Memory usage when repeatedly solving a problem

Your subroutine contains some unary constraints ('is_binary' and bounds on variables) that are not reset. The easiest solution would probably be to rewrite your subroutine as follows:

procedure ND
	
declarations
	z: array(PLANT, DC, PRODUCT) of mpvar
	x: array(DC, CUSTOMER, PRODUCT) of mpvar
	y: array(DC) of mpvar
	penalty_dist: array(CUSTOMER) of mpvar
	CO2: mpvar
end-declarations
 
with mpproblem do
 
forall(i in DC) y(i) is_binary
 
obj:=sum(i in DC, j in CUSTOMER, k in PRODUCT) x(i,j,k)*INT_DISTANCE(i,j)*INT_TRANS_OUT_COST + sum(i in DC) y(i)*INT_WHS_FIX_COST + sum(i in PLANT, j in DC, k in PRODUCT) z(i,j,k)*INT_DISTANCE(i,j)*INT_TRANS_INB_COST + sum(j in CUSTOMER)penalty_dist(j)*9999999 + CO2/1000*INT_CERTIFICATE_COST
 
C0:=CO2=sum(i in DC, j in CUSTOMER, k in PRODUCT) x(i,j,k)*INT_DISTANCE(i,j)*INT_EM_OUT + sum(i in PLANT, j in DC, k in PRODUCT) z(i,j,k)*INT_DISTANCE(i,j)*INT_EM_INB
 
forall(i in DC, k in PRODUCT)
	C1(i,k):=sum(m in PLANT)z(m,i,k) = sum(j in CUSTOMER)x(i,j,k)
 
forall(i in DC, k in PRODUCT)
	C2(i,k):=sum(j in CUSTOMER)x(i,j,k) <= y(i)*INT_BIGM(k)
	
forall(j in CUSTOMER, k in PRODUCT)
	C3(j,k):=sum(i in DC)x(i,j,k) >= EXT_DEMAND_QUANTITY(j,k)*INT_DEMAND_MULTIPLIER
	
forall(i in DC) do
if EXT_DC_STATUS(i)=1 then
	y(i)=1
end-if
if EXT_DC_STATUS(i)=0 then
	y(i)=0
end-if
end-do
 
forall(i in DC, j in CUSTOMER, k in PRODUCT)
	C4(i,j,k):=x(i,j,k)*INT_DISTANCE(i,j) <= x(i,j,k)*INT_MAX_DISTANCE_CUST(j) + penalty_dist(j)
	
minimize(obj)
 
![...] write output to console
 
end-do    ! Leave the subproblem (resets solution info and deletes all constraints)
end-procedure

While working on your model, in order to track the memory consumption, you could add a call to ''memoryuse" after terminating an iteration of the subroutine (remove such calls for production runs as they add to the execution time).