P R O B L E M OF O P T I M I Z A T I O N RAW-MATERIAL V.

OF H Y D R O C A R B O N

BALANCE

M. A n d r i a n o v

and

P. F. S u k h o m l i n o v

UDC

665.55

: 62-508

The rapid growth of the petrochemical industry represents the most characteristic and the most progressive trend of the development of synthetic-products manufacture. The mass-scale chemical processing of petroleum and gas is the basis for increasing the scales and reducing the cost of production of synthetic materials. The proportion of petroleum and gas raw-materials in the manufacture of chemical products increased from 5% in 1958 to about 20% in 1966. The range of hydrocarbon raw materials, which come from chemical processing, increased sharply. The trend of increase in the proportion of hydrocarbon raw material in chemical-products manufacture will be preserved in the current five-year period and much later. The overall hydrocarbon raw-material requirement of the petrochemical industry now exceeds 1.5% of the total production of petroleum and gas. Later, the raw-material requirement by this industry will be stabiKzed at 2-2.5% of the totat petroleum and gas production. The potential sources of C2-C s hydrocarbons, contained in the associated petroleum and refinery gases, are enough to satisfy in full the communal needs, the needs of the petrochemical industry, and for automobile transport. But despite all this, the optimization of the hydrocarbon rawmaterial balance for petrochemical production processes is of great practical interest. This is, firstly, because the potential sources of valuable components of associated and refinery gases are not used more than one-third at present and, secondly, because the petrochemical industry uses mainly the low-molecular hydrocarbons - methane, ethane, propane, butanes, pentanes, and light distillates. The petrochemical industry is not the sole consumer of hydrocarbon gases and light-petroleum distillates. They are used also by the petroleum-processing industry and by automobile transport. A considerable amount of liquefied gases is used for satisfying domestic needs. It is natural that all this seriously restricts the sources of certain types of hydrocarbon raw materials and, therefore, stresses the urgency of the problem of optimizing the rawmaterial balance for petrochemical production processes. In the petrochemical industry itself there are several principal types of use of hydrocarbon raw materials. One of them is the manufacture of acetylene, ammonia, and methanol. These processes require mainly natural gas. The second type is the processing of petroleum raw material into low aromatic hydrocarbons. The narrow fractions of straight-run gasoline are used in the production of benzene, toluene, and xylenes from petroleum. The third type is the processing of hydrocarbon raw material into the basic monomers (divinyl and isoprene) needed in synthetic rubber production. This type of use is based on the processing of the C4 and C5 hydrocarbons contained in the associated and refinery gases. The fourth type of use is the manufacture of ethylene and propylene. It is distinguished by the widest range of the hydrocarbon raw material - from ethane to heavy petroleum residues and crude. The "world and Soviet experience of development of the petrochemical industry shows a considerable demarcation of the spheres of use of hydrocarbon raw materials among the basic productions. This is particularly characteristic of the countries which have a rich raw-material base, i.e., have not only a petroleum-processing but also a developed oil- and gas-producing industry. For instance, in the United States ethylene is produced mainly from ethane and propane, divinyl from butane and butylene, and petrochemical-acetylene from natural gas. A similar demarcation of the spheres of chemical use of hydrocarbon raw materials is true also of the petrochemical industry of our country. The semifinished petrochemical products - a m m o n i a , methanol, and a c e t y l e n e - a r e produced from natural gas. All the new capacities for the production of divinyl are mainly based on dehydrogenation of butane, and those for lower aromatic hydrocarbons on the processing of narrow-gasoline fractions.

Translated from Khimiya i Tekhnologiya Toptiv i Masel, No. 11, pp. 56-59, November, 1967.

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It follows from here that the problem of optimizing the hydrocarbon raw-material balance should be solved within the framework of each basic type of use. Moreover, the problem of optimizing the pyrolysis-feedstock balance is of the greatest importance. It is very imp0rtant to determine the most important consumers of the deficit types of raw materials in the generalplan 0f optimization of the hydrocarbon raw-material balance. The development of the petrochemical industry indicates quite definitely that the most important consumers of butane are the production processes of divinyl and narrow-gasoline fractious, particularly the 62-85~ fractious, i.e., the lower aromatic hydrocarbons. The use of these types of hydrocarbon raw-materials in other processes is possible only when there is an excess after fully satisfying the needs of the most important consumers. Pyrolysis, used for obtaining ethylene and propylene, is the biggest large-scale consumer of hydrocarbon raw material in the petrochemical industry. The rational satisfaction of the demand for pyrolysis feedstock is a complex and important problem. Its solution is further complicated by the fact that economists have not yet agreed upon a method for calculating the comparative economic efficiency of processing different types of hydrocarbon raw materials on pyrolytic units. The systematization and processing of the initial information, particularly the production cost, specific investments, and reduced expenditure, are quite important in soIving this problem. Each of these factors should include three components - expenditure for production and primary processing (Kp and Cp), transport costs (Kt and Ct) and expenditure for obtaining monomers (Ko and Co). K =Kp +K t +K o andC = C p + C t + C o. The reduced expenditures are determined by the formula: Z=C+K.E where K is specific investments, C is production cost of 1 t monomer, and E is efficiency coefficient equal to 0.2 for petrochemical production processes. The second part of initial information should contain information on actual sources of various types of petrochemical raw materials, the volume of monomer output, and the requirement of hydrocarbon raw materials in economic regions and enterprises. The main products of pyrolysis are ethylene and propylene. Besides, butylenes, divinyl, and aromatic hydrocarbons are produced during pyrolysis. As the pyrolysis feedstock becomes heavier, the yield of secondary products increases. On pyrolysis of ethane the yield of secondary products is 25-30% on ethylene. On pyrolysis of propane, straight-run gasoline, Romashkino crude, and Arlan crude the yield of secondary products is 128, 250, 300, and 550%, respectively. Therefore, during processing of initial information it is very important to have an objective approach to the distribution of expenditure not only among the main products, it is also important to correctly evaluate the output of secondary products of the primary chemical processing of the raw material. The extremely high value attached to the secondary products of pyrolysis can be considered as the most common error in calculating the comparative economic efficiencies of processing various types of pyrolysis feedstock. This distorts the actual efficiency of processing the competing types of hydrocarbon raw materials, since the types of raw materials whose processing yields the greatest amounts of secondary products (i.e., straight-run gasoline, crude petroleum, and residuum) appear advantageous with such evaluation. As a result, we meet with a paradoxical situation whereby ethylene and propylene become steadily "cheaper" as the raw material becomes heavier. Recently, some petrochemical economists have proposed to change the distribution of expenditure among benzene and toluene, ethylene and acetylene, which are produced simultaneously, such that the cost of the end product, which is obtained from monomers produced simultaneously, be the same. If such proposals are accepted, benzene and ethylene will be artificially dearer, and toluene and acetylene will be unjustly cheaper, i.e., we shall get a distorted picture of the actual economic efficiency of chemical processing of aromatic hydrocarbons, ethylene, and acetylene. The most acceptable and the nearest objective method of distributing expenditure among the simultaneously obtained basic products of chemical processing of hydrocarbon raw materials should be their distribution in proportion to cost of individual production. In order to reduce error in the conclusions regarding the economic efficiency of processing various types of pyrolysis stock, the secondary products should be evaluated according to the minimum cost for similar products obtained by other methods, Here one should take account of the expenditure on concentration and purification of secondary products. In any case, the cost of finished products obtained from secondary semifinished products of pyrolysis should not exceed the minimum designed cost of the products of similar quality

816

which are obtained in allied processes, For e x a m p l e , the cost of pyrogasoline after hydrogenation should correspond to the cost of reformed gasoline, and the cost of benzene obtained from pyrolytic tar should correspond to that of benzene from reforming the 62-85~ fraction. The selling prices for petroleum products or p e t r o c h e m i c a l feedstock are not a c c e p t a b l e for evaluating the secondary products. The relative efficiencies of processing various types of feedstocks will be grossly distorted if we base it on these prices. The problem of optimizing the pyrolysis feedstock balance should be solved separately for every economic region - the Volga region and the Ural, the Northern Caucasus and Transcaucasia, Siberia and Kazakhstan, and so on. The solution of the general problem of optimizing the b a l a n c e of this type of feedstock is the sum of the solutions of individual (regionwise) problems. The problems of o p t i m i z a t i o n should be solved regionwise, because the economic indices for production and transportation of the feedstock, and therefore those for the manufacture of monomers vary not only according to the type of feedstocks but also from region to region. For optimizing the balance of pyrolysis feedstock one must consider the priority of the requirement of the most vaIuable types of raw materials in the production of synthetic rubber and a r o m a t i c hydrocarbons. It can be taken into account by introducing into the problem severe limitations of sources of such raw materials for processing into ethylene and propylene. There is no decisive p r a c t i c a l value in combining in one problem the problems of optimizing the b a l a n c e of the entire volume of hydrocarbon raw materials for the p e t r o c h e m i c a l industry, since the problem of satisfying the needs of the priority consumers in each economic region is p r a c t i c a l l y known. It is advisable to have individual solution of the problem of optimizing the r a w - m a t e r i a l b a l a n c e for each basic type of use of oil and gas in the manufacture of synthetic products. Here, it is naturally advisable to solve separately the problem of optimizing the satisfaction of r a w - m a t e r i a l requirement for divinyl production, and separately for isoprene production. By solving the first problem it is possible to determine the o p t i m a l limits of using butane from the gasoline plants and c r u d e - o i l refineries and butylene from the secondary product of crude processing. For o p t i mizing the r a w - m a t e r i a l b a l a n c e in isoprene production it is important to determine the rational limits of the use of isopentane, n-pentane, amylenes, isobutylene, and formaIdehyde, propylene, acetylene, and acetone, For optimizing the balance of hydrocarbon raw materials it is necessary to determine not only the range but also the sources of m e e t i n g the raw m a t e r i a l requirement in the case of each consumer enterprise. Every economic region has m sources of hydrocarbon raw materials having n number of types. Besides, there are p consumers, and i=1,2,3

.....

]=1, 2,3 ..... k=-l, 2......

m

n p

The following are given: aij is sources of the i - t h supplier for the j - t h type of hydrocarbon raw material; bjk is amount of the j - t h type raw m a t e r i a l which can be processed at the k - t h enterprise; ejk is specific requirement for the j - t h type raw m a t e r i a l at the k - t h enterprise; A is monomer output according to plan; Zj k is reduced e x p e n d i ture for unit monomer obtained from the j - t h type raw m a t e r i a l at the k - t h enterprise. The o p t i m a l r a w - m a t e r i a l distribution among the various consumers can be acheived by finding the value (xij k) of the j - t h type hydrocarbon raw m a t e r i a l supplied to the k - t h enterprise from i - t h source. The o p t i m a l i t y criterion of r a w - m a t e r i a l distribution is the m i n i m u m expenditure: n

p

Y, ]=1 k=!

rn

~ x i i ~ . Z = rain i=l

Thereby the plan of hydrocarbon r a w - m a t e r i a l distribution among individual consumers should be worked out P

under conditions under which

~xif

k , the sum of deliveries of the j - t h type raw m a t e r i a l from the j - t h source,

does not exceed the present r a w - m a t e t i a l supply from this source: 2 x i l k = b]k,

i.e., the total deliveries of hydrocarbon raw m a t e r i a I of the j - t h type from the i - t h source to the k - t h enterprise should fully satisfy the r a w - m a t e r i a l needs of the enterprise.

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An important condition for solving the problem of optimizing the hydrocarbon raw-material balance for each petrochemical enterprise of a region is integral delivery. It means that the volumes of delivery of the j-th type raw material to the k-th enterprise should not be less than the annual raw-material requirement of one pyrolytic furnace or one dehydrogenation unit. If, for example, the requirement of the j-th type raw material by one pyrolytic furnace is 60,000 tons, the yearly volume of delivery of this raw material should be of this amount. If the solution is not of integral type, it will be necessary to modify the technological scheme of the furnaces. But this will inevitably reduce their output of the final m o n o m e r and, hence, increase the cost of labor and capital per unit annual production. One of the conditions for optimizing the raw-material balance is the rhythmic supply of the j-th type to the k-th consumer. The volumes of delivery of the j-th type raw material to the k-th consumer are non-negative, i.e,,xijk -> 0. On solving the problem of optimizing the raw material balance, at the initial stage one m a y obtain a tentative optimal variant of distribution of the j-th type raw materials of the i-th number of suppliers among k-th consumers. This is due to the fact that the supplies of certain types of raw materials can exceed their actual sources. Therefore, the redistribution of the j-th type raw materials at the second stage of optimization should be such that the rise in reduced expenditure be the minimum. For each transformation of the tentatively optimal variant a portion of the supplies of the j-th type raw materials, which exceeds the limits, is substituted by a moreexpensive, but the cheapest (of the remaining types) type of raw material from the i-th number of suppliers. The optimum variant of raw-material distribution among the consumers makes it possible to achieve the m i n i m u m costs of labor and resources for feeding all the pyrolytic units in the region out of the sources of the j-th type raw materials from all possible suppliers. The problem of optimizing hydrocarbon raw-material distribution among the consumers should be solved in terms of one and ,lot several end products - predominantly monomers obtained on primary processing of crude oil and gas. For example, ethylene can and should be the m o n o m e r for optimizing the balance of pyrolysis feedstock. Taking this m o n o m e r as the main representative, we recognize that the volumes of production of all other pyrolytic products depend on the planned output of ethylene. This premise is fully confirmed by the experience of planning the volumes of m o n o m e r production. This, apparently, is due to the fact that pyrolysis can be considered only as the additional source of obtaining the overwhelming number of other monomers and products. The latter include butylenes, divinyl, and aromatic hydrocarbons. Pyrolysis is practically the sole source of petrochemical ethylene. In determining the sources of propylene - the second main m o n o m e r obtained on pyrolysis of hydrocarbon feedstock, one should take into account that it is produced at the refineries. At present, there is practically no need for optimizing the balance of crude-oil feedstock in the production of aromatics, the narrow gasoline fractions being indisputably that raw material. Later this problem m a y b e c o m e relevant with the development of new methods for producing benzene, toluene, and xylenes from middle distillates and heavier feedstock. And in this case the problem m a y and should be solved with respect to one product - benzene, despite the production of toluene together with benzene. The extended range of hydrocarbon raw materials, the development of new production methods for monomers, the changed scales of chemical processing of raw materials, and changes in the overall demand-pattern of monomers indicate that recommendations based on the solution of the problems of optimizing raw material balance can be applied not only to specific condit.ions of place but also to quite specific conditions of time.

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