Parades Ti Lac Ion

ESL-IE-86-06-63

PARASTII.IMICN PIO:ESS

Frank canfield ChenShare Coq:oration Houston, Texas

rn

OPERATIONS

Chien Jenkins

Distech Limited Stoke-0n-Trent, G.B. will require two-pass trays and two-pass will require four-pass. Another disadvantage is that tcMer internals for the Parastillation process are rrore expensive to manufacture and install. 'llle total installed cost for 2N Parastillation stages is estimated to e::IUa1 1.4 times the installed cost of N distillation stages for a given column height.

The Parastillation process is a new rrethod for ITU.11ti-stage, counter-current contact between vapor and liquid that results in 33% rrore ideal stages than distillation for a given tray spaci.n;J. Patents have been granted in the U.S.A., U.K., Europe and other countries. Perfonnance of the process has been confimm aver the past several years by eat1puter siITU.llation, by laboratory tests and in ccmnercial installations. INrRODUCrION

sm 'llle Parastillation process (1, 2, 3, 4) is a new rrethod for multistage, counter-current contact between vapor and liquid. Like distillation, liquid is contacted stage-by-stage with risi.n;J vapor as ShCMIl in Figure 1. Unlike distillation, the vapor is split into two or rrore parts at the botton of the column, and the liquid altel;natively contacts one vapor then the neKt. In the case of trays with 100% Murphree efficiency, this rrechan­ ical arran:;enent produces 33% rrore ideal· stages than distillation for a given tray spaci~. At 60% Murphree vapor efficiency, the rrechanical enhancement is 18%. In addition, because liquid always flCMs in the same direction with respect to a given vapor, concentration gradients across the trays develop rrore fully (5) and lead to higher Murphree efficiencies. On average, the Parastil­ lation process will produce 25 to 30% rrore sepa­ ration than distillation in a given column height. PARASTn.LATION VS. DISTn.LATION

l-bre separation for a given column height often is the rrost significant advantage of the para­ stillation process. Not to be ignored, ha.vever, is the possible advantage of liquid loa~. Because for single pass trays only one-half of the column is available for liquid flo... , the Parastillation process helps solve the problen of liquid mal-distribution at lCM liquid loa~. Better liquid distribution can lead to higher Murphree efficiencies. Colunns often have lCM liquid l ~ in sare sections and high in others; it is possible to use distillation in sections with high liquid loading and the Parastillation process in sections with lCM liquid loa~.

v

Inability to handle very high liquid loa~, e.g., purrp-around sections of a crude tCMer, . is tJ:e rrost significant disadvantage of the Parastillatl.on process. If liquid loa~ currently controls flooding in a distillation column, a good rule-of­ thumb is that distillation with one-pass trays

Figure 1.

Parastillation Prooess

404

Proceedings from the Eighth Annual Industrial Energy Technology Conference, Houston, TX, June 17-19, 1986

ESL-IE-86-06-63

Parastillation Process for New Towers For new construction, we assume a scenario of performing a given separation at specified conditions. Using the Parastillation process, tower height can be reduced by 20-25%, but the installed cost about 1.6 N Parastillation process stages will be 12 to 14% rrore. on purely econanic grourrls, one must carpare the savings due to a 20-25% reduction in tower height to the cost of 12-14% rrore for tower intervals. On average, we expect the trade-off to be sane­ lNhat in favor of the Parastillation process. In sane cases of very high incremental cost for increased tower height, e.g., offshore, air­ transportable packaged units, or envirorrrental constraints, then the Parastillation process will be strongly favored. Also, it is strongly favored in the case of low liquid loading where half-pass trays would be required for distillation. Parastillation Process Best for RevampS Several reasons may exist for revamping a distil­ lation colUllU1: (a) (b) (c)

save energy Improve separation Increase capacity

In all cases, the parastillation process should be evaluated as a candidate. Because rrore theoretical stages can be obtained, it will improve separation and/or reduce energy cOnstlll'ption. In cases of low liquid loading, there is nearly always significant increase in capacity. Studying the nurrber of existing colUllU1S in various services shows that a large number are candidates for retrofitting to use the Parastillation process. Table 1 shavs the results for a nurrber of geograrhical areas arourrl the world in various irrlustry segments. Alrrost 10,000 colUllU15 in North America, Western Europe and Japan are candidates for Parastillation process retrofit. TABLE 1. Existing Retrofit candidate Towers for the parastillation Process North Western America Europe Japan REFININ:; GAS PRCX:ESSIN:; OLEFINS PErRCCHEMICALS GOO 'TOTAL

2500 1000 230 1500 5230

1200

450

260 1600 3060

100 750 1300

Industry Total 4150 1000 590 3850 9590

before reaching the tray above, there is of capacity due to jet or entraimlent fl approach when carpared to conventional tr hydraulics. In tenus of pure liquid down flood, there is a reduction of tray spaci one-half of the conventional spacing. In order to take full advantage of the po ential improvement when using the Parastillation recess, skill is required to insure the correct t of tray is used. The optimum selection will elim­ inate downa:rner liquid flooding as a poss' ility. Using one-, two- or four-pass arrangement , the designer can utilize available weir 1 per half tray, equivalent to a minimum of 0.3 times column diameter to a maximum of 1.8 times colUllU1 diameter. As there is no loss of C01UllU1 ea, and therefore no constraint in the selection f down­ a:rner size, the design can always ensure t entrairrrent flood governs the capacity l' 't. The use of one-, two- or four-pass Paras stages allows this process to be substitu conventional distillation in nearly all where single-pass trays were formerly rrost cases where two-pass conventional tr used, on a minimun 21-24 inch tray spac' is no loss of capacity, but a rrost signif in fractionation in every case due to the in theoretical stages. It is not suitabl certain applications, such as pumparourrl of crude colUllU15, where liquid load is en the predaninant factor. It is of rrost va the rhase loadings are reasonably balan when the vapor phase load governs the des

llation for ses , and in ys were There cant gain increase for ections . ely ue where , or gn.

The rrost significant design consideration Parastillation process is that all of the tested hydraulic calculation techniques 1 for distillation tray design are equally to Parastillation tray design.

for the time­ ng used pplicable

TESTS

OF THE PARASTILIATION PRCCESS

Total reflux tests of Parastillation pr traditional distillation were conducted a University of Manchester Institute of Sci Technology (UMIST) and have been reported detail (1). Tests were conducted in a 2rreter column with three traditional disti sieve trays and six Parar;tillation ProceS trays of similar design. Spacing between was 24 in. in each case. Average results tests are given in Table 2 where it can that the Parastillation process produced separation than distillation. Table 2. UMIST Total rrol %

Parastillation Process - Design and Use

D~st~llatl.on

Location Each pair of Parastillation stages can be viewed as two half-tray sections separated by a dividing plate; the downstream half is lowered a half tray spacing below the upstream half. Because vapor rising fran one tray traverses a full tray spacing

Reflux Bottan Downa:rner

Reboiler

(3 Trays) 70.8 10.8 2.5

405 Conference, Houston, TX, June 17-19, 1986 Proceedings from the Eighth Annual Industrial Energy Technology

ss vs. the ce and in t. dia­ lation siE·ve trays fran the seen ch rrore

ESL-IE-86-06-63

t1=asured bottom. downcx::Iller and reboiler liquid cx:rnpositions were used to calculate Ev of the reboiler in each case. With these values set, E'mv for distillation and Parastillation process were calculated. Also an apparent Emv for Parastillation process was calculated. This was done using a traditional distillation simulation for three stages above the reboiler and getting Fmv corresponding to the Parastillation process test results, Table 3. Reboiler efficiencies are essentially the same in each case. Apparent Emv for Parastillation process is 78% vs. an actual E'mv of 62% for distillation. Parastillation process gave a 26% increase of theoretical stages. Table 3. Murphree vapor efficiencies calculated fran UMIST tests. Itan

Ev (Reboi1er)

Fmv

(Trays) Apparent ~

Distillation

Parastillation

67.3% 61.7%

67.9% 68.9% 78.2%

*Apparent Fmv is that required of three distil­ lation trays to give the separation produced by six Parastillation process trays. Part of the enhancerrent o::mes fran a higher Fm.1, 69% for Parastillation process as canpared to 62% for distillation. It is higher for tw:::J reasons. First, since wier heights and loadings were about the same on each test, the liquid depth in Para­ stillation process was greater than in distil­ lation. This is a design difference and is not an inherent benefit of Parasti11ation process. Secorrl, liquid always flONS in the same direction on a given side of the co1tmU1. in Parastillation process, i. e., Parasti11ation process approaches Lewis Case II as previously mentioned. l'obst of the enhancanent is due to the inherent Parastillation process advantage which results fran the mechanical arranganent for contacting vapor and liquid. In sumnary, increased Murphree vapor efficiency accounts for the rrove fran 61.7% to 68.9% and the Parastillation process rraterial balance effects account for the renain::ler to get an over all efficiency of 78.2% for the Parasti1­ 1ation process as canpared to 61.7% for distilla­ tion.

Two rrore ccmnercia1 installations were started up

in England in late 1985; both are operating as expected with CXIllp1ete client satisfaction. We have obtained client pennission to publish scaled test results fran one co1tmU1. and general mechanical details about another. One co1unn (Co1tmU1. #2) in which 28 Parastillation trays were installed on 21 inch spacings was 3.5 feet in diameter and 32 feet high. This was a new co1unn so no canparison with previous operations is possible. The co1unn is perfonning as expected with cx:mp1ete client satisfaction. Another co1unn (Co1unn #3) in which 32 Parasti1­ 1ation trays (arrl one conventional seive tray at the feed) were installed on 27 inch spacings was 3 feet in diameter and 50 feet high. Results fran samples taken during normal operations are shown in Table 4. Table 4. Parastillation Results at

Operating Conditions (Co1unn #3)

Reflux ratio = 0.37

weight Fraction

CCI\JX?nent A B

0.75 0.25

0.99883 0.00117

0.00294 0.99706

A reflux ratio of 0.37 is nCM required to perform essentially the same separation that was being perfonned with conventional distillation at a reflux ratio of 0.63. Ccrrp.1ter simulation results shCM that, whereas conventional distillation provided 7 to 8 theoretical stages of separation, the Parastillation process gives 10 to 11 theo­ retical stages in the same column height. Detailed operating results for this column are available on a confidential basis to parties approved by the client. OONCLUSION The Parastillation process has been thoroughly tested in laboratory arrl ccmnercial operations, and nCM is ready for general use in cases where applicable. References

Parastillation Process in Crnmercial Operations (1)

The first ccmrercial installation of the Parasti1­ 1ation was done in Eng1arrl in a co1tmU1. where conventional seive trays above the feed were replaced with Parastillation trays, rot existing Pall rings were left in the section !:;e1CM the feed. The operation of this co1tmU1. (Co1tmU1. #1) met design expectations for increased purities and increased capacity and resulted in a satisfied client.

Canfield, F.B., Chemical Engineering Progress, p. 58, v. 60, February 1984.

(2) Jenkins, A.E.O., U.S. Patent 4,496,430, U.K. Patent 2,093,712 and numerous foreign patents. (3)

Jenkins, A.E.O., U.S. Patent 4,582,569.

(4) Anon., Chemical Week, p. 30, OctOOer 19, 1983. (5) Lewis, Jr., W.K., IE:, p. 399, v. 38, 1936.

Proceedings from the Eighth Annual Industrial406

Energy Technology Conference, Houston, TX, June 17-19, 1986

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