August, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
803
The Development of a Liquid-Phase Cracking Process' By S. J. M. Auld and A. E. Dunstan ANGLO-PERSIAN OIL Co., LTD.,LONDON, ENGLAND
T
This paper, dealing with the A. D. H. cracking were instituted t o determine HE history and techprocess, is devoted to a description of fundamental the pressure-solubility nology of c r a c k i n g principles, which were elucidated experimentally, and tions Of the permanent hyh e a 9 hydrocarbons upon which the design of the plant was based. A drocarbon gases and chosen for the production of light discussion is given of the problem of maintaining the heavy oils such as those genspirit is now so well known liquid phase, and the factors which operate for and erally used as cracking stock. that the discussion of a parOf process against this state are examined; while the actual mechThe apparatus d e si g n e d anism of cracking is also the subject of comment. and used for the purpose was for no further introduction Some experimellts were carried out to determine the a modification of that emthan its apologia. On the relationship between temperature, skin temperaployed bY h d r e w s for the other hand, discussion Of the ture, and furnace temperature, so as to design the measurement of the critical genesis and development, and heating arrangements of the plant to give the desired constants of gases (Figure 1). of the underlying principles, of procerses of commercial effect. . The reaction tube and the utilityis infrequent. I n most high- and low-pressure manc a s e s the patent literature ometer tubes were made of alone provides clues-frequently equivocal-as to the inven- transparent silica. The electrically heated air bath surtors' intentions and their empiricism or otherwise. A general rounding the former was fitted with a plate-glass window, account of the development of a liquid-phase cracking process and the volume changes were observed through this window from the time of its inception, over five years ago, may there- by means of a cathetometer. Temperature could be confore be of interest. trolled to within 1" C. a t 300' to 450" C. Effective disThe rapid supersession of vapor-phase by liquid-phase engagement of gas, on the one hand, and its dissolution in cracking processes during the last few years makes it difficult the heavy oil on the other, were facilitated by stirring with for those not directly interested in the subject to realize how a small steel ball contained in the tube and moved up and cogent was the decision to concentrate on the working out down in the liquid and vapor from the outside by means of a process to he conducted on oils maintained as far as of an electromagnet. possible in the liquid form. I n this connection it must be It was possible with this apparatus to observe actual understood that the rather confused terminology of the cracking taking place under pressures as high as 1000 pounds cracking industry allows no distinction between the modern per square inch. The temperature-solubility experiments liquid-phase processes and the more obviously dual-phase were carried out as nearly as possible under cracking conpressure-distillation operations. There being no intention ditions without causing more than the minimum of actual of developing a pressure-distillation process and plant, the decomposition. This was done by raising the temperature need for maintaining a high pressure on the whole system rapidly to the point chosen and quickly attaining equilibrium was apparent from the first. It was felt, however, and by bringing oil and gas into intimate contact through vigorquickly confirmed by the initial experiments. that the mainous agitation. tenance of a true liquid phase throughout would be a matter Another point affecting the range of pressures to be adopted of great difficulty. The pressures required to prevent sub- is the highly important question of the time of heating. From stantial evaporation of the stock a t cracking temperatures a perusal of successive published accounts of other liquidin the case of heavy distillates and residues are not excessive, phase cracking processes, it appears probable that the imbut the formation of more volatile products by thermal de- portance of the time factor, though now well known, was not composition immediately increases the pressure to an extent sufficiently realized in their early stages of development. which renders the condition less easy to fulfil. This also It was apparent to the present writers from the inception applies to the cracking of volatile oils and light distillates. of their experiments, however, that there was a very conI n fact. many of the hydrocarbons normally present in naphtha siderable time interval between the attainment of cracking and kerosene possess critical temperatures considerably temperature and the setting in of appreciable decomposition lower than the corresponding temperatures of thermal de- as evidenced by the formation of volatile products and carcomposition. Still more is this the case with the products bonaceous material. They were first led to an examination of decomposition, including, as they do, permanent gases of this point because of its bearing on plant design and conlike the lower members of the paraffin and olefin series. struction. The actual experiments were carried out in an autoclave. which was eventually so improved in design that Determination of Pressure Range pressures up to 1500 pounds per square inch could be used with temperatures of 400" to 500" C., and with a temperature It is clear, therefore, that the only factor likrly to assist variation over an indefinite time of only 0.25" C. the attainment of a strictly liquid-phase condition is the At this stage the investigation had taken the form of dereduction of vapor tension of the system consequutmt on the termining the pressure which would give the maximum con&dissolving of the lighter components in the mass of heavier tion of liquid phase, as limited by the practical considerations Oil. since no data appear to be on record regarding the vapor of operating an industrial process a t such pressure, and by tension of such solutions at high temperatures, experiments the benefit to be derived from it in efficiency of operation and nature of the products. Received March 23, 1926. Presented before the Division of PetroThere is little doubt that the higher the pressure used the leum Chemistry a t the 71st Meeting of the American Chemical Society, Tulsa, Okla , April 5 to 9, 1926. better is the nature of the light spirit formed and the smaller
INDUSTRIAL AND ENGINEERING CHEMISTRY
804
the amount of carbonaceous matter and uncondensable gas produced. Under pressure, molecular scission of the paraffinoid hydrocarbons appears to be directed towards the middle of the molecule, thus limiting the amount of gas and carbon formed. This advantage, however, is not directly proportional to the pressure and, although the application of very high pressures res u l t s i n good yields and products, the greatest effect is p r o d u c e d in the early stages of the application of
It can be realized, theref o r e , t h a t whatever the data collected on the points first enumerated, there are so many practical considerations to be taken into account, to which numerical values cannot readily be
Note - Oven is wound with 22 gage n i chrome wire for 100 volts
Figure I-Arrangement
of Andrews' Apparatus
given, that the final choice of operating pressure must be made on a general appreciation of the exigencies of the case. Summing up the situation from every point, pressures of the order of 25 to 30 atmospheres for gas oil and fuel oil and 30 to 35 atmospheres for kerosene seem to be optima, since these give most of the advantages of high pressure without unduly increasing costs and the difficulties of plant design and operation. For individual oils a h a 1 choice of pressure is made after exhaustive static experiments in the autoclave, and trial runs in a miniature continuous plant.2 The possibility of applying high pressures to the cracking system by means other than mechanical was not lost sight of during the investigations, and experiments were conducted
Vol. 18, No. 8
was so far superior to ordinary cracked spirit as to be almost indistinguishable from a straight-run gasoline. Even after three years samples of the gasoline remain water-white, of sweet smell, and without appreciable deposition of gum. So far, however, it has been impossible to maintain the activity of the charcoal owing to slight deposition of carbon within the pores, and after a short period of operation the resultant volatile products become similar to those ordinarily obtained by cracking. Carbon Formation
Possibly the most important point to be considered in developing a cracking process is the formation of carbon. The general experience throughout the history of cracking has been that the higher the temperature used the greater the amount of coke formed. This has led to the conclusion4 that the cause of carbon formation is overheating of the hydrocarbons under treatment, whereby molecules remaining too long in contact with the heated wall of the vessel undergo complete decomposition into permanent gas and carbon. This, however, is only a partial truth. Close examination of the various theories elaborated to explain the mechanism of the thermal reactions of the hydrocarbons indicates clearly that free carbon is almost certain to be formed, even from the decomposition of paraffi hydrocarbons a t comparatively low temperature. It is not necessary to assume an elaborate progressive series of reactions culminating in the formation of high carbon-hydrogen ratio compounds, such as members of the aromatic series, which notoriously decompose a t high temperatures with separation of carbon. Haber's conception of the decomposition of a paraffi hydrocarbon as the formation in the primary stage of a new aliphatic hydrocarbon and an olefin, the relative molecule sizes of which are determined by the temperature, has been substantially confirmed by the work of Engler and others. Since the secondary paraffin will in its turn be decomposed, it is apparent that the thermal behavior of the olefins alone need ultimately be considered. Their modification under the influence of heat includes the formation of
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on the use of the high pressures induced on the surface or within the pores of powerfully adsorbent materials, such as the mineral gels and activated charcoal. Up to a certain point a process developed on these lines3 was very successful. From a gas oil adsorbed into activated charcoal maintained a t a suitable temperature as much as 70 per cent of gasoline could be produced, and this gasoline, even before refining,
* Dunstan
and Pitkethly, J . Insl. Petroleum Tech., 10, 728 (1924). Auld, British Patent 208,569.
a certain amount of carbon. Thus, Bone and Cowardb have shown the production of a definite quantity of carbon by the thermal decomposition of ethylene a t only 570' to 580" C. Similarly, it must be concluded that hydrocarbons less saturated than the paraffins will also, and more readily, produce carbon under $he influence of heat. It follows, therefore, that carbon is a normal product of 4
6
Dag. Handbook of the Petroleum Industry, 11, p. 434. J . Chem. SOC.(London), 934, 1197 (1908).
August, 1926
INDUSTRIAL A N D EAZ'GINEER1NG CHEMISTRY
hydrocarbon thermolysis, and that its invariable formation must be prepared for in advance. This conclusion, taken in conjunction with the writers' previously established existence of a time interval between attainment of temperature and incidence of appreciable cracking, completed a syllogism and indicated that, should it be possible to raise the body of oil to cracking temperature without overheating, it should be possible almost completely to segregate the formation of carbon into a part of the containing apparatus where the carbon could conveniently be handled. Two considerations complicate this issue and prevent ready practical fulfilment of the desired effect. The first is the impossibility of bringing the bulk of the oil to the required temperature, without causing that portion adjacent to the source of heat to acquire a temperature considerably higher than that of the remainder. The second is the heterogeneity of the oils to be cracked, and the consequent difference in the rates of decomposition of each constituent for a given temperature. In general, this effect becomes more marked with rise of molecular weight, and is particularly apparent in the case of asphaltic residue. These considerations are closely interlocked and, since the thermal treatment of Persian fuel oil and asphaltic heavy residues, both for the production of light spirit and the reduction of viscosity, was an early desideratum,e it was apparent that during the heating process the surface or skin temperature of the oil must be kept down to an absolute minimum. Such an enforced relationship between furnace temperature and oil temperature necessitates a knowledge of (a) the individual cracking temperature of the oil a t a fixed rate of heating, ( b ) the temperature gradient, a t different velocities, across the tube carrying the oil through the furnace, and ( c ) the increase in velocity of reaction with rise of temperature. Both static and dynamic experiments showed that, for each hydrocarbon oil, there is within certain limits a definite timetemperature relationship corresponding to any determined extent of production of light spirit. The practical lower limit of temperature is specific for each oil, but in general is not less than 350" C., nor higher than 400" C. For an average Persian gas oil the lower temperature limit is 380" C. The higher temperature limit has not been determined, but probably exceeds 1000" C., since light spirit is still produced to a small extent in the manufacture of oil gas, where the time factor is very small and the cracking temperature exceeds 800" C. Even precedent to the exact determination of the conditions affecting the relationship between oil and furnace temperatures, it had been decided, for practical reasons and others connected with the limitation of yields, in no circumstances to consider, either for the production of light spirit or the reduction of fuel oil viscosity, any temperature higher than 500" C. For oils of Persian origin the prescribed limits were set a t 380" and 450" C . Between these limits the yield of light spirit is a function of the temperature if the time is constant, and within the narrower limits prescribed for particular oils it is substantially doubled for each 10" C. rise of temperature. At constant temperature the yield in the early stages is also a function of the time, being linear in the early stages but ceasing to be so over longer periods. At this point, then, the situation had clarified, and in order to come to the initial stages of plant design, there remained but to elucidate the relationship between oil temperatures, skin temperatures, and furnace temperature. The first named determines the extent of cracking in general, but it is the second which governs the unavoidable carbonization due to increased reaction velocity a t the interface, and which ultimately decides the duration of the process by its limitation of the life of the heated portion of the plant. 6
Dunstan and Thole, British Patent 156.284.
so5
A study of the skin temperatures in oil, through the surface of which heat was being passed, was made by E. S. L. Beale, in a tube wound for electrical heating in three equal sections capable of absorbing 1.5 kilowatts and lagged with magnesia to a depth of 3 inches. Specially designed pyrometers (Figure 3) were screwed into the tube in the three lower spaces, while the mean temperature of the oil was measured a t the top. The total heat input of the oil was calculated from electrical measurements and heat-loss experiments, and the mean temperature of the oil a t the bottom of the tube obtained from the measured pumping rate and the specific heat. Since the heat input was uniform along the length, a straight line could be drawn showing the mean temperature of the oil a t any point. X diagram constructed for a particular run is shown in Figure 4. The abscissas represent distances along the tube as well as heat input, so that the vertical heights above the mean temperature line give the skin effect. The other diagram (Figure 5) indicates the increase of the skin effect with the gradual deposition of a thin layer of carbon on the walls of the tube. I n order, therefore, to avoid the danger of carbonizing due to the skin effect, it was concluded that the oil should be heated in thin streams moving a t a low velocity. The velocity must, however, be sufficiently high to allow the removal of the oil from the heating zone to the segregating zone before appreciable decomposition takes --- -_ -p l a c e . Again, while -_ _ _ aiming by these means &_ to bring the f u r n a c e ---temperature as near to the oil temperature as possible, it is apparent -~~~ that there is a limit s e t b y t h e practical consideration of t h e Figure 3 throughput requiredthat is, by the velocity of the oil and the cross section of the tube carrying the oil through the heating zone. ~
Effect of Yield of Light Spirit
I n the earliest stages of the investfgation attention was paid to the effect of yield of light spirit on the course of the action. On general grounds it was concluded that the yield of light spirit should be strictly limited; that, once formed, light spirit should be removed from the sphere of reaction; and that recycling of the cracked oil should be restricted. Arising from these conclusions it appeared, moreover, that if for economic reasons a large yield of light spirit was required, recracking should be resorted to, rather than profound single-stage cracking. The reasons for these further conclusions were twofold. I n the first instance, it must be recollected that we wished, and still wish, particularly, to crack heavy Persian residues for the production of low-viscosity fuel oil as well as for light spirit. In the second place, whatever view is held of the mechanism of cracking, it appears that in passing from hydrocarbons of high to others of low molecular weight, intermediate reactions must, and do, occur. involving the formation of unsaturated bodies and the elimination of permanent gases and carbon. At each stage a certain proportion of the oil is broken down in this way. On Haber's hypothesis it is probable that with the successive elimination of paraffin molecules the effect will become more marked. In other words, a small yield of light spirit might be obtained continuously and unaccompanied by appreciable formation of carbon and gas, whereas for high yields from the same oil the gas and carbon losses would be disproportionate and possibly prohibitory. On this basis it is apparent, therefore, that the greater the
806
INDUSTRIAL AND ENGINEERING CHEMISTRY
extent of the decomposition the lower will be the quality of the spirit because of the higher proportion of unsaturated compounds present, and the greater will be the proportion of permanent gas and carbon produced. That this explanation is correct, and of general application, is shown by the known fact that a cracked oil is not so suitable as an uncracked one for production of light products, and that the number of times an
Vol. 18, No. 8
Application of Principles
The method of applying these principles and attaining to the prescribed conditions follows readily from the premises. Separate heating and deposition zones are employed, and the stages of treatment are three in number. I n the first zone the oil is heated, at any suitable rate and by any suitable means compatible with efficient temperature control, to within an appreciable distance of the lower predetermined temperature limit. This point is generally not more than 20” to 25” C. below what we may call the “optimum temperature,” which is the temperature a t which the previously fixed extent of decomposition occurs within the time necessitated by the form of plant and the velocity of the oil stream. I n the second, or principal, heating zone, the oil is quickly heated to a temperature exceeding by about 20” C. the predetermined optimum temperature. This stage is the most important and must be carefully controlled. It is here that advantage is taken of the time factor, to give the oil its impetus towards disruption without materially effecting its decomposition. This portion of the apparatus, which we have termed the “thermalizer,” consists solely of a simple tube, or a number of tubes in parallel, of such bore that the oil can be rapidly heated throughout its bulk, and of such length that the oil has Dassed through without actual decomposition, TOTAL “EAT “Pur or a t any raG with no appreciable separation of 0 5 1 IS 2 25 ,425 8 JXW carbon. Figure 4 After leaving the thermalizer the oil passes forthoil can be cracked is limited. Besides this, the retention of with to the “reactor.” This is a vessel of such dimensions a considerable mass of the heavier oil helps to hold up some that, in passing through it, the oil remains for at least the of the carbonaceous matter primarily formed and to carry it amount of time necessary to attain the required extent of deforward to the point most suitable for its deposition. This composition, By suitable means the reactor is maintained a t is of especial importance in the cracking of asphaltic residues. or about the optimum temperature. Owing to the unavoidable heat radiation and the endothermic nature of some of the operaSummary of Principles of A. D. H. Process tions involved, application of a certain amount of external heat is required to do this. Since most of the other variables The basic principles thus deduced, which have governed functions of the temperature, it will be realized that the almost entirely the development of the “A. D. H.” cracking are essence of the process is careful temperature control. For process,’ may be summarized as follows: obvious reasons, however, delimitation of the variables is ( a ) The pressure should be high enough to maintain the oil necessary, Parts of the plant, for example, must remain fixed, to be cracked in the liquid condition, and to reduce the amount and, once the length and diameter of the thermalizer tubes of undissolved gaseous products of reaction t o the lowest point have been adopted, the only true variables are the temperacompatible with readily attainable practical conditions. ture and the rate of flow. (b) The extent of decomposition, as evidenced by the yield I n the development of the plant from its inception, the of the light spirit, should be deliberately restricted. For each oil there is a definite and fairly well defined most important of the earlier experiments were carried out (c) narrow range of temperature at which appreciable cracking com- in a 1.5-foot length of 1-inch steel tube for the purpose of demences. Above t h a t temperature the rate of decomposition increases rapidly, the velocity of reaction in the early stages termining the orientation of carbon deposition a t different rates of heating and flow. The tube was wound for electrical being approximately doubled for each 10’ C. rise in temperature. (d) There is a marked time factor which governs the pro- heating in a number of separate sections, and the temperatures duction of light spirit and which is a function of the temperature. were measured throughout its length by suitably installed This time factor is displayed in two ways: there is a n appreciable thermocouples. By the laborious, and a t times dishearteninterval between the moment of attaining the thermolytic temperature and observation of the formation of noticeable quan- ing, method of deliberately carbonizing the vertically placed tities of decomposition products; and the temperature of the oil tube under various conditions of velocity and temperature, under treatment must be maintained for a definite and consid- and of plotting the position, amount, and nature of the carbon erable time, if the disruption is to proceed to the extent required deposit against the length, it was possible to arrive a t a point a t the temperature agreed upon. It follows from this t h a t the rate of flow must be a function of the temperature and corre- where no difficulty was experienced in avoiding deposition of the carbonaceous material in the heated parts of the tube, spondingly controlled. and eventually of insuring its subsidence in an enlarged (e) The oil should be heated in thin streams a t low velocity in order to reduce the furnace temperature and the skin tem- vessel exterior to the tube. perature. The factor determining the limiting minimum velocA point of considerable importance, which was noticed ity or the length of time the oil is actually heated before passing to the carbon segregation area is the time interval mentioned during the course of the experiments, was that when the above. experimental tube became choked with carbon towards its 7 Auld, Dunstan, and Herring, British Patent 220,864. upper end, it was also invariably partially choked near its
I N D U S T R I A L A l i D ENGINEERING C H E M I S T R Y
bugust, 1926
lower end, there being a long stretch of quite carbon-free tube in between. UTith the segregation of the upper carbon deposit in a “reactor,” the lower deposit also disappeared. This observation was only explained later, when a closer examination was made of the “coke” deposited in the reactors of the many successive types of plant which were constructed and used. Briefly, it appeared that. even when cracking asphalt-free oils, the coke had been formed from intermediate, semifluid, pitchlike substances produced throughout the body of the decomposing oil and slowly deposited on the bottom and lower portions of the sides of the reactor. In the top of the reactor this asphaltic substance builds up pendants of pitchy material, which are left at the close of the operation as carbonaceous stalactite-like formations. The main bulk of the coke occupies the lower half of the reactor and shows, from the level surface, that it was laid down in the fluid condition. It has been possible to separate these intermediate asphaltic bodies in a number of forms, varying from oils of gravity only slightly higher than that of the original stock, up to heavy asphaltic residues, and thence to pitchlike substances almost wholly insoluble in petroleum but largely soluble in benzene and carbon bisulfide. It appears, therefore, that one of the most important changes taking place in an oil under the influence of heat is that of association, either preceding or accompanying the dissociation ultimately sought. It is conceivable that, in the cracking process, the associated bodies successively formed become of progressively higher molecular weight as light spiritous compounds are split off, until heavy bodies of the nature of asphalt or pitch are produced. These pitchlike substances are, in their turn, either similarly condensed or polymerized to cokelike materials, or undergo directly a disruptive dry distillation of a coking nature when in contact with the walls of the reactor. The latter is probably more
837
t o be noticed that, according to Nellensteyn, the presence of sulfur (or oxygen) is unnecessary for the formation, and that neither sulfur nor oxygen is an essential constituent of asphalt. On these views the phenomena observed by the writers may probably best be explained by the formation, during*the hydrocarbon decomposition, of finely divided carbon, its colloidal solubility as the disperse phase in hydrocarbons present in the oil or produced by thermolytic association, and the existence of this stable sol up to a point where one of two things may happen: either the continued decomposition of the hydrocarbons alters the equilibrium between the medium and the disperse phase, and thus results in the observed separation of an insoluble, pitchlike material, which readily cokes and itself contains much free carbon; or else the continued application of heat causes conversion of the asphaltenes into carbenesg and free carbon. The carbenes, like part, a t least, of the intermediate pitchlike bodies, are insoluble in light petroleum products, but soluble in benzene. The writers have already applied the conclusions drawn from these observations to the successful removal, while still in the fluid state, of the intermediate asphaltic material formed during cracking, and, by the use of specially designed vertical reactors, have in some cases almost wholly prevented the formation of carbon within the plant. This highly important result is in course of technical development. The application of external heat to the reactor has already been mentioned. It hardly needs to be emphasized that the amount of heat added is very small, and that there is scarcely any difference in temperature between the shell of the reactor and the hot gases by-passed from the flues. Kevertheless, to avoid any possibility of overheating, even by accident, advantage is taken of the observed orientation of the carbon deposit to arrange that the hot gases shall carry heat only to the top and sides of the reactor. The application of the principles outlined in the foregoing summary of the development of the A. D. H. cracking process has so far proved successful. The writers have been enabled, both in purely experimental and larger scale plants, to crack all varieties of petroleum and shale products including the heaviest asphaltic residues, and in particular to cope with the special dual treatment for the production of light spirit and the reduction of viscosity required by the heavier Persian residues. Acknowledgment
10. IO
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M U R S HEATIN6
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Figure 5
generally the case, for it is noticeable that the coke removed at the close of an experiment is most dense a t the contact surface. On this hypothesis, the whole cracking operation would be one of association preceding dissociation, since practically all the carbon formed is produced by way of asphalt formation. In point of fact, however, a combination of this theory with the older views of direct formation of carbon is most likely, especially since the recent work of Nellensteyns has indicated that solutions of asphalt are to be considered as very stable carbon oleosols. All the conditions enumerated by Nellensteyn as favorable for the formation of asphalt exist in a regulated liquid-phase cracking proc(1) low temperature of carbon formation, ess-namely, whereby the carbon possesses strong adsorbent properties; ( 2 ) finely divided state of the carbon; and (3) presence of a large excess of protective hydrocarbons surrounding the carbon a t the moment of its formation. It is particularly 8
J . Ins1 Petroleum Tech , 10, 311 (1924).
Opportunity is taken of recording, with appreciation, the valued cooperation in this extensive work of E. S.L. Beale, P. H. Herring, and Robert Pitkethly of the Anglo-Persian Oil Company’s research staff. D
Richardwn, J. SOC. Chem. Ind., 24, 310 (1905).
Government Warns against Radioactive Claims The alleged medicinal efficacy of slightly radioactive waters and other preparations has been found by the Bureau of Chemistry t o be much misrepresented. The products analyzed by the bureau included hair tonics, bath compounds, suppositories, tissue creams, tonic tablets, face powders, ointments, mouth washes, demulcents, opiates, ophthalmic solutions, healing pads, and other preparations in solid, semisolid, and liquid form. Only 5 per cent of the products analyzed and claimed to be radioactive contained radium in sufficient quantities t o entitle them t o consideration as therapeutic agents and then only under very limited conditions. Highly exaggerated therapeutic claims are being made for many of these products. One sample examined consisted of a short glass rod coated on one end with a yellow substance and enclosed in a glass bulb. The bulb is designed t o be hung over the bed and the inventor claims that it disperses “all thoughts and worry about work and troubles and brings contentment, satisfaction and body comfort t h a t soon results in peaceful, restful sleep.”
