SE1050619A2 - Drying method where a partial stream of the circulating gas is contacted with a co-current absorption solution - Google Patents

Description

l0 l5 20 25 30 35 40 and diluted solution which leads to lost driving force in the system. Yet another reason may be entrainment of hygroscopic substances with the gas. Purifying the large gas flow without losing the drying capacity is both difficult and expensive.

The invention The invention aims to eliminate several of the disadvantages of the current systems. This is achieved with the method according to the invention with the features stated in claim 1. Further developments and preferred embodiments of the invention are stated in the subclaims.

The invention is thus based on a gas being circulated between dry goods and a unit for solvent removal and reheating. The circulation means that energy and materials are retained in the process. The dry goods can be an unstructured bulk product or a systematically stacked product.

The contact device must create a good contact between gas and goods.

The process can be designed for continuous or batch transport of dry goods. The gas consists of a permanent gas and liquid evaporated from the drying material.

The process can also be designed with several circuits for gas in the same material flow if you want increased capacity or different process conditions.

In the following description, the invention is described in the case where the gas consists of air and the solvent of water.

The principle is that saturated gas from contact with the moist goods is overheated before it is returned to the drying goods for continued absorption of moisture. The overheating is carried out mainly with heat from the absorption of moisture from a smaller part of the circulating gas.

When the solvent is disposed of from the dryer, it is common for other substances to accompany the gas. These substances give rise to pollutants if they are not handled in a controlled manner.

A further object of the invention is therefore to provide an environmentally friendly drying process, in which emissions of environmentally and / or hazardous substances are reduced or eliminated. This is achieved according to the invention by adapting the solvent-absorbent system so that substances released during drying are captured by the absorbent.

The absorbent can be selected so that the contaminant is absorbed simultaneously with the solvent. This effect is particularly powerful if the contaminant is included as part of the absorbent.

When drying wood, organic acids are released. The acids give rise to low pH values and increased risk of corrosion in the system. In the end, they will give rise to an unwanted issue. It is then appropriate to use the corresponding organic salt as absorbent. By controlling the pH by adding the second part of the absorbent in the form of a cheap basic salt (eg potassium carbonate), the released pollutant can be included in the absorption solution. The process will then produce the desired solution of potassium formate and potassium acetate in excess. A eutectic mixture of sodium and potassium salts, e.g. acetates.

For dry goods that emit a basic substance such as e.g. ammonia can be a weakly acidic solution, e.g. ammonium nitrate is used as the absorption solution. If nitric acid is added to control the pH, ammonia will bind in the solution. Instead of giving rise to a harmful emission, a valuable fertilizer is produced.

The same absorption solution can be used if a gas containing nitrogen oxides e.g. a flue gas must be dried. If ammonia is added, the nitrogen oxides will be absorbed into the solution, which is highly desirable. Flue gases with residues of ammonia occur e.g. where ammonia has been used as an additive to reduce nitrogen oxides at high temperatures. Ammonium nitrate can be explosive but these risks can be eliminated by using previously applied methods in the nitrogen industry.

If the absorption solution is alternately used for drying a material which emits acids e.g. wood fuel and for drying the flue gas from the combustion of a fuel containing alkaline substances, e.g. tree fuel, readily soluble substances can be enriched in the solution to such an extent that the intended hygroscopic properties are achieved without foreign additives. Foreign substances in this relatively unspecified solution are characterized by the fact that they precipitate as a solid somewhere in the process, other substances are in principle desirable in the solution. You can thus separate unwanted substances by filtration. The excess of the desired solution can be used in other similar plants or for other purposes e.g. antifreeze, anti-slip or fertilizer.

The same function can also be achieved by contacting a weakly acidic absorption solution from a drying plant with an alkaline ash. After filtration, a basic strong hygroscopic solution is obtained which is suitable in the drying process.

Another substance combination is ammonia - water, where ammonia is the volatile solvent and water is the absorbent with high affinity for the solvent.

Brief description of the drawings The invention is described in the following in connection with exemplary embodiments shown in the accompanying drawings. Fig. 1 shows a basic flow diagram for recovery of moisture and recovery of heat from circulating gas flow in a drying process, Fig. 2 schematically shows a dryer with internal heat exchanger enclosed by dry goods where the absorption takes place in the heat exchanger and heat is transferred directly to Fig. Dryer. Fig. 4 is a schematic flow diagram for extracting moisture and recovering heat from a circulating gas flow in a drying process combined with a compressor, which provides a combined chemical and mechanical heat pump function; the heat exchanger and heat are transferred directly to the dryer in combination with a compressor that provides a combined chemical and mechanical heat pump function, and 10 15 20 25 30 35 40 Fig. 5 recovery of moisture and recovery of heat from a circulating gas flow in a drying process where energy recovery takes place with chemical and mechanical heat pump.

Detailed Description The invention will be described in more detail below in connection with exemplary embodiments of the invention. The solvent is absorbed in a solution consisting of an absorbent having high affinity for the solvent and high solubility therein. If the solvent is water, such substances are called hygroscopic.

Known hygroscopic substances are mineral salts, carbonates, alcohols, glycols and salts of organic acids such as formate and acetate. Salts of lithium, sodium and potassium are frequently used, as they have good solubility and strong affinity for water. Thus, although for the sake of simplicity, water and hygroscopic substances are discussed in the following, the invention thus encompasses all other conceivable combinations of the kind mentioned.

As mentioned initially, in today's systems the entire gas flow is treated by dehumidification and simultaneous reheating in an adiabatic process (latent heat in the steam is transferred to sensitive heat in the gas). Strong hygroscopic materials allow the gas to overheat up to the level of 100 K. Others are much less powerful. Toxicity, corrosion, solubility, etc. often limit the choice to less powerful substances. The hygroscopic potential (often called boiling point increase) constitutes the driving force of the system, which is thus severely limited compared to e.g. with flue gas-powered drying systems that work with gas that can be several hundred K. The consequence of the lower temperature potential is that a significantly greater flow of carrier gas is required and thus consistently larger areas, surfaces and volumes.

The specific heat demand for overheating e.g. air is about 1 kJ / kg, K. At 50 K overheating, about 50 kJ / kg air is thus required. The heat of steam formation in water vapor is about 2200 kJ / kg, which means that about 0.023 kg of steam needs to be absorbed per kg of gas to be heated. If the gas content of the gas is significantly greater than 0.023 kg / kg, it can be stated that only a small part of the gas needs to be used for absorption, provided that most of the water vapor in the smaller stream is absorbed. If the gas consists only of water vapor or if moist air is used in a system which is heated to the boiling point of the water so that the gas will be dominated by steam, the above ratio will be accentuated. The specific heat of the water vapor is about 2 kJ / kg, K which means that the proportion of the gas flow that must participate in the absorption increases a few percentage points to: 50 x 2/2200 = 0.045 kg / kg.

Because the proportion of steam in the gas is high, the proportion of gas that must be treated decreases.

According to the invention, the process is divided into two steps: an absorption step where steam is absorbed in the hygroscopic liquid at the same time as heat is released in the liquid. A second step for transferring heat from the liquid to the gas.

The first step works with concentrated media and requires a small contact area and a relatively small driving force. However, the second step, which involves transferring heat to a permanent gas, requires a large contact area or a large driving force.

As mentioned above, the driving force during dehumidification with hygroscopic liquids is severely limited, which is why significant contact surfaces are required to carry out the process. Thus, in today's procedures it is required that a large contact surface be wetted by a hygroscopic liquid.

The invention is based on the two processes being physically separated so that the moist gas flow 1 is heated in a large but dry heat exchanger 2 of conventional design as shown in Fig. 1. A partial stream 3 (5 - 50% of the main stream) is taken out before or after the heater . The smaller stream is contacted with the absorption solution 4 in co-current in a smaller heat exchanger 5 where the surfaces are wetted by the solution. Already at the initial contact with the solution, gas and solution are heated to a temperature close to the current equilibrium value for the dehumidification. Gas and liquid then pass in the same flow direction through the heat exchanger. The heat is transferred to the large heat exchanger that heats the main flow l.

Thermally, the heat exchangers are connected in countercurrent, which means that the positive effect of countercurrent is surprisingly achieved despite the fact that gas and liquid are contacted in cocurrent. The limiting parameter l0 l5 20 25 30 35 40 for the drying capacity of the air is the temperature increase that can be achieved on the large stream of gas before it is led to the drying chamber 6. The highest temperature is achieved if the most concentrated solution is contacted with the gas with the highest water content. Thus, somewhat surprisingly, a higher absorption temperature is reached in this split process where the absorption takes place in cocurrent than in the previously known combined countercurrent process. The cooling in connection with the absorption means that the absorption process can absorb more moisture and deliver more heat at given flows than during an adiabatic process.

Thus, unlike the prior art, the absorption process is carried out only in a small partial flow to the main stream, it is carried out in co-current and furthermore it is cooled (i.e. not adiabatic). To remove traces of the absorption solution in the product, the smaller partial stream from the absorption can be purified if necessary, as indicated at 7, even with high specific cost methods or wet methods which remove the drying ability of the gas. One can also consider diverting the smaller gas stream after absorption. Such measures are impossible or very expensive in the prior art.

If the dehumidified stream is dehumidified in a purification step, it should then be treated according to point 2 below. If the gas is not wetted, it can be used in any of the following ways: 1. The gas is returned to the main stream after the heater, arrow 8, where the drying capacity of the gas can be utilized 2. The gas is returned to the main stream before the heater, arrow 9, The gas is passed through the finished dry goods, arrow 10, whereby the dry gas recovers heat by cooling (and drying) the dry goods. 4. The gas is first led through the heater and then through a cooling zone in the dryer to achieve more powerful drying and simultaneous cooling of the dryer, arrow ll. If you want a certain negative pressure in the device, some of the gas can be led away from the process, which is not shown in the figure. Since the gas is relatively low in energy, it is advisable to choose this gas for this purpose. Another way to divert gas is also shown below. 10 15 20 25 30 35 40 To achieve the intended operating temperature for the process, steam, arrow 12, is supplied to the circulating gas flow. The steam condenses in the cold dry goods at the same time as heat is transferred to the goods.

The temperature of the goods controls the composition of the circulating gas by a rising proportion of steam rising at a rising temperature displacing another gas (air). High temperature thus gives a high proportion of steam and a low proportion of oxygen in the gas, which counteracts fire and explosion. The working temperature is controlled by a controlled addition of steam. Added steam can also be used to give the material the desired temperature and humidity profile, e.g. as a finishing of the dried product. If the supplied steam is contacted with the solution from the process, volatiles can be returned to the process together with the steam in the manner shown in Fig. 1 by heat supply and evaporation 13. Steam and impurities are added to the hot gas, arrow 12, and purified concentrated solution is diverted. pil 14.

The working temperature can change during the process e.g. for heat-sensitive materials through increased or decreased steam supply.

If the solvent is not flammable, an embodiment is preferred where the working temperature is close to the boiling point of the solvent so that the composition of the gas counteracts fire and explosion. This effort must be weighed against the durability of the dryer for higher temperatures.

The moist gas that passes through the cold dry goods at the goods inlet 15 during continuous goods feeding or initially during periodic operation will lose moisture and energy through condensation whereby the material is heated and moistened. The remaining gas is thus depleted of moisture and energy and mainly contains permanent gas (air) and other substances which the drying material may have released and which in contact with the absorption solution are more volatile than the solvent. Especially the substances that have low affinity for the absorption solution e.g. carbon monoxide and other hydrophobic substances e.g. hydrocarbons are enriched in this stream. A certain proportion of this stream is evacuated from the process, arrow 16, suitably so that the content of combustible substances in the system is limited to a non-combustible level. At the same time, a negative pressure is created in the system that counteracts leakage to the environment from the system's possible leakage points. The evacuated gas can be treated e.g. by combustion to avoid impact on the environment and to utilize the energy content of the gas. The size of the evacuation is controlled partly by the negative pressure in the system and partly by the content of combustible substances in the gas so that fire and explosion are avoided.

The heat exchanger system used can consist of 2 separate regenerative heat exchangers 2, 5 with a heat transfer liquid between the heat exchangers as shown in Fig. 1, but other heat exchanger systems can also be used. The system can also be designed with a heat exchanger so that the absorption can be carried out on one side of the heat exchanger and gas or goods can be heated on the other. Periodically working so-called recuperative heat exchangers are used where a (smaller) part of the heat exchanger is put into contact with the absorption process which heats the heat exchanger material and is then contacted with the main flow of gas which absorbs said amount of heat. Such exchangers can be designed as two or more separate units operating periodically. Another common design is rotating wheels with special sectors for the different media flows. A less common design that can be used is a stationary unit of heating surface that is designed so that the media flows are periodically redirected in the desired manner.

Another embodiment of the invention is shown in Fig. 2. Here the heat exchanger is placed so that it is enclosed by the dry goods. A number of appliance constructions with built-in heat exchangers for traditional heating media are already known. According to the invention, the absorption in this case is carried out on one side of the heating surface and on the other side the heat is transferred directly to the dryer. This is shown in Fig. 2, where a heat exchanger 21 is arranged in a dryer 22. Dryer, arrow 23, is led into the contact device via one end and dried goods, arrow 24, are taken out of the contactor at a second end. Moist carrier gas is circulated from the other end by means of a fan 25 or corresponding to the inlet end of the heat exchanger 21, arrow 26, to which a concentrated absorption solution is also added, arrow 27. Liquefied gas from the heat exchanger is circulated by means of a fan 28 or equivalent, after separating a diluted solution, to the inlet for dry goods in the contactor, arrow 29, possibly together with part of the second circulating carrier gas stream (26). Such a design has advantages in the form of lower need for gas flow and apparatus volumes but also disadvantages in the form of a greater dependence on the properties of the dry goods. Of course, it is also possible to use a heat carrier to transfer heat from an absorption device according to Fig. 1 and a heat exchanger according to Fig. 2. countercurrent is not as important as in the previously described case.

In this design, the thermal coupling in As shown in Fig. 2, it is also affected by the design of the contact device.

Yet another design is shown in Figs. 3 and 4 where the system is supplemented with a compressor 30 and 40, respectively, which raises the pressure of the gas to be participated in the absorption.

As a result, the temperature reached during absorption rises, which i.a. reduces the need for heat transfer surface, which provides increased capacity in a given device. It may be advisable to release the gas from dust and other interfering substances before the compressor. This can be done by filtration, washing or with the help of a separating heat exchanger. If the absorption solution is supplied with high pressure, an ejector that utilizes the energy in the liquid can replace or supplement the compressor. The purified solution can be further concentrated by multiple evaporation and coupled with other processes, such as steam generation, electricity production, etc.

The need to purify the gas before the compressor and the desire to reduce the size and power demand of the compressor can be met if the absorption step takes place in a heat exchanger in the same way as shown in Figs. 1 and 2, but where the heat carrier between mechanical heat pump so that the medium is gasified in the heat exchanger 50 which supplies steam to the compressor 52. After compression, the steam is condensed in a conventional heat exchanger 51 which heats drying gas or dry goods.

This principle is illustrated in Fig. 5.

The mechanical heat pump is a known technology that is combined with the new absorption technology. The chemical heat pump compensates for the mechanical heat pump's inability to handle large polluted gas volumes of low pressure while the mechanical heat pump compensates for the chemical heat pump's inability to withstand large temperature rises. A limited addition of mechanical energy 10 ll via the compressor also reduces the need to supply steam to maintain the temperature in the process. Of course, the two solutions can be combined so that a preheating takes place in one step (chemical or mechanical heat pump) and final heating takes place with heat whose temperature is raised in both steps. The compressor can be operated with low speed, low pressure set and low energy consumption so that mainly the chemical heat pump works.

To increase the capacity of the plant, the capacity of the compressor is increased so that the drying temperature is increased. This provides great opportunities to let production follow the current cost of electricity by varying the compressor's capacity.

Claims (17)

10 15 20 25 30 35 l2 Patent claims 1. l. When drying dry goods using an absorption solution containing an absorbent with high affinity for the liquid to be removed from the dry goods and with high solubility in said liquid, characterized in that a gas is circulated between the drying goods and a unit for removing liquid taken up from the drying goods and reheating of the gas, that a partial stream of the circulating gas is brought into contact with absorption solution in co-current in a heat exchanger and that the resulting heat is used for drying the drying goods. A method according to claim 1, characterized in that the gas is heated in a first heat exchanger after passage of the dry goods, that a partial stream of the gas is taken out of the gas stream before or after said heating and brought into contact with co-absorption solution in a second heat exchanger and heat is used in the first heat exchanger to heat the gas. 3. A method according to claim 1 or 2, characterized in that a heat carrier in the form of a working medium for a mechanical heat pump is circulated between the two heat exchangers, said heat carrier being gasified in the heat exchanger in which said substream is brought into contact with absorption solution in cocurrent. delivered to and compressed in a compressor from which the steam is led to and condensed in the heat exchanger, which heats drying gas or drying goods. 4. A method according to claim 1, characterized in that the circulating gas after passage of the dry goods is brought into contact with absorption solution in cocurrent in a heat exchanger enclosed by dry goods, wherein absorption is carried out on one side of the heat exchanger heat surface and heat is transferred directly to the dry goods on the other side . A method according to any one of the preceding claims, characterized in that said partial current constitutes between 5 and 50% of the circulating gas. Method according to one of the preceding claims, characterized in that the liquid to be wiped off is water and in that the substance is hygroscopic. 10 15 20 25 30 35 40 13 Method according to one of the preceding claims, characterized in that the absorbent is adapted by additives so that undesirable substances released during drying are captured by the absorbent. Method according to Claim 8, characterized in that when drying dry matter which emits organic acids, e.g. wood, the corresponding organic salt is used as absorbent, the pH value being controlled by the addition of a basic salt, e.g. KZCO3. Method according to Claim 7, characterized in that acids are used as additive to the absorption solution when drying dry matter which releases a basic substance. 10. A method according to claim 9, characterized in that when the emitted substance is ammonia, a slightly acidic solution, e.g. ammonium nitrate, is used as an absorbent and nitric acid as an acid to control the pH. 11. ll. Method according to one of the preceding claims, characterized in that the partial current is transferred directly back to the drying process after heat exchange. Method according to one of the preceding claims, characterized in that the partial stream is purified with respect to the absorption solution. Method according to one of Claims 7 to 12, characterized in that the absorption solution is used alternately for drying a material which emits acids, e.g. wood fuel, and for drying the flue gas from the combustion of a fuel containing alkaline substances, e.g. wood fuel. A method according to claim 13, characterized in that the absorption solution is filtered to remove formed solid. A method according to claim 13, characterized in that weakly acidic absorption solution from a drying plant is brought into contact with a basic ash and that after filtration the basic strongly hygroscopic solution thus obtained is returned to the drying process. 14 Method according to one of the preceding claims, characterized in that the partial current is diverted from the system after absorption and heat exchange. A method according to any one of the preceding claims, characterized in that the pressure of said substream is raised before it is brought into contact with the absorption solution.