Ethylene Oxide.pdf

CATAL. REV. - X I . ENG., 23(1&2), 163-185 (1981)

Technology for the Manufacture of Ethylene Oxide J. C. ZOMERDIJK AND M. W. HALL Shell Internationale Chemie Maatschappij B. V. The Hague, Netherlands

I.

11.

III . IV . V.

VI . VII .

VIII.

INTRODUCTION .................................... 163 ETHYLENE OXIDE AND ITS DERIVATIVES ........... 165

ETHYLENE OXIDE PLANTS AND CAPACITIES ........ 166 THE BASIC CHEMISTRY ............................ 169

ETHYLENE OXIDE PROCESS TECHNOLOGY. OXYGEN CASE VERSUS AIR CASE ................... 169 DESIGN ISSUES .................................... 175

TECHNOLOGY IMPROVEMENTS . . s - - * . 182

FUTURE DEVELOPMENTS .......................... 183

* * v . - -.

I. INTRODUCTION

With the advance of modern technology, it is becoming increas- ingly feasible economically to synthesize and produce on a large scale those chemical products, for example plastics, synthetic fibers, dye stuffs and paints, which are s o much a part of our present day life.

Not least in the development of new technologies is the petrochemical industry, where the practice is to build up the complex molecular structures needed from relatively simple building units. Such a building unit is the compound ethylene oxide, which forms

163

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164 ZOMERDIJK AND HALL

the keystone of products as diverse a s detergents, synthetic fibers, paints solvents, and pharmaceutical bases,

Ethylene oxide is a highly reactive, highly toxic, inflammable, in certain circumstances even autoexplosive, chemical. It therefore requires a sophisticated approach in manufacturing and handling.

hydrin route and the modern catalyzed direct oxidation method. As this is a conference on catalysis and surface science, we shall con- centrate on the latter method.

polluting chlorohydrin route, direct oxidation technology has developed along two distinct lines, that of using a i r a s the oxygen sup- ply medium, and that of f i rs t separating the oxygen and then using i t pure. Each route has its own particular problems and advantages. At first sight it might be thought that a i r oxidation would be the more attractive route, a s the necessity for an associated a i r separation unit is avoided, and processes were developed by Scientific Design, Union Carbide, Japan Catalytic, and Chemische Werke Hula. How- ever, when using oxygen as in processes developed by Shell, and later by Scientific Design, the oxygen unhampered by about four times i ts own volume of nitrogen, allows the size, and so the cost of the reactors and associated equipment, to be substantially reduced, therefore offsetting the cost of the a i r separation unit.

Local cost pattern conditions will influence the choice of air o r oxygen, but above a capacity boundary in the region of 20,000 t /a of ethylene oxide the oxygen-type process is definitely favored. For an oxygen-type process, however, high purity ethylene (99.5% min.) and oxygen (99.5% min. ) are needed, and with the progress to higher performance catalysts, requirements for feedstock purity have become more stringent.

oxidation of ethylene over a silver catalyst. The reaction conditions a r e critical. If they are too mild, little oxide is produced; if they a r e too severe, the reaction runs away and carbon dioxide and water are the only end products. The a r t of making ethylene oxide is thus to control the oxidation reaction so that while maintaining an adequate production rate, the wasteful production of carbon dioxide and water is severely limited.

Table 1 shows clearly how ethylene oxide has remained an important ethylene derivative over the last two decades notwithstanding the growth of polyethylene.

There are two basic manufacturing methods, the historical chloro-

With the phasing out of the old-fashioned and environmentally

Essentially, ethylene oxide is produced by the carefully controlled


MANUFACTURE OF ETHYLENE OXIDE 165

TABLE 1

Distribution of Ethylene Utilization in %

EEC only 1960 1970 1980 1978

~

Polyethylene 25 38 44 51 Ethylene oxide 32 21 16 13.5 Dichloroethane (for VC) 5 7 13 17 Ethylbenzene (for styrene) 9 9 8 8 Others (acetaldehyde, ethano 1,

Worldwide ethylene production yinyl acetate, etc. ) 29 25 19 10.5

in millions tons/year 4.3 19 36 9.7a

aNederlandse C hemische Indus trie, December 197 9.

11. ETHYLENE OXIDE AND ITS DERIVATIVES

Ethylene oxide is probably the most versatile petrochemical a t the disposal of industry today. The most important products in terms of volume a r e the glycols. Ethylene oxide is converted by thermal hy- dration into a mixture of glycols, namely monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG), and higher glycols. MEG has two main uses, in fiber manufacture in combination with di- methylterephthalate, and in the cooling systems of motor vehicles where it acts a s a freezing point depressant. DEG i s also used in the fiber industry and as a tobacco humectant. TEG and DEG are both used for gas treatment. TEG is also used in the manufacture of cellophane for food packaging, When MEG and DEG a r e further reacted with ethylene oxide, then polyglycols of specific molecular weight a r e produced. These polyglycols can then be used in the cosmetic and pharmaceutical industry as base material for carrying the active ingredients, in the ink and print- ing industry, a s antifoam agents in distillation processes, and in the textile industry a s spinning lubricants,

Ethylene oxide will react with ammonia to give a mixture of ethanol- amines, and the mono-, di- and triethanolamines so produced can be separately recovered. Mono- and diethanolamine a r e used as absorbents



TABLE 2 ~~ ~~ ~~

1960 1970 1980

Ethylene glycols 46.5 52 55 Polyglycols 6 4 4 E thanolamines 12.5 8.5 7 Glycol ethers 15.3 13.5 12 Surface-active agents 14.5 13 12 Polyols 3 4 - Others 7 6 6

for the removal of acid gases from synthesis gas; triethanolamine is widely used in cosmetic and toilet preparations @air shampoos).

and, depending on the alcohol used, will yield, for example, methyl-, ethyl-, isopropyl-, and butyl glycol ethers, again in the mono, di, and tri forms. In particular the monoethers a re the most important products and find applications as high-boiling solvents in the surface coating industry, The di- and triethers, either separated or as a blend, are used a s components in hydraulic and brake fluids.

Ethylene oxide is also used for the ethoxylation of long-chain alcohols and alkyl phenols. These products are used as the active ingredients in surface-active agents (detergents).

Ethylene oxide will react with sucrose for the production of polyols, precursors for the manufacture of flexible and rigid polyurethane foams.

Ethylene oxide is also employed directly a s a sterilizing agent and a s a fumigation chemical (e.g., insect control of grains).

Table 2 summarizes the utilization of ethylene oxide on a percentage basis for the major applications in Western Europe.

Ethylene oxide will react with alcohols to produce glycol ethers

111. ETHYLENE OXIDE PLANTS AND CAPACITIES

Table 3 lists ethylene oxide plants and their capacities, taking into account that quite a number of the earlier direct oxidation plants have been closed down after the building of new larger plants. A l l figures a re published data, including those for Shell plants. In operation by mid-1980, capacity in kt/a.

Not listed a re plants under design, under construction, or in a serious planning stage. For these the reader is referred to the sum- maries in, e. g., European Chemical News o r Hydrocarbon Processing.



TABLE 3

Europe West Germany

Belgium

Nether lands

France

Italy

U. K.

Sweden

Spain

Eastern Europe East Germany

Bulgaria

CSSR

Poland

Rumania

USSR

USA

BASF Erdoelchemie Hoechst Hills

BASF BP

Shell Dow

Naphtachimie Ethylox

Montedison ANIC

Shell ICI B P

Berol Kemi

IQA A lcudia

BASF-Wyandotte Calcasieu Celanese Dow Eastman Texaco Chemical

(Jefferson)

150a 15@ 170a 135a (Shell/Huls techn.)

15@ 130e

210a 120f

200 (150a, 50b) 80b

gob 40d

1 2 s 24@ 20e

40b

70a 20b

80b

80b

70 (40a, 30d)

30d

60b

260 (200b, 60')

210a 1 ooa zooa 300f 90a

220b

(continued)



TABLE 3 (continued)

Canada

Latin America

Japan

Australia

India

Taiwan

Korea

PR of China

PR of Korea

Northern P C Olin PPG Shell Sunolin ucc

uc c Dow

Brazil (Oxiteno) Mexico

Mitsubishi Yuka Mitsui Nisso Yuka Nihon Shokubai

ICI

Nocil IPCL

Oriental Union CMFC

Honam PC

lOOb 5oa 7 Ob

30@ 45a

1200e

90e 75f

140b 140b

135 (looa, 35b) 200 (150a, 50b) 120 (60a, 60b) 200c

20b

12a 18b

1 ooe 35b

80a

30b

1 oc

a Shell technology. bScientific Design. CNihon Shokubai (Japan Catalytic). ~ S N A M Progetti. eUCC. fDow.



IV. THE BASIC CHEMISTRY

The ethylene oxide process involves the direct vapor phase oxidation of ethylene over a silver containing catalyst at elevated temperatures to give ethylene oxide according to

CH,=CH, + 1 - 0 , d Ag CH,-CH, + 25 kcal/g mol ethylene reacted

‘0’ 2

The required oxygen is normally supplied from an a i r separation plant. The main by-products formed are carbon dioxide and water according to the reaction:

CH,=CH, + 30, + 2C0, + 2H,O + 316 kcal/g mol ethylene reacted

A very small amount of the ethylene oxide formed may isomerize to acetaldehyde, which in turn is in general rapidly oxidized to carbon dioxide and water. Therefore only traces of acetaldehyde are found in the reactor product,

The ethylene oxide product is recovered from the reactor effluent gases by absorption in water in the ethylene oxide absorber.

Carbon dioxide by-product is then removed from the ethylene recycle gas by chemical reaction with hot aqueous potassium carbonate:

CO2 + K,C03 + H,O + 2KHC03

The COz is rejected from the reaction system after steam stripping the bicarbonate-rich solution a t approximately atmospheric pressure.

V. ETHYLENE OXIDE PROCESS TECHNOLOGY. OXYGEN VERSUS AIR CASE

The direct oxidation process for the manufacture of ethylene oxide from ethylene can be divided into two sections, an oxidation section and a purification section. This applies to both the a i r case and the oxygen case process. In view of the commercial dominance of the oxygen case process, we will in general describe mainly the oxygen case technology with limited excursions into the a i r case technology to emphasize the major differences. The typical basic process flow schemes for the oxygen-type and air-type ethylene oxide process are shown in Figs. 1 and 2, respectively,



9--- B

E - u l D - - 4



3

1

I



A s only partial conversion of the ethylene and oxygen will be applied, a large ethylene recycle stream is forced around into which make-up ethylene is fed and oxygen is injected by means of a specially designed mixing nozzle. It is important to minimize equipment volume in which the gas composition is above flammable conditions. Di- luents in the form of nitrogen, methane, and ethane can be added to the system, carbon dioxide can be left in the recycle gas after the removal of part of the carbon dioxide to achieve an optimal concentration. Shell has a patent related to the use of methane, Halcon has patents related to the use of ethane or a mixture of ethane and

This ethylene and oxygen-enriched gas will-after exchanging heat co*.

with reactor exit gas- wter a multitubular reactor where over a silver-containing catalyst the reaction to ethylene oxide and by-products takes place at temperatures varying between 200-300°C. Ethylene concentrations are normally between 20-30% vol (in the a i r case around 5%) and oxygen concentrations a s high as a margin from the flammability limit will allow. A space velocity of between 2500 and 5000 Nm 3/h per m3 catalyst is normally chosen. Reaction pressures vary between 15 and 25 atm.

In the reactor shell a liquid coolant is pumped around to remove the heat of reaction, This can also be achieved by applying a boiling coolant, which after leaving the reactor shell is phase separated in a coolant separator, and the coolant vapor is used to generate medium pressure steam. The coolant liquid is then returned to the reactor. To obtain optimum yields of ethylene oxide, it is necessary to use a moderator which suppresses the complete oxidation reaction. Chlor- inated hydrocarbons like 1,2-dichloroethane o r vinyl chloride are normally used.

A s mentioned earlier, a limited conversion of ethylene and oxygen will be applied. The conversion is defined by the quotient of the number of molecules (ethylene or oxygen) converted to the number of molecules (ethylene or oxygen) fed to the reactor and expressed on a percentage basis. In the oxygen case process it has become common practice to use oxygen conversion in relation to the aelectivity. The selectivity is defined by the percentage of ethylene that produces ethylene oxide.

It is clear that under otherwise stable conditions a certain catalyst will perform according to a specific selectivity vs conversion relationship, Maximum selectivity can be Bchieved at very low conversions. However, to achieve a practical productive operation, higher conversions at a somewhat reduced selectivity will have to be chosen. Low



sensitivity of the selectivity versus the oxygen conversion within the practical commercial range of conversions is an important yardstick for commercial catalyst performance,

The ethylene oxide produced in the reactor is recovered from the reactor effluent gases by absorption in water in the ethylene oxide absorber. The ethylene oxide is then stripped from the fat absorbent in the ethylene oxid; stripper and subsequently sent to the ethylene oxide purification section for the removal of light ends, water, and aldehydes. The ethylene oxide absorber overhead stream is for the larger part sent to a recycle compressor (a small process vent, 0.1 to O.Z%, is necessary to remove inerts) and then to a carbon dioxide absorber. The carbon dioxide is partly removed from the recycle gas by absorption in a hot potassium carbonate solution. The carbon dioxide absorber overhead gas is returned to the ethylene oxide reactor cis reactor feed.

carbon dioxide stripper, where carbon dioxide is rejected to the at- mosphere or recovered for further processing. The stripped carbonate is returned to the absorber by the carbonate pump.

stripper are sent to a light ends column after condensation in a series of heat exchangers. Light ends (e. g., C 0 2 ) are removed from the EO product down to a level to meet final product specifications. Depend- ing on final product specifications, the ethylene oxide is then dehy- drated and aldehydes are removed in the final purification column.

When comparing air-type process versus oxygen-type process, the following conceptual aspects should be considered.

In order to achieve high selectivities, the conversion of ethylene (or oxygen) per pass must be limited. This implies the recycling of a gas that still contains a substantial concentration of ethylene after it has passed through an ethylene oxide absorber to recover the ethylene oxide. In the oxygen case process the ethylene-rich gas leaving the top of the ethylene oxide absorber will be fully recycled to the reaction section (after part removal of carbon dioxide in a carbon dioxide removal section) and after mixing with fresh ethylene and oxygen fed into the reactor. Only a very small vent from the gas loop is needed to maintain the level of inerts below a certain value, and causing an ethylene loss below 1% of the ethylene intake to the plant. In the air case process, however, a substantial portion of the gas from the top of the ethylene oxide absorber is not recycled but sent to a second reactor after mixing with additional air. Subject to economi- cal considerations, a third reaction and ethylene oxide absorption

The fat absorbent from the carbon dioxide absorber is sent to the

The ethylene oxide (EO) and water vapors from the top of the EO



stage may be applied such that finally the loss of unconverted ethylene will be between 2 and 5% of the ethylene intake to the plant.

a re applied, In the first reactor low conversion per pass will result in achieving high selectivity of the catalyst. In the further stages higher conversions with consequential lower selectivities will be aimed for. However, finally the nitrogen and the formed carbon dioxide have to be purged from the system, causing a few percent loss of ethylene from the system.

The differences between the a i r and the oxygen case processes can be summarized briefly as follows. In the air case process, use is made of multiple reaction stages (normally two o r for larger plants three) which operate under different conditions. In the primary reactor a t low conversion per pass the higher selectivities are achieved (70-75%), while in the final reactor a t high conversion levels a selectivity of about 50% is accepted. This is distinct from an oxygen case process where one stage reaction only is applied with a recycle of unconverted ethylene after part removal of carbon dioxide.

As the carbon dioxide produced in the reactors is purged with the nitrogen, a separate carbon dioxide removal system is not needed,

In the air case process an air compression and treatment unit is required, It is also clear that in an air case process the reaction and recovery sections require larger equipment sizes and a considerably higher catalyst inventory which needs regular replacement.

The purification system in an a i r case process is similar to that of the oxygen case process.

Earlier studies by Stanford Research Institute (SRI) have shown that the oxygen case process is economically more attractive than the a i r case process. The exception could be that for very small plants, say below 20,000 t/a (Scientific Design indicated 10,000 t/a), the cost of an air separation plant might not be adequately compensated by the ethylene savings in the ethylene oxide production, because of higher selectivities in the oxygen-type process.

It should be realized that in the a i r case process the catalyst is not working at its most optimal conditions in the different reaction stages, while in the oxygen-type process all conditions (gas composition including the choice of the diluent or ballast gas, temperature range, moderator concentration, gas hourly space velocity, etc. ) can be chosen compatible with optimal operation, bearing in mind the safety considerations. In Hydrocarbon Processing, March 1976, pp. 69, 73, and 78, Nihon Shokubai, Scientific Design, and SNAM Progetti presented results of their studies related to a i r and oxygen case op- erations. Nihon Shokubai reports selectivities of 74 and 71% for an

In these different reaction stages, different reaction conditions



oxygen and an air-type process, respectively, at practical conversion levels in actual plant operation. Scientific Design presents val- ues of 7540% selectivity for an oxygen-type plant and 72-66’3, for an air-type plant. SNAM Progetti mention data of 70 and 65% selectivity for an oxygen and an a i r case plant, respectively.

VI. DESIGN ISSUES

Given the prime objective of a design, namely, that the plant shall be capable of production a t the nameplate capacity, of product of the required purity, the remaining criteria of a good plant design in modern petrochemical technology can be said to be the following. The plant must conform to high standards of safety, it should be capable of flexible operation in long campaigns without interruption, and with feedstock at a premium and high energy prices, produce a t the lowest cost. A l l of these criteria are, however, subject to the constraint that the plant shall be built for the lowest possible capital. The plant design is therefore always a matter of compromise, only in part cal- culable, in which the experience of the designer plays an important role. Aspects of this process of compromise will be examined in relation to the oxygen case ethylene oxide process.

The basic ethylene oxide formation reaction is exothermic. The first consideration of the designer must be how best to remove the heat of reaction in order that the reaction may proceed under iso- thermal conditions. It will be remembered that the partial oxidation reaction of ethylene to ethylene oxide has the characteristic that the lower the temperature, the greater the selectivities to ethylene oxide, but the slower the reaction. Therefore it is of the essence that the temperature can be controlled at a known level where reaction rate and selectivity can be predetermined for each type of catalyst. The first choice is therefore between a fluid-bed reactor, with easy heat removal but with difficult mechanical problems-particularly those connected with catalyst attrition-and a fixed-bed reactor where the provision of adequate catalyst cooling can pose severe problems. These problems may be augmented in a situation of a partial oxidation reaction like the formation of ethylene oxide from ethylene because in the case of a “runaway” (complete oxidation to carbon dioxide and water) the heat generated increases several fold. In practice for ethylene oxide manufacture, mechanical considerations have predominated, as they will tend to in any process where, for reasons of economy, the active catalyst has to be spread over the surface of a substrate. This is because in a gas-solid fluid-bed operation,



problems of attrition of the active surface a re such as almost to dictate the fixed-bed alternative. In fact, all the direct oxidation ethylene oxide processes now in use apply fixed-bed operation in multitubular reactors. The second consideration is the selection of the bed diameter (or inner diameter of a tube). In order to maintain a uni- form temperature across any diameter, there must be an efficient heat exchange between the catalyst and the gases, and then between the gases and the tube wall. This sets a maximum diameter on the bed. The minimum is fixed by considerations of cost-clearly with narrow beds more tubes a re required to hold a given quantity of catalyst-and by considerations of loading (and eventually discharging) the catalyst from the reactor, Commercially chosen diameters have varied between 25 and 40 mm. There is a strong economic incentive to choose the standard pipe size nearest to the diameter desired.

Having, from heat transfer considerations, arrived a t a reactor tube diameter for the catalyst beds, an optimum size for the catalyst beds, an optimum size for the catalyst particle can be determined. Too large a particle will give poor gas distribution; too small a particle will create too great a resistance to gas flow. The tube diameter selected will also have a bearing on the choice of catalyst particle size a s will the nature of ,the substrate itself. It is clear that if the active catalyst is deposited over the interior of an open honeycomb-like substrate, rather than just on an external surface, at a certain diameter, gas diffusion rates within the particle will set an upper limit to its size. From the complex nature of the variables it is clear that the catalyst particle size is best determined by experi- ment.

The bed diameter and catalyst particle size having been determined, the bed volume needs to be established. Again the essential design information must necessarily come from pilot-plant trials, in which the bed height and gas flow rate a re varied. With regard to bed height it is clear that at the top of the reactor bed the chemical potential for reaction is greatest because of the higher reactants concentration. Near the bottom, with part of the reactants already consumed, the reaction rate is lower and therefore a unit volume of the catalyst will be used less efficiently. More serious, however, is the tendency for the oxide which has already been formed to be oxidized further. Thus there is a maximum to bed height. Equally, there is a minimum, set largely by safety and by mechanical considerations. From a safety point of view the effluent gases need to be nonreacting in the absence of a catalyst when they leave the catalyst bed. There is also an interaction of gas flow rate with bed length.



The taller beds will offer more resistance to flow, and thus considerations of energy cost for gas circulation are germane to bed height determination.

On the basis of a certain reactor and catalyst bed configuration, attention can be focused on the type of heat removal (wholly liquid circulation or boiling fluid), and on the heat removal agent. Where- as in setting the bed height data experimentally derived play the ma- jo r role, in determining the heat removal regime the designer has considerable latitude. It is beyond the scope of this paper to give the hydrodynamic and heat transfer calculations which set any detailed final design. However, the items which affect the broad choice between the two alternative schemes can be examined.

The essential difference between the two methods is that in the case of the boiling liquid, the bed is maintained a t an essentially constant temperature: in the case of the liquid circulation there will be a temperature gradient in the bed. This need not be a bad thing as, if the liquid is forced to flow co-current, i t will allow the bed to be hotter a t the bottom where, because of feedstock depletion, the reaction would otherwise be slower. In fact, even with a boiling liquid there will be some increase of temperature a t the bottom of the bed caused by hydrostatic head, The other considerations are that a boiling system will have better heat transfer in the reactor itself, and also the flow of liquid will be significantly smaller, thus saving liquid pumping energy. With the liquid circulation, on the other hand, the degree of increase in temperature down the reactor can be controlled by the liquid flow rate, the internal baffling is easier to arrange, and there is no need for a liquid-vapor separation device. So far the two methods seem about equal. However, in the case of a runaway reaction (i. e., total oxidation to carbon dioxide and water), in a boiling liquid system the liquid inventory of the coolant in the reactor shell and in the coolant separator is available to provide the necessary extra cooling by evaporation, thereby contributing to built-in safety.

choice of coolant liquid. obviously water, Unfortunately, a t the temperatures around which the catalyzed oxidation of ethylene to ethylene oxide occurs with a sufficient ra te to be commercially usable, the pressure of water as a liquid is around 50 bars. Even a stronger drawback is that a t this pressure a small change in temperature causes a large change in pressure. These difficulties are not insurmountable and indeed recent developments in catalysts have allowed operation temperatures to decrease favoring the use of water as a coolant.

In the case of using either system, it is useful to make an early A strong contender for any coolant liquid is



In the boiling system the hydrodynamics clearly have to be right if the system is going to work well. Distribution of the cold fluid has to be even, and the liquid head has to be sufficient to achieve this. Further, the interpipe distances, the "pitch" of the tubes, have to be sufficient to allow the vapor bubbles to rise smoothly and without bumping. There has to be a disengaging zone so that the liquid in the bubbles can be disentrained and sent back to the reactor, and the vapor (steam) can escape. In addition, the mechanical construction has to be such that there are no undue s t resses from expansion or con- traction.

These requirements present a strong challenge to the designer, and call for close cooperation between process engineers, heat transfer experts, and hydrodynamics specialists. Solutions involve measures such as the use of circular collars around the top of the reactor, one each for delivery of return fluid and the collection of hot vapor, the use of a close-coupled vapor-liquid separator to provide liquid a t i ts boiling point (therefore avoiding mechanical stresses), and the careful choice of the intertube pitch.

action cannot simply be vented a s this would cause the loss of too much ethylene, A positive carbon dioxide removal system must be selected. Considerations of catalyst contamination rule against the use of any volatile organic compounds such as amines, and so the potassium carbonate/bicarbonate system needs to be selected-the loss of ethylene in a high-pressure water absorbing system such as used in some gas reformers being too high for this to be a viable alternative, With the agent chosen, it is convenient to operate the system at around 100°C. This allows the use of contaminated very low-pressure steam as a stripping agent for the subsequent stripping off of the bound carbon dioxide. Now comes the problem of optimization. If the whole of the recycling gas were to be put through the carbon dioxide absorber, this absorber would have to be very large; i f only a small fraction were put through, the concentration of carbon dioxide in the gas loop could only be maintained constant a t a rather high level. This would have the effect of increasing the molecular weight of the gas (thus increasing the energy required for recycle), and decreasing by displace- ment the concentrations of active gas (thereby decreasing the reaction potential and the reaction selectivity). Knowing all the relevant economic factors, the designer can calculate the optimum size of the carbon dioxide removal facilities.

Having made the ethylene oxide, and settled on the method and ex- tent of the carbon dioxide waste product removal system, the ethylene

In the oxygen case process the carbon dioxide generated in the re-


MANUFACTUREOFETHYLENE OXIDE 179

oxide has to be recovered from the reaction gases. A s the solubility in water is very good, recovery by water scrubbing is universally used. Usually the water will be slightly alkaline to neutralize small amounts of by-product formic and acetic acids. The absorbing water needs to be recirculated for two reasons; to avoid loss of (and pollution by) the alkaline neutralizing agent employed, typically caustic soda, and to avoid loss of the small amount of glycol that is formed in the absorbent by hydrolysis. The ethylene oxide is recovered from the absorbent by steam stripping, and therefore before the lean absorbent can be used again, i t needs to be cooled. This may be done by direct heat exchange against cooling water o r in a cooling tower. In either case the absorbent may be further cooled by a refrigerant.

to 4OoC, every fall of 10°C in lean absorbent temperature brings about an approximately 25% reduction in the amount of lean absorbent required to achieve a desired scrubbing factor. (The scrubbing factor, which is also set by the designer, is a measure of the recovery of the volatile products within the column. Where there is a danger, as in the ethylene oxide process, that the product can be recycled to de- struction, the product recovery will have to be high. The choosing of a scrubbing factor is in itself a separate design optimization problem.) A lesser volume of absorbent requires less steam to heat it- in a smaller stripper-and less pumping energy to circulate it. Thus the designer will seek to achieve the minimum capital together with the minimum stripping steam, consistent with not pushing the costs of obtaining a cooler lean absorbent to levels so exaggerated that they outweigh the advantage of the lower flow. The relationship is in fact quite complex, and the good designer will check several points within a spectrum, costing out each so that the minimum can be established.

A survey over a period of time of the solutions reached of how best to absorb the produced ethylene oxide gives an interesting insight into how external factors affect designs. In the earlier designs, process cooling towers were almost invariably used. Refrigeration of the stream was hardly attempted in view of the high energy costs associated with compressing the refrigerant gas. With increasing awareness of the need to protect the environment, although the levels of organic emissions from the process cooling tower are usually well below the strictest legal limits, on general environmental grounds moves were started in Europe and in Japan to use indirect cooling rather than a cooling tower, accepting the higher capital charges this would entail. Recently spurred on by rising energy costs and the timely commercialization of lithium bromide cooling systems, which

There is a very satisfying design problem here. In the range 10



can turn low pressure steam or even hot water into "cooling, 'I de- signers can come up with new optima. Cooling of the lean absorbent by successive heat exchange is back in vogue.

The story above leads quite naturally to those aspects of the design related to energy saving. Broadly speaking, every extra heat saving device has two drawbacks. The first is increased capital. Given proper economic yardsticks, the designer can achieve an optimum here setting capital against energy cost, either current or pre- dicted. The second drawback is, however, more difficult to evaluate It is operating flexibility. A typibal example is the following one. In most plants a major portion of the ethylene oxide produced is converted further into glycols by reacting it with excess water. In the ethylene oxide purification column, the oxide is separated from water. Some of this oxide is then remixed with water and sent to the glycol reactor. It is obvious that significant heat (and cooling water) savings a re to be made if the oxide-water mixture can be fed directly to the glycol reactor. The disadvantage is that hot ethylene oxide-water mixtures, particularly those where the ethylene oxide to water ratio is high, a re very reactive, Thus they a re with difficulty or even with hazard stored. If the pure ethylene oxide is separated first from the water, it can be stored easily and safely, cooled, and kept under a nitrogen blanket. Thus in those plants where for energy economy reasons the glycol plant is fed directly from the ethylene oxide purification column bot- toms with a glycol-water mixture, if a temporary shutdown is needed, in the glycols plant, the ethylene oxide plant too must be shut down.

It must be apparent that operating philosophies will dictate in sit- uations like the above where the economics can readily be recognized but with difficulty quantified. The good designer must accept that his design must conform to the general company operating philosophies.

There is a further complication for the designer. All the pres- ently available catalysts offering competitive selectivities decline in performance with use. The decline is accelerated by contaminants such as sulfur in the ethylene, and therefore the use of high purity feedstocks, combined with a guard bed system, is becoming increas- ingly common.

Now the changing of a catalyst inventory in a large plant, in order to replace an "aged" catalyst with a fresh one, is in operation which is best carried out during an annual, or a s the case may be, bi-annual inspection shutdown. A t any other time the cost in lost production caused by the shutdown would be very serious. The designer needs to know, therefore, how the catalyst declines in a commercial plant (which is often different from a laboratory) in terms both of time and of amount



of oxide produced. (This is also linked with considerations of by how much the temperature can be increased to compensate for the falling activity,) This will enable the reactor@) to be so sized that the catalyst can be changed during a shutdown before the declining selectivity has caused the ethylene and oxygen demand to become uneconomically high or the design production to be unachievable. For the above reasons the campaign is usually for 2 years or 4 years, The campaign length is sometimes called the "catalyst cycle time." In view of this time/production related performance decline, the considerations which the designer must take into account include the relative cost of catalyst and feedstock (complicated in different countries by import and excise duties), the period between statutory inspections, the climate for labor (the catalyst discharging and reloading procedures a re extremely labor intensive), and the climate for capital, Clearly a plant designed for a longer catalyst cycle time will have a higher capital cost due to, for instance, a bigger reaction section and a bigger catalyst inventory. In view of the current price of silver, this represents no small amount of tied up "working capital,

The above story has highlighted some of the design considerations, each of which will play its part in the shaping of the finished plant. Some are more fundamental, such a s the choice of reaction condition, and some are more peripheral, such as the design in an oxygen case plant of the oxygen injection facility. (In the latter instance, the designer is concerned above all with safety, and sophisticated very high integrity systems are in operation, capable of stopping the oxygen flow, and therefore the reaction within seconds.)

an established process is selected by an operator will be given. The operating company, often a licensee, will on the basis of market research predictions have decided on the desired plant size, the product specifications, and in the case of a combined ethylene oxide/glycol plant, the proportions of each and the degree of flexibility between them. The site will have been earmarked, and thus the characteris- tics of the utilities will be known as well a s the climatological factors which can influence the design. Once these parameters have been settled, the designer will need to be furnished with a great deal of technical and economic information, including cost factors for capital, feedstock, and utilities; restrictions if any on imported materials; the type of instrumentation preferred; and local regulations on the dis- charge of effluents both in normal operation and in emergency. In addition, the already-mentioned problems of plant integration for energy saving purposes and cycle times for catalyst have to be resolved.

In conclusion, a brief insight into the procedure which occurs when



OftFn a good way to do this is to hold a design conference at which these issues a re settled and the design alternatives systematically investigated .

Thus supplied with information and being aware of those considerations which lie more in the region of policy than calculation, the designer proceeds with his main optimization. This can only be done by means of sophisticated computer techniques. Once the central op- timized skeleton is determined, the designer will then begin to weave in the secondary optimization programs, typically involving such items as the best use of the available heat so as finally to achieve a plant which is safe, reliable, and flexible, and produces on specification product at the lowest cost.

VII. TECHNOLOGY IMPROVEMENTS

Technology improvements during recent years have been concen- trated on improvements related to a more efficient utilization of ethylene and energy (steam and electricity) in view of their greatly increased prices, which in Europe and Japan a r e still above those in the USA. A more detailed inspection of the breakdown of cost elements in the manufacturing costs per unit of ethylene oxide shows that around 70% of those costs a r e attributable to ethylene, assuming a 15% depreciation per annum on investment costs for a large scale s ize plant (say 100,000 t/a o r higher). The remaining 30% is divided over oxygen (lo%), utilities plus catalyst and chemicals (8%), and depreciation and maintenance (12%). It is therefore clear that savings of ethylene using higher selectivity catalyst a r e of major importance.

A lower consumption of ethylene demands a catalyst with higher selectivity. Over the last decade catalysts with improved selectivities have been developed and a r e being used in commercial plants. Selectivities have been improved from 67-700/0 in the early 1970s. to between 75-81% for initial operation a t practical production conversions. In addition, the use of high-purity feedstocks will-in particu- l a r in the oxygen-type process-allow for very small vent streams to purge the inert components (e.g., argon) from the recycle gas loop, thereby minimizing ethylene losses. Further improvements in the degree of recovery of the unintentionally formed amount of glycol in the ethylene oxide unit will add to the total recovered amount of ethylene oxide equivalent.

Savings in energy as far a s they are already not consequential on the selectivity improvements of the catalyst (and here i t should be



noted that a catalyst with a higher selectivity will generate less carbon dioxide which will require correspondingly less energy to remove it) can be found in a more efficient use of heat in the process streams. A s in most cases an ethylene oxide plant is combined with a glycol plant, a heat integration between the two plants will result in further savings in energy, as the ethylene oxide plant is a net heat producer and a glycol plant is a net heat consumer, In a number of designs, generated steam from the reaction section is used to drive the recycle-compressor-turbine and then the lower pressure steam from the turbine is available for further heat exchange purposes. One can imagine that there a r e many variants possible within the constraints of the cost factors for steam, electricity, costs for the additional equipment, and the loss of operational flexibility.

tion the use of water-cooled reactors, generating steam directly from the reactor and not via an intermediate coolant, thereby contributing to improved energy economics,

timization programs, makes tighter and therefore cheaper designs possible and also enables a faster adaptation both to improved catalysts with revised performance relationships as well a s to other conceptual changes.

advanced equipment in the plant; for example, trays with higher ef- ficiencies in distillation equipment, low-pressure drop packing in columns, and advanced designs in heat exchangers.

In order to minimize both a i r and water pollution, ethylene oxide plants a r e now designed in such a way that either no harmful gaseous or liquid effluents leave the plant, or the effluents are such that they can easily go to a fuel gas system, a biotreater, o r an in- cinerator. It is obvious that such measures lead to higher investment costs.

In connection with energy saving, i t is also worthwhile to men-

The use of modern tools in the design phase, e. g., computer op-

Further improvements are the results of the application of more

VIII. FUTURE DEVELOPMENTS

When considering in what directions this technology might develop, we may have to investigate more deeply two aspects for such an analysis. In the first instance, does the size and the growth of the market for ethylene oxide and i ts derivatives for the next two decades justify expenditures for further research in catalyst and process development? There is a second and equally difficult



question to answer: Is there still room for further improvement in process technology and/or catalyst?

still holds, i. e., to compensate for rising costs of feedstock and energy supported by the healthy ambition to stay ahead of the competition as regards manufacturing costs of the product,

Published surveys on the growth of the market for ethylene oxide show that growth rates of 12 up to 20% per annum in the sixties have changed to 6 up to 10% per annum in the seventies. Forecasts for the eighties vary between 3 and 6% per annum. However, the growth in ethylene oxide demand depends for about 50% (in Japan even 75%) on growth in ethylene glycol. It is especially in this field that a number of new technologies are under development, namely direct routes from ethylene to ethylene glycol (Scientific Design, Teijin, Kuraray), routes from synthesis gas to ethylene glycol (Union Carbide), or routes from carbon monoxide and formaldehyde to ethylene glycol (PPG, Chevron, DuPont). More recently a route to ethylene glycol using biotechnology was mentioned. The acetoxylation route as developed by Scientific De- sign and used in Oxirane's Channelview, Texas, plant has apparently met insurmountable problems such as to cause permanent plant shutdown. One must, however, not rule out the possibility that in the nineties a direct route to glycol could find commercial application, e. g., based on synthesis gas. Yield to glycol (by-products are, among others, glycerol, propylene glycol, methanol) still needs a considerable improvement as well a s the set of reaction conditions. Such a breakthrough in technology would then of course affect the growth of the direct oxidation ethylene oxide technology, particularly if this would lead to the installation of large scale "direct" ethylene glycol plants in the USA, Western Europe, and Japan, However, if one realizes that developments in the ethylene oxide technology have caused selectivity improvements from '70% in 1970 to above 80% in 1980, further improvements to say 85% by 1990 might well be feasible. Thus the new routes will have to compete with constantly improved ethylene oxide technology, and, in any case, the demand for nonglycol derivatives will continue to exist.

Therefore, commercialization of a direct glycol route may not develop as smoothly a s some publications make us believe. A good summary of alternative routes to glycol is presented in Nippon Chem- tech Cons Inc.'s report: "Evaluation of new ethylene glycol processes,

that the range of between 3 to 6% annual growth for the eighties will be affected by the introduction of new glycol technologies, while for the

We assume that the general incentive for technology improvement

So, coming back to our market growth margin, we do not believe



last decade of this century marketing forecasts are, of course, not available. It is therefore clear from the above that further development work-in particular on improved catalysts-is fully justified on the basis of product demand in the future market.

Regarding the second question, in relation to the margin for technology improvements we again refer to our earlier comment on manufacturing cost breakdown. The costs attributable to ethylene consumption a r e about 75% of total (15%) depreciated manufacturing cost. It is therefore clear that, although further limited savings in utilities could still be achieved, and further demands to meet more stringent safety and environmental regulations might be compensated for by possible capital saving, the major field for improvements is catalyst performance. In this connection i t is worthwhile to observe that the patent literature on catalysts and process technology could provide you with some pleasant optimistic reading, but that commercialization is in general related to a very small part of this.

Another aspect of catalyst which could be improved is that of the relative production rate expressed as kg EO/per kg of catalyst per hour. Moving into the area of higher production rates will, as said earlier, in general increase decline rates, and a proper optimization of conditions is therefore of major importance. Increase in productiv- ity rate would enable the design of smaller reactors for a certain capacity.

ciated catalyst development activities, we can say that there has been a challenging continuation of new facts and findings that have made working in this field extremely interesting and rewarding, We trust you have understood from the above that further technology improvements often associated with developments in the catalyst field can be expected. Finally we hope that the impression of this technology presented to you has generated a challenge of interest that could lead you to an exchange of useful ideas and opinions in this field.

Having been involved in ethylene oxide technology and the asso-


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