SE1050619A1 - 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 leading to lost power in the system. Yet another reason may be entrapment of hygroscopic substances with the gas. To purify 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 thereof current systems. This is achieved with the method according to the invention with the characteristics specified in claim 1. Further developments and preferred embodiments of the invention are set out in the subclaims.

The invention is thus based on a gas being circulated between dry goods and a solvent removal unit and reheating. Circulation involves energy and materials retained in the process. The dry goods can be 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 from the evaporator evaporated liquid.

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

In the following description, the invention for it is described case that the gas consists of air and the solvent of water.

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

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

A further object of the invention is therefore provided environmentally friendly drying process, in which emissions of environmental and / or hazardous substances are reduced or eliminated. l0 15 20 25 30 35 40 This is achieved according to the invention by adapting the system solvents - absorbents so that substances released during the drying is captured by the absorbent.

The absorbent can be selected so that the contaminant is absorbed simultaneously with the solvent. This effect will be special 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 ultimately, they will give rise to an unwanted issue. The is then appropriate to use the corresponding organic salt as absorbent. By checking the pH value by addition of the second part of the absorbent in the form of a cheap alkaline salt (eg potassium carbonate) may be the pollutant released included in the absorption solution. The process will then produce the desired solution of potassium formate and potassium acetate in excess. It is also suitable as an absorbent 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 a slightly acidic solution, e.g. ammonium nitrate is used as absorption solution. If you add nitric acid to check the pH, ammonia will be bound in the solution. Instead to give rise to a harmful emission, one is produced valuable fertilizer.

The same absorption solution can be used if a gas contains 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 very desirable. Flue gases with residues of ammonia occur for example where ammonia was used as an additive to at high temperature reduce the nitrogen oxides. 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 used alternately for drying one materials that emit acids e.g. wood fuel and for drying the flue gas from the combustion of a fuel containing alkaline substances, e.g. wood fuel, easily soluble substances can be enriched in the solution to such an extent that the intended hygroscopic lO l5 20 25 30 35 the properties are achieved without foreign additives. Foreign substances in this relatively unspecified solution is characterized by the fact that they precipitates 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 the solution can be used in other similar facilities or for other purposes e.g. antifreeze, anti-slip or fertilizer.

The same function can also be achieved by contacting a weak one acidic absorption solution from a desiccator with a basic ash. After filtration, a basic strong hygroscopic is obtained solution suitable for the drying process.

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

Brief description of the drawings The invention is described in the following in connection with the the accompanying drawings show exemplary embodiments. Thereby showing Fig. 1 is a basic flow chart for the 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 the dryer, Fig. 3 is a basic flow chart for the recovery of moisture and recovery of heat from a circulating gas flow in a drying process combined with a compressor, which provides a combined chemical and mechanical heat pump function, Fig. 4 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 the dryer in combination with a compressor that provides one 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 the energy recovery takes place with chemical and mechanical heat pump.

Detailed description The invention will be described in more detail below in connection to exemplary embodiments of the invention. The solvent is absorbed in a solution consisting of an absorbent with large 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 frequent use, because they have a good solubility and strong affinity to water. Although in the following for the sake of simplicity water and hygroscopic substances discussed, the invention thus encompasses all other possible combinations of the kind mentioned.

As mentioned initially, in today's system the whole is treated the gas flow by dehumidification and simultaneous reheating in a adiabatic process (latent heat in the steam is transferred to sensitive heat in the gas). Strong hygroscopic materials allow one overheating of the gas up to the level of 100 K. Others are significant less powerful. Toxicity, corrosion, solubility, etc. often limits the choice to less powerful subjects. 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 which can be several hundred K. The consequence of the lower the temperature potential is that significantly greater flow is required salvaged gas and thus consistently larger areas, areas and volumes.

The specific heat demand for overheating e.g. air is about 1 kJ / kg, K. At 50 K overheating, approximately 50 kJ / kg of air is thus required. The steam generation heat 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 need to be claimed for absorption provided that most of the water vapor in the smaller stream is absorbed. If l0 l5 20 25 30 35 40 the gas consists only of water vapor or if moist air is used in a system that heats up to the boiling point of the water so that the gas will be dominated by steam, the above relationship will 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% units to: 50 x 2/2200 = 0.045 kg / kg.

Since the proportion of steam in the gas is high, the proportion of gas decreases as must be treated.

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

The first step is working with concentrated media and requires one small contact area and a relatively small driving force. The other the step which means that heat is to be transferred to a permanent one however, gas requires a large contact area or large driving force.

As mentioned above, the driving force during dehumidification is included hygroscopic fluids severely limited why it is required significant contact areas to carry out the process. By today procedures thus require a large contact area to be wetted by one hygroscopic fluid.

The invention is based on the two processes being physically separated so that the moist gas flow l is heated in a large but dry heat exchanger 2 of conventional design as shown in Figs. l. A partial current 3 (5 - 50% of the main current) is taken out before or after the heater. The smaller current is contacted with the absorption solution 4 in cocurrent in a smaller heat exchanger 5 there the surfaces are wetted by the solution. Already at the initial contact with the solution is heated gas and solution to a temperature close to the current equilibrium value for the dehumidification. Gas and liquid then passes in the same flow direction through the heat exchanger. The heat is transferred to the large heat exchanger which 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 gas and liquid are contacted in co-current. The limiting parameter l0 l5 20 25 30 35 40 for the air drying capacity is the temperature rise that can reached on the large stream of gas before it is led to drying chamber 6. The highest temperature is reached if you contact it most concentrated solution with the gas that has the highest water content. You thus surprisingly reach a higher one absorption temperature in this split process there the absorption takes place in co-current than in the previously known merged countercurrent procedure. The cooling in connection with absorption means that the absorption process can absorb more moisture and deliver more heat at given flows than at one adiabatic process.

The absorption process is thus carried out in contrast to the known technology only in a small substream to the main flow, the is carried out in co-current and further it is cooled (i.e. not adiabatic). To remove traces of the absorption solution in the product can the smaller partial current from the absorption if necessarily purified, as indicated at 7, even with high methods specific cost or wet methods that remove the drying of the gas ability. You can also consider diverting the smaller one the gas stream after absorption. Such actions are impossible or very expensive in the prior art.

If the dehumidified stream is dehumidified in a purification step, it should then dealt with in accordance with paragraph 2 below. If the gas is not humidified it can be used in any of the following ways: l.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, there drying capacity is further improved by heating. 3.The gas is passed through the finished dry goods, arrow 10, whereby the dry gas recovers heat by cooling (and drying) of the drying goods. 4.The gas is first passed through the heater and then through a cooling zone in the dryer to achieve heavier drying and simultaneous cooling of the dry goods, arrow ll. 5.0m you want a certain negative pressure in the device can some of the gas is led away from the process, which is not shown in the figure. Because gas is relatively low in energy, it is appropriate to select this gas for this purpose. Also one other ways to divert gas are shown below. 10 15 20 25 30 35 40 To achieve the intended working temperature for the process is added steam, arrow 12, to the circulating gas flow. The steam condenses in the cold drying 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 away from other gas (air). High temperature thus gives a high proportion 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 desired temperature and humidity profile e.g. as a finishing of the dried product. About the added steam contacted with the solution from the process can volatile substances returned to the process along with the steam in the way that is shown in Fig. 1 by heat supply and evaporation 13. Steam and pollutants are added to the hot gas, arrow 12, and purified concentrated solution is diverted, arrow 14.

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

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

The moist gas that passes through the cold dry goods at the goods inlet 15 during continuous goods feeding or initially at periodic working will lose moisture and energy through condensation whereby the material is heated and moistened. The remaining the gas is thus depleted of moisture and energy and contains mainly permanent gas (air) and other substances such as the dry goods possibly given and which is in contact with the absorption solution more volatile than the solvent. Especially the substances that have low affinity for the absorption solution e.g. carbon monoxide and others hydrophobic substances e.g. hydrocarbons are enriched in this stream. Of this current, a certain proportion is evacuated from the process, arrow 16, suitably so that the content of combustible substances in the system is limited to one non-combustible level. At the same time, a negative pressure is created in the system which counteracts leakage to the environment from any system leakage points. The evacuated gas can be treated e.g. through combustion to avoid environmental impact and to l0 l5 20 25 30 35 40 utilize the energy content of the gas. The size of the evacuation is partly controlled of the negative pressure in the system and partly of 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 ones regenerative heat exchangers 2, 5 with a heat transfer fluid between the heat exchangers as shown in Fig. 1 but also others heat exchanger systems can 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 it Other. Periodically working so-called recuperative heat exchangers be used where a (smaller) part of the heat exchanger is put in contact with the absorption process that heats the heat exchanger material for to then be contacted with the main stream of gas which takes up said amount of heat. Such exchangers can be designed as two or more separate units that work periodically. Another common prevalent design are rotating wheels with special sectors for the different media flows. A less common design that can used is a stationary unit of heating surface designed so that the media flows are periodically redirected in the desired manner.

Another embodiment of the invention is shown in Fig. 2. Here place the heat exchanger so that it is enclosed by the dryer. One several appliance constructions with built-in heat exchangers for traditional heating media are already known. According to According to the invention, the absorption in this case is carried out on the heating surface one side and on the other side the heat is transferred directly to the dry goods. This is shown in Fig. 2, where a heat exchanger 21 is arranged in a dryer 22. Dryer, arrow 23, led into the contact device via one end and dried goods, arrow 24, are removed the contactor at one end. Moist salvage gas is circulated from the other end by means of a fan 25 or equivalent to the input end of the heat exchanger 2l, arrow 26, to which also concentrate concentrated absorption solution, arrow 27. Salvaged from the heat exchanger is circulated by means of a fan 28 or correspondingly, after separation of a diluted solution, to the inlet for dry goods in the contact device, arrow 29, if necessary together with part of the second circulating carrier gas stream (26). 10 15 20 25 30 35 40 10 Such a design has advantages in the form of lower need for gas flow and device volumes but also disadvantages in the form of a larger one depending on the properties of the dry goods. Of course it is too possible to use a heat carrier to transfer heat from one 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 is in As can be seen from Fig. 2, it is also affected by the contactor design.

Yet another design is shown in Figs. 3 and 4 therein the system is supplemented with a compressor 30 and 40 respectively increases the pressure of the gas to be absorbed.

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 appropriate to release the gas from dust and other interfering substances before the compressor. This can be done by filtration, washing or with using a separating heat exchanger. About the absorption solution applied with high pressure can an ejector that utilizes the energy in replace the fluid or supplement the compressor. It purified the solution can be further concentrated by multiple evaporation and interconnected 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 of the compressor and power requirements 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 the two heat exchangers 50, 51 consists of a working medium in a 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 into 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 the inability of the mechanical heat pump to handle large polluted gas volumes of low pressure at the same time as the mechanical the heat pump compensates for the inability of the chemical heat pump to large temperature rise. A limited addition of mechanical energy lO ll via the compressor also reduces the need to supply steam to maintain the temperature in the process. Of course, the two can the solutions are combined so that a preheating takes place in one step (chemical or mechanical heat pump) and final heating takes place with heat whose temperature rises in both stages. The compressor can operated at 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 through that the capacity of the compressor is varied.

Claims (17)

A method of drying drying goods using an absorption solution containing an absorbent with high affinity for the liquid to be removed from the drying material and with high solubility in said liquid, characterized in that a gas is circulated between drying materials. and a unit for removing liquid taken up from the dry goods and reheating 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 dry goods. Method according to claim 1, characterized in that the gas is heated in a first heat exchanger after passage of the dryer, that a partial stream of the gas is taken out of the gas stream before or after said heating and is brought into contact with co-absorption solution in a second heat exchanger and that 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 partial stream is brought into contact with absorption solution in cocurrent. is 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. 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 substream constitutes between 5 and 50 6 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. l0 l5 20 25 30 35 40 l3 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. Set according to claim 8, characterized in that when drying dry goods which emit 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 emits a basic substance. lO. 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. ll. Method according to one of the preceding claims, characterized in that the partial current after heat exchange is led directly back to the drying process. l2. Method according to one of the preceding claims, characterized in that the partial stream is purified with respect to the absorption solution. l3. 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. l4. Process according to Claim 13, characterized in that the absorption solution is filtered to remove the solid formed. l5. 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 Set according to one of the preceding claims, characterized in that the substream is diverted from the system after absorption and heat exchange. Set according to any one of the preceding claims, characterized in that the pressure on said substream is raised before it is brought into contact with the absorption solution.