WO2001072493A1

WO2001072493A1 - Method and equipment in a cable vulcanization process

Info

Publication number: WO2001072493A1
Application number: PCT/FI2001/000298
Authority: WO
Inventors: Ali Harlin
Original assignee: Nextrom Holding S.A.
Priority date: 2000-03-29
Filing date: 2001-03-27
Publication date: 2001-10-04
Also published as: FI20000724A; FI108411B; AU2001250445A1; FI20000724A0

Abstract

The invention relates to a method for removing by-products produced in a cable vulcanization process. In the method circulation gas is led from a gas space of vulcanization equipment into a gas circulation means, which is preferably thermally insulated, after which the gas is led into a condenser where it is cooled, the by-products are condensed into a purification cell comprising a condensation surface, and the substantially purified circulation gas is reintroduced into the gas space of the vulcanization equipment. The invention also relates to equipment for removing the by-products. The equipment comprises a pipeline (10) leading from the outlet area of the vulcanization equipment to its inlet area, and, connected to the pipeline, a gas circulation means (2), a condenser (3) for cooling the gas, and one or more purification cells (4) comprising a condensation surface (5) for separating the by-products from the circulation gas.

Description

METHOD AND EQUIPMENT IN A CABLE VULCANIZATION PROCESS

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and equipment for removing by-products produced in a cable vulcanization process. According to the method, circulation gas which used in the vulcanization and which contains by-products is removed from the gas space of vulcanization equipment for purification, the purified gas being then reintroduced into the gas space of the equipment. By-products are thus continuously removed from the continuous vulcanization process. [0002] The vulcanization of an extruded cable is known to produce undesirable by-products which damage the equipment and may have a harmful effect on the properties of the cable. Such products must therefore be removed from the circulation gas as completely as possible. The amount and composition of the by-products naturally depend on the vulcanizing agents used, but also on the vulcanization temperature and the manner of treating the gas space. For example, when dicumyl peroxide is used as a vulcanizing agent, the resulting by-products include alpha-methylstyrene, asetophenone, cumyl alcohol, xylene, phenol, methane and ethane. Water is also produced in the reaction which significantly deteriorates the final result of the vulcanization as well.

[0003] A prior art method and equipment have been disclosed for separating degradation products produced during the manufacture of a continuously produced cable. According to EP Patent 603363, such a separation is carried out by continuously leading circulation gas containing gaseous deg- radation products produced during a vulcanization process away from the outlet area of the vulcanization heating-up zone, by purifying the gas of the degradation products, and then leading the purified gas to the inlet area of the heating-up zone. The gas is washed by subjecting it to direct contact with water, whereby the degradation products are transferred from the gas into the water. After the washing and the subsequent drying, the temperature of the gas is raised to the operational temperature of the heating-up zone before it is led to the inlet area of the zone. A disadvantage of the method is that such a direct method of washing the gas does not allow the by-products to be efficiently recovered. In addition, significant amounts of water classified as prob- lem waste are produced during the washing, the proportion of degradation products to the amount of water being extremely low in such water. Moreover, the further processing of such a great amount of waste water is difficult and also expensive. In addition, the water content of the circulation gas is continuously high. [0004] Publication WO 96/13367 teaches a method which attempts to avoid the above shortcomings. According to the method disclosed in WO 96/13367, the circulation gas from the gas space of the vulcanization equipment is led to a condenser where the gas is cooled indirectly, the by-products and circulation gas of the produced multiphase mixture are separated from each other and the purified circulation gas is reintroduced into the gas space of the vulcanization equipment. In the method, the gases produced in the process are condensed into liquid by-products which are removed from the run during the production process. Although this allows by-products to be recovered without any major amounts of waste water being produced, the waste collected must nevertheless be taken to a hazardous waste treatment plant. Another weakness of the method disclosed in WO 96/13367 is the poor separability of liquid by-products from the circulation gas flow. A yet further drawback of the method is that the means circulating the gas is located after the condenser due to which by-products left in the circulation gas collect into the gas circulation means.

[0005] It is an object of the present invention to provide a method that allows the above shortcomings to be avoided and by-products to be efficiently removed from the process. This is achieved with a method of the invention which is characterized in that circulation gas is led from a gas space of vulcanization equipment into a gas circulation means, which is preferably thermally insulated, after which the gas is led into a condenser where the gas is cooled, the by-products are condensed into a purification cell comprising a condensation surface, and the substantially purified circulation gas is reintroduced into the gas space of the vulcanization equipment. [0006] The advantages to be gained with the present method are essentially due to the arrangement of the equipment to be used. The fact that the gas circulation means is installed before the condenser in the direction of circulation of the gas has a significant effect on the amount of by-product collected into the gas circulation means. The gas circulation means of the present method contains hot gas which is why practically no by-products are condensed into the gas circulation means. This allows service downtime needed for the cleaning of the gas circulation means to be avoided. Moreover, in the method of the invention the by-products condense onto the surfaces of the purification cell which provides a significantly more effective purification of the gas than the prior art methods. For a by-product to be separated with the prior art methods, it must be cooled to form droplets. In the method of the present invention, even circulation gas by-product in the form of mist is condensed onto the surfaces of the purification cell.

[0007] An embodiment of the invention comprises an additional phase in which the by-products condensed into the purification cell are cleaned therefrom by disconnecting the flow of the circulation gas into the cell and by introducing into the cell an excess mount of air or pure oxygen needed for complete equimolar burning, the cell being heated at the same time by means of a flame or electricity, whereby the by-products condensed into the cell burn up substantially completely and the purified flue gases are led into the atmosphere. The advantage of this embodiment, compared with the prior art, is that no waste is produced in connection with the purification of the gas.

[0008] In the method of the invention the substantially purified flue gases produced can be led directly into the atmosphere whereby no secondary waste or waste waters are produced the further processing of which would be both problematic and expensive.

[0009] The equipment of the invention also allows to reduce the amount of emissions into the atmosphere in case of a rapid shutdown of the equipment. In connection with a shutdown, the circulation gas in the equipment is usually led directly into the atmosphere, but with the equipment of the invention the gas can be guided through the purification equipment which allows at least some of the by-products to be removed from the gas before it is released into the atmosphere. Further, the discharge pipe may be provided with a burner that can be used for burning any by-products that may have passed the purifier which further reduces the amount of harmful substances released into the atmosphere.

[0010] The method of the invention can be implemented using the mathematical modelling of the entire gas space of the vulcanization equipment known from WO96/13367. This means that the flow of the circulation gas containing by-products and to be removed from the vulcanization equipment, and thereby the entire removal of the by-products from the process, are mathematically optimized by using thermodynamic phase equilibrium models. The amounts and compositions of the by-products produced in the vulcanization can be theoretically calculated on the basis of the run parameters of the vulcanization equipment. These run parameters include vulcanizing agents, the speed of the production line, the amount of plastic raw material used and the temperature of the heating-up zone. In other words, the plastic raw material and the vulcanizing agent to be used in each particular case have on effect on the choices made with regard to the run parameters.

[0011] On the basis of the theoretically calculated compositions and amounts of by-products, the flow of the circulation gas in the process is opti- mized by applying the Wegstein optimization model. In the model, variable q (also known as an acceleration parameter) representing a change in the amount of moles in the gas flow to be circulated in the different iteration rounds is calculated for each by-product component (i.e. tear stream variable) from the following equation:

s- l

G(X_k) - G(X_k_ ) s = -

X_h — X k; -\

[0012] wherein X represents the estimated amount of moles in each by-product component, G(X) is the calculated mole amount value obtained for a particular iteration round and k is the number of the iteration round in the optimization calculation. The new estimate of the mole amount calculated using the Wegstein model is

X_k+1 = q(X_k) + (l - q)G(X_k)

[0013] The iteration continues until the amounts of moles in the byproduct components of the gas flow to be circulated are within the required error limits. This produces an optimized amount of circulation gas flow wherein the proportions of by-product components are advantageous with regard to the operation of the equipment used for removing by-products.

[0014] The actual cable vulcanization equipment is known in the art and described in greater detail for example in US Patents 4,035,129 and 4,155,695. According to them, the equipment comprises a vulcanizing tube where the actual vulcanization takes place. The vulcanizing tube comprises a heating-up zone and a cooling zone which are in pressure contact with each other, both the zones containing the same protective gas of the same pressure. [0015] According to the present invention, the circulation gas into which water and other by-products are produced during the vulcanization, is led from the gas space of the vulcanization equipment into the gas circulation means and from there further to a condenser where the gas is cooled indirectly. The circulation gas is preferably extracted from the process at the for- ward end of the heating-up zone of the gas space and the gas circulation means is thermally insulated to prevent the by-products in the circulation gas from cooling too much before they enter the condenser. This allows to substantially avoid the by-products from condensing into, and thereby dirtying, the gas circulation means. The temperature of the circulation gas entering the gas circulation means is usually about 200-400°C and its pressure about 700-1200 kPa (7-12 bar). When needed, the pressure is increased using the gas circulation means. The more by-product is collected into the purification cell, the greater is the pressure loss in the equipment, and the gas circulation means must be used to increase the pressure to produce a desired gas flow. The gas circulation means is preferably a centrifugal blower or a single-phase turbo compressor. When for example a low-density polyethylene (LDPE) is used as a starting material and a dicumyl peroxide as a vulcanizing agent in the vulcanization, the flow rate of the gas is suitably adjusted to be less than 1 me- tre/s, preferably less than 0.1 metres/s. [0016] The indirect cooling of the gas can be carried out using for example water or cooled gas. The preferred temperature of water cooling is about 10-15°C. The temperature of circulation gas entering the condenser is typically 150-200°C and when the gas leaves the condenser its temperature is usually about 60°C. The preferred gas for the circulation is nitrogen. The cooled gas mixture produced in the cooling is led into the purification cell where the by-products condense onto the surfaces of the cell. As a result of the condensation, a thin layer of by-products is formed onto the surfaces. A normal thickness of the layer is of the order of 0.1 mm, but it may vary from 0.01 to 1 mm, depending on the condensation surface used. [0017] The condensation surface of the purification cell arranged after the condenser can be implemented for example by filling the cell with steel wool, or the like. Other preferred condensation surfaces include different net tubes, lamellae, membranes and screens. The condensation surfaces are preferably made of metal, but ceramic surfaces can also be used. Specialty steel types resisting high temperatures are preferred. Condensation surfaces that facilitate the burning of the by-product when the cell is being cleaned are particularly preferred. These include metal nets or lamellae coated with vanadium oxide or nickel.

[0018] The circulation gas purified in the purification cell is reintroduced into the gas space of the vulcanization equipment. The gas is preferably returned to the rear end of the heating-up zone in the gas space. The temperature of the circulation gas returned into the vulcanization equipment is usually no more than about 60°C, preferably about 20-30°C.

[0019] The invention also relates to equipment for removing byproducts produced in a cable vulcanization process. The equipment is char- acterized in that it comprises a pipeline 10 leading from the outlet area of the vulcanization equipment to its inlet area; and, connected to the pipeline, a gas circulation means 2; a condenser 3 for cooling the gas; and one or more purification cells 4 comprising a condensation surface 5 for separating the byproducts from the circulation gas. [0020] In addition, the equipment may comprise a pure oxygen or air feed 51 and possibly an organic hydrocarbon feed 52 into the purification cell 4, a means for heating the condensation surface 5, and a piping 53 leading from the purification cell 4 into the atmosphere. The pure oxygen or air feed is preferably adjustable, the flow of the pure oxygen or air being adjusted on the basis of the temperature measurement of the condensation surface 5. In addition, to the piping 53 may be connected a free flame or glowing net 6 that assists the burning, the gas flowing in the pipe getting into contact with the net or the flame.

[0021] In the following the invention will be described in greater detail with reference to a preferred embodiment according to the present invention and to the accompanying drawing which is a schematic view of the preferred embodiment of the equipment of the invention.

[0022] The drawing shows a vulcanization tube 1 , which is part of the vulcanization equipment and in which the plastic coating of a cable is vul- canized. The tube comprises a heating-up zone A and a cooling zone B. An arrow above the tube shows the direction of travel of the cable. As stated above, the structure and operating principle of the vulcanization equipment are described in greater detail in US Patents 4,035,129 and 4,155,695.

[0023] From the forward end of the heating-up zone A, circulation gas, preferably nitrogen, comprising by-products is led into the gas circulation means 2 after which the gas is led into the condenser 3 where it is cooled indirectly by means of water to a temperature of about < 70°C. The cooling water is circulated in the condenser through a supply line 31 and discharge line 32. The flow rate of the circulation gas entering the cooling is about 1-3 m/s, its temperature no more than about 200°C, and its pressure 1600 kPa (16 bar). After the blower, the temperature of the gas is typically about 150°C and its pressure is 100-300 kPa (1-3 bar) higher than the pressure in the vulcanization tube. The circulation of the cooling water in the cooling is adjusted such that the temperature of the circulation gas leaving the condenser is about 60°C. On the condensation surface, the flow rate of the gas is reduced to < 0.3 meters/s, preferably even to < 0.1 meters/s, by increasing the cross-sectional area of the flow.

[0024] Most of the by-products condense at a temperature of 70°C. The cooled gas mixture produced led from the condenser into the purification cell where the by-products are condensed onto the surfaces of the cell. As a result of the condensation, a thin layer of by-products is formed onto the surfaces. The thickness of the layer is of the order of 0.1 mm, but it may vary depending on the condensation surface used.

[0025] From the purification cell, the purified circulation gas is reintroduced into the gas space of the vulcanization equipment, preferably to the rear end of the heating-up zone of the gas space. The temperature of the circulation gas returned into the vulcanization equipment is usually no more than about 60°C, preferably about 20°C.

[0026] The circulation gas can be circulated in the vulcanization tube also in another direction than described above. The circulation gas is then taken from the rear end of the gas space of the vulcanization equipment and, after the purification, the gas is led into the forward end of the gas space of the vulcanization equipment.

[0027] The regeneration of the condensation surface preferably starts at a temperature of 500-600°C and at low supply of air, the air contain- ing about 22% of oxygen. As the net warms up, the flow of air is increased and its temperature adjusted typically to 700-800°C. This allows a sudden outburst of fire and any equipment damages thereby caused to be avoided. Depending on the structural materials used, temperatures as high as 1000-1200°C may be used. In such cases ceramic condensation surfaces are preferred. High temperatures further improve the completeness of the burning.

Claims

1. A method for removing by-products produced in a cable vulcanization process, characterized in that circulation gas is led from a gas space of vulcanization equipment into a gas circulation means, which is pref- erably thermally insulated, after which the gas is led into a condenser where the gas is cooled, the by-products are condensed into a purification cell comprising a condensation surface, and the substantially purified circulation gas is reintroduced into the gas space of the vulcanization equipment.

2. A method according to claim 1, characterized in that the condensation surface comprises one or more steel wool layers, metal nets, metal lamellae, membranes or ceramic structures, or a combination thereof.

3. A method according to claim 1 or 2, characterized in that the method further comprises a phase where some of the circulation gas that has passed the purification cell is reintroduced into the purification cell.

4. A method according to any one of claims 1 to 3, characterized in that the method further comprises a phase where the byproducts condensed into the purification cell are cleaned therefrom by disconnecting the flow of circulation gas into the cell and by introducing into the cell an excess amount of air or pure oxygen needed for complete equimolar burn- ing, the cell being heated at the same time with a flame or electricity, whereby the by-products that have condensed into the cell burn up substantially completely and the purified flue gases are led into the atmosphere.

5. A method according to claim 4, characterized in that the supply of pure oxygen or air is controlled on the basis of the temperature of the condensation surface.

6. A method according to claim 5, characterized in that organic hydrocarbon is also introduced into the purification cell to assist complete burning of the by-products.

7. A method according to claim 6, characterized in that the organic hydrocarbon comprises propane, butane or a mixture thereof.

8. Equipment for removing by-products produced in a cable vulcanization process, characterized in that it comprises a pipeline (10) leading from the outlet area of the vulcanization equipment to its inlet area; and, connected to the pipeline, a gas circulation means (2); a condenser (3) for cooling the gas; and one or more purification cells (4) comprising a con- densation surface (5) for separating the by-products from the circulation gas.

9. Equipment according to claim 8, characterized in that the gas circulation means (2) is thermally insulated.

10. Equipment according to claim 8 or 9, characterized in that it further comprises a pure oxygen or air feed (51) and possibly an organic hydrocarbon feed (52) into the purification cell (4), a means for heating the condensation surface (5), and a piping (53) leading from the purification cell (4) into the atmosphere.

11. Equipment according to claim 10, characterized in that the pure oxygen or air feed (51 ) can be adjusted.

12. Equipment according to any one of claims 8 to 11, characterized in that to the piping (53) leading into the atmosphere is connected a free flame or a glowing net (6) assisting the burning, the gas that flows in the pipe coming into contact with the net or the flame.