Surface Reactions Occurring During Pyrolysis of Light Paraffins

16 Surface Reactions Occurring During Pyrolysis of Light Paraffins CHUNG-HU TSAI and L Y L E F. ALBRIGHT

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch016

Purdue University, Lafayette, Ind. 47907

Coke and carbon oxides, both undesirable by-products, are always formed to some extent in commercial pyrolysis units. The carbon oxides, are produced when part of the coke reacts with steam that is used as a diluent with the hydrocarbon feedstock. Most, if not all, of these undesired products are formed by surface reactions that reduce the yields of olefins and other desired products. Coke also acts to increase heat transfer resistances through the tube walls, and most pyrolysis units must be periodically shut down for decoking of the tubes. During decoking, pure steam or steam to which a small amount of oxygen (or air) is added is fed to the reactor, and the coke is oxidized to produce carbon oxides. At pyrolysis conditions, steam and oxygen react with iron, nickel, and chromium to form oxides of these metals (1,2). Crynes and Albright (3), in their 304 stainless steel reactor, found that metal oxides were formed on the inner wall of the reactor used for propane pyrolysis. These metal oxides apparently often promote secondary and undesired reactions that reduced the yield of products. More recently, Dunkleman, Brown, and Albright(4,5) have reported more evidence confirming the undesirabllity of these metal oxides. Several gaseous components present during most commercial pyrolysis runs react with or at the surface. For example, hydrogen reduces the surface oxides (6), desulfurizes coke (7), and reacts with the coke Itself to produce methane (8). Cleaning coke from the surface may act to promote more coke formation, but reduction of surface oxides presumably often decreases the rate of coke formation. Carbon monoxide also is a reducing agent for metal oxides and is sometimes employed during the manufacture of steel. Hydrogen sulfide and various sulfur-containing hydrocarbons result in complex surface reactions. Treating the inner surface •Present address:

The Lummus Co., Bloomfield, New Jersey

274

Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

16.

TSAi A N D A L B R I G H T

Surface Reactions

275


of a 304 stainless steel tubular reactor with hydrogen sulfide reduced the subsequent formation of coke during propane pyrolysis (4, 9, 10). Hydrogen sulfide often acts to form metal sulfides (11, 12). More information i s certainly needed as to the complete role of the hydrogen sulfide that 1s sometimes added in small amounts when ethane or other l i g h t paraffins are used as feedstocks. In the present Investigation, considerable information has been obtained to c l a r i f y the role of the surface reactions that occur during pyrolysis. Reactions investigated include the formation and destruction of metal oxides and metal s u l f i d e s . Information has also been obtained relative to coking and decoking. Experimental Details Tubular reactors constructed from 304 stainless steel and Incoloy 800* were used to investigate surface reactions that occur during the pyrolysis of l i g h t hydrocarbons. Flows to the reactor of various gases including oxygen, hydrogen, carbon monoxide, and hydrogen sulfide were controlled to within about 2% on a relative basis using conventional metering equipment ( i n cluding pressure regulators, d i f f e r e n t i a l manometers, and metering valves). The gas stream to the reactor could be bubbled, i f desired, through water maintained at temperatures varying from about 0 C to 90 C; by this technique, the desired amount of steam could be added to the gas stream entering the reactor. The Inlet l i n e to the reactor was heated to prevent condensation of water. The reactors used were 1.09 cm I.D. and about 80 cm long, and 45.7 cm (equivalent to 42.8 cc) of the reactor was positioned inside a Hoskins e l e c t r i c furnace, type FC-301. The ends of the furnace were insulated with f i r e brick. Five chrome!-alumel thermocouples were attached to the outer surface of the reactor at various positions. Temperature variations along the reactor were in general less than 90 C when the temperature was controlled 1n the 700 to 900 C range. The temperatures reported in the result section are the maximum temperatures for the various runs. The e x i t gas from the reactor was cooled, and i t was then analyzed in a gas chromatograph employing three columns. Carbon monoxide, carbon dioxide, s u l f u r dioxide, water, nitrogen, o l e f i n s , and methane were determined on a relative basis to within about 2-3%. Analysis of hydrogen was probably accurate on a •Incoloy 800 obtained from Huntington Alloys 1s an alloy reported by them to contain 30-35% n i c k e l , 19-23% chromium, 1.5% (max) manganese; 1.0% (max)s1l1con, and the remainder mainly Iron. 304 stainless steel contains 8-12% n i c k e l , 18-20% chromium, and the remainder primarily i r o n .



276

INDUSTRIAL AND LABORATORY PYROLYSES

relative basis to within about 10% (A thermal conductivity c e l l and helium were used, and hydrogen sometimes resulted in reverse peaks). The e x i t gas stream was cooled to almost 0 C using the ice bath, and most of the water vapor was condensed. The remaining gas was metered using a soap-bubble meter. The general experimental procedure employed was to flow f i r s t one gas or mixtures of gases to the reactor at a controlled rate; the reactor temperature was adjusted to a desired l e v e l . After a desired period of time, the flow of the f i r s t gas was terminated, and a second gas flow was started. Frequently helium was used to flush the f i r s t gas from the reactor before the second one was started. Chromatographic analyses were made in most cases at frequent i n t e r v a l s , and the results were used to make material balances of a l l atoms into and out of the reactor. This general technique indicated which gases resulted in the production or destruction of coke, surface oxides, or surface s u l f i d e s . Experimental Results Oxidation of the Inner Surface of the Reactor. When oxygen was passed through unoxidized reactors ( i . e . reactors with r e l a t i v e l y few metal oxides on the inner surface), the following reaction occurred: Surface °2 " Surface When coke (or possibly metal carbides) were present on the surface, the following reaction also occurred: { m e t a 1

< surface C)

+

+

°2 *

{ m e t a l

2

C

0

{ 2 )

In order to study the surface oxidations, a series of experimental runs were made in which the surface was f i r s t oxidized and then reduced. This procedure was repeated numerous times in both reactors investigated. Starting either with a reduced and clean reactor or with a coke-covered reactor, the rate of oxidation was i n i t i a l l y r e l a t i v e l y high in a l l cases, and the rate then decreased becoming essentially zero after a l l carbon on the surface was converted to carbon oxides and after a surface layer of metal oxides was formed. The following describes the phenomena noted when a new (and non-coked) Incoloy reactor was oxidized at 800 C. With an i n l e t flow of oxygen of 30 s t d . cc/m1n., the e x i t flow was i n i t i a l l y 19 cc/min, indicating that 11 cc/m1n were being reacted on the surface. After about 20 minutes, the i n l e t and e x i t flows were almost identical indicating that the rate of oxidation was very low. Graphical integration of the graph of the rate of surface oxidation versus time Indicated that 33 millimoles 0 /sq. 2


16.


277

Surface Reactions

meter had reacted (see Table I ) . The surface area was calculated based on the assumption that the Inner surface of the reactor was smooth, and i t made no allowance for surface roughness or porosi t y that developed as the reactor was repeatedly oxidized and reduced. Key findings are summarized in Table I for runs in both the Incoloy 800 and 304 stainless steel reactors. F i r s t , the amount of oxygen that reacted with the surface increased as the reactor tube was used, i . e . as oxidation-reduction sequences were repeated. Although a l l runs were not d i r e c t l y comparable since somewhat different operating conditions were sometimes used, the amounts of oxygen that reacted (calculated as mill 1moles 0 /sq. meter) increased by a factor of about 12 as a result of the f i r s t 30 oxidation-reduction sequences in the stainless steel reactor and by factors of about 4 because of the f i r s t eight sequences in the Incoloy reactor. The largest increases in the amount of oxygen reacted occurred in both reactors as a result of the f i r s t few oxidation-reduction sequences. As the reactors were repeatedly oxidized and then reduced, the time required to complete the oxidation of the surface with oxygen increased s i g n i f i c a n t l y . For runs in r e l a t i v e l y new reactors at 800 C, about one t h i r d of an hour resulted in complete oxidation. Longer times, up to several hours, were required for older reactors that had s i g n i f i c a n t l y roughened surfaces. The following f i r s t - o r d e r kinetic equation correlated reasonably well the oxidation results of this investigation after i n duction periods of several minutes that were sometimes noted:


2

d (metal oxides) . dt " ox K

where k

Q x

[metal J

I oxidesJ[at a

(metal ]"] l°*

i d e s

(3)

ij

• f i r s t - o r d e r rate constant (metal o x 1 d e s )

(metal oxides)

sat

s

concentration of metal oxides when surface i s completely oxidized (second from l a s t column in Table I)

• concentration of metal oxides at time t

In general for r e l a t i v e l y unused (or new) reactors, a constant value of k occurred during the entire run, but for older reactors, two values were as a rule noted (see Table I ) . Whenever oxygen was used as the oxidant, the k value for the i n i t i a l portion of the run was always larger, and perhaps i t represented the oxidation of the inner and more easily oxidized layer of the surface. The k value for the second phase of the run probably represented the oxidation of the metal layers somewhat below the



0

5

8

800

800

800

800

2

8300

40

800

2030

383

24***

oxygen

2

38

800 2

1.0

oxygen

1

34

800

steam-N

0.33

oxygen

20

800

125

4090

0.33

oxygen

0

3

810

17

128

60

33

Total Oxygen Reacted with Surface, Millimoles/ sq. meter

5.0

1.0

steam-He

0

1

0

1.0

oxygen

0.33

oxygen

0

Time for Complete Oxidation, hours

Oxidant Used

0.67

0

0

Prior Sulfiding Treatments

oxygen

°c

Prior Oxidation Runs

Temp,,

Oxidation of Inner Surfaces of Reactors

n y

k , * ox* min."*

0.0008 (0.002)

0.021 (0.021)

**

0.64 (0.18)

**

0.06 (0.013)

**

0.31 (0.15)

0.24 (0.24)

* Values are reported for i n i t i a l (and l a t t e r ) portions o f run. ** k values could not be calculated since i n i t i a l surfaces were coke covered. *** Oxidation with steam was not complete even at end of run.

304 SS

Incoloy 800

Reactor

Table I.


16.


279

Surface Reactions

surface; diffusion of oxygen through pores to these layers was probably occurring. An additional explanation i s that i r o n , chromium, and nickel are transition metals each with more than one oxide. The i n i t i a l stages of oxidation may have involved the formation of lower oxides. As a reactor was repeatedly o x i dized and then reduced (the reduction step w i l l be described l a t e r ) , the values of (metal oxides) , also expressed as m i l l i moles 0 /sq. meter, also Increased οτΙβη very substantially. Furthermore, the rate of surface oxidation increased, but as a rule, k decreased. Increased temperature was found in runs made in both the Incoloy and 304 stainless steel reactors to result in increased values of both k and (metal oxides) . For runs in the In coloy reactor at°700°, 800°, and 900 C, the k values for the i n i t i a l (and l a t t e r ) stages of the runs were 8.20 (0.09), 0.31 (0.15), and 0.54 (0.54) min respectively and the values of (metal oxides) . were 34, 60, and 91 millimoles oxygen/sq. meter. These runs were made after 3 , 5 » and 4 oxidation-reduction sequences respectively. The k value for a run in a 304 stainless steel reactor , that had been oxidized and reduced 38 times was only 0.021 min" , but (metal oxides) . was 4090 millimoles oxygen/sq. meter. (A values much greater than those noted for the Incoloy 800 reactor). This 304 stainless steel reactor, however, had a highly porous and corroded surface as indicated by subsequent inspection of the reactor. Runs made using steam indicated that metal oxides were pro duced as follows: t

2


t

6

{ m e t a 1

Surface

+

H

2°

7~^

metal

0 x 1 d e s )

surface

+

H

2

( 4 )

Tests with steam were made using mixtures of essentially 50% steam and the remainder helium or nitrogen. Such mixtures have a steam content s i m i l a r to steam-hydrocarbon mixtures used as feed stocks in many pyrolysis units. For reactors that had no coke deposits on t h e i r surface, the rate of oxidation of the surface can be calculated from the rate of hydrogen formation. When coke was present on the walls of the reactor, the coke was also oxidized by the steam as follows: < >surface C

+

H

2°

*

C 0

Surface Reactions Occurring During Pyrolysis of Light Paraffins

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