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A disjunctive cutting plane based branch-and-cut
algorithm for O-1 mixed-integer
nonlinear programs
Yushan Zhu“ Takahito Kuno
February 27, 2003
1SE-TR一一〇3-190
Institute of Information S ciences and Electronics, University of Tsukuba, lbaraki
305-8573, Jap an
Institute ofProcess Engineering, Chinese Academy of Sciences, Beij ing 100080,
China
“ Tel:+81-298-536791,:Fax:+81-298-535206, e-mail:yszhu@syou.is.tsukuba.ac.jp
Key words. MIAILP, bTanch-and-cut,
plane, chemlcalprocess des ign.
1哲一and「ρr(~1セ。乙 読砂,unction, cuttlng
A disjunctive cutting plane based branch-and-cut algorithm for O-1
mixed-integer nonlinear programs
*Yushan Zhu Takahito Kuno
Institute of Information S ciences and Electronics, University of Tsukuba, lbaraki
305-8573, Japan
Institute of Process Engineering, Chinese Academy of Sciences, Beij ing 100080,
China
Abstract.
A branch-and-cut algorithm is presented for solving O-1 mixed-integer nonlinear
programming problems. lt is shown that the optimal solution of the continuous
relaxation problem of the O-1 mixed-integer nonlinear programming at an
enumeration node is an optimal point of the linear programming over the linear
approximation set of the O-1 mixed-integer convex set, therefore an effective
disj unctive cutting plane to cut away this point can be generated by solving a
proj ection problem due to the disj unction on a free binary variable. The
proj ection problem is further converted into a linear programming problem, and
its dual problem is applied to lift the cut generated at some node to become valid
throughout the enumeration tree. A strengthening process is given to improve
the coefficients of the di sj unctive cut by imposing integrality on the left free
binary variables. The produced cutting plane is either a facet-defining inequality
of the convex hull of the disj unctive set or a stronger one. Preliminary numerical
experiments are reported on two typical chemical design problems, and the
re sults show that the proposed cutting plane is effective for the bounding
property in a branch-and-bound framework, then makes the resulting
branch-and-cut algorithm promising for large scale practical O-1 mixed-integer
nonlinear programming problems widely encountered in chemical processes.
Keyvvords: maNLP, bTanch-and-cu4 lzft-and-projec4 disl’unction, cutting
plane, chemlcalprocess des ign.
’ Corresponding author, Tel.: 81’298-536791. Fax: +81’298’535206. e’mail: yszhu@syou.is.tsukuba.ac.jp
1
1. lntroduction
The mixed-integer nonlinear programming ( MINLP ) has played a crucial role
for the chemical process design via superstructure that always involves discrete
and continuous variables, especially in the recent two decades ( Grossmann and
Westerberg, 2000 ). The fomierly much investigated MINLP problems involves
continuous variables and binary ones, and the main characteristics defining their
mathematical structures are linearity ofthe binary variables and convexity of’ 狽?
nonlinear functions involving continuous variables ( Duran and Grossmann,
1986 ). Generally, this particular class ofMINLP problems can be described by
(po)
min!(κ)+のノ
ちア
s.t.9(κ)+By≦0,
x∈x⊂E) ln ,
アd⊂{o,1}q
where the nonlinear functions f:Sl n-ER and those in the vector function
g:g{” 一〉 mM are assumed to be continuously differentiable and convex on the
n-dimensional compact polyhedral set X = [x:x E 9{n,Ax s a,]. The finite discrete
set is given by Y=/y:yG{O,1}q,By sb,}, and c and B are constant vector and
matrix defined in Sl q and s{MXq. This separable and convex mathematical
programming structure described by PO has many variants, such as only f
nonlinear or only g nonlinear, and its application arises in several areas,
especially the synthesis problems in chemical process design ( Floudas, 1995;
Zhu and Kuno, 2003 ). ln this paper, a more general formulation for this kind of
O-1 MINLP problems is considered, as
( Pl )
minプ(X,ア)
ぎヲア
s.’.9(x,)ノ)≦0,
Ax+Gン≦わ,
x∈ERn,ア∈{o,1}q
2
where, the functions f:ER”’q 一〉 9{ and g:ER”’q 一一〉 ER M are assumed to be convex
and once continuously differentiable. Note that all linear constraints have been
incorporated into the polyhedral set described by Ax+Gy s b. The solution to
this generally formulated MINLP problem was greatly motivated by the recent
need from chemical process design with control ( Mohideen, perkins, and
Pistikopoulous, 1996; B arton, Allgor, Feehery, and Galan, 1998; Bansal, Perkins,
and Pistikopoulos, 2002 ) and process synthesis based on global optimization
( Zhu and Kuno, 2003; Zhu and Xu, 1999; Zhu and lnoue, 2001; Zhu and Kuno,
2001 ), where a reliable and fast solver for large scale MINLP problem is
necessary. MINLP problems described by PO can be solved by two well-known
techniques, i.e. the general Benders decomposition method ( Geoffrion, 1972 )
and outer approximation method ( Duran and Grossmann, 1986 ). The latter
method was further extended by Fletcher and Leyffer ( 1994 ) to solve problem
Pl. Generally, those methods solve the MINLP by computing an NLP primal
problem at some fixed integer combination and an MILP master problem
alternatively in order to get the upper bound and lower bound of the original
MINLP problem, respectively. For each round, the general Benders
decomposition method j ust generates one. cut to update the constraint set of the
MILP master problem, then it yields a pretty loose lower bound. But for outer
approximation method, since all constraints will be incorporated into the former
constraint set of the MILP master problem, then the cutting plane constraint
number grows exponentially, which is a great burden, for the solution of the
MILP problem at each round though it produces quite tight lower bound.
Although the branch-and-bound method is most popular for solving integer
programming or mixed-integer linear programming problems ( Nemhauser and
Wolsey, 1999 ), it was found that the brutal-force implementation for
mixed-integer nonlinear programming is quite inefficient due to the large gap
between the solution to the MINLP problem at an enumeration node and that of・
its continuous relaxation NLP problem. However, the successfu1 employment of
the branch-and-cut methods for O-1 integer programming ( Sherali and Adams,
1990; Lovasz and S chrij ver, 1991 ) and O-1 mixed-integer linear programming
( Balas et al., 1993; 1996 ) has spurred great interest in its application for O-1
mixed-integer nonlinear programming due to the significant progress of interior
point algorithm for convex programming problems. The problem formulation of
Pl can be rearranged to have a linear obj ective function by introducing an
additional variable, as
3
( P2 )
rnln X η+1あア
s.t.!(x,ア)一Xn+1≦o,
9(x,ア)≦o,
癬+(か≦ゐ,
x∈Sln,ア∈{o,1}q
A linear obj ective function is necessary for cutting plane kind of algorithm
because for general convex function it is possible that the minimization of the
relaxation problem takes in the strict interior of the feasible set, therefore we
cannot add a valid inequality in this situation. But, for the above fomiulation, it
is easy to see that the valid inequalities can be added since the minimization of
the relaxation problem always acquires at some extreme point of its feasible set.
Stubbs and Mehrotra ( 1999 ) first generalized the lift-and-proj ect method for
O-1 integer linear programming proposed by Sherali and Adams ( 1990 ) and
Lovasz and S chrij ver ( 1991 ), and extended their method into a branch-and-cut
algorithm for the O-1 mixed-integer convex programming. The lifting idea for
O-1 linear programming can be traced to the classic work by Balas ( 1974 ), but
not formally published until 1998, for disjunctive programming. lt is interesting
to note that the generalized idea proposed by Stubbs and Mehrotra ( 1999 ) can
be taken as an natural extension of the Theorem 2.1 in B alas ( 1974 ). The
lift-and-proj ect cut presented by Stubbs and Mehrotra ( 1999 ) i s obtained by
solving a convex proj ection problem, then it is computationally expensive. lt is
shown in this paper that a reliable disj unctive cut can be produced by solving a
linear programming for the proj ection problem over the convex hull of the
disj unctive linear approximation to the convex feasible region of the continuous
relaxation problem at each node in an enumeration tree. It should be noted that
the cut generation linear programming proposed by B alas et al. ( 1993; 1996 ) is
the dual problem of our proj ection problem with a different norm obj ective
function. But, their dual problem is always unbounded without a normalization.
Unfortunately, no normalization method now can guarantee to get a
facet-defining inequality based on the solution to the cut generation linear
programming ( B alas et al., 2002 ). This disj unctive cut constructed in this paper
can be further lifted to be valid throughout the enumeration by virtue ofthe dual
problem of the above proj ection problem, moreover, a strengthening procedure
is presented when the gap between the dual multipliers exists. Some preliminary
computational experiments are presented for two typical process design
4
problems, it is shown that the disj unctive cuts generated in this paper i s efficient
for solving the MINLP problems in a branch-and-cut framework.
2. 0-1 MINLP problem and the general branch-apd-cut procedure
The general O-1 mixed-integer nonlinear programming problem considered in
this paper can be formulated as
min礁
x,ン
s.t.ノlx十(}y≦b (P)
g,(x,y)一くO, i=1,...,l
x∈貌〃,ア∈{o,1}q
where, the nonlinear functions g,:S{”’q一一>9{, i=.1,...,1 are assumed to be
continuously differentiable and convex, and the constant vectors and matrices
are defined as d G ER”,AE9{MX”,GEERMXq,bES{M. The standard continuous
relaxation ofproblem (P) is presented as
聖艸
s.ムAx+Gン≦わ,
g,(x,y)SO, i=1,...,1,
0≦ア≦1,
(x,ア)∈E)ln+q,
Let the feasible region of the .above NLP relaxation be defined as
Ax+Gン≦b,
C一(x,ア)∈91n+9&(x,ア)≦0, i-1,_,1,
0≦ア≦1,
Hence, the feasible set ofthe O-1 MINLP ( P ) can be formulated as
5
co一{(x,ア)∈C:アノ∈{・,1},ノー1,…,9}.
and the following assumptions are made
(Al) The continuous variables are bounded, i.e. lx, i s L, i= 1,...,n, which can be
taken as a part of the constraints in problem ( P ) and L is a large positive
number.
(A2)For any(κ,ア)∈C, g’(x,ア)≧一五, i=1,...,1.
(A3) A constraint qualification holds at the solution of every resulting NLP
problem which is obtained from problem ( P ) by fixing some or all ofthe binary
variables.
With above assumptions held, we know that problem ( P ) is well-defined in the
sense that if it is feasible and does not have unbounded optimal value, then it has
an optimal solution. At a generic step ofthe branch-and-cut algorithm ( B alas et
al., 1996; Stubbs and Mehrotra, 1999 ), let (S,」) be a solution to the current
NLP relaxation of (P). If any of the components of the binary variables y are
not in {O,1}, then we can add a valid inequality to the current continuous
relaxati・n such that this・ inequality is vi・lated by⊂i・y〕・At血e same ti鵬it
should be noted that some of the binary variables are fixed either at their upper
bound or at their lower bound in an enumerative tree. We denote by c the
current family of inequalities for describing the continuous relaxation and the
newly enriched ones. We denote by F,,F, g {1,...,q} the sets of binary variables
that have been fixed at O and 1, respectively. Let
6
and let
K(加州o(x・ア)∈c
;1ゴ溜副
NLP(C, F,,F,) denote the nonlinear program
mln CX ズシア
s.t.(x,ア)∈κ(c,F。,Fi)
which is assumed to be either bounded with a finite minimum or infbasible at the
current node by virtue ofthe above mentioned assumptions. The active nodes of
the enumeration tree are represented by a list of S of ordered pairs(Fo,Fi).Let
UBD stand f()r the current upper bound, i.e., the value of the best一㎞own
solution to the MINLP problem(P).
B70nch-and-cu11フrocedureプ~)T convex O-1ルtlN.乙」P
Input:d, n, g,.4,G,~5,ノ;,ノ=1,_,1,
1. ln itialization. Set 8ニ{(Fo=Φ,・Fi=Φ)}, let C consist of the nonlinear
programmi:ng relaxation of(P)and UBD=。○.
2.1>∂de Selection. If SニΦ,stop. Othサrwise, choose an ordered pair
(Fo,F,)∈S and remove it from S.
3.」乙ower、Boundlng 8t(?p. Solve the nonlinear program M,P(0, Fo,F,). If the
pr・blem is infeasible・ 9・t・Step 2・・the職1et〔S・P〕den・teits・ptimal
s・luti If翻D・9・t・Step 2・lf iノ∈臨L…・9・let (x*・ア*)一〔晃の・
乙IBD=dx,and go to Step 2.
4・ Branching v(2アrs・us cutting declslon. Should cutting Planes be generated?If
yes, go to Step 5, else go to St(?P 6.
7
5. Cut generatlon. Generate cutting plane aoc+th s l valid for the (P) but
violated by (i,Y).Add the cuts to C and go to Step 3.
6. Branehing Ste)o: Pick an index 7’E{1,...,q} such that O〈y,〈1. Generate
the subproblems corresponding to
into the node set S. Go to Step 2.
(Fo v{i F,) and (F,,F, u{/’}), add them
When the algorithm stops, if UBD〈co, then (x“,y*) ‘is an optimal solution to
(P), otherwise (P) is infeasible. All of the steps in the above algorithmic
description are completely defined except for Step 5. And the cut generation
step introduced in the underlying section is divided into two stages, first a valid
inequality is generated at the current node on the basis of its MP relaxation
problem, then this cut is lifted to be valid throughout the enumeration tree.
3. Separating inequality
The li負一and-pr(刀’ect cut is used in this paper to solve the MINLP problem(P),
which is obtained by a procedure that lifts the constraint set of the continuous
relaxation into a higher dimensional space by introducing new variables, adds to
it some equations implied by O-1 conditions, and then proj ects it back onto the
original space by eliminating the new variables ( Balas et al., 1993; Balas et al.,
1996; Stubbs and Mehrotra, 1999 ). The newly enriched inequality is still
satisfied by every feasible solution to MINLP, but it can cut off some parts ofthe
original space as implied by the O-1 conditions. Then, the outcome of the
process of lifting-strengthening-proj ecting is a constraint set tighter than the
original one. For the MILP problem, the lift-and-proj ect can be directly
generated form the original linearly constrained set, but it i s not convenient for
MINLP problems since it concerns of the nonlinear constraints. Stubbs and
Mehrotra ( 1999 ) proposed an excellent way to generate lift-and-proj ect cut by
solving a convex minimum distance problem after reformulating the proj ection
problem by using the perspective functions ( Hiriart-Urruty and Lemarechal,
1993). ln this paper, it i s shown that strong lift-and-proj ect cut can be obtained
by solving a linear programming ( LP ) rather than convex programming for the
8
O-1 MINLP problems.
3.1 A linear approximation of the NLP relaxation
The general continuous relaxation for MINLP at some node in an enumeration
tree can be described by
min dx
x,ア
3。t.ノIx十Gア≦ろ,
(NLP)&(x・ア)≦・・i-1・・…1・
アノ=o,ノ∈F。,
アノ=1,ノ∈F,,
(x,ア)∈E}ln+・,
where the newly reformulated linear constraint set consists of the original linear
constraint set and the upper and lower bound constraints for binary variables,
then A E E}1(’”“2q)Xn,G E s{(M“2dyq,b G g{M’2q. Assume that the above NI.P contihuous
problem is feasible and has a finite minimum at (2,Y), since otherwise the node
is done. Then, a linear approximation problem at (il,P) for the above NLP
problem can be given by
min dx
x,ア
s.t.ノ4 x十Gア≦b,
(LP)&〔i・y〕+▽嗣/二1〕≦い一一
アノ=o,ノ∈F。,
アノ=1,ノ∈F,,
(x,.y) E E}ln+q,
where the original convex and differentiable functions are replace by their
9
first一・rder Tayl・r apPr・ximati・n at〔x・ア〕・Acc・rdingいMILP pr・blem
corresponding to the MINLP’ 垂窒盾b撃?香@at the current node can be described by
min礁
x,ア
s.乙・4λ:+Gア≦ゐ,
(MILP)嗣+▽&欧二1〕一一一
アノ=o,ノ∈F。,
アノ=1,ノ∈F,,
x∈1)ln,ア∈{o,1}q,
The following theorem tells us the relationship between the NLP and its LP
approximation at (2,Y), and helps to realize that (il,P) is an extreme point of
the relaxed feasible set ofthe above MILP in the sense that it can be cut away by
introducing a valid disjunctive cut to the above MINLP feasible set.
伽一3・L A∬随一ゐ・vε皿P漁e剛胆al・s・一’〔x・ア〕・
so it is also an optimal solutlon ofthe above LP.
Proof. lt is obvious to observe that (i,Y) is a teasible solution to the primal LP.
And the dual formulation ofthe above LP can be presented by
( DLP )
鍵)+μ〔9(s・ア〕一▽練〕〕
&乙蜘▽9〔x, y〕一d・
一ノしG+δo+δ1=0,
iZ E ER?,,a E E}lzl,60 E E}ltlFol,6i E fi){tlFil,
10
By virtue ofthe first-order Kuhn-Tucker condition ofthe NLP, we have
一〃一μ▽k一d・ む
一ノLG+δ+δ=0
む
λ∈s{f,μ∈蝋,δ∈㌶凧δ∈㌶1君「
Then,(S, 」)
is a feasible solution to the dual LP. By virtue of the strong dual
theory, we know ⊂動 is an optimal solution to the above LP. O
The geometrical explanation of the above linear approximation is presented in
the Figure 1, and it is obvious that the mixed-integer set is expanded after linear
approximation. lf we introduce the subdifferential concept of the convex
function into our problem, then the assumption that the nonlinear function is
continuously differential is not needed. For a nonlinear inequality constraint
g,(x,y)gO defining C, let
嗣≡恥〕+ξ七二1〕鯛蝋ア)∈c}
Then, the linear approximation set of C can be constructed by choosing any
element from this subdifferential set. The geometrical explanation is to use the
tangent cone at the point (i,Y) to apProximate the convex set. According to
Theorem 3.1, it i s interesting to note that the Gomory mixed-integer cut can be
easily generated to cut away the point (S,Y) since the optimal simplex tableau
can be easily recovered when the optimal solution is available. And the lifting
procedure for Gomory mixed-integer for MILP is provided by Balas et al.
( 1996 ). But, the efficiency ofthe Gomory mixed-integer cut is not comparable
by the disj unctive cut which is elaborately generated by solving a linear
progranmiing in the consequent section.
11
3.2 Lift-and-project cut generated based on LP approximation
For problem ( P ), it i s very prospective to generate the lift-and-proj ect cut based
on the LP introduced in the former s ection rather than the NLP, but it still can
cut away the point (2,P). Such cut can be derived by imposing the O-1
condition on a single binary variable yj while O〈 y」 〈1. ln Figure 1, the short
dashed line represents the cut generated directly by using the convex hull of the
mixed-integer convex set presented by Stubbs and’ lehrotra ( 1999 ), and the
long dashed line stands for the cut to be generated in this paper based on the
linear approximation. The direct cut generation needs to solve a convex program,
which is computationally expensive though it might yield better quality than that
described here if measured by some norm distance between the cut hyper-plane
and the current NLP solution (i,」).For cut generation, the node sets Fo and Fi
can be expanded to include additional binary variables whose optimal solutions
are obtained at O or 1 for the NLP problem at the current node. Then, we have
F・一o1:ア’一・}and F,一{i:ア’一1}・lt is n・t difficult t・veri幽the ab・ve
NLP and LP problems have the same optimal solutions if we change their
original node sets to be the expanded ones. Then, iet the feasible region of the
above LP be defined as
K =K(C, F,,F,)= (x,ア)∈1)1”+・
・4x+Gア≦ゐ
ア,=0,ノ∈1㌔
ア1=1,ノ∈Fi
where A E s{(”’“2q’i)×”,G E s{(””29“ihe,b E g I M’2q’i, i.e., the newly reformulated linear
constraint set consists of the linear approximation set besides the original ones,
as
12
Ax+Gy≦ゐ,
A.+Gア≦5≡▽ガ向κ+▽9・〔s・ y)ア≦▽9@〔1〕一9㈲
一ア1≦O i=1,_,(1,
ア,≦1 i=1,_,9,
It should be noted that the node sets now have changed to the expanded ones. ’ 撃
we impose the integrality on some binary variable y」 while O〈y,. 〈1, the
lift-and-proj ect cut can be obtained’by choosing the valid inequality for
P」・ (K) :conv(K A ((x,y)E E}ln+q :yy. E {o,1}]〉
And the convex hull of this union set can be further described by its disjunctive
form as
P、’(K)一・・nv({κ∩勧∈ERn+・:アノ≦・}}∪(K∩{(切∈ER・n+・:一アノ≦一1}}}
Note that the disjunction (yj 〈一 O)v (一 y」 一く 一1) instead of (y」 = O)v (y」 =1) is used
to describe p」 (K), though both of them are equivalent to each other. A key idea
of disjunctive programming is that it is equivalent to a linear program in a
higher-dimensional space, as that described in Theorem 2.1 by Balas ( 1974 ). ln
this paper, it i s shown that the proj ection problem to generate’the cut can be
achieved by using this reformulation. First, we introduce the following notations
to simply the description of the constraint set of the above LP by using a linear
tran’唐?盾窒高≠狽奄盾氏D Let F = {1,...,g}X(F, u F,) denote the set of free variables at node
(F,,F,), and the vector corresponding to those free variables can be defined as
yF=yN{y,:ie(F, v F,)}.. The columns of matrix G corresponding to those
13
fixed binary variables can be removed from the constraint set by defining
GF@= Gx(Gi :iE (F, v F, )/, and the right-hand side can be presented accordingly
-F ” 一 一 一17 Fas b ==b-2Gi.Finally, the rows in matrices A and G , and vector S that
iEFI
correspond to the upper and lower bounds of the fixed binary variables are
removed. After doing the above operations, we can assume without loss of
generality, that F, =¢. S ince if F, #¢, then all the variables yk in Fi can be
complemented by 1-yk which amounts replacing the columns Gk and the
right-hand side with 一Gk and b-Gk, respectively. B efore we go to the next part,
we assume that we have already done those complementing operations on the
linear constraint set of the above LP. Mu,ch more clearly, we use the following
equations to show the above reduction process for the linear constraint set ofthe
LP approximation at the current node. After complimenting, the LP constraint
set can be presented as
鰍Σ戸烈一G’ア・〕≦5-iδ’
一 一e 一Q
which we denote by A x+G y一くb . The reduced LP constraint set after
removing the fixed binary variables becomes
ア ダ ア
・4κ+Gア≦b ≡
Ax+ΣGノァノ≦ゐ一ΣGノ,
iEF iEFI
▽嚇+聾▽嗣アノ≦▽9暁〕嗣溜▽イえラ}
一Y, 一くO iG F,
ア,≦1i∈F,
ア ア アwhere Z∈m(m+2iFl“’)×n,G∈E)1(m+21FI+1)×IFI,S∈Dl・・21・FI・1.Then, the feasible regi・n・f
the above LP can be reformulated by
14
K十一i∴+G冨}
Then by virtue of the Theorem 2.1 in Balas ( 1974 ), we have the following
theorem.
Theorem 3.2. The convex hull of Pj(K), i.e. conv(P,.(K)), can be described by
X= Uo 十 Ul,
.Jy F :vo +vi,
一F 一F 一F ・4Uo+G Vo一わえ’o≦0,
Vo,ノ≦0,
conv(p」 (K)) =/(x,.y”)G si”’iFilZ” u, + (1>F v, 一一 5” ,z, s o, L.
一一 vl,」 一く 一/1,
Z, 十Z, =1;
!o,/, 2 O,
Uo,ui E Sl”,vo,vi E sllFl,
Proof. The bounded property of P,(K) is implied by the former assumptions
that all continuous variables are bounded. Obviously, the same is true for
conv(P」 (K)). Then the above Theorem is a direct result of Theorem 2.1 ( Balas,
1974)applied into the disjunctive set P,(K). O
Let (il,」) be the optimal solution wh’en solving NLP(C,F,,F,). First, we
assumet泣堰lisn・t・feasible・t・pr・blem(P)・ and let/be the binary variable
index such that O〈y」〈1. ln order to find a strong valid inequality for p」(K)
15
that cuts away (i,Y) as much as possible, Balas et al..( 1993 ) proposed to
solve a cut generation linear program which maximizes the Euclidean distance
between (S,Y) and some facet of P」(K). ln fact, such generation procedure is
exactly the dual linear program of the proj ection problem, i.e. the problem of
minimizing the distance between the point (i,Y) and the convex hull of p,(K)
measured by some norm ( Stubbs and Mehrotra, 1999 ). ln this paper the
l, 一norm・is used to generate a valid cut for p,. (K). As
( CP(F) )
min f(x,アF)
s・t・κ=Uo+u1,
ゾ=v。+Vl,
ア ア ア
・4Uo+G Vo-b 2,0≦0,
Vo,ノ≦0,
ア ア ア
・4Ul+G VI一ゐ2、1≦0,
一Vl,ノ=一Zi,
λ。+!,=1;
・Z。,Zi≧0,
U。,Ul∈㌶〃,V。,Vl∈㌶IFI.
whe瞭ゾ)掴一〔 Fx,ア〕1丘濤κ’一ii+暮減・The-f・ll・wing
separation theorem from convex analysis ( Rockafeller, 1972; Hiriart-Urruty and
Lemarechal, 1993 ) ensures that a valid inequality can be obtained after solving
the above proj ection problem. Note, the only convexity of the above program is
implied by the piecewise obj ective function.
Theorem 3.3. Let (S,」F)be an optimal solution to the above CP(F) problem.
Then, there exi sts a valid inequality apc+/3 F Jy”s/, which cuts away〔静り・
16
The coefficients ofthis valid inequality are given by
α一一嗣}
β㌧鱗同
7一一嗣〕唖アF〕デ・
Proof. Since
〔 Fx,ア〕
is an optimal solution to the above minimum distance
pr・blem・ and we kn・w that〔x・・yりd・es n・t be1・ng t・
virtue ofthe separation theorem, we have
り ぶ ズ 一~γκ,ア
.lyF一ア
(s,yF)(1;iy.)so
is the subdifferential・f!(x,ジ)
cony(P,(K)), then by
where k at〔 Fx, y〕・andthenweget
the cut stated in the above Theorem by doing some rearrangements. lt is obvious
to observe that function f(x,yF) is convex, so according to the definition ofthe
subdifferential for convex function, we have
〆+傘∵.≦!(x・ゾ)
ア ーア
By letting@ゾ)一〔 Fx,ア〕
F)(1;i,F)
andnoting (i,YF)#(S,YF),wehave
17
・f
k Fx,ア〕/初≧飼〕一!〔i’デ〕≧〔 Fx, y〕一〔ガ〕
1
>o
Then, the valid inequality denoted by
(N,YF)from p,(K). e
ax+fi F y F s r in the theorem cuts away
The objective function defined in CP(F), i.e. f(x,yF), is convex, but not
differentiable at point (i,YF). And, it is easily to observe that the
subdifferential at this point belongs to a cone set. But there exists only one
element for the subdifferential set at any other point, i.e. the gradient of f(x,yF).
since the relation (i,」F)#(i,YF) always holds for the above separation
problem, then we can change the subdifferential stated in the above theorem’ @into
the gradient, as
α一施F}
18 F一一畷ガ}
γ一幅茂アF}一点ガ〕∴
The 1,一norm convex function f(x,yF), or piecewise linear function, can be
easily transformed into linear function by introducing additional variables, and
18
the resulting proj ection problem is equivalent to a linear program by increasing
some additional linear constraints on those newly introduced variables, denoted
by LP(F), we have
min 2 Zi+Σw、
iE V iEF
S・1・X=UO十Ul,
アF=Vo+v1,
_F _F _F
・4Uo+G Vo一わノし。≦0,
Vo,ノ≦0,
ア ア ア
AUI+GVI-b!,≦0, 一VI,ノ≦一!l,
/o十!,ニ1;
一一 z十x≦λ:,
一z-x≦一x,
ア
一W+アF≦ア ,
ア
一w一アF≦一ア ,
Zo,!i≧0, z, w≧0,
X,U。,Ul,.Z∈ERn,ノ,V。,Vl,W∈㌶囚.
This linear program has 4n+41Fl+2 variables with 2m+3n+71Fl+21+3
equality’@or inequality constraints. By noting the coefficient ofthose correlations,
it is clear that the scale of the LP(F) can be largely reduced by decreasing the
free binary variable set. By solving this linear program, we can obtain its
solutions denoted by (i,5)F,ilo,ili,Vo,Vi,」io,」ii,2,W) as well as the dual
multipliers. Note that those dual multipliers are equivalent to the Lagrange
multipliers ifwe solve the convex program directly. Denote by z.”,z,F,6,F,6,F,6,”
for the equality constraints, pt,F,pt,F for the inequality original constraints, and
s.F CE.” Cgpf,〈pf for the additional constraints in LP(F), then we have the following
19
dual linear program for LP(F), denoted by DLP(F), as
ア ア
max一δf一εfκ+εf x一ψfア+ψfア
s.t.πf+εf一εf≧o,
πf+ψf一ψf≧,
ア
一πf+,a8 A≧o,
ア
一πf+μ侮≧o,
ア
一πゴ+μ8G+δ♂θノ≧o,
ダ
一πゴ+〆G一δrθノ≧o,
ダ
一μ♂b +δノ≧0,
ア
一μfb +δガーδ’≧0,
一ε卜εf≧一1,
一ψf一ψf≧一1,
μ8,μr,εf,εf,ψf,グ≧o,
π詞,εf∈ERn,πゴ,ψf,ψf∈㌶IF1,
μ8,〆∈Sl”’+2「F「+’,δ♂,δr,δガ∈㌶,
The corresponding dual multipliers to the solution of the LP(F)can also be
obtained by solving this dual linear program directl)1. Following the above
n・t就i we have〆・論∴ラ霜・Sl・5凶as血s・luti・nt・
above DLP(F). Since we have assumed that the MINLP is bounded, then the
corresponding LP(F)generated at each node is bounded too. Then, the fbllowing
relation can be easily obtained by using strong duality theorem, as
り ア ア ア ア リア ア ふア ア
1・z+1・w=δ1+δz-s+x+s一:κ一ψ+ア +ψ_ア ,
By virtue ofthe Kuhn-Tucker condition ofthe CP(F), we have
20
施り一π∴
一病アF〕一πゴ・
Then the cut generated in Theorem 3.2, i.e. ex+βy≦γ,can be reformulated
by the dual multipliers and primal solutions to LP(F), as
ア π∫κ+ゆF≦πfκ+πゴァ
By virtue ofthe dual linear program, i.e. DLP(F), we have
ア 一πf+,“8 A≧Ol
ア 一πガ+μ侮≧o,
ダ 一πゴ+,Lt8 A+δ8θノ≧o,
ア 一πゴ+μ一一δfθノ≧o,
ア δf一μ8ゐ ≧0,
ア δガーμrZ) +δf≧0,
Note that in any optimal solution to LP(F), which is also dual feasible since
LP(F)is always bounded according to the former assumptions. Then, we have
α,=mi晦1,αi}加∈N,
fii=min(fii,β2}加∈F,
where
ア ダ
α1=,a8 A,α’=μ侮,
and
21
ヘア/i11 ・・ ,u8 A+δ8θノ,
ヘダ
β2=μ侮一δ’θノ,
By virtue of the above derivation, we have that the cut denoted by
cxM+6FyF 一く7 at the node (F,,F,) is a valid inequality for the MILP problem at
that node, for it was generated according to the linear relaxation of the MILP
problem at that node together with a O-1 condition imposed on a additional
relaxed binary variable. S ince every nonlinear function in the formulation of the
MINLP is convex, then we have
gi
ii,Y)+vgi(il,Y)(1:Xl.)一くg,(x,y)so, i-i,...,i.
This relation implies clearly that the feasible set of the MINLP at the node
(F,,F,) is contained in the feasible set of the MILP at that node. Then, the cut
ex+fi”yF s/ is a valid and proper inequality for the MINLP problem at that
node denoted by (F,,F,), as well as its descendents since where the variables in
(Fo,Fi) remain fixed at their values.
3.3 Cut lifting
In general, a cutting plane derived at a node in an enumeration tree defined by a
subset (F,,F,) of the binary variables, where (F,,F,) index those variables
fixed at O or 1, respectively, is ’盾獅撃凵@valid at that node and its descendants in an
enumeration tree since where ’the variables in (F,,F,) remain fixed at their
22
values. Such a cut may not be valid throughout the enumeration tree and
therefore might be violated by other nodes. Then, it is extremely important to lift
the cut generated at some node to be valid throughout the enumeration tree for it
not only can reduce the need for extensive bookkeeping, but also can possibly
improve the bounds at the other nodes in the enumeration tree. A cut generated
at some node can be made valid for the whole MINLP problem by computing
appropriate coefficients for the fixed binary variables belonging to the subset
(Fo,F,), but the sequential li甜ng procedure(Nemhauser and Wolsey,1988)in
general is a daunting task, which may require the solution of an integer program
for every coefficient. An important advantage of the cuts generated by the
lift-and-proj ect way is that’ the multipliers, i.e. zf,7vS,6,F,6,F,6,F and ,a8,pt,F,
・btained al・ng with the s・luti・n〔”りby s・lving LP(F)can be used t・
calculate closed form expressions of the coefficients fi, for the variables
iE F, v]F,. First, we lift the inequality obtained at the current node into the
complemented original space ofthe MILP problem, that is
{“ア)一:x
Let 1’E{1,...,q} be an index such that O〈y」〈1 and consider the inequality
aqx+6qy m〈rq generated over the complemented original space of the MILP
problem, this is to solve the linear program LP(Q). For clarity, the corresponding
CP(Q) to this linear program can be expressed as
23
( CP(Q) )
min f(x, y)
Sl. X= Uo 十 Ul,
ア=VO十Vl,
m 一e 一e Au,+G v, 一一一b Z,gO,
Vo,ノ≦0,
一 一e me Au,+G v,一b !,SO,
一Vi,」 〈一 ’一iZi,
λ。+ろ=1;
!,,Z, ) O,
κ,U。,Ul∈ERn,ア, V。,Vl∈91q.
C・mpare this c・nvex pr・gram with CP(F), the new cut vect・r (a・,/g q,1・)can
be calculated by the cut vect・r@β・,γ)and its multipliers, i.e.ゐ∴〆. By
virtue ofthe Kuhn-Tucker condition ofthe above convex programming, we have
ム ム ムリ
一門 κ,ア =πκ,
ム ム ムリ
一▽ろん,アニπ・,
wherkarethes・luti・nst・theab・vec-vexpr・gra叫and〔劃are
the corresponding Lagrange multipliers. Those values can be obtained by the
corresponding LP(Q) to the above CP(Q), but, first we have the following
notations compared with those appeared in LP(F), as
k一 ヨ♂一〔で1 }嗣アー〔チ〕
where the set 2F == F, v F, for node (F,,F,). Then, the linear program LP(Q)
24
corresponding to CP(Q) can be presented as
min 2 zi +2 wi
i∈ノ〉 ’∈O
S.t. X = Uo 十 Ul,
ア=VO十Vl,
一 一e 一e Au,+G v, 一一b !,gO,.
Vo,ノ≦0,
. .e 一e Au,+G vi-b !, gO,
一vl,」 一く 一/1,
!o 十/i =1,
一z十xS-x ,
一z-xS-x ’
一一・w+ア≦ア,
一W一ア≦一ア,
!o,4 2 O,
X,U。,Ul,Z∈E)ln,ア, V。,Vl,W∈91q.
Note that more variables and constraints are added into this linear program
compared with LP(F). But, by using the solutions to LP(F) and its multipliers,
we can obtain the optimal solution to the above LP(Q). The following theorem
shows how to get those solutions and how the inequality apc+fi F y” s/ can be
extended into a valid cut a q x + fi q y s /9 throughout the enumeration tree.
Theorem 3.4. Assume that denoted by (},i,Do,Di,90,9i,Ao,Ai,2,W) be the
solution to the linear program LP(Q), then it can be constructed by the solution
t・the LP(F)as潟一〔ラF嚇一ii・・Dl一憂1鍾…)・01一阿
Ao 一io, Ai 一ii, 2-2, and O=(W,o). And the corresponding dual multipliers
25
・fLP(Q)den・tedby〔身1・2∵・凝ゆ1・甜・31・綱・als・thes・1uti・n
of the dual linear program DLP(Q), can be constructed by those of DLP(F)as
∴易1・油蝉∈F・㌶一min{齢1∂1}圃uF,・㍍
and♂1伺・Then the inequalityαWア≦・・described by
ムリ ムリ んリバ ムリム
π。x+π〃≦π。κ+π〃is valid fbr the entire enumeration tree, and cuts away
〔えの・
Pr・・f・ The feasibility・fシ堀・・Dl・磁・・Al・2・h) t・LP(Q)canbejusti且ed
by showing that the more variables and constraints are satisfied by the above
construction compared with those of LP(F). The more variables hold since fbr
一 _9F
i¢ Fwe haveア、=0・The more constraints are satisfied by noting that∂,≧O
fbr ノ¢F and !o,ノli≧0,expressed as
び
一∂ !o≦0,
びり
一ろ !i≦0,
The feasibiiity of the duai soiution (fi9’,gg,g9,fi9’,dig,di9,b8,Q9’,3g,39,39’) t.
dual linear program DLP(Q), described by
26
max一δ9一ερκ+εξκ一ψρア+ψ皇ア
S.t. Z.O一+S.O一一s-O-20,
z,O. +q.O. m¢一〇. 2,
一z.O一 + ,Lt8一 A 2 O,
一z.O’ + ,Lt9 A) O,
一e -z,O一 +pt,O一 G +6,0’e, 〉一一 O,
一e 一一 z,o一 +pt,e G 一一 6,ee」 〉一 o,
一e 一一 pt,O一 b +6,0一 SO,
一e -pt,O-b +6,0一 一一 6,e so,
一s.O一 一s-O一 2-1,
一q.O一 一q-0’ 2-1,
,“8’,,Lti2,s.O’,E.O’,〈p9,q9一 ;}r o,
7r9一,s.O一,s-O’ E ER”,7z79’,gp9’,cp9’ G ERq,
,Ltg一?,,Lt9一 G 91M+2q+i,6,0一,6,e,6,0一 E 91,
can be verified by its constnlction fbr the more constraints and variables
compared with those ofDLP(F), and the former ones are expressed clearly as
〈9 ~F_9
一πアノ+μ。G・≦0, for’∈1㌔uF,,
ムリ ア リ
一π,,’+μIG≦0,f・ri∈F。uF,,
The optimality can be indicated by the strong dual theorem by noting that the
fbllowing equality holds, as
A A Ae Ae Ae一 A9一 Ae一 Ae-1・z+1・w = 6i + 6z-s+ x+ s一 x一 q. y+ q一 y,
Then, by virtue of the Theorem 3.3,
the entire complimented MILP, as
we have the following valid inequality for
AO AO AOA AOAn. x+zy y S z. x+ zy y
27
Clearly, this inequality rewritten by a q x+fi q y g/q
that for i¢F we have y,=O. D
cuts away (i,Y)by noting
It should be noted that the cut is “optimally” lifted by Theorem 3.4, in the sense
Ae Ae Ae A Ae Athat the resulting cut z. x+z, y s z. x+zy y is identical to the one that would
be obtained by solving LP(Q) directly.
3.4 Cut strengthening
The mixed-integer inequality a q x+6qy 一く 7 q obtained in the preceding section
can be further strengthened by imposing integrality on the left binary variables
which are still not fixed at zero or one, and this process can be easily
implemented by using the Gomory mixed-integer rounding ( Nemhauser and
Wolsey, 1988 ). But, B alas et al. ( 1993 ) introduced a cut strengthening process
( Balas, 1979 ) originated from disj unctive program for mixed-integer cut, and
showed that the Gomory mixed-integer rounding is just a special case of this
general method. Here, we apply the original ideal of the disj unction by Balas
( 1979, Theorem 7.1 ) into the primal problem and strengthen our inequality
generated and lifted in the former sections by using the dual feasible conditions.
Theorem 3.5. The inequality Qpc+fiy g 7 which is strengthened from the valid
inequality a C’x+6qy s 7 q is also valid for MINLP, where
ai = ai,
/3’i =
,6,i
/ == 7q,
プ~)7i=1,...,n
maxoβ1+δ柿2一小’」/・ f・ri∈鴫
。」, for i E F, v Fi v {7’},
28
where 6
3.2, and
,6,2,6,F,6,F are obtained by the solution to LP(F) described in S ection
r一一 ’ U,2-iB,i
mi = (5’c5 + ,s,F ’
f・r i∈F鵬
Proof. To show that the strengthened cut is valid for MINLP can be done by
proving that it arises from a valid disjunctidn for the O-1 solution of its linear
approximation, i.e. the MILP. Suppose that in order to derive a cutting plane we
use the following valid disjunction (Balas et al., 1996 ). as
Pj(κ)一。・nv({K∩勧∈ERn+・:アノ吻≦・}}∪{K∩((x,ア)∈ERn+・:一アノー、my≦一1}})
instead of
p」, (K)= conv((K A ((x,y)E E)ln+q :y」 〈一 o]]v [K A [(x,y)G g{n+g :一y」, 〈一 一1]]),
where m is a vector of all integer components. The LP(F) based on this
disj unction can be obtained j ust by changing the two inequalities corresponding
to the above disjunction, as
v。,ノ槻v。≦0,
一vザ脚1≦一1,
Then, the corresponding changes in the DLP(F) for cut vector can be presented
as
ア
ーπゴ+μ8G+δ8θノ+δ8駕≧o,
ア
ーπゴ+μ8G一δ、Feノー 6、Fm≧0,
29
By virtue ofthe optimality ofLP(F) or DLP(F), we have
β万一m.4x min [6+δ8吻,,βノーδr吻,}加∈F\{ノ}
m
The maximum is implied by that m,,iEFX{/’} can be chosen as integers that
make 6,”,iEFX{/’} as large as possible. The inner minimization can be simply
eliminated by making the two terms in the bracket equal to each other, then we
have
1. 6,2-6,i
mi = 60” + 6iF
Take-m’一 m司・r-mi一同・
for i E F X {/’ }・
whichever yields the maximum value for
6,F ,i E FX {7}, as
β万一max{β1+δ伸・一δr國}加∈F魁
Then, the theorem follows by noting that the cut vector for other variables does
not change during this process. O
By virtue of the argument of the above Theorem, the strengthening gap is
determined by lfi,2 一fi,1, and this absolute value becomes zero when the ith
binary is lying at O or 1 (Balas et al., 2002 ). As that commented by Balas et al.
( 1993 ) for cut strengthening, a linear transformation, i.e. complementing, ofthe
linear constraint set A x+G)ノ≦b leaves K, Pj(K)and its facets unchanged, but
it can change the effect of the strengthening procedure. Then, it should be noted
that the above cut strengthening can be done after the solution to LP(F) is
defined, then the disjunction is changed only a posteriori. Remember that we
30
ever assume that the index set Fi i s empty. If this i s not true for the MINLP
problem at some, we do complementing on this set to change it become empty.
Then, finally doing a simple reverse linear transformation, we can get the cut
which is valid for the original MINLP problem before complementing, as
cvc+ 2 i6,.]y, + ]Ei (一 i(3’i ).yi 〈一7+2(一 iBi)
iEFUFo iEFI iEFI
And the following section is devoted to a theoretical comparison of the cut
generation method in this paper and that ofBalas et al. ( 1993; 1996 ).
3.5 Cut Comparison
The reverse polar set presented by B alas ( 1979 ) can be used to characterize all
valid inequalities for P」(K), and the facets of P,(K) are strong inequalities
( Balas et al., 1993, 1996 ) for eliminating the current optimal solution, i.e.
G,」), in a o-1 mixed-integer linear program. Then, it is very interesting to give
a comparison between the two kinds of cuts described by Balas et al. ( 1993,
1996) and that proposed in this paper, since the latter can be used independently
to solve MILP problems. Given (2,Y) be the current optimal solution to the
continuous NLP, then we assume that (i,i) is the closest extreme point of
P,(K) relative to (S,P), and P,(K) can be described by
乃(K)一{(x・ア):繭≦5}・Let the index set・be defined as
・一oi:爺+∂面}・then we have a c・nvex c・ne・riginated fr・m the p・int
〔i・y〕・i・e・k一{(x・ア):Z’x+∂’ア≦蜘∈・}・・bvi・usly we have P」(K)⊆瓦
since the latter is obtained by dropping off a number of constraints from the
former one. The polar cone ( or negative polar cone ) of K can be defined by
31
kO = ((u,v)E m”’q :〈(u,v), (x,」y)〉 sg o for all (x,」y)G k/.
Then, the following lemma ensures that the projection of (i,」) onto p,(K)
can be obtained at (S,i).
Lemma 3.6. Assume that we have the above definitions. and ,
itsprojectiononto P」(K) is (i,Y),andviceversa.
(il,P) G kO, then
Proof. Since we have P」(K)gK, then the assertion holds if the proj ection of
(S,Y) onto the cone k is (i,Y). This is implied by Proposition 3.2.3
( Hiriart-Urmty and Lemarechal, 1993 ) by virtue of (i,Y)Ek,
(5e,」P)一(i,5))G kO, and ((i,5))一(N,」V),(i,5))) =o, where the latter two relations
are satisfied by n・ting〔i・y〕isthe・rigin・f血ep・1arc・ne k.・C・nversely, by
virtue o f Theorem 3.1.1( Hiriart-Urruty and Lemarechal, 1993 ), we have
〈⊂i・P〕一⊂S・y}唖」〕〉≦・綱∈k・then〈〔i・y〕一〔え夕洞〉一・
implies thatq⊂i・P〕一⊂動(の≦・姻∈k・Then・ the lemma f・ll・ws by
notingthat (i,Y) istheoriginofthecone k anditspolar. O
It is very difficult to compare the cut quality and its role played in an
enumeration tree, since some cut may not work well j ust after its generation, but
will become very strong after some iterations. Then, even we use the closed gap
gained by the introduction of the cut, it is still not certain to measure the cut
quality,definitely. Here, we use the distance between the point and the cut to
32
evaluate the cut quality, which is first proposed by B alas et al. ( 1993; 1996 ) for
solving MILP problem by using branch-and-cut method. Very roughly, the
following theorem shows that the cut generated in this paper is the same as or
somewhat better for some Cases than the lift-and-proj ect cut proposed by Balas
et al. ( 1993; 1996 ). But, it should be carefu1 to note that the Euclidean distance
or 1,一norm is used in their method instead of 1,一nomi in this paper, then the
following comparison is just qualitative, and sketched simply in Figure 2.
Theorem 3.7. Assume that the projection of the solution point (i,」) onto
P,. (K) is not an extreme point of the P」 (K), then the cut generated in this paper
is either a facet-defining inequality of P」(K), or a stronger inequality in the
sense ofthe separation distance.
Proof. lfthe projection ofthe solution point (2,Y) onto p,(K), without loss
of generality denoted by (i,Y), is not an extreme point of the P/(K), then it
must lie on some facet of P」 (K). Then the distance between point (i,Y) and its
proj ection onto P」 (K), i.e. the minimal separation di stance, is equivalent to that
of the largest distance between that point and the facet defining P」(K) which
separ訊ing向丘・呪(K)・・bvi・usいhe l飢ter is the飴ceレde丘ning
inequality presented by Balas et al. ( 1993; 1996 ). Otherwise, we know (i,」)
is an extreme point of P」(K), or the origin of the cone K, and different from
(2,i), the projection point of (i,Y) onto some hyperplane defined by some
facet of p,(K) or cone k, i.e. the facet-defining inequality. Since (i,Y) also
33
lies on that hyperplane, then the distance between (i,Y) and (i,」) is always
longer than that between (S,P) and (2,P), which minimizes the distance
betweenpoint (il,Y) anditsseparatinginequality. O
4. Preliminary computational experiments for process design problems
In this section, the effectiveness of the disj unctive cut or lift-and-proj ect cut in a
branch-and-bound framework is demonstrated on the basis of the algorithm
developed at the preliminary stage. The algorithm has been implemented by
using standard C language on a Pentium III with 800 MHz CPU and 128 MB of
RAM, but the CPU time for two process synthesis problems are not reported
here due to the instability of the cut generation linear programming solver used
our branch-and-cut algorithm. Then the iterations or the enumerated nodes and
the LPs solved become the main comparison between a pure branch-and-bound
algorithm and the proposed branch-and-cut algorithm which has imbedded the
disj unctive cut into an enumeration process. Concerning of the specific
implementation aspects, the binary tree is searched according to the depth-first
principle and the cuts are generated at each enumerated node ifthere is a binary
variable for disj unction in its feasible solution. lt should be noted that the latter
choice has answered to the question given in the algorithm described in Section
2, i.e. branching versus cutting decision. But, for large practical problems, it is
recommended for MILP by B alas et al. ( 1996 ) that a skip factor obtained by
the calculation results at the root node be introduced in order to promote the
efficiency of the branch-and-cut algorithM. However, in this paper, we j ust
simply chose that parameter to be unity so as to observe the roles played by the
disj unctive cuts in an enumeration tree.
The continuous relaxation problems are solved by using an NLP solver
LSGRG2C ( Smith and Lasdon, 1992 ), and the cut generation linear
programming problems are solved by the same solver as above but all its
variables are set to be linear and a Phase 1 subroutine, i.e. a perturbation problem
in order to find a feasible solution, is first done before running this revised LP
solver. The two problems tested are related to the problem of synthesizing a
process system ( Duran and Grossmann, 1986 ) which is the one of
34
simultaneously determining the optimal structure and the operating parameters
for a process so as to satisfy the given design specifications. Since these two
MINLPI problems have nonlinear obj ective function, then the additional
continuous variables are introduced to transform them into the standard fomi
introduced in S ection 1. The first MINLP problem has 3 nonlinear constraints
and 4 linear constraints with 7 variables, where 3 of them are binary. The
transformed fomiulation ofthis problem is presented as
Test problem No. 1
mmlmlze X4
subj ect to 5y, +6y, +8y, +10x, 一7x, 一181n(x, +1)一19.2 ln(x, 一 x, +1)+10 一 x, sO
一 O.8 ln(x, + 1)一 O.961n(x, 一 x, + 1)+ O.8x3 g O
一 ln(x2 + 1)一1.2 ln(xi 一 x2 + 1)+ x3 + Uy3 一一 2 S O
X2 一一 Xl SO
κ2一の1≦O
x「κ2一乙ケ2≦0
ア1+ア2≦1
ア∈{0,1}3,a≦x≦b,」・c∈9{4
aT = (o, o, o,m), bT == (2, 2, 1,一), U =2
The solution to the continuous relaxation NLP problem at the root node for the
above MINLP problem is given by (2,Y)={1.14640, O.54581, 1.0, O.75919,
O.27290, O.30030, O.O}, then two disj unctive cuts can be obtained by solving the
cut generation linear programming problem, as
一x:1+0.01135x,+x3一κ4+Yi一ア2+8)ノ3≦一1.37445
一κ1-x2+x3一κ4一)ノ1-0.15312ア2+8)ノ3≦一2.85375
35
It is obvious to observe that the MP solution point, i.e. (i,Y), is violated by the
above two cuts, then this point will be cut away after adding those cuts into the
constraint set of the continuous relaxation problem of the original MINLP. By
checking the optimal solution of this MINLP problem to the above two cuts, it is
found that the optimal solution point, i.e. (x*,y“)={1.30097, O.O, 1.0, 6.00972,
O.O, 1.0, O.O}, is still feasible. The proposed branch-and-cut algorithm in this
paper converges on the optimal solution after 5 iterations ( enumerated nodes ),
the optimal solution was found at the 5‘h iteration and the number of the total
cuts generated is 2. The pure branch-and-bound algorithm converges on the
same optimal solution after 5 iterations and the oPtimal solution was found at 5th
iteration. The advantage of the proposed algorithm is not obvious for this small
example, but for the following slightly bigger problem, we will see that the
proposed algoritlm is more advantageous than the pure branch-and-bound
algorithm.
Test Problem No. 2
ロ
mlnlmlze XIO
subj ect to 5)ノ1・+8ア2+6.Y3+10)ノ4+6)ノ5+7)ノ6+4)ノ7+5)ノ8一一10κi-15x2+15x3+80x4
+ 25xs +35x6 一40x7 +15xs 一35xg + exp(xi)+ exp(x2 /1.2)一 651n(x3 + x4 +1)
一 90 ln(x, + 1)一80 ln(x, + 1)+120 一 xio S O
-1.5 ln(x, + 1) 一一 ln(x6 + 1)一 xs S O
-ln(x3 + x4 +1) 一く O
exp(x,)一10y, 一一1SO
’ exp(x, /1.2) 一一一10.Jy, 一一1 sil O
一一一 xl 一一一 x2 + x3 +2jx 4 + O.8x, + O.8x6 一〇.5x7 一 xs 一2xg 一く O
一 xl 一 x2 +2x4 + O.8xs + O.8x6 一2x7 一 xs 一2xg S O
-2x4 一〇.8xs 一〇.8x6 +2x7 + xs +2xg S O
-O.8x, 一一 O.8x6 +xs SO
-X4 +X7 +Xg gO
一 O.4x, 一 O.4x, +1.5x, S O
x3 一一 O.8x, SO
一一 x3 +O.4x4 sO
36
x, 一10y, go
o.sx, + o.sx, 一一loy, g o
2x4 一一 2x, 一2xg 一10y, SO
x, 一loy, go
x, 一10y, SO
X3 + X4 一10ys S O
ア1+ア2-1≦0
一アーア2+1≦0
ア4+ア5-1≦0
一ア4+ア6+ア7≦0
ア4一ア6一ア7≦0
ア3一ア8≦0
ア∈{0,1}8,α≦x≦b,x∈9Z・’O
aT@一 (o,o,o,o,o,o,o, o, o,一), bT 一 (2, 2,1,2,2,2,2,1,3, 一)
The second MINLP problem has 5 nonlinear constraints and 20 linear
constraints with 18 variables, where 8 of them are binary. lt should be noted that
no only the obj ective function was transformed into linear function in this
formulation by introducing new continuous variable compared with that
described by Duran and Grossmann ( 1986 ), the equality constraints in their
formulation were also changed to two equivalent inequality constraints. The
optimal values of this MINLP and its continuous relaxation NLP problem at the
root node are 68.00974 and 15.08219, respectively. For this middle scale
problem, the proposed branch-and-cut algorithm converges on the optimal
solution after 17 iterations ( enumerated nodes ), i.e. 17 NLPs were solved, and
optimal solution was found at 16’h iteration after generating 21 cuts, i.e. 21 LPs
were solved. The branch-and-bound algorithm found the optimal solution at 21St
iteration after fathoming 23 nodes, i.e. altogether 23 NLP were solved. At this
preliminary stage, it is found that the proposed algorithm has saved 6 NLPs at
the cost of solving extra 21 LPs. Then, it is very prospective for large-scale
process synthesis MINLP problems to use this proposed branch-and-cut
algorithm since the LP i s always much computationally cheaper than the NLP
for large problems.
37
5. Conclusion
A branch-and-cut algorithm is developed in this paper for solving O-1
mixed-integer nonlinear programming problems where the lift-and-proj ect cuts
generated at each enumeration node are incorporated into a branch-and-bound
framework. lt should be noted that the lift-and-proj ect cut generation due to the
disj unctive property of the binary variable can not directly extended into the
general mixed-integer programming problems. A linear approximation scheme
is used to replace the continuous relaxation nonlinear programming problem by
a linear programming problem, and it is proved that the disjunctive cut yielded
over this linear set is able to cut away the solution point to the continuous
relaxation nonlinear programming problem of the .O-1 mixed-intgger nonlinear
programming at an enumeration node. The cut generation linear programming is
derived according to the separation theorem of the convex analysis, and the dual
problem of this linear programming is applied to lift the disj unctive cut at some
node to be valid throughout the enumeration tree. F or the cut coefficients of the
free binary variables, a strengthening process is proposed by imposing
integrality on those variables after the cut generation process and lifting. The
theoretical comparison of the disjunctive cut developed in this paper and that
according to the dual description states that the former i s either coincides with
the latter as the facet-defining inequality ofthe convex hull of the disj unctive set
or a stronger one as’separation inequality. Two typical chemical design O-1
mixed-integer nonlinear programming cases are solving by using a tentative
implementation of the branch-and-cut algorithm developed in this paper. The
results show that the branch-and-cut algorithm i s very promising for large scale
O-1 mixed-integer ponlinear programming problems which are widely
encountered in the chemical engineering community for process design and
synthesis.
Acknowledgement
Y.Z. gratefully acknowledges financial support from Japan society for the
Promotion of Science ( JSPS ) and JSPS Fellow Project O I O40, and sincerely
appreciates the kind help from Prof. E. Balas ( Carnegie-Mellon University ) for
his patient explanation of the lifting and strengthening process. Y.Z. is a JSPS
Fellow. He is also gratefully supported by a Grant-in-Aid for Scientific Research
且om JSPS.
38
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41
Figure 1. Linear approximation of the MINLP problem, where the obj ective
function 一x is minimized subj ect to a mixed-integer convex set described by a
continuous variable and a binary variable, respectively.
y
1
/
ノ
ノ1
;s’ /
s/ /
ノ1
/ 1
s’ 1ノ
ノ 1
(s, 」)
o/
/
42
Figure 2. A simple geometrical sketch for cut comparison.
P, (K)g K
〔籾
’ . //
/ /
/ /
/t /
/ /
./ // /
/
;モ、 〔i・P〕
1 ’N N..
1’ N. 一〉,.,×
il 〉..x“”“”“’NxN
11...,.......,..,“・:・・“’XNN ”Nx..
li,“““’ ’m N
N
o
K
43