US2119091A - Process and apparatus for indirect heat transfer between two liquid materials - Google Patents

Description

May 31, 1938. L. A INSON AL 2,119,091

PROCESS APPAR S FOR IND ECT HEAT TRANSFER WEEN TWO LIQUI ATERIALS d Nov. 29. 193

- [ax/44A 2 y Men/uh FOT MED/UM EXCHANGE!- Patented May 31, 1938 UNITED STATE a'uaosi PROCESS AND APPARATUS ron 1 HEAT TRANSFER BETWEEN 'rwo mourn TERIALS ration of Delaware Application November 29, 1935, Serial No. 52,204

3 Claims.

This invention relates to a method and means for transferring heat from one fluid to another,

and has for its particular object the provision of a system in which heat can be transferred from one medium to another when both mediums have physical properties such that under the operating conditions they tend to deposit solid matter on the heat transfer surfaces whereby it is necessary that both mediums be flowed thru tubes which can be easily cleaned. In the appended claims both of these types of materials will be referred to as materials which tend to deposit solid matter since such action usually accompanies decomposition.

A typical example of a heat exchange system to which the present invention is applicable is one in which heat is transferred from cracking coil tar, which is at a hightemperature, to a reduced crude which is at a lower temperature. Both of these materials contain a considerable quantity of dirty materials and tend to deposit sediment more or less readily, which tendency is apparently enhanced by the presence of the dirt which may contain minerals which accelerate coking. Attempts to transfer heat from one of these mediums to another in a conventional tube and shell heat exchanger have led to grave operating difficulties due to plugging up of the shell side of the tube bundle which have either discouraged attempts to'transfer the heat from the hot medium to the cool'medium or have necessitated the employment of special equipment which ordinarily involved considerable expense.

According to the present invention both mediums of the character described above are passed through straight, easily cleanable tubes, and heat is transferred from the one to the other through the agency of an auxiliary medium. One of the most important advantages arising from the present invention is that it can be carried out by: the employment of conventional tube and shell heat exchangers connected in such a manner as topermit the flow of the auxiliary medium over the heat transfer surfaces. Another important advantage of this system is that by simple manipulation of the paths of flow either or both mediums can be passed through the heat transizable at the temperature of the heating medium.

When such an auxiliary medium is employed it is used in a quantity suflicicnt to cover the heating tubes but insuflicient to contact, in the liquid state, the tubes which bear the medium to be heated. The customary arrangement with such an auxiliary medium consists of two horizontal tube and shell heat exchangers in superimposed relation, the auxiliary'medium. covering the tubes in the lower shell only, the heating medium being passed through the tubes in the lower unit and the medium to be heated being passed through the tubes in the upper unit- The system; in which heat is transferred by vaporization and condensation of the auxiliary medium, has several advantages over a system in which the heat transfer zone is completely filled with liquid. Chief among these advantages is the fact that a much better rate of heat transfer is possible when the auxiliary medium con-. tacts the tubes to be heated in the vapor phase and condensesthereon than is obtainable when the heating medium contacts the'tubes to be heated in the'liquid phase thus reducing the area of heat transfer surface required. In the latter case, where circulation of the heating medium is efiected purely by temperature differential, the circulation isusually too sluggish to effectuate any rapid transfer of heat. To increase the rate and thus reduce the surface required it is necessary to resort to the employment of circulating pumps to impart higher velocities to the transfer medium. This adds considerably to the cost and operation of the unit. Furthermore,

when the auxiliary heating medium is maintained in the liquid phase it only transfers sensible heat to the material to be heated, whereas when it is vaporized it transmits to the material to be heated both sensible heat and latent heat of vaporization.

Various materials may be employed as the auxiliary medium, the only requirements being that it be chemically heat-stable and vaporizable within the temperature range under which it is desired to operate, that it be in, the liquid state at the lowest operating temperature, and that it have a suiiiciently high boiling point to permit .the operation of the unit at a practicable pressure. In a system in which the heating medium enters at a temperature of about GOO-800 F., diphenyl has proven very satisfactory as an auxiliary medium. The medium selected for any given unit, of course, must boil at a temperature below that at which the heating medium enters the unit.

When the present invention is practiced with an auxiliary medium which is maintained in the liquid state throughoutthe heat exchange operation, there is no limitation upon the relative positions of the heat input chamber and the heat absorption chamber except when the circulation of the auxiliary medium is natural and dependent upon gravity, in which case it is oviously necessary that the heat absorption chamber be at a higher level than the heat input chamber.

The preferred embodiment of the present invention is illustrated in vertical longitudinal section in the accompanying drawing in which I and 2 are conventional tube and shell heat exchangers joined together by enlarged conduits 3 which are Jointed together at 4. In each of units l and 2, horizontal parallel tubes 5 are mounted with their ends secured in tube plates 6 and 1. Covering the ends of the tubes in plate 6 is a manifold 8 having on one side an inlet 9 and on the other side an outlet l0, and having a partition II mounted between the inlet and outlet points to divide its interior into two compartments, each of which has the same number of tube openings. Mounted over each plate 1 and covering the ends of the tubes secured therein is a manifold l2 providing intercommunication between the ends of the tubes. As is apparent, by employing additional partitions in heads 8 and 12 thus making suitable connections between the ends of the tubes mounted in plates 6 and I, the medium passing through the tubes may be caused to travel the length of the shell several times before it leaves the unit, thus controlling the veloc- I ity of flow in the tubes.

In the arrangement shown on the drawing, the auxiliary medium completely fills the lower shell. In order to avoid entrainment of auxiliary liquid with the vapors evolved therefrom, a floating perforated plate l3 can be arranged in the passage formed by conduits 3. The heating medium passes through the tubes of the lower unit and the medium to be heated passes through the tubes of the upper unit. For example a cracking coil tar entering the lower unit at a temperature of 780 F. and at a rate of 72,000#/hr., di-phenyl being the auxiliary medium, will leave the lower unit at a temperature of 640 F. and will heat a reduced crude entering the upper unit at a temperature of 470 F. and at a rate of 254,000#/hr. to a temperature of 505 F.

While the present invention has been described with particular reference to the exchange of heat between dirty, readily cokable oils, it is apparent that it is of general appliaction. It is a fact, however, that the advantages attending the present invention become most significant when both the heating'medium and the medium to be heated are of a nature to either deposit sediment, or dement thereof given, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. A process for indirect heat transfer between two liquid materials each of which tends to form carbonaceous deposits when heated comprising passing the hot liquid material through a series of narrow elongated passageways in heat exchange with a body of heat stable liquid in a shell, vaporizing said last mentioned liquid and conducting the vapors substantially freed of entrained liquid, into a second shell, passing the relatively cool liquid material through a series of narrow elongated passageways in said shell in heat exchange with said vapors, whereby they are condensed on said passageways, and returning the condensate to said liquid body.

2. A process according to claim 1 in which the liquids tending to form carbonaceous deposits when heated between which an indirect transfer of heat is accomplished are a hot cracking coil tar and a relatively cool reduced crude oil.

3. Apparatus for the indirect transfer of heat between two liquid materials tending to form carbonaceous deposits when heated by means of an auxiliary liquid heat exchange medium, comprising, in combination, a pair of heat exchanger shells disposed one above the other, an upwardly extending conduit of enlarged cross section on the lower shell and a downwardly extending conduit on the upper shell, which conduits are joined to form a passageway between them, a plurality of interconnected elongated passageways for a relatively cool liquid material disposed within said upper shell and a plurality of interconnected elongated passageways fora hot liquid material disposed within said lower shell, the passageways within each shell consisting of a plurality of manifold connected tubes in which the outer manifold of the bundle forms a closure for the shell within which it is contained, and provides a series of inlet and a series of outlet tubes in each bundle, and a body of a heat stable liquid heat exchange medium vaporizable at the temperature of the hot liquid material in said tube bundle contained in the lower shell and entirely covering the tube bundle therein.

VERNON L. ATKINSON. CLARENCE W. FOSTER.

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