US2028432A - Combined rectification and refrigeration process - Google Patents

Description

P. D. BARTON Jan. 21, 1936.

COMBINED RECTIFICATION AND REFRIGERATION PROCESS v Filed March 19, 1954 ATTORNE i Patented` Jan. 2l, 1936 UNITED STATES PATENT DFI-lcs COMBINED nEc'rlFlcA'rIoN AND BEFBIGERATION raocnss poration of Delaware Application March 19, 1934, Serial No. '116,281

5 Claims.

My invention relates to a combined rectification and refrigeration process and more particularly to a process for utilizing by-product liquid ethane-propane hydrocarbon mixtures from recovery and stabilization plants for refrigeration and particularly the chilling of hydrocarbon oils undergoing treatment at sub-atmospheric temperatures while submitting the fluid hydrocarbons to rectification.

The by-product liquid which is obtained from recovery and stabilization plants consists largely of a mixture of ethane and propane together with other hydrocarbons. This'by-product liquid material is of little commercial use unless the highly volatile ethane is separated by rectification from the less volatile propane. The propane is usually recovered from the rectification system and transferred to steel containers for shipping. Commonly this rectification is carried out at a relatively high pressure in order that the normally gaseous ethane may be condensed with water as the cooling medium. The condensation of ethane is necessary in order to obtain reiiux liquid for the rectification. The factors which control the condensation of hydrocarbon vapors are temperature and pressure. If the rectification operation is carried on at a lower temperature, the pressure Will be shifted downwardly.

In general, my invention contemplates rectifying at low temperatures and at comparatively low pressure in such a manner that the heat of reboiling necessary for rectication is supplied from material to be refrigerated and is dissipated as latent heat of the overhead or exit fluids from the rectifying system.

It happens that the latent heat of ethane compares very favorably with that of both propane and ammonia at the temperature range which would be employed for obtaining refrigeration temperatures below 0 F. `As an illustration, -at 40 F. and assuming that for purposes of comparison, the evolved refrigerant vapor must be recompressed and condensed at F. the following table will represent the B. t. u. per mol, dlvided by the number of Icompressions through the compression range.

Ammonia '741 Ethane--- 954 Propane 977 Due to the fact that the proportion of ethanel (Cl. (i2-,4755) ethane in the by-product liquid is due to the-fact that the exit vapor from a stabilization unit, which is the source of the by-product liquid, is relatively rich in ethane. As the stabilization pressure is increased, the by-product liquid obtained therefrom becomes richer in ethane.

When a material such as the by-product liquid just described is allowed to boil freely at reduced pressure, (for example when a sample of the liquid is drawn into an open Dewar flask) the temperature rapidly falls to the average boilingtemperature of the mixture at that pressure. I have observed that the same composition' liquid when expanded across a throttle valve to the same reduced pressure produces a. temperature far lower than the average boiling temperature of the mixture and approaches the boiling point of the lighter component. For example, a mixture consisting mainly of ethane, propane, and butane, the proportion of propane being large, was observed to produce temperatures as follows:

a. Temperature of a freely boilingbody of liquid at atmospheric pressure 16 F. b. Temperature in outlet of stream from throttle valve at atmospheric pressure 50 F.

I consider the discovery of this elect an important feature of this invention and the process as described is designed to take advantage of this effect. This effect will be hereafter referred to as preferential vaporization.

The accompanying drawing illustrates an apparatus capable of carrying out my process.

Referring now to the drawing, refinery gases from any suitable source are collected and enter the pressure gas line I and pass through line 2 to the absorber 3. ,f Low pressure gases enter through the vacuum gas system. In the absorber, a menstruum enters through line 4 and passes in contact with the renery gases. The fat oil is withdrawn through'line .5 and passes through heat exchanger 6, through line 1, into the still 8 in which heat supplied through coil 9 denudes the menstruum, which is pumped by pump I0 through line II, through exchanger 6, through cooler I2, through line 4 for recycling. AThe vapors from still 8 are condensed in condenser I4 and pass through line I5 to the accumulator I6 from whence they are pumped by pump I1 through line I8 to the stabilizer I9. The vapors from the stabilizer I9 pass through line 20, through condenser 2I, through linek 22, into separator 23. Separator 23 serves as an accumulator for reflux liquid which is returned to the tower by pump 24.

The gases and lighter hydrocarbons pass overhead through line 25, through condenser 26 into separator 21. It is to be understood that the stabilization system is under a relatively high pressure which may be between 200 and 300 pounds gauge under normal conditions. It is to be understood further that any suitable pressure may be supplied' to the by-product liquid collected from the stabilizer system. This liquid consists 0f a mixture of hydrocarbons which are normally gaseous and chiefly of ethane and propane.. The by-product liquid from separator 21 passes through line 28 into surge tank or accumulator 29. From the surge tank 29, the by-product4 liquid, which is still under high pressure as above indicated, passes through line 3U, through pump 3l, through heat exchanger 32, ,through line 33, through expansion Valve 34, into the shell of reflux condenser 35. The reduction of pressure on this liquid will produce a great refrigerating effect through preferential vaporizaton. The system is so .operated that the temperature existing within shell 35 is about -60 F. The reduction of pressure on the liquid by-products is to within a few pounds of the tower pressure, only enough pressure being maintained to insure flow into the tower. The vaporized and unvapcrized liquids will automatically separate in the. condenser shell. The vaporous fluids will leave the condenser shell through line 36 and pass through pressure controller 31, through line 38, into the rectiiication tower 40. The liquid component of the by-product fluid will pass from condenser shell 35 through line 39 in which control valve 4I is interposed. The liquid may be introduced into the rectification tower at a point above the introduction of the yvaporous component so that the vapors will rise countercurrent to the liquid in the rectication tower, thus taking the fullest advantage of the partial pressure effect of the lighter fraction on the boiling point of the heavier. The pressure in the tower is held at a lower point than that of the condenser shell and will be determined by temperaturedesired at the bottom of the tower. The pressure can be easily controlled by means of pressure control valve 42, which is interposed in line 43 leading from the rectication tower. At the bottom of the tower 40, I provide a thermocouple 44 which is connected by conductors 45 to a pressure control device 46 which, through line 41, controls valve 42. It is to be understood that the possible refrigeration temperature obtainable by my system is controlled by two factors. Since, in any rectifying system, material on any plate is actually at its boiling point, the temperature on any plate is controlled by the pressure of the system. Assuming, then, that the rectifying column 40 is run at atmospheric pressure and that the ma-l terial at the bottom of the tower is pure propane, the temperature at that point will be 44 F. If the temperature become higher, the thermocouple 44 would react to operate control device 46, to open valve 42, thus reducing the pressure in the tower. With a drop in pressure, the temperature will drop. Should the temperature become lower than 44 F. the operation of pressure control device will tend to close valve 42 which will increase the pressure within the tower, which will tend to raise -the temperature existing therein.

The rectied propane collecting in the bottom of tower 40 is withdrawn through line 48 and pumped by pump 49 through coil 50 in tank 5|. If it is desired, for example, to refrigerate a hydrocarbon oillto undergo'l treatment at subatmospheric temperatures, the oil in tank 52 will be pumped by pump 53 through line 54, through heat exchanger 51, through line 58, into tank 5I. A bleeder line 60, controlled by valve 6I, admits propane to tank 5l to act as a diluent. The chilled, diluted oil is withdrawn from tank 5I through valve controlled line 62. The heat exchanger imparts heat to the propane which. was withdrawn from the rectication tower. The heated propane leaves heat exchanger 51 and passes through line 10. A portion of the propane passes through line 1| which is controlled by valve 12, through expansion valve 13 into the tower in order to supply the heat of reboiling. The remaining rectified propane passes through line 14, which is controlled by valve 15, and valve 16, through heat exchanger 32, through line 11. from which it is stored in steel cylinders for shipping. Valve 16 is controlled by the liquid level device 18.

As was pointed out above, the proportion of ethane is controllable. It is believed that this fact enables the process to be conducted with great flexibility. In operation, practically all the ethane, together with a considerable proportion of propane, leaves the rectiiication tower at substantially atmospheric pressure and -60 F. In order to vaporize these products. heat must be' supplied for reboiling. If the heat -were supplied in the usual method by means of steam, a loss of propane to fuel wouldbe occasioned, along with rejected ethane. As the proportion of ethane is increased, there is a corresponding drop in the top temperature of the column. This results in the knocking down 4of an increased amount of reux and the proportion of propane in the exit vapors decreases. It will be observed that, combining this process with a stabilization process, the ethane leaving through line 43 and passing through line 80, is recycled to the absorption system. In this way, we have an added factor giving flexibility of control. It will be observed, therefore. that the two factors which control refrigeration temperature, namely, the pressure of the system and the condensation of the material, are both under the control of the operator. It will be observed that the sensible heat picked up by the cold circulating fluid in the refrigeration exchangers not tend to increase the temperature of the liquid at the bottom ot the tower when it is returned thereto. This is true because the liquid was at its boiling point at the lower temperature and, as soon as the pressure on the circulating liquid is released, there is immediate evaporation or flashing of a portion of the returning circulated liquid. This results in a drop in the temperature of the unvaporized portion, bringing it back to the temperature before it left the tower.

It willr be seen that I have accomplished the objects of my invention. I am enabled to rectify the by-product iiuid from refinery operations at a low pressure in a relatively inexpensive unit. I save the heat load which would be normally required for reboiling and supply this heat load by refrigerating material undergoing low temperature treatment. I am thus enabled to obtain a refrigerating eiect at the desired temperature with a negligible expenditure of energy.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope oi' my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. A process for rectifying normally gaseous hydrocarbons obtained from refinery operations including the steps of expanding compressed hydrocarbon fluids in a reiiux condensing zone, introducing the fluid from said expansion step into a rectiiication zone, withdrawing a selected fraction from said rectification zone, supplying heat to the withdrawn fraction from a material being refrigerated and reintroducing a portion of the thus heated hydrocarbon fraction into the rectification zone.

2. A combined process of rectifying normally gaseous hydrocarbon mixtures and chilling a material being refrigerated which includes the steps of rectifying a mixture of hydrocarbon gases at reduced temperatures, withdrawing a selectedv fraction from the rectification zone, transferring heat from the material to be refrigerated to the fraction so withdrawn and rein-v troducing a portion of the fraction so heated into the rectication zone to supply the heat of reboiling.

3. In a rectification process in which hydrocarbon iluids are rectied by fractional condensation and reliux condensate is supplied by condensing exit vapors of the rectification zone, the steps comprising expanding hydrocarbon fluids in heat exchange with the exit vapors, introducing the fluids after said expansion step into the 5 rectication zone, passing condensed vfluids countercurrent to uncondensed lluids in the rectification zone while controlling the pressure Within said zone in response to the temperature existing in said zone. 10

4. A process as in claim 3 in which liuids of said expansion step comprise liquid and gaseous components, including the additional steps of introducing the liquid component into the rectification zone at a point above that at which the gase- 15 ous components are introduced, and separately introducing the gaseous components.

5. In a process for rectifying normally gaseous hydrocarbons, the steps of establishing predetermined conditions for selective condensation of 2o desired products comprising expanding compressed normally gaseous hydrocarbons to extract heat from eduction fluids of a rectification zone to form a reflux condensate, withdrawing a selected fraction from said zone and supplying 25 heat thereto from a material being refrigerated, returning at least a part of said fraction so heated to the rectiiication zone to supply the heat of reboiling and supplying heat to the remainder of said fraction from said compressed hydrocar- 30 bons to be expanded.

PAUL D. BARTON.

Images