Thermal Cracking: When Quantity Matters More Than Quality
Thermal cracking unit at Novokuibyshevsk Oil Refinery
If you are not yet tired of the boiling of various pungent-smelling liquids with difficult-to-pronounce names, then we should consider another oil refining technology - cracking.
Shukhov's Diverse Talent
The father of cracking is Vladimir Grigorievich Shukhov, engineer, architect, inventor. Yes, the same engineer who built the Shukhov Tower on Shabolovka in Moscow in 1919-1922. He was a very versatile person, which is now condemned and often persecuted, but he made a great contribution to the oil business.
V.G. Shukhov and his famous radio tower
He studied at the Imperial Moscow Technical School (Bauman Moscow State Technical University), in particular, he was taught theoretical mechanics by N.E. Zhukovsky himself. After graduating from the school, he worked in the management of the Petersburg-Warsaw Railway, designing locomotive depots. Between school and work, Shukhov visited the 1876 World's Fair in Philadelphia, USA, where he met engineer Alexander Bari, who was engaged in the construction of metal structures at the exhibition.
Bari soon returned to Russia and began to build the first oil pipeline from Balakhany to the oil refinery in the Black City, in Baku, for Ludvig Nobel. He had many adventures, the pipeline was repeatedly set on fire and destroyed by oil carriers, and eventually guards were posted. The pipeline paid for itself in a year, and then Bari remembered Shukhov and invited him to join him.
Shukhov became the chief engineer of the Bari company, built iron oil tanks, oil pipelines, and approached everything from a scientific point of view and developed a theory of oil pipelines, which is still used today. Shukhov built the Baku-Batumi (883 km) and Grozny-Tuapse (618 km) oil pipelines.
After the revolution, Shukhov quickly became very much in demand by the Soviet government; already in 1924, he was the chairman of the USSR State Planning Committee for oil pipelines, then the chief engineer and technical consultant in a number of trusts.
Actually, Shukhov invented cracking back in 1890 for more complete processing of oil and obtaining kerosene from fuel oil. However, in those years when wells were flowing, the cost of oil was so low that cracking was unprofitable. Cracking for the production of gasoline was developed in 1912 by S.K. Kvitko.
But when the automobile era began, cracking came into its own. In 1913, the first cube cracking unit with a furnace under the cube appeared in the USA using similar solutions, and then in 1916, tubular units with a heating furnace were built. Cracking in the USA developed rapidly due to motorization and the rapid growth of gasoline consumption.
In the USSR, experiments with cracking were carried out in 1925–1928, but then a decision was made to purchase foreign Vickers, Jenkins, and Winkler-Koch units. In 1931, there were 22 cracking units in the USSR and 207 cracking units in the USA.
However, large foreign currency expenditures on the acquisition of foreign installations and the general course of replacing foreign technology with Soviet technology led to the creation of a domestic cracking installation based on Shukhov's design, better known as "Soviet Cracking". Its construction began in February 1930, a trial run was launched in April 1931, and the installation itself operated until 1935 for experimental industrial purposes.
"Soviet Cracking"
However, the Soviet Cracking unit was used for research purposes, and the Winkler-Koch units were chosen for industry as technically more advanced. Based on this unit, the Nefteproekt cracking unit project was developed in 1935. During the first five-year plan, 23 thermal cracking units were built, and during the second five-year plan, 73 cracking units.
Types of cracking
A brief presentation of cracking in its various varieties is given in general from the second volume of the textbook “Oil Technology”, written shortly before his death by Professor Sergei Nikolaevich Obryadchikov of the Moscow Oil Institute.
Obryadchikov, after graduating from the chemistry department of the Nizhny Novgorod University in 1927, began working at the Grozny Research Institute, established under Grozneft. There he developed methods for calculating the main equipment of oil refineries. His doctoral dissertation, defended in 1941, was on the topic of "Material balances of cracking and cracking depth per cycle." He died in September 1951 at the age of 48 from tuberculosis.
So, Professor Obryadchikov divided cracking into thermal and catalytic.
Thermal cracking involved the processing of distillation residues and heavy distillates into gasoline by heating, which resulted in the breakdown of heavy molecules into lighter ones, as well as the partial synthesis of new heavy molecules.
Thermal cracking was divided into low-temperature cracking (raw materials: fuel oil, tar; conditions: 490 degrees and 20 atmospheres; product: gasoline), high-temperature cracking, also known as reforming (ligroin, kerosene; 500 degrees and 50-70 atmospheres; gasoline), vapor-phase cracking (fuel oil, tar; 550 degrees and 2-3 atmospheres; gas rich in alkenes), pyrolysis (fuel oil, tar; 700 degrees, atmospheric pressure; gas rich in alkenes). Coking was also used to obtain petroleum coke (high-purity graphite made from petroleum coke was used in the nuclear project) and electrocracking in a voltaic arc to obtain gas rich in acetylene.
In catalytic cracking, contact cracking (ligroin, gas oil; 510-540 degrees; aromatized gasoline), catalytic aromatization under hydrogen pressure (ligroin; 500 degrees and 20-50 atmospheres; aromatized gasoline), cracking on aluminosilicate catalysts (gas oil, kerosene; 450 degrees, atmospheric pressure; base for aviation gasoline and motor gasoline) were distinguished.
Alkylation methods were also used, in this case the synthesis of alkene molecules (ethylene, propylene, butylene, etc.) and alkanes (ethane, propane, butane, pentane, etc.): thermal alkylation of isobutane with ethene at a temperature of 500 degrees and a pressure of 300 atmospheres; catalytic alkylation of isobutane with isobutene at a temperature of 20 degrees, a pressure of 15 atmospheres and the presence of sulfuric or hydrofluoric acid to obtain alkylate or alkylbenzene. We will return to this later, but not now.
These are just the main types of cracking, while industrial installations have many more variations, features and nuances. In my opinion, no two plants have exactly the same cracking process.
Is one oven enough? Or would three ovens be better?
In the previous series "Is it easy to distill oil?" we stopped at the fact that as a result of distillation there remains quite a lot of residue in the form of fuel oil, tar or asphaltenes. They can be burned or used in some way, but the need for the same tar is much less than for gasoline.
The distillation of oil itself turned out to be a complex process, both in terms of stages and equipment design. But this was clearly not the end of the matter. Cracking is in no way inferior to distillation in complexity.
We have fuel oil or distillates that are in low demand, such as ligroin. Ligroin is heavier than gasoline, boiling point is 120-240 degrees. It was previously used in tractor engines, but was replaced by diesel fuel, and is now used as a raw material for further processing.
This raw material must be heated to approximately 380 degrees. In cracking units, this is done using heat exchangers heated by the products, then the raw material is fed into a rectification column, where it is mixed with cracking phlegm - a heavy distillation residue, and only after this is the mixture of raw material and cracking phlegm fed into the furnace under a pressure of 15 atmospheres.
The very first cracking units included the following chain in the flow direction: heat exchanger, furnace, rectification column, then stripping column, gas separator and stabilization column for gasoline.
However, since it was not possible to completely crack the raw material in one pass, they began to drive it in a circle, feeding the cracking phlegm from the column into the furnace until it disintegrated into an indecomposable residue.
Thermal cracking was improved in two directions. The first direction is rational circulation of raw materials and cracking phlegm in order to reduce the number of cycles as much as possible. The second direction is maintaining raw materials under heating or deepening cracking.
The thing is that in the furnace, which we already know from the process of distillation of a tubular furnace, the raw material is not kept for as long as is required for chemical transformation. For example, to obtain 38% gasoline from a straight-run solar fraction, it must be kept at a temperature of 440 degrees for 122 minutes or about two hours. It is difficult to do this in a furnace. That is why they did it this way. The furnace was made with a high-pressure tube in order to drive the largest possible volume of raw material into it, and the holding was carried out in an evaporator-reactor - a large heat-insulated container in which the raw material was kept for the required time. Fuel oil or other distillates in the reactor broke down into light fractions and phlegm. Vapors of light fractions were fed into a rectification column, where they were divided into fractions, and the residues from the reactor and column were fed back into the furnace for heating before the next cracking cycle.
The peculiarity of the Winkler-Koch unit was that the fuel oil was first dispersed into a broad solar fraction and tar, and only the solar fraction was fed into the cracker. This increased the yield of gasoline and reduced the number of cycles, since the most stubborn molecules ended up in the tar, which was removed from the process.
On this basis, a new direction of cracking improvement began. It is necessary to first divide the raw material into narrower fractions and crack them separately. A unit with two furnaces appeared.
Two-furnace cracking unit
The fuel oil is first heated and fed to the first rectification column, where the light fraction is separated and sent straight to the second rectification column. The cracking phlegm of the first column is sent to the first light cracking furnace, where it is heated and fed to the evaporator-reactor. From the reactor, the light fraction goes to the second rectification column, and the phlegm is fed to the second deep cracking furnace, from there to the evaporator-reactor, and so on in a circle through the second furnace until there is no residue.
Then came installations with three, four, and even five furnaces, many of which began cracking directly from crude oil, which was driven through several heating and distillation loops until an indecomposable residue remained.
For example, a three-furnace installation consisted of the following devices:
heavy gas oil cracking furnace,
light gas oil cracking furnace,
light fuel oil cracking furnace,
evaporator-reactor at 20 atmospheres of pressure,
17 atmosphere evaporator-reactor,
7 atmosphere evaporator-reactor with plates, as in a rectification column,
distillation column,
heat exchanger,
gasoline condenser,
gas separator for separating dissolved gases.
And all this is often not instead of distillation, but together with distillation, although a number of plants were cracking plants and did not have a pronounced stage of crude oil distillation. The deeper the oil processing, the more the structure of furnaces, columns and reactors increases. In addition to this, pumps, valves, gate valves and various control and measuring equipment are also required.
The gasoline is of so-so quality
Cracking is usually spoken of in the most elevated tone, ignoring such aspects as the low quality of the product. Yes, much more gasoline is obtained than with direct distillation of oil. Only in thermally cracked gasoline there are from 15 to 25% unsaturated hydrocarbons, in particular alkenes (butene, pentene, hexene, heptene, etc.), which are almost absent in straight-run gasoline.
On the one hand, alkenes increase the octane number of cracked gasoline to 60-66 at the end of boiling 200-225 degrees and even to 70 at the end of boiling 175 degrees. For comparison: straight-run gasolines: Grozny - 57, Ishimbay - 42. Only Balakhani oil on a straight run gave gasoline with an octane number of 76. But, on the other hand, alkenes easily oxidize during storage and form resins that either settle in containers or form a sticky sediment in pipes and on engine valves. As a result of oxidation, the octane number is also greatly reduced. If fresh cracked gasoline has an octane number of 77, then after two months - 65.
Cracking gasolines had to be purified in several stages. First, washing with water to remove hydrogen sulfide, then treatment with sulfuric acid and removal of acid tar - those very unsaturated hydrocarbons prone to polymerization, then washing with an alkali solution, then secondary distillation and after that another wash with water and alkali and settling in large containers. Gasoline distillation was carried out in a tubular unit with heating to 200-210 degrees, and then the vapors were fed into a column with 19 plates, into the bottom of which superheated steam with a temperature of 250-270 degrees was fed. During purification, 81% of purified gasoline with an octane number from 58 to 65 was obtained, and 17% fell out as polymers and undercuts.
Then antioxidants such as hydroquinone or phenols from wood raw materials began to be added to cracked gasoline. Hydroquinone, when added at 0,01%, maintained the octane number of gasoline at 75 and prevented the formation of resins.
Thermally cracked gasoline was not suitable for aviation, and was used as a car fuel. For cars of the 1930s and 1940s, it was pretty good. If you had told drivers of that time that in 50 years cars would run on gasoline with octane ratings of 92, 95, 98 and even up to 100, that is, aviation gasoline by the standards of the 1930s, they would have said that we were a little crazy.
The problem of obtaining high-octane gasoline was solved only with the advent of catalytic cracking, which was developed just before the start of World War II and came into widespread use during the war years. And even then, it was a very complex technology, including obtaining the base of gasoline, various additives and anti-knock additives.
The struggle for a monopoly position
In light of this entire review of the technology, processes and hardware design of oil refining, the talk of some inherent cheapness of petroleum gasoline and the high cost of synthetic gasoline fades considerably. Such a complex process using so many devices that it is difficult to even list them, is unlikely to have such a low cost price to easily win the competition from other methods of obtaining liquid motor fuel.
There is one factor here that is little noticed. The oil industry already in the 1920s had a monopoly in the supply of liquid motor fuels for cars, aircraft, fuel oil and diesel fuel for fleet. The importance of these new types of transport was greatly emphasized by the First World War, which was won by the most motorized countries at that time: the USA, Great Britain and France. Therefore, the attention to the oil industry was very great, and oil workers were allowed a lot, because because.
As people well versed in chemistry, the oilmen understood perfectly well that the process invented by Franz Fischer and Hans Tropsch was much better than their endless fuss with furnaces, rectification, stripping and stabilization columns, evaporator-reactors and other devices of very intricate design. And the product of their oil refining is such that it has to be “conjured” to meet standards.
The Fischer-Tropsch process is variable and potentially controllable, right up to obtaining narrow target fractions of the product, which do not even have to be accelerated. Not to mention that the Fischer-Tropsch unit is much more compact and safer to operate than the tubes, columns and all kinds of evaporators of oil refineries.
If the Fischer-Tropsch process reaches its technical specifications, then all this pile of columns, reactors, pipes at the refinery will quickly turn into scrap metal. And the oil industry from the current monopoly supplier of all motor fuel, lubricating oils and a number of chemical products will turn into simple drilling of wells and pumping of oil from underground, and into sale at the well, since the drillers and pumpers will not be able to handle the oil pipelines. Whether the producers of synthetic fuel will buy oil from them is still a question. They have a choice: natural gas, synthesis gas from coal, including from underground gasification, from wood, organic waste, and so on. Oil from a depth of 1,5-2 km and more (such wells already existed in the 1940s) will not be the most popular commodity.
This is where the solution comes from: to trample synthetic fuel by all available means, first of all, with heart-rending talk about its high cost compared to oil refining, in the hope that people who don’t know the details, including those making decisions, will buy into it.
Information