Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28G—CLEANING OF INTERNAL OR EXTERNAL SURFACES OF HEAT-EXCHANGE OR HEAT-TRANSFER CONDUITS, e.g. WATER TUBES OR BOILERS
  • F28G13/00—Appliances or processes not covered by groups F28G1/00 - F28G11/00; Combinations of appliances or processes covered by groups F28G1/00 - F28G11/00

Definitions

• Our invention involves a method for removing scale from heater tubes. More particularly, the present invention is directed toward effecting the removal of scale from heater tubes having served as charge heaters for various petroleum refining processes. Although well-suited for use in descaling heaters which have been employed in processes for dehydrogenation, hydrogenation, catalytic reforming, hydrocracking, isomerization, etc., the method is most advantageous when applied to a process for hydrorefining.
• Hydrorefining processes are effected for the principal purpose of removing various contaminating influences from a wide variety of hydrocarbonaceous charge stocks.
• charge stocks include naphtha fractions, kerosene fractions, light and heavy gas oils, both vacuum and atmospheric, and extremely heavy material commonly referred to in the art as black oils.
• black oils are not only contaminated by the inclusion of high molecular weight sulfurous and nitrogenous compounds, but also contain high-boiling (above 1,050 F.) asphaltenes, some of which are complexed with metallic components principally comprising nickel, vanadium and iron.
• high-boiling above 1,050 F.
• a principal object of our invention resides in a method for descaling heaters.
• a corollary objective is to remove scale from heater tubes having seen service in processes charging hydrocarbonaceous black oils.
• Another object of the'present invention is to increase the time interval between shut-downs for equipment maintenance.
• more than one series of altemating oxidation and reduction techniques are conducted, the first being at a temperature in the range of Oxides, sulfides and sulfates of other metals.
• the heater tube scale after three burnings over an approximate 13-month operating period, was actually affixed to the interior of the tube walls in two distinct layers. These were 1) an extremely hard black inner layer bonded to the metal, and (2) a softer brown'outer layer bonded to the inner layer.
• the analytical analyses of the two. layers is presented in the following Table II:
• Our invention for effecting the removal of this scale involves not merely an oxidation technique, but a series of alternating oxidation and reduction techniques.
• the use of the term series in the present specification and appended claims, is intended to connote one or more oxidation techniques, each of which is followed by a reduction technique.
• a single oxidation followed by a single reduction is considered a series.
• Oxidation and reduction are effected at a temperature in the range of 950F. to about 1,250F., and preferably from 1,050F. to about 1,250F.
• the first may be effected at a lower temperature i.e., 1,000F. while the succeeding oxidations and reductions are carried out at a higher temperature i.e., 1,100F.
• Oxidation is effected under an imposed pressure in the range of about psig. to about 100 psig. in a vaporous atmosphere of 1.0 percent to about 15.0 percent oxygen, on a mo] basis. It is understood that this range is applicable when the greater share of burning is being effected, as evidenced by the quantity of carbon dioxide in the effluent gases. 'As the carbon dioxide content attains a value of nil," the concentration of oxygen may be increased until a level of 100.0 percent is attained. This high oxygen content is not essential to our invention, but may be employed to further insure complete scale removal. Caution must be exercised in not increasing oxygen content while carbon dioxide emanates in the effluent gases. The concentration of oxygen may be readily controlled through the use of steam or nitrogen in admixture therewith, steam being preferred.
• the heater tubes are flushed with either steam or nitrogen, steam being preferred for this purpose.
• the tubes may be dried with air and subsequently purged with nitrogen prior to conducting the reduction technique.
• Reduction is effected in the pressure range above set forth, and in a vaporous atmosphere of 5.0 to 50.0 percent for a period of from minutes to 10 hours, and is followed by flushing, preferably with steam, to remove loosened scale.
• concentration of hydrogen in the reduction step, and the duration thereof is generally determined by the scale formation and its removal rate. A relatively short time and low hydrogen concentration, initially, is recommended since too much scale removal at one time may result in plugging of the tubes.
• this particular heater had been burned by prior art methods three times within an approximate 13-month operating period. If each time a porous layer of iron oxide had been left on the tube wall, the succeeding start-up would fill the interstitial spaces with oil which would coke-up immediately. Continued operation would then result in deposition of another layer of scale of the composition of the outer brown layer until the next burn operation. This would account for the fact that the inner layer was virtually twice as thick as the outer brown layer.
• the acidizing is effected using a 1.0 percent to about 5.0 percent citric acid solution which has been buffered to a pH from about 2.0 to about 6.0.
• hydrogen constitutes the preferred reducing agent
• either producer gas, or the off-gas from a catalytic cracking unit may be utilized. Both of these comprise up to about 15.0 percent carbon monoxide, another vaporous reducing agent. These could be employed where hydrogen is either scarce, or not at all available.
• thermocouple which had been indicating the excessively high skin temperatures.
• the thermocouple proved to be accurate to within 7F.
• One of the smaller sections was set aside as a blank, and a second section subjected to 10.0 percent sulfuric acid at 175F. for a period of about four hours. Scale solubility was only 3.16 percent by weight.
• the third and fourth sections were used in effecting the oxidation-reduction techniques of the present invention.
• the equipment consisted of a furnace with a variac control and a quartz tube with inlet and outlet manifolds. A pyrometer was used to calibrate the control dial. The two sections were centered in the tube within the furnace which was heated to a temperature of 1,000F'. Air was passed through the tube and over the two sections at a rate of cc./min. for about 18 hours. The tube was then purged with nitrogen for a 1- hour period. Visual inspection indicated that both layers appeared to be a reddish-brown. Hydrogen was then introduced into the tube for three hours, at 1,000F., at a rate of about 300 cc./min. The scale immediately turned black. One section was removed and stored under a nitrogen blanket. The scale on both pieces appeared to be loose on most of the surface.
• the second piece was subjected to a second oxidation-reduction cycle as above described, but at a furnace temperature of 1,100F. and a hydrogen reduction period of 8 hours. Upon inspection, the scale was found to be extremely loose and would reduce to powder at the slightest touch. The entire layer of scale was homogeneous, in consistency, and there was no evidence of rigid scale near the metal tube wall.
• Solubility tests were conducted on the piece which had been subjected to two cycles of oxidation and reduction.
• the scale was 95.0 percent soluble in 10.0 percent sulfuric acid and 94.0 percent soluble in 3.0 citric acid, at 175F. for 4 hours.
• a method for the treatment of a heater employed in the hydrorefining of hydrocarbon oil contaminated with high molecular weight sulfurous and nitrogeneous compounds and high-boiling asphaltenes which comprises descaling said heater by a series of alternating scale oxidation and reduction techniques effected at a temperature in the range of from 950F. to about l,250F.
• the method of claim 3 further characterized in that the first alternating oxidation and reduction technique is effected at a temperature in the range of about 950F. to about 1,150F., and the succeeding oxidation and reduction techniques are effected at a temperature from about l,O50F. to about 1,250F.

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Production Of Liquid Hydrocarbon Mixture For Refining Petroleum (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=22280121

Family Applications (1)

Country Status (13)

Cited By (14)

Families Citing this family (1)

Citations (5)

• 1970
  • 1970-12-21 US US00100507A patent/US3732123A/en not_active Expired - Lifetime
• 1971
  • 1971-12-06 CA CA129411A patent/CA935076A/en not_active Expired
  • 1971-12-07 ZA ZA718201A patent/ZA718201B/xx unknown
  • 1971-12-09 AU AU36652/71A patent/AU456450B2/en not_active Expired
  • 1971-12-11 DE DE2161567A patent/DE2161567C3/de not_active Expired
  • 1971-12-17 IT IT54833/71A patent/IT945524B/it active
  • 1971-12-17 AR AR239667A patent/AR192729A1/es active
  • 1971-12-20 YU YU3171/71A patent/YU34910B/xx unknown
  • 1971-12-20 ES ES398145A patent/ES398145A1/es not_active Expired
  • 1971-12-20 GB GB5896871A patent/GB1363927A/en not_active Expired
  • 1971-12-20 FR FR7145720A patent/FR2119481A5/fr not_active Expired
  • 1971-12-21 JP JP46104057A patent/JPS513309B1/ja active Pending
  • 1971-12-21 BR BR8468/71A patent/BR7108468D0/pt unknown

Patent Citations (5)

Also Published As

Similar Documents

Legal Events