US2478863A - Heat exchange - Google Patents

Description

Aug.9,1949; 5 H. R AWS 2,478,863

x v-VLHEAT EXGHANGE.V

Filed Jan. 2, 1948 IN VEN TOR.

Patented Aug. 9, 1949 HEAT EXCHANGE Hyman n. Davis, Jackson Heights, N. Y., assignmto The Lummus Company, New York, N. Y., a

corporation of Delaware Application January 2, 1948, Serial No. 64

This invention relates to heat exchange and more particularly to sub-cooling of multiple phase mixtures that has an inherently low heat transfer rate and to a temperature below the solidication point of one of the components.

Fractional crystallization of mixtures for the separation of the respective components having different freezing points is accomplished by subcooling of the mixture. Solvents which would aid in the dilution of one of the components while tending to throw out the other component from the solution areusually 'added to reduce viscosity. The mixtures are then sub-cooled to such an extent that the higher melting point material congeals and can be separated by filtration, settling or centrifuging, etc. and this frequently requires 4 Claims. (Ci. ia-114) minimum temperatures as low as 25 F. An

example of this operation is in the separation of various saturated and unsaturated components of fatty acids and another is in the separation of Wax from oil as in the production of lubricating oil.

There are several serious difficulties which result during the sub-cooling of viscous mixtures of this type. Not only is the heat exchange rate extremely low when the mixtures are moved slowly through the sub-cooler, but the wax or cake tends to congeal out on the heat exchange surface and further reduces the heat transfer rate. Heat transfer rates as low as 5 to 8 B. t. u.s per hr. per sq. ft. per F. temperature difference are common and substantial superatmos`pheric pressures are usually required.

One of the principal objects of this invention is to provide an improved sub-,cooling method and apparatus which will be economical.

More specifically, it is an object of the invention to provide a very rapid chilling step. for a multiple phase mixture in which the mixture repeatedly moves through an improved heat exchange cycle, such cycle materially reducing the heat exchange surface and more elciently cooling the mixture.

A further object `of the invention is to provide an improved sub-cooler for a multiple phase mixture, one of the components of which solidies at the sub-cooled temperature, such sub-cooler including a fixed heat exchange surface and a high capacity recycle pump establishing a high velocity of the mixture in the sub-cooler.

Further objects and advantages of the invention will appear `from the following description of a preferred form of embodiment thereof taken in connection withthe attached drawing in which:

Fig. 1 is a schematic flow diagram of the important pieces of apparatus in 'accordance with my invention.-

Fig. 2 is a horizontal cross section on the line 2--2 of Fig. 1.

In accordance with a preferred form of embodiment of my invention, I provide a shell and tube type of heat exchange unit generally indicated at I0 having a shell I2, a suitable bottom header I4, and atop header I6. Continuous tubes I8 extend between these headers and form `one path of heat exchange cycle.

Preferably, the tubes which are conveniently of small diameter such as 3A in. or l in. O. D. are connected into a refrigerant system which is c ommonly of normally gaseous type such as propane or ammonia. In such a System, an accumulator drum 20 is provided with a refrigerant outlet 22 connected with the bottom header I4. A shut oif valve 24 may be provided in the line 22 if desired. The refrigerant will evaporate in the tubes I8 and will discharge through the upper header I6 and through the outlet pipe 26 to the accumulator y2|).l A refrigerant make-up line will be provided at 28 with a liquid level control at 29 and the vapors will be removed at 30 to a refrigerant compressor. This part of the cycle is of Well-known and characteristic construction.

If a waxy oil and solvent charge is to be cooled, for example, it may be reduced in temperature by independent heat exchange to low atmospheric temperatures and usually as low as 30 F. before it becomes so viscous as to hinder further cooling in conventional heat exchange equipment. At such time,y additional oil solvent may be added. The charge of oil-wax-solvent mixture is then introduced at 32 to cycle pump 36, the solution substantially filling the space between the tubes I8 within the shell I2.

The pump 36 is inter-connected at 39 with the lower part of the shell l2, such pump discharging into distributor pipe 40 which preferably extends vertically through the heat exchanger shell. This distributor pipe 40 may be provided with a series of differentially sized openings 42, the openings increasing towards the top of the closed end of the pipe for uniform discharge. The openings may be oriented to assure flow of material between tubes I8.

The pump 36 is preferably a high capacity low head type and is provided with suction line 38 also connected to a low part of the shell I2 so as to recycle the material within the shell I2. The recycle rate may be as much as 10 to 100 times the net discharge through the product outiet u. It win thus be apparent that the solution is repeatedly l,passed through the shell and moves rapidly across the tubes I8 with the result that it is possible to obtain heat exchange transfer rates from to 100 B. t. u./hr./sq. ft./F. temperature difference orhigher.

Not only is it possible under the circumstances to have from l/3 to 1/20 the customary surface in view of the materially greater heat transfer 4 rates, but it is not necessary to have the comavoiding the tendency of the material to congeal on the tubes, it is also possible to minimize the exchange tubes in a refrigerant circuit; a cham. ber surrounding said heat exchange tubes; a pump for removing material from said chamber; a perforated discharge element communicating with the outlet of said pump and extending into said chamber whereby the mixture may be repeatedly circulated across said tubes, said chamber' having a product outlet and pressure responsive means to control said outlet, said pressure responsive means being dependent upon the pressure within the chamber..

2. A heat exchanger for cooling a multiple phase mixture below the freezing point of one of the components which comprises a shell; a series of heat exchange tubes in parallel arrangement within 'the shell; a refrigerant circuit interconnected through the tubes and an external iluid "circuit for the multiple phase mixture located pressure drop through the unit which may be Y kept to as low as 410 to 15 pounds p. s. i. g. as cornpared to pressures of .'15f pounds or more now common. Advantagemay be also taken of the high velocity discharge through holes 42 to provide deep penetration of the charge between the tubes I 8 of the tube bundle.

A further advantage of a unit of this type is that the difference in temperature between refrigerant and mass need only be in the order of about 15`= F., it being possible to obtain temperatures as low as 10 F. by using propane or ammonia operating at a temperature of F. This also provides substantial economies in operation.

Temperature and pressure controls are suitably provided for the unit. The discharge line 34 for example, has pressure controlled valve 44 operated by regulator 46 from the interior pressure in the shell. This closely follows the rate of feed at 32 so that the valve 44 will allow more or less discharge based on fresh feed, the recycle rate being substantially constant.

The temperature may be controlled by regulator 48, interconnected to a temperature responsive element 49 and operating a suitable control through line 50 with refrigerant discharge outlet 30. l

It will thus'appear that I have provided an economical and positive apparatus for sub-cooling a fluid mass below the solidication temperature of one of the components. Not only is advantage taken of the high velocity heat transfer rate but deposition of the solids on the heat exchange walls is nearly entirely avoided.

While I -have shown and described a preferred form of embodiment of my invention, I am aware that modifications `may be made thereto and I therefore desire a broad interpretation of my in vention within the scope and spirit of the description herein and of the claims appended hereinafter.

1. Apparatus for the cooling of a multiple phase mixture below the freezing point of one of the components which comprises a series of heat within the shell surrounding the tubes. means to maintain a temperature within the tubes below the freezing temperature of the component of the mixture to be solidied,.said Vexternal fluid circuit including a pump, the intake oi' which is in terconnected with the shell and the outlet of which includes a perforate discharging pipe extended centrally within andvalong the heat exchange tubes, means to withdraw a part ofthe sub-cooled mixtureA and means to operate the pump at from lOvto 100 times the amount of material withdrawn. y

3. A process vfor the sub-cooling of a liquid hydrocarbon oil-wax-solvent mixture at a temperature which would normally be below the congealing-point ofthe wax which comprises passing such mixture under high velocity in indirect heat exchangewith anormally gaseous refrigerant4 and at a rate of recycle su-illcient to prevent substantial deposit of the material'on the heat exchange surface and undery small temperature diierential conditions whereby advantage, is obtained of a high heat transfer rate with a relatively small heat exchange surface and under conditions that there is a relatively low pressure drop, .and continuouslyV withdrawing the subcooled mixturein the ratio of not more than one tenth that of the circulating rate.

4. 'Ihe apparatus as claimed in claim 1 in K which the pump is a low head, high capacity type adapted to recirculate the material through the chamber at a rate not 'less than 10 times the amount which is withdrawn as sub-cooled product. r

v HYMAN a. Davis. vnnlrnmritms crrxn4 Thefoilowing references are of record in the iile of this patent:

UNITED STATES PATENTS

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