US3484277A - Tank for storage and transportation of low boiling liquefied gas - Google Patents

Description

United States Patent 3,484,277 TANK FOR STORAGE AND TRANSPORTATION OF LOW BOILING LIQUEFIED GAS Rene Sieders, Arnhem, and Leo Taal, Amersfoort, Netherlands, assignors to Algemene Kunstzijde Unie N.V., Arnhem, Netherlands, a corporation of the Netherlands No Drawing. Filed May 8, 1967, Ser. No. 636,627

Claims priority, applicatigogzmNetsherlands, May 12, 1966,

Int. 'Cl. B50c 7/00 US. Cl. 117-95 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a tank for low-boiling liquefied gas which is provided with a layer of insulating material consisting of a foam of poly-(2,6-dimethylparaphenylene-oxide) having elongated cells which are directed substantially perpendicular to the tank wall.

BACKGROUND OF THE INVENTION Field of the invention The present invention lies in the field of a new construction for a tank for the storage of liquefied gases such as methane or nitrogen which brings about new and unexpected results.

Description of the prior art Tanks of this general character are already known from British patent specifications Nos. 817,012 and 851,266.

DESCRIPTION OF THE INVENTION It has been found according to the present invention that tanks of the type referred to above are with advantage constructed in a particular way. Accordingly, a tank according to the present invention is characterized in that the insulating material substantially consists of a foam of poly-(2,6-dimethyl-paraphenyleneoxide) having elongated cells which are directed substantially perpendicular to the tank wall.

A foam of this type, as well as methods for the manufacture thereof are described and claimed in the copending U.S. patent application Ser. No. 494,782 of S. Pezarro and B. L. Boogaard, filed Oct. 11, 1965, and assigned to the same assignee as the present invention, the con tents of which are included herein by reference. The foam disclosed in said application is characterized by a highly anisotropic structure. That is, the majority of the pores or cells in the foam are elongated, as distinguished from spherical, and are substantially parallel to each other along a given axis. In a preferred embodiment, the majority of the cells of the foam have a length which is at least five times the largest width. Details regarding the structure of the foam and the manner of making the same can be found in the above referenced patent application.

For the sake of brevity a foam of poly-(2,6-dimethylparaphenyleneoxide) having elongated cells will hereinafter be referred to as anisotropic PPO foam. A foam of this type need not purely consist of poly-(2,6-dimethylparaphenyleneoxide). It may contain various additives such as pigments, fibres, fire-retarding substances, and also other polymers such as polystyrene.

The use of anisotropic PPO foam for the lining of tanks according to the invention has considerable advantages over the use for this purpose of previously proposed insulating materials.

For instance, over the insulating material described in German patent specification No. 1,006,875 it has the advantage that the cross-sectional area of the elongated cells in the foam is considerably smaller than that of the cells formed by the battles described in said German patent specification. As a result, with a tank according to the present invention the effectiveness of the insulation is considerably greater. An additional advantage is that anisotropic PPO foam can be manufactured and applied to the tank wall in a very simple manner.

The use of anisotropic PPO foam according to the invention has great advantages over the known use of balsa wood in that it is a far more uniform product than the natural product balsa wood. Moreover, balsa wood is, naturally, only available in pieces of limited dimensions.

Anisotropic 'PPO foam can be manufactured in slabs of very large dimensions, which not only simplifies mounting them in the tanks, but also makes it possible to obtain an insulating layer which shows a considerably smaller number of seams. It is the seams in the insulating material that form the weak spots in tanks of the abovedescribed type, because they may burst upon shrinkage of the material. With tanks lined with the anisotropic PPO foam the number of seams need be very small. Moreover, the foam may be given such a structure that there is no longer any risk of the seams bursting. These structural details will be described further on.

Anisotropic foam may be manufactured in the foam of slabs in such a way that the longitudinal axis of the cells is not quite perpendicular, but at a slight tilt to the surface of the foam. The provision of such a foam on vertical walls in such a way that the cells extend at a slight downward tilt from the tank wall inwards, has the additional advantage that gas present in the cells cannot escape therefrom. Therefore, if in this connection mention is made of cells that are directed substantially perpendicular to the tank wall, this includes cells that are positioned in some direction at an angle to the tank wall which deviates from In the German patent specification No. 1,006,875 (column 1, lines 6 to 13) it is mentioned that at low temperatures the usual insulating materials are very brittle and are unsuitable in that they crack. In this respect anisotropic PPO foam compares favourably with the usual insulating materials in that also at l w temperatures it is sufficiently tough to withstand shocks.

In the German patent specification No. 1,106,350 (column 1, lines 45 to 54) it is mentioned that no foams of synthetic material are known which are resistant to the extreme conditions of, for instance, to C., and also to hydrocarbons. In this respect PPO excels the known foams, because it is resistant. to low temperatures as well as to aliphatic hydrocarbons. Tanks lined with PPO foam may therefore be used for the storage of most gases liquefied at a low temperature. Some halogen containing hydrocarbons, such as some Freons, however, must be excluded, because they may swell or dissolve the PPO.

In addition to the above-mentioned reasons, many of the existing foams of synthetic materials are unsuitable because of their closed cell structure. The cells which are in contact with the liquid fill up with liquid, for instance by diffusion. When the tank is discharged, the temperature abruptly rises, as a result of which the liquid contained in the cells will evaporate as it were explosively, so that the cells burst.

If use is made of PPO foam according to the invention, the elongated cells of the foam will, when the tank is being charged, fill up partly with liquid and partly, namely in the cell parts nearest to the metal tank wall, with gas formed by evaporation of the liquid. In each cell this gas forms a kind of cushion, which has an insulating effect. The cross-sectional area of the cells is so small that there is hardly any heat transmission by means of convection. When the tank is discharged, the liquid present in the cells may readily emerge, which will prevent bursting of the cells by abrupt evaporation. As the cells are not in communication with one another, no gas or liquid can pass from one cell to another.

What makes anisotropic PPO foam so particularly suitable for the purpose of lining tanks is that it has a low specific weight as well as a high compressive strength in longitudinal direction of the cells. In this respect the anisotropic PPO foam excells any other hitherto known foam of equal density.

In a direction perpendicular to the longitudinal direction of the cells the foam may be compressed fairly easily. This property of the anisotropic PPO foam may be taken advantage of in applying it to the tank wall. By applying the foam while it is slightly compressed in transverse direction it is possible to retain some stress therein. In this way it is achieved that although said stress decreases when the foam is cooled down, there does not occur any shrinkage of the foam. As a result, strong cooling of the foam does not lead to bursting of the seams. A preferred embodiment according to the invention is therefore characterized in that at least at its free surface the foam is at room temperature under pressure in directions lying in said surface.

Anisotropic PPO foam may be manufactured in the form of slightly bent slabs. If these slightly bent slabs are so applied that their concave sides are in contact with a flat tank wall, then the free sides of the slabs will be somewhat compressed, so that in said free sides stresses are produced, Since the cooling of the foam slabs is strongest there, the insulating layer will in the cold state be subject to only slight stresses.

A preferred tank according to the invention, in which this is the case, is characterized in that in the foam the pressure in directions parallel to the free surface decreases with the distance to the tank wall.

A similar effect is, of course, also obtained if a flat slab of anisotropic PPO foam is adhered to the concave side of a bent tank wall.

The anisotropic PPO foam may be applied to the walls of a tank in a known manner, for instance by gluing.

In general slabs of anisotropic PPO foam are on both sides provided with a thin PPO skin. On the side of the tank wall this skin is preferably left intact. On the side of the liquid the skin may be removed, for instance, by cutting or grinding. But this is not necessary.

For safety purposes it has advantages to incorporate in the foam layer one or more impermeable sheets. These sheets, which are hereinafter referred to as barriers may consist of any material which will withstand the conditions prevailing in the tank, such as metal foil, plastic foil or sheet, plywood etc. For this purpose use is preferably made of FPO-sheet or foil, which may be coated with metal foil such as aluminum or copper foil. The FPO-sheet has a high elongation at low temperatures and is easy to connect to the PPO foam. Moreover, it is possible to produce PPO foam slabs in such a way that they are provided with an impermeable FPO-skin which may act as a barrier. If use is made of these slabs, the insulation layer is composed of at least twoof these. slabs.

If desired, the parts of the insulation layer which form the outside, may be composed of an insulating material other than PPO foam, provided that the insulating layer is so composed that the liquid gas does not contact these parts.

To prevent damage to the insulating layer it may be advantageous to cover the PPO foam with a material which protects it against mechanical damage. Such damage may occur when the empty tank is to be inspected and people are to go inside the tank. This protecting material may form a continuous lining of metal or plastic. In the case where this protecting material forms a continuous wall it is to be regarded as an inner tank. In that case too one has the full advantages of the use of the anisotropic foam according to the invention in that the construction of the inner tank is much less critical than the construction of a conventional inner tank. For, if in this inner tank a leak should occur, the FPO-foam layer acts as a secondary safeguard and a leak in the inner tank does not imply any danger.

Tanks of the above-indicated type may be used on their own or form part of a plant.

The invention relates not only to a tank of the aboveindicated type, but also to means of conveyance, more particularly a ship equipped with such a tank. Tanks according to the present invention may also find application in space and aircraft.

What is claimed is:

1. A tank for the storage of low boiling liquefied gas having its inner surface lined with a layer of cellular insulating anisotropic foam of poly-(2,6- 'methylparaphenylene oxide), said anisotropic foam havin elongated cells having a length which is at least five times the largest width of a cell, said cells directed substantially perpendicular to the tank wall.

2. A tank according to claim 1, in which in the insulating material there is incorporated at least one impermeable sheet parallel to the surface of the material.

3. A tank according to claim 2, in which the impermeable sheet consists of poly-(2,6-dimethylphenyleneoxide).

4. A tank according to claim 1, in which at room temperature the foam, at least at its inner surface, is under pressure in directions parallel to said surface.

5. A means of conveyance equipped with a tank according to claim 1.

References Cited UNITED STATES PATENTS 2,769,205 11/1956 Pfieumer 2602.5

2,911,380 11/1959 Doedens.

3,382,212 5/1968 Price et a1.

MURRAY TILLMAN, Primary Examiner MORTON FOELAK, Assistant Examiner US. Cl. X.R.