WO1996013367A1

WO1996013367A1 - Method and equipment for removing by-products produced during a cable vulcanization process

Info

Publication number: WO1996013367A1
Application number: PCT/FI1995/000588
Authority: WO
Inventors: Jukka Karppo; Raili Kajaste
Original assignee: Nokia-Maillefer Holding S.A.
Priority date: 1994-10-27
Filing date: 1995-10-25
Publication date: 1996-05-09
Also published as: FI945072A; FI97035B; AU3747695A; FI97035C; FI945072A0

Abstract

The present invention relates to a method for removing by-products produced during a cable vulcanization process. In the method, circulation gas is passed from the gas space of a vulcanization equipment into a condenser, wherein the gas is cooled indirectly, the by-products and the 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. The invention also relates to an equipment for removing these by-products. The equipment comprises a condenser (2) for cooling gas, a separating means (5), connected with the condenser, for separating by-products from circulation gas, and a gas circulation means (8), connected to the vulcanization equipment, for reintroducing the purified gas into the gas space of the vulcanization equipment.

Description

Method and equipment for removing by-products produced during a cable vulcanization process

The present invention relates to a method and equipment for removing by-products produced during a cable vulcanization process. According to the method, circulation gas which is used in vulcanization and which contains by-products is removed from the gas space of a vulcanization equipment for purification, and the purified gas is reintroduced into the gas space of the equipment. By-products are thus continuously removed from the continuous vulcanization process.

As is well known, undesirable by-products are produced in vulcanizing an extruded cable, such by- products having a harmful effect on the properties of the cable. Such products must thus be removed as completely as possible from the circulation gas. The amount and the composition of the by-products are naturally affected by the vulcanizing agents used, but also by the vulcanization temperature and the manner of treating the gas space. For instance in using cumyl peroxide as a vulcanizing agent, the resulting by-products include for instance alpha- methylstyrene, asetophenone, cumyl alcohol, xylene, phenol, methane and ethane. The reaction also produces water, which also significantly deteriorates the final result of the vulcanization.

In the present field, a method and equipment have previously been disclosed for separating degradation products produced during the preparation of a continuously produced cable. According to Finnish Patent Application 941,105, such a separation is carried out by continuously passing circulation gas containing gaseous degradation 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 passing the purified gas to the inlet area of the heating- up zone. Washing the gas is carried out by means of a direct contact of the gas with water, whereby the degradation products are transferred from the gas into the water. After the washing and the subsequent drying, the gas temperature is raised to *-he operational temperature of the heating-up zone before the gas is passed to the inlet area of said zone. A disadvantage of the method is that an effective recovery of by-products is not possible with the use of this type of direct gas washing method. In addition, significant amounts of water classified as problem waste water are produced during the washing, the proportion of degradation products to the amount of water being extremely low in such water. As to further processing of such a great amount of waste water, it is difficult to implement and also expensive.

The object of the present invention is to provide a method by means of which the above-mentioned disadvantages can be obviated and which enables by- products to be efficiently removed from the process. This object is achieved with the method of the invention, characterized by passing circulation gas from the gas space of a vulcanization equipment into a condenser, wherein the gas is cooled indirectly; separating the by- products and the circulation gas of the produced multiphase mixture from each other; and reintroducing the purified circulation gas into the gas space of the vulcanization equipment. In the method, the gases produced during the process are condensed into liquid by-products, which can be removed from the run in a controlled manner during production. In this manner, the by-products are very efficiently recovered without producing great amounts of waste water. According to the invention, circulation gas is circulated and by-products are removed from the circulation continuously during vulcanization. The idea of the method of the invention is based on the fact that the entire gas space of the vulcanization equipment is mathematically modelled. The flow of the circulation gas which is removed from the vulcanization equipment and which contains by-products, and thus the entire removal of the by-products from the process, are mathematically optimized by utilizing thermodynamic phase equilibrium models. The amounts and compositions of the by-products produced in vulcanization can be theoretically calculated on the basis of the run parameters of the vulcanization equipment. Such run parameters include for instance vulcanizing agents, the speed of the production line, the amount of the plastic raw material used, and the temperature of the heating-up zone. In other words, the plastic raw material and the vulcanizing agent used in each particular case affect how run parameters are selected.

By means of the theoretically calculated compositions and amounts of by-products, the flow of circulation gas in the process is optimized by using the Wegstein optimization model. In this model, variable q (a so-called acceleration parameter) , which represents a change in the amount of the moles of the circulated gas flow at different iteration stages, is calculated for each by-product component (i.e. tear stream variable) according to the following equation:

q = s - 1

s = fiiZ - Q tXi .il

where X is the estimated amount of moles of each by- product component, G(X) is the obtained calculated value of this amount of moles at a determined iteration stage, and k is the number of the iteration stage of the optimization calculation. The new estimate of the amount of moles, calculated with the Wegstein model, is:

The iteration is continued until the amounts of moles of the by-product components of the circulated gas flow are within the required error limits. The obtained result is thus the optimized amount of circulation gas flow, wherein the proportions of the by-product components are advantageous as regards the operation of the equipment used for removing the by-products.

The actual equipment used for the vulcanization of a cable is previously known. Such an equipment is described in more detail for instance in Finnish Patents 52,299 and 57,367. According to these publications, the equipment comprises a vulcanizing tube, in which 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, in such a manner that the same protective gas, under the same pressure, is provided in both zones.

According to the present invention, circulation gas, in which water and other by-products are produced during vulcanization, is passed from the gas space of the vulcanization equipment into a condenser, wherein the gas is cooled indirectly. Preferably, circulation gas is extracted from the process from the starting part of the heating-up zone of the gas space. Indirect cooling can be carried out with water or cooled gas, for instance. Condensation with water is preferably performed to a temperature of 70 °C. The temperature of the circulation gas to be condensed is usually about 200 - 400 °C, and the pressure thereof is about 1100 kPa (11 bar) . The temperature of the circulation gas coming out of the condenser is usually about 70 °C. The circulation gas used is preferably nitrogen.

The majority of the by-products are condensed at a temperature of about 70 °C. Depending on the manner of condensation, the produced condensed mixture may comprise several phases. For instance in performing an indirect cooling with water, a two-phase mixture is obtained, said mixture comprising circulation gas and liquid by-products.

The multiphase mixture produced in the condensation is passed into a separating means, wherein the by-products are separated from the circulation gas. The by-products accumulated in the separating means are then removed in a controlled manner, for instance into a waste container. The separating means is preferably a drop separator; it is however also possible to separate the multiphase mixture by means of a cyclone. The circulation gas to be returned to the vulcanization equipment must be as dry as possible. It is therefore possible to pass the partially purified circulation gas obtained from the separating means to a dryer before the gas is returned into the gas space of the vulcanization equipment. The dryer may comprise for instance the following components: a cooler, cyclone separator, prefilter, active carbon filter, adsorption dryer, and an afterfilter. The circulation gas is preferably dried. The dry, purified circulation gas is returned into the gas space of the vulcanization equipment by means of a gas circulation means. The gas is preferably returned to the final end of the heating-up zone of the gas space. The gas circulation means is preferably an inverter controlled blower. The temperature of the circulation gas returned to the vulcanization equipment is usually no more than about 90 °C, preferably about 72 °C. In using for instance a low density polyethylene (LDPE) as a starting material and a cumyl peroxide as a vulcanizing agent in vulcanization, the speed of gas is suitably adjusted to be less than 1 meter/s, preferably less than 0.1 meters/s.

The invention also relates to an equipment for removing by-products produced during a cable vulcanization process. The equipment is characterized in that it comprises a condenser 2 for cooling gas, a separating means 5, connected with the condenser, for separating by¬ products from circulation gas, and a gas circulation means 8, connected to the vulcanization equipment, for reintroducing the purified gas into the gas space of the vulcanization equipment.

The equipment may also comprise a dryer 7, connected between the separating means and the gas circulation means, for drying gas.

In the following, the invention will be described in more detail with reference to the preferred embodiment of the method of the invention and to the accompanying drawing, which is a schematic representation of the preferred embodiment of the equipment of the invention.

The drawing shows a vulcanizing tube 1, which is part of the vulcanization equipment and in which the plastic coating of a cable is vulcanized. The tube comprises a heating-up zone A and a cooling zone B. An arrow indicates the direction in which the cable moves. The structure and the operating principle of the vulcanization equipment are described in more detail in Finnish Patents 52,299 and 57,367, as mentioned above.

Circulation gas, which is preferably nitrogen, containing by-products is passed from the starting part of the heating-up zone A to the condenser 2, wherein the gas is cooled indirectly with water to a temperature of about 70 °C. The cooling water is circulated in the condenser through an inlet line 3 and an exhaust line 4. The velocity of flow of the circulation gas to be condensed is about 0.1 m/s, the temperature no more than about 200 °C, and the pressure 1100 kPa (11 bar) . The cooling water circulation of the condensation is adjusted in such a manner that the temperature of the circulation gas leaving the condenser is about 70 °C.

The majority of the by-products are condensed at a temperature of 70 °C. The produced two-phase mixture is passed into a drop separator 5, wherein the liquid by¬ products are separated from the circulation gas. The by¬ products accumulate in the bottom of the drop separator. A liquid level controller automatically opens and closes a valve located at the bottom at suitable intervals, whereby the by-products can be collected in a controlled manner in a waste container 6.

The partially purified circulation gas is passed from the drop separator into the dryer 7, which comprises a cooler, cyclone separator, prefilter, active carbon filter, adsorption dryer, and an afterfilter, as mentioned above.

The purpose of the prefilter and the active carbon filter is to remove possible by-product vapours from the circulation gas before the actual drying. The actual dryer is a cold-regenerative, automatic adsorption dryer, the reject of which is clean condensing water which can be disposed in the sewer. The purpose of the afterfilter is to prevent possible impurities spreading from the adsorption dryer from reaching the circulation gas blower.

From the dryer, the circulation gas is passed to the gas blower 8, by means of which the circulation gas is pressurized back to the operating pressure of the vulcanization equipment. If a dryer is not used, the circulation gas purified in the drop separator is passed directly to this blower. The controllable blower blows the circulation gas to the final end of the heating-up zone A of the vulcanizing tube 1.

Claims

1. A method for removing by-products produced during a cable vulcanization process, c h a r a c t e r - i z e d by passing circulation gas from the gas space of a vulcanization equipment into a condenser, wherein the gas is cooled indirectly; separating the by-products and the circulation gas of the produced multiphase mixture from each other; and reintroducing the purified circulation gas into the gas space of the vulcanization equipment .

2. A method according to claim 1, c h a r a c - t e r i z e d in that the flow of the circulation gas which is removed from the vulcanization equipment and which contains by-products is mathematically optimized by utilizing thermodynamic phase equilibrium models.

3. A method according to claim 2, c h a r a c ¬ t e r i z e d in that the flow of the circulation gas is optimized by utilizing the Wegstein optimization model.

4. A method according to any one of the claims

1 - 3, c h a r a c t e r i z e d in that circulation gas is passed into the condenser from the starting part of the heating-up zone of the gas space of the vulcanization equipment .

5. A method according to any one of the claims

1 - 4, c h a r a c t e r i z e d in that the purified circulation gas is returned to the final part of the heating-up zone of the gas space of the vulcanization equipment .

6. A method according to any one of the claims

1 - 5, c h a r a c t e r i z e d in that the circulation gas is cooled indirectly with water in the condenser.

7. A method according to any one of the claims 1 - 6, c h a r a c t e r i z e d in that the by-product and the circulation gas are separated from each other in a drop separator.

8. A method according to any one of the claims 1 - 7, c h a r a c t e r i z e d in that the circulation gas is dried before it is returned into the gas space of the vulcanization equipment.

9. A method according to any one of the claims 1 - 8, c h a r a c t e r i z e d in that the circulation gas is cooled indirectly to about 70 °C.

10. A method according to any one of the claims 1 - 9, c h a r a c t e r i z e d in that the speed of the circulation gas to be cooled is adjusted by means of a blower to be less than 1 m/s, preferably less than 0.1 m/s .

11. An equipment for removing by-products produced during a cable vulcanization process, c h a r ¬ a c t e r i z e d in that it comprises a condenser (2) for cooling gas, a separating means (5) , connected with the condenser, for separating by-products from circulation gas, and a gas circulation means (8) , connected to the vulcanization equipment, for reintroducing the purified gas into the gas space of the vulcanization equipment.

12. An equipment according to claim 11, c h a r a c t e r i z e d in that the separating means is a drop separator.

13. An equipment according to claim 11 or 12, c h a r a c t e r i z e d in that the gas circulation means is an inverter controlled blower.

14. An equipment according to any one of the claims 11 - 13, c h a r a ct e r i z e d in that it comprises a dryer (7) , connected between the separating means and the gas circulation means, for drying gas.