A PROCESS OF ADSORPTION

Abstract: A process of adsorption wherein an adsorbate compris­ ing an azeotrope of water and an organic compound having a positive deviation from Raoult's Law, e.g. n-propanol and water, is adsorbed by an adsorbent comprising a layer of material which preferentially adsorbs water e.g. Y-type zeolite molecular sieve, and an adjacent layer of material which preferentially adsorbs organic compounds without substantiail dehydration e.g. activated charcoal, and wherein heat of adsorption is evolved.

Description: A process of adsorption This invention relates to a process of adsorption wherein an adsorbate is adsorbed by an adsorbent. It has been found that in absorption processes where methanol is used as absorbate and aqueous zinc bromide is used as absorbent, after a.time the zinc bromide will decompose the methanol by extracting water thereby converting it to dimethyl ether. There is a strong possibility that the same type of decomposition will occur if the azeotrope of n-propanol and water is adsorbed by a zeolite molecular sieve. According to this invention such difficulties are overcome by the use of an adsorption process wherein an adsorbate comprising an azeotrope of water and an organic compound,said azeotrope having a positive deviation from Raoult's Law, is adsorbed by an adsorbent comprising a layer of material which preferentially adsorbs water and an adjacent layer of material which preferentially adsorbs organic compounds without substantial dehydration and wherein heat of adsorption is evolved. when using this process most of the water will be adsorbed by the layer of material preferentially adsorbing water although some of the water will be retained by the other layer. In this manner''the organic compound will be prevented from being adsorbed by the layer of material which preferentially adsorbs water provided sufficient of the material preferentially adsorbing organic compound is present and any tendency towards decomposition will be avoided. Various organic compounds can be used as the component of tlie azeotrope provided the azeotrope has a positive deviation from Raoult's Law, i.e. it displays a minimum boiling point relative to the surrounding mixture compositions. Although homogeneous binary azeotropes are preferred it is possible to use a heterogeneous binary azeotrope. Azeotropes of water and an organic compound having a relatively high minimum boiling point e.g. above 70'C at 760 mm Hg and a relatively high content of water e.g. above 40 mole % are preferred. Thus the organic compound can be certain alcohols, e.g. those having 3 to 6 carbon atoms per molecule, ethers, carboxylic acids, ketones or hydrocarbons for example. The suitable water-containing azeotropes in approximate increasing order of preference are Heterogeneous azeotropes Mole % of Minimum b.p Second Component second component Temp 'C (760 mm Hg) Ethyl ether 95.00 34.2 Ethyl acetate 76.00 70.4 Benzene 70.40 69.3 Toluene 44.40 84.1 Tert Amyl alcohol 35.00 87.0 n Butyl alochol 25.00 92.3 iso Amyl alcohol 17.21 95.2 Homogeneous azeotropes Mole fi of Minimum b.p. Second Component second component Temp 'C (760 mm Hg) Ethanol ' 89.43 78.2 Isopropyl alcohol 68.54 80.4 Methyl ethyl ketone 67.00 73.5 Tert Butyl alcohol 64.59 79.9 Allyl alcohol 45.50 88.2 n Propyl alcohol 43.17 87.7 sec Butyl alcohol 34.00 88.5 iso Butyl alcohol 32.86 89.9 iso Butyric acid 5.50 99.3 Propionic acid 5.30 100.0 An adsorption process using an azeotrope of water and n-propanol is described and claimed in our U.K. patent application 8115312. The azeotrope has about 28 to 32 weight percent of water according to the temperature. The material which preferentially adsorbs water hereinafter referred to as material A, can be a zeolite molecular sieve, silica gel or a salt capable of hydration such as sodium sulphate, copper sulphate, lithium nitrate, magnesium chloride, magnesium nitrate or calcium chloride. The preferred material is A or Y type zeolite molecular sieve. The material which preferentially adsorbs organic compounds without substantial dehydration hereinafter referred to as material B can be for example activated charcoal. It could also be silica gel or an A or Y-type zeolite molecular sieve. It is to be understood that materials A and B must be different substances.so that silica gel for example'cannot be used for both layers. Usually both materials A and B will be used in powder or particulate form. It is essential that when the azeotrope contacts the two layers of materials A and B it first contacts the layer of material B i.e. layer B is superimposed on layer A if the contacting is from above. When the adsorbate is adsorbed by the adsorbent, heat of adsorption will be evolved and this heat can be transferred elsewhere, for example to another part of a heat pump or for delivery to the load. This transfer is usually carried out by heat exchange, by the use for example of a heat exchanger, the coils of which are housed inside the adsorber and which are surrounded by adsorbent. After adsorption the azeotrope will have to be desorbed from the adsorbent if the process is to be cyclic. This is usually achieved by insertion of a heat exchanger in the adsorbent housed in the adsorber. Alternatively an electrical resistive heater can be located in the adsorbent housed in the adsorber. The azeotrope of -n-propanol and water has a rather large positive deviation from Raoult's Law and this means that after the n-propanol has been removed by passage through material B the water will be subject to the full pressure of the system i.e. at a pressure higher than the partial pressure it would otherwise display. This means that the heat pump will work at high efficiency and in fact it should be possible to pump heat from -5'C to 70'C at a coefficient of performance (C.O.P) approaching 1.5. Very often systems employing an adsorber also make use of a heat of fusion heat store. In such systems the adsorbate vapour which is desorbed from the adsorber is usually condensed in the heat of fusion heat store. In such heat stores there are a plurality of receptacles housing a substance with a relatively low melting point and a relatively high heat of fusion, for example stearic acid. The heat of condensation of the adsorbate as it condenses in the heat of fusion heat store is transmitted to this relatively low melting substance which then melts. In this way heat is stored in this substance housed in the receptacles. When heat is abstracted from this heat store the substance freezes, evolving latent heat of fusion. It is possible by the use of the process of this invention to operate an alternative heat of fusion heat store at low temperatures i.e. below O'C and water can be used as the substance of relatively high heat of fusion which is of course very cheap. Alternatively phenol could be used as this high heat of fusion substance. Processes and systems involving the adsorption of an adsorbate by an adsorbent and where an n-propanol-azeotrope can be used as the adsorbate include those claimed in our prior U.K. patent applications 8038283, 8039157 and 8111392. U.K. Application 8038283 claims a heating system wherein solar energy is used to supply heat to a heat of fusion heat store, adsorbate is stripped from adsorbent in an adsorber and thereafter condensed, the heat of condensation being delivered to the load, condensed adsorbate is sent to an evaporator, adsorbate is vaporised in the evaporator and vaporised adsorbate is adsorbed in the adsorber, the heat of adsorption being delivered to the load. U.K. Application 8039157 claims a combined adsorption and absorption process in which an external source of heat is used to desorb adsorbate from adsorbent housed in an adsorber. The adsorbate vapour is condensed and the latent heat of condensation used to remove absorbate from the working fluid in the generator of an absorption system. U.K.Application 8111392 claims a method of obtaining heat wherein adsorbate is evaporated from an evaporator and thereafter adsorbed by an adsorbent in an adsorber, the latent heat of adsorption providing heat to the load and thereafter the adsorbate is desorbed from the adsorbent and desorbed adsorbate condensed in a heat of fusion heat store and the substance housed therein melting and thereafter solidifying, the latent heat of fusion providing heat to the load. Example An adsorber was provided with a series of trays, each tray containing as adsorbent a bottom layer of powdered Y-type molecular sieve and an upper layer of activated charcoal. Vaporised azeotrope of n-propanol and water was passed over the trays in the adsorber where it was adsorbed and it was found that the Y-type molecular sieve preferentially adsorbed nearly all the water component of the azeotrope whilst the activated charcoal adsorbed substantially all of the n-propanol component of the azeotrope. Heat of adsorption was evolved and this was transferred to a domestic hot water system by means of a heat exchanger through which heat transfer fluid circulated and the coils of which passed through the adsorbent in the adsorber. The system used was cyclic and when adsorption was complete the azeotrope was desorbed by means of heat supplied to the adsorbent from the coils of a resistive heater. The desorbed azeotrope was allowed to condense in a condenser giving up its heat of condensation. Thereafter the condensed azeotrope was evaporated by--heat exchange and the vaporised azeotrope once more passed to the adsorber where it was adsorbed giving off heat of adsorption.

Claims: Claims 1. A process of adsorption wherein an adsorbate comprising an azeotrope of water and an organic compound,said azeotrope having a positive deviation from Raoult's Law, is adsorbed by an adsorbent comprising a layer of material which preferentially adsorbs water and an adjacent layer of material which preferentially adsorbs organic compounds without substantial dehydration and wherein heat of adsorption is evolved. 2. A process according to claim 1 wherein azeotrope is a homogeneous binary azeotrope. 3. A process according to either of claims 1 and 2 wherein the azeotrope has more than 40 mole Z water and a minimum boiling point of above 70'C at 760 mm Hg. 4. A process according to claim 3 wherein the azeotrope is an azeotrope of water and n-propanol. 5. A process according to any one of the preceding claims wherein the material which preferentially adsorbs water is P-type zeolite molecular sieve. 6. A process according to any one of the preceding claims wherein the material which preferentially adsorbs organic compounds without substantial dehydration is activated charcoal. 7. A process according to any one of the preceding claims which is cyclic and wherein after adsorption the azeotrope is desorbed from the adsorbent. 8. A process according to any one of the preceding claims wherein after adsorption adsorbate vapour is desorbed from the adsorbent and condensed in a heat of fusion heat store.

Inventors: MOSS, GERALD
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1983
Main classification: C09K5
IPC codes: C09K5