TECHNICAL FIELD

This disclosure relates to improvements in corrosion resistant heat exchangers.

BACKGROUND ART

Heat exchangers are extensively used in the chemical industry.

Carbon steel is a commonly used material of construction as this material provides a good balance of thermal conductivity and mechanical strength properties at a reasonable cost. However, the corrosion resistance of carbon steel is comparatively low. Accordingly, it is a common practice to fabricate heat exchangers from stainless steel when the process fluid is corrosive. We have observed that heat exchangers fabricated from stainless steel can become severely fouled - even at comparatively low rates of corrosion - particularly when the process fluid is both corrosive and reducing. The present disclosure mitigates this problem.

DISCLOSURE OF INVENTION

In an embodiment, provided is an improved method for heat exchange between a hot process fluid and a coolant stream; characterized in that a heat exchanger having a process fluid side and a coolant stream side is employed in said method and wherein the materials of construction used on said process fluid side are Cr-Ni-Fe-Mo alloys that contain from 50 to 65 weight % Ni.

In an embodiment, the hot process fluid creates a reducing environment

In an embodiment, the hot, reducing process fluid contains chlorides and substantially free of oxygen.

In an embodiment, the Cr-Ni-Fe-Mo alloys are selected from INCONEL ^® 625 and HASTELLOY ^® C-276.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a side view of a cross section of part of a shell and tube heat exchanger.

DESCRIPTION OF EMBODIMENTS

Heat Exchangers

In an embodiment, the heat exchanger is a shell and tube exchanger. The term“shell and tube exchanger” as used herein is meant to convey its conventional meaning, namely a heat exchanger in which a plurality of tubes (referred to as“tube bundle”) is contained within a shell. The tubes are conventionally held in place by a tube sheet (where the term“tube sheet” refers to a flat plate which corresponds in size and number to the tubes). Accordingly, the tube sheet defines the

arrangement of the tubes in the bundle according to the pattern of the holes in the sheet.

Shell and tube exchangers are commonly used to exchange heat between two fluid streams by passing one fluid stream through the shell (hereinafter, the “shell side”) and another fluid stream through the tube bundle (hereinafter, the“tube side”).

Exchangers of this type are designed to keep the shell side fluid stream separate from the tube side fluid stream. Corrosion can defeat this design if it produces a hole in a tube. Corrosion can also cause fouling of the tubes as corrosion products deposit within the tubes. Corrosion can also lead to loss of process fluid containment if the integrity of the shell side is severely compromised by corrosion.

Materials of Construction

Table 1 provides a description of some metals and alloys that may be used to fabricate heat exchangers. The term“bal” in Table 1 means balance - for clarity. Carbon steel is (about) 2 weight % other materials with the balance (almost 98 weight %) being Fe; the HASTELLOY C-276 alloy is about 15+5+15+8 (43 weight % of Cr-Fe-Mo and others (respectively) with the balance (about 57 weight%) being Ni.

TABLE 1

Metals/Allovs for Fleat Exchangers (weight %)

DSS = duplex stainless steel Process Conditions

The process fluid may be one or more gases; one or more liquids or a combination thereof.

In an embodiment, the heat exchanger is located in a solution polymerization process for the preparation of polyethylene. In a particular embodiment, the heat exchanger is used in the“recovery” section of a solution polymerization process - i.e. the section where solvent and unreacted monomers are“recovered” after the polymerization reaction. In an embodiment, the heat exchanger is used as a condenser - i.e. it receives a hot vapor mixture (containing unreacted ethylene monomer and gaseous process solvent; the vapor may also contain unreacted comonomer and some water vapor) as the“process” stream and this hot vapor mixture is condensed with cooling water. In an embodiment, the hot vapor mixture is directed through the tube bundle and the cooling water is directed through the shell side of the exchanger. In an embodiment, the presence of chlorides produces a corrosive process fluid such as a corrosive hot vapor mixture. The corrosive process fluid may be described as oxidizing if it contains oxygen. Alternatively, the corrosive process fluid may be described as reducing if it is substantially free of oxygen. The term reducing has its conventional meaning, it is a condition in which oxidation is prevented by the removal or lack of oxygen gas and the presence of reactive gas such as hydrogen that willing to provide electrons needed for reduction. The term“substantially free of oxygen” as used herein means a composition that contains no oxygen that was deliberately added to the mixture. Figure 1

An embodiment will now be described with reference to Figure 1 , which shows a side view of a cross section of part of a shell and tube heat exchanger.

A plurality of tubes 1 is held in place by tube sheet 2. The shell 5 defines a confined cylindrical space in which cooling water flows across the tubes 1. The cooling water enters the shell 5 through inlet 30 and exits the shell through outlet 31. Process fluid enters the heat exchanger through inlet 40 and flows through the tubes 1 in the direction indicated by arrows 41 . A hemispherical dome 42 contains the process fluid at the entrance to the heat exchanger. It will be apparent from Figure 1 that the process fluid is in contact with: a) the interior surface 42a of the hemispherical dome; b) the front surface 2a of the tube sheet and c) the interior of the tubes 1 which carry the flow of the process fluid. For convenience, these surfaces may be collectively referred to as being on the process fluid side.

In an embodiment, the process fluid is a corrosive vapor from a solution polymerization process. The vapor contains vaporized solvent; unreacted monomers (for example ethylene) and chlorides. The vapor is substantially free of oxygen and hence may be referred to as“reducing”. In this embodiment, the surfaces of the heat exchanger that are in contact with the process fluid are fabricated from a high Ni alloy, for example INCONEL 625 and/or HASTELLOY C-276 - i.e. the dome 42 (or, at a minimum, the interior surface 42a of the dome); the tube sheet 2 (or, at a minimum, the front surface 2a of the tube sheet) and the tubes 1 are made from high Ni alloy. In an embodiment, the shell 5 may be fabricated from carbon steel.

Figure 1 describes a non limiting embodiment. It will be recognized by those skilled in the art that many different types of shell and tube exchangers are potentially suitable for use in the process of this disclosure, provided that the surfaces of the exchanger that are in contact with the process fluid are fabricated from the specified high Ni alloys.

As previously noted, Figure 1 shows part of a heat exchanger and the line 50 confirms this - i.e. a description of details of the heat exchanger to the right of line 50 is not essential to provide a description of the invention because many different designs are suitable - with the proviso that the materials of construction on the process fluid side are high Ni alloys.

EXAMPLES

Example 1 (Comparative)

This example describes the use of a shell and tube heat exchanger that is fabricated from ordinary carbon steel as a condenser in the recovery section of a solution polymerization process.

The process fluid is a hot vapor that contains vaporized solvent (a mixed aliphatic solvent); and unreacted monomers (for example ethylene). The process fluids also contain chlorides which are present because of residual catalyst - i.e. the catalyst included one or more metal chlorides. The process fluid was directed through the tube side of the heat exchanger. Cooling water was used on the shell side to condense the process fluid, thereby causing some liquid flow through the tube side. This type of comparative heat exchanger was subject to severe corrosion. The useful service life of this type of heat exchanger was observed to be approximately 2 years.

Example 2 (Comparative)

The process conditions in this example are essentially the same as the process conditions of example 1 - i.e. in both examples, the process fluid is a hot vapor in the recovery section of a solution polymerization process; the process fluid contains metal chlorides; the process fluid is directed through the tube side of the exchanger and is condensed with cooling water.

In this example, the heat exchanger is fabricated from duplex stainless steel. Based on comparative corrosion resistance (i.e. it is well known that duplex stainless steel is generally more corrosion resistant than carbon steel), it was expected that the useful service life of this heat exchanger would be about 10 years.

The duplex stainless steel heat exchanger failed within 6 months. As noted above, the process conditions were essentially the same as those of Example 1. It is noted that the level of catalyst residues could have been marginally different between Example 1 and Example 2 because the products being produced were not exactly the same at all times, however, it is believed that this difference is not important and did not cause the failure of the duplex stainless steel heat exchanger after only 6 months.

Example 3 (Laboratory Experiments)

The stainless steel heat exchanger of Example 2 failed in an unusual manner: the tubes became heavily fouled when in use, leading to unacceptable pressure drops across the tube bundle. The foulant was analyzed and found to contain a combination of about 60% metals (Cr-Ni-Fe-Mo, consistent with stainless steel) and 40% organics. The foulant can also cause fouling/plugging of downstream process equipment.

A series of corrosion experiments was then completed using three different types of corrosion inducing solutions as shown in Table 2.

Solution 1 is referred to as“reducing only”;

Solution 2 is“reducing and high metal chloride”;

Solution 3 is“reducing and low metal chloride”. These solutions were used to test the corrosion resistance of the alloys. The tests were conducted using immersion test coupons that were prepared in accordance with ASTM G48 (dimensions = 2.5 x 5.0 x 0.2 cm) and were then ground polished in accordance with ASTM G1 -03.

The immersion tests were conducted in the corrosive liquids at 80°C. The tests were conducted under a nitrogen purge to exclude oxygen.

Test results are provided in Table 2. The numerical values are expressed in millimeters (of loss) per year (mmy). The“NA” value for carbon steel indicates that it dissolved very quickly (about one hour) and hence was excluded from further study. Similarly, the NA values for duplex stainless steel indicate that it failed very quickly (it was dissolved after about 14 hours) in solutions 2 and 3.

These laboratory tests suggest that INCONEL 625 and HASTELLOY C-276 should provide superior corrosion resistance in a process environment that is both reducing and oxidizing contains metal chloride.

While not wishing to be bound by theory, it is believed that the presence of oxygen can improve the corrosion resistance of duplex stainless steel by a mechanism that causes a thin layer of chromium and/or nickel oxides to form on the surface. In the absence of oxygen, this protective layer of Cr and/or Ni oxides does not form.

TABLE 1

Corrosion Rate Values (mmy) for Carbon Steel, DSS 2507,

INCONEL 625 and HASTELLOY C-276

Example 4

A heat exchanger having tubes made from duplex stainless steel, a tube sheet cover made from INCONEL 625 and demister made of HASTELLOY C-276 was used under process conditions that were substantially the same as those described in Example 1. The tube sheet and demister were observed to be corrosion resistant although the stainless steel tubes were subject to fouling. This observation is consistent with the results shown in Example 3.

INDUSTRIAL APPLICABILITY

The service life of a heat exchanger is improved through the use of selected materials of construction.