the original liquid feed and may now be supplied as liquid feed to the top of the final liquid feed compartment 4 for a second round of evaporation using another boiling point for the partially concentrated liquid feed and thus for further concentration. The supply of liquid feed may be accomplished by internal or external piping, possibly using pumps 32. In general, circulating pumps may be required to maintain proper evaporator liquid flow. In the present example, the final liquid feed compartment 4 comprises a smaller number of vertical tubes than the first liquid feed compartment 3 in order to ensure optimal wetting of the inside of the tubes, thus avoiding film breakdown and consequently fouling due to drying out of liquid film during evaporation. After the second round of evaporation, a concentrate is discharged from the final bottom compartment 20.

Fig. 2 shows a simplified scheme of a possible embodiment of the falling film tubular evaporator of the present invention. The figure shows the schematic drawing of Fig. 1 with the container of the evaporator comprising two steam compartments and a MVR and/or TVR 35. The steam compartment illustrated in Fig. 1 has been separated into two steam compartments by a partitioning plate 34. Each of the steam compartments have separate inlets 8, 16 for steam for heating the liquid feed and outlets 9, 17 for condensate. This allows for different steam temperature and/or pressure in each steam compartment, thereby enabling different conditions of evaporation to be established in each of the two liquid feed compartments. The evaporator further comprises two vapor droplet separators 37. After separation in the bottom

compartment 12/20 of the mixture of liquid and vapor obtained from the tubes, the vapor is led through the vapor droplet separator 37 for separation of droplets contained in the vapor from the vapor. The thus obtained vapor exiting the vapor droplet separator is compressed in the MVR and supplied to the steam inlet 8 and the inlet 16 of the second steam compartment. The obtained droplets, on the other hand, can be supplied to the liquid product discharged from the bottom compartment 12/20.

In a specific use of the evaporator shown in Fig. 2, milk is concentrated to approx. 40% total solids (TS) at a boiling temperature of 62°C followed by final evaporation to approx. 50%TS at

55°C.

In another specific use, the evaporator is configured for concentrating whey permeate in the production of non-caking whey permeate. Whey permeate is widely used in the global food and beverage industry as a cost-effective milk ingredient in food formulations. Whey permeate may be concentrated in a first stage at a boiling temperature of 65°C and in a final stage at a boiling temperature of 78°C.

Fig. 3 shows a simplified scheme of a possible embodiment of the falling film tubular evaporator of the present invention. Compared to the embodiment shown in Fig. 2, the bottom

compartment of the first liquid feed compartment is segregated into a number of sections 36. Each of the sections is via a conduit connected to an inlet to the top of the tubes for either recirculating the partially concentrated liquid feed in the same section 22 or forwarding the partially concentrated liquid feed to a subsequent section. The number of tubes in each section is generally decreased in the downflow direction to ensure sufficient wetting of the inner surface of the tubes as the liquid is further evaporated.

The various aspects and implementations have been described in conjunction with various embodiments herein. Flowever, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The reference signs used in the claims shall not be construed as limiting the scope.