Suppression Of Coke Formation In A Thermal Hydrocarbon Cracking Unit

Abstract

Coke formation and deposits in the inline heat exchanger of a thermal hydrocarbon cracking unit comprised of a thermal-cracking zone, a quenching apparatus and the heat exchanger are removed and prevented by introducing an aqueous solution of an alkali metal salt or hydroxide into the hydrocarbon flow at a point downstream of the cracking zone of the unit.

Parent Case Text

Cross-Reference to Related Application

This application is a continuation-in-part of our copending application Ser. No. 872,220 filed Oct. 29, 1969, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is a method for prolonging the cycle time between shutdowns of a thermal-cracking unit in which ethylene and propylene are made by the thermal cracking of lower alkanes. Specifically, this invention is a method for decreasing coke buildup in the heat exchanger employed in the cracking unit by introducing an aqueous solution of alkali metal salt or hydroxide into the unit at a point downstream of the cracking zone.

2. Description of the Prior Art

Kohfeldt's s U.S. Pat. No. 2,893,941 (1959 ) discloses a process for treating liquid hydrocarbons such as heavy naphtha, kerosene, and gas oil which involves the steps of thermally cracking the hydrocarbon in the presence of potassium carbonate and steam wherein the potassium carbonate is added as an aqueous solution preferably at a point upstream from the thermal-cracking zone. The preferred Kohfeldt teaching was followed by adding an aqueous potassium carbonate solution to a thermal-cracking unit for lower alkanes at a point upstream of the cracking zone and the unit became plugged with coke formation at the in-line heat exchanger within 7 days. The operating time in a unit wherein no potassium carbonate is used is about 7 days; therefore, Kohfeldt's preferred process is unworkable in our unit for all practical purposes.

We have found that by adding an aqueous potassium carbonate solution to the process in a unit for making ethylene and propylene or other olefins at a point downstream of the cracking zone, the furnace can operate without plugging for as long as 74 days, a more than ten-fold improvement over introducing the carbonate to the cracking unit as taught by Kohfeldt's preferred method. And the reason for shutdown of the unit after 74 days operation was not because of plugging due to coke formation in the heat exchanger but because of coke formation in the reactor coil upstream of the quench fitting. Comparable improvements are shown using other alkali metal salts or hydroxides of our invention.

SUMMARY OF THE INVENTION

The invention is an improvement in a thermal hydrocarbon cracking process comprised of a thermal-cracking zone, a quenching apparatus and an in-line heat exchanger arranged in series such that the gaseous effluent from the cracking zone flows through the quenching apparatus and then through the exchanger. The improvement is introducing an aqueous solution of alkali metal salt which yields an alkaline product on hydrolysis or an alkali metal hydroxide into the hydrocarbon flow at a point downstream of the cracking zone.

DESCRIPTION OF THE DRAWING

The invention will be further illustrated with reference to the accompanying drawings. In FIGS. 1 and 2, a feed gas is introduced by means of line 1 into a cracking heater 2 where the gas is heated at a temperature above 1400.degree. F. The effluent gases, at a temperature about 1,530.degree. F., pass from the cracking heater 2 by means of line 3 to the quenching apparatus 4. Sufficient water to cool the gases to 1,000.degree. -1,400.degree. F. is introduced at the top of quenching apparatus 4 in a fine spray by means of line 5 or alternatively by line 3. The addition of an alkali metal salt or hydroxide to the prequench water successfully suppresses the formation of heavy hydrocarbon and prevents coke and polymer formation in the transfer line 6 which feeds the gases to the heat exchanger 7 where they are cooled from the temperature of 1,000.degree. -1,400.degree. F. to a temperature of 400.degree. -1,000.degree. F. Quenching apparatus 4 can contain a baffle 10 against which the quenched gases impinge. In the exchanger, the heat removed from the gases is used to generate steam, which may then be used in other plant operations. These cooled gases then pass from heat exchanger 7 by means of line 8 to a quench tower which forms no part of the present invention. Beneath quenching apparatus 4, there can be located a coke collecting vessel 9 which is shown in FIG. 1 as an integral part of the quenching apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the production of olefinic hydrocarbons such as ethylene, propylene and other cracked products by thermally cracking gaseous and vaporizable liquid hydrocarbons in a tubular pyrolysis furnace at high temperatures in the presence of steam and using short residence time, often referred to as high severity cracking, which is followed by immediate cooling or quenching of the cracked effluent to minimize secondary reactions by direct or indirect cooling means; coke deposits and other cracked deposits often form at critical points in the apparatus. Such deposits cause interruption of onstream time of the cracking furnace and produce a serious economic problem. We have discovered that an aqueous solution of an alkali metal salt which yields an alkaline product on hydrolysis or an alkali metal hydroxide, for example, potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, lithium carbonate, barium carbonate, potassium hydroxide, sodium hydroxide, magnesium hydroxide, barium hydroxide or lithium hydroxide injected in small quantities into the hydrocarbon flow path of prequenching and subsequent cooling apparatus removes or prevents coke deposits and other cracked deposits downstream from the cracking zone. These deposits have been such a problem prior to our discovery that it was necessary to shut down the unit at least every 7 days in order to remove the coke deposits. Aqueous solutions of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium hydroxide or potassium hydroxide are preferred in the practice of our invention. By adding an aqueous solution of these salts or hydroxides downstream from the cracking zone, i.e., in the cooling or quenching apparatus including the quench and coke pots, the quench boilers and transfer line exchangers and connecting piping in the cracked hydrocarbon flow path, coking problems have been reduced significantly. In addition, the quench water is cleaner and less oils are formed when our improvement is used, illustrating a reduction in secondary reactions which thereby abates water pollution because of the reduction of total carbon going into the quench water. By reducing the amount of carbon added to the quench water, less oxygen is removed from the receiving water bodies which are, for example, lakes and rivers. The determination of the amount of total carbon in the quench water is determined by the Beckman Carbonaceous Analyzer and shows a 28 percent reduction in organic carbon in the water. The carbonates or hydroxides may be introduced into the unit in a proportion of 1 to 200 parts per million of the hydrocarbon feed to the cracking furnace. The preferred range of carbonate is about 20-30 p.p.m. of the hydrocarbon feed.

To further illustrate the improvement of our invention, a test-cracking furnace was put on stream, after decoking, under normal operating conditions and an aqueous solution of potassium carbonate, 25 to 30 p.p.m. based on the hydrocarbon feed, was continuously injected into the quench pot via the quench water nozzle. After 62 days of operation, the unit was shut down for inspection and repairs. Inspection revealed less coke fouling than normally found after only a few days operation with no potassium carbonate treatment.

A second test showed similar results to those described above. A test-cracking furnace was put on stream after decoking under normal operating conditions. An aqueous solution of potassium carbonate, 25 to 30 p.p.m. based on hydrocarbon feed, was continuously injected into the quench pot via the quench water nozzle. After 41 days on stream, the furnace was shut down prematurely for inspection. Inspection revealed no coke deposits in the quench pot or on the surface of the connecting pipe up and down stream of the transfer line exchanger and there was less coke on the inlet tube sheet of the transfer line exchanger that previously encountered after only a few days operating time without using potassium carbonate. A similar furnace run was made for cracking lower alkanes wherein an aqueous solution of sodium carbonate, 25 to 30 p.p.m. based on hydrocarbon feed, was continuously injected into the quench pot via the quench water nozzle. After 35 days of operation, the unit was still running with no indication of coke buildup in the in-line heat exchanger. A further similar furnace run was made for cracking lower alkanes wherein an aqueous solution of potassium hydroxide, 25 to 30 p.p.m. based on hydrocarbon feed, was continuously injected into the quench pot via the quench water nozzle. After 35 days of operation, the unit was still running with no indication of coke buildup in the in-line heat exchanger. Comparable results are obtained using other salts or hydroxides of our invention.

In an attempt to carry out the addition of potassium carbonate to the cracking zone as taught in the preferred method of Kohfeldt's U.S. Pat. No. 2,893,941, a test-cracking furnace was put on stream after decoking under normal operating conditions and an aqueous solution of 25 to 30 p.p.m. potassium carbonate based on the hydrocarbon feed was continuously injected into the cracking zone with the hydrocarbon. The unit had to be shut down after 7 days of operation due to plugging caused by coke formation in the transfer line exchanger.

To further illustrate the improvement of our invention, test-cracking furnaces for cracking lower alkanes were operated under normal conditions while an aqueous solution of potassium carbonate, 25 to 30 p.p.m. based on the hydrocarbon feed, was continuously injected into the quench pots via the quench water nozzles for a period of 3 months. The formation of heavy oils, hydrocarbon polymer and aromatic distillate was measured and compared to the same formation accumulated in 3 months in a unit where no potassium carbonate was used. The data in the following table illustrate the decrease in heavy hydrocarbon production when our improvement is used. Even though the aromatic distillate decreases when our process is used, better quality products in the aromatic distillate are obtained. ##SPC1##

Claims

We claim:

1. In a thermal hydrocarbon cracking process wherein the hydrocarbon cracking apparatus comprises a thermal-cracking zone, a quenching apparatus and an in-line heat exchanger arranged in series such that the gaseous effluent from the cracking zone flows through the quenching apparatus and then through the exchanger, the improvement which comprises

introducing an aqueous solution of alkali metal salt which yields an alkaline product on hydrolysis or an alkali metal hydroxide into the hydrocarbon flow at a point downstream of the cracking zone.

2. A process according to claim 1 wherein the aqueous solution is of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium hydroxide or potassium hydroxide.

3. A process according to claim 2 wherein the aqueous solution is of potassium carbonate, sodium carbonate, potassium hydroxide or sodium hydroxide and the solution is introduced into the hydrocarbon flow at the quenching apparatus.

4. A process according to claim 3 wherein the potassium carbonate, sodium carbonate, potassium hydroxide or sodium hydroxide is fed to the apparatus in a proportion of 1 to 200 p.p.m. of the hydrocarbon feed to the cracking zone.

5. A process according to claim 4 wherein the potassium carbonate, sodium carbonate, potassium hydroxide, or sodium hydroxide is in a proportion of about 20 to 30 p.p.m. of the hydrocarbon feed.

6. A process according to claim 3 wherein potassium carbonate is introduced into the hydrocarbon flow at the quenching apparatus.

7. A process according to claim 6 wherein potassium carbonate is fed to the apparatus in a proportion of 1 to 200 p.p.m. of the hydrocarbon feed to the cracking zone.

8. A process according to claim 7 wherein the potassium carbonate is in a proportion of about 20 to 30 p.p.m. of the hydrocarbon feed.

9. A process according to claim 8 wherein a coke collecting vessel is mounted beneath the quenching apparatus and open thereto.