The Double Contact Process For Sulfuric Acid Production PDF

 

 Journal of the Air Pollution Control Association

ISSN: 0002-2470 (Print) (Online) Journal homepage: https://www.tandfonline.com https://www.tandfonline.com/loi/uawm16 /loi/uawm16

The Double Contact Process For Sulfuric Acid Production W. Moeller & K. Winkler To cite this article:  W. Moeller & K. Winkler (1968) The Double Contact Process For  Sulfuric Acid Production, Journal of the Air Pollution Control Association, 18:5, 324-325, DOI: 10.1080/00022470.1968.10469134

To link to this article: https://doi.org/10.1080/00022470.1968.10469134

Published online: 16 Mar 2012.

Submit your article to this journal

Article views: 9933

View related articles

Citing articles: 11 View citing articles

Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journa https://www.tandfonline.co m/action/journalInformation?journ lInformation?journalCode=uawm20 alCode=uawm20

 

W. Moeller K Winkler

The Double Contact Process

Farbenfabriken Boyer  AG

For Sulfuric Acid Production

 to   2 - 3 nonoxidized  SO2 In the usual usual contact contact plants 97 - 9 8 of the SO2 is is oxidize d to    SO3 SO3 while the remaining 2 are emitted.   Th The    stack e stack   gas   generally contains 0.2-0.3 by  volume SO2. Through changing to the   Bayer  Bayer Double Contact process, tthe he conversion can  be  raised to  to  9 9 . 5 or highe r. The sulfuric sulfuric acid can  be  be  produced a t  the  same cost  as  with the  the  single  si ngle co ntact metho d, si since nce  th   the e additional capital investment  is  compensated by  by higher yield   and   throughput (this applies  to   Germ any). Bas Based ed  on  on   equilibrium considerations,  the   rates of  of conversion obtainable with   the  contact   and   double contact processes  are   shown.   The Th  e theoretical results are compared with   th the   e values obtained   in   practical operations.  The  fundamentals of   of the  process are  are explained   and tthe he  experience so  so far  accumulated  is  discussed.   Th  efir first st d ouble contact contact plant  wa was  started s  up  up in March   1964. 196 4. By the end of 1966 as  many  as   sixteen double contact plants were  in  operation and   and a further   ten  will   go   into production during  1967.   These plan ts  us sulfur,   e sulfur, pyrite, zinc sulfide,  or   sulfates as  as raw material materials. s.

The major part of sulfuric acid is In terms of quantities, sulfuric acid is by far ahead of all other chemicals with world production running to 72 million tons (short tons) in 19 1965. 65. This means that approximately one million tons SO2 are being emitted per year by sulfuric acid plants and that the share of the total SO2 emitted to 2 per cent at the most. From this it is obviou obvious s that the SO2 emit emitted ted duri during ng sulfuric acid production is not a global problem. In the neighborhood of sulfuric acid plants, however, substantial SO2  levels are often found in the air since the emitted gases seldom contain less than 2000 ppm SO2, and sometimes even over 3000 3000 ppm. Disper Dispersion sion in the a atmotmo-

produced by the contact proce process ss - in the U.S.A. approximately 90% and in the Federal Republic of Germany approximately 85%. 85 %. Bes Besides ides other sulfur-containing fur-containin g materials, it is primarily elemental sulfur and pyrites which are used as start ing materials. An  SO2 formed when O2 - N2  gas mixture is formed burning the sulfur or pyrites with air. SO 2   contents of 7 to 10% by volume are normal. The gas gas should still have sufficien suff icientt oxygen for for the ensuing oxidation of SO2  to SO SO3. 3. This oxidatio oxidation n is effected effe cted catalytica catal ytically lly on vanadium vanadi um pentoxide toxi de conta cts ; it is is a typical equilibrium reaction :

sphere is not very good since the waste gas temperature is approximately 60°C and this temperature being relatively low result results s in sulfuric aci acid d waste gases being encountered on the ground at short distances from the smokestack much more more frequently frequently than hot waste gases.  Although high chimneys have proved successful with hot waste gases from power stations, our experience has shown them to be a rather unsatisfactory approach to the problem where sulfuric acid waste gases are concerned. In light of this, research was started by our company as early as ten years ago with the aim of materially reducing the SO 2   levels in the waste gases from sul-

The equilibrium depends very strongly on temperature and it is furthermore influenced by b y an excess of oxygen:

furic acid production.

Drs.  Moeller Moeller and Winkler are associated with Farberfabriken Bayer AG, 509 Leverkusen, Bayerwerk, Germany.

2SO2 + O2 •=* 2S  2SO O3

SO3 SO2   '

0,90 40 0  

Figure  

1

rium   as

a 

420 

440   460  480 500   520  540°C

C onversion   of

reaction temperature

SO2 to SO3 at  

equilib-

function   of  t e m p e r a t u r e .

  SO2 from sulf sulfur ur combustion) Parameter: SC >2-concentration entering converter

SO ,  in stack gas Vol.  Vol. 

0,6 SO , 0,5

*-96V

0,4

 

1

0,3

^98

  s A \^

0,2

.....

— 

0,1 2 

—

— 99 L 12   Vol. SO, SO concentration entering converter 4

6

8

10

Figure   2.   Emitt Emitted ed perce ntage  of SO2 as a function   of SO2  concentrati concentration on befo re  the  converter. O   US   contact plants  contact   \ . . c  ., ,  , r raw   material; Sulfur • double contact plants) — emission, when   the gas   reached equilibrium

The resultant SO3  is then taken up in concentrated sulfuric acid in absorption towers while the nonoxidized SO2 passes this absorption tower and is vented through the chimney. For the purposes of the followingconsiderations let it be assumed that the start ing material is elemental sulfu sulfurr which is burned with air containing 2 1 % oxygen. oxyg en. The resultant gas mixture iis s to be passed on to the contact in undiluted form. form. In Figure 1 the deg degree ree of reacti reac tion on of S SO O2 to SO3 has been plotted for this gas versus the reaction temperature, assuming that equilibrium has been reached. The SO SO2 2 concentration of the starting gas has been selected selec ted as the parameter. parameter . The graphs show that the degree of reaction increases with decreasing temperature.

324

Journal  of the Air   Pollution Control Association

 

AIR FOR SULFUR BURNING

HE= HEAT EXCHANGER

SO ,  GA S

CONVERTER

TO STACK

TOWER FOR INTERMEDIATE ABSORPTION

ABSORBING TOWER AIR FROM DRYING TOWE TOWER R

 

Figure 4: Flow Flow diagra m for sulfur-bu sulfur-burning rning double-contact plant with intermedia te SO3 absorption

.450°C •400

4 6 8 10 12 12 Vol. '/oSO , SO ,  concentration entering converter

Figure 3. Concentrat Concentration ion of emitted SO2 as a function of SO2 before the converter. Parameter: Degree of reaction reaction O US conta contact ct plan plants ts / . . _ ., . . . . f raw material: Sulfur Sulfur U double contact plants

It is better for low SO2 contents of the starting gas than for high  SO  SO2 contents. With the catalysts normally used today reasonable reaction speeds can be reached at temperatures in the range of 440°C. The  SO2  content of of the s tarting gas cannot be kept as low as desired since this would call for an increase of equipment portions. dimensions in inverse proIt may be seen fr from om Figure   that the theoretically possible degree of reaction is approximately 98 %. The degrees degrees of reaction reached in commercial operations are slightly below those which would be theoretically possible. In Figure 2 the percentages emitted by 1 ten U. S. plants   are plotted versus the SO 2   concentration of the starting gas. From 1. 1.55 to 4.4% of the  SO2  are emitted; in other words, the degrees of reaction are between 95.6 and   98.5%.  Figure 3 shows that the concentrations emitted by these plants are between 0.13 and 0.54% by volume SO 2. the Bayer double contact process theInreaction is interrupted after approximately 90% SO2 has been reacted to form SO 3   which is removed in a first absorption stage, the so-called intermediate absorption, and the remaining

state of the remaining gas mixture departs from the equilibrium and permits further reaction, which means higher degrees of reaction . The theo retical proportions are shown in Figure 1. Theoretically, the double contact process allows the degree of reaction to be

process, it would correspond to O2/SO2 proportions of 1.1. No satisfactory degree of of reaction can  be  be achie  achieved ved in th is way. Wh ile, at first, the same O2/ O2/SO2 proportions are, of course, encountered with the same gas in the double contact process; the O2/SO2 O2/SO2 prop ortions hav e,

increased from approximately 98% for the contact process to approximately 99.8%,  and thus the amount of SO2 emitted is reduced by a factor of ten. The higher yield of SO2 (and thus of sulfuric acid) in the Bayer double contact process, in conjunction with the lower percentage of emitted SO2, is of course achieved at a cost. Th e process calls for an additional heat exchanger and an additional absorption tower. Figure 4 shows a schematic diagram of the process. process. The two pieces pieces of apparatus on the right of the flow diagram must be provided as additional fixtures for for the double contact process. A cost cost calculation has shown that, in the Fed-

however, improved to 6.5 after the intermediate absorption, which is generally carried out after 90% reaction. These improved O 2 /SO 2   proportions after intermediate absorption enable the operator to use a starting gas mixture of high  SO  SO2 content. A phenomenon discovered after the first trial run by the double contact principle was of great importance: On completion completion of intermedite absorption the temperature at which the catalyst begins to respond was about 50°C lo lower wer than norm al. This greatly helps our efforts at controlling air pollution since the favorable equilibrium state cuts down the quantity of emitted SO2

eral Republic of Germany, thishigher extra investment is made good by the sulfuric acid yield and savings in plant equipment since the double contact process permits higher SO2 concentrations in the starting gas resulting in a higher SO 2   throughput, or smaller equipment dimensions. Thu s the double contact plants in Europe, which use pyrites as a starting material, today operate with up to 10%  SO2 in the starting gas as compared with the formerly normal leve levell of 6. 6.55 to 7% . When using using elemental sulfur, commercial-scale trials with a starting gas mixture of 13.4% SO2 still gave good degrees of reaction for the Bayer double contact system; while the conventional contact process in such cases only allows gas concentrations of approximately 10% SO 2. These high starting concentrations of SO2 of the double contact process have been made possible by the intermediate

by50°C 50% lower. or more at a temperature which is The first double contact plant has been commissioned in March 1964. Eighteen double contact plants were on stream in May 1967 and another eight are scheduled to go into production by the end of 1967. Further plants have also also been ordere ordered. d. The process proce ss has come up to expectati expectations ons and in particular, the problem of SO 2  being emitted during sulfuric acid production may be considered solved. The SO SO22 concentration emitted by the double contact plant, based on elemental sulfur, which is located in the district supervised by us does not exceed more than 240 ppm under 20% overload and as little as 100 to 120 ppm SO 2   under normal load. References 1. Atm osph eric Emissions from Sulfur Sulfuric ic